1. Analyze the control requirements of the control system
Familiar with the process requirements of the controlled object, determine the actions that must be completed and the sequence in which the actions are completed, and summarize the sequence function diagram.
2. Choose the appropriate type of PLC
According to the requirements of the production process, determine the number of I/O points and the type of I/O points (digital, analog, etc.), and list the I/O points. Estimate the memory capacity and leave a margin appropriately. According to experience, for a general switch control system, the memory capacity required by the user program is equal to the total number of I/Os multiplied by 8. For a control system with only analog input, each analog quantity needs 100 memory words; for existing analog quantities The input and analog output control system, each analog quantity needs 200 memory words. When determining the model, it is necessary to review the comprehensive situation of the PLC manufacturer's products and its after-sales service, technical support, network communication, etc. in combination with the market situation, and select a PLC model with better performance and price.
3. Hardware design
According to the selected PLC product, understand its performance. According to the information provided with the machine, combined with actual needs, while considering the software programming, the external circuit is designed, and the principle wiring diagram of the electrical control system is drawn.
4. Software design
(1) The main task of software design is to convert the sequential function diagram into a ladder diagram according to the requirements of the control system. It is best to list the used software components (such as internal relays, timers, counters, etc.) when designing the program, and indicate the purpose. In order to facilitate program design, debugging, system operation, maintenance, and inspection.
(2) Simulation debugging. Download the designed program to the PLC main unit. The test signal is added from an external signal source, the input signal can be simulated by a button or a small switch, the load can be simulated with an indicator light, the status of the program operation can be understood through the brightness of various indicators, and the change relationship between input/output and the logic state can be observed Whether it meets the design requirements, and modify and adjust the procedures in time until the design requirements are met.
5. On-site debugging
On the premise of qualified simulation debugging, connect the PLC to the field device. Before on-site debugging, check the entire PLC control system, including power supply, grounding wire, equipment connection wire, I/O connection, etc. Power can be sent only when the entire hardware connection is correct. Set the working mode of the PLC to "RUN". Debug repeatedly to eliminate possible problems. After a certain period of trial operation and the system is running normally, the program can be solidified in a memory with long-term memory function for backup.