In the fields of automation and industrial control, PLCs (Programmable Logic Controllers) are indispensable components. Through programming, PLCs can implement various complex control logics, optimize production processes, and improve production efficiency. This article provides 10 tips on PLC programming, from basic to advanced, to help you better master PLC programming.
I. Control Mode Selection
In monitoring systems, we often need to switch between control modes such as "manual" and "automatic". In programming, we can use the MOV instruction to achieve this. For example, when "manual" mode is selected, the constant 1 is MOVed into register VB10; when "automatic" mode is selected, the constant 2 is MOVed into the same register VB10. By checking the data in the registers, we can determine the current control mode of the system. This method is simple, easy to understand, and less prone to errors.
II. Analog Control
When the program involves analog signal control, if the error in the read analog data is small, time filtering can be used for data processing; if the error is large, other filtering methods, such as average value filtering, are required. This necessitates consulting relevant materials to understand the applicable scenarios and advantages and disadvantages of various filtering methods.
III. Program Debugging
During program debugging, especially when modifying equipment, if certain conditions are met but the output coil is not connected, it is necessary to check whether the program is affected by other statements (such as JUMP, GO TO, etc.). Additionally, after interrupting the program, situations may arise where conditions are met but the output is not connected; this is usually because the relevant program segment has not been scanned. Therefore, unscanned program segments should be avoided as much as possible during programming.
IV. Sequence Control
For sequential control programs, a +10+10 control mode can be used. A register is preset, with its value set to 0 during initialization. When the system starts, the register is incremented by 10, now displaying a value of 10, indicating the start of the first action. After completing this action, the register is incremented again by 10, now displaying a value of 20, indicating the start of the second action; and so on. The register value determines the action to be executed. This control mode is simple, clear, easy to understand, and easy to maintain.
V. Troubleshooting
When designing a program, potential process failures should be considered. In the event of a failure, the fault condition should be displayed and an alarm signal should be issued until the operator resets the system. This helps operators to promptly identify and handle faults, preventing production accidents caused by program issues. This point should be given special attention when designing new systems.
VI. Sub-modular design
For frequently called subroutines, they can be designed as submodules to facilitate frequent calls. This can improve program reusability, reduce code redundancy, and improve program maintainability.
VII. Overbeat Protection
Since each step in a production machine's work cycle has a specific time limit, a timer can be started simultaneously with the beginning of each step, using these times as a reference. The timer's set time is 20% to 30% longer than the normal duration of the action to prevent the equipment from running beyond its set time. If the machine fails to proceed to the next step within the specified time, the timer sends a fault signal, stopping the normal work cycle and triggering an alarm or shutdown procedure. This over-cycle protection effectively prevents equipment overload and production accidents.
VIII. Input Signal Design
For safety detection switches (such as emergency stop buttons, safety light curtains, limit switches, etc.), it is recommended to use a normally closed (NC) input signal design. This ensures that the input signal is low-level when the switch is in the normal operating state, effectively reducing the risk of malfunction.
IX. Output Design Principles
When designing output devices, the principle of "better to do nothing than to tamper with them" should be followed. Ensure that output devices only operate when necessary to avoid unnecessary misoperation that could impact equipment and production. At the same time, care should be taken to avoid prolonged operation or frequent start-ups and shutdowns.
10. Rotation and Cumulative Functions
For single-unit equipment control, regular rotation is necessary to ensure stable operation and extend service life. Each unit must be rotated after 24 hours of continuous operation. Furthermore, a cumulative running time function is also essential. Unless the start/stop sequence is set by a host computer, operators can manually configure the start/stop time sequence and cumulative running time. These measures ensure stable equipment operation and extend its service life.
