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DMASP Question

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David Durham posted on Tue, Apr 5 2011 9:39 AM
We have bought a package of [Bruker] DMASP Active probes.
My question is: what is the range of voltage that can be produced as a result of deflection of ZnO stack in DMASP. We are going to use it as a sensor not actuator. Therefore, we need to know the range of voltage that can be sensed when the DMASP deflects.
 
I appreciate your time and help.
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Verified by David Durham


As phrased, this is a bit of a difficult question. When you deflect the DMASP piezo, it will create a charge (not a voltage). The equations that relate the conjugate variables of  cantilver: angle, deflection, piezo volume, and piezo charge to applied: bending moment, Uniform load (both usually 0 for SPM), Tip Load, and Piezo Voltage are well described in the paper by Smits:

Smits JG, Choi W., IEEE Trans Ultrason Ferroelectr Freq Control. 1991;38(3):256-70.
http://www.ncbi.nlm.nih.gov/pubmed/18267583

From this you can calculate the generated charge, and then convert that to a voltage depending on the circuitry you attach to accomplish the measurement.

For your experiment, your best approach is to make a charge sensor (infinite input impedance).

If you use a voltage sensor, the voltage, and decay, you measure will be proportional to the input resistance and capacitance of your sensor. You will essentially get the derivative of your signal (as the generated charge is depleted by the measurement amplifier).

A way to get around this is to measure the change in capacitance of the piezo (rather than the voltage or charge). You can find several papers where people have used the DMASP in this type of self sensing mode. The basic concept is a “capacitive” Wheatstone bridge.

Hope this helps,
Steve

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