The course
Process Control
is taught during the eighth
semester in the Master of Science program in Engineering Physics at
Uppsala university. In this course, groups of four students use Matlab
to design and implement a real time control system.
The students use Java to design a graphical user interface for
their controller. Via this interface, it should be possible to view
the state of the process and interactively change some of the most
important parameters. The graphical user interface communicates with
the controller over the network.
The controller is (of course) designed using Matlab. What is new is
that the real-time control loop is implemented
in Matlab, using
a single m-file. Execution of the control loop is initiated by a timer
once per sampling interval. At the end of the control loop iteration,
the Matlab process enters a waiting state, which is terminated by the
timer at the beginning of the next sampling interval.
The Matlab process executing the m-file runs under
Linux, a
public domain Unix dialect available for e.g. Intel PC:s. The Linux
kernel has been extended with real-time capabilities. Unix is not an
ideal operating system for real-time applications, but for our
purposes it is quite adequate. By using Linux, all features of a
Unix system can be used, including built-in IP support.
The students implement the graphical user interface as a Java
applet, which can be run either using a WWW browser or a
so-called applet viewer. Matlab is not used in this case, since
the real-time update of the complex interface would be too
time-consuming. Also, Java is a more powerful language for
implementation of graphical user interfaces. Last, but not least,
there is currently a great demand for experienced Java programmers.
The graphical user interface presents important properties about the
running control process, e.g.
- current values of output signals, control signals and reference
signals.
- current type of controller.
- possible alarm events.
- current values of regulator parameters.
From the interface, it should also be possible to interactively change
certain of the above entities.
To communicate, the Matlab process and the Java applet exchange
UDP packets. These packets are transmitted
from one of the two
process over Ethernet to a specific port on a specific computer. This
makes it possible for the controller and the user interface to reside
on different computers. Such an arrangement has two advantages:
- One computer can be dedicated to running the computationally
intensive controller loop.
- The separation makes the distinction between the two tasks
clearer.
From the control process, Matlab arrays with elements of type
``double'' are sent to the user interface process, where they are
received as two-dimensional Java arrays of ``doubles''. Up to four
matrices can be sent using a single Matlab instruction. An arbitrary
string can be sent along with the matrices, containing information
about the matrices. When sending data in the opposite direction, from
the user interface to the controller loop, two-dimensional arrays of
doubles are sent from the Java applet to the Matlab control loop,
where they are received as two-dimensional Matlab arrays of doubles.
Also, communication between two Matlab processes is possible using
these UDP functions.
By using Matlab for both design and implementation, the development
process could be simplified considerably. The final results were very
encouraging, and the problems we had feared due to real-time timing
constraints could be avoided completely: even with an indirect adaptive
controller the controller loop lasted less than 10 ms on a 200 MHz
Pentium Pro computer.
