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Rotate joint around given axis (steer car)

asked 2017-01-23 13:37:39 -0600

kaldo gravatar image

updated 2017-01-24 04:02:33 -0600

Hello,

I have a car whose front wheel are connected to chassis in this manner :

<joint name="wheel_front_left_steer_spin" type="universal">
  <parent>chassis</parent>
  <child>wheel_front_left</child>
  <axis>
    <xyz>0 -1 0</xyz>
    <limit>
      <lower>-0.5235987755982988</lower>
      <upper>0.5235987755982988</upper>
    </limit>
  </axis>
  <axis2>
    <xyz>0 0 1</xyz>
  </axis2>
</joint>

In order to steer my car, I want to set the position around axis2 to 30 degrees. So I tried this (inside my plugin):

 this->joint_ = model_->GetJoint("car::wheel_front_left_steer_spin");

 this->steer_pid_ = common::PID(0.1, 0, 0);

 this->model_->GetJointController()->SetPositionPID(joint_->GetScopedName(), steer_pid_);

 double position = -3.14159265 / 6;
 this->model_->GetJointController()->SetVelocityTarget(joint_->GetScopedName(), position);

But it sends my car bouncing and jumping uncontrollably. So how do I set the angle of my wheels? I haven't found clear examples and couldn't figure it out from the doc.

Thanks in advance,


EDIT: So I split my joint into two revolute joints, but can only get one moving at a time. How can I link the two so I keep the 2 degrees of freedom?

<joint name="wheel_front_right_steer" type="revolute">
    <parent>chassis</parent>
    <child>wheel_front_right_steer_link</child>
    <axis>
        <xyz>0 -1 0</xyz>
        <limit>
            <lower>-0.5235987755982988</lower>
            <upper>0.5235987755982988</upper>
        </limit>
    </axis>
</joint>

<link name="wheel_front_right_steer_link">
     <!-- Not sure what to put here -->
</link>

<joint name="wheel_front_right_spin" type="revolute">
    <parent>wheel_front_right_steer_link</parent>
    <child>wheel_front_right</child>
    <axis>
        <xyz>0 0 1</xyz>
    </axis>
</joint>
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answered 2017-01-23 15:41:29 -0600

chapulina gravatar image

It looks like the joint controller doesn't have good support for multi-axis joints (such as universal). I ticketed an issue for this.

I would suggest you change to a revolute joint. If you need the 2 DOF, you could add 2 revolute joints with a dummy link in between them.

Also, when things start bouncing uncontrollably like this, it's a good idea to run Gazebo in verbose mode to see if any error messages are being printed.

I'd also inspect the model by visualizing the inertias and masses for each link, to make sure there are no tiny or huge links in the model making the physics engine crazy. Finally, you can try playing with different PID gains to see if things become more stable.

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Comments

Thank you :) Gazebo is already verbose and said nothing. And I'm using an adapted version of the "standard" cart with front steering, so I don't expect to have any extravagant link. I split my joint as you suggested, but can get only one to move at a time. Could you please elaborate on the dummy link?

kaldo gravatar imagekaldo ( 2017-01-24 03:20:48 -0600 )edit
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Asked: 2017-01-23 13:37:39 -0600

Seen: 1,104 times

Last updated: Jan 24 '17