Shielding from electrical drive of piezolelectric at low frequency

Thread Starter

Rani Barbara

Joined Jun 1, 2019
2
Hi

I am driving a piezolelectric element (ceramic) at alternating voltage between 10V and 20V at a frequency of 8hz. The piezo is attached to my skin. Meanwhile I am recording Electromyography from my skin to capture the effects of the piezo on my muscles. However the 8hz alternating driving voltage is being picked up and want to get rid of it. How do I shield from this artifact?

Thank you
Rani
 

DickCappels

Joined Aug 21, 2008
6,591
I take it this is a single-ended signal. If it were differential -that is two signals of opposite phase using two conductors you could start by twisting the wires together to cancel out much of the capacitively coupled voltage.

If it is single-ended the first thing I would try is driving the transducer through a shielded cable such as small diameter coaxial cable. The shield connected to ground near the driver circuitry. You might want to AC couple your transducer with a capacitor since DC across piezolelectric is not supported in all of the datasheets I have seen.

Also, be careful that your skin is not coupling the transducers. You may have to place a shield between the transducer and your skin. That would probably be the last countermeasure to try because the shield between the transducer and your skin is likely to "mess with" coupling of the transducer to your skin.

Since it appears that you equipment is electrically coupled to your skin, please be sure that your equipment is properly earthed and that there is not more than a few micirocamps leakage current to earth.
 

MisterBill2

Joined Jan 23, 2018
7,033
I had a project that used a piezo element to vibrate an aluminum plate, and I had to keep the drive signal from coupling to the aluminum plate. The method required an intermediate conductor between the piezo element and the plate, and then an insulation layer between the grounded intermediate conductor and the aluminum plate. In your instance the grounded side of the piezo element must be insulated from your skin, and make sure that the ground is low impedance. That should prevent the coupling of the drive signal into your body, so that you can measure the response.
 

MisterBill2

Joined Jan 23, 2018
7,033
Thank you for your suggestions. I will try and return with feedback
One thing more, which is after seeing results, I would temporarily disconnect the isolated shield and verify that the results were different. This served as a "reality check" to verify that the isolation was really being isolation.
The whole effort started because another researcher had obtained results that did not make sense to me, in that his signals seemed to travel instantly instead of at the speed of sound. That made no sense and so I discovered that the isolation was vital to seeing what was really happening.
 
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