Calibrating The Compass; Compass Background - Teledyne WORKHORSE Operation Manual

May 2015

Calibrating the Compass

Compass calibration corrects for distortions in the earth's magnetic fields caused by permanent magnets
or ferromagnetic materials near the H-ADCP. These magnetic field distortions, if left uncorrected, will
create errors in the heading data from the H-ADCP. A compass calibration should be conducted at each
measurement location, and whenever the mounting fixture or ancillary equipment are changed or rear-
ranged. You should be aware of the following items:
•
Ferromagnetic structures such as platform hull, bridges, or sheet piling in the measurement loca-
tion may interfere with proper compass operation. The compass calibration procedure can
NOT correct for heading errors due to these types of structures. If the H-ADCP is
mounted next to a large ferrous material such as the hull of a platform, external heading data
MUST be input to the system software running on a customer-supplied computer, from a custom-
er-supplied heading device. Most commercially-available heading reference instruments will pro-
vide a digital (serial) NMEA $HDT or $HDG output suitable for use with H-ADCP software.
•
A good compass calibration requires slow, smooth movement to allow the compass to collect data
at each point.
•
Calibrate the compass as close to the location that it will be deployed and as far away as possible
from objects that have magnetic fields that could result in a poor calibration. Common objects
to avoid calibrating the compass near include steel reinforced concrete, buildings,
and automobiles.
•
Completing the calibration rotation(s) does not guarantee an acceptable compass
error. Compass error is based not only on the quantity of measurements made during the cali-
bration but also the quality of the magnetic environment. Attempting to calibrate the compass in
a poor environment, e.g., near fixed ferrous objects, will likely result in an unacceptable compass
error regardless of how well the calibration is performed.
•
The Single-tilt calibration is intended for applications where tilting the unit is not practical. This
calibration is only applicable to the tilt orientation the unit is rotated about during the calibration.

Compass Background

The compass calibration algorithm collects magnetic field vector information for various measured head-
ings during the calibration. Hard and soft iron effects rotating with the compass are made observable dur-
ing the calibration by causing the local field to be perturbed as the compass is spun during the calibration.
That is, each component of the hard and soft iron has to alternately increase the local field for some orien-
tations and decrease for orientations 180 degrees (or 90 degrees for soft iron) from those orientations for
the algorithm to "notice" it. Tilting and rotating the compass about the vertical axis is sufficient to do this.
There are three compass calibrations to choose from; one only corrects for hard iron while the second cor-
rects for both hard and soft iron characteristics for materials rotating with the H-ADCP. The third method
provides calibration for a
and cause single cycle errors while soft iron effects are related to magnetic permeability that distorts the
earth's magnetic field and causes double cycle errors. In general, the hard iron calibration is recommend-
ed because the effect of hard iron dominates soft iron. If a large double cycle error exists, then use the
combined hard and soft iron calibration.
Page 60
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
single tilt orientation. Hard iron effects are related to residual magnetic fields
WorkHorse H-ADCP Operation Manual