Joint order in JointGroupVelocityController 'command' topic

melodic
ROS

asked 2022-06-01 07:47:31 -0500

CroCo
153 ●24 ●39 ●51

updated 2022-06-01 13:35:28 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

I'm working now with UR5. When I echo the states of joints I get this

galf@galf:~/Desktop/ur5/catkin_ws$ rostopic echo /ur5/joint_states -n1
header: 
  seq: 9465
  stamp: 
    secs: 94
    nsecs: 662000000
  frame_id: ''
name: [elbow_joint, shoulder_lift_joint, shoulder_pan_joint, wrist_1_joint, wrist_2_joint,
  wrist_3_joint]
position: [-3.1415916305807485, 0.30983008014827096, 1.9488655889645834, 3.1415983918040062, 1.845209559572325, -3.141592812638949]
velocity: [3.621569639258467e-06, -9.772236883207112e-06, 0.00011501897260136615, -2.0719156311055294e-07, 0.06444979561546602, -6.340667138213241e-08]
effort: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
---

Are the positions match the joints in the same order as they appear in the names? I'm asking this because when I retrieve the info for the command topic, I get

galf@galf:~/Desktop/ur5/catkin_ws$ rostopic pub /ur5/joints_group_velocity_controller/command std_msgs/Float64MultiArray "layout:
  dim:
  - label: ''
    size: 0
    stride: 0
  data_offset: 0
data:
- 0"

So to command the joints, I need fill out data as data: [0,0,0,0,0,0] but I don't know the order of joints. How this works in ROS? Does data[0] represent elbow_joint?

The tree appears as shoulder_pan_joint > shoulder_lift_joint > elbow_joint > wrist_1_joint > wrist_2_joint > wrist_3_joint. I don't know why ROS puts them in alphabetical order.

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I've updated the title to better reflect your question. "topic command" is too ambiguous.

gvdhoorn ( 2022-06-01 13:36:20 -0500 )edit

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answered 2022-06-01 13:27:26 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2022-06-01 13:32:24 -0500

From this:

galf@galf:~/Desktop/ur5/catkin_ws$ rostopic pub /ur5/joints_group_velocity_controller/command std_msgs/Float64MultiArray ...

it would appear you're using a JointGroupVelocityController with your driver (note: this cannot be ur_driver, as that's not a ros_control compatible driver).

That controller requires a configuration stanza similar to this:

joint_group_vel_controller:
   type: velocity_controllers/JointGroupVelocityController
   joints:
     - shoulder_pan_joint
     - shoulder_lift_joint
     - elbow_joint
     - wrist_1_joint
     - wrist_2_joint
     - wrist_3_joint

The order in which you should provide the data to the joints_group_velocity_controller/command would be the same order as specified in the configuration stanza I've included here.

So to command the joints, I need fill out data as data: [0,0,0,0,0,0] but I don't know the order of joints. How this works in ROS? Does data[0] represent elbow_joint?

this is not really "in ROS". It's just a consequence of how the ros_control developers implemented their infrastructure.

The tree appears as shoulder_pan_joint > shoulder_lift_joint > elbow_joint > wrist_1_joint > wrist_2_joint > wrist_3_joint. I don't know why ROS puts them in alphabetical order.

For this, see #q356347, #q282097, #q221560 and #q351105.

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this is exactly my ymal file in the same order you've posted it.

CroCo ( 2022-06-01 17:06:11 -0500 )edit

Could you please confirm if the names match the positions' indices in the command topic?

CroCo ( 2022-06-01 17:11:23 -0500 )edit

I'm not sure what is unclear about this:

The order in which you should provide the data to the joints_group_velocity_controller/command would be the same order as specified in the configuration stanza I've included here.

gvdhoorn ( 2022-06-02 01:41:54 -0500 )edit

@gvdhoorn, I'm talking about /joint_states.

CroCo ( 2022-06-02 06:42:33 -0500 )edit

Your question is still a bit ambiguous (which "names" are you referring to in your comment), but: please see #q356347, #q282097, #q221560 and #q351105.

Summarising: no, they don't have to, they probably don't, and you shouldn't assume they do or will.

gvdhoorn ( 2022-06-02 06:45:57 -0500 )edit

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answered 2022-06-01 10:34:12 -0500

ljaniec

3064 ●14 ●297 ●50 https://www.linkedin.c...

By checking the driver code in their repository:

https://github.com/ros-industrial/uni...

and

https://github.com/ros-industrial/uni...

it seems to be

JOINT_NAMES = ['shoulder_pan_joint', 'shoulder_lift_joint', 'elbow_joint',
               'wrist_1_joint', 'wrist_2_joint', 'wrist_3_joint']

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thanks for the answer. But why I'm getting incorrect order?

CroCo ( 2022-06-01 11:03:06 -0500 )edit

Add more details about your setup, please - right now it's divination from a glass ball :) Can you link repository with packages you are using, maybe even the project? You can also have a different configuration than the default: https://github.com/ros-industrial/uni...

ljaniec ( 2022-06-01 12:44:06 -0500 )edit

1

@ljaniec: @CroCo doesn't mention it, but I'm pretty sure (s)he's using UniversalRobots/Universal_Robots_ROS_Driver, not ur_driver.

You can't use code inur_driver to answer this question.

@CroCo will have to first clarify which driver (s)he is using.

gvdhoorn ( 2022-06-01 13:24:06 -0500 )edit

@gvdhoorn, yes I'm not using ur_driver. Actually, currently I'm using Gazebo for simulation. The algorithm I'm working with requires the joints to accept velocity inputs that is why I've created my own yaml.

CroCo ( 2022-06-01 17:08:04 -0500 )edit

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Joint order in JointGroupVelocityController 'command' topic

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