Environment; Power; Error Correction; Centrifugal Force Compensation - WATSON INDUSTRIES AHRS-E304 Owner's Manual

Symmetry is very important in such mount designs (consult Watson Industries for application
notes on this if needed).

Environment:

High level AC magnetic fields, such as from large transformers, motors, or soldering guns, are to
be avoided as being potentially damaging to the circuitry, even if the system is not powered.
Exposure to high DC magnetic fields are to be avoided since this can produce a lingering self-
magnetization of the sensor, which can cause distortion of the heading reference.

Power:

This unit has an internal regulator to allow operation over a wide voltage input range. Best
operation is obtained at either 12 or 24 VDC level, although operation is fully satisfactory down
to 10 VDC and up to 30 VDC. Power draw of the unit is about 4 Watts. Internal capacitors are
provided to remove a reasonable level of power line noise, however, capacitors should be added
for long power line wiring or if noise is induced from other loads on the circuit.

ERROR CORRECTION

The correction systems, which all aircraft attitude systems use, unfortunately also allow errors
from the forces on a maneuvering aircraft to enter the system. To control these effects, a balance
of time constant and error limits are used:
Limiting Factor Causing Error Reduction:
Axis of Bank
Error Limit Angle Error
Bank ±10º
Elevation None
Heading None
It would not be a problem to cross over into these limits since the rate of deterioration from the
original reference should be about 0.03 degrees per second. However, prolonged strong
maneuvers should be avoided if accuracy is to be minimized. In normal attitudes, error is
corrected with a 30-second time constant.
While this AHRS is "all attitude", the accuracy deteriorates rapidly within about 5 degrees of
straight up or straight down.

Centrifugal Force Compensation:

The compensation for centrifugal force is based on calculating the horizontal turn rate and
multiplying it by the forward velocity. The result is subtracted from the vertical reference
accelerometers for the roll axis. This system directly depends on the quality of the velocity
signal.

Delta Velocity Compensation:

The compensation for forward acceleration is based on calculating the changes in average
forward velocity. The result is subtracted from the vertical reference accelerometers for the pitch
axis. This is limited in unusual attitudes and the error correction system is locked out when the
AHRS-E304 Watson Industries, Inc
Elevation Heading
Angle Error Angle Error
None None
±10º None
None ±20º
4
Centrifugal Acceleration
Force Offset Force Offset
±5 g None
None ±5 g
None None
09/06/05