Velocity controller does not stop on Slopes

asked 2019-10-24 05:27:32 -0500

Weasfas
1695 ●12 ●91 ●41

updated 2019-10-24 05:28:13 -0500

Hi,

I am currently working with ROS Kinetic and Gazebo 7 in a vehicle model.

To simplify things, the model has 4 continuous joints to apply a certain velocity to the model. With the <transmissions> tag I linked the joints in gazebo with a velocity_controller interface from ROS_control. In a planar environment the controller just works fine, the problem comes when I test the controllers on terrain with slopes. The vehicle model is huge and has a high mass value (2000 Kg), and that may be the problem.

If I input, let's say and X velocity to the controller, it is unable to reach the target velocity. And it get worse when the model is in a slope and I command it to stop (0 velocity), because the model is unable to stop having always a very small velocity that moves the model very slowly in Gazebo.

I tried to adjust the pid gains of gazebo_ros_control but always with the same effect depending on the environment: The higher the slope is the more velocity error It outputs.

I am just wondering if there is a way to decrease error in this environment of there are similar problems or works in this community.

I will post here the config file for controllers:

joint_state_controller:
 type: joint_state_controller/JointStateController
 publish_rate: 50

control_vel:
  type: velocity_controllers/JointGroupVelocityController
    joints:
      - front_left_wheel_joint
      - back_left_wheel_joint
      - front_right_wheel_joint
      - back_right_wheel_joint

/gazebo_ros_control:
  pid_gains:
      front_left_wheel_joint:
          p: 2000.0
          i: 0.0
          d: 0.0

      back_left_wheel_joint:
          p: 2000.0
          i: 0.0
          d: 0.0

      front_right_wheel_joint:
          p: 2000.0
          i: 0.0
          d: 0.0

      back_right_wheel_joint:
          p: 2000.0
          i: 0.0
          d: 0.0

And the joint description:

<joint name="wheel_joint" type="continuous">
  <parent link="base_link"/>
  <child link="cont_wheel"/>
  <origin xyz="0 0 0" rpy = "0 0 0"/>
  <axis xyz="0 1 0" rpy="0 0 0" />
  <limit effort="1000" velocity="20"/>
  <dynamics damping="0.1" friction="0.1"/>
  <joint_properties damping="1.0" friction="1.0"/>
  <safety_controller k_velocity="200"/>   
</joint>

<transmission name="wheel_joint_trans">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="wheel_joint">
    <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
  </joint>
  <actuator name="wheel_joint_motor">
    <mechanicalReduction>1</mechanicalReduction>
  </actuator>
 </transmission>

Thanks in advance.

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Comments

Just a note: velocity_controllers/JointGroupVelocityController is a forwarding command controller. It's in => out. There is no actual control loop there. So PID values are ignored.

If you want / need a closed-loop controller, you'll have to either switch your vehicle to an effort interface and use effort_controllers/JointVelocityController, or you could create a customised version ..

gvdhoorn ( 2019-10-24 07:41:01 -0500 )edit

.. using the ClosedLoopHardwareInterfaceAdapter.

I tried to adjust the pid gains of gazebo_ros_control but always with the same effect depending on the environment: The higher the slope is the more velocity error It outputs.

are you referring to the p, i and d values in your .yaml configuration? As I wrote above, those are not used by the JointGroupVelocityController that you're using.

gvdhoorn ( 2019-10-24 07:42:08 -0500 )edit

Hi @gvdhoorn,

Thanks for your answer. I decided to implement a new controller to have a close loop. The problem is that Gazebo is always reporting a small velocity on the joint and I am unable to control the platorm in a planar ground.

Besides, I do not understand the difference betweern the gazebo_ros_control pid gains and the effort_controller pid gains. I can confirm the pid gains defined in the file I posted are used by gazebo to output new velocity commands because depending on the values I assing there, I have different velocity output for the joint.

Weasfas ( 2019-10-25 17:43:02 -0500 )edit

Gazebo runs PID controllers itself, internally, depending on how the hardware_interface interfaces with Gazebo itself.

I don't know how that is implemented. But I can definitely state that PID gains are not used at all by fowarding command controllers (ie: those in ros_controllers).

gvdhoorn ( 2019-10-26 04:47:18 -0500 )edit

Ok, I am going to try to implement a custom controller knowing this.

Thanks..

Weasfas ( 2019-10-26 04:55:39 -0500 )edit

Is there anything preventing you from switching your simulated vehicle to an effort interface? That would seem to offer a way forward without requiring any custom code. You'd just have to switch the Gazebo description to an effort hardware_interface and use effort_controllers/JointVelocityController in your gazebo_ros_control configuration.

If your response is going to be: but my actual hw only has a velocity interface, then I don't think that would matter, depending on the level at which you are simulating your vehicle.

In the end the controllers should bring your vehicle to a desired velocity, exactly as it would when using the hardware_interface/VelocityJointInterface it would seem to me.

gvdhoorn ( 2019-10-26 04:57:43 -0500 )edit

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