[time-nuts] Capacitive temperature sensing

[Bob Paddock]

>   Bruce, very interesting. I didn't know capacitive sensors  went down
>   that low. That could be useful in other areas.
>
>   I searched google but found nothing. Do you have any urls?

http://www.eetimes.com/showArticle.jhtml?articleID=175801455

"EE Times: SENSORS: Quake detector preps for market

...
"We are finishing the prototype now," said Les LeZar, president of Zoltech 
(Van Nuys, Calif.). "As it stands, its case is just under 2 feet tall and 
houses an 18-inch pendulum."

Peters' design uses a novel means of varying the surface area of a capacitor. 
Rather than varying its gap as in standard capacitive sensors, Peters' design 
varies the capacitor's surface area. Because the capacitor's gap is constant, 
detection is not accompanied by a drop-off in sensitivity, as is the case 
with other capacitive sensors. Most of those become less sensitive when their 
gap widens. "I have a patent on several variations of what I call a symmetric 
differential capacitive sensor — what microelectromechanical-systems 
designers call 'fully differential,' " said Peters.

Since sensitivity and dynamic range don't have to be treated as a design 
trade-off — as in traditional capacitive sensors — Peters' design sets 
sensitivity by the constant size of the gap. By changing the surface area, it 
separately determines dynamic range.

In a nutshell, the dangling pendulum has a grounded Faraday shield on the end, 
hanging between the four plates of a symmetric differential capacitor wired 
in a diamond like a Wheatstone bridge, but with series diodes and parallel 
resistors to rectify its output into direct current. Parallel printed-circuit 
boards house the plates of four square capacitors.

The grounded Faraday shield initially covers all capacitors equally. When the 
Faraday shield is jiggled, it increases the surface area of two capacitors on 
opposite sides of the bridge, and decreases the other two opposing 
capacitors' area, thereby giving the bridge its differential sensitivity. A 
50-MHz signal is then pumped across two opposing capacitor leads in the 
bridge, while the differential inputs to the sensing operational amplifier 
are wired to the other two opposing capacitor leads. ..."



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