SPI Sensors
The ASALT vehicle uses the Serial Peripheral Interface (SPI) bus to
communicate with two sensing devices. These devices allow the vehicle to acquire
information about its tilt and heading. For tilt sensing, the vehicle uses an Analog
Devices ADIS16300 Inertial Measurement Unit (IMU). To obtain heading, a PNI V2Xe
2-axis digital compass is used.
IMU
The ADIS16300 contains a 3-axis accelerometer as well as a gyroscope which
allows the device to accurately calculate pitch and roll readings. Pitch is described as the
rotation about the y-axis of the vehicle, and roll is described as rotation about the x-axis.
The vehicle’s code receives pitch and roll readings from the IMU and converts them to
degrees. These readings are used for diagnostic data that will be returned via status
updates to the UI as well as for calculating a tilt-compensated heading.
Figure X: Pitch and Roll as it relates to the ASALT vehicle
We chose the ADIS16300 since it was relatively low cost, and it was capable of
calculating pitch and roll internally using its own X,Y,Z-axis acceleration values as well
as gyroscope readings. This allows us to quickly query the IMU for these values as they
are needed during the navigation routine.
Digital Compass
We chose a V2Xe to serve as our digital compass since it is low cost and small in
size. Typically 3-axis compasses take up a large amount of space. Since the V2XE is a
2-axis sensor, we are able to obtain heading information accurately while conserving
space and cost.
The digital compass contains two PNI SmartSens magnetometers oriented
perpendicular from each other to sense X and Y magnetic field readings from the
surrounding environments. Using these readings and an onboard microcontroller, the
V2Xe is capable of computing a heading with a resolution of 0.01 degrees when the
device is flat. The V2Xe implements hard and soft iron corrections to the raw X and Y
readings. This means that it can be calibrated in an environment that may have external
magnetic forces. For the ASALT vehicle, the magnetic fields generated from the chassis
and motors were significant enough to cause the un-calibrated compass to give false
readings. As a result, we included functionality to calibrate the compass in the vehicle’s
command mode. By calibrating the compass in its host environment, we were able to
obtain accurate heading readings while the compass was exposed to these external
magnetic forces. In the future, the iNRG researcher may choose to relocate the compass
position, and as a result of the calibration functionality, they will be able to successfully
re-calibrate the compass to obtain accurate readings.
Tilt Compensation
Like with traditional compasses, digital compasses only provide accurate heading
information when it is oriented horizontal with the Earth’s surface. When the compass is
tilted, the magnetic readings from the X and Y axis sensors provide readings relative to
the tilted reference frame. This causes significant error in heading readings. To deal
with this issue, we implemented a tilt compensation algorithm described by a paper
written by Samsung engineers Seong Yun Cho and Chan Gook Park. Their paper was
targeted at mobile applications where a compass unit could be oriented to any direction,
but pitch and roll are available. Their equations generate X and Y readings that have
been corrected to the horizontal reference frame. The equations are as follows:
a  Xmag *cos   Ymag * sin  * sin   Zmag * sin  * cos 
b  Ymag * cos  Zmag * sin 
b
heading  arctan( )
a
Where  and  denote pitch and roll, respectively. These values are obtained directly
from the IMU and applied to the sensor magnitudes obtained by the digital compass. The
V2Xe only provides Xmag and Ymag , fortunately Cho and Park provide the following
equation to approximate Zmag :
sin   Xmag * sin   Ymag * cos  * sin 
Zmag 
cos  * cos 
Where  denotes the dip angle at the present location. Using a calculator provided by
the National Geophysical Data Center (NGDC), we were able to determine the dip angle
for Santa Cruz, CA is about 60.65 degrees.
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
As a result of the compensation algorithm, the heading is improved, although some error
still exists. The following graph shows the error of the uncompensated heading and
compensated heading for the same sample. For this test, we pointed the compass to 278
degrees, then tilted it to ±20 degrees of roll.
Compensated and Uncompensated Compass Readings
60
Heading Error (degrees)
50
40
30
20
Compensated
10
Uncompensated
0
-10
1
51
101
151
201
251
-20
-30
Data Samples
The results show that the tilt compensation algorithm reduces the significant error
observed with the uncompensated heading. The correction is not perfect, however, it still
demonstrates an average error of about 10 degrees, even when compensated. This error
is substantial, but satisfactory for our application because the GPS module provides an
accurate heading that is independent of vehicle tilt. The compass will still be used when
the vehicle is stationary because the GPS heading can only be used when the vehicle is in
motion.