Sensors, Electronics and Prototyping

Units Confusion

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  • #866
    Jason Greenberg

    I am confused as to what the units are for the processed accelerometer data. On pages 65 & 66 of the UM7 datasheet it says under the DREG_ACCEL_PROC_(X,Y,Z) registers that “Contains the actual measured acceleration in m/s/s after calibration has been applied.” However on in the paragraph right above the Section 3. Estimating Velocity and Position, it says “Note that CH Robotics sensors typically output acceleration in units of gravities, not m/s/s or ft/s/s.”

    I am more inclined to believe the website (the processed acceleration is in gravities) because when I plot the processed accelerometer data when the IMU is still, through CHR Serial Interface or MATLAB (after extracting from microprocessor) I get x and y are zero and z is close to 1. The exact numbers are actually x, y and z are -0.01, -0.016 and -0.96 respectively. Nevertheless based on this I am inclined to believe the acceleration is in gravities and not m/s/s.

    However I am having issues with distance accuracies when running my algorithms with this data and so I am starting to think that the IMU is actually reporting m/s/s but off by a factor of 10.

    A clarification of what units are actually being reported by the UM7 for the processed acceleration data would be appreciated.




    Hi Jason,

    The processed accelerometer data on the UM7 is reported in gravities. You should expect to see some slight errors based on accelerometer bias and scale factor error on the UM7, since it isn’t calibrated.

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