animalTrack (version 1.0.0)

calibrate.axes: Calibrate two accelerometer/magnetometer axes. Axes are scaled and centered.

Description

Hard iron distortion compass calibration. Hard iron error arises from magnetized iron or steel on the compass platform that creates a constant offset for the compass axes. This offset is additive and constant.

Usage

calibrate.axes(x, y, scale = 1, xcal = NULL, ycal = NULL)

Arguments

x

a numeric vector of axis values. This should be calibration rotations (i.e. yaw, pitch, roll)

y

a numeric vector of axis values. This should be calibration rotations (i.e. yaw, pitch, roll)

scale

the maximum value x and y will have after being scaled (i.e. values will range from -scale to +scale. Defaults to 1.

xcal

a numeric vector of axis values (corresponding to x). This should be track data that should be scaled according to the scale factor, offset, and scale variable. Optional.

ycal

a numeric vector of axis values (corresponding to y). This should be track data that should be scaled according to the scale factor, offset, and scale variable. Optional.

Value

xsf

scale factor for x.

ysf

scale factor for y.

xoff

offset value for x.

yoff

offset value for y.

xh

calibrated x value. Scale factor and offset applied.

yh

calibrated y value. Scale factor and offset applied.

xhscaled

calibrated x value. Scale factor and offset applied, and scaled using scale.

yhscaled

calibrated y value. Scale factor and offset applied, and scaled using scale.

xcal

calibrated independent xcal value. Scale factor and offset applied to xcal, and scaled using scale. Optional. Will be NULL if xcal is not provided.

ycal

calibrated independent ycal value. Scale factor and offset applied to ycal, and scaled using scale. Optional. Will be NULL if ycal is not provided.

Warning

If any dynamic acceleration is present during the calibration procedure or track, it will result in an erroneous calibration. This can be avoided by keeping the sensor still and by pausing during rotations. Once the calibration constants are known for the instrument, they can be applied to the raw signal from the deployment on the animal. The raw accelerometer signal will include both static and dynamic acceleration, so the accelerometer signal must be filtered using a low-pass filter to extract the "static" portion of the signal.

Details

A common source of compass error is hard and/or soft iron disturbance. Hard iron error arises from magnetized iron or steel on the compass platform that creates a constant offset for the compass axes. To account for the additive shift, an offset value is determined for each axis and applied by adding or subtracting the offset value from x or y. Soft iron error results from the interaction of soft iron near the compass and the earth's magnetic field. Soft iron error is a much more complicated distortion to account for and is not handled in this function.

References

Caruso, M. J. (2000), Applications of magnetic sensors for low cost compass systems. Proc. Position Location and Navigation Symp., IEEE 2000, Mar. 2000, pp. 177-184

Grygorenko, V. (2011), Sensing - magnetic compass with tilt compensation. Cypress Perform, AN2272, Document No. 001-32379 Rev. B.

See Also

calibrate.axis

Examples

## Import the missionbay_calib2 dataset. There is a calibration sequence 
## at Mission Bay, CA. Use help(missionbay_calib2) for info.

data(missionbay_calib2)

# Uncalibrated raw values
mx <- missionbay_calib2$mx_raw
my <- missionbay_calib2$my_raw

plot(mx,col='blue',type='l',lwd=3)
lines(my,col='green',lwd=3)

## Scale and center the x and y magnetometer axes against one another. Also, make 
## sure that each axis is in the NED (north-east-down) orientation.
cal <- calibrate.axes(mx, my)
attributes(cal)

plot(cal$xhscaled,col='blue',type='l',lwd=3)
lines(cal$yhscaled,col='green',lwd=3)