charge-based piezo drive

It's super helpful with high-hysteresis piezo materials, e.g. PZT-5H. Turns out that the hysteresis is mainly in the capacitance and not in the piezoelectric effect. Using charge control lets you take advantage of the much higher piezoelectric sensitivity of 5H.

Another useful trick is that you can get the piezo to respond faster than its lowest longitudinal resonance by launching pulses down the stack--you get a wave of compression going down the stack, and the launch end moves almost instantly by comparison. Not too much range, but it makes a coarse-fine strategy possible.

A third thing is the utility of notch filters in PZT control loops. You can get a 10x improvement in the loop bandwidth that way, by notching out the huge resonant peak.

I did that on my atomic & magnetic force microscopes in 1988-9. Sped the instrument up by a lot. The cantilever tip was driven by a PZT bimorph, which had a first bending mode with a Q of about 30 at ~200 kHz iirc. To avoid oscillation without a notch, you have to roll off the loop gain by 30 dB at that frequency, making the loop schloooowww. Notching it out let me run the loop up to about f_bend/3 with good performance.

Cheers

Phil Hobbs

On a sunny day (14 Jul 2019 10:37:47 -0700) it happened Winfield Hill wrote in :

What is a 'charge based' piezo drive?

I am driving some big 40 kHz piezos hard with a MOSFET push pull circuit and transformer, PIC as signal generator. These:

Class E efficiency and simplicity vs Class D?

The piezo is capacitive. I'm assuming Win is treating it like a Mosfet gate (Qg), and designing the driver so that the piezo mechanically oscillates in as little time as the driver can handle.

Cheers

Sorry, gang! I should have said, piezo actuator. OK, now you can carry on.

Is that some sort of threat?

Hi,

Can you put in some type of feedback, ie measurement of capacitance to correlate to the strain and position?

I don't know if any other piezo drives have direct feedback built in, but it may give more accurate positioning compared to other ways of measuring the extremely small position changes.

cheers, Jamie

The first lab car on mars had 4 piezo drives in its wheels. Survived very much longer then estimated before launch.

Do you have a section on Piezo Transformers?

Cheers

Good idea. Sorry, no.

When I worked at a small company called Ultrasonic Energy Systems. We drove a 2" round x 0.80" thick PZT8 ceramic bonded to a 1/16" aluminum plate at 1000 watts pulsed and 250 continuous. We also had it in an ice water bath. to keep the transducer cool. We ran the transducers around 660kHz, whereever the maximum efficiency was. We treated solution in a vessel through an FEP Teflon 0.003" window. One of our customers substituted PTFE for the FEP window and could not repeat results. He has killing bacteria in milk, after the teflon sub, his kill rate went way down. FUN fact; we could watch sonoluminescence go across a 20" aquarium, reflect off the glass go to the other end and reflect again before dying off. First design was a class A amplifier, second one we used class E. The class E driver design was stupid, we had an osc and a couple of tuned preamps, to drive four Mosfets. By then there were good mosfet drivers, the tuned circuits were a PITA, I was the one winding coils and transformers. Probably OK for a physicist designing electronics. (Not me) Our claim to fame was the sale of a 4000 watt unit to Cal Tech, it used 4- 4" x 4" ceramics, with about a 1 gallon flow through treatment vessel. I remember those years fondly. Mikek

It's hard to say anything about that section before you show it to us :)

all flippancy aside...

I had an issue using piezo wafers as a sensor, because rectification in the sensor pulse stretcher caused strange charge accumulation effects.

That would not have occurred if I knew I should be treating them as a charge source (rather than current or voltage source). I think that might be a relevant comment to make.

Clifford Heath.

Well I was curious (having used voltage drive piezo nano-positioning stages in the past and noticing position drift) so I guess you are doing this:

"These actuators are normally driven by voltage amplifiers; however, the relationship of the applied voltage on the actuator and its position is nonlinear and complex due to phenomena such as hysteresis. This adversely influences actuator?s position control which is the core component of nanopositioning. However, the electrical charge applied on a piezoelectric actuator has a significantly less complex relationship with its position."

cheers, Jamie

On a sunny day (Sun, 14 Jul 2019 15:48:20 -0500) it happened amdx wrote in :

I always wanted to do that sonoluminescence experiment, but am a bit afraid of all that high power 40 kHz for my ears.

The PIC I programmed for this does at start up do a range of frequency increments, At the same time it measures output voltage for each frequency and remembers the best one representing resonance. It then uses that one, takes about a 2 seconds run to find it. It can do an automatic range of other pings too, is for anti-fauling in boats.

I still have no idea what a 'charge based' piezo drive is? It is all about resonance AFAIK.

What I find amazing is those little beepers, I have one from China, that make a +120 dB 3 KHz or so beep hardly using any power, have one as alarm here, you cannot live in that noise.. few mW?

Hi,

I just realized "section" is referring to AOE.

That makes more sense than requesting guesses on a section of a circuit board you designed which is what I assumed you meant!! :D

cheers, Jamie

Way back when... (Tim Wescott was still posting here.) I tried putting a notch in a piezo control loop, (I think on your advice) And didn't have much luck making it work. How deep a notch do you need?

george H.

Several times 1/Q. You want the open loop gain at the resonance to be well below 1.

You also want the notch to be sharp, so that its phase funnies don't extend too low in frequency.

Cheers

Phil Hobbs

Should have said 1/(several times Q).

Cheers

Phil Hobbs

You sense the integrated charge and feed back on that. Conceptually like running the piezo with a very small capacitor in series, and jacking up the drive voltage.

Of course you have to get the DC from someplace, but that can be a capacitive gauge or something like that--much slower than the piezo. The reason that charge control is a good idea is that it reduces the nonlinearity and hysteresis of the piezo actuator, so you can get better control over a wider displacement range.

One way of looking at it is that the capacitance goes down as the piezo material starts to saturate, so an increment of charge dq produces a larger voltage change dV, helping compensate the nonlinearity.

There's an analogous process that reduces hysteresis, but I'm less clear on how that works.

Cheers

Phil Hobbs