Author Topic: Piezo disc impedance matching  (Read 2266 times)

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Offline dimbmwTopic starter

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Piezo disc impedance matching
« on: June 14, 2020, 09:36:43 pm »
Hello
I am building a piezo transducer and have some questions on matching.
I am using radial resonance of the pzt-5a disc.
There are resonance and antiresonance frequencies.
I use the same piezo for transmit and receive. I use burst excitation. It is a range measuring application.
Max response frequency is somewhere between resonance and antiresonance frequencies.
Are there any hints on what is the best way to do impedance matching and if it is useful at all if i am not planning on using long cables.
I tried to do the matching by using series inductor and parallel capacitor (calculated via smith charts) but wasn’t impressed with improvement of pulse echo response. Also it is hard to match without using adjustable inductors and capacitors as this thing is pretty sensitive to particular values... it will be nearly impossible to use standard L and C values in production.
Does anyone have similar experience and some hints?
Thanks!
 

Online coppercone2

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Re: Piezo disc impedance matching
« Reply #1 on: June 14, 2020, 11:21:05 pm »
you tend to use impedance analyzers for piezos devices, I don't think the VNA works well at their load impedance

$$$

Have you considered using tunable inductors and capacitors to at least determine what the optimum effect is? What do your values required look like? Not many people will want to get a impedance analyzer. I don't think its very useful or standard.


« Last Edit: June 14, 2020, 11:31:59 pm by coppercone2 »
 

Offline dimbmwTopic starter

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Re: Piezo disc impedance matching
« Reply #2 on: June 15, 2020, 12:54:48 am »
Hello
I am building a piezo transducer and have some questions on matching.
I am using radial resonance of the pzt-5a disc.
There are resonance and antiresonance frequencies.
I use the same piezo for transmit and receive. I use burst excitation. It is a range measuring application.
Max response frequency is somewhere between resonance and antiresonance frequencies.
Are there any hints on what is the best way to do impedance matching and if it is useful at all if i am not planning on using long cables.
I tried to do the matching by using series inductor and parallel capacitor (calculated via smith charts) but wasn’t impressed with improvement of pulse echo response. Also it is hard to match without using adjustable inductors and capacitors as this thing is pretty sensitive to particular values... it will be nearly impossible to use standard L and C values in production.
Does anyone have similar experience and some hints?
Thanks!

I have somewhat similar experiece driving electro acoustic modulators and piezo fiber stretchers (fiber optic device used in interferometors) in those cases I was driving them with a fast sub nano-second pulse or slewing a very high voltage into a very capacitive load. In both cases the impedance match was really poor since The manufacturer of these devices had such poor control of the impedance over frequency, they often didn’t spec it or provided a typical, non-guaranteed performance number just for reference.

Understandably, in your case, matching is a lot more difficult. Perhaps the best way to match is by selecting the frequecies of interest and do your best with L’s an C’s to get the most stable operating point and match to that.  How are you measuring the piezo device, experimentally or via the data sheet  ( assuming it exists)

Also, do you have a VNA or impedance analyzer that could help measure and characterize the device?
And could you explain how you drive the device now and the issues you expect to have without a proper or optimized match

Bob


Bob,
I am using impedance analyzer in Analog Discovery 2, it is good enough for my frequencies.
For driving, i use siglent sdg1025 generator, it has burst mode i need.
There is no datasheet for the piezo transducer, as I am the one who tries to design it, so the datasheet should follow, lol :)
Problem is - matching does not seem to bring any significant improvement in perfomance. I am puzzled with that actually.

 

Offline dimbmwTopic starter

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Re: Piezo disc impedance matching
« Reply #3 on: June 15, 2020, 01:01:43 am »
you tend to use impedance analyzers for piezos devices, I don't think the VNA works well at their load impedance

$$$

Have you considered using tunable inductors and capacitors to at least determine what the optimum effect is? What do your values required look like? Not many people will want to get a impedance analyzer. I don't think its very useful or standard.





yea, i use impedance analyzer in analog discovery, and then throw Rs and Xs values  (they are like Rs=500ish ohm, and Xs =-750ish ohm) , throw them in SimSmith software where I get values of L and C for matching network. For my particular case I am getting about 300 uH series inductor and about 3nF parallel capacitor... but even slight changes do change the tuneup dramatically..... so not a great setup overall.  i bought some variable Ls and Cs on digikey, but it isn’t practical for production anyways, hence my post here.... maybe I need something completely different like matching transformer? feel like brain stuck :)
« Last Edit: June 15, 2020, 01:10:31 am by dimbmw »
 

Online coppercone2

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Re: Piezo disc impedance matching
« Reply #4 on: June 15, 2020, 01:19:27 am »
well you use the variable parts to see what matching actually does, then if you like it you figure out how to match it cheaply (i.e. parallel/series components)

How are you measuring performance of the device during match tuning? I suspect your IA will have a difficult time (not sure how well it can mimic a real Keysight IA).

You should be able to produce a graph of output power vs tuning some how

I do see values of nF for capacitor, but I don't know about series L. I only worked with one piezo device and I never finished the project.

Not sure if you want to say tuning/adjustment is bad for production, its best avoided.. but it might be unavoidable. Plenty of good profitable things get adjusted. You need to test products before sale typically, especially if they do a process (you can't just plug it into a card, you need to make sure the transducer works and is in spec), so having the test technician trim something is not outlandish.

I assume you will have to test a glue bond to the device being stimulated anyway. Don't pull a harbor freight and sell a ultra sonic cleaner which has the horn fall off and melt the chassis and try to light tables on fire..............
« Last Edit: June 15, 2020, 01:24:09 am by coppercone2 »
 

Offline dmendesf

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Re: Piezo disc impedance matching
« Reply #5 on: June 15, 2020, 03:38:41 am »
First let's remember what impedance matching brings to us: when a source is matched to a receiver then you have the biggest transfer of POWER possible (and it's 50%). Question is: you don't want the power. You want the signal. So you need to maximize the signal to noise ratio, not the power coupled.
 

Offline dimbmwTopic starter

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Re: Piezo disc impedance matching
« Reply #6 on: June 15, 2020, 06:03:29 am »
First let's remember what impedance matching brings to us: when a source is matched to a receiver then you have the biggest transfer of POWER possible (and it's 50%). Question is: you don't want the power. You want the signal. So you need to maximize the signal to noise ratio, not the power coupled.

I need maximum response (transmit->reflection from target->receive). I guess you can say that it's better to have max power.

On a side note, I don't think it has to be 50%.  I can have the output impedance lower than 50 ohm, let's say 10 ohm, and can have better %.
« Last Edit: June 15, 2020, 06:07:13 am by dimbmw »
 

Offline dmendesf

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Re: Piezo disc impedance matching
« Reply #7 on: June 15, 2020, 01:15:24 pm »
Power is V*I. If you put a voltage buffer at the sensor output then as long as you have enough current to drive the resistors and Op Amp input then you want to maximize the voltage, not the product V*I, so perfect power matching isn't what you need.
 

Offline dmendesf

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Re: Piezo disc impedance matching
« Reply #8 on: June 15, 2020, 01:18:23 pm »
Ok I think I got it: I'm talking about the receiver, you're talking about the transmitter. For transmission you want good matching, for receiving it's not important.
 

Offline dimbmwTopic starter

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Re: Piezo disc impedance matching
« Reply #9 on: June 15, 2020, 08:20:27 pm »
Ok I think I got it: I'm talking about the receiver, you're talking about the transmitter. For transmission you want good matching, for receiving it's not important.

In my case the same transducer is transmitting and receiving, it is a pulse - echo setup.
 


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