TABLE OF CONTENTS This chapter extends the examples given in previous chapters to show programs developed to complete specific tasks. These include tasks which involve temperature control and a number involving pneumatic valves.
Consider the task of using a PLC as an on-off controller for a heater in the control of temperature in some enclosure. The heater is to be switched on when the temperature falls below the required temperature and switched off when the temperature is at or above the required temperature. The basic algorithm might be considered to be:
IF temperature below set valueTHEN DO switch on heaterELSE DO switch off heaterENDIF
The sensor used for the temperature might be a thermocouple, a thermistor or an integrated chip (see Section 2.1.5). When connected in an appropriate circuit, the sensor will give a suitable voltage signal related to the temperature. This voltage can be compared, using an operational amplifier, with the voltage set for the required temperature so that a high output signal is given when the temperature is above the required temperature and a low output signal when it is below. Thus when the temperature falls from above the required temperature to below it, the signal switches from a high to a low value. This transition can be used as the input to a PLC. The PLC can then be programmed to give an output when there is a low input and this output used to switch on the heater.
Figure 14.1 shows the...
