The Collision Tags will be the same as that for the Visual Tag.

Code for the Collision Tag for the base_link:

    <link name="base_link">
        <visual>
            <geometry>
                <box size="${base_length} ${base_width} ${base_height}" />
            </geometry>
            <origin xyz="0 0 ${base_height / 2.0}" rpy="0 0 0" />
            <material name="blue" />
        </visual>
        <collision>
            <geometry>
                <box size="${base_length} ${base_width} ${base_height}" />
            </geometry>
            <origin xyz="0 0 ${base_height / 2.0}" rpy="0 0 0" />        
        </collision>
        <xacro:box_inertia m="5.0" l="${base_length}" w="${base_width}" h="${base_height}"
                           xyz="0 0 ${base_height / 2.0}" rpy="0 0 0" />
    </link>

Code for the Collision Tag for the wheel_link:

    <xacro:macro name="wheel_link" params="prefix">
        <link name="${prefix}_wheel_link">
            <visual>
                <geometry>
                    <cylinder radius="${wheel_radius}" length="${wheel_length}" />
                </geometry>
                <origin xyz="0 0 0" rpy="${pi / 2.0} 0 0" />
                <material name="grey" />
            </visual>
            <collision>
                <geometry>
                    <cylinder radius="${wheel_radius}" length="${wheel_length}" />
                </geometry>
                <origin xyz="0 0 0" rpy="${pi / 2.0} 0 0" />
            </collision>
            <xacro:cylinder_inertia m="1.0" r="${wheel_radius}" h="${wheel_length}" 
                                    xyz="0 0 0" rpy="${pi / 2.0} 0 0" />
        </link>
    </xacro:macro>

    <xacro:wheel_link prefix="right" />
    <xacro:wheel_link prefix="left" />

Code for the Collision Tag for the caster_wheel_link:

    <link name="caster_wheel_link">
        <visual>
            <geometry>
                <sphere radius="${wheel_radius / 2.0}" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <material name="grey" />
        </visual>
        <collision>
            <geometry>
                <sphere radius="${wheel_radius / 2.0}" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
        </collision>
        <xacro:sphere_inertia m="0.5" r="${wheel_radius / 2.0}"
                              xyz="0 0 0" rpy="0 0 0" />
    </link>