Execute Code Based on ROS Time

Using a rosrate object allows you to control the rate of your code execution based on the ROS Time /clock topic or system time on your computer. By executing code at constant intervals, you can accurately time and schedule tasks. These examples show different applications for the rosrate object including its uses with ROS image messages and sending commands for robot control.

For other applications based on system time, consider the rateControl (Navigation Toolbox) object.

Send Fixed-rate Control Commands To A Robot

This example shows to send regular commands to a robot at a fixed rate. It uses the Rate object to execute a loop that publishes std_msgs/Twist messages to the network at a regular interval.

Setup ROS network. Specify the IP address if your ROS network already exists.

rosinit

Launching ROS Core...
Done in 0.62554 seconds.
Initializing ROS master on http://172.30.144.24:53841.
Initializing global node /matlab_global_node_20860 with NodeURI http://dcc796195glnxa64:45171/ and MasterURI http://localhost:53841.

Setup publisher and message to send Twist commands.

[pub,msg] = rospublisher('/cmd_vel','geometry_msgs/Twist');
msg.Linear.X = 0.5;
msg.Angular.Z = -0.5;

Create Rate object with specified loop parameters.

desiredRate = 10;
rate = robotics.Rate(desiredRate);
rate.OverrunAction = 'drop'

rate = 
  rateControl with properties:

         DesiredRate: 10
       DesiredPeriod: 0.1000
       OverrunAction: 'drop'
    TotalElapsedTime: 0.1214
          LastPeriod: NaN

Run loop and hold each iteration using waitfor(rate). Send the Twist message inside the loop. Reset the Rate object before the loop to reset timing.

reset(rate)

while rate.TotalElapsedTime < 10
   send(pub,msg)
   waitfor(rate);
end

View statistics of fixed-rate execution. Look at AveragePeriod to verify the desired rate was maintained.

statistics(rate)

ans = struct with fields:
              Periods: [0.1058 0.0944 0.1000 0.1001 0.0999 0.1000 0.1000 0.1000 0.1000 0.1000 0.0999 0.1000 0.1000 0.1001 0.0999 0.1001 0.0999 0.1000 0.1000 0.1000 0.0999 0.1002 0.0998 0.1000 0.1001 0.1000 0.1000 0.1000 0.1000 0.1002 ... ] (1x100 double)
           NumPeriods: 100
        AveragePeriod: 0.1000
    StandardDeviation: 8.8298e-04
          NumOverruns: 0

Shut down ROS network.

rosshutdown

Shutting down global node /matlab_global_node_20860 with NodeURI http://dcc796195glnxa64:45171/ and MasterURI http://localhost:53841.
Shutting down ROS master on http://172.30.144.24:53841.

Fixed-rate Publishing of ROS Image Data

This example shows how to regularly publish and receive image messages using ROS and the rosrate function. The rosrate function creates a Rate object to regularly access the /camera/rgb/image_raw topic on the ROS network using a subscriber. The rgb image is converted to a grayscale using rgb2gray and republished at a regular interval. Parameters such as the IP address and topic names vary with your robot and setup.

Connect to ROS network. Setup subscriber, publisher, and data message.

ipaddress = '192.168.203.129'; % Replace with your network address
rosinit(ipaddress)

Initializing global node /matlab_global_node_10650 with NodeURI http://192.168.203.1:50899/

sub = rossubscriber('/camera/rgb/image_raw');
pub = rospublisher('/camera/gray/image_gray','sensor_msgs/Image');
msgGray = rosmessage('sensor_msgs/Image');
msgGray.Encoding = 'mono8';

Receive the first image message. Extract image and convert to a grayscale image. Display grayscale image and publish the message.

msgImg = receive(sub);

img = readImage(msgImg);
grayImg = rgb2gray(img);
imshow(grayImg)

writeImage(msgGray,grayImg)
send(pub,msgGray)

Create ROS Rate object to execute at 10 Hz. Set a loop time and the OverrunAction for handling

desiredRate = 10;
loopTime = 5;
overrunAction = 'slip';
rate = rosrate(desiredRate);
r.OverrunAction = overrunAction;

Begin loop to receive, process and send messages every 0.1 seconds (10 Hz). Reset the Rate object before beginning.

reset(rate)

for i = 1:desiredRate*loopTime
    
    msgImg = receive(sub);

    img = readImage(msgImg);
    grayImg = rgb2gray(img);
    writeImage(msgGray,grayImg)

    send(pub,msgGray)
    
    waitfor(rate);
end

View the statistics for the code execution. AveragePeriod and StandardDeviation show how well the code maintained the desiredRate. OverRuns occur when data processing takes longer than the desired period length.

statistics(rate)

ans = struct with fields:
              Periods: [1×50 double]
           NumPeriods: 50
        AveragePeriod: 0.1000
    StandardDeviation: 0.0083
          NumOverruns: 0

Shut down ROS node

rosshutdown

Shutting down global node /matlab_global_node_10650 with NodeURI http://192.168.203.1:50899/