Moog Crossbow VG700MB Series - Manual (Page 14)

VG700M User’s Manual
Page 8 Doc.# 7430-0280-01 Rev. F
To convert the acceleration data into G’s, use the following conversion:
accel = data*(GR * 1.5)/(2
15
-1)
where accel is the actual measured acceleration in G’s, data is the digital
data sent by the DMU, and GR is the G Range for your DMU. (The data is
scaled so that1 G = 9.80 m s
-2
.) The G range of your DMU is the range of
accelerations your DMU will measure. For example, if your DMU uses a
±4 G accelerometer, then the G range is 4.
To convert the angular rate data into degrees per second, use the following
conversion:
rate = data*(AR*1.5)/(2
15
-1)
where rate is the actual measured angular rate in
°/sec, data is the digital
data sent by the DMU, and AR is the Angular rate Range of your DMU.
The angular rate range of your DMU is the range of angular rates your
DMU will measure. For example, if your DMU uses a ±200
°/s rate sensor,
then AR range is 200.
3.4.3 Angle Mode
In angle mode, the DMU will act as a vertical gyro, and output the
stabilized pitch and roll angles along with the angular rate and acceleration
information. The angular rate and acceleration values are calculated as
described in the scaled sensor mode. It will also provide the relative
heading calculation output.
In angle mode, the DMU uses the angular rate sensors to integrate over
your rotational motion and find the actual pitch and roll angles. The DMU
uses the accelerometers to correct for the drift in the rate sensors. This is the
modern equivalent of an analog vertical gyro that used a plumb bob in a
feedback loop to keep the gyro axis stabilized to vertical. The DMU takes
advantage of the rate gyros’ sensitivity to quick motions to maintain an
accurate orientation when accelerations would otherwise throw off the
accelerometers' measurement of the DMU orientation relative to gravity.
The DMU uses the accelerometers to provide long term stability to keep the
rate gyro drift in check.
The DMU gives you control over the weighting between the accelerometers
and rate gyros through a parameter called the “erection rate.” This term is
derived from analog vertical gyros, and refers to the rate at which the
system can pull the gyro spin axis back to vertical as measured by gravity.
With a small erection rate, you are depending more on the rate gyros than
the accelerometers; with a large erection rate, you are forcing the rate gyros
to follow the accelerometer measurement of vertical more closely. In
general, for dynamic measurements, you will want a low erection rate. But