Moog Crossbow VG700MB Series - Manual (Page 28)

VG700M User’s Manual
Page 22 Doc.# 7430-0280-01 Rev. F
4.5 Advanced Strategies for Adjusting the Erection Rate
The DMU attitude estimation algorithm is divided into two separate
entities. Gyro angular rate information is integrated in time to propagate
the DMU body attitude with respect to the tangent plane. If the initial
attitude of the vehicle was known exactly and if the gyros provided perfect
readings then this integration process would suffice. However, the initial
state is seldom known to great precision, especially a vehicle's attitude, and
the gyros usually provide corrupted data. Rate gyro bias, bias drift,
misalignment, acceleration (G-sensitive), nonlinear (square term), and scale
factor errors will be present in the angular rate measurements. The largest
error is typically associated with the bias and bias drift terms. Without a
correction algorithm and separate independent sensors, the attitude
estimation algorithm would diverge off the true trajectory. Accelerometers
provide the separate measurements, which help keep the attitude estimates
on track.
The correction algorithm involves deriving an estimate of the roll and pitch
angle from the accelerometer’s gravity reference, comparing this estimate
to the gyro propagated quaternion Euler angles, and providing a linear
feedback gain to the quaternion propagation to take out the errors observed
from the gyro angular rate measurements. The correction feedback is also
referred to as the “erection rate” implying that the attitude errors are erected
out by moving the estimated orientation more towards the absolute attitude
measurements derived from the accelerometer measurements. It is also
given the name T-Setting to describe the user interface which allows the
user to command the DMU to use a desired erection rate. Sensed dynamic
accelerations can introduce error into the accelerometer absolute attitude
reference. The angle calculation algorithm has no way of knowing whether
the sensed acceleration change is being caused by an attitude tilt change in
the gravity vector, or from external translational accelerations.
For this reason a user selectable erection rate is available which allows for
the possibility of a rapidly maneuvering mission. There is a tradeoff
between how much error in the gyros the algorithm can overcome with a
low erection setting, compared to the errors induced from having a high
erection rate while experiencing large maneuvering accelerations. The gyro
zeroing command is useful in maintaining the gyro bias errors down to a
minimum, which allows a lower T-Setting to be used during the mission. It
must be noted that for the zeroing command to work properly, there must
be no external disturbance to the unit (engine noise, wind disturbance, etc.)
If the user has knowledge of the intensity of upcoming maneuvers or
complete control of the flight profile, and can maintain constant serial
communications with the DMU unit, then an adapted erection setting
profile can be developed.