Moog Crossbow VG700MB Series - Manual (Page 26)

VG700M User’s Manual
Page 20 Doc.# 7430-0280-01 Rev. F
drift rate of the rate gyros, or the calculated angles will drift off with
increasing error. If the erection rate is too high, however, the calculated
angles will be forced to follow the accelerometers too closely. This will
lead to inaccuracies when the unit is under dynamic conditions.
One way to start is to set the erection rate about twice as fast as the worst
rate gyro drift rate. This is appropriate for a dynamic environment, when
the unit will be under significant acceleration. Estimate the drift rate by
looking at the offset on the rate gyro output. Use the zero command first to
zero the rate gyros. The rate gyro output is in degrees per second; the
erection rate is set in degrees per minute. So take the rate gyro offset;
multiply by 60 to turn it into degrees per minute; multiply by two and use
this as a starting value for the erection rate. As an example, if the rate
sensor offset is 0.1 degrees per second, we would set the erection rate to 0.1
x 60 x 2 = 12. The stabilized pitch and roll output will be responsive to
actual rotations, and relatively insensitive to linear accelerations.
You can set the T-Setting in a qualitative way using GyroView. Graph the
pitch and roll. Start with the T-Setting at about 100. Lower the T-Setting in
increments of 10 – 20 until the roll and/or pitch starts to drift. When the
angle outputs start to drift, the T-Setting is just a bit lower than the rate
sensor offset. Increase the T-Setting by about 5. This should keep the
angle outputs stable.
If you expect the DMU to be subject to changing temperatures, or to have to
operate for long periods without re-zeroing, you should increase the T-
Setting further. You may have to experiment some to find the best erection
rate for your situation.
If the DMU is used in a less dynamic environment, the erection rate can be
set much higher. The DMU angles will stabilize quicker to the gravity
vector. So if the motion is slow, or if you sit in one position for a long time,
then you should probably use a high erection rate.
A more advanced approach to the erection rate would take advantage of
both regimes of operation. Use a low erection rate when the unit is subject
to dynamic motion; use a high erection rate when the unit is relatively
stable. You can use the DMU itself to distinguish between the two cases by
looking for changes in the accelerometer outputs. For example, in an
airplane, you could use a low erection rate when the airplane executes a
banked turn; and a high erection rate (100+) when the plane is flying
straight and level.
Unfortunately, there is no single ideal erection rate for all applications. We
can suggest a starting point based on past experience with similar
applications, but you should be prepared to experiment some in the
beginning to find the best setup for your DMU in your application.