initial joint values for JointStateBroadcaster

[christianrauch]

Description

Add new parameters to set the initial interface values for JointStateBroadcaster.

Fixes # (issue)

Is this user-facing behavior change?

Yes. It adds new parameters. The defaults stay the same, i.e. the default behaviour does not change.

Did you use Generative AI?

No.

Additional Information

The tests:

colcon test --event-handlers=console_direct+ --packages-select joint_state_broadcaster

fail. But they currently also fail on master (at least for me locally).

TODOs

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• Fork the repository.
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• Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.

[saikishor]

Could you explain the motivation behind this change?

[christianrauch]

Could you explain the motivation behind this change?

When your system only exposes velocity state interfaces, but you need the position for the rotor_state_broadcaster, it makes snese to initialise these values with 0, for example. This is akin to the joint_state_publisher, which allows you to send 0 for joint values.

[saikishor]

When your system only exposes velocity state interfaces, but you need the position for the rotor_state_broadcaster, it makes sense to initialise these values with 0, for example. This is akin to the joint_state_publisher, which allows you to send 0 for joint values.

I understand the need. If this is what you need, then the intial value should be per joint name instead of everything together, so it is explicit

[christianrauch]

When your system only exposes velocity state interfaces, but you need the position for the rotor_state_broadcaster, it makes sense to initialise these values with 0, for example. This is akin to the joint_state_publisher, which allows you to send 0 for joint values.

I understand the need. If this is what you need, then the intial value should be per joint name instead of everything together, so it is explicit

This seems a bit excessive to me. For a 7 DoF arm, this amounts to setting 21 values. You do not have to do this with the joint_state_publisher. Isn't there an easier way to do this?

[saikishor]

You only set those joints, who lack the feedback not for everything right?

[christianrauch]

You only set those joints, who lack the feedback not for everything right?

The initial values of the joints with feedback will be overridden anyway, so you can as well just set all joints at once. But I see your problem. I guess when a robot's default configuration is not all-0, then initialising all joints to the same value does not make sense.

[saikishor]

Moreover, 0 is a valid value and I don't think the JSB should initialise them to a valid value by default. There could be a case where 0 is not within your joint limits too.