Set initial robot joint values

[karl-exwzd]

I'm setting up an environment with a UR robot and its default zero-joint position is to be completely flat, as in the example.

I searched and looked through all the samples, but I could not figure out how to modify the initial joint values of the robot. It will always start straight, which is an issue with what I would like to do.

It is possible to set the joint values after the simulation starts, then the robot will move there, but I would like to have the configuration set prior to starting the simulation.

Is this possible through some parameter setting? Do I have to change the PROTO file for that?

[omichel]

Yes, you will have to change the PROTO file for that. You should rotate each joint so that it is in the initial position you wish.

[karl-exwzd]

I could not figure out how to set the initial rotation of a joint for the UR robot. I tried using the position field of the HingeJoint and under the HingeJointParameters. Both do not work... at this point I feel there must be a bug. When trying other parameters, such as translation, and reloading the simulation, I could see the changes.

The only way to achieve this in the PROTO file is to change the rotation of the Solid object, but this is not ideal: the zero rotation position should not differ between the model and the controller. This changes the zero position and the controller is not accurate anymore.

[ad-daniel]

It can be done programmatically as well in theory by using wb_supervisor_node_set_joint_position. You still need to set the position of each joint, but at least you can do it from the controller (supervisor)

[karl-exwzd]

Thanks for the answer. I like the supervisor method, as it does not involve PROTO file editing.

However, because of the lack of examples/documentation, I have not successfully been able to acquire the robot joint node instance, in the supervisor script, in order to use the wb_supervisor_node_set_joint_position function. It seems the supervisor can only work with generic nodes, it cannot have access to a robot instance from which I could get to the joints via wb_robot_get_device.

When I try to create a Robot instance in the supervisor I get the following message

Only one instance of the Robot class should be created

More simply: how would one use the wb_supervisor_node_set_joint_position function in the supervisor script?

[ad-daniel]

A Supervisor is just a Robot with extra powers, you don't need to create an instance of both. You can simply do

robot = Supervisor()

Concerning the node instance, you can retrieve it using the DEF mechanism, so if in the PROTO you have something like: DEF BALL_JOINT BallJoint { ... }, you can retrieve it using WbNodeRef ball_joint_node = wb_supervisor_node_get_from_def("BALL_JOINT")

and then you set the position to whatever you want: wb_supervisor_node_set_joint_position(ball_joint_node, POSITION, 3), the last parameter specifying which of the three axis of the BallJoint you wish to change.

Since you're using a pre-existing robot, you might need to change/set the DEF names inside it