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Piezoelectric motor drive

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ALFAR

New Member
Hello everyone,

I am completely new in this field so my apologies in advance if I make elementary questions:

My project summary: I need to use the reverse piezoelectric effect and make a piezoelectric motor drive to create vibrations. The piezo element is going to attach the external wall of beaker (liquid container) and creates vibrations with a push button to produce ULTRASONIC (> 80 kHz in my case) vibrations and detaches micron size bubbles that are attached to the beaker wall. I have found manufacturers who can provide me with tiny (2*2*2 mm) piezos but suggest me to use an amplifier.

And here are my questions:

1) Why do I need an amplifier to amplify the output from my ac power?
2) What are the main components I need to make this kit and attach it to the walls of the liquid container?

Thank you so much and sorry for my limited knowledge and probably naive questions.
 

MikeMl

Well-Known Member
Most Helpful Member
You need relatively high voltages (hundreds of V) to make a piezo material bend. Depending on your budget, you can buy some very high-voltage opamps with a sufficiently high slew-rate and GBP to drive a 80KHz Piezo from Apex

Building one from scratch would be difficult for a beginner...
 
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ALFAR

New Member
Hello MikMl,

Thank you for your response,

Did you mean that I need a piezo bender? Actually, I just need a small piezo plate (say 5*5 mm) that I can attach (by means of an adhesive) to the walls of my liquid container. Considering this, do you mean that I need an opamps to amplify the output from the ac supply to create vibrations beyond 80 kHz? and finaly, do you think this arrangement will make sensible ultrasonic vibrations?

I think I need to find a custom kit manufacturer to save time as this is only a small part of a bigger project and we need to advance rapidly.

I welcome any suggestion regarding custom kit manufacturers.

Thank you so much for your advice,
 

MikeMl

Well-Known Member
Most Helpful Member
What I wrote about voltage required to drive a Piezo applies equally to bars or plates. At 80KHz, the Piezo looks like a slightly lossy capacitor to the driver. The driver needs to have sufficient current to be able to charge/discharge the Piezo capacitance through hundreds of Volts in one-half of Period of a 80KHz sine wave...

For Example, a sinosoidal +-1Apeak current source will drive a 5nF capacitance to +-400V. An Opamp would have to run on +-500V supplies, and be capable of delivering +-1A (at a slew rate of 170V/1us). That is gonna be one expensive Opamp.

8.jpg

You need to tell us what is the capacitance of the Piezo you are trying to drive, and what voltage is required to drive it to produce your bubbles...
 
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dr pepper

Well-Known Member
Most Helpful Member
Mike, how about driving a piezo with a 555 and a fet, and put a choke across the piezo, the choke charges and then when the fet disconnects the voltage across the choke will rise so that the current through the piezo equals what the end of the charge cycle was, therefore generating a lot of voltage.
I've tried this small scale to increase the nosie made by a piezo buzzer and it worked, might work larger scale too.
Probably a bit of jiggery pokery to get resonances matched up though.
 
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ALFAR

New Member
Hello Mike and dr pepper,

I think I am a bit confused and I also believe that I was not as clear as I should be.
I am attaching a paper which uses a device that is very similar to what I want (please kindly look at the highlighted parts on pages 1 and 2).

The are using a PZT that is used to detach bubbles from a container that has liquid inside.

Based on the paper, do you still think that I can have a PZT with a similar functioning as on the paper to get 80 kHz vibrations?

Thank you so much,
 

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dr pepper

Well-Known Member
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To be honest I dont know if the point I raised will work or not its just a thought, I was going to discuss the idea with mike and see if we can come up with a solution for you.
The technique already mentioned a high voltage amplifier is a difficult circuit to make, my idea is real simple but might not work.
 

MikeMl

Well-Known Member
Most Helpful Member
I worked on a wearable artificial kidney machine at the University of Utah Artificial Organs Lab under Dr. Willem Kolff (the inventor of kidney dialysis) in the 197os... We got our machines to the point that we could take chronic kidney dialysis patients on > 1week vacation trips to places hundreds of miles from the nearest dialysis center...

The paper you linked confirms what I am trying to tell you: That the drive voltage needs to be >100Vp-p. What is not stated, is the effective capacitance of the transducer, or how much current is required to drive it.

I found the data sheet that covers the C-82 transducer cited in the paper. It would take a mechanical engineer (beyond me) to figure capacitance or drive current requirement based on the parameters on the data sheet. It is likely easier just to measure it after you build a fixture.

I do remember playing with ultra-sonic PZT transducers to make a nebulizer. It took a surprising amount of power to drive it...
 

tomizett

Active Member
What you are describing is (as I am sure you know) what is commonly done in ultrasonic cleaning systems, although I believe they usually use somewhat lower frequencies. Such systems are now available very cheaply for domestic use.

I think the usual technique is to drive the transducer at its mechanical resonant frequency, using a conventional power amplifier stage and a transformer. I presume that the power amplifier hard-switches, producing a square wave rather than a sine, but this is only my guess.

While this would be an interesting project (it's one I indend to tackle myself one day), I'm sure there are manufacturers of ultrasonic baths who could supply you with a driver off-the-shelf, if you have the budget.
 

tomizett

Active Member
A quick search turns up results like this:
https://www.steminc.com/PZT/en/ultrasonic-generator-100w-40-khz-asic
http://www.allendale-ultrasonics.co.uk/100w-ultrasonic-transducer-drive-pcb-assembly.html

As I suspected though, these are all specified for 40kHz operation - it's quite possible that they employ a tuned circuit between amplifier and transducer to reduce harmonics and (I'm guessing) improve efficency. It may or may not be possible to modify them for your 80kHz requirement... if you could get hold of a schematic before purchase then I'm sure someone here would be able to advise you as to whether they are suitable or not. If you're lucky, the manufacturer might even be willing to help.

Are the transducers you are using broad-band or specified for a particular frequency? I know very little about ultrasonics, but it's my understanding that most transducers are designed to work at one frequency only.
 

ALFAR

New Member
Thank you Mike and tomizett,

Well, I am actually beginning to understand thanks to your useful comments.

For my project, it's the small size that matters. I myself found these piezo actuators which sound very attractive:

http://www.piceramic.com/product-detail-page/pl0xx-100800.html
http://www.noliac.com/products/actuators/plate-actuators/show/nac2013/


The first one is quite small (2*2*2 mm) and has the resonant frequency of over 600 kHz (even beyond my expectation as my ideal frequency was 120 kHz actually) and the second one is also impressive.

My idea was to attach one of these small actuators to the wall of my liquid container and by the use of a small push button I make intermittent vibrations (each for like half a second), sorry if you feel I am being inaccurate in describing my idea, to detach bubbles from my surface (the idea of detaching bubbles from a surface by vibration comes from a doctor's practice when he taps the syringe before injection to make bubbles migrate to the top of the syringe).

Do you think these actuators will be suitable for my purpose?
 

tomizett

Active Member
to detach bubbles from my surface (the idea of detaching bubbles from a surface by vibration comes from a doctor's practice when he taps the syringe before injection to make bubbles migrate to the top of the syringe).
So the purpose of the project is to de-gas the fluid, if I'm understanding correctly?

Do you think these actuators will be suitable for my purpose?
As for that... I'm afraid I've got no idea! That sounds like a physics/chemistry problem to me. There are some fairly diverse technical backgrounds on this forum though, certainly a few peoplw who are well-versed in chemistry, so we'll see what others have to offer.
 

ALFAR

New Member
Tomizett,

Actually, there is no chemistry behind the project. All that matters is to produced ultrasound vibrations to detach the bubble sticking to the surface. The biggest help I can get from this forum and professionals like Mike, dr pepper and you is to give me an idea of the components I need to make this circuit.
 

MikeMl

Well-Known Member
Most Helpful Member
The PiCeramic data sheet you linked has some data that is useful. e.g. the PL022.30 has a capacitance of 25nF, and a free resonance>600kHz. The drive voltage is restricted to -20V to +100V.

Driving it brute force would require C*ΔV = I*t, or I = C*ΔV/t.

At 80kHz, one half the period = t = 0.5*(1/80000) = 6.25us
so, the required current from the amplifier is I = 25e-9*100/6.25e-6 = 0.4A

It is possible that you could drive it at less than 100V and still knock loose the bubbles??

It is also possible that by the time you mount the transducer, that will lower its resonant frequency to where you can sweep the drive frequency to match its resonance, and then it will take much less power to drive it...

Note that PiCeramic makes power amplifiers like this one. I understand all of its ratings, except where it says: "Amplifier bandwidth, small signal = 3.5kHz". I would expect that it should have a bandwidth well into the MHz in order to drive your stack at near 80kHz???
 

ALFAR

New Member
Thank you Mike for your detailed explanations,

I am trying to digest what you mentioned in your latest reply.

Assuming that I use the PL022.30 and fix it to the wall of my liquid container by means of an adhesive, for example, it would require a function generator, both to convert DC current into AC and tune to the desired frequency and amplitude. Am I right?

I will really appreciate it if we could review the components (amplifier, capacitor, etc) required for this circuit assuming that all the components will function as we expect. Unfortunately, I am a chemical engineer and have very limited knowledge in this part and I am gradually learning. This task is just a small part of my project and I am much more knowledge in the other sections. Sorry if in this section I ask very elementary question as I just want to learn from all of you.

Thank you so much,
 

Externet

Well-Known Member
Why 80KHz or ultrasonics ? To detach bubbles on a beaker wall, is there a more convenient frequency ? What studies/calculations determine such ?
Banging on the beaker with a spoon should do it too.
Or a door bell hammering on the beaker? Or a whatever 60 Hz vibrator ?
 

ALFAR

New Member
Hi Externet,

It seemed easy to me as well. However, I am not dealing with large bubbles which will detach by just a simple tapping. I am dealing with micron size bubbles which have a great adherence to the surface. So, the frequency to detach them, should also be high.

Just to give you a better understanding, I attach this spreadsheet. As you can see the frequency that corresponds to detaching a micron size bubble is pretty high. The equation in the excel file is based on physical study on the mechanism of bubble detachment from vibrating wall.
 

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Tony Stewart

Well-Known Member
Most Helpful Member
  1. 100Vpp square wave drivers are the easy part. Look at any full bridge 100W MOSFET driver design with a varable DC-DC power control or lab supply
  2. Since the mechanical Q will be >>1000, excitation frequency accuracy will be critical to within 1/10Q or 0.01%f since Q. defines the half power bandwidth
  3. Also since double-amplitude or p-p displacement is inverse squared with rising frequency, You must choose size wisely for nano- cavitation. as rising f may also increase energy source.
  4. From what I read, use ceramic in flexural mode for highest mode of displacement.
  5. Dissipation Factor defines the tan delta of resistance/reactance impedance ratio in series so ESR and peak current will be also defined by drivng volage over Z(f) = ESR + 1/2piCf,
  6. ESR of your ceramic, siver contacts and RdsON of driver dissipates ALL your heat,
  7. then drive it with twisted pair or coax for suitable EMC , electro-magnetic compatiblity, and impedance control for both differential mode , DM transmission of max power and common mode, CM emissions (for mnimum interference)
  8. A CM choke or balun is used for raising CM impedance while keeping DM low.
  9. For HV isolation a special transformer might be used whch can then be used to transform Z and VI , only if YOU know all the values
  10. You can optimize f using a phase shift feedback oscillator, Colpitts or Hartley , whichever works for flexural mode using a Class D MOSFET amp.
  11. Using a variable 25-50V SMPS you can vary the power to the drivers and ceramic and
  12. monitor temp of contact point with an epoxy bonded SMD thermistor coated with silicone for thermal and fluid isolation.
  13. As indicated already C of ceramic will be important to know driving current
  14. also the stray capacitance of polar fluids which also have high dilectric constant (80-100) and C with contact area of electrodes and wire insulation.. We call this stray capacitance.
  15. The Japanese reearchers used an open loop approach (easy but tuning is critical method) with gen+ power amp
  16. They failed thermally with high ESR silver contact. Perhaps more area or higher concentration required to reduce ESR to well below that of ceramic ESR. Getting FET RdsON low is an easy choice but make sure bridge has a dead time of1us during commutation.
  17. You can treat ceramic like any crystal oscillator, which has so much info on web, to define equivalent circuit of LCRC passive equivalent resonator, where one of the two C's is called the motional C and there are slightly different freq for series or resonant mode. I just dont know yet which gives the most flexural mode
Once you learn these factors, you can begin to master this EXCITING :) nano-cavitator

good luck with your learning process
 
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Tony Stewart

Well-Known Member
Most Helpful Member
I have a couple rock err.. fish finders with high peak power, I suppose I could test it for cavitation. My eyes may not see it, what is best test? immerse in H2O2?
 
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