How to Use Variable Lever Arm

The variable lever arm functionality is simple to use in a SPAN system. It requires the input of gimbal angles from the camera mount or platform that the IMU is mounted on. After that is provided, the system will automatically compute the variable lever arm and produce several messages for output.

Ensuring a proper variable lever arm is important for SPAN performance.

Basic I/O

The variable lever arm functionality is based on the input of the INPUTGIMBALANGLE command. This command specifies the right-handed rotation angles from the mount body frame (m) to the gimbal body frame (g), with the order of rotations Z-X-Y. Entering this command will automatically cause the system to rotate the static lever arm into the latest gimballed frame. The update rate of the variable lever arm depends on the rate of the gimbal commands which can be entered at 1 to 50 Hz. If an INPUTGIMBALANGLE command is not received for over 1 second then the system will return to using the static lever arm.

In addition to using the variable lever arm internally, the following information is available to the user.

Logs used with Variable Lever Arm

Log	Description
VARIABLELEVERARM	This log displays the calculated variable lever arm. The VARIABLELEVERARM log is output in IMU Body frame. The VARIABLELEVERARM log will not be published or used internally unless a SETINSROTATION RBM command is sent.
GIMBALLEDPVA	This log has the same format as the INSPVA log, but contains the attitude rotated back to the null gimbal position. When the gimbal mount is in its zero locked position, the GIMBALLEDPVA attitude will match the attitude given in the INSPVA log. If the translational offset to the gimbal center of rotation has been provided, the position and velocity output are offset to this position. Otherwise, the position and velocity values are given at the IMU center of navigation.
INSPVA, INSPVAS, INSPVAX	The INSPVA log is synchronous so does not depend on the entry of the INPUTGIMBALANGLE command. However when the gimbal angles are non-zero, the INSPVA log no longer represents the orientation of the gimbal mount / vehicle, but rather the current orientation of the unlocked gimbal platform. If the orientation of the vehicle is important, use the GIMBALLEDPVA log.

Rotations and Translations

For the information provided by the system to be meaningful, the following offsets have to be set correctly. By default the system assumes that the IMU Body frame, gimbal mount and Mount Body frame are coincident. If this is not the case then additional commands are necessary to relate all of the different frames together using the commands below. If attitude output is desired relative to an external camera, this can also be configured using these commands.

When entering the SETINSROTATION commands described in the following table, the gimbal mount should be locked (INPUTGIMBALANGLE rotations are zero).

Commands used with Variable Lever Arm

Command	Purpose
SETINSROTATION RBV	This command provides the orientation of the IMU Body frame while the gimbal frame is in the locked null position. This is required information before the SPAN system can complete an inertial alignment. The SETINSROTATION RBV command relates the IMU Body frame (b) to the Vehicle frame (v). Once an RBV rotation has been provided, default attitude output will be referenced to the Vehicle frame.
SETINSROTATION RBM	This command relates the IMU Body frame (b) to the Mount Body frame (m). This information is required for interpreting the rotation angles from the VARIABLELEVERARM log correctly.
SETINSROTATION USER	By default, attitude information output by SPAN is for the vehicle frame. If attitude output is required to be referenced to another frame (typically the axes of a gimbal-mounted sensor), the USER offset can be used to rotate the attitude output in general SPAN logs to this frame.
SETINSROTATION MARKx	Similar to the user rotation, this rotation can be used to change the attitude output reference frame for the MARKxPVA logs.
SETINSTRANSLATION GIMBAL	This command provides the offset from the IMU center of navigation to the gimbal center of navigation. This is an optional translation used by Inertial Explorer for post-processing.