I am trying to simulate a robot I do not want fixed to the Gazebo world I am spawning it into. However, when I make the joint between my base link and the world floating, I get unexpected behavior in the way children frames of reference are established.
To preface, here is all the relevant XACRO
<?xml version="1.0"?> <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="frye_arm_1"> <xacro:include filename="/home/jackfrye/Documents/Robotics/workspace/src/frye_arm_1/urdf/base_wide.xacro" /> <xacro:include filename="/home/jackfrye/Documents/Robotics/workspace/src/frye_arm_1/urdf/polou_4801.xacro" /> <link name="world"></link> <joint name="world_to_base" type="floating"> <parent link="world" /> <child link="a_base_wide" /> <origin rpy="0 0 0" xyz="0 0 0" /> </joint> <xacro:base_wide prefix="a" x="0.0" y="0.0" z="0.374" /> <joint name="base_to_motor_body" type="fixed"> <parent link="a_base_wide" /> <child link="a_robot_body" /> <origin rpy="0 0 0" xyz="0 0 .359" /> </joint> <xacro:polou_4801 prefix="a" x="0.0" y="0.0" z="0.0" phi="0.0" theta="0.0" psi="0.0" /> </robot> base_wide.xacro
<?xml version="1.0"?> <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="frye_arm_1"> <xacro:macro name="base_wide" params="prefix x y z"> <link name="${prefix}_base_wide"> <visual> <origin xyz="${x} ${y} ${z}" rpy="0 0 0" /> <geometry> <mesh filename="file:///home/jackfrye/Documents/Robotics/FryeArm1/Mechanical/ROS2_Prep/base_widen/base_widen_x10.dae"/> </geometry> <material name="blue"/> </visual> <collision> <origin xyz="${x} ${y} ${z}" rpy="0 0 0"/> <geometry> <mesh filename="file:///home/jackfrye/Documents/Robotics/FryeArm1/Mechanical/ROS2_Prep/base_widen/base_widen_x10.dae"/> </geometry> </collision> <inertial> <origin xyz="${x} ${y} ${z}" rpy="0 0 0"/> <mass value="131.0971" /> <inertia ixx="0.1811" ixy="0.0" ixz="0.0" iyy="0.1809" iyz="0.0002" izz="0.18" /> </inertial> </link> </xacro:macro> </robot> polou_4801.xacro
<?xml version="1.0"?> <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="frye_arm_1"> <xacro:macro name="polou_4801" params="prefix x y z phi theta psi"> <link name="${prefix}_robot_body"> <visual> <origin xyz="${x} ${y} ${z}" rpy="${phi} ${theta} ${psi}" /> <geometry> <mesh filename="file:///home/jackfrye/Documents/Robotics/FryeArm1/Mechanical/ROS2_Prep/polou_4801_body/polou_4801_body_x10_no_wire.dae"/> </geometry> <material name="blue"/> </visual> <collision> <origin xyz="${x} ${y} ${z}" rpy="${phi} ${theta} ${psi}" /> <geometry> <mesh filename="file://file:///home/jackfrye/Documents/Robotics/FryeArm1/Mechanical/ROS2_Prep/polou_4801_body/polou_4801_body_x10_no_wire.dae"/> </geometry> </collision> <inertial> <origin xyz="${x} ${y} ${z}" rpy="${phi} ${theta} ${psi}" /> <mass value="94" /> <inertia ixx="3.4438" ixy="0.0" ixz="0.0" iyy="3.4438" iyz="0.0" izz="0.7338" /> </inertial> </link> <joint name="${prefix}_body_to_rotor" type="revolute"> <parent link="${prefix}_robot_body" /> <child link="${prefix}_robot_rotor" /> <axis xyz="0 0 1" /> <!-- <origin xyz="${x} ${y} ${z}" rpy="${phi} ${theta} ${psi}" /> --> <origin xyz="0 0 0.374" rpy="0 0 0" /> <limit lower="0" upper="${4*pi}" effort="10" velocity="3"/> </joint> <link name="${prefix}_robot_rotor"> <visual> <!-- <origin xyz="${x} ${y} ${z+0.75}" rpy="${phi} ${theta} ${psi}" /> --> <origin xyz="${x} ${y} ${z}" rpy="0 0 0" /> <geometry> <mesh filename="file:///home/jackfrye/Documents/Robotics/FryeArm1/Mechanical/ROS2_Prep/polou_4801_shaft/polou_4801_shaft_x10.dae"/> </geometry> <material name="blue"/> </visual> <collision> <!-- <origin xyz="${x} ${y} ${z+0.75}" rpy="${phi} ${theta} ${psi}" /> --> <origin xyz="${x} ${y} ${z}" rpy="0 0 0" /> <geometry> <mesh filename="file:///home/jackfrye/Documents/Robotics/FryeArm1/Mechanical/ROS2_Prep/polou_4801_shaft/polou_4801_shaft_x10.dae"/> </geometry> </collision> <inertial> <!-- <origin xyz="${x} ${y} ${z+0.75}" rpy="${phi} ${theta} ${psi}" /> --> <origin xyz="${x} ${y} ${z}" rpy="0 0 0" /> <mass value=".05" /> <inertia ixx="0.00000" ixy="0.0" ixz="0.0" iyy="0.0000" iyz="0.0" izz="0.0000" /> </inertial> </link> </xacro:macro> </robot> Now in the top level for the joint "world_to_base", when I used floating, which is the setting I want to use, it seems like the transforms are not being done as expected. If I say I want to make a joint at 0.374 +Z, that means I want that joint at +.374 from the parent's frame of reference. If I create a floating joint the move the child +Z .359, that should create the frame of reference from which the joint is created. In that case, the joint would then be at +.359+.374 in the "world" frame. When I do a floating joint, I do not see this behavior.
However, when I used fixed for the connection between my base link and world, I do see this relationship in the link-joint tree.
It makes it difficult to write portable XACRO if I cannot assume that my make first joint in the XACRO (eg the motor body joint) is going to be offset WRT to its parent not and not world. Maybe I just need to refactor and add logic into the polou xacro that takes into account that all joints are being layed out wrt the world frame, but I would prefer not to and just have it calculate like it does for the "fixed" case.
