DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER

International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 3-6, 2013)

DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER
Tanmai Bhargava, Shivam Aggarwala, Sangram K. Dasb, Arshad Javedc*
a Graduate Student, Department of Mechanical Engineering, Birla Institute of technology and Science, Pilani, Rajasthan, India
b Research Scholar, Center for Robotics and Intelligent System, Birla Institute of technology and Science, Pilani, Rajasthan, India
c Lecturer, Department of Mechanical Engineering, Birla Institute of technology and Science, Pilani, Rajasthan, India
* corresponding author e-mail: arshadjaved2@gmail.com

ABSTRACT
Walking on uneven terrain is always a benchmark problem for autonomous guided vehicles. In the present work, the
same issue is dealt with the help of a legged mobile robot. Various comparisons are made among two, four, and sixlegged walking machine and a four-legged walking machine is selected based on the suitability criterion. In this paper,
the emphasis is given for minimization of the design and controlling complexities for the four-legged walking machine.
A prototype devised to test various gaits. For the walking and turning, an improved gait is presented. The legs are
designed with one degree of freedom each. The actuation is tested on normal DC geared motors as well as DC servo
motors. A comparison is made between the two actuators. For proper walking, a control scheme is prepared and real
time tests are performed by implementing it on the Arduino microcontroller. The present work is helpful to analyze the
performance of a legged autonomous walking machine on unstructured environment.
Keywords: Walking Machining, Legged AGV, Mobile Robotics, Servo Motor Control
leg to reduce the controlling to its minimum level.
Keeping this condition, the present work is focused
upon the design of a four legged walking robot with 1DOF in each leg. The major task to design this
walking machine is to analyze the walking pattern. To
serve this purpose a prototype of four legged walking
robot is made using DC geared motors with manual
switch control. By doing different experiments, the
correct walking patterns are identified. Based on the
obtained walking patter the final control is made
through micro-controller on RC servo motors.

1. Introduction
A smooth and controlled navigation in an unstructured
environment is a benchmark problem for an
Autonomous guided vehicles (AGVs) [1]. The modes
of navigation are primarily provided by either wheels,
or legs. Wheels are important to provide smooth
motion in an even terrain. On the other hand, the
legged motion provides flexibility to walk on the
uneven terrain efficiently. In legged motion-based
AGV, there are several designs, which provided
different levels of smoothness and stability. A twolegged walker similar to the human legs, provides
high speed of walking and quick turn capabilities [2].
However, its stability and dynamic control are highly
complex. A four-legged walking machine provides
smoother motion than the two legged and less
complicated control [3, 4]. In the similar way the
smoothness of walking and precision in motion can be
achieved by the six-or higher number of legged
machines. In order to provide a controlled motion the
controlling complexities again increased in six or
higher numbers of legged machines. In addition to the
number of legs, the degree of freedom (DOF)
associated with each leg also plays a major role in the
motion of an AGV. By increasing the DOF, the
walking precision can be improved and greater
capability of obstacle avoidance and uneven terrain
navigation can be achieved. However, the controlling
becomes more complex. Considering these issues, it is
evident to employ four legs with single DOF in each

The presented manuscript is organized in
following way. In section 2, the design of the
prototype and gait experiment analysis is presented. In
section 3, the construction, working and controlling of
the RC servomotor based walker is explained. Finally,
the conclusion is made.

2. Gait Experiment and Analysis for
Four Legged Walker
The motion of a four legged walker is the most vital
aspect of this paper. A lot of experimentation is done
on the walking sequence of a four legged walker,
which is required to be controlled by servo motors.
The aim of these experiments are to find out the exact
sequence of the movements of the four legs relative to
each other. For conducting experiments, a DC geared
based model is devised as shown in Fig. 1. It is
controlled using manual switching. For each motor,

Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA)

48


forward and reverse motions are generated through the
switch as shown in Fig. 2.
The gait sequence, which is obtained after this
experimentation, is given here. Assuming the four legs
are initially still at their respective positions (Fig.3).
The steps of walking are described below:
Step 1: Let the left front leg moves ahead by a small
distance. The front right leg will be at its same
position but it will be having a tendency to go
backward. The left and right back leg will also remain
in its initial position but it will also be a tendency to
move forward.
Step 2: Now the left front leg is at its same new
position. The right back leg steps forward. When the
right back leg steps forward, the left back leg remains
in its same position but it tends to move backward. At
the same time, the right front leg lifts up in a
backward direction in order to move forward.
Step 3: Now the right front leg starts moving forward.
At this time, the left front leg is at its same position
but it tends to move backward. The left back leg is
inclined backward and is on the verge of moving
forward. The right back leg is at its same position and
tends slightly backward.
Step 4: Now the right front leg is at its new position.
Now the left back leg also lifts from the ground to
move forward while front left leg will be at its same
position but it will be having a tendency to go
backward. The right back leg will also remain in its
initial position but it will also be a tendency to move
forward.

Fig. 1 DC geared motor-based four-legged walker

Fig. 2 Switch control of four legged walker


49


(a)

(d)

(b)

(e)

compact form of close loop controlled actuator. It has
four modules packed in a single system. These
modules are,
Controller circuit- It is the main controlling part,
which implements the controlling scheme by taking
the signals coming from feedback element.
Feedback Potentiometer- The feedback of the rotation
of the motor shaft is taken by a potentiometer. It is
connected through a gear train to the motor shaft.
The Motor- This is a small DC motor, which is
controlled by an H-bridge inbuilt in the main
controller.
The Gearbox- Normally a small DC motor comes with
a very high RPM. A small gearbox is provided to
reduce the RMP of the motor.

(c)

(f)

Fig.3 Gait for four-legged walker

3.1 Operation of RC servomotor

The above cycle (Fig. 3) continues to cover the
specified distance. Taking the maximum angle that
can be moved by the leg-link to be 100 and link length
to be 5 cm the distance moved by the walker in each
cycle is 1.74 cm (17.4 mm).

The operation of RC servomotor is based upon the
feedback signal. When there is no input signal from
the controller, the servo does not move. By giving an
input signal motions are made. These signals are in
terms of a pulse of a specified pulse-width and
frequency, for a required angle of motion (Fig. 6). The
controller will de-code this signal into a reference
voltage. Thus, each voltage corresponds to different
positions of the drive shaft. The controller then reads
the actual drive shaft position by reading the voltage
of the feedback potentiometer connected to the drive
shaft. By making the comparison between the required
voltage to the actual voltage, the rotations are
performed. If the reference voltage is lesser than the
potentiometer- voltage, the motor rotates in one way.
If it is greater than the potentiometer-voltage, the
motor rotates the in the other way. Then the
comparison of voltages made again. After the
comparison, if the two voltages are found to be equal,
it means that the required position has been achieved.
The servomotor works continuously. It is never
idle. It always checks if the voltage differences of the
potentiometer and required voltage and try to correct
the position. Hence, the input signal is always
required.

Fig.4 turning motion by walker
The above cycle (Fig.4) shoes turning moment for
the walker. Taking the maximum angle that can be
moved by the left link to be 100 and right link to be
50, the length of the arc subtended by the walker in
each cycle is 1.305 cm (13.05 mm) and an angle of
780.

3.2 Constructional details
The main parts of four-leg walker are, chassis,
servomotors, leg links, controller and battery. In the
present work, the main emphasis is given to the
reduction of controller task. Hence, the torque
computations for the drive motor etc. are neglected by
selecting an adequate amount of motor-torque. In this
regard, the chassis design is also simplified by
selecting a hollow rectangular box in which all
components will be accommodated. The over

3. Design of Four Legged Walker
Based on the walking pattern analyzed in the
prototype experiment an ARDUINO microcontroller
based control is developed [5]. In this four-legged
walking machine, actuation is achieved by radio
controlled (RC) servomotor. RC servomotor is a


50


construction is shown in Fig. 5. Four V3003
servomotors are used in the present work. The
specifications of the servomotor are given in Table 1.
The required position signal is given in terms of a
pulse. For the selected motor the values of these
pluses corresponding to the different position is shown
in Fig. 6.

Fig. 6 Pulse signal ranging

3.3 Controlling
For the controlling ARDUINO microcontroller is
used. It has the capability to provide multiple signals
simultaneously. In the present work, it sends the
separate controlling signal to four servomotors. As
stated earlier, these controlling signals are the pulses
corresponding to the required position of the motor
shaft. Based on the gait analysis the motion sequences
of the legs are already identified. For a straight
motion, left, and right turning, the modules of
corresponding signals can be made. In the
microcontroller, these modules are coded by the
means of programming [6, 7]. For illustration the
codes of the straight-line motion is given in Appendix.
In the present work the provision of obstacle
avoidance etc. are not provided. However, by making
simple changes in the controlling scheme and by
incorporating an Infrared or Ultrasonic proximity
sensor it can be easily done.

Full View
Fig. 5 Servo motor-based four-legged walker

4. Conclusion
A four-legged walker has been an area of interest due
to its varied applications and better stability. The gait
or the manner of walking is one of the vital parts of
the analysis of the four-legged walker. In the present
work, a gait analysis is performed be the means of a
various experiments on a prototype four-legged
walker. The relative positions and motions of the legs
are figured out which is explained. In addition, the
walking pattern for forward and turning is found out.
The four-legged walker can be controlled by any of
the varied number of types of electric motors but RC
servomotor has better properties as compared to other
motors like DC motors. A RC servomotor based
walker is developed for the autonomous walking. In
this, the control is implemented by using
microcontroller. The presented work proposes a
design of a walker with the minimum controlling
complexities. It is helpful to analyze the walking of
legged AGVs.

Table 1: Specification of the servomotor

Operating Voltage

4.8-6.0 V

Stall Torque

3 Kg-cm at 4.8V
3.2 Kg-cm at 6V

No load Operating
Speed

0.2 sec/ 60Â° at 4.8V
0.18 sec/ 60Â° at 6V

Weight

38gm

Size

41.3mm×20.3mm×38.7mm


51


}
for(pos4 = 90; pos4>=80; pos4-=1)
{
myservo4.write(pos4);
delay(15);
}
if(i = 2)
for(pos2 = 80; pos2 < 111; pos2 += 1)
{
delay(15);
}
for(pos1 = 111; pos1>=80; pos1-=1)
{
delay(15);
}
if(i = 3)
for(pos4 = 80; pos4 < 111; pos4 += 1)
{
delay(15);
}
for(pos3 = 111; pos3>=80; pos3-=1)
{
delay(15);
}
if(i = 4)
for(pos2 = 111; pos2 < 89; pos2-= 1)
{
delay(15);
}
for(pos1 = 80; pos1>=90; pos1+=1)
{
delay(15);
}
for(pos4 = 111; pos4 < 89; pos4-= 1)
{
delay(15);
}
for(pos3 = 80; pos3>=90; pos3+=1)
{
delay(15);
}
}
}

REFERENCES
1.

Siegwart R., Nourbakhsh I. (2004), 'Introduction to
Autonomous Mobile Robots', The MIT Press, Cambridge,
Massachusetts, London, England, pp 5-152.

2.

Tlalolini D., Chevallereau C., Aoustin Y. (2011), 'HumanLike Walking: Optimal Motion of a Bipedal Robot With ToeRotation Motion',
IEEE/ASME Transactions on
Mechatronics, Vol. 16, pp 310 - 320.

3.

Gurfinkel V. S., Gurfinkel E. V., Shneider A. Yu., Devjanin E.
A., Lensky A. V., Shtilman L. G. (1981), 'Walking Robot with
Supervisory Control', Mechanism and Machine Theory, Vol.
16, pp 31–36.

4.

Peng S., Lam C. P., Cole G. R. (2003), 'A Biologically
Inspired Four Legged Walking Robot', Proceedings of IEEE
International Conference on Robotics and Automation,
Taipei, Taiwan, Vol. 2, pp 2024 – 2030

5.

Getting
Started
with
http://www.arduino.cc/

6.

Servo Control tutorial on,
http://playground.arduino.cc/Learning/SingleServoExample

7.

Servo library, available at,
http://arduino.cc/en/reference/servo

Arduino,

available

on,

Appendix
ARDUINO program to run servo motor
#include <Servo.h>
Servo myservo1;
Servo myservo2;
Servo myservo3;
Servo myservo4;
int pos1 = 90, pos2 = 90, pos3 = 90, pos4 =
90, i = 0;
void setup()
{ myservo1.attach(9);
myservo2.attach(10);
myservo3.attach(11);
myservo4.attach(12); }
void loop()
{
for(i = 0; i < 5; i+=1)
{
if(i = 0)
for(pos1 = 90; pos1 < 111; pos1 += 1)
{
delay(15);
}
for(pos2 = 90; pos2>=80; pos2-=1)
{
delay(15);
}
if(i = 1)
for(pos3 = 90; pos3 < 111; pos3 += 1)
{
delay(15);


52

DESIGN OF A SIMPLIFIED FOUR LEGGED WALKER

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