Control Engineering - A guide for beginners - Chapter 6 doc

JUMO, FAS 525, Edition 02.04 101

6 Improved control quality through special controls

So far, we have only considered single-loop control circuits, where controller and process form a

closed signal loop. However, when using such single-loop control circuits, there are limits to the

control quality which can be achieved in certain control processes. It is possible to go beyond the

control quality limits imposed by the single-loop control circuit by using multi-loop control circuits,

or by switching auxiliary variables on and off. To some extent, relatively simple solutions can lead

to considerable improvements in control quality.

6.1 Base load

With a base load setting, the controller only influences part of the total manipulating variable, and a

fixed proportion is continuously supplied to the process (combination of control and operation). It

could then be the case that, for example in an electrically heated furnace, one section of the heat￾ing elements is controlled by the controller, whereas another section is supplied at full supply volt￾age (see Fig. 65).

Fig. 65: Base load setting

Essentially, base load setting offers the following advantages:

- The actuator, e.g. a thyristor controller, can be more compact and less expensive, as it only

needs to control low power.

- Load fluctuations on the supply network, as a result of power consumption in bursts caused by

the switching controller, are similarly reduced.

- If the controller fails, the process can still be operated with the base load component of the total

power.

Furnace

R1

K1

N

L1

x

t

100 % power

with base load

without base load

R2

6 Improved control quality through special controls

102 JUMO, FAS 525, Edition 02.04

Against the advantages shown, there are also a number of disadvantages:

- The dynamic control action is impaired, especially with regard to disturbances. As the controller

now no longer provides the full manipulating variable, the cooling curve, for instance, is not only

shifted by the amount of the base load setting, but is also clearly flatter (see Fig. 65). If, for any

reason, the power requirement suddenly becomes less than the base load setting, the controller

is helpless in this situation, as it cannot reduce the manipulating variable below the value of the

base load.

- In addition, the base load setting must also be matched to the setpoint. If the setpoint is

changed downwards, for instance, the excess power could suddenly be too large; with an

upward change, the excess power could be too small. In such cases, the base load should be

changed at the same time as the setpoint.