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Controlling the AutoRally Platform

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Brian Goldfain edited this page Jul 29, 2016 · 30 revisions

Introduction

This tutorial describes how to control a physical or simulated AutoRally platform with your own controller.

The AutoRally platform has 3 actuators, all controlled by the same message:

  • steering
  • throttle/rear brake
  • front brake

The autorally_chassis nodelet allows any program to control any actuator by following the steps described below. At any one time, there can be many nodes publishing autorally_msgs/chassisCommand messages to their own chassisCommand topics. The autorally_chassis receives all of these messages and selects the highest priority valid command for each actuator. This design allows different programs to control different actuators at the same time such as independent steering and throttle controllers. This structure supports seamless, automatic switching between controllers according to their priorities by having high priority controllers cede control to lower priority controllers as desired. The autorally_chassis translates each actuator command to a PWM signal taking into consideration the arChassisConfig chassis calibration .yaml file and sends them to the actuators.

alt text You must choose a string name to refer to your controller. The easiest choice is the node name, but it really doesn't matter as long as it is unique. This string will be referred to as YOUR_SERVO_COMMANDER for the rest of the tutorial.

alt text The maximum meaningful control rate for all of the actuators is 50 hz, limited by the PWM period that the actuators accept. There is no torque or position control available through the autorally_chassis for the same reason. If you send control commands faster than the maximum control rate, the most recent message received during the previous time window will be used, and all others will be ignored.

ChassisCommand Message Details

The autorally_chassis will only accept the autorally_msgs/chassisCommand message that satisfies the following format:

  • Header stamp contains current timestamp (msg->header.stamp = ros::Time::now())
  • sender is YOUR_SERVO_COMMANDER
  • Valid actuator values are [-1, 1], anything out of this range signals to the autorally_chassis you do not want to control the actuator
  • steering
    • -1 is full left
    • 0 is straight
    • 1 is full right
  • throttle
    • -1 is full electronic brake
    • 0 is no throttle (coast)
    • 1 is full throttle

alt text Throttle and rear brake are part of the same signal and therefore cannot be controlled independently.

  • frontBrake
    • less than 0 is ignored
    • 0 is no brake
    • 1 is full front brake

How to Control the AutoRally Platform

  • Add YOUR_SERVO_COMMANDER with a unique priority number to your copy of autorally/autorally_util/config/servoCommandPrioritites.yaml. Valid priority values are unique, positive integers. Lower values have higher priority and 0 has the highest priority. Priority values do not have to be sequential.
  • Configure your controller to publish an autorally_msgs/chassisCommand message to the topic /YOUR_SERVO_COMMANDER/chassisCommand at a rate between 10Hz-50Hz with your desired actuator values and configured header
  • If you do not want to control an actuator, set it's respective command value in the chassisCommand message to something outside the valid range (-5 is a good choice)
  • With the autorally_chassis (started as part of autorally.launch) or autorally_controller (started as part of the simulation) and your controller running, verify the chassis interface is subscribed to your chassisCommand topic using rqt_graph.
  • runstopMotionEnabled in the /chassisState topic by autorally_chassis and displayed in the OCS must be 1 to enable autonomous throttle control. The runstopMotionEnabled in /chassisState is an or of all valid runstop messages received. The OCS and joystick nodes both publish runstop messages.

Tips

  • A simple example node that publishes a chassisCommand message is the autorally_control joystick node.
  • The physical and simulation platform are controlled by the same message.
  • For the lowest latency control, design your controller as a nodelet and load it into autorally_core_manager nodelet manager, which is the same nodelet manager the autorally_chassis nodelet loads into. Make sure to follow the ROS Intraprocess Publishing instructions instructions for publishing chassisCommand.

alt text If your control nodelet loaded into the autorally_core_manager nodelet manager crashes, it will take down the autorally_chassis with it.

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