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Weighing Scale - Auto Zero option ON FLY (not during power on)

Jayaraj A
Prodigy 200 points
Other Parts Discussed in Thread: ADS1230, ADS1232, STRIKE

Dear Sir,

           I had used ADS1232 & ADS1230 for my weighing scale application and I am able to see good results from those. One of my problem is load cell drift. My load cell gives the proper zero output and calibrated output, for 1 or 2 days. After that, I am finding a small drift of +/-4-5 counts. My application does not have a power on zero (tare). It consists of fixed tare (stored in eeprom). So my zero as well as span drifts, totally.

          I don't want to experiment in my Hardware further, for these drifts. So one thing strike me is auto zero. Subtract or add the drift counts of load cell ON FLY to the raw ADC counts, based on the zero(tare) value stored in eeprom. 

           So how to implemt this auto zero logic in FIRMWARE, on weighing scale, on fly? Eg., My load cell zero has drifted to +/- 6 counts, which I won't know, drifted due to load cell or small weight residing on the load cell?

          Any good firmware logic, to avoid / rectify this drift?

 

With regards,

Jayaraj.A

  • 0
    TI__Guru** 113765 points

    Jayaraj,

    Do you know for sure what is causing the drift and what type of drift it is?  How have you determined that the load cell itself is drifting?  It is possible you are seeing some offset drift of the ADC.  It is always wise to run a periodic offset calibration to remove offset due to drift.  Are you running the offset calibration?

    Basically the auto zero feature you are inquiring about is a change of the TARE value.  The problem with doing this is knowing when the load cell is unloaded to make the adjustment.  The calculation is just an adjustment to the TARE value so that there is either an add or a subtract of a value from the previous TARE that brings your result back to zero.  If you are not careful you can be constantly adjusting the value due to noise.

    I had a similar conversation a while back where the poster added some example code for auto zero.  You may find this post helpful:

    http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/119662/427852.aspx#427852

    Best regards,

    Bob B