Arm moves when zero torques are applied and gravity torques are disabled

[Ravi]

Hi,

I am facing an unexpected behavior with Baxter Arm. I noticed that arm moves when zero torques are applied and gravity torques are disabled. Below is the code snippet:

from baxter_interface import Limb

limb = Limb('right')
command = dict.fromkeys(limb.joint_angles(), 0)
limb.set_joint_torques(command)

I noticed that the arm starts moving in the upward direction. 

Since no torque is being applied, it should not move at all (arms are kept at resting position). Isn't it? What is the workaround to tackle this issue?

[Ian McMahon]

Quote

Since no torque is being applied, it should not move at all (arms are kept at resting position). Isn't it?

Applying a zero torque command to the arm in free space will *not* keep it resting. There is always a certain amount of positive or negative torque acting on each joint due to gravity. When completely stationary, the arm is constantly trying to apply the exact amount of torque required to achieve zero motion. With Baxter, there is the added complexity that the S1 spring passively adds an additional torque to the S1 joint in addition to gravity. You can account for both gravity and the S1 spring torque by subscribing the the gravity_compensation topic, looking at the gravity_model_effort and histeresis_model_effort respectively: http://sdk.rethinkrobotics.com/wiki/Gravity_Compensation

[Ravi]

Hi Ian,

Thank you very much. I got it. Now I applied zero torques without disabling gravity compensation torques. The trajectory of Baxter's arm changes based on starting position. Below are the two starting positions:

1. Arm resting position
2. Arm neutral position

Please see attached videos of the trajectory of Baxter's arm.

The behavior of arm is still not understandable to me. The gravity_compensation torques along with hysteresis model effort are being applied by the robot since I haven't disabled them this time. My question is why the arm is still moving?

Arm neutral position.mp4

Arm resting position.mp4

[Ravi]

Hi Ian,

Any comments? Am I missing something obvious here?