Implementing 2D Dynamics in PyBullet

[emrecemal]

I am currently working on a PyBullet simulation and I am looking to implement different modes for the simulation.

• The first mode I want to enable is called "1D". In this mode, the robot is only allowed to move in the direction of gravity (z-axis).
• The second mode is called "2D". In this mode, the robot can move along the X and Z-axes and can also rotate around the Y-axis (pitch).
• Finally, the third mode is called "3D" where there are no limitations on the robot's movements.

Initially, I thought about adding constraints to the robot for the "1D" case.

if self.mode == "1D":
            # Add a constraint to enable movement only in z-axis (vertical)
            constraint_ID_z = p.createConstraint(parentBodyUniqueId=self.robot_ID, 
                                               parentLinkIndex=-1, 
                                               childBodyUniqueId=-1, 
                                               childLinkIndex=-1, 
                                               jointType=p.JOINT_PRISMATIC, 
                                               jointAxis=[0, 0, 1], 
                                               parentFramePosition=[0, 0, 0], 
                                               childFramePosition=[0, 0, 2])
            p.changeConstraint(constraint_ID_z, maxForce=max_force)

This works well for the 1D case. Where. an arbitrary number max_force is used along with the childFramePosition for the initial position.

However, when attempting to apply the same approach for the 2D case, I encountered some difficulties. Specifically, I added two prismatic joints for the x and z axis, respectively, according to their axis with the same childFramePosition. Unfortunately, the robot remained fixed at the childFramePosition and did not move. Additionally, I was unable to enable the pitch and could not determine how to proceed. How can I implement 2D dynamics?

I came across an approach mentioned in issue number #2336 (2D dynamics in PyBullet). They have implemented this approach in the URDF file. However, I do not want to modify the URDF file. Could you please suggest how I can use this approach conditionally in my case?