'what(): Actuation input port for model instance XXX must be connected

I am trying to build and simulate a simple diff-drive robot under drake to get familiar with the components offered. I created a simple URDF which bases on existing PR2 URDF. I load the model with AddMultibodyPlantSceneGraph. With following snippet,

  std::string vox_nav_drake_ros_package_path =
    ament_index_cpp::get_package_share_directory("vox_nav_drake_ros");

  drake::systems::DiagramBuilder<double> builder;
  auto [plant, scene_graph] =
    drake::multibody::AddMultibodyPlantSceneGraph(&builder, 0.1 /* time_step */);
  const std::string & pathname = vox_nav_drake_ros_package_path + "/urdf/botanbot.urdf";
  drake::multibody::Parser parser(&plant);
  parser.AddModelFromFile(pathname);
  plant.Finalize();

  auto context = plant.CreateDefaultContext();

  auto zero_torque =
    builder.AddSystem<drake::systems::ConstantVectorSource<double>>(
    Eigen::VectorXd::Zero(3));

  builder.Connect(
    zero_torque->get_output_port(),
    plant.get_actuation_input_port());

  auto diagram = builder.Build();

  drake::systems::Simulator<double> simulator(plant);
  simulator.Initialize();
  while (1) {
    simulator.AdvanceTo(simulator.get_context().get_time() + 0.1);
  }

The error says that I need to connect the Actuation input port for model instance, which I already do, but the error persists and I don't understand why.

Here is the URDF I use;

<?xml version="1.0"?>
<robot name="botanbot">
    <!-- BEGIN LINKS                      -->
    <!-- BEGIN LINKS RELATED TO ROBOT BODY-->
    <link name="base_link">
        <visual>
            <origin xyz="-0.019375 0.005287 0.340756" rpy="0 0 0" />
            <geometry>
                <mesh filename="/home/atas/colcon_ws/install/vox_nav_drake_ros/share/vox_nav_drake_ros/meshes/base_simplified.obj" />
            </geometry>
            <material name="Cyan">
                <color rgba="0 1.0 1.0 1.0" />
            </material>
        </visual>

        <inertial>
            <mass value="1326.0" />
            <inertia ixx="2581.13354740" ixy="0" ixz="0" iyy="591.30846112" iyz="0" izz="2681.9500862" />
        </inertial>
    </link>

    <!-- END JOINTS RELATED TO ROBOT BODY -->
    <!--//////////////////////////////////-->
    <!-- END JOINTS                       -->

    <link name="base_footprint">
        <inertial>
            <mass value="1.0" />
            <origin xyz="0 0 0" />
            <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <box size="0.01 0.01 0.01" />
            </geometry>
            <material name="Cyan">
                <color rgba="0 1.0 1.0 1.0" />
            </material>
        </visual>
    </link>

    <joint name="base_footprint_joint" type="fixed">
        <origin rpy="0 0 0" xyz="0 0 0.051" />
        <child link="base_link" />
        <parent link="base_footprint" />
    </joint>

    <link name="base_link_0" />
    <link name="base_link_1" />
    <!-- RigidBodyTree only supports fixed or unactuated joints to the
         world.  Prismatic joints with a transmission don't work
         correctly. -->
    <link name="base_link_for_rbt_compat" />
    <joint name="world_joint_for_rbt_compat" type="fixed">
        <parent link="world" />
        <child link="base_link_for_rbt_compat" />
    </joint>
    <joint name="x" type="prismatic">
        <parent link="base_link_for_rbt_compat" />
        <child link="base_link_0" />
        <axis xyz="1 0 0" />
    </joint>
    <joint name="y" type="prismatic">
        <parent link="base_link_0" />
        <child link="base_link_1" />
        <axis xyz="0 1 0" />
    </joint>
    <joint name="theta" type="revolute">
        <parent link="base_link_1" />
        <child link="base_footprint" />
        <axis xyz="0 0 1" />
        <limit effort="100" lower="-3.14" upper="3.14" velocity="100" />
    </joint>

    <transmission name="x_tran">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="x">
            <hardwareInterface>PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="x_motor">
            <hardwareInterface>PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>

    <transmission name="y_tran">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="y">
            <hardwareInterface>PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="y_motor">
            <hardwareInterface>PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>

    <transmission name="theta_tran">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="theta">
            <hardwareInterface>PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="thata_motor">
            <hardwareInterface>PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>

</robot>
drake


Solution 1:[1]

The problem is that you passed the plant to the Simulator. You need to pass the diagram. (The plant doesn't have the input port connected, only the diagram does).

Sources

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Source: Stack Overflow

Solution Source
Solution 1 Russ Tedrake

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