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### Topic: Piezo disk vibration sensor - Stability and resolution (Read 443 times)previous topic - next topic

#### snai091

##### Nov 10, 2017, 03:57 amLast Edit: Nov 10, 2017, 04:03 am by snai091
Hi All,

I have a piezo disk sensor connected to an Arduino Duo (see setup in figures 1a and 1b). When looking at the response using the serial monitor and serial plotter, the signal is sometimes stable on zero, and then shows a spike in signal response when it is feels a vibration. These signals show excellent baseline resolution and baseline stability. An example of this can be seen in figures 2 and 3.

The problem I have is that sometimes instead of a nice stable baseline and good resolution, I get a random baseline with no baseline resolution as the signal continues to randomly increase or decrease (see figures 4 and 5). When this happens, I do however continue to get a spike in signal in response to a vibration however with an unstable baseline I can't do much with the information.

Does anyone have any advice on how to stabilise my signal and baseline?

The attached pdf contains figures referred to.

Much appreciated!!

#### Wawa

#1
##### Nov 10, 2017, 05:24 am
A piezo disc is essentially a capacitor.
If there is a DC voltage on that capacitor, from e.g. pin leakage or clipping, then it could stay there.
A drifting or not defined (floating) pin.

Normally a (1Megohm) resistor is used across the sensor (pin to ground) to force the baseline to ground level.
But for a vibration sensor, you could bias the piezo mid-voltage (with a 2-resistor voltage divider).

A Due uses 3.3volt logic, so use one resistor from pin to ground, and one resistor from pin to 3.3volt.
Use equal high value resisors (1-10Megohm) if you also want to detect low frequency vibrations.
You should now have an A/D value of ~512 (or 2048) as baseline.
Leo..

#### MarkT

#2
##### Nov 10, 2017, 12:26 pm
In fact piezo materials are similar or identical to the composition of some ceramic capacitors, the
same class of titanate minerals are used which have very large dielectric constant and are ferro-electric,
meaning the mechnical distortion of the crystal lattice pushes charge across the crystal.  So piezo elements
are perfectly serviceable ceramic capacitors in their own right, but engineered to bend easily - ceramic
capacitors made from these materials are "microphonic", so it you give them a sharp tap you'll see a
voltage spike from them - they are not used in audio circuits for this reason.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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