Moog Crossbow GNAV540 - Manual (Page 27)

GNAV540 User Manual
7430‐0808‐01 Rev. B Page 27
VG Function adaptively tunes the EKF feedback in order to best balance the bias estimation and attitude correction
with distortion free performance during dynamics when the object is accelerating either linearly (speed changes) or
centripetally (false gravity forces from turns). Because centripetal and other dynamic accelerations are often
s filtered y associated with yaw rate, the unit maintains a low‐pas aw rate signal and compares it to the turnSwitch
threshold field (user adjustable).
When the platform the unit is attached to exceeds the turnSwitch threshold yaw rate, the unit lowers the feedback
gains from the accelerometers to allow the attitude estimate to coast through the dynamic situation with primary
reliance on angular rate sensors. This situation is indicated by the softwareStatus→turnSwitch status flag. Using the
turn switch maintains better attitude accuracy during short‐term dynamic situations, but care must be taken to
ensure that the duty cycle of the turn switch generally stays below 10% during the vehicle mission. A high turn
switch duty cycle does not allow the system to apply enough rate sensor bias correction and could allow the attitude
estimate to become unstable.
The VG Function algorithm has two major phases of operation. The first phase of operation is the initialization
phase. During the initialization phase, the unit is expected to be stationary or quasi‐static so the EKF weights the
accelerometer gravity reference heavily in order to rapidly estimate the roll and pitch angles, and X, Y rate sensor
bias. The initialization phase lasts approximately 60 seconds, and the initialization phase can be monitored in the
softwareStatus BIT transmitted by default in each measurement packet. After the initialization phase, the unit
operates with lower levels of feedback (also referred to as EKF gain) from the accelerometers to continuously
estimate and correct for roll and pitch errors, as well as to estimate X and Y rate sensor bias. To reset the algorithm
or re‐enter the initialization phase, sending the algorithm reset command,
, will force the algorithm into the reset
AR
phase.
In addition to the scaled sensor packets described in the IMU Function section, the VG Function has additional
measurement output packets including the default A2 Angle Packet which outputs the roll angle, pitch angle, and
digital IMU data. N0 and N1 packets are also available for use with an external GPS receiver. For more information,
refer to Chapter 9. Communicating with the GNAV540 Unit and Chapter 10. Programming Guidelines for packet
descriptions.
VG Function Advanced Settings
In addition to the configurable baud rate, packet rate, axis orientation, and sensor low‐pass filter settings, VG
unction provides additional advanced settings that are selectable for tailoring the unit to a specific application
equirements. The settings are listed in
F
r
Table 8 below.
Table 8 VG Function Advanced Settings
Setting Default Value Comments
Baud Rate 38,400 baud 9600, 19200, 57600 also available
Packet Type A2 S1, S2, N0, N1 also available
Packet Rate 25Hz This setting sets the rate at which selected Packet Type, packets are output.
If polled mode is desired, then select Quiet. If Quiet is selected, the unit will only
send measurement packets in response to GP commands.
Orientation See Figure 4 on
page 24. To configure the axis orientation, select the desired measurement for each axis:
NAV‐VIEW 2.2 will show the corresponding image of the unit, so it easy to
visualize the mode of operation. Refer to Orientation Field on page 86 for the
twenty four possible orientation settings. The default setting points the
connector AFT.