The Nanoscale World

# DMASP Question

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42 Posts
Points 485
David Durham posted on Wed, Sep 14 2011 9:30 AM

Hi,

Is there a spec for the nominal sensitivity of Bruker's DMASP cantilevers for sensing?  As in the charge or current per meter of applied force or displacement to the cantilever tip, as opposed to the displacement of the tip per applied voltage?

Thanks,

Dave

• | Post Points: 12

288 Posts
Points 3,905
Stephen Minne replied on Wed, Sep 14 2011 10:36 AM
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.

The following paper show different options for doing this, and from it you can get an idea of the deflection sensitivity of the DMASP.

Hope this helps,
Steve

• | Post Points: 13

#### All Replies

288 Posts
Points 3,905
Stephen Minne replied on Wed, Sep 14 2011 10:36 AM
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.

The following paper show different options for doing this, and from it you can get an idea of the deflection sensitivity of the DMASP.

Hope this helps,
Steve

• | Post Points: 13
42 Posts
Points 485
David Durham replied on Wed, Sep 14 2011 10:47 AM

Thanks Steve!

• | Post Points: 10
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