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Fixed joint vs Continuous joint

asked 2019-04-01 13:07:43 -0500

VansFannel gravatar image

updated 2019-04-02 02:39:34 -0500

I'm using Gazebo 9.7.0.

I'm adding a bumper sensor to my robot, and it appears different in Gazebo editor.

When the joint to the chassis is fixed:

image description

It seems that the gripper, the yellow box, it is part of the chassis, the blue rectangle.

And, when the joint is continuous: image description

The gripper appears as a link in the editor.

Which is the best type of joint to a contact sensor?

Maybe, it would be better to set the sensor in the chassis.

And also, if I add a plugin, the topic appears as part of the chassis if the joint is fixed:


The joint is named sensor_joint, and the SDF generated from an URDF Xacro file is:

<?xml version="1.0" encoding="utf-8"?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from nav_odom_description/urdf/mybot.xacro | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot name="mybot" xmlns:xacro="">
  <!-- If a <gazebo> element is used without a reference="" property, it is
       assumed the <gazebo> element is for the whole robot model. -->
    <plugin filename="" name="gazebo_ros_control">
  <gazebo reference="chassis">
  <gazebo reference="caster_wheel">
    <!-- Equals to zero to remove the friction. -->
    <!-- Equals to zero to remove the friction. -->
  <!-- BUMPER -->
  <gazebo reference="gripper">
    <sensor name="bumper_contact_sensor" type="contact">
      <plugin filename="" name="gripper_bumper">
  <material name="black">
    <color rgba="0.0 0.0 0.0 1.0"/>
  <material name="blue">
    <color rgba="0.0 0.0 0.8 1.0"/>
  <material name="green">
    <color rgba="0.0 0.8 0.0 1.0"/>
  <material name="grey">
    <color rgba="0.2 0.2 0.2 1.0"/>
  <material name="orange">
    <color rgba="1.0 0.423529411765 0.0392156862745 1.0"/>
  <material name="brown">
    <color rgba="0.870588235294 0.811764705882 0.764705882353 1.0"/>
  <material name="red">
    <color rgba="0.8 0.0 0.0 1.0"/>
  <material name="white">
    <color rgba="1.0 1.0 1.0 1.0"/>
  <!-- BOX INERTIA -->
  <!-- WHEEL MACRO -->
  <link name="base_link"/>
  <joint name="base_joint" type="fixed">
    <parent link="base_link"/>
    <child link="chassis"/>
  <!-- CHASSIS -->
  <link name="chassis">
      <origin rpy="0 0 0" xyz="0 0 0.1"/>
        <box size="0.4 0.2 0.1"/>
      <origin rpy="0 0 0" xyz="0 0 0.1"/>
        <box size="0.4 0.2 0.1"/>
      <material name="orange"/>
      <origin rpy="0 0 0" xyz="0 0 0.1"/>
      <mass value="50.0"/>
      <inertia ixx="0.208333333333" ixy="0" ixz="0" iyy="0.708333333333" iyz="0" izz="0.708333333333"/>
    </inertial ...
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2 Answers

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answered 2019-04-01 15:01:08 -0500

chapulina gravatar image

I suspect you're using URDF to describe your robot, as opposed to SDF, correct?

Gazebo automatically lumps URDF fixed joints when converting it to SDF. You can prevent that by adding the following to your URDF:

<gazebo reference='joint_name'>

Which is the best type of joint to a contact sensor?

I think it boils down to personal preference, both ways should work.

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Thanks for your answer. I've added your suggestion but I'm still have the same result. The gripper link doesn't appears on Gazebo Editor. I have added the SDF generated to my question.

VansFannel gravatar imageVansFannel ( 2019-04-02 02:20:09 -0500 )edit

answered 2019-11-09 06:51:50 -0500

schulze_18 gravatar image

Hello, maybe it is too late now, but when you define a "fixed" joint, Gazebo will make both links together, removing the child from the list of links. This answer describes this behavior a litter better.

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Asked: 2019-04-01 13:07:43 -0500

Seen: 4,603 times

Last updated: Nov 09 '19