Capacitive Sensors – Part III

Written by: Bjoern Schaefer

Typical Dielectric Material Factors
Typical Dielectric Material Factors

The general sensing principle across this myriad of applications is nearly the same. As seen in last months post, the total amount of capacitance, as we remember, the ability to store a charge within an electrostatic field, depends on mainly three factors. Those factors are the ones which determine the success of your application.

1.  The size of the electrode area
A larger electrode will cause a larger and deeper field. Therefore, larger capacitive proximity sensors offer a longer sensing range as smaller ones.

2.  Distance between electrodes
The larger the distance, the smaller the capacitance – capacitive proximity sensors have usually maximum sensing distances of up to 30mm.

3.  Dielectric material factor
The dielectric factor is material specific and is probably the most important variable in your capacitive application. The table below shows typically dielectric values for common liquids and materials. A higher value indicates a higher capacitance given that all other variables stay constant.

Given these three variables, capacitive sensors can perform a variety of tasks:

Proximity – Sensor area and target material stay constant, the sensor will provide changing signal depending on the distance to the target.

Material composition – Sensor area and distance stay constant while the materials dielectric factor changes. This would allow the sensor to differentiate for example between two or more plastic materials of same color and shape.

Indirect sensing – Keeping again the sensor area and distance constant, capacitive sensors are capable to sense material through other materials. For example detect water through a non-metallic glass or plastic wall. This is possible, as long as the wall material has a lower or equal dielectric constant as the target.

In the next part, we will look at possible problems you have to be aware of when using capacitive sensing technology.

Previous posts:
Capacitive Sensors – Part I
See Capacitive Sensors – Part II

Jack Moermond has more than 41 years of experience in the manufacturing and automation industry. His roles have included controls engineer, systems specialist, systems department manager, and product manager. His product experience covers sensors, PLCs and drives, steel and paper industries, packaging, food and beverage industries, semicon and life sciences. In addition to his roles at various automation suppliers, Jack has taught PLC programming and various other training classes on automation devices.

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