Software PLC and NC on PC systems
PC-based automation systems are currently used in the form of
- PLCs, process computers or NC controllers combined with a PC,
- Industrial PCs with a coprocessor card for PLC/NC tasks.
Typically, PCs are not entrusted directly with control tasks. Instead, these are executed by additional processors. This structure is disadvantageous. A new approach consists of a pure software solution. A processor runs all automation tasks in a single-processor mode of operation. Contrary to the traditional approaches, with TwinCAT a separate processor system including memory and operating system is substituted by real time-compatible deterministic processing of functions for the PLC and NC with the PC processor and in its memory. The advantages of this solution are obvious: what is no longer there can also not produce any errors.
Automation with four standard components
Thus, an automation system now only consists of 4 components:
- Industrial PC,
- Open field bus system for I/O links,
- Standard Windows NT operating system for the user interface (HMI),
- TwinCAT system software "IEC1131-PLC and NC on the PC"
The advantages of this solution are:
- almost unrestricted memory space for programs and data, constant
- performance development for the automation task and full
- integration in the operating system: PC resources are accessed with methods of the operating system instead of with driver software and
- a low number of components results in high system reliability.
Software PLC and NC on the PC: the PC executes the controller
For the PC to execute the control task, besides the programs for operator control the automation software must be executed in the usual fashion. A "classical" PLC runs programs cyclically. Input and output alternate with program execution:
In a software PLC on the PC, programs are run in exactly the same way as in a conventional PLC / NC, i.e. in exactly timed (deterministic) cycles, which are executed with the same length and regardless of the program length. The user interface is executed in the cycle pauses, for which computing capacity can be reserved:
The PLC, NC and user interface are executed simultaneously by "overlaying" tasks for PLC, NC and operating system tasks with a multitasking system. Each task ("server") for a specific purpose operates with its own cycle time and priority. Sketch 3 shows overlaying of the PLC, the NC and the operating system. TwinCAT ensures the operation of Windows NT together with TwinCAT programs by a special implementation integrated into the operating system:
