First Let’s have a look on the URDF File Documentation that is available by ROS:

https://wiki.ros.org/urdf/XML

Following the above link we will get to the URDF File Documentation and if we inspect the joint or link in the Documentation we will find the different Attributes and Elements that is applicable for either of them.

So if we go to the joint Documentation under Attributes→type, we will find the different types that can be applied. Till now we have only used fixed type but revolute type allows us to rotate links around a specified axis.

Revolute Joint Type

Let’s try to implement revolute joint type on the cylinder so that it can rotate.

Note that if we scroll down under Elements we find that it says:

<limit> (required only for revolute and prismatic joint)

The modified code is the following:

<?xml version="1.0"?>
<robot name="my_robot">

    <material name="blue">
        <color rgba="0 0 0.5 1" />
    </material>

    <material name="grey">
        <color rgba="0.5 0.5 0.5 1" />
    </material>

    <link name="base_link">
        <visual>
            <geometry>
                <box size="0.6 0.4 0.2" />
            </geometry>
            <origin xyz="0 0 0.1" rpy="0 0 0" />
            <material name="blue" />
        </visual>
    </link>

    <link name="second_link">
        <visual>
            <geometry>
                <cylinder radius="0.1" length="0.2" />
            </geometry>
            <origin xyz="0 0 0.1" rpy="0 0 0" />
            <material name="grey" />
        </visual>
    </link>

    <joint name="base_second_joint" type="revolute">
        <parent link="base_link" />
        <child link="second_link" />
        <origin xyz="0 0 0.2" rpy="0 0 0" />
        <axis xyz="0 0 1" />
        <limit lower="-1.57" upper="1.57" velocity="100" effort="100" />
    </joint>

</robot>

Explanation of the code:

<axis xyz="0 0 1" /> This line specifies the axis the rotation will take place on.

<limit lower="-1.57" upper="1.57" velocity="100" effort="100" /> This line specifies:

• lower="-1.57"

  • This sets the minimum (lower) limit of the joint's position, typically in radians for a rotational joint or in meters for a prismatic joint.
  • A value of -1.57 radians corresponds to -90 degrees, indicating the joint cannot move below this angle.

• upper="1.57"

  • This sets the maximum (upper) limit of the joint's position.
  • A value of 1.57 radians corresponds to 90 degrees, indicating the joint cannot move beyond this angle.

  Note: These limits apply only to certain joint types, such as:

  • Revolute joints (rotational movement).
  • Prismatic joints (linear movement).
  • Fixed joints and continuous joints do not have position limits.

• velocity="100"

  • This specifies the maximum allowable velocity for the joint, typically in radians per second (for rotational joints) or meters per second (for prismatic joints).
  • In this case, the joint can move at a maximum speed of 100 radians per second or 100 meters per second, depending on the type of joint.

• effort="100"

  • This sets the maximum allowable effort that can be applied to the joint, typically measured in Newton-meters (Nm) for rotational joints or Newtons (N) for prismatic joints.
  • A value of 100 indicates that the joint can handle up to 100 Nm or 100 N of force/torque.