PLC and related equipment should be integrated and standardized, in accordance with the principle of easy integration with industrial control systems and easy expansion of their functions. The selected PLC should be a mature and reliable system with operational performance in relevant industrial fields. The system hardware, software configuration, and functions of the PLC should be suitable for the device scale and control requirements. Familiarity with programmable controllers, functional diagrams, and related programming languages can help shorten programming time. Therefore, during the selection and estimation of engineering design, a detailed analysis of the characteristics and control requirements of the process should be carried out, and the control tasks and scope should be clarified to determine the required operations and actions. Then, based on the control requirements, the input and output points, required memory capacity, PLC functions, external equipment characteristics, etc. should be estimated. Then, a PLC with high performance price ratio should be selected and corresponding control systems should be designed.

1、 Estimation of input/output (I/O) points

When estimating I/O points, appropriate margin should be considered. Usually, based on the input and output points counted, an additional 10% to 20% of the expandable margin should be added as the input and output point estimation data. When placing an actual order, it is also necessary to round off the input and output points based on the product characteristics of the manufacturer's PLC.

2、 Estimation of memory capacity

Memory capacity is the size of the hardware storage unit that the programmable controller itself can provide, while program capacity is the size of the storage unit used by user application projects in memory, so program capacity is smaller than memory capacity. During the design phase, as the user application program has not yet been developed, the program capacity is unknown during the design phase and needs to be determined after program debugging. In order to have a certain estimation of program capacity during design and selection, memory capacity estimation is usually used instead.

There is no fixed formula for estimating the memory capacity of memory. Many literature sources provide different formulas, which are generally based on 10-15 times the number of digital I/O points, plus 100 times the number of analog I/O points. This number is used as the total number of words in memory (16 bits are a word), and a margin of 25% of this number is considered.

3、 Selection of control functions

This selection includes the selection of features such as calculation function, control function, communication function, programming function, diagnostic function, and processing speed.

1） Arithmetic function

The calculation functions of a simple PLC include logic operation, timing, and counting functions; The calculation functions of ordinary PLC also include data shift, comparison, and other calculation functions; More complex computational functions include algebraic operations, data transmission, etc; Large PLCs also have PID calculation and other advanced calculation functions for analog quantities. With the emergence of open systems, communication functions are now available in PLCs. Some products have communication with lower level computers, some products have communication with the same or upper level computers, and some products also have data communication functions with factories or enterprise networks. The design and selection should start from the requirements of practical applications and reasonably select the required computational functions. In most applications, only logical operations and timing and counting functions are required, while some applications require data transmission and comparison. When used for analog detection and control, algebraic operations, numerical conversion, and PID operations are only used. To display data, operations such as decoding and encoding are required.

2） Control function

The control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., which should be determined according to the control requirements. PLC is mainly used for sequential logic control, therefore, in most cases, single or multi loop controllers are often used to solve the control of analog quantities. Sometimes, dedicated intelligent input and output units are also used to complete the required control functions, improving the processing speed of PLC and saving memory capacity. For example, using PID control units, high-speed counters, analog units with speed compensation, ASC code conversion units, etc.

3） Communication function

Large and medium-sized PLC systems should support multiple field buses and standard communication protocols (such as TCP/IP), and should be able to connect to the factory management network (TCP/IP) when needed. The communication protocol should comply with ISO/IEEE communication standards and should be an open communication network.

The communication interfaces of the PLC system should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication ports, industrial Ethernet, commonly used DCS interfaces, etc; Large and medium-sized PLC communication buses (including interface devices and cables) should be configured with 1:1 redundancy, and the communication bus should comply with international standards. The communication distance should meet the actual requirements of the device.

In the communication network of the PLC system, the communication speed of the superior network should be greater than 1Mbps, and the communication load should not exceed 60%. The communication network of PLC system mainly takes the following forms: 1) PC is the main station, and multiple PLCs of the same model are the slave stations, forming a simple PLC network; 2) One PLC serves as the master station, while other PLCs of the same model serve as slaves, forming a master-slave PLC network; 3) The PLC network is connected to a large DCS through a specific network interface as a subnet of the DCS; 4) Dedicated PLC network (dedicated PLC communication network of each manufacturer).

To reduce CPU communication tasks, communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) should be selected based on the actual needs of network composition.

4） Programming function

Offline programming method: The PLC and programmer share a common CPU. When the programmer is in programming mode, the CPU only provides services for the programmer and does not control on-site devices. After completing the programming, the programmer switches to the running mode, and the CPU controls the on-site equipment, which cannot be programmed. Offline programming can reduce system costs, but it is not convenient to use and debug. Online programming method: The CPU and programmer have their own CPUs, and the host CPU is responsible for on-site control and data exchange with the programmer within a scanning cycle. The programmer sends the online program or data to the host, and the host runs according to the newly received program in the next scanning cycle. This method has a high cost, but is convenient for system debugging and operation, and is often used in large and medium-sized PLCs.

Five standardized programming languages: Sequential Function Diagram (SFC), Ladder Diagram (LD), Functional Module Diagram (FBD), and Statement List (IL) and Structured Text (ST).