JAVA-BASED REAL-TIME PROGRAMMING

trolled periodically to prepare the material for casting with the proper
quality.
2. The piston of the cylinder should be controlled sequentially to actually
perform the casting.
Note that the tasks according to items 1 and 2 need to be handled concurrently,
and each task has to be carried out in real time. Otherwise the machine will
not work.
Item 1 is a simple temperature regulator. Since reading the measured
value (temp) and computing the control signal (doHeat) takes some time,
such feedback control, when done in software, by necessity has to be carried
out periodically. In each period (also called a sample), the controlled system in
measured (sampled), and the computed control signal is sent to the controlled
system. For reasons of control performance, the time between each sample
should not be too long. For computing (CPU-load) reasons, we want to have
as long sampling period as possible. Hence, a tradeoff between control and
computing efficiency has to be made. This type of control is time driven since
it is run with respect to (real) time without waiting for conditions/events.
Item 2 is an example of sequencing control, which is event driven (execution
advances when certain events occurs and arrives to the software); writing
this as a periodic controller would imply unnecessary sampling and evaluation
of the state of the application. On the other hand, if execution only advances
on incoming events, how should then the temperature control be carried out
(in the same program)?
The reader could try to write one sequential program performing both
these tasks. Note that neither the temperature control nor the sequencing
doPush
doHeat
~
doPull
isIn
isOut
temp
isEmpty
Figure 1.1: Schematic figure of a machine for manufacturing of LEGO bricks
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