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what does joint_state_controller read?

asked 2019-04-14 15:25:46 -0600

zoidberg2010eaf gravatar image

Hello, I'm fairly new to ros_control and I'm really confused about what are the data joint_state_controller acquires from the Robot's Encoders, are they ticks, ticks per meter or rad? and if it was ticks, where can I provide the Radius of the wheels to the Controller?

Thanks.

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answered 2019-04-15 02:05:45 -0600

gvdhoorn gravatar image

updated 2019-04-15 02:13:06 -0600

I'm really confused about what are the data joint_state_controller acquires from the Robot's Encoders

To start (and this is slightly pedantic), but joint_state_controller (JSC) does not read from your robot's encoders. It retrieves joint state through something called a JointStateInterface and this state is comprised of position, velocity and effort.

The "only" task of the JSC is to take that state, convert it into a sensor_msgs/JointState message and publish it.

As this "controller" (and I place this in quotes as it doesn't really control anything, but that's just ros_control nomenclature) has to do that in a generic way: it cannot know anything about any robot's specific implementation, or we could not all reuse the same JSC. Hiding such details as "ticks" and "encoders" from the JSC (and other controllers in ros_control) is the task of the hardware_interface.

This piece of the ros_control stack is the interface to the hardware, and it performs a number of functions:

  1. it takes are of converting whatever units the robot hw uses into ROS standard units (REP-103)
  2. it provides a way to communicate with the hw (ie: using a serial bus, a fieldbus (ethercat, canbus, powerlink, profinet, profibus, ..), plain ethernet, TCP or UDP sockets, GPIO, direct memory registers, etc, etc)
  3. it takes care of any special initialisation routines or procedures that are needed to put the hardware in a state where it can actually be controlled
  4. it deals with error handling and recovery

And most (all?) of this is intended to abstract away implementation details about the robot for one purpose: if we hide all (most) knowledge of the actual hardware being used/addressed behind the hardware_interface, we can reuse as much of ros_control as possible, as none of the controllers (for instance) need to know anything about "encoders", "ticks" or any such things. This is the main idea behind ros_control (and in fact all of software engineering where abstraction is used to hide "implementation details" as they are called).

I'm really confused about what are the data joint_state_controller acquires from the Robot's Encoders, are they ticks, ticks per meter or rad?

I hope you already know the answer to this now, but just to make it clear: they are not ticks, but the units that we've standardised on in ROS. So:

  • position: radians or metres (for revolute and prismatic joints respectively)
  • velocity: radians/sec or metres/sec (idem)
  • effort: Newtons or Newton metres (idem)

The hardware_interface will have to already have performed the conversion. And again: this is read from the hardware_interface, not encoders or the robot directly.

and if it was ticks, where can I provide the Radius of the wheels to the Controller?

I hope this one is already clear now as well: the controller does not do any conversion of units, so it does not need to know about ticks or wheel radii.

If you're thinking about odometry, then keep in mind that there are (at least) two levels of ... (more)

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For some additional words about the joint_state_controller and the joint_state_publisher, see #q303358.

gvdhoorn gravatar imagegvdhoorn ( 2019-04-15 02:06:11 -0600 )edit
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And I don't like linking to off-site resources (as they may change and/or disappear), but perhaps How to implement ros_control on a custom robot can also help.

gvdhoorn gravatar imagegvdhoorn ( 2019-04-15 02:07:08 -0600 )edit
1

While that guide is definitely a good start. Please be aware that it registers SoftJointLimits which are not given in in the joint_limits.yaml. Make sure to either not register the softlimits or specify them in your joint_limits.yaml as they will otherwise default to 0 and the enforcing of the limits in the write() method will not allow you to send position commands other than 0.

I'm not sure if the guide is outdated or simply incomplete but this caused me quite the headache while following it the first time.

Isha Dijcks gravatar imageIsha Dijcks ( 2019-04-15 02:39:40 -0600 )edit

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Asked: 2019-04-14 15:25:46 -0600

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Last updated: Apr 15