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Load cell project: wheatstone bridge signal ampl., arduino ADC, EM NOISE from brushless DC motor

Discussion in 'Sensors and Actuators' started by pdehairs, Apr 12, 2014.

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  1. pdehairs

    pdehairs

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    Apr 12, 2014
    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 :D !! 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 :eek:.

    Thanks in advance for your time!!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,900
    2,097
    Nov 17, 2011
    Welcome to our forum.

    A direct answer to your question: yes, a low pass filter is a way of getting rid of high frequency noise Whether it will work for you depends mainly on two things:

    • The lowest frequency of the noise from the motor.
    • The highest frequency of your measurement signal (I know you stated DC, but the signal will change, albeit very sowly, otherwise you wouldn't have to sense it).
    As long as the signal frequency is a good measure below the noise frequency spectrum, a low pass filter can remove the noise. Depending on the difference between highest signal frequency and lowest noise frequency you may need a filter of a higher order. Let's assume the noise is well above the signal spectrum. I'd suggest putting two filters in place: one before the amplifier, one after the amplifier. The first will reduce noise entering the amplifier, the second will remove residual amplified noise. You can kill two birds with one stone if you build the amplifier from active filter stages. The amplifier filters and amplifies at the same time.

    As for the OpAmp specs: most important is low offset (and offset drift with time and temperature) as well as low input bias current. Here is just one selection page from TI, other manufacturers like Maxim, Linear Technology, Analog Devices etc. offer comparable products. You may consider using a chopper stabilized amplifier, although in that I case I recommend not to use it for an active filter because the chopper operation may disturb the filter characteristic. It is a good idea to place a low pass filter after a chopper stabilized amplifier anyway to remove residual noise from the chopper's operation within the amplifier.

    The "voltage step" you mention is called offset. Here we don't mean the unwanted offset which is a parameter of the amplifier, we mean a well specified offset, 1V or 2V in your application. ou can create such an offset using a difference amplifier. With Vout = Rf/R1*(V2-V1) (see article linked), setting V2=1V and V1=measured signal you achieve the effect you envision.

    Regards,
    Harald
     
  3. pdehairs

    pdehairs

    2
    0
    Apr 12, 2014
    Thank you Harald for all the information! I'm sure I can start working with what you gave me. I'll let you guys know how the project is advancing!

    Pieter
     
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