Regarding the HingeJoint presence issue

[zhy3221]

Hello, in webots, HingeJoint adds the motorof the device to control endpoint, why does position control of the motor cause the parent point to make a small movement?

[omichel]

This is simply due to the laws of physics.
If you do the same on a real system, you will observe the same movement.
The bigger the mass of the parent, the smaller the motion.

Note: if the parent is the static world, then obviously no motion will occur at the parent.

[zhy3221]

Thank you very much for your reply, I can understand the reason, but for robotic arm kinematics control then each arm is movable, how to design the webot so that the HingeJoint doesn't affect the parent point's arm.

[omichel]

The base of the robot should be either heavy enough or linked to the static environment.

[zhy3221]

For example, for the two arms L-1 and L connected by the robotic arm joint K, how can the control of the joint K enable the movement of only L, while the movement of L-1 is only the effect produced by the joint K-1.

[omichel]

The joint K-1 could be attached to the static environment.