Relay Control
The relay was the first control system component to be invented, originally to boost telephone signals. It is an electromechanical switching device, which allows a high power load to be controlled by a small input current, using an electromagnetic coil to operate a set of changeover switches. Relays can be wired up to operate in sequence, with time-delayed switching if required, to operate as a process controller. Before the development of transistors and digital logic, even before the development of valves, relays could be used to make simple industrial controllers. For example, a relay can be used to switch on a machine tool, using on and off push buttons and safety interlocks to make its operation safer.
The main components of a relay are shown in Figure 14.6(a). The small input current through the coil creates an electromagnetic field that attracts a steel yoke, which operates a set of contacts, which in turn switch a load (motor, heater, pump, etc.) on and off. The coil typically operates at 12 V or 24 V dc, but a 5 V coil allows the relay to be connected directly to a digital or microcontroller system (see the temperature controller, Section 13.1, Chapter 13).
A relay circuit for controlling a machine tool is illustrated in Figure 14.6(b). The system is designed to provide push-button operation and to prevent the main motor starting unless the machine guard is closed and the cutting fluid pump is on. There is also a thermal torque overload sensor, which disables the machine if the tool jams or the motor is stalled for some other reason. The relays operate in latched mode, and the system will ‘fail safe’ if the power goes off. Relay 2 (motor) is controlled from relay 1 (control), operated at 24 V. The motor and pump are connected to a 240 V supply via contacts in relay 2 and 1, respectively.