The most important thing in industrial control is stability, stability, and stability. Stability is not only about hardware; software is also very important, as is immunity to interference. Here, we will only discuss PLC software.
I have standardized the PLC program into the following components:
I. System Initialization
II. Description of the process or state (equivalent to intermediate relays in relay and contact control),
III. Human-machine operation control (manual operation, parameter modification, etc.)
IV. Equipment control outputs (motors, valves, etc.)
V. Communication (interlocking and data exchange between various control devices)
VI. Fault description of the process or state,
VII. Display of alarm output and fault location information,
8. Production process reports (output, quality, etc.).
The following discussion will focus on the description of processes or states and the output of device control, as these are essentially the main parts of the program.
The criteria for good or bad programs are: stability, ease of debugging, ease of modification, ease of expansion, ease of reading, and real-time performance (speed). In my years of practice, I've found that first obtaining various state variables from the description of the process or state, and then programming the device output, achieves these goals relatively well. While it may lack speed (because the program is longer), this can be easily addressed through other means (such as interrupts). This idea primarily stems from the mathematical state equation: Q = f(S1, S2, S3, ..., Sn), where Q is the device output, S1, S2, S3, ..., Sn are the state variables (including inputs and outputs), and f is the algorithm composed of the instruction set.
Generally, in a system, state variables are deterministic and invariant (depending on the state description), just as the elements that make up the world are essentially unchanging. Therefore, different control outputs can be obtained simply by changing the algorithm, making expansion and modification very convenient. During debugging and troubleshooting, the cause of the fault can be quickly found based on the state progression. Because the control output is generally a function (algorithm) of several states with redundant relationships, stability, reliability, and anti-interference capabilities are greatly enhanced.
