Taming an Inclinometer

A project we are working on requires a MEMs inclinometer to assure coplanarity between a flat surface and a sensor.

The Problem

The sensor has a high gain section before an analog-to-digital converter feeds data to the microprocessor. Unless the sensor is located in a Faraday cage, the 120 hz hum generated causes severe instability in the digital data. It was determined that the center-line of this signal is the stable signal output.

At the extreme high and low outputs, the AC signal is compressed, but the center-line signal is still correct.

The code takes 20 asynchronous X and Y axis readings, finds the highest and and lowest, determines their difference, and divides by two to get the current X and Y inclination.

This code snippet just covers the search for highest and lowest value. We have left off the hardware code as it is different for each platform.

    tiltDataXhiAccum = tiltDataX[0];
    tiltDataYhiAccum = tiltDataY[0];
    tiltDataXloAccum = tiltDataX[0];
    tiltDataYloAccum = tiltDataY[0];

  //this one finds the highest value
   for ( j = 1; j < 20; j++ )
  {
       if ( tiltDataXhiAccum < tiltDataX[j] )
     {
            tiltDataXhiAccum = tiltDataX[j];
     }
       if ( tiltDataYhiAccum < tiltDataY[j] )
     {
            tiltDataYhiAccum = tiltDataY[j];
     }
  }

    //this one finds the lowest value
   for ( j = 1; j < 20; j++ )
  {
       if ( tiltDataXloAccum > tiltDataX[j] )
     {
            tiltDataXloAccum = tiltDataX[j];
     }
       if ( tiltDataYloAccum > tiltDataY[j] )
     {
            tiltDataYloAccum = tiltDataY[j];
     }
  }

  tiltDataXAccum = ( tiltDataXhiAccum - tiltDataXloAccum ) / 2;
  tiltDataYAccum = ( tiltDataYhiAccum - tiltDataYloAccum ) / 2;

This solution allowed us to go from a realtime bobble rate of over 5 per second with differences of 10 or more, to a stable reading that might change every few seconds with a worst case difference of 1.