Bob,
Thank you for your response!
I really appreciate your help.
Sorry for the slow response, I have been trying to digest this as much as possible - as well as trying to figure out exactly what I don't understand and doing more online research...
I am rather green when it comes to load cells, amplifiers and how to set them up. However I am very much willing to learn. If you would please bear with me, as I may have some silly questions.
Yes it will work, but it will be less sensitive than a load cell with a lower maximum range.
This load cell has a sensitivity of 2 mv / volt of excitation. You would typically apply a 10 volt excitation voltage and if you did, you would have a 20 mv signal change when you put a 2000 pound weight on the load cell. This would make the response equal to 2000 pounds / 20 mv = 100 pounds per mv.
Bob
I think I understand for the most part up to this point and maybe a little further.
Basically the excitation voltage is a reference voltage and determines your full range.
The excitaion voltage and "sensitivity" are both from the load cell datasheet:
https://www.intertechnology.com/Revere_Transducers/pdfs/SSB.pdf
Let me see if I really get this...
At 0 lbf load the response in mv from the load cell = 0 mv
At 2000 lbf (Full Capacity of load cell) the response in mv from the load cell = 20 mv [2mv/1V = Xmv/10V, solve for X = 20 mv]
Correct?
This number is low so you really need a load cell amp to increase the signal from the load cell to match the input of the digitizer your are using to record the data.
Bob
Correct me if I am wrong, but this statement means that to be able to accurately measure 50 lbs, we would some how need to be able to measure 0.5 mv. And to be able to measure 1 pound, I'd need to be able to decipher 0.01 mv? To be able to obtain the submilivolt signals and read them we need some sort of amplifier, which you go into in your next section...
If you have a 0-10 volt or a -5 to +5 volt digitizer, the load cell amp ideally needs a gain of 10 volts /0.020 volts = 500 to match the full scale range of the load cell to the full scale range of the digitizer. If you had a 12-bit digitizer, the sensitivity of your measurement is 2000 pound / (4096-1) bit steps = 0.488 pounds / bit step. That's ~ 400 steps for a 200 pound max thrust curve.
You need the load cell amplifier to provide gain to amplify the load cell signal to a reasonable set size so you have good resolution in your thrust curve. For example if you set the gain to 5000, the resolution is 4000 bit steps with matches the resolution of the ADC. The problem can be that you need to adjust the output offset or you will go off scale which is the reason for the load cell amplifier.
Bob
When you say "digitizer", are you referring to some sort of analog to digital converter (ADC) Like a DATAQ unit? Or is it part of the amplifier circuit?
Could you please explain the concept of a gain and how you came up with the value of 500?
Let me see if I understand the overarching principles though...
With the gain of 500, we have a "resolution" of ~0.5 lbs over the full range of the load cell up to 2000 lbs. However, if we are only testing motors up to 200 lbs, our data would very coarse.
Using a gain of 5000, we can obtain a "resolution" of ~0.05 lbs, but only up to about 200 pounds. If our motor produces 250 lbs of thrust, that data will be truncated?
Either way, if a load cell with a capacity closer to the intended max thrust of the data we want to capture is selected, a much smaller gain is required and the better the data is, if I am starting to wrap my head around this correctly?
A load cell amplifier supplies the bias voltage for the load cell, amplifies the signal from a submillivolt signal level to a volt signal level, puts in a voltage offset to allow you to adjust the zero signal level voltage to match the minimum value of the digitizer, and a low pass filter to eliminate high frequency noise. A 400 Hz low pass filter is optimum of a test stand.
Most load cells have a full scale sensitivity between 0.5 to 5 mv per volt of excitation, and can have an excitation voltage between 5 to 10 volts.
A good load cell amplifier will supply excitation voltage of 5 or 10 volts, have continuous gain adjustment form 100 to 5000, have -10 to +10 volt offset capability, and may also have a 1 to 15 scaling amplifier with a 400 hz low pass filter. The best use a TI IN125 chip as the load cell amp, and if it is built to the recommended circuit, the gain, excitation voltage, offset, scaling and frequency are independently adjustable. Sadly many load cell amps are not designed this way and are a pain to calibrate.
Bob
Is the low pass filter applied real time as the data is being captured or can it be applied after the data has been gathered?
Please let me know if I can clarify any of my statements. Also, please let me know if I am understanding the basic principles so far. Like I said this is all relatively new to me. Do you know of any other resources you could point me to to help with my understanding as well?
I hope I didn't complicate matters with all of the quotes...
Thanks so much for your help.
Dave