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kou merged 1 commit into from
Oct 26, 2026
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website: Add FAQ entry for the arm not reaching the commanded position #639

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29 changes: 29 additions & 0 deletions website/docs/faq/index.md
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Expand Up @@ -45,6 +45,35 @@ Turn off the power, then check the screws.
- Vibration during operation tends to loosen the screws around the wrist.
- Loose screws around the elbow can cause play in the joint and make the arm's motion unstable.

## The arm doesn't reach the commanded position or the end effector sags.

The default control gains are set low for safety.
With low gains, gravity leaves a steady-state error in each joint, and the error grows at the end effector when the arm is extended.

Increase the position gain (Kp) first.
This fixes most of the steady-state error and the sagging.

We recommend tuning in the following order:

1. Kp
2. Kd
3. Gravity compensation
4. Friction compensation
5. Integral gain (Ki) and disturbance observer (DOB), only if still needed

OpenArm software controls the arm motors only in MIT mode (PD control with feedforward torque).
It doesn't provide an inner position or velocity loop with integral control or a disturbance observer.
If you implement them yourself, we don't recommend combining Ki and DOB because they can interfere with each other and cause oscillation.

For reference, see the gains used in our software:

- [Default gains for ROS 2](https://github.com/enactic/openarm_description/blob/main/assets/robot/openarm_v2.0/config/arm/control/control_gains.yaml)
- [Higher gains used with OpenArm Cell](https://github.com/enactic/openarm_driver/blob/5c73d835b31875057695f1b8688e0bf9c0bf87b0/src/openarm_driver/configs/openarm_cell_higher_pd_zero_offset.yaml#L87-L89)

See [Control Gains](../api-reference/ros2/control.mdx#control-gains) for how to change the gains.
Start with low gains and increase them gradually.
With high gains, a command far from the current position can move the arm dangerously fast.

## The cameras disconnect.

In the standard configuration in the [OpenArm 2.0 BOM](https://drive.google.com/drive/folders/14RM-fIPml0HTMnt2zcTXjNxJj9s2R_mG), the head stereo camera (ZED mini) and both wrist cameras share one USB hub and one USB repeater cable.
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