Constrained z-prismatic point joint state keeps increasing

[ajordana]

In our setup we have a 1D hopper. We model the hopper using a 1D prismatic joint. See the setup here 1.

When we try to fix the base using something like

p.createConstraint(self.robotId, 0, -1, -1, p.JOINT_FIXED, [0, 0, 0], [0, 0, 0.],[0 ,0 , fixed_height])

and query the joint state later (while the leg is swinging with zero torque), we observe the p.getJointState(robot_id, 0) to increase from the initial fixed_height value=0.4 to values like 4.9. Looking at the visualization, the base stays fixed.

Any idea what is going wrong here? Is the p.getJointState(...) returned before taking the constraints into account?

[ajordana]

+\cc @jviereck

[erwincoumans]

Please provide a small self-contained full working pybullet example (without other dependencies).