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Object slips out of the gripper after jitters

asked 2016-06-21 09:15:33 -0500

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I am trying to make robot arm grasp a cylinder object but when ever the gets in the grasp and the gripper closes, it starts vibrating and the object appears to slip out of it. I am using a position_controller for gripper and the rest of the arm. The problem I can think of could be with the controller as it tries to reach a position which it cannot due to collision with the object so the pid controller starts shaking it.

Pls answer if you have any insight on this. Thanks

<joint name="${prefix}gripper_joint" type="prismatic"> <parent link="${prefix}wrist_roll"/> <child link="${prefix}gripper_finger1"/> <origin xyz="-0.01331 0.00485 0.137225" rpy="0 0 0"/> <limit lower="-0.08" upper="0.008" effort="20.0" velocity="5.0"/> <axis xyz="1 0 0"/> <dynamics damping="0.7" friction="0.0"/> </joint>

<link name="${prefix}gripper_finger1">
  <visual>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <geometry>
      <!--box size="0.015 0.0229 0.0475"/-->
      <mesh filename="package://cyton_gamma_1500_description/meshes/gripper_finger1.dae"/>
    </geometry>
  </visual>
  <collision name="${prefix}gripper_finger1_collision">
    <origin xyz="0 0 0" rpy="0 0 0" />
    <geometry>
      <box size="0.015 0.0229 0.1"/>
  <mesh filename="package://cyton_gamma_1500_description/meshes/gripper_finger1.dae"/>
    </geometry>
  </collision>
  <inertial>
    <mass value="0.01"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <inertia ixx=".0000027" iyy=".0000027" izz=".0000027"
              ixy="0" ixz="0" iyz="0"/>
  </inertial>
</link>

 <gazebo reference="${prefix}gripper_finger1">
  <kp>1000000.0</kp>
  <kd>1.0</kd>
  <mu1>200.0</mu1>
  <mu2>100.0</mu2>
  <minDepth>0.001</minDepth>
</gazebo>


<transmission name="tran8">
<type>transmission_interface/SimpleTransmission</type>
<joint name="${prefix}gripper_joint">
  <hardwareInterface>EffortJointInterface</hardwareInterface>
</joint>
<actuator name="motor8">
  <hardwareInterface>EffortJointInterface</hardwareInterface>
</actuator>

</transmission>

<joint name="${prefix}gripper_joint2" type="prismatic"> <parent link="${prefix}wrist_roll"/> <child link="${prefix}gripper_finger2"/> <origin xyz="0.0098 0.00485 0.137225" rpy="0 0 0"/> <limit lower="-0.008" upper="0.08" effort="20.0" velocity="5.0"/> <axis xyz="1 0 0"/> <dynamics damping="0.7" friction="0.0"/> </joint>

<link name="${prefix}gripper_finger2">
  <visual>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <geometry>
  <!--box size="0.015 0.0229 0.0475"/-->
      <mesh filename="package://cyton_gamma_1500_description/meshes/gripper_finger2.dae"/>
    </geometry>
  </visual>
  <collision name="${prefix}gripper_finger2_collision">
    <origin xyz="0 0 0" rpy="0 0 0" />
    <geometry>
      <box size="0.015 0.0229 0.1"/><!--0.0475-->
  <!--mesh filename="package://cyton_gamma_1500_description/meshes/gripper_finger2.dae"/-->
    </geometry>
  </collision>
  <inertial>
    <mass value="0.01"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <inertia ixx=".0000027" iyy=".0000027" izz=".0000027"
              ixy="0" ixz="0" iyz="0"/>
  </inertial>
</link>

<gazebo reference="${prefix}gripper_finger2">
  <kp>1000000.0</kp>
  <kd>1.0</kd>
  <mu1>200.0</mu1>
  <mu2>100.0</mu2>
  <minDepth>0.001</minDepth>
</gazebo>



<transmission name="tran9">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="${prefix}gripper_joint2">
      <hardwareInterface>EffortJointInterface</hardwareInterface>
    </joint>
    <actuator name="motor9">
      <hardwareInterface>EffortJointInterface</hardwareInterface>
    </actuator>

This is the updated urdf of the gripper

  <?xml version="1.0"?>
<robot name="mug">
<link name="coke_can">
    <inertial>
      <mass value="0.1 ...
(more)
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Comments

please post your world file with your gripper demonstrating the issue so we can help

Peter Mitrano gravatar imagePeter Mitrano ( 2016-06-21 10:57:47 -0500 )edit

1 Answer

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answered 2016-06-21 13:38:36 -0500

hsu gravatar image

updated 2016-06-21 13:40:36 -0500

Some suggestions:

1) add following blocks to your urdf for every joint:

  <gazebo reference="[joint name]">
    <implicitSpringDamper>1</implicitSpringDamper>
  </gazebo>

Sometimes I think we should have these settings on by default.

Also, try to come up with as realistic as possible material properties kp, mu1, mu2. Set contact dissipation coefficient kd to 1 to help stabilize contacts. Also, setting minDepth margin to a small non-zero number prevents contact from jumping in and out of existence. For example:

<gazebo reference="[links in contact]">
  <kp>1000000.0</kp>
  <kd>1.0</kd>
  <mu1>1.0</mu1>
  <mu2>1.0</mu2>
  <minDepth>0.001</minDepth>
</gazebo>

2) The cylinder has mass value of 0.1kg, but Ixx of 1.0 kg m^2, which is not very physically realistic. One should always start with a more physically realistic object model.

3) As a rule of thumb, if the inertia ratio across contact constraint formed by the object and the gripper finger is on the order of or greater than 1e3, consider increasing number of inner iterations from default of 50, do a quick ad-hoc search by doubling iterations and look for improvement in behavior. Alternatively, plot iterations vs rms_error and check trend.

4) Consider increasing joint damping for the fingers. Without modeling high gear ratio gearbox joints and the underlying motor and gear inertia, the stabilizing effects of such mechanisms is easiest replaced by larger joint damping in the time being. Physically, my go to damping coefficient for triaging a hard to converge system ranges anywhere between 0.1 to 30 (effort/joint_vel). Until such time validation or more accurate model data are available.

5) Are the joint effort limits realistic? 0.1N seems low for holding up a 0.1kg cylinder (under gravity, downward force is 9.81N).

6) Consider tuning the PID gains of your controller. We can go into details on this as needed.

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Comments

Thanks for such a detailed answer. But could you elaborate on point 3. For, example what exactly are inner iterations and rms_error

rilch gravatar imagerilch ( 2016-06-21 14:09:49 -0500 )edit

It still doesn't work! When ever gripper gets a grasp on the object it slips over it while lifting and other times object passes through the gripper

rilch gravatar imagerilch ( 2016-06-22 07:15:34 -0500 )edit

I am having the same issue. @rilch did you manage to solve this?

hamzamerzic gravatar imagehamzamerzic ( 2017-06-15 07:50:12 -0500 )edit

I am having the same issue. [@rilch](http://answers.gazebosim.org/users/2676/rilch/) did you manage to solve this?

hamzamerzic gravatar imagehamzamerzic ( 2017-06-15 07:50:55 -0500 )edit
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Asked: 2016-06-21 09:15:33 -0500

Seen: 2,406 times

Last updated: Jun 22 '16