Summary of Siemens PLC selection method 1

author: chen

2023-08-03

In the design of the PLC system, the control scheme must be determined first, followed by the selection of PLC engineering design. The characteristics and application requirements of the process flow are the main basis for design and selection. PLC and related equipment should be integrated and standardized. According to the principle that it is easy to form a whole with the industrial control system and expand its functions, the selected PLC should be a mature and reliable system with good operating performance in related industrial fields. The system hardware and software configuration and functions of the PLC should be compatible with the device scale and control requirements.

Familiarity with programmable controllers, menu diagrams and related programming languages will help shorten programming time. Therefore, when selecting and evaluating engineering design, the characteristics and control requirements of the process (Guòchéng) should be analyzed in detail, the operations and actions required for the control task and scope should be clarified, and then the input and output points and the required storage capacity should be estimated according to the control requirements, Determine the functions of PLC and the characteristics of external equipment, and finally select a cost-effective PLC and design a corresponding control system.

1. Estimation of input and output points

When estimating IO points, an appropriate margin should be considered. In general, the input and output points plus a 10%-20% expandable margin are used as input and output points. When actually ordering, the input and output points should be rounded up according to the product characteristics of the manufacturer's PLC.

2. Estimation of storage capacity

Memory capacity is the size of the hardware storage unit provided by the programmable controller itself. The program size is the size of the storage unit used by the user's application program items in memory, so the program size is smaller than the memory size. In the design stage, because the user's application program has not been compiled, the program capacity is unknown at the design stage, and it needs to be known after the program is debugged. To estimate program capacity, memory capacity is usually used.

There is no fixed calculation formula for the storage capacity of PLC memory. Many literatures give different formulas, generally 10-15 times the number of digital IO points, plus 100 times the number of analog IO points. This number is the total number of words in memory. Also, the surplus is considered to be 25% of this figure.

3. Selection of control functions

Including operation function, control function, communication function, programming function, diagnosis function and selection of processing speed.

an operating function

The operation functions of simple PLC include logic operation, timing and counting functions; the operation functions of ordinary PLC also include operation functions such as data shifting and comparison; more complex operation functions include algebraic operation, data transmission, etc.; large-scale PLC also includes PID analog operation and other advanced operation functions. With the emergence of open systems, PLC has communication functions. Some products communicate with the lower computer, some communicate with the same computer or upper computer, and some products also communicate with the factory or enterprise network.

In the design and selection, it is necessary to proceed from the actual application requirements and reasonably select the required operating functions. In most applications, only logic operations and timing counting functions are required. Some applications require data transfer and comparison. When used for analog quantity detection (detection and control), algebraic operation, conversion and PID operation are used. In order to display the data, decoding and encoding operations are required.

Two control functions

The control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., which are determined according to the control requirements. Since PLC is mainly used for sequential logic control, in most cases, single-loop or multi-loop controllers are usually used to solve the control of analog quantities, and sometimes special intelligent input and output units are used to complete the required control functions, thereby improving PLC. processing speed and save storage capacity. Such as PID control unit, high-speed counter (JìshùQì), analog unit with speed compensation, ASC code conversion unit, etc.

3 communication functions

Large and medium-sized PLC systems should support a variety of field buses and standard communication protocols, and should be able to connect to the factory management network if necessary. The communication protocol shall conform to the ISOIEEE communication standard and shall be an open communication network.

The communication interfaces of the PLC system should include serial-parallel communication interfaces, Rio communication interfaces, industrial Ethernet, general-purpose DCS interfaces, etc.; large and medium-sized PLC communication buses should adopt 1:1 redundant configuration, the communication bus should meet international standards, and the communication distance should meet actual requirements of the device.

In the communication network of the PLC system, the communication rate of the superior network should be greater than 1Mbps, and the communication load should not be greater than 60%. The main forms of PLC system communication network are:

1) The 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 as a subnet of the DCS through a specific network interface; 4) A dedicated PLC network. In order to reduce CPU communication tasks, PLC should choose communication processors with different communication functions according to the actual needs of network composition.

Four programming functions

Offline programming mode: PLC and programmer share one CPU. When the programmer is in programming mode, the CPU only provides services for the programmer and does not control the field devices. After the programming is completed, the programmer switches to the operation mode, and the CPU controls the field devices and cannot be programmed. Offline programming can reduce system cost, but it is not easy to use and debug.