Controlwhose program is stored in a program memory. The functional elements necessary to process the program and their connections are regarded as factors that cannot be altered. PLCs may be equipped with control units which work by bits or by words (EDIN 19 226 T 5).
In contrast to the hard-wire programmed control, the PLC has application-independent standard hardware with application-independent wiring. The individual logical connections are achieved by triggering a sequence of individual program steps, which are run through step by step by a fast microprocessor and are logically connected. The entirety of the steps – the control program customised for the respective task – is stored in the memory.
When the PLC and the numeric control of the machine are working together, the numeric control specifies the geometry of the processing, i.e. the movement control (e.g. line motion control), while the PLC performs the function control, i.e. the coordination and linking of the functions (Figure S 68).
The main advantages of the PLC are as follows:
- standardised hardware, even for complex control tasks,
- adjusting to the respective control task using software variations,
- non-problematic program changes by changing or replacing the stored program,
- short commissioning times as changes can be made quickly,
- automatic and error-free program documentation,
- identifying and localising errors outside the control using additional diagnostics programs.
The PLC is primarily used for workflow and logic control system, therefore as a replacement for relay and contactor controls. However, it can normally be enhanced for control technology tasks.
Figure S 68: Interaction of PLC and NC