PLCs come in a wide variety of types, differing in structure, performance, capacity, instruction set, programming method, price, and application scenarios. Therefore, selecting the right PLC plays a crucial role in improving the technical and economic performance of a PLC control system.
1. Selection of PLC Model
The selection of a PLC model should prioritize meeting control requirements while ensuring reliability, ease of maintenance and use, and the best performance-price ratio. Specifically, the following aspects should be considered:
(1) Performance should be adapted to the task: For small, single-unit equipment that only requires digital control, a general small PLC can meet the requirements. For application systems that mainly use digital control with a small amount of analog control, a small PLC with strong computing and data processing functions should be selected. For engineering projects with more complex control and higher control function requirements, a mid-range or high-end PLC can be selected depending on the scale and complexity of the control.
(2) The structure should be reasonable, the installation convenient, and the model should be uniform. For more complex and demanding systems, modular PLCs should generally be selected. For an enterprise, the control system design should strive to achieve uniformity in model.
(3) Whether the response time requirement is met. In order to reduce the I/O response delay time of the PLC, a PLC with a high scanning speed can be selected to process this type of function instruction using high-speed I/O, or a fast response module and an interrupt input module can be selected.
(4) Requirements for network communication function. As the main control device for factory automation, most PLCs have communication networking capabilities. When selecting, the communication method should be chosen according to the needs.
(5) Other special requirements. For systems with extremely high reliability requirements, consideration should be given to whether to adopt a redundant control system or a hot backup system.
2. PLC capacity estimation
The capacity of a PLC refers to both the number of I/O points and the storage capacity of the user memory. When selecting a PLC model, one should not blindly pursue excessively high performance indicators. However, in terms of I/O points and memory capacity, in addition to meeting the requirements of the control system, a margin should be reserved for backup or system expansion.
3. Selection of I/O modules
In a PLC control system, to control the production process, various measured parameters of the object must be sent to the PLC in the required manner. After calculation and processing, the PLC outputs the results in digital form. This output must then be converted into a quantity suitable for controlling the production process. Therefore, an information transmission and conversion device must be installed between the PLC and the production process. This device is the input/output (I/O) module. Different signal types require different types of I/O modules. For PLCs, signal types can be divided into four categories: digital input signals, digital output signals, analog input signals, and analog output signals.
4. Assign input/output points
After selecting the PLC model and input/output (I/O) modules, firstly, design the overall configuration diagram of the PLC system. Then, based on the process layout diagram and referring to the relevant PLC manuals or handbooks, draw the I/O wiring diagram, i.e., the PLC input/output electrical schematic diagram, by matching the input signals with input points and the output control signals with output points.
After selecting the PLC model, the number of input/output points is an important factor in determining the price and design rationality of the control system. Therefore, the number of input/output points can be simplified through reasonable design to achieve the same control function.
5. Safety circuit design
A safety circuit is a loop that provides interlocking control to protect the load or controlled object and prevent operational errors or control failures. While directly controlling the load, the safety protection circuit also provides input signals to the PLC for protection processing. Safety circuits generally consider the following aspects: short-circuit protection, interlocking and interlocking measures, undervoltage protection and emergency stop measures, and limit protection.
