Capacitive Sensors

As noted earlier, the sensor traces can be any number of different shapes and sizes:

• Buttons,

•wheels,

•scroll-bar,

•joypad, and

•touchpad

Buttons

Capacitive Sensors

8-way switch

Slider

Wheel

Capacitive Sensors

Keypad

Touchpad

Capacitive sensors

Capacitive sensors are noncontact devices capable of high-resolution measurement of the position and/or change of position of any conductive target. The nanometer resolution of high-performance capacitive sensors makes them indispensible in today's nanotechnology world. They can also be used to measure the position or other properties of nonconductive targets.

Basic Theory

Capacitive sensors use the electrical property of

"capacitance" to make measurements. 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. High-performance displacement sensors use small sensing surfaces and as result are positioned close to the targets (0.25-2mm).

Capacitive Sensors

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. A newer application, coming into widespread use, is in human-to-machine interfaces. Mechanical buttons, switches, and jog wheels have long been used as the interface between the user and the machine. Because of their many drawbacks, however, interface designers have been increasingly looking for more reliable solutions

How does it work

The goal is to develop a capacitive sensor that has the right response and meets ergonomic requirements. In some applications, the sensor may have to be small, resulting in small changes in capacitance levels upon user contact.

www.zaoqiche.com/qczz/qcyy/ywlw/110946.as

Alignment of plates Aligning of two plates on the basis of capacitive

value:Capacitive value between electrodes represent

, where A is changing as the two plates are moved for alignment. Therefore one could exploit this for making accurate alignment.

4

AC

de

p=

Fluid Pressure

A

B

A and B being electrodes representing a capacitive value between two plates. While the area is changing due to pressure on B as diaphragm, therefore a change in d is experienced. Such change is proportional fluid pressure on this diaphragm. This as bridge out reading through moving coil meter could be calibrated for pressure measurement.

4

AC

de

p=

Linear Displacement

Capacitive value is dependant upon electrolyte of the fluid in the cylinder. While due to linear displacement of the piston the fluid matter is pushed out/pushed in, therefore causing change in Capacitance due to r:

4

AC

de

p=

Fluid Level

Inner float is representing fluid level in the tank. Movement of the float is causing change in area under outer plates of the capacitance therefore could be exploited to measure the level.

4

AC

de

p=

Frequency

Capacitive transducer is used as part of active oscillator, where a change in frequency is reflected in terms of voltage for a meter scale to give frequency readings.

1

1

2

LC

fLC

w

p

=

=

1

CD

Fluid Pressure

A diaphragm is pushed out as a result of fluid pressure, causing respective changes in capacitive values of C1

and C2. Both of these capacitors are part of two legs of a Wheatstone Bridge therefore reading could be measured as voltage, scaled for fluid pressure

Linear Displacement

The sensing shaft in a capacitive transducer changes the position of the dielectric between the capacitor's plates in the transduction element, or it changes the distance and area between the plates. A change of these three parameters leads to a change in capacitance, which is a measure of the quantity to be measured.Capacitive displacement transducers with variation in :

4

AC

de

p=

(a) dielectric constant

(b) gap between plates, and (c) area of capacitor's plates

Capacitive Sensor Advantages

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 subnanometer resolutions

Not sensitive to material changes: Capacitive sensors respond equally to all conductors

Less expensive and much smaller than laser interferometers.

Applications

Position Measurement/Sensing: Automation requiring precise location Semiconductor processing Final assembly of precision equipment such as disk drives Precision stage positioning

Dynamic Motion: Precision machine tool spindles Disk drive spindles High-speed drill spindles Ultrasonic welders Vibration measurements

Applications …

Thickness Measurement: Silicon wafer thickness Brake rotor thickness Disk drive platter thickness

Nonconductive Thickness: Label positioning during application Label counting Glue detection Glue thickness Assembly testing

Applications …

Assembly testing: Capacitive sensors have a much higher sensitivity to

conductors than to nonconductors. Therefore, they can be used to detect the presence/absence of metallic subassemblies in completed assemblies. An example is a connector assembly requiring an internal metallic snap ring which is not visible in the final assembly. Online capacitive sensing can detect the

defective part and signal the system to remove it from the line.