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The Predictive Maintenance Tale - piezo element voltages

Jan Cumps

For ' Tales for Makers Amusement Park, I'm making a predictive maintenance design for the rides and Ferris wheels. They have to be kept safe and in good condition. 

The project is a little piezo buzzer. In this little forum post , I show that these can generate high voltages that can damage microcontroller inputs.

image

The artwork in this story is an ode to Benoît Sokal. Comic Artist and Designer of the Syberia game.

A piezo transducer can act as speaker or sensor. I'm using it as a sensor. Piezo can generate high voltages - an element is used to generate the spark in piezoelectric stove igniters. Although these high voltages aren't generated when measuring usual mechanical vibrations, a protection circuit is advised. Even a medium tap on the sensor generates 10s of volts.

image
image source: product page of the kit I bought on Conrad

There are breakout kits from several companies for Arduino and Raspberry Pi, that come with a piezo element and a protection circuit. They all seem to deploy the same protection circuit:

image
The diode is a 5.1V Zener Diode. But let's see what happens if I hit a test probe on the board where sensor sits on:
image
It's a "plastic on cardboard" action. I can easily generate 24 V peaks with the protection circuit in place. Not good for a 5V UNO. Most likely unhealthy for 3.3 V Arduino MKR or Raspberry Pi. If this would be mounted on metal, and a metal part would be used to hit, I'd expect the peaks to go higher. There's almost no energy - it's all voltage and for a short time. But it's voltage that often does the damage.

also check out this related post:  Piezo disk as Vibration Sensor: input buffer and filter 

Additional Syberia art from this walktrough, from PlayStation store. 

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  • 0

    Hello Jan,

    Not very suprsied by the result or the badness of some of the Arduino and Pi support stuff - it's attracted a lot of very low end suppliers.

    There is a very old charge amp/filter fronmt end ciruit on my website and I've adapted it for your sounder sensor.

    I've guessed your source capacity at 25nF.

    This circuit gives real and fast protection and band pass filtering and the ability to drive an ADC properly.

    I've modelled it with dual supplies but you can use and RRIO type CMOS amplifier with a single supply to match the ADC. (with suitable biasing of course)

    Putting the protection diodes on the summing node of the charge amp means that there is no voltage (almost) across them in normal operation so they don't screw up the LF response (which the zener across the sensor can.)

    C1 and V1 represent the sensor

    R1 sets a HF rolloff (dependent on the source capacity as well (yes the source capacity))

    C2 sets the charge gain (if C1 = C2 the output is the same as the open circuit outpuy of the sensor would have been))

    Because R1 is between the sensor and the protection diodes they will clamp the maximum input to the amplifer effectively.

    The blue trace is the voltage on the summing node.

    image

    MK

  • 0 in reply to michaelkellett

    I've been putting it to the test. Creating high energy peaks by dropping things right on the piezo. 
    The circuit behaves well, containing the signal within the +- Vcc:

    image

  • 0 in reply to Jan Cumps

    Here a flow where both the industrial sensor and the piezo disk with  's front end are fed into a DAQ.

    image

    To check if the phase difference was related to the DAQ, I switched both channels.
    It's not related to the DAQ. It's either caused by the front end  (Michael's for the disk, inside the sensor housing for the industrial sensor).
    Or by the positions of the two sensors. The industrial one sits on top of the vibration hotspot. The Piezo disk a few centimetres away.

    Setup:

    image

    And a statistical view, of the piezo disk with Michael's front end only:

    image

    The acceleration calculation value isn't correct, because I haven't specified/calibrated the disk & front-end voltage vs acceleration.
    You can see that the FFT graph finds the correct resonance frequency at 40 Hz.

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  • 0 in reply to Jan Cumps

    Here a flow where both the industrial sensor and the piezo disk with  's front end are fed into a DAQ.

    image

    To check if the phase difference was related to the DAQ, I switched both channels.
    It's not related to the DAQ. It's either caused by the front end  (Michael's for the disk, inside the sensor housing for the industrial sensor).
    Or by the positions of the two sensors. The industrial one sits on top of the vibration hotspot. The Piezo disk a few centimetres away.

    Setup:

    image

    And a statistical view, of the piezo disk with Michael's front end only:

    image

    The acceleration calculation value isn't correct, because I haven't specified/calibrated the disk & front-end voltage vs acceleration.
    You can see that the FFT graph finds the correct resonance frequency at 40 Hz.

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