1. Content of PLC Programs
PLC application programs should maximize the control requirements of the controlled object. After conceptualizing the framework of the main program, it should serve as the main line, with each control function or sub-task program written one by one. Through continuous adjustments and improvements, the program should be able to complete the required control functions. Additionally, PLC application programs typically include the following aspects:
1. Initialization Program After powering on the PLC, some initialization operations are generally required. The purpose is to prepare for the startup and avoid system malfunctions. The main content of the initialization program includes: clearing certain data areas and counters; restoring required data in certain data areas; setting or resetting certain outputs; displaying certain initial states, etc.
2. Signal Detection, Fault Diagnosis, Data Display, Device Communication These can all be reserved as conventional content. They can be added after the basic program design is completed and can also be relatively independent program segments.
3. Protection and Interlocking Programs Protection and interlocking are essential parts of various application programs. They can prevent control logic confusion caused by illegal operations, ensuring that the system operates more safely and reliably. Therefore, careful consideration of protection and interlocking issues is necessary. Typically, interlocking and protection measures should also be set up externally to the PLC.
4. Main PLC Program and Subprograms This involves the logic and calculations directly related to process control, which can generally be categorized into subprograms based on space or time. Each subprogram should ideally be allocated sufficient independent data storage areas or data blocks in advance, with a unified naming convention for program variables, and detailed comments in complex areas. Understanding the process and fully collecting equipment parameters and operator requirements is essential for writing a program that is easy to accept.
2. Quality of PLC Programs
For the same control requirements, even if the same model of PLC is used, the programs written using different design methods may have different structures. Although several programs can achieve the same control function, the quality of the programs can vary significantly. The quality of a program can be measured by the following aspects:
1. Correctness of the Program:
The fundamental criterion for evaluating an application program is its correctness. A correct program must withstand the test of practical system operation; any evaluation of the program that disregards this criterion is meaningless.
2. Reliability of the Program:
A good application program ensures that the system can operate safely and reliably under both normal and abnormal (short power outages, certain controlled quantities exceeding limits, faults in certain links, etc.) working conditions, and can also prevent system control errors in the event of illegal operations (such as pressing or accidentally triggering buttons that should not be activated).
3. Ease of Parameter Adjustment:
One of the advantages of PLC control is its flexibility, allowing certain system functions to be changed easily by modifying programs or parameters. For example, some systems may need to change certain control parameters (such as timer or counter settings) under certain conditions, so the program design must consider how to write it for easy modification.
4. Conciseness of the Program:
The written program should be as concise as possible, reducing the number of statements. This generally reduces program scan time and improves the PLC’s response speed to input signals. However, excessive use of instructions with longer execution times may result in a shorter program statement count but not necessarily shorter execution times.
5. Readability of the Program:
Programs are not only for the designer but also need to be readable by system maintenance personnel. Additionally, to facilitate communication, programs should have a certain level of readability.