Creating PID Tuning Graphs

Overview

This page documents a Python program designed to graph the first few seconds of power applied to the motors and robot speed. It is used to help calibrate PID parameters.

I didn't know how to configure rqt_plot to scale the lines reasonable snapshots. For example, entering the follow command resulted in a wheel speed line that was on the zero x-axis because the count measurements dominated the smaller wheel speed measurements. This was solved with a custom Python program described below.

     rqt_plot /cmd_vel/linear/x /wheels/data[1] /wheels/data[2] /phidgets/encoder/351547/count_change

Message Additions to the Motor Controller

Code

A "wheels" message of type Float64MultiArray is published in the setMotorPower function. It has five elements.

• data[0]: time is seconds since the motor controller was started.
• data[1]: wheel 0 speed in m/sec
• data[2]: percent of full power applied to wheel 0
• data[3]: wheel 1 speed
• data[4]: wheel 1 power

Wheel published global declaration

   ros::Publisher wheel_pub_;
   ros::Time beginning_time_;

Publisher initialization in the main.

   wheel_pub_ = n.advertise<std_msgs::Float64MultiArray>("wheels", 100);

Initializing beginning time in InitMotorPower

   beginning_time_ = ros::Time::now();

Publishing a wheel status message in setMotorPower

    std_msgs::Float64MultiArray wheel_data;
    wheel_data.data.clear();
    ros::Time odom_time(odom_.header.stamp.sec, odom_.header.stamp.nsec);
    ros::Duration elapsed_time = odom_time - beginning_time_;
    wheel_data.data.push_back(elapsed_time.toSec());
    wheel_data.data.push_back(wheel0_speed);
    wheel_data.data.push_back(power_wheel0_);
    wheel_data.data.push_back(wheel1_speed);
    wheel_data.data.push_back(power_wheel1_);
    wheel_pub_.publish(wheel_data);

Message Reference

General

• General Messages Primitive Data Types in Messages
• Common Messages ROS Specific Messages
• Standard Messages Primitive Data Messages

Float Array Specific

• 64 bit float array messages Message Detail
• Array message layout
• Array message example

Time

• Time Overview
• Time in a message header
• Header in an odometry message

The Python Plot Code

Plot termination Criteria

There are two termination criteria 1) cnt-c and 2) time. After one of the criteria is met, the graph is drawn.

• Capture Ctl-C in Python
• capture timer event

Plotly Graphing Tools

Installing

• Getting Started

 sudo apt-get install python-pip
 sudo pip install plotly

Authentication

• getting Started
• Creating an Authentication String

See authentication file in

   ~/.plotly/.credentials

Authentication statement in Python code

   plotly.tools.set_credentials_file(username='xxx', api_key='yyy')

Coding

• Coding Lint Charts
• Multiple Y Axis

Working around a bug in iplot. Change it to plot.

• Keyerror
• Keyerror

Code

pid_graph.py

Execution

This is done under load - the robot is on the floor and running.

Terminal 2 - Start up the motor controller with new pid parameters

 roslaunch floor_hugger motor_control_nav.launch

Terminal 3 - Start up odometry

 roslaunch floor_hugger odometry.launch

Terminal 4 - Collect 8 seconds of data and save the plot on the plot.ly site as 153020pidplot. ^c to create graph or wait for 8 seconds for the program to terminate.

 python pid_graph.py 153020pidplot 8
 ^c

Terminal 5 - Publish continous cmd_vel Twist meassages. ^c to terminate and stop motors.

 python motors_go_full.py 0.2 0.0
 ^c

Example Graph

These measurements are under load.

Setting

• P = 15
• I = 15
• Target speed 0.2 m/sec

Observations

• Notice the odometry glitches that report a 0 m/sec wheel speed. See Odometry Glitches.
• It also suggest a power dead zone from 0 to 10%.