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Moon Rover Project

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Moon Rover Project

  1. 1. 22/04/2013 Lunar Rover Design Project | DM401 APPALING 13 – DRIVE TEAM – INTEGRATED MECHATRONIC DESIGN PROJECT – LUNAR ROVER Alexander Clayton Catriona Provan Elaine Macqueen Kerrie Noble Simon Walls
  2. 2. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 2 Appalling 13 – Lunar rover Drive System Sub-Group Abstract This report details the design process undertaken by the drive team sub-group element of the Appalling 13 mechatronic design project team. This report will guide you through the market research conducted by the team, including discussion on previous missions and the technology they incorporated, how this has left a legacy within space flight and mission to the lunar surface, and also subsequently mars. This market research then leads to the discussion of requirements which must be met by this design in order for it to be sanctions fit for a lunar mission. The requirements cover an extensive range of areas including design for the moon, standards and references, fasteners, bearings, lubricants, motors, power systems and other general considerations. These outlined requirements, combined with the team’s Product Design Specification will help to outline the design constraints and user requirements within this design project. The project will then enter discussions and exploration of the concept development of the lunar rover design. This includes exploration through a function means tree and concept drawings of initial ideas. Following this a detailed section will discuss the selected components required for this design, including the Maxon RE30 motor and the radioisotope power system. A visual exploration of the design, of both a theoretical physical model and a realistic CAD model will further detail the chosen design.
  3. 3. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 3 Contents Appalling 13 – Lunar rover Drive System Sub-Group ....................................................................1 Abstract...............................................................................................................................................2 1. Introduction ................................................................................................................................5 2. Market Research.........................................................................................................................5 2.1 Existing Products.......................................................................................................................6 2.2 Technology Research ..............................................................................................................11 3. Requirements............................................................................................................................15 3.1 Design for the moon ...............................................................................................................15 Issues for Lunar Machinery...........................................................................................................16 3.2 Standards and References ......................................................................................................16 3.3 Fasteners.................................................................................................................................17 3.4 Bearings...................................................................................................................................17 3.5 Lubricants................................................................................................................................18 3.6 Motors.....................................................................................................................................18 3.7 Power System Components – Solar Arrays.............................................................................18 3.8 Power System Components – Batteries..................................................................................19 3.9 General Considerations...........................................................................................................19 3.10 Product Design Specification ................................................................................................19 4. Function Means Tree ............................................................................................................24 5. Concept Generation..................................................................................................................24 6. Selection of Components For Chosen Design...............................................................................27 6.1 Moon Rover Wheel Motors and Gearing................................................................................27 6.2 Rocker Bogie Suspension ........................................................................................................30 6.3 Power Requirements for the Mission.....................................................................................30 6.4 Power Source ..........................................................................................................................31 6.5 Materials .................................................................................................................................36 Aluminium.....................................................................................................................................36 High Strength Plastics ...................................................................................................................36 Carbon Fibre..................................................................................................................................37 Composite Materials.....................................................................................................................37 Chosen Material............................................................................................................................37 6.6 Joining methods......................................................................................................................37 6.7 Shape.......................................................................................................................................37
  4. 4. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 4 6.8 Fasteners.................................................................................................................................38 7. Calculations...............................................................................................................................38 7.1 Static Calculations...................................................................................................................38 7.2 Power Requirements for Planned Mission .............................................................................40 7.3 Power Transmission Calculation .............................................................................................41 8. Prototyping ...............................................................................................................................42 8.1 The Theoretical Prototype ................................................................................................42 8.2 The Realistic Prototype...........................................................................................................45 9. Project Management ................................................................................................................50 10. Conclusion.............................................................................................................................54 More Flexibility .............................................................................................................................55 Minimise Energy Usage.................................................................................................................55 Navigate Difficult Terrain..............................................................................................................55 References ........................................................................................................................................56
  5. 5. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 5 1. Introduction Moon exploration is still not considered a thing of the past. Future exploration missions to the moon will look at creating global maps of unprecedented quality, captured by at least four robotic missions which will orbit the moon. These exploration missions to the moon, and its mysterious surfaces, especially those situated around the Polar Regions, will require soft landings in order to map the surface, examine the volatile deposits and characterise the unusual environment which exists there. Other plans for moon exploration also include plans to return humans to the moon, however this time it will not be to prove what man-kind can do, as in the Apollo mission in 1969, but instead it will ultimately explore how the moon could be used to support a new and growing spacefaring capability. Whilst on the moon the main aims will be to learn the skills and develop the technologies which are needed to live and work on another world. This report outlines the steps taken to design a more advanced moon rover to help with future exploration missions to the moon. The project has taken inspiration from previous moon rover designs, however these past designs are still very basic and there are various highlighted instances where improvements could be made. Such areas for improvements are; designing so that the moon rover has a more flexible ability to navigate around different terrains, to design to minimise energy usage to enable a maximum distance to be covered during the mission, and finally to design in the ability for the moon rover to negotiate terrain surfaces with different terrain surface quality and hardness. The decision taken within this project was to concentrate on these main areas and use the advancement in technology within these areas to help design a moon rover with substantial capabilities in these three areas. It is hoped that with this design consideration a more advanced, modular moon rover design with more efficient drive systems can be achieved. The main outcomes from this initial project brief are detailed below with evidence of further research and development throughout the design process. The design process used throughout this project was the standard Pugh process. This allowed for quick and wide-ranging development of ideas while also leading to detailed technical development in a minimised period of time. 2. Market Research In order to develop a greater understanding of existing products within the space market, a market research portfolio was produced, detailing some current and past moon rover designs which have been developed by NASA. The information gathered from this process concentrates heavily on the dimensions and aspects of the drive systems of all existing products covered. The outcomes from this research portfolio are shown below.
  6. 6. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 6 2.1 Existing Products Sojourner Micro Rover The Sojourner Micro Rover was one of the first Mars exploration rovers operating in the late 1990’s for a duration of 83 earth days. It operated using a non- rechargeable battery and a solar panel in order for the rover to operate through the day. The design is comprised of 6 wheels, each of diameter 130mm. In this design each wheel is independently actuated resulting in high torque to allow for inclines and variances within the rough terrain. Due to the high torque being incorporated within the design the Sojourner Micro Rover can only achieve a top speed of 0.4m/min. This may seem like a slow speed, however when considered within the context of this project a slow speed with high torque seems necessary in order for the moon rover to have the ability to negotiate terrain surfaces with different terrain quality and hardness. At this stage of the project the team have therefore identified that the final calculations for the design of the more technically advanced moon rover should illustrate an achievable high speed which is slow in comparison to some other moon rover designs and it should also incorporate a high toque system. (NASA, 1996) Spirit Rover The Spirit Rover is another NASA designed Mars Rover. Originally designed for a 90 day mission, the Spirit Rover operated for more than 6 years due to environmental events resulting in activity between the years of 2004-10. In terms of the power requirements within this design, the rover used solar arrays and rechargeable batteries which in turn allowed for 4 hours of activity during one Martian day. This energy could be stored for use at night. With 6 independently motorised wheels the rover could achieve a maximum speed of 3m/min and stability on tilts of up to 30 degrees. Within the context of the moon rover design for this project, it is important to note what may be important power requirements which have been illustrated through this rover design. For this design project the team have highlighted that it will be necessary to consider the storing of energy for use at night throughout the missions. (NASA, unknown date) Figure 1 – The Sojourner Rover Figure 2 – The Spirit Rover
  7. 7. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 7 ATHLETE Rover The ATHLETE (All-Terrain Hex-legged Extra-Terrestrial Explorer) is a system which is currently under development for use on lunar rovers. Its 6 legs offer the design the ability for movement with 6 degrees of freedom and the design of these legs allow the rover to both roll and walk over difficult terrains. With a leg reach in the region of 6 meters this allows the rover to operate on slopes of 35 degrees, this is a larger slope than conventional rocker-bogie drive systems can obtain. (NASA, 2008) In the context of the moon rover design project, this existing design again highlights the importance of flexible travel and the part this plays in how the rover will be able to negotiate the different terrains which it may face. As this is an on-going development the team feel that this design is important as some of the included hardware in this design is more technically advanced that what has been expressed previously through the research shown on the other rover designs from NASA missions. In this case the team feel that a lot of lessons can be learnt from this development and therefore refining and further development of these ideas within the design project could prove to be very beneficial. Curiosity Rover The latest in mars rovers, the Curiosity rover, landed on the Martian surface in august 2012 to begin a two year mission, although this has been extended indefinitely due mainly to its platonium-238 core. This allows for thermoelectric energy generation, meaning the rover can operate day and night during any season for a minimal duration of 14 years. The heat energy generated from the decay of the isotope is the converted to electrical energy which then recharges two lithium-ion batteries. The rover uses a rocker-bogie suspension system equipped with 6 independently actuated and geared wheels of diameter 500mm. The vehicle can withstand tilts of up to 50 degrees however on-board sensors limit this angle to 30 degrees. (NASA, 2012) In terms of Figure 3 – The Athlete Rover Figure 4 – The Curiosity Rover
  8. 8. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 8 flexibility in movement, efficient power source generation and the rover’s ability to traverse very rough and difficult terrain, this deign proves to be a good benchmark for the design of the moon rover within the context of this project. Any design generated by this project will be ultimately assessed against the abilities of this rover as this is currently the most advanced design which NASA have used on any mission. (NASA, 2010) The Successful Legacy of the Lunar Rover The Apollo Project has left a very important legacy in terms of missions to the lunar surface. The first legacy, which is less significant within the context of this project, was the successful accomplishments, politically, for which the Apollo project had first been created. The second legacy left by the Apollo project was the triumph in managing and meeting difficult engineering and technologically integrated requirements. The Apollo project proved that although the technological challenge posed by the mission was sophisticated and impressive, the result which was being targeted was very much in the grasp of the lunar rover and NASA. However, to achieve this, the access to required resources was a necessity. This legacy can provide some necessary insight for this design project. Although the requirements posed by this design project are sophisticated, a successful outcome is within grasp. The final legacy left by the Apollo programme was the way in which the program forced every person of the World to look at planet Earth in a different way. As the newly designed moon rover emerging from this project has the potential to complete future exploration missions to the lunar surface, this is also an important legacy to consider. (NASA, 1999) (Beale, D., unknown year) Figure 5 – The lunar rover in space
  9. 9. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 9 Frame details from the first Apollo Programme mission show that the frame was constructed from Aluminium 2219 welded tube. The frame was a fully foldable frame and this is illustrated in the original design diagram shown to the left. (Kurt, 2004) Drive Details The drive details for the first lunar rover are listed below; • Four, ¼ horsepower, electric motors were located at each wheel, meaning each wheel had an individual drive system • Top achievable speeds of up to 17 km/h could be achieved (Baker, D., 1971) (Kudish, H., 1970) • The motors were geared in order to reduce the speed of the motor. The gearing ratio was 80:1 with a harmonic drive gearing. This reduced the overall speed that each of the wheels within the design would be travelling, allowing for more considered movement of the buggy, especially beneficial when moving over difficult terrain. (Gear Product, 2006) • The motors and harmonic drive were hermetically sealed and pressurized to 7.5 psia (pounds per square inch absolute) to protect from lunar dust and for improved brush lubrication. • Braking was both electodynamic by the motors and from brake shoes forced against a drum through a linkage and cable. (Kudish, H., 1970) It is essential to remember that at the time this rover was at the top of technological development. However with growing development, much of the technology used within this design has now been out-dated by further progression in this area. Although this highlights a simple design solution, in the context of this design project, it is essential that this simplicity is kept but integrated with more developed technological concepts. (Beale, D., unknown year) Figure 5 – The frame design of the first lunar rover in space Figure 6 – The frame design of the first lunar rover in space Figure 7 – The frame design of the first lunar rover in space
  10. 10. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 10 Other Rover Details Other details concerning the construction and drive system of the first Apollo Lunar Rover have been listed below; • The suspension was a double wishbone suspension system, each wishbone feature was attached to a torsion bar and a damper was placed between the chassis and upper wishbone. • The wheels consisted of an aluminium hub, a tire made of zinc coated woven piano wires and titanium chevron treads attached to the rim and discs of formed aluminium. Dust guards were mounted about each wheel • Front and rear wheel steer was accomplished by an Ackermann-geometry steering linkage system, driven by an electric motor servo-system that amplifies the left and right joystick motion from the astronaut. • In order to protect the LRV against the thermal environment of the moon several different thermal control systems were incorporated into the LRV design. These systems consisted of MLI blankets (Multi-layer insulation) covered by Beta Cloth, space radiators, mass heat sinks, special surface coatings and finishes, and thermal straps. • One of the main problems that the LRV encountered was an issue concerning lunar dust lying on the surface of the moon. Degradation of thermal and electronic components was a problem as well as the wear and tear of components and other surfaces from the abrasive lunar dust. (Beale, D., unknown year) At the end of the market research section it became apparent that there are many different types and designs of rover which have been designed and manufactured by NASA. The main difference between each of these designs is the technicality of the design, the ability with which the technological aspect provides the rover and the differing configurations which produce different results in certain circumstances. As it became apparent that technological aspects of the design were some of the main issues and features the team moved into the technological review process. Figure 8 – The wheel design of the first lunar rover in space
  11. 11. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 11 2.2 Technology Research The main technological areas, mainly identified from the previously conducted research, which were highlighted by the team as areas which would be the key focus areas for this design project, were the drive system requirements and the use of a Rocker Bogie suspension system. The team felt that by combining these two elements the most satisfactory outcome, in terms of flexibility and the ability to move over differing terrains would be achieved through the use of this type of design. Drive Mechanisms Research has shown that many common drive mechanisms for rover design include the following capabilities; • 4, 6 and 8 wheel drive systems are commonly used to help provide stability within the design, especially where traversing over inclined terrain is a possibility. • Walking and rolling drive systems have been tested in locations such as Mount Spurr Alaska or on active volcanoes to prove terrain navigation concept and ability. • All battery operations (normally lithium battery) are now controlled with recharge systems such as solar radiation. • Engineers are in the early development stage of skid steer. This is a drive system where wheels on either side are synchronised allowing each side of the frame to move independently. This provides a 0 degrees pirouette capability within the rover design resulting in good movement control however this often results in the rover tearing up the surface of the ground on which it is moving. This type of system does not involve the use of a rigid frame in order to provide better balance capabilities within the rover. This is shown through the Ratler design example. • Over the past decade development of the rocker bogie suspension system, which is now used in Mars exploration, has been rapid. This design is regarded as the best design for vehicle stability and obstacle climbing capability. (NASA, 2003) Figure 9 – Initial development of the mars exploration rover
  12. 12. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 12 Due to the apparent benefits of using a Rocker Bogie suspension system the team have also completed some research on the technical capabilities of this type of suspension system. Rocker bogie suspension The main technical aspects of a Rocker Bogie Suspension system are discussed below; • There are two primary components within a Rocker Bogie suspension system, the Rocker and the Bogie. This shown clearly in one of the images below. • These two elements are connected via a free rotating pivot; this is again demonstrated in the image shown below. • The design has 6 joints that must be reliably locked after deployment. One joint is motor driven for deployment. • Yoke and clevis design for a rocker bridge joint (motorised deploying joint) withstands 714 N-m bending load and 506 N-m torsional load. • Latch pawl locks into place on the deployed arm, this changes the state of the micro switch used within the design and subsequently sends electrical signals to state that the arm is successfully locked and ready for deployment. (NASA, 2003) Figure 10 – Rocker Bogie joint design and movement
  13. 13. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 13 Extreme Tyres The nature of the lunar surface has already been highlighted extensive throughout the research which has been conducted. It has become apparent that there are extreme conditions to overcome through the design of the wheel on the moon rover. Within the design of the drive system of the moon rover the team have decided that it will be necessary to include wheel design within this project. The object of this wheel design is to provide a tire that can withstand multiple punctures in addition to being high-performance, efficient, high capacity, and long lasting. This is an extreme which must be met as this is critical to the overall success or failure of the overall lunar rover design. Previous lunar rovers have also been designed with these wheel requirements having been considered. Some lunar rovers had wire mesh tires made from zinc-coated piano wire, and the tires on military vehicles are often outfitted with special inserts that enable them to drive on a flat tire. However, the demands on tires for both types of vehicles are rising, requiring tires that can support more weight and travel further under stress. The team have identified similarities between the design for military vehicles and the lunar rover. The tolerances and extremities to be met by both designs are much the same. For this reason the research below outlines some technological advances within the military design world which, the team believe, would be greatly beneficial within this lunar rover design task. Tire companies, such as Michelin and Pirelli, have developed a few promising prototypes for these extreme applications, some working in partnership with NASA. From a distance these newly developed tires may not look very different than +300 million tires that are discarded annually in the United States. On closer inspection, the design reveals that these tires are not filled with air, and in some cases do not even use rubber. In 2005 Michelin introduced the Tweel. The Tweel consists of a central hub connected to a rigid outer rim by flexible spokes. The spokes and hub are made of plastic structures that deform when the tire goes over rough terrain, within the context of this project, the terrain discussed here would be similar to the differing quality and hardness of terrain of the lunar surface. This enables a large portion of the tire to stay in contact with the surface, even over uneven terrain, which provides traction and stability. As previously discussed this is a major requirement of any new developments within lunar rover design as flexibility and the ability of the lunar rover to move over differing types of surface are now key design requirements. The deformable hub and spokes also act as shock absorbers that help reduce the vibrations felt by the vehicle. Additionally, the tire functions well even if some of the spokes are damaged. At high speeds the Tweel has run into some problems with noise, vibration, heat, and wear. With designing for the lunar rover in mind, this would not cause many issues as the intention of the design is to travel at low speeds with high torque. Figure 11 – Apollo 11 mission
  14. 14. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 14 Currently, the plan for Michelin’s Tweel design is to introduce it in smaller vehicles first, such as scooters, small construction equipment, and wheelchairs. In 2009 a small prototype of the new lunar rover, driving on Tweels, was trialled and formed part of President Obama’s inaugural parade. (American Physical Society, 2012) In other tire developments, a next generation tire for the Humvee is made of polymers and has the interior structure of a honeycomb. The honeycomb structure can support heavy loads and has lots of room for shrapnel to pass through, while still offering a relatively smooth ride. Additionally, if as much as 30% of its cells are damaged, the tire suffers little performance loss. Humvees can currently travel on a flat tire because of their insert design, but testing shows that the honeycomb design enables them to travel significantly faster and further. This style of design would obviously also be beneficial to any lunar rover design due to its strong durability and ability to withstand large amounts of force and pressure. (American Physical Society, 2012) Within the context of lunar rover design, the Apollo lunar rovers, collaboration between the NASA Glen Research Centre and the Goodyear Tire & Rubber Company, resulted in a tire made entirely from springs. Each Spring Tire consists of 800 helical springs that are woven together by hand. As the tires travel over rough terrain, the springs flex and relax in conjunction with changes in the surface. The springs are woven in such a way that they can support heavy loads and continue functioning even when some of the springs are damaged. This design produces little heat and little energy loss, and is not affected by vast temperature differences like those between night and day on the moon. Figure 12 – Tweel in use on rough terrain Figure 13 – Tweel in use on military vehicles
  15. 15. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 15 In 2009 the Spring Tire was installed on NASA’s lunar test vehicle, which had a successful ride through the Johnson Space Centre Planetary Analogue Test Site, (aka “rock yard”). There are obvious benefits to using this tire design within any design produced within the context of this project. Firstly this tire has been developed for specific application on the moon, therefore the functional aspect of this design will have great focus and detail within the design to produce the best result once the rover has reached the lunar surface. The design in this tire also means that if any design changes are required, then extensive testing of specific expected conditions can take place with appropriate mechanical test values being applied. This will ensure the wheel is fit for purpose. In the case of the other designs, testing may not be specific to use on the lunar surface, therefore caution will have to be applied, and resulting in a longer testing time period before the tire can be used within any lunar application. (American Physical Society, 2012) The research presented here has ultimately led to the identification of many design requirements for this mechatronic design project. 3. Requirements Many requirements for the design of the lunar rover drive system have been identified within many areas, such as design for the moon, specification standards and fasteners. These areas of design requirement have been discussed in detail below. 3.1 Design for the moon As the identified environment in which this design has to operate successfully, then designing for lunar requirements is the most important design aspect within this project. The focus is on choosing and testing components that may be of concern to a mechanical designer. This includes any component which may fail under loading during any operational aspect of the lunar rover functionality, such as deployment or simply navigating the lunar surface. These components can vary from mechanical components (bearings, fasteners, and lubricants), motors, materials and an Figure 14 – Goodyear spring tire
  16. 16. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 16 overview of power systems. Component design and selection for use on the moon is driven by the application and the environment. As has been discussed already, the lunar environment can vary depending on the region of the lunar surface which the rover may be navigating. This environment includes moon dust, uneven surfaces, slopes and regions which little information is known about, i.e. the Polar Regions. For this reason it is important to consider legacy. Legacy “refers to the original manufacturer’s level of quality and reliability that is built into the parts which have been proven by (1) time and service, (2) number of units in service, (3) mean time between failure performance, and (4) number of use cycles.” If a candidate component has a successful legacy, then a designer should strongly consider using it. As the research presented above has highlighted many instances where several components have been used successfully within the design of a rover, then these components will be considered for use in this design due to their proven ability to operate within the intended environment. (Robotics Institute, 1995) (Beale, D., unknown year) Issues for Lunar Machinery Considering the harsh environment in which the lunar rover will be used, there are clearly going to be issues which may affect the functionality of the lunar rover design and the components which have been used to construct it. Some of these key issues include; • Abrasion and wear on parts that contact regolith • Vacuum welding of metals, which may require special coating and treatments. • Electrostatic properties of regolith will cause it to adhere to and penetrate bearings, structural connections, viewing surfaces, solar panels, radiators and antennas. • Strategies must be put in place to create effective vacuum seals (e.g. for door locks) and effective bearings (including lubricants, filters, and seals for bearings). These points all represent possible modes of failure for the design and therefore these issue need to be considered in depth while proceeding through the design process. In order to achieve a successful, fully functioning lunar rover, testing will be required at every stage to ensure these potential issues have all been accounted for within the mechanical design of the rover. (American Society of Civil Engineers, 2002) (Beale, D., unknown year) 3.2 Standards and References As with any design project, there are many standard specifications and requirements from professional bodies which the design must meet in order to be certified as ‘fit-for-purpose’. The design standards, including space and lunar specific standards which apply to this design project for a lunar rover are listed below; • AIAA S-114-2005, “Moving Mechanical Assemblies for Space and Launch Vehicles” • The Proceedings of the Aerospace Mechanism Symposium. • NASA/TP-1999-2069888 - NASA Space Mechanisms Handbook. • MIL-HDBK-5 Metallic Materials and Elements for Aerospace Structures, Other Standards: • DOD-HDBK-343 Design, Construction, and Testing Requirements for One of a Kind Space Equipment • MIL-STD-100 Engineering Drawing Practices
  17. 17. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 17 • MIL-STD-1539 Direct Current Electrical Power Space Vehicle Design Requirements • DOD-E-8983 General Specification for Extended Space Environment Aerospace Electronic Equipment • MIL-S-83576 General Specification for Design and Testing of Space Vehicle Solar Cell Arrays • DOD-STD-1578 Nickel-Cadmium Battery Usage Practice for Space Vehicles The lunar rover, when complete, will be tested against the various technical data outlined in each of these standard specifications, therefore it is essential that these requirements are met throughout the design of the lunar rover to avoid costly redesign at a later stage in the process. (NASA, 2012) (Beale, D., unknown year) 3.3 Fasteners Fasteners are an integral component within any design. A failure with a fastener could cause the failure of the lunar rover and its ability to navigate across terrain. It is therefore necessary to choose the correct fasteners for use under the loading and pressure shown within the lunar design. Space fasteners design choices, with attention given to aerospace applications, materials and temperature ranges, are presented in the Fastener Design Manual (Barrett, 1990), NASA Report RP-1228 (NASA, 1990) MIL-HDBK-5 also contains allowable strengths for many fasteners including those used for MS (military standard) and NAS (national aerospace standard) (Standard Aero Parts Inc., 2013) (Beale, D., unknown year) 3.4 Bearings Bearings are essential to generating the smooth movement of the lunar rover across varying types of terrain. If the bearings used within the design cease, or become jammed due to moon dust, then the rover movement will ultimately fail. Rolling-element bearings for lunar applications must capably withstand the challenges of the lunar environment (temperature extremes, penetrating regolith and the vacuum environment) and be highly reliable to minimize repairs. For space flights the AISI 440C (a high hardness, corrosion resistant steel) and AISI 52100 (not as hard or corrosion- resistant, but better wear resistance) are the most common bearing materials. Shields and seals Figure 15 – Table of space flight suitable materials
  18. 18. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 18 cover the rolling element so they are not exposed and protected to a certain degree from outside contaminates like regolith. Shields and seals are attached on a bearing’s outer race, and move with the outer race. A shield will not touch the inner race because of a small clearance gap. Seals do rub against the inner race but will be less likely to allow regolith particles inside. Thermal control is a concern in a lunar environment where convection is not an available heat transfer mechanism. Thermal conductivity through a bearing is increased by the presence of a lubricant. (Robotics Institute, 1995) (Beale, D., unknown year) 3.5 Lubricants Bearings require lubricant to work to the functionality in which the design intended to be used. For bearings to work to their full potential and free and easy movement to allow flexible motion of the lunar rover lubricant is required to be used on all moving parts of the lunar rover. This will help overcome any restrictive and wearing movement caused by the moon dust on the lunar surface. In the past, Lubricant inadequacies have been implicated as a cause of a number of space mechanism failures. The three types of lubricants are liquids (lubricating oils, lubricant greases) and solid films, any of these three types may be present within the lunar rover. An ideal lubricant would retain the desired viscosity over a wide temperature range in response to the varying and often extreme temperatures which may be experience in the lunar environment. Lubricants are more volatile in vacuum and heat than higher molecular weight lubricants, this makes the choosing of a non-volatile liquid for use in the lunar rover design a key consideration as the lunar rover will be used within a vacuum setting. Solid films, such as soft metal films, polymers and low-shear strength materials, find use in bearings, bushings, contacts and gears. (Robotics Institute, 1995) (Beale, D., unknown year) 3.6 Motors Motors are the essential element which provides the drive mechanism for the wheels. The motor is the source of speed and torque, determining if the lunar rover has the ability and flexibility to navigate difficult terrain. The types of motor most commonly used include DC brush, DC brushless and stepper motors. (Beale, D., unknown year) 3.7 Power System Components – Solar Arrays The most widely used and cost efficient form of energy conversion is the photovoltaic solar array. There are three different types of solar array, each provide different power outputs and are structured in a different way. The three types of solar array are; Single-Crystal Silicon Cells, Gallium Arsenide Cells, and Semi-Crystalline & Poly-Crystalline Cells. There is currently not enough data on amorphous cells in order for these to be selected as a serious candidate for space applications as this a new and emerging technology. When this type of technology has undergone more testing then this may constitute a power source of the future. Multi-junction cells offer high efficiency and good manufacturability. Solar arrays can provide power requirements from tens of watts to several kilowatts with a life span of a few months to fifteen years. The life of a solar array degrades due to the space environmental effects on the photovoltaic cells. (Beale, D., unknown year)
  19. 19. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 19 3.8 Power System Components – Batteries The use of solar arrays or nuclear power are used to provide the lunar rover with a supply of energy, it is one of the other components, the battery, within the power system which is used to store the energy being generated by this energy source. Some of the common types of battery used include; • Silver Zinc Batteries • Nickel Cadmium (NiCd) • Nickel Hydrogen (NiH2) – Currently used in place of Nickel Cadmium for space applications • Nickel Metal Hydride (NiMH) • Lithium-Ion (Li-Ion) (Beale, D., unknown year) 3.9 General Considerations Having considered the more detailed requirements of many aspects of the drive system design within the lunar rover, there are a few general requirements which must also be met. • Any hardware or materials used for lunar missions will need to be of a special variety known as "Flight Qualified". • Flight qualified materials and parts are always flight proven hardware with program heritage. • The process to get any new material or part flight qualified is an arduous and long task. (Beale, D., unknown year) This highlights the final design considerations which the group must consider within the lunar rover design for this project. With all of this information documented, the other design considerations were also listed within the Product Design Specification which is detailed below. 3.10 Product Design Specification The PDS below outlines points 16 of the most influential areas when considered in the context of the design of a lunar rover. Each of the 16 areas is clearly identified and all of the specification points related to that area are listed clearly in each section. 1. Performance 1.1 The moon rover must be able to navigate a variety of different terrains that include surface quality and hardness. 1.2 The moon rover must be able to withstand atmosphere in space. 1.3 The moon rover must be able to operate within a gravitational pull of -1.6ms-2. 1.4 It must minimise energy consumption during operation. 1.5 The moon rover must anticipate obstacles such as hole, rocks, walls and ditches with on-board analysis to prevent the vehicle becoming stuck on the surface on the moon. 1.6 The moon rover must be able to negotiate out of unforeseen locations that may cause the rover to become stuck. 1.7 The moon rover must be able to detect the inclination angle when it is on slope (the angle between the horizontal direction of movement and the road surface or the rover base surface) so that the intelligent controller will be able to detect if the centre of gravity is within the safe region to avoid tipping-over accidents.
  20. 20. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 20 1.8 The moon rover must be able to move forward for 200cm approximately and backward for 50cm. 1.9 The system must detect a possible obstruction while it is moving forward, it must stop and move backward to re-route its movement. 1.10 The moon rover must be able to carry a load of up to 150kg. 2. Disposal 2.1 The device will not be returning to earth hence all parts should be non-hazardous to moon environment. 2.2 The device should be able to be left or destroyed to ensure no design secrets can be obtained by other governments. 2.3 Careful consideration should be taken when choosing the materials for the device (See ‘Materials’) 3. Processes 3.1 There are no limitations to the manufacturing processes as there are no constraints on the manufacturing facility. 4. Time Scale 4.1. 01/10/12 Project Introduction 4.2. 22/10/12 Project Specification and Project Planning 4.3. 05/11/12 Concept Development 4.4. 03/12/12 Concept Evaluation in Sub-groups 4.5. 10/12/12 Detail Design 4.6. 10/12/12 Milestone: Final Concept chosen 4.7. 11/01/13 FEA optimization 4.8. 18/03/13 Prototyping 4.9. 22/04/13 Final Presentation 4.10. 22/04/13 Milestone: Report and Partial Prototype submission 5. Quality and reliability 5.1 The quality of finish of the product is negligible unless it affects any performance or engineering constraints. 5.2 As this is a one off production the quality of each part is highly important. To ensure reliability in function each part will be rigorously checked during and after production, non- destructively. This will increase manufacturing costs however the cost of failure outweighs this. For example if a part is to be cast, a mould flow check will be carried out, the mould will be checked for imperfections and the cast part will be x-rayed after to check for defects. 5.3 The system reliability will be calculated and displayed in a block diagram, to determine what subsystems are likely to cause the most problems. If it is below 0.8 then decisions will be made on the need for preventative systems. 5.4 If a standby system is put in place, the reliability of the sensing and switch unit must be over 0.9.
  21. 21. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 21 5.5 A fail safe approach will be taken when designing the product. Using root cause and effect analysis and FMEA, any weak spots in the system/ components will be indentified and action for monitoring this weakness will be put into effect. 5.6 Computer and physical prototypes will be tested for the most extreme conditions of loading and environment where possible (see environment and performance). 5.7 As there is no end customer as such, guarantees, warranty service and claim adjustments are unnecessary. 5.8 Operator training will have to be provided in a precise and fool proof document. It is assumed that the design team will be on hand but a document of operation must be provided. 5.9 The company carrying out inspection must have a documented ISO 9000 or ISO 9001 to ensure the organisations quality systems meet written standards. If checked in house, an ISO 9000 or ISO 9001 audit must be carried out. 5.10 The centre of the tolerance range of the component would ideally be at the centre of the range of dimensions produced by the chosen manufacturing machine. This will give the greatest capability without reducing the process standard deviation. 5.11 The quality of the materials used will be determined by the life cycle; environment conditions and performance (see each of these sections). 5.12 The supplier’s material quality guarantees will be checked over before commencement of any contracts. 6. Materials 6.1 The frame should be made of a light-weight aluminium alloy 6.2 Tubular components used to; 1. Decrease weight 2. Use readily available manufactured parts 6.3 Tyre chevrons should be made of titanium to provide traction & resist wear on the lunar surface 6.4 The engine and fans must be covered in a fine cloth (or Mylar blanket) to filter out lunar dust 7. Politics 7.1 All parts of the designed concept should be bought in or purchased from British companies 7.2 Careful marketing policy will eliminate the possibility of unintentional social prejudice or unwanted implications 7.3 It is not expected that the product will have any social implications 7.4 Not expected that the product launch would have any political implications 8. Weight 8.1 The moon rover’s frame should be designed to carry a load of 150kg and the total weight of the rover including the frame and any scientific samples recovered and should not exceed 300kg. 8.2 The vehicle needs to have a low centre of mass to minimise the risk of tipping. 8.3 The moon rover must be made from the lightest material possible with sufficient material properties to complete that parts function to minimise cost on fuel getting to the moon.
  22. 22. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 22 Note: The weight should be kept to an absolute minimum while still being able to complete all its functions completely, as sending 1kg of mass to the moon costs roughly £68,065. 9. Ergonomics 9.1 The controller of the moon rover should be designed to fit the users hand comfortably and allow them to control all the buttons easily. 9.2 The moon rover does not have a human driver on board and will not need transported at any point by a human so does not need time focussed on the ergonomics of seating or handles. 10. Standards Specifications 10.1 The Moon Rover will conform to set communication standards to enable reliable transmission of the expected data that will be gathered 10.2 The standards were developed by the Consultative Committee for Space Data Systems (CCSDS) 10.3 CCSDS is made up of leading space communications experts representing 30 countries, its founding member space agencies, 28 observer space agencies, and over 140 private companies. CCSDS members include national space agencies from Japan, the United Kingdom, France, Germany, Italy, Brazil, Russia, Canada, China, and the United States, as well as the multinational European Space Agency. In doing this, data transfer will be straightforward, reliable and robust albeit through a weak-signal relay or direct-to-earth space links. 10.4 Must meet all standards of cleanliness before sent into space to avoid a risk of contamination. 10.5 Software must conform to JPL Institutional Coding Standard for the C programming language. 11. Documentation 11.1 A detailed user manual and maintenance instructions should be included with the product. 11.2 All testing certifications should be retained and recorded. 11.3 The mission aims and mission itself should be well documented 11.4 Build process should be well documented 11.5 Documentation on the product’s specifications should be produced to allow others to understand the workings and construction of it. 11.6 Sourced material should all be documented. 11.7 Project planning should all be well documented and displayed in an effective manner 12. Manufacturing Facility 12.1 Manufacturing of prototypes will be done on university premises with university machines. 12.2 Final design Manufacturing will be outsourced to other manufacturing facilities in the UK.
  23. 23. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 23 12.3 The company who can build our design for the cheapest while still ensuring the desired quality and reliability of the product will be used 13. Packing 13.1. Must be of high quality to protect the rover while in transport 13.2. Minimal cost 13.3. Minimal size 13.4. Minimal weight 13.5. Packaging must be environmentally friendly 13.6. Should be waterproof 13.7. Easily removable 14. Aesthetics 14.1 Must visually convey a sense of durability and high manufacturing quality 14.2 Aesthetics of the rover should contribute to its stability while it is operating in rough terrain 14.3 Should be coloured contrastingly to its environment for ease of detection 14.4 Shaped in such a manner so as to not obstruct vision from sensors and camera/video etc. 14.5 External appearance should be shaped in such a way as to protect vital internal components from impact or other such forces 15. Testing 15.1 As the parts required for the moon rover will be used in a highly demanding and precise application, visual inspection of all parts being used within the design will be visually inspected. 15.2 Due to the intensive requirements placed on the functionality and quality of the components manufactured for the moon rover, visual and mechanical inspection of each part of the moon rover will be inspected closely. 15.3 In terms of the strength of the material, a standard strength test will be carried out to ensure the loading limits experienced by the moon rover are well within the limits of the material. 15.4 The structure of the material is integral to the performance and longevity of the moon rover and so ultrasonic testing of all metal components will be required. 15.5 The ability of the driving system to cope with the surroundings which will be faced on the mission will require testing. An appropriate testing facility, providing a rough, cratered and mountainous surface will be used to test the drive, control and steer ability. 16. Installation 16.1 The frame must have brackets suitable for mounting various sensors onto and have appropriate space available for said sensors. 16.2 Standardised nuts and bolts should be used to fix parts where appropriate. 16.3 The frame must have ample room in its construction to accommodate the installation and removal of both the drive system and sensors.
  24. 24. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 24 16.4 Cable management should be considered within the frame design to allow for easy installation. 16.5 The drive system should be designed with easy removal and installation in mind so that adjustments can be made to the system when necessary. 16.6 The sensor systems should be easily accessible while installed to allow for calibration. Additional Points • Return continuous video with minimal interruption • Accomplish an unprecedented 1000 km traverse spanning two years of operation in the extreme conditions on a surface of fine electrostatic dust. • Survival in radiation, -180 deg C cold, vacuum, and operations in the heat of +130 deg C Bibliography for Product Design Specification George E. Dieter, Linda C.Schmidt; 4th Edition; Engineering Design; 2009; Mcgraw-Hill International Edition. John Corbett, Mike Dooner, John Meleka, Christopher Pym; Design for Manufacture; Strategies, principles and techniques; 1993; Pearson Education; Addison-Wesley Publishers. By combining all of the gathered information on standards and requirements with the PDS which has been generated by the team, there are now a comprehensive set of design guidelines which can be used in relation to the lunar rover design. The drive sub-group will concentrate on those which affect the drive system components and will design the drive system to strictly adhere to the guidelines which have been highlighted in this report. 4. Function Means Tree A function means tree was developed as the initial process in the concept generation stage within the design process. This identified the key functional areas to be addressed through design in order to make a successful and functioning lunar rover. The developed function means tree can be seen in Appendix 1. The drive team sub-group used this function tree, concentrating on the areas which mainly affected the drive system within the design and used this as a basis to generate some concepts which are discussed below. 5. Concept Generation The entire team decided in an early stage of the project to utilise the robotics kit available and use it’s functionality to drive the development of the physical design of the rover. In a way this was limiting as it was felt that the kit prevented major innovation to the overall design as it was important that the parts and functions were not reconfigured in a drastic manner. In light of this it was realised that the kit was a representation of the moon rover that was being designed for the purpose of going to the moon and that the eventual design did not have to be limited by the prototype. With this in mind the team continued with the project developing two final outputs to the design, a realistic design which shows all the detail of the lunar rover design which would ultimately be sent on an exploration mission and a theoretical design which will be illustrated within
  25. 25. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 25 the physical prototype which is produce. This theoretical design will aim to identify key features and explore how they might work on the realistic design if it were to be manufactured. Various concepts were therefore suggested to explore different ways the theoretical moon rover could be driven. The main focus of this exploration was to find creative solutions to the issues the rover would encounter while in use on the moon, primarily obstacles it may not be able to navigate around such as craters or rocks. The resulting concepts are presented below: 1. This is the most basic concept created and features the use of six wheels rather than four. It was theorised that the additional wheels would provide additional traction helping the rover move through the moon’s environment. The wheels are heavily treaded for optimum grip and the frame is symmetrical to prevent the rover becoming off balanced. 2. This concept uses tank tracks to drive the rover forward. It was heavily influenced by military and construction vehicles which regularly operate in rough, rocky and sandy terrain, similar to that of the moon. The tracks are intended to be larger than the frame of the rover to allow the rover to drive if flipped upside down. 3. This concept is influenced by the trolleys that can be used to transport heavy items up stair cases by attaching 3 wheels to each corer which can rotate independently. In this way one wheel is always in contact with the ground surface, even when moving over an obstacle. 4. This concept is based around the idea of splitting the frame into an articulated structure with each section’s wheels being driven independently. In this way if a section of the rover becomes stuck another section is still able to function and can free it. In addition the articulation allows the rover to be more flexible and able to respond to overcoming obstacles such as sudden changes in gradient.
  26. 26. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 26 5. This concept was created in response to the issue of large rocks on the moon’s surface and has been designed with a large amount of ground clearance to avoid the body becoming damaged by them. It does have the disadvantage of potentially being fragile or unstable however. 6. This concept once again uses tank tracks for extra traction on the moon’s surface. It has long stabilising arms which are intended for use if the rover is moving along steep or precarious terrain as these arms can reach out, for example downhill below the rover and provide resistance against the gradient to prevent the rover slipping. 7. This concept is very similar to the previous idea, number 6, as it features tank tracks as the main method of driving the rover and stabilisers. In this case the stabilisers are in front of the rover and are have a very thick tread intended to grip into the surface of the moon. They would be used in cases where the rover was struggling to cross an area of ground and required additional grip. The stabilisers could then be applied to the surface and their additional grip would pull the rover forward. 8. This concept again makes use of a tank track but has replaced two outer tracks with a single central one. This would allow the rover to drive both upright and upside down. Two wheels are included for additional stability. 9. This concept again attempts to solve the issue of the rover potentially becoming upside down. The shape shown has been sketched as a way to allow the rover to function in any of the three orientations where four wheels touch the ground. In addition the extra wheels would give extra leverage against obstacles which the rover may encounter. 10. This final concept depicts a traditional rocker bogie drive system used on current rovers. It was felt that in this case the conventional drive system was the most suitable for the project as it is a successful design which could be replicated across both the conceptual moon rover and the physical prototype. This consistency
  27. 27. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 27 was a desired feature for both designs across the team. For this reason this concept was chosen as the final design which would be developed in detail for the design of the lunar rover. 6. Selection of Components for Chosen Design This section of the report will detail the components, mainly the motors and power source which will be used within the final design. The detail design of these elements is discussed below. 6.1 Moon Rover Wheel Motors and Gearing Overview During this section of the moon rover report there will be an in depth description into the types of motors used to operate the rovers driving and steering capabilities. Along with the selection of the motors used and their specifications there will also be an analysis of the gearing used within the operations of driving and steering the rover through the use of harmonic drives. Motor Selection and Specifications After undertaking research into previous exploration rovers, key information was obtained. A large number of the exploration rovers used motors supplied by Maxon Motors to operate functions such as wheel rotation and wheel steering along with positioning of other key components such as cameras and sensors. The Maxon motor used in the 2003 rovers Spirit and Opportunity steering and driving operations was the RE25 model, which was capable of producing 6170 rpm of non-load speed. Identical motors and gear configurations were used in the wheels steering and turning actuators of these rovers. Similar to the Spirit and Opportunity rovers our rover will utilize a singular Maxon motor model for both operations. In the past ten years since these rovers were launched there have been newer motor models produced which is why our rover will use the Maxon motor RE30 which has the following characteristics:  Supply voltage: 48V  No-load speed: 6180rpm  No-load current: 73.6mA  Nominal current: 1.72A  Terminal resistance: 2.52ohm  Torque constant: 53.8mNm/A  Bearing type: Ball Bearings As the rover will be using a motor with a similar non-load speed value as the Spirit and Opportunity models, a similar style of actuators can be used. As in the previous rovers there is the problem of producing the high gear ratio required which will allow the motor to provide the ground speed requirement to enable the lunar rover to navigate over difficult terrain. As the gear ratio will undergo high output torques in the operations of steering and driving the decision was made to utilize harmonic drives in this area. Standard DC motors use an iron core which results in its magnetically soft cogs, which are polarized, to be attracted to the nearby permanent magnets. This means that re-magnetization is required so that the motor doesn’t stop in specific places; this is known as a detent. The drawback of this detent Figure 16 – The Maxon RE30 motor
  28. 28. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 28 is that a larger amount of energy needs to be used to operate the motor. The advantages of the Maxon motor compared to standard motors are as follows; • The Maxon RE30 motor contains no iron but instead uses a copper winding which rotates naturally within the magnetic field supplied by high performance Neodymium magnets. • The advantages of this include low mass inertia; low inductance; sort-run-up time; smooth motor running at all speeds; high performance control characteristics; minimal vibration; and quiet operating volumes. • As there is no iron present inside the Maxon motor this means that it can perform at a high efficiency level (above 90%) and a low zero load current (less than 80mA). Additionally the motor will use less electrical energy during its lifetime because of the lack of iron components within it and will also have an extended life expectancy than other motors because it does not suffer from sparking. Another useful aspect of the Maxon motor is its compact design which includes such features as its centrally arranged magnets. As the magnets are positioned in this way it allows for a more efficiently designed magnetic circuit which creates a very strong induction field in the air gap. The motor is also light weight because of its hollow cylinders rather than full iron cylinder which produces a high performance to space ratio. In summary the Maxon RE30 motor will be able to provide consistently high performance ratings through an extended life in service as well as fast acceleration, to allow for fast reaction to changing terrain, and fast run-up times of only a few milliseconds. (Maxon Motor, 2010) (Maxon Motor, 2010) (Maxon Motor, 2013) Harmonic Drives The harmonic drive has been used in numerous precision positioning applications from robotics to aerospace because of its high positioning accuracy. This is due to its high reduction ratio; high torque transmissibility; compact size; and near zero backlash (clearance between the teeth on the flexspline and circular spline). The reason for there being zero backlash, is due to the naturally occurring radial pre-loading in the tooth engagement. This means that the characteristics of the components will not change over their entire lifetime. This becomes a very desirable benefit as it allows for consistent manoeuvring of the rover over its entire mission duration. The most typical harmonic drive includes three main components. These are shown in the images below. As the wave generator, circular spline, and a flexspline, this is placed between the first two components. The wave generator is an elliptical shape which has a thin walled ball-race fitted around it. The flexspline is a thin flexible steel cylinder which has one end attached to an output shaft while its other end comprises of external teeth that fit tightly over the wave generator. The teeth on the flexspline engage with the fixed circular spline across the points of the wave generators major axis. The pitch of the teeth on the circular spline and flexspline are the same and the way the reduction ratio is achieved is by having a smaller number of teeth on the latter.
  29. 29. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 29 One of the components of the harmonic drive, the Flexspline, is a deformable device which can cause problems in the dynamics of the component. However recent advances in this area have solved the problem of deformability through large amounts of friction (which the harmonic drives in this system would undergo as it will be running at a gear reduction of 1500:1) by incorporating an adaptive joint torque controller. (DM942, 2013) (Tadayoni, A., Xie, W., & Gordon, B., 2011) (QTC Gears, unknown date) (IEEE, 2013) Use of Harmonic Drives in the Wheel Actuators The design for the moon rover will utilize a rocker-bogie suspension system that will have the Maxon motors powering the 6 wheels that will be in operation. Along with these motors there will be an additional 4 motors connected to the front two and rear two wheels which will allow the rover to turn from stationary. As these motors run at a non-load speed of around 6000rpm the rover will need to have some way to reduce the speed of these motors into a much slower rate to allow it to navigate the terrain on the moon. Through using harmonic drives with these high rpm motors the moon rover can now perform precision positioning applications. A common actuator will be used for both the wheel drive and the wheel steering as the same motors will be used in each. The rover itself will be required to operate at a speed of 5cm/s which will produce a rotational speed in the region of 3.6rpm. This mean that with using the Maxon RE30 motor, which is capable of producing 6180rpm, a gear ratio of 1500:1 will have to be used. As this ratio is so large that the most effective method of producing this is through the use of harmonic drives which can operate under the high torque values produced by the motor. With the motor and gearing methods now selected, to be placed within the wheel actuator, design considerations had to be altered to allow them to be packaged in the given area. For this to be accomplished the inner bore diameter of the harmonic drives wave generator had to be increased to allow for the gear motor to be positioned through the inside of the drive. As the motor will now be placed through the harmonic drive, due to area constraints, changes had to be made to components of the harmonic drive. The changes made to the harmonic drive, which ultimately would reduce the overall mass of the actuator, would also reduce the radial stiffness and torque capabilities of the harmonic drive. However the decrease in capability of the harmonic drive is acceptable enough for it Figure 17 – Examples of harmonic drive systems
  30. 30. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 30 to still be able to perform its actions with the given Maxon RE30 motor. (NASA, 2004) (Maxon Motor, 2010) (Maxon Motor, 2013) 6.2 Rocker Bogie Suspension The aim of this design project was to endeavour to improve the ability of the lunar rover navigation, minimise energy usage and improve the ability to negotiate different types of terrain. The team feel that to achieve these aims it was essential to use a Rocker Bogie suspension system. Many developments to suspension systems on lunar rover designs have taken place since the initial Apollo Programme exploration. The most beneficial advancement within the space industry is the development of the rocker bogie system. This system was designed to allow stability even at extreme angles of up to 45 degrees for Mars exploration. Additional design constraints which have been overcome include the ability to compact the overall size of the design for travel which then deploys into a functional frame on landing, withstand the large impact load of landing and incorporate a suspension system which can suffice for negotiation of rocky terrain. (Harrington, B. D., & Voorhees, C., 2004) 6.3 Power Requirements for the Mission Each area of the theoretical rover which will require a power source is included in the table below. Although it has been hard to estimate the power required by each section theoretically, a value has been given and the rationale has been stated. Figure 18 – A detailed view a harmonic drive system
  31. 31. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 31 Please note this table assumes that the levelling system is accommodated by the rocker bogie frame by distributing the weight and providing stability up to 45 degrees. Incorporated into the sensor programme is a safety check to ensure that this tilt angle will not be exceeded. It is also noted that the frame design can withstand a force of up to 714 N-m bending load and 506 N-m torsional load created by centre wheels falling into a 20cm hole. See reference (Harrington, B. D., & Voorhees, C., 2004) It is also assumed that the exploration rover will not be drilling to collect samples or analysing them. If this were to be included then additional power source requirements should be included in the calculations. Addition of all the estimated power requirements suggests the maximum total output power required is 636W for continuous operation. For deployment, upwards of 333N is necessary, please note that this does not account for the weight of the frame or complexity impacts upon landing, before fabricating the final realistic lunar rover design these instances will need to be considered within the design. These calculations for power requirement have all been considered within the design and prototype of the theoretical lunar rover with the desire to test all of these calculations in order to gain more reliable information on the forces and effects of deployment, landing and motion during the life cycle of the lunar rover. At this stage of development it would then be acceptable to use this generated data to help further detail the design of the realistic lunar rover before proceeding with further building and testing of this design. Also, for consideration within calculations it should also be noted that MER drive speed is 34 metres per hour. (NASA, unknown date) 6.4 Power Source Research conducted into the previous Apollo missions reveals that out of the successful moon landings Apollo 11 was the only mission with equipment that was powered by solar radiation converted through photo-voltaic cells. All other lunar exploration equipment and rovers have been powered by a radioisotope thermoelectric generator. From NASA resources, it is suggested the Figure 19 – A table of power requirements for the mission Sensors: Gyrometer Shaft encoders Touch sensors Ultrasound Accelerometer Light sensor/ laser Hazard cameras Navigational cameras P=IV P=1.72A* 48V P= 82.56W * 6 40W 35W 5W 20W 1W 5W 10W 20W 5W 495.36W Rationale Rocker deployment actuator N/A One time use Based on typical values for similar sytems using reference [3] Power Req.Section Area Controlled Part Heat regulation for electronics preservation Control System Drive System Frame Wheel motor x6
  32. 32. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 32 power source change took place due to the longevity and efficiency of the solar cells being surpassed by the plutonium source. Similar changes can be seen with Mars exploration rovers where the first two rovers were solar powered and from there onwards it has been powered by decaying plutonium. These two technologies will be explored and compared to suggest the best power source for the moon rover with respect to advances in solar power and the aim of improving the ability of navigation, negotiating different types of terrain and minimising energy usage. Solar Power Photo-voltaic cells have increased in efficiency since 1969 when first used in Apollo 11's EASEP experiments package. Shown below is a diagram of the seismic experiment equipment which was powered by solar panels and was manually operated. It is estimated that the efficiency of these solar cells is as little as 14% accuracy due to available technology of the 1960's. (NASA, unknown date) In more recent years exploration mission to Mars have also utilised solar power, such as with the 1997 landing of Sojourner. The picture below shows the solar array which was mounted on top of the small 11.5kg rover. (NASA, 1997) The solar cells utilised are made of Gallium Arsenide and are2x4x5.5mm. Overall, the 13 parallel strings with 18series cells per string are only 18% efficient and could produce only 16.5 watts on Mars at noon, the equivalent of 45 watts on Earth at noon. The impressive quality about the solar cells is the ability to withstand temperatures of -140 and +110 degrees. This is all within the size of 0.22 m2 weighing only 0.340kg. Figure 20 – An example of a solar array for use on a lunar rover Figure 21 – An example of a solar array for use on a lunar rover
  33. 33. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 33 A controversial issue with solar cells on the moon is that although the moon does spin and therefore the entire moon is lit by the sun, the axis of rotation may leave certain polar areas in darkness. If these are the areas to be explored then solar power cannot be used. There is a possibility that an automated rover on the moon may come into areas of shade if entering craters or bypassing very large rocky terrain. It is suggested that the steepest angles experienced are 35 degrees and therefore the height of the rover and solar array will need to be changed to circumvent potential shadows. This is only suggested to apply to times of potential shadow that will last for more than the rechargeable batteries can suffice and recharge. The general downfall of solar cells is their rate of degradation and efficiency. It seems that solar panels are only used for very short missions as it is easy for the solar cells to become covered in a thin layer of dust or debris which limits the light that can be changed. For a relevant example, considering the moon's geology, Regolith (or lunar soil - a fine dust that covers the moon's surface) may get dislodged by the rover vehicle's tires and settle on the solar panels due to Van Der Waals adhesive forces. Although it is unproved, it is estimated that dust may cause 22%- 89% of degradation. (NASA, 2007) It is for these reasons and for beneficial advancement for Earth's use of solar cells, research and developments has been conducted by NASA and accompanying organisations. In 2011 a patent was released for a waterless dust removal device. Using dielectrics a blowing stream is generated by electrical fields to remove dust or debris off the top of the cells. (NASA, 2007) This works by applying an alternating current across two electrodes which are separated with a dielectric. A time varying voltage creates a waveform. Collisions between the ions and neutrals create the induced flow because plasma generates a body force creating velocity of the surrounding atmosphere. (NASA, 2007) Further to this, recent reports of solar cell efficiency have increased to as high as 43% =/- 2.5%. (Green, M. A., 2011) This Multi-junction cell is made of GaInP/GaAs/GaInNAs and is a two terminal solar junction, triple cell and is produced by Solar Junction. (Green, M. A., 2011) It is disappointing to state however, that although there have been many advancements in efficiency for the purpose of the moon rover project the temperatures that the highly efficient solar cells can withstand are incapable of surviving the temperatures experienced in the moon's atmosphere. Further developments would need to be taken to apply a more efficient solar panel on a lunar rover. To summarise the solar discussion, although there have been many advances in efficiency and ability to self-clean the solar cells, these advancements do not exist with the extended temperature ranges suited to the temperature experienced in the moon's environment. For this reason the solar panels will not be used as the mission will last longer than 2 years which may be the extent of the solar cells lifespan. An alternative power source is a radioisotope power system. Radioisotope Power Systems (RPS) RPS systems have been used successfully on Apollo NASA missions and others such as Galileo, Voyager and Viking. The success of the power source is due to its high reliability, long life of a maximum 14 years and non-reliance on the operational environment. The system works by harnessing the heat transfer from the degradation of Plutonium (Pu-238) which has an 87.7 year half life.
  34. 34. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 34 There are two recent developments, the first of which has never before been used in a mission by NASA or any other space exploration institutions. The first of these developments is an Advanced Stirling Radioisotope Generator (ASRG). The aim of this generator is to improve the efficiency of the conversion of heat to increase it from around 5-7% to around 38% and therefore use less Pu-238. This would in turn reduce the costs and weight utilised by each mission. To give a perspective of the suspected efficiency increase, it is expected to produce 140-160 Watts of power using less than 1kg of Pu-238, this is a quarter of the mass used in the original converter. This technology has been undergoing reliability testing since 2008. (NASA, 2008) Advanced Stirling Convertor -E2. Courtesy Sunpower (NASA, 2008) The above diagrams have been included to help describe the function of this convertor. The heat source is the material plutonium which has an alpha decay of 5.593MeV. As the material decays it produces a large heat transfer through radiation. The heat generated through the radiation is then transferred in to electrical energy by the Seebeck effect. This effect is generated when a set of solid- state thermocouples react to a temperature difference and generate an electrical current; this is known as a thermo-electric effect. The electricity is then stored in the rechargeable lithium ion battery that is then used to power the rover. (University of Waterloo, unknown date) This creates a constant and reliable source. Figure 22 – An example of a radioisotope power system for use on a lunar rover Figure 23 – An example of a radioisotope power system for use on a lunar rover
  35. 35. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 35 (University of Waterloo, unknown date) (Glen Research Centre, 2008) In some exploration cases more than one battery is used dependant on the amount of energy needed to be stored and utilised. The secondary system that has been developed recently is that of the Multi-mission Radioisotope Thermoelectric Generator (MMRTG) used in the 2009 Mars mission. This works off the same principle except there are multiple plutonium cells which can generate a much larger power potential. (Glen Research Centre, 2008) A further benefit of the radioisotope power system is the ability to channel any residual heat into beneficial energy use. A pipe system should be put in place to channel any excess heat back into the Figure 24 – The seebeck effect Figure 25 – Construction of a radioisotope power system
  36. 36. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 36 rover chassis to re-heat the system if necessary. It is required that the rover be heated during the night cycle on the planetary surface, otherwise the system will freeze over and cease to operate. It is essential that the electronics involved in the control system are kept at a stable temperature to ensure that they can operate. As such, a heating and cooling system must exist and be regulated. Previous missions to mars have suggested that the optimum temperature range for the circuitry to work without issue is between 40 degrees Celsius and -40 degrees Celsius. (NASA, unknown date) If this system was not used further heaters/ coolers would have to be installed and powered to keep the system running. All the main electronics must be protected within this system: batteries, electronic circuitry and the motherboard due to unfiltered solar radiation. To protect the system from solar radiation and help increase the efficiency of the system gold paint is applied to the inside and outside of the surrounding box for the circuitry and batteries. The gold paint acts as a reflector and hence it helps keep heat in and radiation out. This system will be used due to its reliability, improved efficiency and commercial viability. When creating a rover which will be going into space there are a lot of considerations that must be made when choosing technologies. We have looked at materials, shapes and some of the important components of the moon rover. For the areas we have studies we have looked at what other moon/mars rovers have used as well as other industries. 6.5 Materials Since the cost of getting the rover into space is very high, weight is a crucial aspect of the material as the lighter the frame, the lower the transport cost will be. During the take off process the frame will be put under a lot of different forces and vibrations so as well as being light the material must also be strong. We have described some of the materials which the group have created a research portfolio of below. Aluminium Aluminium is a good choice of material for many reasons as it is very light compared to most metals and has a good resistance to corrosion. It is more expensive than other standard metals but is cheaper than some of the other materials in this list such as carbon fibre. Aluminium is workable but fastening should be done with bolts and rivets where possible as it is difficult to weld. As well as difficulties welding it is also difficult to machine. (ALU, 2012) High Strength Plastics This is the cheapest of the options highlighted. Plastics have been around for a long time and are well known for being very light. They have, for a long time, suffered from poor strength qualities, in relation to their weight especially. With new ways of producing the plastic parts this is changing, however, as the mould can be designed to strengthen the weak areas. It is becoming evident that plastics are starting to be used in new applications such as a car shell in some models for example a Chevrolet Corvette. Figure 26 – Aluminium material
  37. 37. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 37 Carbon Fibre This is another material which has a very high strength to weight ratio, it is primarily used in F1 and the high end of the automotive industry. It is becoming more common but is still not widely used because of its high cost. (Carbon Mods, 2010) Composite Materials Composite materials are commonly used in aircraft construction because of their high strength to weight ratio while also having high resistance to corrosion. These are the key aspects to be considering for the lunar rover design due to the previously mentioned cost of sending the craft into space, as well as that it will not receive any maintenance once the rover is on the moon, it is therefore important that the material chosen is resistant to corrosion from any dust on the moon. Depending on the materials used, and the way in which the composite is constructed, the prices can vary, they are generally quite expensive but this is not crucial. (About.com, 2013) Chosen Material After weighing up the options of the previously researched materials the team have decided the frame will be made from aluminium. This is because of its high strength to weight to ratio while also having an excellent resistance to corrosion, this was identified as being important as the rover will not be cleaned or serviced once it’s out on the moon. Aluminium offers all these strengths at the most competitive cost. 6.6 Joining methods There will be two main forms of joining methods in this moon rover design. The first will be nuts and bolts, these are for parts which will be moving in some capacity and will require locking nuts so the nuts will remain on the rover. This is essential as throughout the process of getting the rover to the moon there will be a lot of different kinds of vibrations and forces acting on the rover which could lead to nuts shearing off. The second kind of joining method will be rivets. This method is very similar to nuts and bolts in terms of dealing with forces and stresses but is a more permanent option as the rivets can’t be removed without physically breaking the component. (Athena, 2005) 6.7 Shape The shape of the rover does not directly affect technology, but it is a crucial aspect of the design as the rover will be tackling various terrains which may involve gradients. The rover must be able to travel along steep gradients in all directions as well as have a high ground clearance. While having these aspects it must also be compact enough to fit through gaps between rocks and other obstacles. The frame must work together with the drive and sensors to overcome these obstacles. The propulsion will come from the drive in the difficult terrains and the sensors will plan a route which will have as little obstacles as possible. With regards to technology, having a frame which has some degrees of movement, allowing its shape to change, would allow for the frame to be both sure-footed and stable, while also being compact and nimble. Some examples of previous mars and moon rovers have been discussed earlier in the report which all has flexible moving frames. (Athena, 2005) (NASA, unknown date) Figure 27 – Carbon Fibre material
  38. 38. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 38 6.8 Fasteners The fasteners that hold various sections of the power supply casing obviously also have to be suitably designed so that they operate as required on the moon surface, withstanding high temperatures if necessary and dealing well with high loads and tensions. The most suitable material found for the fasteners is Inconel 718, a metal super-alloy. Inconel is primarily a Nickel, Chromium and Molybdenum alloy which has very high heat and corrosion resistance. The chemical structure of this alloy is shown in the table below. (ALCOA, 2012) (HPAlloys, 2007) Bolts made from this material have excellent strength and can operate efficiently in a temperature range of -253 degrees Celsius to 705 degrees Celsius. This material can be hard to manufacture but machining bolts is moderately simple provided that the tool is kept sharp, held rigid and not allowed to weld to the material surface. Along with the Inconel718 bolts, Keenserts would be utilised to keep the bolts securely held in the parent material. Keenserts, shown below, are small studs that are driven down through the threads of the joined part to mechanically lock the bolts into place. A special tool is used to drive the pre- assembled keys into the material. These inserts are designed to transfer high axial loads into the base material as efficiently as possible and provide high resistance to pull-out and torque loads. (ALCOA, 2013) 7. Calculations The following calculations outline some of the key technical aspects required for this design of a lunar rover. 7.1 Static Calculations The calculations below illustrate the lunar rover’s weight and distribution. These values rely on the friction coefficient in order to provide traction to propel the lunar rover along the lunar surface. Figure 28 – Chemical requirements for Inconel Figure 29 – Keenserts bolt
  39. 39. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 39 Weight and Weight Distribution Weight = Mass*Gravity The weight of the lunar rover design has been calculated for both the loaded and non-loaded rover. It was stated that the lunar rover must be capable of carrying a weight of up to 150 kg. Both of these calculations are detailed below. Mass of non-loaded lunar rover = 2.766487e+11 kg Gravity on Earth = 9.81 m/s2 Non-loaded weight = 2.766487e+11*9.81 Non-loaded weight = 2.713923747e+12 N This calculation gives a total non-loaded weight, on Earth, of 2.713923747e+12 N Mass of loaded lunar rover = 2.7664870015e+11 Loaded weight = 2.7664870015e+11*9.81 Loaded Weight = 2.7139237484715e+12 N The calculations above have detailed the weight of the loaded and unloaded lunar rover on the Earth’s surface. The following calculations will detail the lunar rover’s weight on the moon’s surface. Mass of non-loaded lunar rover = 2.766487e+11 kg Gravity on the Moon = 1.622 m/s2 Non-loaded weight = 2.766487e+11*1.622 Non-loaded weight = 4.487241914e+11 N This calculation has detailed the non-loaded weight of the lunar rover on the surface of the moon as 4.487241914e+11 N. The calculation below details the weight of a loaded lunar rover on the surface of the moon. Mass of loaded lunar rover = 2.7664870015e+11 kg Loaded Weight = 2.7664870015e+11*1.622 Loaded Weight = 4.487241916433e+11 N The weight of a loaded lunar rover on the surface of the moon is therefore 4.487241916433e+11 N. Moment and Friction Coefficient The frictional force is key to the movement of the lunar rover as the surface provides grip, allowing the rover to navigate over the terrain in the lunar environment. Research presented earlier in the report suggests the presence of moon dust on the surface of the lunar environment. In order to help calculate the frictional force present it has been assumed that the frictional coefficient present on the surface is similar to that of fine sand on the Earth’s surface. The coefficient of friction of fine
  40. 40. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 40 sand is given as 0.35-0.45. (USACE, 1992) The coefficient of friction of rubber on a wet asphalt surface is stated as 0.25-0.75. (Engineering Toolbox, 2013) The value associated with wet asphalt has been chosen as it was thought this was the closest scenario which was similar to that represented by the surface and friction scenario which would occur on the lunar surface. The lower frictional value given for each of these materials will be used in the calculation as this represents the worst case scenario. This will therefore produce a ‘worst-case’ frictional force value. This will then indicate to the team that this is the lowest frictional force value which can be expected. Force of Friction = Weight*µ (rubber-sand) Force of Friction = 4.487241916433e+11*(0.25-0.35) Force of Friction = 4.487241916433e+11*0.1 Force of Friction = 4.487241916433e+10 N (when loaded) Force of Friction = 2.766487e+11*(0.25-0.35) Force of Friction = 2.766487e+11*0.1 Force of Friction = 2.766487e+10 N (when un-loaded) The key concern with the frictional force in the design of the lunar rover occurs when the rover is loaded and in the lunar environment. For this reason this is why the values shown above have been selected. It is also for this reason that calculations for friction on the Earth’s surface are not being conducted. Moment In order to indentify the force required to move the lunar rover along the lunar surface the moment force was calculated for both the loaded and un-loaded rover on the surface of the moon. These calculations are shown below. To ensure this is within safety limits a factor of safety of 2 has been applied to the weight values calculated previously. Moment = Force*Distance from lunar surface Moment = 8.974483832866e+12*396 Moment = 3.553895596149360e+15 N (loaded) Moment = Force*Distance from lunar surface Moment = 8.97448382e+11*369 Moment = 3.31158452958e+15 N (un-loaded) These calculations prove that the design has a great enough moment to provide motion along the lunar surface as these values are greater than the frictional force values obtained above. 7.2 Power Requirements for Planned Mission All power requirements for this planned mission have been covered extensively in a previous section of the report. A summary of the power requirements from this section is included below.
  41. 41. Integrated Mechatronic Lunar Rover Design Project Appalling 13 – Drive Team Sub-Group Integrated Mechatronic Design Project Report 19/04/2013 41 Sensors: Gyrometer Shaft encoders Touch sensors Ultrasound Accelerometer Light sensor/ laser Hazard cameras Navigational cameras P=IV P=1.72A* 48V P= 82.56W * 6 Section Area Controlled Part Heat regulation for electronics preservation Control System Drive System Frame Wheel motor x6 10W 20W 5W 495.36W Rationale Rocker deployment actuator N/A One time use Based on typical values for similar sytems using reference [3] Power Req. 40W 35W 5W 20W 1W 5W 7.3 Power Transmission Calculation Gear Ratio The selected motor, the Maxon RE 30. This motor has a gear ratio of 1500:1. It has already been discussed that this motor has the ability to produce 6180 rpm. With this gear ratio it produces an output speed of 3.6 rpm. This is further discussed in a previous section of the report. Circumference Calculation The power requirement for this lunar rover design is 636 W for continuous use. The current being used within the electrical power supply is 1.72 A. This gives a voltage requirement of 369.7 V. For the purpose of this calculation a value of 370 V will be used. It is essential that this design includes appropriately dimensioned wheels for use with this power requirement. This calculation is shown below. Circumference = Velocity/RPM The rover has a velocity of 5cm/s = 0.05m/s at an rpm of 3.6 Circumference = 0.05/3.6 Circumference = 0.0138m = 1.38cm The value obtained from this calculation is obviously very low as the speed and velocity of the lunar rover is particularly low to provide the ability for the rover to negotiate difficult terrain. The wheel sizing the actual and theoretical prototypes, shown in later sections, are much larger than this to provide the momentum for the movement of the rover and to support the weight of the design. Key Components All key components have already been indentified in the previous sections. Wheel Sizing The wheel diameter which is proposed for use in the realistic design is 300mm. This is very similar to values which have been used in previous moon rover missions.

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