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Inside of a piezo buzzer is a little unimorph/bimorph disk which converts voltage to mechanical movement. To function as a buzzer, you just feed it an ac signal at its resonant frequency. However, John Alexander, the author of this project: STM Project, patented a design that uses the unimorph disk inside of a cheap buzzer as a fine-adjustment mechanism (US5866902).

Basically, if you apply a DC voltage to the across the ceramic part of the disk and the brass part, the disk will flex a very tiny amount (around 100nm/V).

My question is this: Piezo electric elements are given a polarity by applying a strong electric field that aligns the dipole moments inside of the piezo material. Thus, I cannot a priori know if the piezo disk that I remove from a buzzer will extend or contract when I apply a positive voltage bias. Keeping in mind that an application of 15V would yield a flex of around 1um, what is a clever way to test a disk for its polarity?

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  • \$\begingroup\$ One way to observe 1um is with a layer of glass (microscope slide?) over the brass surface and monochromatic light ( say from a red LED), observing the interference fringes between them. \$\endgroup\$ – Brian Drummond Apr 30 '16 at 10:24
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    \$\begingroup\$ I ended up doing something different, as recommended by my professor: since I used the piezo for the fine adjustment of a scanning tunneling microscope, I had a wire rigidly attached to the buzzer. I brought the wire near to the sample and ran a high frequency wave through the sample. Since the sample and the wire are capacitively coupled, I could pick up the high frequency wave on the wire (hooked up to a scope). By then applying a bias the the piezo, I could observe the signal grow and shrink (grows when wire is closer to sample). \$\endgroup\$ – Hunter Akins May 8 '16 at 5:13
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Normally when polarization is done a dot of paint or a scratch or some similar sign is written on one surface of piezoceramics to indicate positive polarity. To view the deformation probably the best system it shine a light ray on the surface and see how the reflected ray goes. Another system could be to mechanically couple a reference bimorph to unit under test and measure the output current from known unit when you give a drive to unit under test.

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I have purchased specific piezo-ceramic brass disks in quantity. Typically, a batch is mostly polarized the same. :) However, some may be reversed. You can test by flexing the edge of the disk and observing the direction of voltage change. In my application, I allowed for either polarity by selective "strapping", wires are added to connect in either direction.

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Another method is to apply a little pressure on the disk and observe the generated voltage on a scope.

As per this thread:

At frequencies below resonance their impedance is capacitive...about 20,000pf avarage. Bigger ones are higher.

Easiest way is if you have something like a charge amp. A fet input follower (unity gain) op amp will do. My lab scopes are very high impedance so I can just hook the piezo directly to the scope.

Hook input/ grd to the piezo. Then just lay them on a support (like a little ring about 2/3 the diameter) and press on it with a pencil eraser or something. See if the op amp (or scope) output goes positive or negative. Even with a fairly light pressure you should see 5-20 volts.

For this to work RC has to be a second or so, so that means the op amp (or scope) input has to be at least 50 meg ohms . A bit higher would be better. The input impedance of the amplifier and the capacitance of the element form a high pass filter.

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Simply connect to a multimeter set to Volts dc. Press on the centre of the disc (with a pencil eraser). The readout will go +ve or -ve very quickly (and then probably reverse) so watch the +/- sign NOT the actual volts. You need to notice the initial response not subsequent changes.

You will need to change the Vdc scale for best results

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