Load cell project, wheatstone bridge, EM noise, signal ampl.

[pdehairs]

Hi everyone,

I'm new to this forum and have never really been on a forum yet. I'm a student in aeronautical engineering and I've been asking myself some questions concerning the electronics implementation for an interesting project I'm working on.

I'll give a small introduction first:
The goal is to design a force transducer or load cell which will measure the couple (0-60Ncm) and traction(0-30N) of a propeller mounted brushless DC motor (840W max continuous, 15VDC).

To measure the loads I'm using 2 DC excited (Vexcitation=5VDC) fully active wheatstone bridges (operating with strain gauges) with an estimated output range of 1mV/Vex & 0.42mV/Vex for respectively the torque & thrust sensing wheatstone bridge. The respective precision required is 2.5µV/Vex & 1.4µV/Vex (this corresponds to 401& 301 values resp.)

A negative bridge output is possible, and the ADC has a range of 0-5V (10bit resolution=>5mV resolution) it thus can't measure negative voltages. Moreover, I specify that the maximum resolution I'll need on the ADC is 10mV, therefore I'll need a voltage step of 1V & 2V resp. after the amplifier (negative voltages bridge outputs will thus yield in measurements smaller than 1V & 2V resp.). The maximum output signal thus needs to be amplified to 4V& 3V resp. before the step. This corresponds resp. to an amplification factor of Av=800& Av=1450.
In short: The analog signal of interest will thus lie in the range of 1-5V & 2-5V resp. and must be measured with a 10mV precision.

Question:
-The electronics are operating near the brushless DC motor (electronically commutated motor), there will thus be EM noise that will be picked up in the electronics and the sensing circuit. However all the usefull information is DC, if I a use a low pass filter to filter out the noise and further filter with software, should this be enough or do I need physical isolation like a faraday cage maybe and coaxed cables for powering the motor? And should I put the low pass filter before or after the amplifier?
-What are other specifications I should look at for operational amplifiers except for Av range, CMRR, offset with t° and time?
-What's the simplest way to implement a voltage step in the circuit?

I think this is basic stuff for you guys, but I'm not used to working with electronics! The more I learn about it the more I'm interested !! Which is nice because I used to find it not so much of interest o_O. I was way more passionate about mechanics and aeronautics. But to play around and experiment at home, electronics are WAY less expensive, it's amazing .

Thanks in advance for your time!!

[pebe]

Hi and welcome to the forum.
I have read through your posting several times and I cannot fully understand para 3.
Is ‘Vex’ the excitation voltage, 5v?
If so 1mV/Vex = 200µV. Or did you mean 1mV per volt of excitation giving 5mV output? I don’t understand where you are getting the ‘401& 301 values’ from.

It would be better if you called the two measuring nodes of the bridge ‘A’ and ‘B’, and stated the maximum voltage and polarity swing to be measured at B relative to A, for each gauge – and with what accuracy. We would then know what gain each amplifier needs to be to give an appropriate output to the ADC.

If both bridge and ADC are powered by the same source, can we assume that three arms of the bridge have fixed values of resistance so one node is at 2.5V, and in order to minimize noise that node will be grounded? Or is the ADC grounded in which case both nodes are above ground voltage?

(Edited 13/04 to correctly define A and B).