Capacitive Sensing

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

Overview

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.

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.

digital-calliper-capacitive-encoder-strip-up-close-photo 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.
capacitance-strip-dimensions-for-linear-encoder 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.

capacitive-encoder-photo-cui-amt 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/.
rotational-cap-encoder-photo-netzer-ds-25-280x263 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