PLC Control
Programmable logic controllers are often used for sequential control in industrial systems. The PLC is a self-contained sequence controller, built around a microprocessor or microcontroller,
but with all the interfacing built in. PLCs also use more user-friendly programming techniques, such as ladder logic. A small Mitsubishi PLC is shown in Figure 14.7.
The PLC can be programmed to act like a set of relays to give a particular output sequence in response to switched inputs, which can be manual inputs or derived from sensors. It is suitable for controlling systems where motors, heaters, valves and other loads must be switched directly from a power supply. The same machine tool seen in the previous example is now shown under PLC control in Figure 14.8.
The PLC has inputs labeled X0, X1, X2 and X3. These are detected as ‘on’ when connected to 24 V via an external switched sensor or control input. The PLC is programmed to operate the outputs, labeled Y0 and Y1, according to the input sequence. The outputs are also simple switched contacts, as in the normally open contact of a relay, which operate a load circuit with an external supply. They are typically designed to handle high power loads operating with mains voltage, or three-phase supplies. If necessary, the PLC outputs can control external
contactors (load relays) if the load current exceeds the PLC output contact rating. The control and load circuits are electrically isolated from each other, for safety, reliability and ease of use. The PLC inputs use opto-isolators, where the on/off signal is passed as infrared light, giving complete electrical isolation between the input and controller internal circuits.
The program for the PLC can be created in ‘ladder logic’ form (Figure 14.9), which allows the control program to be defined as if the PLC contained the relay system shown in Figure 14.6. The graphical program corresponds to the wiring diagram of the low-voltage (control) side of the relay system. Ladder logic uses a basic set of three symbols: normally open contacts, inverted contacts and output coils. These are associated by label with a physical input (Xn) or output (Yn). The normally open contacts represent external normally open contacts connected to the corresponding input; when the real contact closes, the contact in the program is closed. An inverted contact (X1) simply reverses the polarity of the external switch. The sides of the ladder correspond to the 24 V supply rails in the real circuit, so an output goes on when there is a closed path through the contacts in that rung of the ladder to switch on the coil, which operates the associated output in the PLC. The graphical program is entered on a host PC and converted to a machine code program, which is downloaded to the microcontroller in the PLC, in the same way as an assembler program.
In the ladder diagram, the system will come on when the ‘Start’ input is pressed, if the ‘Stop’ button is open and the ‘Guard’ switch is closed (guard closed). The ‘Stop’ button itself is normally open, but is inverted in the ladder program so it operates as if normally closed. The contact labeled Y1 (pump) closes because the virtual circuit is complete. The associated contact Y1 therefore also closes, which ‘holds on’ the output, even when the start button is released. A second Y1 contact then switches on the motor, as long as the overload cut-out is closed (no overload). The machine is then running. If the motor is overloaded, the thermal cut-out operates and switches off the motor, but the pump stays on to maintain
coolant feed. If the guard is opened or the stop button pressed, then both motor and pump go off. Output Y1 corresponds to relay 1 coil in the relay-controlled system, and Y0 to relay 2 coil.
Ladder programming was designed as a user-friendly method for creating this type of sequential control program, for engineers used to dealing with hard-wired relay systems. It was the first graphical programming method, of which there are now many, such as Flowcode for the PIC.