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Interpret Voltage-time graphs to relate the ouput voltage of a piezoelectric crystal

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kvnstv1
#1
Oct2-11, 06:17 PM
P: 5
Hi,
I am doing my Extended Essay in Physics(so yeah, I am doing the IB). I am really interested in taking up electrical engineering at university, and so, I had done an EE in piezoelectricity. I have taken the crystal from a barbecue lighter and connected it to a vernier( a machine that can read potential differences and transfer it to a computer).

So, to investigate the effects of input force on output voltage, I have dropped masses onto a stick pressing onto the crystal using a pulley from known heights( so that I know the value of the impulse) and then recorded the voltages generated as a function of time.

I am just very worried about whether I am interpreting my data properly. I am planning to calculate the average of the highest peaks in each graph(but only in the positive, as I feel that the stick pressing down on the crystal may have affected the voltage in the negative region. Then, I intend to compare these values. Is this correct? Does anyone have any suggestions? Am I doing anything anything scientifically wrong? Also I have noticed that at the beginning of each graph, there is one period of the function(which is not as high as the next one, which is almost always the highest), which I am finding hard to explain. Any ideas here?

Thanks in advance for helping me.
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dlgoff
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Oct2-11, 08:28 PM
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Quote Quote by kvnstv1 View Post
...to investigate the effects of input force on output voltage, I have dropped masses onto a stick pressing onto the crystal using a pulley from known heights( so that I know the value of the impulse) and then recorded the voltages generated as a function of time.
I'm wondering why you are using an impulse instead of just putting a known mass on the crystal and measuring the steady state output.



You could use some weights of know masses as a load and then measure the potential produced. Since the crystals impedance is very high, you would need a good high impedance meter to read its output voltage (so the meter doesn't load the circuit down). The equivalent circuit looks like:



A piezoelectric transducer has very high DC output impedance and can be modeled as a proportional voltage source and filter network.
Piezoelectric sensor

Also the converse is true.



...an applied mechanical stress will generate a voltage and an applied voltage will change the shape of the solid...
piezo materials

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kvnstv1
#3
Oct3-11, 04:00 AM
P: 5
Hi dlgoff,
Thanks a lot for the help. I did not think of that. I think I will modify the apparatus I am using so that I can load a mass on it and then measure a steady voltage. Would I be right in assuming that I would get a straight line for the voltage-time graph then?
Because I find this slightly confusing, are you saying that placing a mass on a piezoelectric crystal will cause the crystal to produce a voltage indefinitely? I thought it was the deflection of the shape of the crystal that produced the voltage and the voltage would stop being produced once the deflection stopped.
I will be using a voltage divider to step down the voltage to a level that can be measured.
Thanks so much for helping me.

NascentOxygen
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Oct3-11, 08:36 AM
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Interpret Voltage-time graphs to relate the ouput voltage of a piezoelectric crystal

Quote Quote by dlgoff View Post
I'm wondering why you are using an impulse instead of just putting a known mass on the crystal and measuring the steady state output.
Energy out, but no energy in? Free energy!
jim hardy
#5
Oct3-11, 09:09 AM
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do not be surprised if the crystal does not produce constant voltage, instead gives you only a brief deflection on your voltmeter (or measuring apparatus)

what level of voltage are you measuring with your impulse test? Barbecue striker must make kilovolts to give that spark.

Might it be interesting to measure the charge produced when the crystal is pushed?

By discharging the piezo through a diode into a large capacitor you could capture the charge (i'm guessing it's microcoulombs) and measure the resulting voltage on the capacitor.
Q = C X V

start with ten microfarads, that'll give a long time constant when read with a 10 megohm voltmeter so you have time to get a reading.
Each microcoulomb will give 1/10 volt.
I'd use a plastic film style, maybe the polypropylene type popular for speaker crossovers.
Experiment with capacitor size and find one that gives repeatable results for a known impulse on your drop test. You want to produce tens of volts on the cap so that the loss across diode is small in comparison.

Sounds like fun - i hope you share your results with us.

old jim
dlgoff
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Oct3-11, 01:14 PM
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Quote Quote by dlgoff View Post
I'm wondering why you are using an impulse instead of just putting a known mass on the crystal and measuring the steady state output.
Quote Quote by NascentOxygen View Post
Energy out, but no energy in? Free energy!
kvnstv1
#7
Oct3-11, 05:27 PM
P: 5
Hi,
Thank you all for your help. Jim, I was thinking of trying to find the charge on the piezoelectric crystal too! I am searching our lab for a capacitor that can carry the required charge now. I will definitely let you know the results of my experiment.
Thanks again
NascentOxygen
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Oct3-11, 11:53 PM
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I'd just like to thank digoff for posting this neat animation he found somewhere. It takes time to search out these things, and include them, and it does make reading the forums so much more interesting. I'm sure the OP's appreciate the extra effort some people go to (even though there seems to be a dearth of thank you posts, nay, replies even, to many responses.)

Yes, I couldn't resist having a dig at digoff for not fully understanding what it was himself, with my "Free Energy!" quip. You can't have a crystal indefinitely delivering DC power .....



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