FBX Import Forces Elbow-Up Even When Rig is Pre-Positioned Elbow-Down¶

CONFIRMED BOTH BOLT BOLT X BOLT JR+

Summary¶

Pre-positioning the Bolt robot in an elbow-down configuration before an FBX import does not reliably force the import to use elbow-down. The robot may attempt to run the imported move elbow-up and fail. A workaround exists: after the failed import, manually type an Arm/Angle value above 90 (e.g. 100) in the first waypoint, attempt a run (which will fail), then re-import — the robot will then accept the elbow-down solution. General best practice: always pre-position the rig in the target orientation before any CGI import.

Symptoms¶

FBX move requires elbow-down (Arm/Angle > 90)
Robot is pre-positioned elbow-down before import
After import, robot still attempts to run the move elbow-up
Move fails because the elbow-up path is out of reach or collides

Systems Affected¶

BOTH

Bolt, Bolt X, Bolt Jr+ — any robot arm with elbow-up / elbow-down dual solution
FBX / CGI imports where the intended configuration is elbow-down

Background¶

When Flair solves a robot position from Cartesian FBX data, there are typically two valid inverse kinematics solutions for each position: elbow-up and elbow-down. Flair uses the current robot position as a hint to choose between them — whichever solution is closer to where the robot currently is should be selected.

In practice, pre-positioning the rig elbow-down does not always cause Flair to consistently choose the elbow-down IK solution for the entire imported move. The solver may revert to elbow-up partway through or on first import.

"When importing an FBX move that requires the bolt's elbow to be down (arm/angle >90). Even if you have the arm set up in an elbow down situation on import it may still want to try and run the move elbow up." — Brendan Isaac, 2022-03-30

Workaround — Force Elbow-Down Via Waypoint Value¶

If the robot is attempting elbow-up after import:

After the failed import, open the move waypoints.
On waypoint 1 (the first point), find the Arm/Angle axis value.
Type in a value above 90 — for example, 100. This manually places the first point in elbow-down territory.
Attempt a run — it will fail (the arm/angle value is not a valid continuous solution yet), but this is expected.
Re-import the FBX move.
The robot should now solve the import using the elbow-down configuration.

"The fix for this is to type an arm/angle value above 90 (say 100) on the first line of this move. Then try and run the move which it wont allow. On your next attempt of trying to import this move the robot will behave and do it elbow down." — Brendan Isaac, 2022-03-30

Best Practice — Pre-Position Before Any CGI Import¶

Even when pre-positioning doesn't guarantee the correct elbow solution (as above), it remains important to put the rig into the approximate starting position and orientation before any CGI import:

"With any import it is a good idea to put the rig in the position you think it should be at the start of the move. This will give the software a starting point for the positions solve. If the arm should be extended or angled out, then put it in that position and point the camera in more or less the direction you think it should go. This should help." — Simon Wakley, 2022-12-24

Best practice steps before FBX import:

Manually drive the robot to the approximate position and orientation the move starts at.
If the move needs elbow-down, drive elbow-down first.
If the move starts with the arm extended, extend it.
Point the camera roughly in the starting direction of the move.
Then import the FBX.
If the robot still chooses the wrong configuration, use the Arm/Angle waypoint workaround above.

Source: talk.mrmoco.com — FBX import and elbow down findings — Posts by Brendan Isaac and Simon Wakley, 2022-03-30 and 2022-12-24