The model selection of PLC should pay attention to what matters - Industrial control -

With PLC and the popularization, more and more types and models of PLC products, functional improvement, currently imported from abroad and domestic manufacturers assembly or PLC products have been developed dozens of series. Hundreds of models. Its structure, performance, capacity, instruction system, programming method, and the price is different, also each have emphasize particularly on the applicable occasions. Therefore, the rational selection of PLC products, to improve the technical and economic indexes of PLC control system plays an important role. Each manufacturer produces the product can pass on the quality of how to choose to meet his needs, both economical and practical, and waste not machine capacity. The choice of PLC controller mainly includes model selection, capacity selection, input and output module choice, power supply module choice.

( A) The type of PLC PLC according to the structure is divided into integral type and module type two kinds, according to the application environment can be divided into the control room and on-site installation install two classes; According to the CPU word length is divided into one, four, eight, 16, 32, 64, etc. From the perspective of application, usually can press control function or input/output points selection. Integral type PLC I/O points is fixed, so the user choice is small, for small control system; Offers a variety of module type PLC I/O card or card, so the user can be a reasonable choice and configuration control system I/O points, convenient and flexible for function extension, is generally used in large and medium-sized control system.

( 2) The selection of input and output module should consider the choice of input and output module and application requirements. The input module, for example, should consider the signal level, signal transmission distance and the way of signal isolation, signal power supply application requirements. The output module, should consider to choose the output of the module type, usually relay output module with low prices, using voltage range and long service life is short, response time; Silicon controlled rectifier output module is suitable for the switch is frequent, inductive load, low power factor but the price is more expensive, overload ability is poor. Output module and output dc, ac output and analog output, etc. , should be consistent with application requirements. According to the application requirements, a reasonable choice intelligent input/output module, in order to improve the level of control and reduce the cost of application. Considering whether to need to extend frame or remote I/O rack etc.

( 3) The power of the choice of PLC power supply, in addition to the introduction of equipment at the same time the introduction of PLC should be according to the design and selection of product specifications, general PLC power supply should choose 220 vac power source design, in accordance with the domestic power grid voltage. Important applications, uninterruptible power supply or regulated power supply should be adopted. If the PLC itself can be used with a power supply, should check the current offer whether meet the application requirements, otherwise you should design an external power supply. In order to prevent external PLC, introduced high voltage power supply due to wrong operation, the input and output signal isolation is necessary, sometimes also can use a simple diode or fuse tube.

( 4) Memory due to the development of the computer integrated chip technology, the choice of the price of memory has fallen, therefore, in order to ensure the normal application project put into operation, the general requirements of PLC memory capacity, choose by 256 I/O points at least 8 k memory options. Need complicated control functions, should choose a larger capacity, higher storage class.

( 5) The control unit of the redundant

1) Important process unit: CPU ( Including storage) And the power supply shall be 1 b1 redundancy.

( 2) Also can be used when there is a need for PLC hardware and hot standby hot standby redundancy system consisting of software, 2 or 3 heavy redundancy fault-tolerant system, etc. The I/O interface unit redundancy

1) Control circuit more I/O card should be redundant configuration.

( 2) Important check point more I/O card can be redundant configuration.

( 3) According to the need of important I/O signals, and can choose 2 or 3 heavy I/O interface unit.

( 6) Economic considerations when selecting a PLC, should consider price performance ratio. Considering economy, should consider the application of extensibility, operability and ratio of input and output factors, comparison and consideration, finally choose the satisfactory products. Input and output points have a direct impact on prices. Every piece of input and output card is a fee may be increased. When points increased to a certain value, the corresponding storage capacity, frame, motherboards and so on also will increase, therefore, to the points on the selection of CPU and memory capacity, control scope, has an effect choice. Should be given due consideration in estimation and selection, make the performance of the whole control system has more reasonable prices.

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