Programmable Logic Controller Control Function Selection

Control function selection
This selection includes the selection of characteristics such as operation function, control function, communication function, programming function, diagnostic function and processing speed.
1. Operation function
The operation function of simple programmable logic controller includes logical operation, timing and counting function; the operation function of ordinary programmable logic controller also includes data shift, comparison and other operation functions; more complex operation functions include algebraic operation, data transmission, etc.; large programmable logic controllers also have analog PID operation and other advanced operation functions. With the emergence of open systems, programmable logic controllers have communication functions. Some products have communication with lower computers, some products have communication with peer computers or upper computers, and some products also have data communication functions with factories or enterprise networks. When designing and selecting, we should start from the requirements of actual applications and reasonably select the required operation functions. In most applications, only logical operation and timing and counting functions are required, and some applications require data transmission and comparison. When used for analog detection and control, algebraic operation, numerical conversion and PID operation are used. Decoding and encoding operations are required to display data.
2. Control function
Control functions include PID control operations, feedforward compensation control operations, ratio control operations, etc., which should be determined according to control requirements. Programmable logic controllers are mainly used for sequential logic control. Therefore, single-loop or multi-loop controllers are often used in most cases to solve analog control. Sometimes, dedicated intelligent input and output units are used to complete the required control functions, improve the processing speed of programmable logic controllers and save memory capacity. For example, PID control units, high-speed counters, analog units with speed compensation, ASC code conversion units, etc. are used. [5]
3. Communication function
Large and medium-sized programmable logic controller systems should support a variety of field buses and standard communication protocols (such as TCP/IP), and should be able to connect to the factory management network (TCP/IP) when necessary. The communication protocol should comply with ISO/IEEE communication standards and should be an open communication network. [5]
The communication interface of the programmable logic controller system should include serial and parallel communication interfaces, RIO communication ports, commonly used DCS interfaces, etc.; the communication bus (including interface equipment and cables) of large and medium-sized programmable logic controllers should be 1:1 redundantly configured, the communication bus should comply with international standards, and the communication distance should meet the actual requirements of the device.
In the communication network of the PLC system, the communication rate of the upper network should be greater than 1Mbps, and the communication load should not exceed 60%. The main forms of the communication network of the PLC system are as follows:
1) PC is the master station, and multiple PLCs of the same model are slave stations, forming a simple PLC network;
2) One PLC is the master station, and other PLCs of the same model are slave stations, 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).
In order to reduce the CPU communication task, according to the actual needs of the network composition, communication processors with different communication functions (such as point-to-point, fieldbus) should be selected.
4. Programming function
Offline programming mode: The PLC and the programmer share a CPU. When the programmer is in programming mode, the CPU only provides services for the programmer and does not control the field equipment. After programming is completed, the programmer switches to the running mode, and the CPU controls the field equipment and cannot be programmed. Offline programming can reduce system costs, but it is inconvenient to use and debug. Online programming: The CPU and programmer have their own CPUs. The host CPU is responsible for field control and exchanges data with the programmer in one scan cycle. The programmer sends the online program or data to the host. In the next scan cycle, the host runs according to the newly received program. This method is more expensive, but the system is easy to debug and operate, and is often used in large and medium-sized programmable logic controllers. Five standardized programming languages: sequential function chart (SFC), ladder diagram (LD), function block diagram (FBD) three graphical languages ​​and statement list (IL), structured text (ST) two text languages. The selected programming language should comply with its standard (IEC6113123), and at the same time, it should also support multiple language programming forms, such as C, Basic, etc., to meet the control requirements of special control occasions.