piezo vibration sensor

[engineheat]

Hi,

I'd like to measure the vibration (frequency, intensity) of an object thru contact. I have heard about piezo sensors that can do this. However, I wonder if the result would vary based on the pressing force between the sensor and the object?

The object position can vary a bit so if I use an actuator to push the sensor to the object, the contact force might vary.

Please advise on any feasible approach.

Thanks

[Renate]

A piezo is measuring pressure, so obviously the contact pressure and resilience of the backing makes a difference.

I think that you just need to go try this and see how the results work for you.

Other trivia:
Piezo buzzers are a cheap and easy source of sensors.
They often have 3 terminals, one face has a little area reserved for feedback to make it peep at the natural mechanical resonance.
If for some reason they get depolarized (or you want the sense to go the other direction) simply apply a few hundred volts to them for a second.

[Marco]

If the object moves more than say micron range then the entire piezo will have to start vibrating with the object. I'd mount the piezo in a harness on springs, including a spring attached to the actuator. Use the DC output from the piezo to adjust the actuator to get the same pressure every time.

It will be hard to calibrate.

I'd look at non contact optical or ultrasonic, doppler or distance measurement devices.

[Conrad Hoffman]

Why not just stick a piezoelectric accelerometer on/against the device? It's really nothing more than a pzt element with a mass on one side. The device vibrates, the mass tries not to, so you get a signal out of the pzt. I've made them out of multilayer stacks for good sensitivity. There are several on the 'bay, usually 4 or so in a pack. Glue it to a small base and glue a small brass mass on top.

[Henrik_V]

What you look for is called an accelerometer. DIY one (as Conrad explained) need a piezo disc and seismic mass .
The output of the force to the piezo is a charge. Together with the capacity of the piezo disc (and the cable!) you can measure a voltage.
To reduce the influence of the capacities a common way is to use a charge amplifier.

What do you want to explore?
(frequency range? expected acceleration range? )

Contact less methods could range from a simple CNY70 (some cent) up to laser doppler vibrometer (>10k$|€)

[voltsandjolts]

Without detailed requirements from you it's not possible to suggest an exact sensor, but since your username is 'engineheat' it seems appropriate to suggest piezoelectric knock sensors. They are just as described above, piezo disc and a mass packaged up with a spec. E.g. this Bosch unit rated at 26mV/g at 5kHz.

[Renate]

Oh, and you might want to use a charge amplifier.
https://www.researchgate.net/figure/Charge-amplifier-for-piezoelectric-sensors-with-balanced-impedances-at-both-operational_fig6_265036087

[mrflibble]

Maybe a contactless solution using an inductance-to-digital converter fits the bill?

See for example:

• https://www.ti.com/lit/an/snoa931/snoa931.pdf
  
• https://training.ti.com/sites/default/files/docs/Sensing%20Solutions%20_0.pdf
  
• https://training.ti.com/ti-precision-labs-magnetic-sensors-designing-analog-proximity-sensor?cu=1137749
  
• https://training.ti.com/sites/default/files/docs/designing_an_analog_proximity_sensor.pdf
  
• https://duckduckgo.com/?q=site%3Ati.com+contactless+distance+inductance+to+digital
  

[Zero999]

Piezo buzzers are a cheap and easy source of sensors.
They often have 3 terminals, one face has a little area reserved for feedback to make it peep at the natural mechanical resonance.
If for some reason they get depolarized (or you want the sense to go the other direction) simply apply a few hundred volts to them for a second.

What do you mean by depolarised? I don't think they're polarised. They're non-polar, as far as I'm aware and nothing in the manufacture's application notes I've found states otherwise.

[Conrad Hoffman]

AFAIK, piezo ceramics don't do anything unless they're polarized, other than be capacitors. You can tell the direction, if you need to know, by squeezing it and noting the polarity of the signal.

[Renate]

What do you mean by depolarised?

I can't speak to the physics exactly, but I have re-polarized piezo crystals a few times before.
(Piezo servoes with a feedback section.)

https://www.comsol.com/blogs/piezoelectric-materials-crystal-orientation-poling-direction/

Consider this, a piezo crystal with two faces, two wires.
You hit it with a hammer, which wire should be positive if a piezo crystal was entirely uniform?

It's like magnetizing something. Put it in a strong field, hopefully some of the "polarization" remains when you remove the field.
Heating above the Curie point kills both magnetism in materials and piezo polarization.