C2.1 - Capacitive Sensing

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings SENSOR 2009, Volume I
C2 - Sensor Electronics II (Capacitive Sensors)
G. Brasseur - Graz University of Technology, Graz, Austria
275 - 280


Capacitive sensors determine the electric field distribution in a measurement volume to capture the measurand like a geometrical quantity, a mass distribution or a volume stream. Since a long time scientists have worked in this field to design highly reliable capacitive sensors but could not obtain a visible market penetration. Thanks to microelectronics, capacitive sensing has become more and more popular in the recent years even for failure critical systems in an industrial and automotive environment. State-of-the-art airbag systems and electronic stability programs for vehicles would almost be impossible without capacitive sensing. Today’s capacitive sensing applications include touch pads, proximity and rotary switches, occupant detection, parking aids, inclination, encapsulated and not encapsulated position, nano-positioning, tank fill level, film and layer thickness, oil quality, smart textiles, flow measurement, electrical capacitance Tomography.
Some achievements from mobile communications contribute to capacitive sensing like low noise amplifiers, low power analogue-to-digital converters, frequency hopping spread spectrum and time division multiplex algorithms. Companies would have never spent money to design such optimised structures for the use in capacitive sensors as the sales volumes are much too small compared to mobile communications. As such building blocks are available now CDCs get a remarkable thrust from mobile communications and the semiconductor platform grows fast for small, reliable, robust and low cost capacitive sensing circuits tailored to an industrial environment.
A huge benefit of capacitive sensing is the simplicity of the front-end: the sensing elements are numerous conductive areas attached to an insulator. The sensor electronics measures the capacitances between these areas and the electronics calculates each measurement result from numerous capacitance measurements. As the conductive areas can be made out of almost all types of materials the sensing elements can be fabricated in a way to be both, inexpensive and robust.
One challenge of capacitive sensing are the small capacitance values to be measured between corresponding plates, usually in the pF to fF range with a resolution as low as aF. Consequently, the measurement electronics has to be located as close as possible to the transmitter and receiver electrodes to avoid leakage effects and disturbances from the environment. Typically, the front-end electronics, commonly called “Capacitance-to-Digital Converters (CDC)”, encompasses the entire analogue and digital signal processing circuitry as well as transmitters and output drivers to communicate with a master.
The aim of the paper is to give a summary of already published challenges and answers related to electrode topologies, sensor circuitries and algorithms for robust capacitive sensors tailored to an industrial environment.