A capacitive sensor uses changes in capacitance to detect proximity or contact with a target. The sensor has a sensing area that produces an electric field, and a change in the gap between this area and a target alters the capacitance in a way that can be measured. Capacitive sensors are used for applications like touchscreens, thickness measurement, and position sensing due to their non-contact operation and insensitivity to the target material. They provide flexibility and cost advantages over mechanical switches in human-machine interfaces.

2. •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. •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.

4. • 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 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
signal
Primary
transducer
Secondary
transducer
SENSOR
Usable
Values

7. •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 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 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. 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. 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. Figure 10.
Nonconduct
ors can be
measured
by passing
the electric
field through
them to a
stationary
conductive
target
behind
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. Other Factors to be considered for optimization:
•Maximizing Accuracy
•Target Shape
•Surface Finish
•Parallelism
•Environment

16. 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. 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)

19. 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. 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. 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. 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 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.

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.

28. •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. •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.

31. •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.

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.

34. • http://www.analog.com/library/analogdialogue/
• http://www.lionprecision.com/tech-library/
• http://electronicdesign.com/Articles/
• http://www.sensorsmag.com/sensors
• http://en.wikipedia.org