| Process Control |
2. Basic Concepts
Let's use an electrically heated tank to define some of the basic concepts in
Process Control.
In the drawing, T and F
are temperature and flow rate, respectively. The subscripts e and
s stand for input and output, respectively. The process goal is to
heat a liquid (initially at a temperature Te) to a desirable
temperature, say TR.
2.1 The designer view
When you design the tank, the requirement is to provide the adequate power in
order to heat the liquid.
Material balance: Fe = Fs = F
Heat balance: Q = F.c.(TR - Te) so that Ts = TR
2.2 The operator view
Since the process does not operate in an unchanging environment, the operator
needs to compensate for external perturbations.
Suppose that Te is subject to perturbation. An approach to
maintaining the process goal may be chosen from those below:
| controlled variable
|
measured variable
|
manipulated variable
|
| TR |
Ts |
Q |
| TR |
Te |
Q |
| TR |
Ts |
F |
| TR |
Te |
F |
| TR |
Te e Ts |
Q |
| TR |
Te e Ts |
F |
Note that in some situations it is possible to attain the goal without control
-> larger capacitance (= larger tank volume)
Feedback control: a comparison between the desired result and the actual result
is used to make a control decision.
Feed-forward (sometimes also predictive) control: the input to the process are
used to make a control decision. For this to work, the controller needs to have
a good understanding of the cause/ effect relationships between these inputs and
the process behavior.
2.3 Simplified automatic control
Q = Qdesign + K. (TR - Ts)
Sketch
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