Help to drive 2 axes from wheatstone bridge?

Discussion in 'Electronics & Electrics' started by v81, Dec 15, 2020.

  1. v81

    v81 Member

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    SE Vic
    Helping a mate build a fancy joystick, has a DACO force sensor in it.
    I believe it comprises a whetstone bridge to derive force.

    Looking to turn this into 2 x analogue (0-3.3v) to feed an STM32 running Freejoy.
    SPI or I2C are not much use as the existing Freejoy firmware onyl supports a few basic things like shift registers etc, and adding code is beyond the scope.

    Ideally looking for a very very compact device, i believe an instrumentation amplifier might be what we're after.
    Just beginning the trip down the rabbit hole.

    Creating a PCB and soldering are both things im more than comfortable with.
    Given limited room in the hand grip of the joystick we're looking for a QFN or similar part.

    Attached is a datasheet, we have the 4 wire version of the device.

    Any help appreciated.

    TLDR;
    Want convert Bridge to 2 x analogue axes with ~ 0-3.3v range.
    Logic is 3.3v
     

    Attached Files:

  2. Technics

    Technics Member

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    An Inamp usually is the way to go for a wheatstone but it doesn't appear to be one. A wheatstone would have the 2 excitation inputs and two outputs for a single quantity. This has one output for each quantity (X axis and Y axis position) making it two half bridges. So you really have a two single ended outputs.

    Edit: You can complete the two bridges with external resistors and that would probably be a good idea as you can add some adjustment for nulling the offset.
     
    Last edited: Dec 15, 2020
  3. mtma

    mtma Member

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    I would create a 1/2 excitation reference voltage (same as 'completing the bridge' but note unlike a typical wheatstone sensor setup there's no dynamic compensation being applied) and then use that with a diff-amp circuit to gain up the signal centred on 1/2 of the ADC voltage (where the ADC measurement reference equals the excitation reference). If you don't gain up the ADC will probably be a limiting factor to the overall resolution.

    You may need to buffer the ADC reference to produce the excitation if it's not already so (if it's hard wired to 3.3V it's probably sufficient).

    You could provide a trim on the hardware but given the specifications of the stick I would say just trim digitally if that's available.

    If you're putting the opamp/inamp circuit together it might also be prudent to implement some basic filtering and if the stick is remote to the converter some protection on both excitation and inputs.
     
  4. OP
    OP
    v81

    v81 Member

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    Sorry for the delay, just wanted to say thanks guys.
    Turns out the sensor i got was faulty hence the minute output.
    But thanks for the time and effort to help.
     

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