April
2004, Issue 165
Mini
Rover 7
Electronic Compassing fo Mobile Robotics
by
Joseph Miller
EFFECTS
OF TILT
Tilting
an electronic compass can create heading errors. When
tilted, the sensors no longer receive the magnetic field
in the proportions measured during calibration. Equations
1 and 2 characterize the sensor output value with respect
to the magnetic field’s angle to the sensor’s orientation
in the horizontal plane. Tilting the sensor would expose
the sensor to portions of the Earth’s vertical field
component and reduce its exposure to the horizontal
field component, which could either be an increase or
decrease in field density depending on local inclination
angles and direction of tilt.
Equation
6 gives the relationship of heading error versus tilt
angle when a compass is tilted in the north-south rotational
axis (also known as pitch):
[6]
where
qERR is the heading error caused by tilt, a is the pitch
angle compass, and j is the inclination angle of the
Earth’s magnetic field.
To
use an example, if your compass were located in San
Francisco, which has a magnetic field inclination angle
of 61°, you could expect a heading error of up to 1.8°
for every degree of pitch for the first 10° of tilt.
Tilt in the east-west direction (roll) or any compound
angle of the two tilt axes creates similar errors, although
the worst-case errors could still be characterized by
Equation 6 by simply substituting the pitch angle, a,
with a tilt angle in any direction. Another accuracy-degrading
factor would be the lack of hard iron and soft iron
distortion correction in the pitch and roll axes. The
two most common ways to make a compass insensitive to
tilt is to mount a two-axis compass on a gimbal and
to use a three-axis, tilt-compensated compass.
COMPASS
FEATURES
A
good compass should have high dynamic range sensors.
The need to compensate large hard iron offsets is not
uncommon, and can account for 75% to 90% of the sensor’s
operating range. A good compass will have an operating
range that is at least four times the Earth’s magnetic
field density.
A
compass also should be able to resolve the Earth’s horizontal
magnetic field component to 1:115 for 1° resolution
and 1:1146 for 0.1°. These numbers don’t account for
soft iron distortions that effect sensor gains and hard
iron offsets, which would increase these requirements.
A
good compass should be temperature-compensated. Magnetic
sensors have temperature dependencies like most sensors.
Fortunately, the sensor’s common temperature effects
drop out of the heading calculations because the arctangent
function’s input value is a ratio of the sensor pair
(y/x). Sensor nonlinear temperature effects, external
hard iron offsets, and soft iron distortions do not
share this temperature cancellation characteristic.
Of course, a compass that has hard iron and soft iron
compensation is essential. The necessity of tilt compensation
depends on the robot’s requirements and your budget.
V2Xe
COMPASS MODULE
I
used the V2Xe compass in the Mini Rover 7 robot. This
is a 1² square electronic compass module that uses an
SPI interface as a means of communication. It consumes
less than 3 mW of power, and has an output resolution
of 0.01° with a heading accuracy of 2°. Its field measurement
range is about 20 times that of the Earth’s field, which
means that it can operate with extremely large hard
iron offsets that are common in robotic applications.
The
V2Xe compass can be calibrated using one of two methods.
Distortion-compensation coefficients along with declination
settings are stored in nonvolatile memory. The V2Xe
can provide raw sensor data and compensated field magnitude.
It has an adjustable digital low-pass filter for heading.
A
continuous calibration is the simplest calibration method
to perform on the V2Xe. Send the calibration start command
to the V2Xe to begin the calibration process, rotate
the robot in one or two complete circles, and then send
a calibration stop command to end the calibration process.
After completing the calibration, you can retrieve the
heading data from the V2Xe as necessary.
