Hi everyone,
First and foremost, thank you for any help/advice you can provide. I am a mechanical engineering student and my knowledge of circuitry is very basic and it has been a good 4-5 years since I last had a course in electrical engineering, so please bear with me.
As the title suggests, I am trying to signal condition a load cell that I have (+/- 3,000 lbs http://www.futek.com/product.aspx?stock=FSH00672&acc2=acc) but after working on it for the past week, I have gotten nowhere. I am afraid I might accidentally damage the load cell (if I haven't already done so). I found a few other posts on this forum about this particular subject but I was not able to solve my problem using that advice. Here is the website I used as a place to start http://www.nakka-rocketry.net/strainlc.html. I've also attached my modification of the circuit presented on that website here: http://imageshack.us/photo/my-images/85/circuitd.jpg/. Basically I modified it to +/- 10V instead of the 5V the site uses. Also, all GNDs are connected.
The way I chose 423 Ohms to be my amplifier's gain is by using the load cells theoretical output at max load (3k lbs) which by the calibration certificate's mV/V output chart comes out to 20.918 mV at 10V excitation. I wanted this to be at 10V, so 10/20.918e-3 = 478. Rg = 200,000/(478-5) = 423 Ohms. Now, I'm pretty sure this is wrong since I didn't read the tare voltage first but still, this isn't where I'm having my issues as far as I can tell. Next, I connected all of the grounds (which is where I think maybe I messed up because the voltage regulator chips are getting burning hot) and at first the voltage read about 10.3V DC but after screwing around a bit I got it to level out at about 0.3###V. Now theoretically (not sure if this is the right calculation) I should get 3,000/2^12 ~ 0.75 lbs resolution since I have a 12 bit ADC I'm reading the voltage with. However, even if i put a load of 40+ lbs on it, the reading doesn't change whatsoever besides the oscillations it goes through naturally with or without load (same #s regardless of applied load).
So based on the above, can anyone tell where I'm going wrong? Also, is my resolution calculation correct? I think my DAQ can read 0-10V but I noticed it does read negative voltages too down to -10V. So would my resolution span the 20V gap or just the 10V? i.e. 6,000/2^12?
Thanks for your time and help guys!
First and foremost, thank you for any help/advice you can provide. I am a mechanical engineering student and my knowledge of circuitry is very basic and it has been a good 4-5 years since I last had a course in electrical engineering, so please bear with me.
As the title suggests, I am trying to signal condition a load cell that I have (+/- 3,000 lbs http://www.futek.com/product.aspx?stock=FSH00672&acc2=acc) but after working on it for the past week, I have gotten nowhere. I am afraid I might accidentally damage the load cell (if I haven't already done so). I found a few other posts on this forum about this particular subject but I was not able to solve my problem using that advice. Here is the website I used as a place to start http://www.nakka-rocketry.net/strainlc.html. I've also attached my modification of the circuit presented on that website here: http://imageshack.us/photo/my-images/85/circuitd.jpg/. Basically I modified it to +/- 10V instead of the 5V the site uses. Also, all GNDs are connected.
The way I chose 423 Ohms to be my amplifier's gain is by using the load cells theoretical output at max load (3k lbs) which by the calibration certificate's mV/V output chart comes out to 20.918 mV at 10V excitation. I wanted this to be at 10V, so 10/20.918e-3 = 478. Rg = 200,000/(478-5) = 423 Ohms. Now, I'm pretty sure this is wrong since I didn't read the tare voltage first but still, this isn't where I'm having my issues as far as I can tell. Next, I connected all of the grounds (which is where I think maybe I messed up because the voltage regulator chips are getting burning hot) and at first the voltage read about 10.3V DC but after screwing around a bit I got it to level out at about 0.3###V. Now theoretically (not sure if this is the right calculation) I should get 3,000/2^12 ~ 0.75 lbs resolution since I have a 12 bit ADC I'm reading the voltage with. However, even if i put a load of 40+ lbs on it, the reading doesn't change whatsoever besides the oscillations it goes through naturally with or without load (same #s regardless of applied load).
So based on the above, can anyone tell where I'm going wrong? Also, is my resolution calculation correct? I think my DAQ can read 0-10V but I noticed it does read negative voltages too down to -10V. So would my resolution span the 20V gap or just the 10V? i.e. 6,000/2^12?
Thanks for your time and help guys!