IEEE Robotics & Automation Magazine - June 2015 - 20

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The Desired (X, Y) Position of the Robot
(Colored Blocks Indicate Tunable Parameters)
ROS

Scope

IsNew
Subscribe
Msg
/odom

IsNew
[-6 8]

Subscribe

Desired Position

Desired Position


Position Y







Position X

x

Orientation (i)

Linear
Velocity (v)

w
Angular
Velocity (w)

y
z

Yaw
quat2eul

v

Command Velocity
Publisher

Proportional Controller

w

The /odom Topic Represents Orientation as a
Quarternion, So Convert It to a Yaw Angle
Conversion
Copyright 2014 The MathWorks, Inc.

Figure 1. A robot controller implemented in Simulink with ROS. (Image used with permission from MathWorks.)

preview = bag.select
('Topic', 'preview')
subset = preview.readMessages(500:599)
images = cellfun(@readImage, all, 'UniformOutput',
false);

We could also extract inertial measurement unit (IMU) data and place
the x-axis translational and z-axis angular velocity, for example into a
timeseries object, which automatically picks up the message time stamps
and has various methods for analysis
and display.
imu = bag.select('Topic',
'fcu/imu');
ts = imu.timeseries('Linea
r A c c e l e r a t i o n . X ',
'AngularVelocity.Z');
ts.plot

A number of interactive commands are available in the MATLAB
environment, which mimics the ROS
command line utilities to find messages, nodes, topics, parameters, or
services, e.g.,
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IEEE ROBOTICS & AUTOMATION MAGAZINE

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june 2015

>> rosmsg list
>> rosmsg show geometry_
msgs/TwistStamped
>> rosnode list
>> rostopic list
>> rosparam list
>> rosservice list.

This provides all the programmatic
tools required to write code in MATLAB that can fully participate in an
ROS-based robot control system. A
powerful advantage of MATLAB ROS is
its platform independence-this code
will work on a Mac, Windows, or Linux
system. The real-time update rate is, of
course, going to depend on the size of
the messages, the complexity and efficiency of your MATLAB code, and the
performance of your computer, but tens
of hertz is feasible.
An alternative to programmatic implementation is to use the Simulink block
diagram modeling environment, as
shown in Figure 1. An RST provides a
palette of blocks that includes Publish
and Subscribe. The Msg output of the
Subscribe block is a bus type, and we
can use a Simulink Bus Selector to pull
out the particular message fields in which
we are interested. We use a triggered subsystem to ensure that a message is pub-

lished only after a message is received.
With the appropriate Simulink settings,
this controller can run in "real time" (see
the aforementioned caveats), and the
Scope blocks allow us to conveniently
see what is happening. We can, of course,
log signals to workspace variables for later
analysis and graphical display.
Finally, and most significantly, we can
export this diagram as code. Simulink
generates a .tgz archive file that contains all the code necessary to build a
standalone real-time ROS node on a
Linux system.
In this short article, we can only skim
the surface of the new MATLAB capability for ROS integration. More details
can be found at mathworks.com/ros.
Over time, the RST will gain additional
functionality that will allow students, engineers, and researchers to create robotic
systems more quickly and leverage the
large and mature ROS code base.
References
[1] P. I. Corke, Robotics, Vision and Control: Fundamental Algorithms in MATLAB. Berlin Heidelberg, Germany: Springer-Verlag, 2011.
[2] P. Corke. Robotics toolbox for MATLAB. [Online]. Available: http://www.petercorke.com/robot
http://www.mathworks.com/ros http://www.petercorke.com/robot
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - June 2015
