AWS DeepRacer simulation: Car control

Roadmap

• Setup
• Launch race track in Gazebo
• Car model
• Car control
  • Controllers
  • Add car control step by step with ros_controls
• Teleoperation

Controllers

Differential drive controller

The most popular and easiest to implement controller. Transforms angular velocity w and linear velocity v to wheel velocities v_l and v_r. Ideal for small robots and robot vacuums.

It already has implementation in ROS1 and Gazebo, you just need to add libdiffdrive_plugin.so, set distance between wheels, wheel radius, what URDF joints to use, etc. Then send v and w through cmd_vel topic and that’s pretty much all.

But….. Our car model has six continuous joints instead of two:

• left steering hinge joint
• right steering hinge joint
• front left wheel joint
• front right wheel joint
• rear left wheel joint
• rear right wheel joint

Ackermann steering drive controller

This controller is good for bicycles and cars. It should transform speed s and steering angle of the virtual center wheel φ, like on a tricycle, to two steering and two wheel commands.

ROS1 has an implementation of this controller, but it:

• uses cmd_vel topic with Twist message instead of AckermannDrive
• doesn’t work good with more than two wheels

So we will implement our version of this controller. A good tutorial shows us how simulating a robot’s controllers in Gazebo can be accomplished using ros_control and a simple Gazebo plugin adapter.

Add car control step by step with ros_controls

Add transmission elements to a URDF as macros. The most important part is hardwareInterface. Read more about transmission and hardware interfaces.

<!-- deep_ws/src/deepracer_car/urdf/xacro/macro/macros.xacro -->
  
  <xacro:macro name="wheel_transmission" params="name">
    <transmission name="${name}_transmission" type="SimpleTransmission">
      <type>transmission_interface/SimpleTransmission</type>
      <joint name="${name}_joint">
       <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
      </joint>
      <actuator name="${name}_motor">
       <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
        <mechanicalReduction>1</mechanicalReduction>
      </actuator>
    </transmission>
  </xacro:macro>

  <xacro:macro name="steering_hinge_transmission" params="name">
    <transmission name="${name}_transmission" type="SimpleTransmission">
      <type>transmission_interface/SimpleTransmission</type>
      <joint name="${name}_joint">
       <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
      </joint>
      <actuator name="${name}_motor">
       <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        <mechanicalReduction>1</mechanicalReduction>
      </actuator>
    </transmission>
  </xacro:macro>

Add the gazebo_ros_control plugin that actually parses the transmission tags and loads the appropriate hardware interfaces and controller manager

<!-- deep_ws/src/deepracer_car/urdf/xacro/control/deepracer_ros_control.xacro -->
  <gazebo>
    <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
      <robotNamespace>/</robotNamespace>
      <robotSimType>gazebo_ros_control/DefaultRobotHWSim</robotSimType>
      <legacyModeNS>true</legacyModeNS>
    </plugin>
  </gazebo>

Create a deep_ws/src/deepracer_car/config/racecar_control.yaml config file

# Publish all joint states -----------------------------------
joint_state_controller:
  type: joint_state_controller/JointStateController
  publish_rate: 60

# add four wheel controllers
left_rear_wheel_velocity_controller:
  type: velocity_controllers/JointVelocityController
  joint: left_rear_wheel_joint
  pid: {p: 1.0, i: 0.0, d: 0.0, i_clamp: 0.0}

# add two steering controllers
right_steering_hinge_position_controller:
  type: position_controllers/JointPositionController
  joint: right_steering_hinge_joint
  pid: {p: 1.0, i: 0.0, d: 0.5}

Also add PID gains for gazebo_ros_control in that same config file

gazebo_ros_control/pid_gains:
  left_rear_wheel_joint: {p: 0.1, i: 0.0, d: 0.0, i_clamp: 0.0}
  right_rear_wheel_joint: {p: 0.1, i: 0.0, d: 0.0, i_clamp: 0.0}
  left_front_wheel_joint: {p: 0.1, i: 0.0, d: 0.0, i_clamp: 0.0}
  right_front_wheel_joint: {p: 0.1, i: 0.0, d: 0.0, i_clamp: 0.0}
  left_steering_hinge_joint: {p: 0.05, i: 0.0, d: 0.0}
  right_steering_hinge_joint: {p: 0.05, i: 0.0, d: 0.0}

And finally create a racecar_control.launch file

<!-- deep_ws/src/deepracer_car/launch/racecar_control.launch -->
<?xml version='1.0'?>
<launch>
<!-- Load joint controller configurations from YAML file to parameter server -->
  <rosparam file="$(find deepracer_car)/config/racecar_control.yaml" command="load"/>

  <!-- convert joint states to TF transforms for rviz, etc -->
  <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher" output="screen"/>

  <!-- The controller_spawner node starts controllers by running a python script that makes a service call to the ros_control controller manager. The service calls tell the controller manager which controllers you want. It also loads a controller that publishes the joint states of all the joints with hardware_interfaces and advertises the topic on /joint_states. The spawner is just a helper script for use with roslaunch. -->
  <node name="controller_manager" pkg="controller_manager" type="spawner" respawn="false"  
  output="screen" args="left_rear_wheel_velocity_controller
              right_rear_wheel_velocity_controller
              left_front_wheel_velocity_controller
              right_front_wheel_velocity_controller
              left_steering_hinge_position_controller
              right_steering_hinge_position_controller
              joint_state_controller"/>
</launch>

Fantastic. Now include this file to racecar.launch

<!-- deep_ws/src/deepracer_car/launch/racecar.launch -->
<include file="$(find deepracer_car)/launch/racecar_control.launch"/>

and we can start simulation with control

roslaunch simulation simulation.launch

and even send commands to each joint

rostopic pub -1 /left_rear_wheel_velocity_controller/command std_msgs/Float64 "data: 1.5"

So far, so good. But do we really need to send 6 commands to control a car?

Control node

Lets add a node for more convinient control. Create a control_deepracer_car.py in a deep_ws/src/deepracer_car/scripts folder. Make a CarController with six command publishers and one AckermannDriveStamped subscriber

#!/usr/bin/env python3
import rospy
from ackermann_msgs.msg import AckermannDriveStamped
from std_msgs.msg import Float64MultiArray, Float64

from collections import OrderedDict
import math
import threading
      
class CarController():

    def __init__(self):
        self.max_speed = rospy.get_param("~max_speed", 4.0)
        self.max_steering_angle = rospy.get_param("~max_steering_angle", 0.523599)
        self._wheel_radius = rospy.get_param("~wheel_radius", 0.03)
        self._wheel_separation = rospy.get_param("~wheel_separation", 0.159202)
        self._wheel_base = rospy.get_param("~wheel_base", 0.164023)
        self.update_rate = rospy.get_param("~update_rate", 50) # Hz

        self._cmd_lock = threading.Lock()
        # Car speed (m/s)
        self.speed = 0
        # Steering angle (rad)
        self.steering_angle = 0
        # Zero steering angle velocity means change 
        # the steering angle as quickly as possible.
        self.steering_angle_velocity = 0

        # Create publishers for controlling the car
        self._velocity_pub_dict_ = OrderedDict()
        self._steering_pub_dict_ = OrderedDict()

        # 4 wheel publishers
        self._velocity_pub_dict_["l_rear_wheel"] = rospy.Publisher('/left_rear_wheel_velocity_controller/command', Float64, queue_size=1)
        self._velocity_pub_dict_["r_rear_wheel"] = rospy.Publisher('/right_rear_wheel_velocity_controller/command', Float64, queue_size=1)
        self._velocity_pub_dict_["l_front_wheel"] = rospy.Publisher('/left_front_wheel_velocity_controller/command', Float64, queue_size=1)
        self._velocity_pub_dict_["r_front_wheel"] = rospy.Publisher('/right_front_wheel_velocity_controller/command', Float64, queue_size=1)
        
        # 2 steering publishers
        self._steering_pub_dict_['left'] = rospy.Publisher('/left_steering_hinge_position_controller/command', Float64, queue_size=1)
        self._steering_pub_dict_['right'] = rospy.Publisher('/right_steering_hinge_position_controller/command', Float64, queue_size=1)

        self.cmd_sub = rospy.Subscriber("/ackermann_cmd", AckermannDriveStamped,
                             self.ackermann_cmd_cb, queue_size=1)

        self.control()

if __name__ == '__main__':
    try:
        rospy.init_node('control_deepracer_car', anonymous=True, log_level=rospy.INFO)
        node = CarController()
    except KeyboardInterrupt:
        print("Shutting down ROS control_deepracer_car node")

Subscriber will update current speed and angle from ackermann_cmd topic

    def ackermann_cmd_cb(self, msg):
        with self._cmd_lock:
            self.speed = msg.drive.speed
            self.steering_angle = msg.drive.steering_angle
            self.steering_angle_velocity = msg.drive.steering_angle_velocity

Control will spin the node with a set rate

    def control(self):
        rate = rospy.Rate(self.update_rate) 
        update_period = 1/self.update_rate
        
        last_time = rospy.get_time()

        while not rospy.is_shutdown():
            t = rospy.get_time()
            delta_t = t - last_time
            
            if delta_t>=update_period:
                #  Calculate target velocity and position values and publish the commands
                with self._cmd_lock:
                    t_speed, t_left_steering, t_right_steering = self.calc_target_speed_steering(delta_t)
                self.publish_commands(t_speed, t_left_steering, t_right_steering)
                last_time = t

            rate.sleep()

will also calculate target speed and steering, ignore numbers out of bounds stabilize a stopped car

    def calc_target_speed_steering(self, delta_t):
        
        # don't go backwards for speed < 0
        target_speed = max(min(self.max_speed, self.speed), 0)
        target_steer_angle = max(min(self.max_steering_angle, self.steering_angle), -self.max_steering_angle)

        tanSteer = math.tan(target_steer_angle)

        t_left_steering = math.atan2(tanSteer, 1.0 - self._wheel_separation / 2.0 / self._wheel_base * tanSteer)
        t_right_steering = math.atan2(tanSteer, 1.0 + self._wheel_separation / 2.0 / self._wheel_base * tanSteer)

        t_speed = target_speed / self._wheel_radius
        
        if self.steering_angle_velocity == 0:
            t_left_steering = t_right_steering = target_steer_angle

        # when speed==0 center wheels, else a car will spin
        if t_speed==0:
            t_left_steering = t_right_steering = 0

        return t_speed, t_left_steering, t_right_steering

and will finally publish wheel and steering commands to the corresponding command topics

    def publish_commands(self, t_speed, t_left_steering, t_right_steering):
        '''Publishes the given action to all the topics in the given dicts
        velocity_pub_dict - Dictionary containing all the velocity joints
        steering_pub_dict - Dictionary containing all the movable joints
        t_left_steering, t_right_steering - Desired amount, in radians, to move the movable joints by
        t_speed - Angular velocity which the velocity joints should rotate with
        '''

        for _, pub in self._velocity_pub_dict_.items():
            pub.publish(t_speed)

        self._steering_pub_dict_['left'].publish(t_left_steering)
        self._steering_pub_dict_['right'].publish(t_right_steering)

Next: Teleoperation