Moog Crossbow GNAV540 - Manual (Page 26)

GNAV540 User Manual
7430‐0808‐01 Rev. B
Page 26
NOTE: The Delta‐Theta, Delta‐V packet is only recommended for use in continuous output mode at 5Hz or greater.
Polled requests for this packet will produce values accumulated since the last poll request; they are subject to
overflow (data type wrap around).
IMU Advanced Settings
The IMU advanced settings are described in Table 7 below. All of the advanced settings are accessible thru NAV‐
VIEW 2.2 under the Configuration Menu→Unit Configuration settings. For information about using NAV‐VIEW 2.2,
refer to Chapter 7. Configuring GNAV540 with NAV­VIEW 2.2.
Table 7 IMU Function Advanced Settings
Setting Default Value Comments
Baud Rate 38,400 9600, 19200, 57600 also available
Packet Type S1 S2 also available
Packet Rate 100Hz This sets the rate at which the 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.
Vertical Gyroscope (VG) Function
The Vertical Gyroscope (VG) Function provides dynamic roll and pitch measurements in addition to the IMU
Function data. The dynamic roll and pitch measurements are stabilized by using the accelerometers as a long‐term
gravity reference. The VG Function can also output a free integrating yaw angle measurement that is not stabilized
by a magnetometer or compass heading.
At a fixed 100Hz rate, the VG Function continuously maintains both the digital IMU data as well as the dynamic roll
and pitch data. As shown in the software block diagram (Figure 2 on page 23), after the Sensor Calibration block, the
IMU data is passed into an Integration to Orientation block. (If using external GPS, refer to Figure 4 on page 24.) The
Integration to Orientation block integrates body frame sensed angular rate to orientation at a fixed 100 times per
d singul ealing with the cosine rot
thm to p e propagation.
second. For improved accuracy and to avoi arities when d ation matrix, a
quaternion formulation is used in the algori rovide attitud
Also shown in the software block diagram (Figure 2, page 23) the Integration to Orientation block receives drift
corrections from the Extended Kalman Filter or Drift Correction Module. In general, rate sensors and accelerometers
suffer from bias drift, misalignment errors, acceleration errors (g‐sensitivity), nonlinearity (square terms), and scale
factor errors.
The largest error in the orientation propagation is associated with the rate sensor bias terms. The Extended Kalman
Filter (EKF) module provides an on‐the‐fly calibration for drift errors, including the rate sensor bias, by providing
corrections to the Integration to Orientation block and a characterization of the gyroscope bias state. In the VG
Function, the internally computed gravity reference vector provides a reference measurement for the EKF when the
unit is in quasi‐static motion to correct roll and pitch angle drift and to estimate the X and Y gyroscope rate bias.
Because the gravity vector has no horizontal component, the EKF has no ability to estimate either the yaw angle
error or the Z gyroscope rate bias.