Page 20 - DCAP608_REAL TIME SYSTEMS

Unit 2: Introduction to Real-time Applications

The computer was connected to the plant by mean of A/D and D/A converters in order to obtain Notes
the real-time system.

!

Caution Analog plants should be connected to the computer through A/D and D/A
converters. This link with the “real world” does not lead to a real-time system. On the
other hand, it is possible to find real-time systems in complete digital contexts.
Our plant is so slow that real-time is actually no problem. A slow control system, which does not
need a fast computer, can require critical time constraints. It is also possible that a control system
does no need any hard real-time requirements but it is not necessarily a consequence of the slow
plant.

It is not meaningful to talk about guarantying real-time performance. It is true that occasionally
time constraints can be relaxed without introducing additional problems in the control loop.
This particularly applies to nice designed laboratory experiments. However, this actually depends
on the application, and the time criticality should be proved for each individual case.

!

Caution Real-time performance cannot be 100% guaranteed while hardware and software
failures cannot be avoided at all.

We do not care about real-time in our digital control system and even though it works. This
statement is similar to the previous one. The problem here is that you are not able to know when
your system can fail.
Real-time programming is assembly coding, priority interrupt programming and device driver
writing. It is true that some code is still writing in assembler. However, high programming
languages like C, Ada 95, Modula 2 and Real-time Java are normally used to develop real-time
software. Device driver programming is necessary for real-time as well as non-real-time systems
but they should be provided by the operating system or by the device manufacturer. Interrupt
programming should be in principle avoided as much as possible.

Digital controllers make three assumptions:
 Sensor data give accurate estimates of the state-variables being monitored and controlled
– noiseless

 The sensor data gives the state of the plant – usually must compute plant state from
measured values

 All parameters representing the dynamics of the plant are known.
If any of these assumptions are not valid, a digital controller must include a model of the correct
system behaviour

 Estimate actual state based on noisy measurement of each iteration of the control loop
 Use estimated plant state instead of measured state to derive control output
 Often requires complex calculation, modelling

Task Apply various assumptions to a digital controller and note down the observations
you have.

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