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1. Capacitive SensorsCapacitive SensorsCapacitive SensorsCapacitive Sensors

2. SensorsSensors
A brief IntroductionA brief Introduction
• A sensor; is a device that measures a physical or abstract quantity and
converts it into a signal which can be read by an observer or by an instrument.
• A sensor's sensitivity indicates how much the sensor's output changes when the
measured quantity changes.
• Sensors need to be designed to have a small effect on what is measured,
making the sensor smaller often improves this and may introduce other
advantages.
• In most cases, a micro-sensor reaches a significantly higher speed and
sensitivity compared with the one with a macroscopic approach.

3. Characteristics Of sensorsCharacteristics Of sensors
•Sensitive to the measured property
•Insensitive to any other property likely to be encountered in its application
•Does not influence the measured property
•Sensitivity is defined as the ratio between output signal and measured
property.
•Resolution of a sensor is the smallest change it can detect in the quantity
that it is measuring.
Terms used in sensor technologyTerms used in sensor technology

4. Sensor systemsSensor systems
• Transducer
• a device that converts a primary form of energy into a corresponding signal with
a different energy form
• take form of a sensor or an actuator
• Sensor (e.g., thermometer)
• a device that detects/measures a signal or stimulus
• acquires information from the “real world”
• Actuator (e.g., heater)
• a device that generates a signal or stimulus
real
world
Sensor
Actuator
Intelligent
Feedback
system

5. Sensor
Input
signal
(measurand)
Microcontroller
Signal processing
communication
Sensor data
analog/digital
Network
display
Electronic sensorsElectronic sensors
Electronic Sensors are those which convert desired parameter into electrically
measurable signal
A General Electronic Sensor consists of
Primary transducer: changes “real world” parameter into electrical
signal. They include
•Resistive Sensors
•Inductive Sensors
•Capacitive Sensors
•Piezoelectric Sensors

6. Secondary transducer converts electrical signal into analog or digital values.
They include
• Wheatstone Bridge
• Amplifiers
Real
world Analog
signalPrimary
transducer
Secondary
transducer
SENSOR
Usable
Values

7. Capacitive Displacement SensorsCapacitive Displacement Sensors
An overviewAn overview
• Capacitive displacement sensors are noncontact devices capable of high-
resolution measurement of the position and/or change of position of any
conductive target.
• Used for detecting proximity, position, etc., based on capacitive coupling effects.
• Capacitance sensors detect a change in capacitance when something or
someone approaches or touches the sensor.
• The technique has been used in industrial applications for many years to measure
liquid levels, humidity, and material composition.

8. Capacitive Sensor measurement systemCapacitive Sensor measurement system
Capacitive sensor dimensional measurement requires three basic
components:
•a probe that uses changes in capacitance to sense changes in distance to
the target
•driver electronics to convert these changes in capacitance into voltage
changes
•a device to indicate and/or record the resulting voltage change.

9. Capacitance theoryCapacitance theory
• Capacitance is a property that exists between any two conductive surfaces within
some reasonable proximity.
• Changes in the distance between the surfaces changes the capacitance.
• It is this change of capacitance that capacitive sensors use to indicate changes in
position of a target.

10. C= Area X Dielectric
Gap
In ordinary capacitive sensing,
• the size of the sensor probe,
• the size of the target,
• and the dielectric material (air) remain constant.
• The only variable is the gap size. Based on this assumption, driver electronics
assume that all changes in capacitance are a result of a change in gap size.
C ≈ 1
Gap

11. Focusing the electric fieldFocusing the electric field
When a voltage is applied to a conductor, an electric field is emitted from every
surface.
For accurate gauging, the electric field from a capacitive sensor needs to be
contained within the space between the probe’s sensing area and the target.
If the electric field is allowed to spread to other items or other areas on the target,
then a change in the position of the other item will be measured as a change in the
position of the target. To prevent this from happening, a technique called guarding is
used.

12. To create a guarded probe, the back and sides of the sensing area are surrounded
by another conductor that is kept at the same voltage as the sensing area itself.
When the excitation voltage is applied to the sensing area, a separate circuit
applies the exact same voltage to the guard. Because there is no difference in
voltage between the sensing area and the guard, there is no electric field between
them to cause current flow.
Any conductors beside or behind the probe form an electric field with the guard
instead of the sensing area. Only the unguarded front of the sensing area is allowed
to form an electric field to the target.

13. Optimizing the sensorOptimizing the sensor
Effects of Target Size
When the sensing electric field is focused by guarding, it creates a slightly
conical field that is a projection of the sensing area. The minimum target
diameter is usually 30% of the diameter of the sensing area. The further the
probe is from the target, the larger the minimum target size.
Range of Measurement
The range in which a probe is useful is a function of the size of the sensing
area. The greater the area, the larger the range.
A smaller probe must be considerably closer to the target to achieve the
desired amount of capacitance. In general, the maximum gap at which a
probe is useful is approximately 40% of the sensing area diameter

14. Optimizing the sensorOptimizing the sensor
Multiple Channel Sensing
Frequently, a target is measured simultaneously by multiple probes.
Because the system measures a changing electric field, the excitation
voltage for each probe must be synchronized or the probes will interfere
with each other.
Effects of Target Material
The sensing electric field is seeking a conductive surface. Provided that the
target is a conductor, capacitive sensors are not affected by the specific
target material; they will measure all conductors brass, steel, aluminum, or
salt water as the same.

15. Optimizing the sensorOptimizing the sensor
Other Factors to be considered for optimization:
• Maximizing Accuracy
• Target Shape
• Surface Finish
• Parallelism
• Environment

16. Measuring Non-conductor sMeasuring Non-conductor s
Capacitive sensors can be very effective in measuring
• density,
• thickness, and
• location
of nonconductors as well.
The dielectric constant determines how a nonconductive material affects the
capacitance between two conductors.
When a nonconductor is inserted between the probe and a stationary reference
target, the sensing field passes through the material to the grounded target .
Capacitance will change in relationship to the thickness or density of the non-
conducting material.

17. High performance sensorsHigh performance sensors
Simple capacitive sensors
•used in inexpensive proximity switches or elevator touch switches,
•simple devices and in their most basic form could be designed in a high
school electronics class.
In contrast, capacitive sensors for use in precision displacement
measurement and metrology applications use complex electronic designs
to execute complex mathematical algorithms.
Unlike inexpensive sensors, these high-performance sensors have outputs
which are
• very linear,
• stable with temperature,
• and able to resolve incredibly small changes in capacitance resulting
in high resolution measurements of less than one nanometer.

18. Compared to other noncontact sensing technologies such as optical, laser,
eddy-current, and inductive, high-performance capacitive sensors have
some distinct advantages
•Higher resolutions including sub-nanometer resolutions
•Not sensitive to material changes: Capacitive sensors respond equally to all
conductors
•Less expensive and much smaller than laser interferometers.
Capacitive sensors are not good choice in these conditions
•Dirty or wet environment (eddy-current sensors are ideal)
•Large gap between sensor and target is required (optical and laser are
better)
Pros & ConsPros & Cons

19. ApplicationsApplications
Position Measurement/Sensing
Capacitive sensors are basically position measuring devices. The outputs always
indicate the size of the gap between the sensor's sensing surface and the target.
When the probe is stationary, any changes in the output are directly interpreted as
changes in position of the target. This is useful in:
• Automation requiring precise location
• Semiconductor processing
• Final assembly of precision equipment such as disk drives
• Precision stage positioning

20. ApplicationsApplications
Dynamic Motion
Measuring the dynamics of a continuously moving target, such as a rotating
spindle or vibrating element, requires some form of noncontact measurement.
Capacitive sensors are ideal when the environment is clean and the motions are
small, requiring high-resolution measurements:
• Precision machine tool spindles
• Disk drive spindles
• High-speed drill spindles
• Ultrasonic welders
• Vibration measurements

21. ApplicationsApplications
Thickness Measurement
Measuring material thickness in a noncontact fashion is a common
application for capacitive sensors. The most useful application is a two-
channel differential system in which a separate sensor is used for each side
of the piece being measured.
Capacitive sensor technology is used for thickness measurement in these
applications:
•Silicon wafer thickness
•Brake rotor thickness
•Disk drive platter thickness

22. ApplicationsApplications
Nonconductive Thickness
Capacitive sensors are sensitive to nonconductive materials which are
placed between the probe's sensing area and a grounded back target. If
the gap between the sensor and the back target is stable, changes in the
sensor output are indicative of changes in thickness, density, or composition
of the material in the gap. This is used for measurements in these
applications:
• Label positioning during application
• Label counting
• Glue detection
• Glue thickness
• Assembly testing

23. Capacitive sensing in hidCapacitive sensing in hid
• Capacitive sensing as a human interface device (HID) technology, for example
to replace the computer mouse, is becoming increasingly popular.
• Capacitance sensors detect a change in capacitance when something or
someone approaches or touches the sensor.
• Capacitive sensors are used in devices such as laptop track-pads, MP3 players,
computer monitors, cell phones and others.
• More and more engineers choose capacitive sensors for their flexibility, unique
human-device interface and cost reduction over mechanical switches.

24. Sensing capacitance in hidSensing capacitance in hid

25. The three parts to the capacitance-sensing solution:
• The driver IC, which provides the excitation, the capacitance-to-digital converter,
and compensation circuitry to ensure accurate results in all environments.
• The sensor a PCB with a pattern of traces, such as buttons, scroll bars, scroll wheels,
or some combination. The traces can be copper, carbon, or silver, while the PCB
can be FR4, flex, PET, or ITO.
• Software on the host microcontroller to implement the serial interface and the
device setup, as well as the interrupt service routine. For high-resolution sensors such
as scroll bars and wheels, the host runs a software algorithm to achieve high
resolution output. No software is required for buttons.
Basic Components of capacitance sensing technologyBasic Components of capacitance sensing technology

26. Basic Components of capacitance sensing technologyBasic Components of capacitance sensing technology

27. Ad714x integrated circuitsAd714x integrated circuits

28. Ad714x integrated circuits featuresAd714x integrated circuits features
• These capacitance-to-digital converters are designed specifically for
capacitance sensing in human-interface applications.
• The core of the devices is a 16-bit sigma-delta capacitance-to-digital
converter (CDC), which converts the capacitive input signals (routed by a
switch matrix) into digital values.
• The on-chip excitation source is a 250-kHz square wave.
• The devices can be set up to interface with any set of input sensors by
programming the on-chip registers.
• One of the key features of the AD714x is sensitivity control, which imparts a
different sensitivity setting to each sensor, controlling how soft or hard the
user’s touch must be to activate the sensor.

29. ResponseResponse
• When the sensor is not active, the capacitance value measured is stored as the
ambient value.
• When a user comes close to or touches the capacitance sensor, the measured
capacitance decreases or increases.
• Threshold capacitance levels are stored in on-chip registers. When the measured
capacitance value exceeds either upper or lower threshold limits, the sensor is
considered to be active and an interrupt output is asserted.

30. Shapes & sizes of sensorsShapes & sizes of sensors

31. Design procedureDesign procedure
• Decide what types, number and dimension of sensors are needed in the
application.
• Place the AD7142 or AD7143 on the same PCB as the sensors to minimize the
chances of system errors due to moving connectors and changing capacitance.
• Other components, LEDs, connectors, and other ICs, for example, can go on the
same PCB as the capacitance sensors
• The sensor PCB must be glued or taped to the covering material to prevent air
gaps above the sensors.
• For applications where RF noise is a concern, then an RC filter can be used to
minimize any interference with the sensors.
• Calibration of capacitance sensing

32. • Capacitance sensors are more reliable than mechanical sensors.
• Humans are never in direct contact with the sensor, so it can be sealed
away from dirt or spillages.
• Capacitive touchscreens are highly responsive
• A standard stylus cannot be used for capacitive sensing unless it is tipped
with some form of conductive material.
• Capacitive touchscreens are more expensive to manufacture.
AdvantagesAdvantages
DisadvantagesDisadvantages

33. • Capacitance sensors are an emerging technology for human-machine interfaces
and are rapidly becoming the preferred technology over a range of different
products and devices.
• Capacitance sensors enable innovative yet easy-to-use interfaces for a wide
range of portable and consumer products.
• They give the industrial designer freedom to focus on styling, knowing that
capacitance sensors can be relied upon to give a high-performance interface
that will fit the design.
ConclusionConclusion

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