Moog Crossbow VG700MB Series - Manual (Page 25)

VG700M User’s Manual
Doc.# 7430-0280-01 Rev. F Page 19
4 DMU Operating Tips
4.1 The “Zero” Command
The “z” command is used to zero the angular rate sensor biases. This
command does not “zero” the angle output! This should be an essential part
of your strategy in using the DMU effectively. Stabilized pitch and roll
angles are calculated by integrating the output of the angular rate sensors.
Rate sensors are subject to small offsets in the angular rate measurement. A
constant offset error in angular rate will integrate into an error in angle that
increases linearly with time resulting in angular drift. The VG700M uses
accelerometers to correct the calculated angle, but in a dynamic situation,
the accelerometers will be an inaccurate indication of the angle due to
motional accelerations. The DMU rate sensors should therefore be zeroed
to maintain the best accuracy.
Zeroing the rate sensors allows you to use a smaller value for the erection
rate (T-Setting), which gives you better performance in dynamic
environments.
The rate sensors need to be zeroed more often when subject to large shocks
or extremes of temperature.
The DMU unit should be still (motionless) during the zeroing process, but
need not be level. You should let the DMU warm-up for 5 minutes before
issuing the zero command. Zeroing the DMU turns on the high-pass filter
rate bias estimation that takes approximately 3 minutes.
The zeroing command does not level the stabilized angle output.
; NOTE
Remember that the VG700M stores the rate sensor zero calibration in non-
volatile memory. If you cycle power to the DMU, the zero calibration will
not be lost. However, if is set to 0, then the previously stored gyro
biases are erased.
The Gyro zeroing validity bit defaults to the high condition upon power up
or any restart of the system. If the previous zeroing performed was
successful, the user may choose to ignore the gyro zeroing validity bit.
If you find that the DMU zeroing algorithm does not work well in your
particular application, please contact Crossbow to discuss possible options.
4.2 The Erection Rate
The erection rate parameter controls the weighting between the rate gyro
sensors and the accelerometers. This is the rate at which the direction of
vertical as measured by integrating the rate gyros is forced to agree with the
direction of vertical as measured by the accelerometers. The erection rate is
specified in degrees per minute. The erection rate must be higher than the