Flexiforce sensors LabView Program

[Hatef.fouladi]

as I told you before it is better to ask just experts to involve in
also see this links about why it is better use 24 bit  daq for strain gauge and even load cell sensors  for  industrial works not playing game with based on ADC full range problem

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[Dennis_Knutson]

No one is arguing that 24 bit measurements will be more accurate. The point was the comment was off topic since the op does not have a 24 bit device and the question was how to use what was available.

[mecheng16]

First off, thanks everyone for your help.

Second off everything is working fine, thanks to labview honestly.

The people who were originally trying to build this Flexiforce sensor circuit and program had it wrong all along. They didn't know what they were doing with the OpAmp and were putting the drive voltage into the ground and vice versa; thus blowing up every OpAmp they tried. 

So I built a very primative labview program (5V drive output, -1V excitation output, an analog input, and a waveform chart to view the return voltage). Using this program I could immediately see that the OpAmp wasnt working, as I could see our return voltage was in the mV range not V range. 

Swapped out the OpAmp and consulted its circuit diagram, and what a suprise it all worked. 

So basically thank you LabView for being so simple that I didn't have to mud up the problem using an Arduino.

[hi7]

Hi mecheng16, 

I am currently doing the samething (Using flexiforce sensors to get force measurement). What did your circuit diagram look like with our the OpAmp?

Thanks, 

[mecheng16]

@hi7

Without the opamp? I don't know if that's feasible as signal amplification is required for using these sensors. We just used the simple circuit diagram that Flexiforce provides on their website.

The only modifications we made were a 100uF capacitor bridging the drive voltage and ground of the opamp chip. And if you have a chip with more than one opamp in it its a good idea to loop the rest of the unused opamps to keep the chip happy. Although this shouldn't be required.

[hi7]

Thanks for responding @mecheng16, 

Sorry I was not clear. I was not sure how to use the opamp, but I will add the capacitor as you suggested. I am using NI PXIe-4330/4331 and use force bridge setup on DAQmx. But before I can use that, it is my understanding that I have to get the voltage readings at different loads so that I can use these values to configure my scale in DAQmx.  Please advise if there is a better way to do the scaling?

Did you use the circuit you posted above? or the circuit they have on the quick start board? 

Thanks for your time!!

[mecheng16]

@hi7

You pretty much have the right idea. We built our own signal conditioning circuit on a breadboard using the diagram I posted above. Opamps are very simple it's a little black caterpillar looking chip usually with 8 or more pins. Just look up the manufacturer diagram for your opamp and it should be a lot clearer. I think I have a picture of what my setup looked like so I'll try to find that for you.

That is also exactly how we calibrated out sensors. However, make sure when you calibrate the sensors that you're using calibration loads that are in the ballpark of what you will be measuring (don't calibrate with 1, 2, and 3 lbs then try to measure 500lbs), make sure that you're sensors are about at the temperature that you will be measuring them with (the sensors are a bit temperature sensitive), and finally whatever metal or weight you put on top of the sensor that is applying the weight has to be about the same size as the sensor. So we used 1" diam sensors so we sanded a quarter perfectly flat. If you used something to large it will spread the force out too wide and the sensor will only be reading pet of the weight.

Finally it is important to remember that these sensors are actually measuring pressure, not force. So after you get all your measurements you'll need to do some simple calculations using the are of the sensor and some simple physics stuff to be able to convert psi to lbf or Newtons or whatever you like.

Hope that helped.

[hi7]

@mecheng16

First thanks so much for your help. I really apprieciate it. I added the capcitor and set up the circuit. I am using LMN741 opamp(8chips). Instead of my sensor, I first attempted using a known resistor but I see the voltage reading was not accurate. How exactly do I connect the batteries to the opamp? And also if you happen to find your set up, I would greatly appreciate it. I am really new to this and any help would be great. 

Thanks again,