PhD-Level Ultrasonic Engineering • Medical • Manufacturing • Consumer
The Shock Show #1: The two "direct" way to measure the piezoelectric charge coefficient
Welcome to the SHOCK SHOW!
I am starting a everyday live stream event (365 days a year) where I discuss different topics regarding ultrasonic transducer measurement, characterization, and development. Please follow the link to watch the first video on where I discuss:
The two "direct" way to measure the piezoelectric charge coefficient"
Here is the link: https://youtu.be/Y_nZk9_dnas
You can read on here as well!
What is the piezoelectric charge coefficient?
You would be surprised to know how many engineering working on ultrasonic devices cannot explain what the piezoelectric charge coefficient is. It's not their fault. This is not a concept taught in schools and it does not come front and center when doing ultrasonic resonance experiments.
Some engineers think it is a material property that you get when you multiply the coupling factor by the permittivity and compliance. Let's dig into this one.
The direct and the converse piezoelectric charge coefficient
Question: is there a difference in the piezoelectric charge coefficient measured by applying a force and measuring charge (units: Coulombs/Newton) vs applying an electric field and measuring strain (units: Meter/Volt). NO!
C/N = m/V
That's cool! So let's dive into the measurement.
How to measure the charge coefficient, the straight forward way
Yup, I could take you through some hoops on how to measure the charge coefficient using a resonance frequency sweep. However, let me get you straight to the heart of the matter.
Measurement using charge and force (direct effect)
Hook up your piezo material in such a way that you can place a large known force on it. You can even put it between two clamps with a load sensor in series with the piezo. Apply the clamping force. Make sure to measure the voltage with a large parallel capacitor attached, as voltage over a capacitor can be used to measure charge generation. The equation that relates charge per area generated by pressure (force/area) is:
charge/area = force/area *d33
Why don't we use this measurement method instead of a resonance frequency sweep? The charge coefficient has some frequency dependence and requires some cycling to stabilize. Some meters apply a cyclical compressive force on the piezo, but the contact conditions between the piezo and the clamps can be variable, thus increasing measurement error.
Measurement through displacement (converse effect)
When I explain this method to most engineers, they have a "why didn't I think of that moment". The piezo charge constant relates strain (and hence displacement) to applied electric field (or voltage). So, measuring displacement tells us strain - we are now off to the races!
However, putting 100V over a typical PZT ceramic generates only 40 nanometers of displacement. You can only measure that with a precise fiber optic light displacement sensor, such as those offered by Philtec.
An alternative solution is to use a laser vibrometer and drive the piezo at a high but still off resonance frequency, say 5kHz. Even though displacement will be the same between 10Hz and 5kHz, the velocity increases dramatically, thus allowing for accurate measurement of velocity and hence calculation of displacement.