Problems with controlling joint velocity

[Raghav Malik]

Hi everybody,

I followed along with the Gazebo and ROS JointPositionController tutorial for RRBot here, and got it working just fine. However, when I tried modifying it to control velocity instead of position the setup immediately broke. The only change I made to the original was replacing "type: effort_controllers/JointPositionController" with "type: effort_controllers/JointVelocityController". Loading the controller through a service call returns success, but the visualization in gazebo immediately breaks and the joint velocities are set to "nan". Any ideas what I am doing wrong? Perhaps there are additional steps one must take when creating a velocity controller? I've been looking online for some kind of explanation but haven't had much luck so far. 

Any help is appreciated.

Cheers,

Raghav Malik

[Toni Oliver]

Hi Raghav,

I think you will need to modify your xacro so that the transmissions have a VelocityJointInterface instead of an EffortJointInterface.

<transmission name="tran1">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="joint1">
      <hardwareInterface>VelocityJointInterface</hardwareInterface>
    </joint>
    <actuator name="motor1">
      <hardwareInterface>VelocityJointInterface</hardwareInterface>
      <mechanicalReduction>1</mechanicalReduction>
    </actuator>
  </transmission>

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[Raghav Malik]

Hi Toni,

Thank you for your response. To clarify, I would like to be able to achieve a certain angular velocity to a joint by changing the effort about that joint. I was under the impression that the hardware interfaces represent what the controller is outputting? I would still like the controller to give me values for effort, but this time to maintain a velocity instead of a position. Do I still need the hardware interface to be a velocity interface?

Cheers,

Raghav Malik

[Andriy Petlovanyy]

Hi Raghav,

Please find my small project here

It is basically vertical motor with attached wheel to it (I have used it as demo for simple ROS serial integration).
I have managed to use effort_controllers/JointVelocityController with EffortJointInterface.
Here you can find joint definition.
Hope you'll find it useful.

Thanks

Best regards,
Andriy

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[Toni Oliver]

Hi Raghav,

You are right, I misread your email and thought you were trying to use velocity_controllers/JointVelocityController.

Andriy's example should help you find the problem.

Cheers,

Toni

Hi Toni,

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[Raghav Malik]

Hi Andriy,

Thank you very much. It was looking at your project that got me to the answer. My mistake was in not adjusting the PID gains for velocity control which is why it quickly grew out of bounds and became unstable. Adding the effort limits and adjusting my gains fixed the issue.

Cheers,

Raghav