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Does ros_control support limit switches

asked 2015-07-08 10:16:24 -0500

elsp1991 gravatar image

Hello everyone, I've just get my robots arms running with ros_control.

My robot read joints position using incremental (quadrature) encoder. These sensor doesn't provide absolute position but only relative to the initial position.

So to set the real position I just sent to joint a small amount of effort and waiting to hit a limit switch, then I set the real position value.

I was wondering if ros_control has something (service or interface) to support initialization processes like this described above, before load the controller, or even better to be part of the load service call. Do you think that is a hardware related issue so it must handled from my node after all?

Thank you

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answered 2015-07-09 02:07:47 -0500

gvdhoorn gravatar image

updated 2015-07-09 02:13:11 -0500

tl;dr: no, there is no support for limit switches in ros_control, but I'd argue that it is also not something that should be there (as something like initialisation of a robot to me seems like a higher-level behaviour).

At the moment ros_control is mostly concerned with providing an efficient and re-usable way to setup a (relatively simple) controller architecture that is able to interface with a ROS node graph. Most (if not all) default controllers operate at the joint-space level, and the package can be said 'to live' at the (or at least very near to) Controller layer (in a 3T architecture, just an example).

It would make more sense to me to make something like initialisation a higher-level behaviour, perhaps part of a larger overall statemachine (again, just an example, there are others) that encodes the Executive part of your architecture. At start-up, your application then always passes through the initialisation (sub)states first, before accepting any other requests for execution of behaviours. That initialisation phase would then interact with ros_control to switch controllers to the appropriate types at the appropriate times, search for the limit switches and setup your encoders properly.

ros_control (and more specifically, the hardware_interface part of it) could perhaps be used to implement an interface to your limit switch hardware, similar to the joint_state_controller (which isn't a controller at all, it's a sensor interface, publishing JointState msgs). Whether that is something that makes sense depends on preference and how you've setup the rest of your system.

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answered 2015-07-09 02:35:51 -0500

Adolfo Rodriguez T gravatar image

There is currently no support for limit switches in ros_control. If you need to use limit switch information inside a controller, it should be rather straightforward to support them. You would need to create a new type of hardware interface that wraps the limit switch raw data (boolean), register such an interface in your RobotHW specialization, and use it in a controller.

OTOH, if you're using limit switch info in your RobotHW implementation, and not in your controllers, there's no need to expose it as a ros_control interface (as mentioned in the original answer).

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Asked: 2015-07-08 10:16:24 -0500

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Last updated: Jul 09 '15