Capacitive Sensing

Overview

This page is about capacitive sensing methods.
For information about capacitors in general, see the Capacitor page.

Capacitive sensing is a widely used technique for proximity detection, touch-screens, level measurement, and distance measurements. It is used in the more expensive touch-screens found in today’s smart phones (as opposed to the cheaper and poorer performing resistive touch screens), for contact-less buttons (see page on Capacitive Touch Sensing), the distance measuring mechanism in digital calipers, liquid level measurement and much more.

The CUI AMT100 series capacitive encoders with up to 2048 counts per revolution. Image from http://www.engineerlive.com/Asia-Pacific-Engineer/Time_Compression/Capacitive_encoder_offers_versatility/22119/.

Applications

  • Level measurement
  • Proximity sensing
  • Impurity detection
  • Touch sensing (see page on Capacitive Touch Sensing)
  • Flow sensing
  • Position sensing

Capacitance Sensors

Linear

Linear capacitive sensors measure a linear position change (i.e. not rotational).

The following image shows the typical dimensions of the linear encoder typically used in digital calliper design.

A zoomed in photo of the capacitive linear encoder pads on a digital calliper. Image from http://www.iceinspace.com.au/forum/showthread.php?t=80356.
The dimensions for a capacitive linear encoder commonly found in digital callipers. Image from http://www.iceinspace.com.au/forum/showthread.php?t=80356.

Rotational

Rotational capacitive sensors can be used in similar situations to those that use optical encoders.

Examples of two small capacitive encoders.

The CUI AMT100 series capacitive encoders with up to 2048 counts per revolution. Image from http://www.engineerlive.com/Asia-Pacific-Engineer/Time_Compression/Capacitive_encoder_offers_versatility/22119/.
The Maccon DS-25, with only a 25mm outer diameter, and 7mm thickness! Image from http://www.maccon.de/en/sensors/electrical-encoders.html.

Examples

PSoC chips with built in cap-sense hardware.

AD7150 with Er compensation

External Resources

Section 3.1 of the “Basic Linear Design” book by Analog Devices. Freely available from http://www.analog.com/library/analogDialogue/archives/43-09/linear_circuit_design_handbook.html