Control Engineering - A guide for beginners - Chapter 3 pptx

JUMO, FAS 525, Edition 02.04 45

3 Continuous controllers

3.1 Introduction

After discussing processes in Chapter 2, we now turn to the second important element of the con￾trol loop, the controller. The controller has already been described as the element which makes the

comparison between process variable PV and setpoint SP, and which, depending on the control

deviation, produces the manipulating variable MV. The output of a continuous controller carries a

continuous or analog signal, either a voltage or a current, which can take up all intermediate values

between a start value and an end value.

The other form of controller is the discontinuous or quasi-continuous controller in which the manip￾ulating variable can only be switched on or off.

Continuous controllers offer advantages for certain control systems since their action on the pro￾cess can be continuously modified to meet demands imposed by process events. Common indus￾try standard output signals for continuous controllers are: 0 — 10V, 0 — 20mA, 4 — 20mA. On a

continuous controller with a 0 — 20 mA output, 10% manipulating variable corresponds to an out￾put of 2mA, 80% corresponds to 16mA, and 100% equals 20mA.

As discussed in Chapter 1, continuous controllers are used to operate actuators, such as thyristor

units, regulating valves etc. which need a continuous signal.

3.2 P controller

In a P controller the control deviation is produced by forming the difference between the process

variable PV and the selected setpoint SP; this is then amplified to give the manipulating variable

MV, which operates a suitable actuator (see Fig. 29).

Fig. 29: Operating principle of a P controller

The control deviation signal has to be amplified, since it is too small and cannot be used directly as

the manipulating variable. The gain (Kp) of a P controller must be adjustable, so that the controller

can be matched to the process.

The continuous output signal is directly proportional to the control deviation, and follows the same

course; it is merely amplified by a certain factor. A step change in the deviation e, caused for exam￾ple by a sudden change in setpoint, results in a step change in manipulating variable (see Fig. 30).

Process value (x) Control

deviation

e = (w - x) Amplifier

Manipulating

Setpoint (w)

(Kp)

variable (y)