Digital scale + multiple strain gauges help

Discussion in 'The Projects Forum' started by JBernard, Jun 24, 2014.

  1. JBernard

    Thread Starter Member

    Aug 8, 2013
    46
    0
    I've been working on a project lately and running into some hiccups on the latest revision. Basically we are trying to reverse engineer a digital scale. And have made 1 prototype using a load cell (beam type) with a 4 wire interface. This was easy to hook up into an INA125P instrument amplifier and works great.
    However, they are very bulky, so we are looking at using a much thinner style 3 wire strain gauge sensor like this for our next version:
    https://www.sparkfun.com/products/10245

    Very simple in design, and easy to use by itself, but we are trying to use 4 of them (1 for each foot) like in this scale system:
    http://www.amazon.com/American-Weigh-ONYX-5K-BK-11-Pound-0-1-Ounce/dp/B001S0ZJUM

    Thats where the hangup on how it's supposed to be wired is. As much info as I have gathered, the 4 need to be wired up to make a complete wheatstone bridge. By tying some of the 4 wires together, then taking + and - readings off the others.
    The best thread I've found so far regarding wiring 4 of the sensors is this one, but still unclear on how it's done as it seems some of the sensors have to be wired to give opposite readings as the others so they don't cancel each other out? (that was a horrible explanation!)

    http://www.nerdkits.com/forum/thread/900/

    here is the details of the sensors i have on hand:
    resistance readings give me 1K ohm between blk/wht
    500 ohm between red/wht
    500 ohm between blk/red

    and here's a link to my ins amp datasheet:
    http://www.ti.com/lit/ds/symlink/ina125.pdf

    here's a current diagram of how i have things wired up. no idea if this is the direction i should be heading or not:

    [​IMG]


    on the + side i get a decent swing on ouput of the amp, .06V resting to decent press of .5V on each of the + sensors.
    on the - side i get .06V resting, pressing hard on each goes down to .048V


    I'm very frustrated at this point, as i know the device is simple, and we already have a working prototype with a similar device, just the lack of documentation on using multiple of these gauges is becoming tiresome. You would think there would be tons of info out there and tutorials. As digital scales, even with 4 sensors, are found everywhere.

    Any help is greatly appreciated. My electronics knowledge is definitely mid pack beginner so if I could find a hookup diagram that would be awesome.

    Thanks!
    Taylor
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    First the disclaimer - I don't have any hands-on experience with this and can only speculate, based on an understanding of things I've read.

    My understanding is that each cell is an independent bridge, and each produces a tiny voltage proportional to the weight applied. Your challenge is to add the 4 voltages together in a way that minimizes noise, corrects for zero, and is scaled for slope. My approach would be to use a separate INA125 for each, and perform the calculations in software. Effectively this is 4 scales. For the price of the extra op-amps, I think you would gain a lot of control.

    If there's some clever way to add the small bridge voltages as inputs to a single INA125, someone may jump in here and tell us. In the meanwhile you might look at basic op-amp adder circuits to accomplish this. I'm just not sure that's a good idea with such small signals. Although 0.5V is plenty, so maybe this is a fine approach.
     
    Last edited: Jun 25, 2014
  3. JBernard

    Thread Starter Member

    Aug 8, 2013
    46
    0
    thanks for the reply.
    reason trying to use 1 amp is that i'm running out of digital inputs on my device (Electric Imp, its an Arduino flavor). But if need be this may be the only option.
    I also thought about summing amps, but i guess i'd be in the same situation hook up wise still correct or would they all feed repeatedly into a summing amp?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    Take a look here, and here. Correct me if I'm wrong, but your sensors output a positive voltage proportional to weight, right? Adding those voltages together should give you the total weight applied to all 4 sensors.
     
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