Control Engineering - A guide for beginners - Chapter 5 pptx

JUMO, FAS 525, Edition 02.04 79

5 Switching controllers

5.1 Discontinuous and quasi-continuous controllers

With the continuous controllers described previously, with P, PD, I, PI and PID actions, the manipu￾lating variable y can take on any value between the limits y = 0 and y = yH. In this way, the control￾ler is always able to keep the process variable equal to the setpoint w.

In contrast to continuous controllers, discontinuous and quasi-continuous controllers do not have

a continuous output signal, but one that can only have the state ON or OFF. The outputs from such

controllers are frequently implemented as relays, but voltage and current outputs are also com￾mon. However, unlike the continuous controller, these are binary signals that can only have a value

of 0 or the maximum value. These signals can be used to control devices such as solid-state re￾lays.

Fig. 51: Continuous, discontinuous and quasi-continuous controllers

In addition to these controller types with binary outputs, there are also 3-state and multi-state con￾trollers, where the manipulating variable output can have 3 or more levels. A tri-state controller

would, for instance, be used for heating and cooling tasks, or humidification and dehumidification.

It might be assumed that controllers with outputs which can only be in the ON or OFF state would

only produce an unsatisfactory control action. But surprisingly enough, satisfactory results for the

intended purposes can be achieved in many control processes, particularly with quasi-continuous

controllers. Discontinuous and quasi-continuous controllers are very widely used, because of the

simple construction of the output stage and the actuators that are required, resulting in lower

costs. They are found universally in those areas of process control where the processes are rela￾tively slow and can be readily controlled with switching actuators.

The simplest controller with a binary output is the discontinuous controller, which is effectively a li￾mit switch that simply switches the manipulating variable on or off, depending on whether the pro￾cess variable goes below or above a predetermined setpoint. A simple example of such a control￾ler is the two-state bimetallic temperature controller in an electric iron, or a refrigerator thermostat.

Quasi-continuous controllers can be put together, for example, by adding a switching stage to the

output of a continuous controller (see Fig. 51), thus converting the continuous output signal into a

switching sequence. P, PD, I, PI and PID actions can also be implemented for these controllers

(Fig. 51) and the foregoing remarks about continuous controllers are also applicable.

fine graduation

of manipulating variable

( 0 – 100 %)

coarse graduation

of manipulating variable

( 0 or 100 %)

continuous

controller

switching

fine graduation stage

of manipulating variable

( 0 – 100 %)

continuous

controller

y

y y R w

-x

comparator

with hysteresis

y

continuous

controller

discontinuous

controller

quasi-continuous

controller

w

-x

w

-x

5 Switching controllers

80 JUMO, FAS 525, Edition 02.04

5.2 The discontinuous controller

The discontinuous controller has only 2 switching states, i.e. the output signal is switched on and

off, depending on whether the process variable goes below or above a predetermined limit or set￾point. These devices are also often used as limit monitors, which initiate an alarm message when a

setpoint is exceeded.

A simple example of a mechanical discontinuous controller is, as previously explained, the bimetal￾lic switch of an electric iron, which switches the heating element off when the set temperature is re￾ached and switches it on again when the temperature falls by a fixed switching differential (hystere￾sis). There are other examples in the field of electronic controllers. For example, a resistance ther￾mometer (Pt 100), whose electronic circuitry switches heating on if the temperature falls below a

certain value, say 5°C, to provide frost protection for an installation. In this case, the resistance

thermometer together with the necessary electronic circuitry takes the place of the bimetallic

switch.

Fig. 52: Characteristic of a discontinuous controller

The discontinuous controller shown here supplies 100% power to the process until the setpoint is

reached. If the process variable rises above the setpoint, the power is taken back to 0%. Apart

from the hysteresis, we see that the discontinuous controller corresponds to a continuous control￾ler with no proportional band (XP = 0) and therefore “infinite” gain.