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VMU931 bias drift

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Bartosz Meglicki edited this page Jun 4, 2018 · 4 revisions

Summary

Written with VMU931 User Guide 1.3 and firmware 1.0.1.

There are two calibration procedures for VMU931:

  • bias calibration command documented in the manual (X and Y axes)
  • undocumented magnetometer calibration (essential for Z axis, "heading")

Calibration command

Command estimates the bias of accelerometers and gyroscopes. Performing this type of calibration will give you reasonable drift on X and Y axes.

To send calibration command you can use vmu931-calibrate example

./vmu931-calibrate /dev/ttyACM0

Magnetometer calibration

While powering up the sensor move it around in multiple orientations until the Euler angles indicates 0 degrees while the sensor has its Y axis pointing North. You can do it blindly (not reading data from the device).

As of firmware 1.01:

  • the result is not stored
  • it is only possible when powering the device on

So you have to repeat it every time you power on the device.

Without magnetometer calibration the device drifts more than 100 degrees per hour on Z axis ("heading")!

Experiment

  • VMU931 firmware version 1.01
  • device fixed to flat surface with 2 screws
  • surface put on the floor
  • results gathered with vmu931-estimate-bias for 60 seconds
  • device calibrated with vmu931-calibrate
  • results gathered again with vmu931-estimate-bias for 60 seconds
  • device powered off, magnetometer calibrated while powering on
  • results gathered yet again with vmu931-estimate-bias for 60 seconds
  • result csv files data plotted in R

Results

Absolute deviation in degrees after 60 seconds

condition/axis X Y Z
not calibrated -0.003906 0.020050 2.021255
calibrated 0.058228 0.120087 2.044922
magnetometer calibrated -0.024506 0.003082 -0.186813

Scaled for degrees per hour

condition/axis X Y Z
not calibrated -0.234375 1.203003 121.275330
calibrated 3.493652 7.205200 122.695312
magnetometer calibrated -1.470337 0.184937 -11.208801

z_drift

x_drift

y_drift

Methodology

Shell instructions to collect data

./vmu931-estimate-bias /dev/ttyACM0 60 > bias.csv
./vmu931-calibrate /dev/ttyACM0
./vmu931-estimate-bias /dev/ttyACM0 60 > calibration-and-bias.csv

Disable device, enable, perform magnetometer calibration

./vmu931-estimate-bias /dev/ttyACM0 60 > magnetometer-calibration-bias.csv

R code used for the plots

bias=read.csv("bias.csv", header=TRUE, sep=";", dec=".")
bias$t=(bias$t-bias$t[1])/1000

cal=read.csv("calibration-and-bias.csv", header=TRUE, sep=";", dec=".")
cal$t=(cal$t-cal$t[1])/1000

mag=read.csv("magnetometer-calibration-bias.csv", header=TRUE, sep=";", dec=".")
mag$t=(mag$t-mag$t[1])/1000

plot(bias$t, bias$z, type="l",  xlab="time [s]", ylab="angle [deg]", col="red", ylim=c(0,4), main="Z drift in time")
lines(cal$t, cal$z, col="green")
lines(mag$t, mag$z, col="blue")
legend("topleft", legend=c("before calibration", "after calibration", "magnetometer calibration"), lwd=c(1,1,1), col=c("red", "green", "blue"))

plot(bias$t, bias$x, type="l",  xlab="time [s]", ylab="angle [deg]", col="red", ylim=c(0,1.5), main="X drift in time")
lines(cal$t, cal$x, col="green")
lines(mag$t, mag$x, col="blue")
legend("left", legend=c("before calibration", "after calibration", "magnetometer calibration"), lwd=c(1,1,1), col=c("red", "green", "blue"))

plot(bias$t, bias$y, type="l",  xlab="time [s]", ylab="angle [deg]", col="red", ylim=c(0,2), main="Y drift in time")
lines(cal$t, cal$y, col="green")
lines(mag$t, mag$y, col="blue")
legend("left", legend=c("before calibration", "after calibration", "magnetometer calibration"), lwd=c(1,1,1), col=c("red", "green", "blue"))
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