Summary of Siemens PLC selection method 2

author: chen

2023-08-03

Online programming mode: CPU and programmers have their own CPU. The host computer CPU is responsible for on-site control and data exchange with the programmer during the scan cycle. The programmer sends the online program or data to the host computer. During the next scan cycle, the host runs according to the newly received program. This method is costly, but the system is easy to debug and operate (f ng-BI), and is often used in large and medium-sized PLCs.

There are five standard programming languages: Sequential Function Chart, Ladder Diagram, Function Block Diagram, Statement List, and Structured Text. The selected programming language should conform to its standards, and should support C, basic and other programming languages to meet the control requirements of special control occasions.

5. Diagnostic function

PLC diagnosis function includes hardware diagnosis and software diagnosis. Siemens PLC is also widely used in my country's metallurgy, chemical industry, printing production lines and other fields. Siemens PLC products include logo, S7-200, S7-1200, S7-300, S7-400, etc. Siemens S7 series PLC has the advantages of small size, fast speed, standardization, strong network communication ability, strong function and high reliability. Hardware diagnosis is to determine the fault location of the hardware through the logical judgment of the hardware. Software diagnosis includes internal diagnosis and external diagnosis. Diagnosing the internal performance and functions of PLC through software is internal diagnosis, and diagnosing the information exchange function of CPU and external input and output of PLC through software is external diagnosis.

The strength of the PLC diagnostic function directly affects the technical ability requirements of operators and maintenance personnel, and affects the average maintenance time.

6. Processing speed

PLC works in scan mode. In terms of real-time performance, the faster the processing speed, the better. If the signal duration is less than the scan time, the PLC will not scan the signal, resulting in loss of signal data.

Processing speed is related to user program length, CPU processing speed, software quality, etc. Siemens PLC agent memory is a semiconductor circuit with storage function. Its function is to store information such as system programs, user programs, and logic variables. The system program is a program that controls the PLC to realize various functions. It is written by the PLC manufacturer and solidified into a read-only memory, which cannot be accessed by users. At present, the PLC contact response speed is fast, and the execution time of each binary instruction is about 0.2-0.4ls, which can meet the application requirements, high control requirements, and fast response speed. The scanning cycle should meet the following requirements: the scanning time of small programmable logic controllers is not greater than 0.5msk; the scanning time of large and medium-sized programmable logic controllers is not greater than 0.2msk.

1. Selection of programmable logic controller

The type of PLC is divided into integral type and modular type according to its structure. According to its application environment, it can be divided into field installation and control room installation. According to the word length of the CPU, it is divided into 1 bit, 4 bit, 8 bit, 16 bit, 32 bit and 64 bit. From the application point of view, the type can usually be selected according to the control function or the number of input and output points.

The IO points of the integrated PLC are fixed, so users have fewer choices in small control systems; the modular PLC provides various IO cards or cards, so users can reasonably select and configure the IO points of the control system, and the function expansion is convenient Flexible, usually used in medium and large control systems.

2.IO module selection

The selection of IO modules should take into account the unification of considerations and application requirements. For example, the input module should consider application requirements such as signal level, signal transmission distance, signal isolation, and signal power supply method. For output modules, the type of output module should be considered. Relay output modules generally have the characteristics of low price, wide voltage range, short service life, and long response time. The thyristor output module is suitable for occasions with high switching frequency and low inductance power factor, but it is relatively expensive and has poor overload capacity.

Output modules include DC output, AC output and analog output, which should meet the application requirements.

According to the application requirements, intelligent input and output modules are reasonably selected to improve the control level and reduce the application cost. Consider whether you need an expansion rack or a remote IO rack.

3. Power selection

PLC (the device that powers electronic equipment) power supply. In addition to the design and selection of programmable logic controllers when introducing equipment, the power supply of general programmable logic controllers should be designed and selected as 220VAC power supply, which is consistent with the domestic grid voltage. For critical applications, an uninterruptible power supply or regulated power supply should be used.

If the PLC itself has an available power supply, check whether the provided current meets the application requirements, otherwise an external power supply should be designed. In order to prevent the external high-voltage power supply from being introduced into the PLC due to misoperation, the input and output signals must be isolated. Sometimes a simple diode or fuse can be used for isolation.

4. Memory selection

Due to the development of computer integrated chip technology, the price of memory has dropped. Therefore, in order to ensure the normal operation of the application project, it is generally required that the storage capacity of the PLC should be at least 8K for 256 IO points. When complex control functions are required, a memory with large capacity and high grade should be selected.

5. Redundant function selection

Important process unit: CPU and power supply should be 1B1 redundant. Siemens PLC is a digitally operated electronic system designed for industrial environment applications. It uses a programmable memory to store instructions such as logic operations, sequence control, timing, counting, and arithmetic operations, and controls various mechanical equipment or production processes through digital or analog inputs.

If necessary, you can also choose a hot standby redundant system composed of PLC hardware and hot standby software, a 2-fold or 3-fold redundant fault-tolerant system, etc.

Control loop IO interface unit

The multi-point IO cards to be detected can be redundantly configured. 3) According to the needs of important IO signals, you can choose 2 times or 3 times the IO interface unit.

six economic factors

When choosing a Siemens PLC, you should consider the cost performance. While considering the economy, factors such as the scalability, operability, and input-output ratio of the application should also be considered, compared and considered, and finally a more satisfactory product is selected.

The number of input and output points has a direct impact on the price. Each additional IO card requires a certain cost. When the number of points increases to a certain value, the corresponding storage capacity, racks, motherboards, etc. will also increase accordingly. Therefore, the increase in the number of points has an impact on the choice of CPU, storage capacity, and control function range. When estimating and selecting, full consideration should be given to make the entire control system have a reasonable performance-price ratio.