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Multiple (3) Analog Inputs (DAQ) Ai3 is copying Ai1 Why?

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Hi All,

 

I have been a LabVIEW user for about 4-5 years, but this is my first post on the forums.  I wrote some code that will collect three inputs, an acceleration (from an accelereometer) measured in volts, a Force (from a force meter) also measured in volts and a voltage single from a piezoelectric thin film.   For simplicities sake you can just think about them as three voltage inputs well under the 10V maximum.  Each voltages is fed into Ai1, Ai2 and Ai3 into my DAQmx measurement board.

 

The purpose of this code is to study how a 3d printed vibration energy harvesting array of piezoelectric coated beams performs under different excitation conditions. (If we shake these beams harder or softer at different frequencies, how much energy can we colelct from our array)

 

However, I have noticed that my piezoelectric measurement is often completely wrong.  The data from the acceleration measurement is for some reason coppied over and displayed in my graphs instead of the correct data from the piezoelectric sensor!  I don't understand why it happens.  If I short circuit Ai1 and Ai2, then Ai3 (which has the piezoelectric sensor) will work just fine and give me the correct measurement.  But when I try to measure all three simultaneously - it always gives me the wrong result.

 

I was wondering if someone could take a look at my VI to see if I am possibly using any of the data collection code incorrectly.  Or if there is another method to perform the same task that could isolate Ai1, 2 and 3 in a more clear and well defined way. (but they still need to all get collected at the same time!)

 

Thanks in advance for any recommendations that you have.  My code is attached. 

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Accepted by topic author EngineerEvan

I have not looked at your VI because the problem is almost certainly in hardware. You probably have "ghosting." Your piezo sensor connected to Ai3 likely has a large source impedance while the other sources have much lower impedances.  Search the Forums for ghosting for more details.

 

The best solution is to provide signal conditioning (basically an amplifier with a low output impedance) between your piezo device and the DAQ input.

 

Another possibility, suggested by your observation about shorting Ai1 and Ai2, is to add one or two unused channels between the low impedance inputs and the piezo input. Short those inputs (in the same way you did Ai1 and Ai2). Read all the channels. Discard the data for the shorted channels. This is the way it would look:

Ai1 accelerometer, Ai2 Force, Ai3 short, Ai4, short, Ai5 piezo. Read channels Ai1:5. Discard the data for Ai3 and Ai4.  While this may work for prototyping purposes, it may not be a good long term fix.

 

Lynn

 

 

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Lynn,

 

Thanks, I really appreciate your advice.  Your idea of shorting Ai3 and Ai4 and collecting data in Ai5 worked great, this was certainly a hardware problem related to ghosting.  Both the force sensor and accelereometer had a very low output impedance and the piezo has a very high output impedance.  Since I have 30 of these sensors to test on the array, I may even be able to measure more of them simultaneously by adding shorted analog inputs between the measurements.  

 

Based on the goal of this particular project I can't insert an operational amplifier in before the piezo.  One of our goals is to maximize the quantity of voltage and power we can harvest from the array.  An operational amplifier consumes more power than would be gained by including it into the circuitry.  We need to be able to test the system under a variety of loading conditions to determine how quickly we can charge up a variety of capacitors and batteries.

 

Thanks,

 

Evan

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You can source the buffer OP from any available power source, including the DAQ device 😉

And depending on your piezo device and it,s circuit you can have a control of the bias current by choosing type and selecting..... 😉

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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I think the point Henrik is making, and I agree, is that the op amps would be considered part of the measurement system, not part of the energy harvesting system.  You are not powering your DAQ device or the computer from the piezo devices.  Apply the same logic to the amplifiers.

 

I just read "Autonomous Multisensor System Powered by a Solar Thermoelectric Enrgy Harvester With Ultralow-Power Management Circuit" by P. C. Dias, et. al., in IEEE Transactions on Instrumentation and Measurement, November 2015.  The system they described starts charging the storage capacitor at input as low as 20 mV and 5.5 uA.

 

Lynn

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