[time-nuts] Capacitive temperature sensing
bob.paddock at gmail.com
Sat Aug 23 09:58:55 EDT 2008
> 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?
"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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