Multiplexing an array of strain gauges?

Discussion in 'The Projects Forum' started by Olaf, Jul 30, 2008.

  1. Olaf

    Thread Starter New Member

    Jul 30, 2008

    I'm currently working on creating an array of strain gauges / gages. Currently I have a single strain gauge sensor working away happily as in the first picture attached (apologies for the terrible drawings..). Due to the small signal of the sensor, amplification is required before passing the signal to a DAQ card connected to a PC

    The amplifier portion of this is a strain guage amp circuit from RS set to a gain of 1000 and the output goes to a DAQ card (NI USB-6008) connected to my PC.

    From this initial design, I wish to expand this into an array of (initially 2x2) strain gauges. The quickest way to do this would be to have a separate amp for each sensor but as I'm trying to keep things small (and cheap!) this isn't feasible.

    My next thought is therefore to somehow multiplex the Bridge Supply from the amp circuit to each different strain gauge in the array.

    Not being amazingly confident in my theory here, I would like to know if this would be possible? and what kind of hardware would be recommended?

    As you can see in the second pic, I'm thinking there'll be some timing issues involved; I need to somehow tell the DAQ card / Labview which signal belongs to which sensor but am unsure how to go about this / if it's even possible. the DAQ card has multiple digital and analogue inputs which I can utilise.

    Any ideas / thoughts / criticisms would be greatly appreciated!

    (Also, this is for a university project, I hope this is the correct forum for this.)

  2. SgtWookie


    Jul 17, 2007
    Multiplexing your strain gauge array is going to be rather difficult.

    Most strain gauges only output a small fraction of their excitation voltage, usually expressed as mV per 1V. Trying to switch supply, return, + and - outputs amongst multiple strain gauges is going to be quite a chore.

    Even just leaving the supply and return connected to all the strain gauges and switching the + and - returns will be problematic. You'd need FETs or MOSFETs with extremely low Rds(on), otherwise it'll interfere with your readings. You'd also get wild "blips" when switching between strain gauges; the instrumentation amp's output will slam against the rails.

    The FETs or MOSFETs would also have to have matched Rds(ON) to keep from throwing the readings off.
    Last edited: Jul 30, 2008
  3. kender

    Senior Member

    Jan 17, 2007

    How quickly do you need to switch between your strain gauges? If the rate doesn't need to be very high, I would suggest multiplexing them with reed relays. I've done something like that many times with thermocouples, where I was able to multiplex sub-mV signals.

    - Nick