Watson Industries, Inc
AHRS-S305 Rev J 03/22/2018
4
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 35 VDC. Power draw of the unit is about 3 Watts. The AHRS power
system is isolated from the AHRS signal system. 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.
Calibration
The AHRS is calibrated at the factory before it is shipped to the user. It is recommended that
the unit be returned to the factory annually for evaluation and recalibration.
Error Correction
The correction systems, which all attitude systems use, unfortunately also allow errors from the
forces from vehicle maneuvers 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
Error Limit
Bank
Angle Error
Elevation
Angle Error
Heading
Angle Error
Centrifugal
Force Offset
Acceleration
Force Offset
Bank
±10º
None
None
±5 g
None
Elevation
None
±10º
None
None
±5 g
Heading
None
None
±20º
None
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
