T

Fig. 8.12. A feedback control loop (Rolf and Lim, 1985).

ture changes from the set point on the thermometer, the operative will take appropriate action and adjust the steam valve to correct the temperature deviation. Should the temperature not return to the set point within a reasonable time, further action may be necessary. Much depends on the skill of individual operatives in knowing when and how much adjustment to make. This approach with manual control may be very costly in terms of labour and should always be kept to a strict minimum when automatic control could be used instead. A justifiable use of manual control may be in the adjustment of minor infrequent deviations.

The process parameters which are measured using probes described in the previous sections may be controlled using control loops.

A control loop consists of four basic components:

1. A measuring element.

2. A controller.

3. A final control element.

4. The process to be controlled.

In the simplest type of control loop, known as feedback control (Fig. 8.12), the measuring element senses a process property such as flow, pressure, temperature, etc., and generates a corresponding output signal. The controller compares the measurement signal with a predetermined desired value (set point) and produces an output signal to counteract any differences between the two. The final control element receives the control signal and adjusts the process by changing a valve opening or pump speed and causing the controlled process property to return to the set point.

Manual control

A simple example of control is manual control of a steam valve to regulate the temperature of water flowing through a pipe (Fig. 8.13). Throughout the time of operation a plant operative is instructed to monitor the temperature in the pipe. Immediately the tempera-

Automatic control

When an automatic control loop is used, certain modifications are necessary. The measuring element must generate an output signal which can be monitored by an instrument. In the case of temperature control, the thermometer is replaced by a thermocouple, which is connected to a controller which in turn will produce a signal to operate the steam valve (Fig. 8.14).

Automatic control systems can be classified into four main types:

1. Two-position controllers (ON/OFF).

2. Proportional controllers.

3. Integral controllers.

4. Derivative controllers.

Fig. 8.13. Simple manual-control loop for temperature control.

Steam -

Control-

valve

Controller

Water

-Signal to operate valve

- Measured value

-Thermocouple

Pipe

Fig. 8.14. Simple automatic control loop for temperature control.

TWO-POSITION CONTROLLERS (ON/OFF)

The two-position controller, which is the simplest automatic controller, has a final control unit (valve, switch, etc.) which is either fully open (ON) or fully closed (OFF). The response pattern to such a change will be oscillatory. If there is instant response then the pattern will be as shown in Fig. 8.15.

If one considers the example of the heating of a simple domestic water tank controlled by a thermostat operating with ON/OFF, then there will be a delay in response when the temperature reaches the set point and the temperature will continue rising above this point before the heating source is switched off. At the other extreme, the water will continue cooling after the heating source has been switched on. With this mode of operation an oscillatory pattern will be obtained with a repeating pattern of maximum and minimum temperature oscillating about the set point, provided that all the other process conditions are maintained at a steady level (Fig. 8.16).

If this type of controller is to be used in process control then it is important to establish that the maxi

Valve or switch position

100% closed (off)

mum and minimum values are acceptable for the specific process, and to ensure that the oscillation cycle time does not cause excessive use of valves or switches.

ON/OFF control is not satisfactory for controlling any process parameter where there are likely to be large sudden changes from the equilibrium. In these cases alternative forms of automatic control must be used.

In more complex automatic control systems three different methods are commonly used in making error corrections. They are: proportional, integral and derivative. These control methods may be used singly or in combinations in applying automatic control to a process, depending upon the complexity of the process and the extent of control required. Since many of the controllers used in the chemical industries are pneumatic, the response to an error by the controller will be represented by a change in output pressure. Pneumatic controllers are still widely used because they are robust and reliable. In other cases, when the controller is electronic, the response to an error will be represented as a change in output current or voltage.

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