PLC and related equipment should be integrated, standard, easy and industrial control system in accordance with the formation of a whole, to expand the principles of its function. Chosen PLC should be in related industries have put into operation performance, proven system, PLC system hardware, software, and device configuration and functions shall meet the requirements of size and control. Familiar with the programmable logic controller, menu plans and related programming languages, help reduce programming time. Therefore, the engineering design selection and estimation, should the characteristics of a detailed analysis of the process, the control requirements, specific control tasks and determine the scope of operations and actions required, then under the control requirements, estimating input and output points, the required memory capacity is determined PLC functions, external device characteristics, the final choice of a higher cost performance and to design appropriate PLC control system.

　　Estimating an input-output (I / O) points

　　After when I / O points to consider appropriate margin estimates, generally based on the statistics of the input and output points, another 10% to 20% margin, scalable, estimates as input and output points. Actual orders, according to the manufacturer needs to PLC product characteristics of input and output points were rounded.

　　二、Estimate the memory capacity

　　Memory capacity is a hardware storage unit programmable controller itself can provide the size, program size is the size of the memory storage unit used by the user application program, the program capacity is less than the memory capacity. The design phase, since the user application has not been prepared, therefore, program capacity is unknown at the design stage, need to know after the program debugging. In order to be able to design and selection procedures when a certain capacity estimates usually estimate the memory capacity instead.

　　Estimate the memory capacity of the memory is no fixed formula, the literature gives many different formulas, in general, are by digital I / 10 ~ 15 times O points, plus analog I / 100-fold O points, in order the total number of words of memory (16-bit word), an additional number of 25% Click to consider margin.

　　三、Select the control functions

　　The selection includes an operational function, control selection, communication, programming, diagnostics functions and processing speed characteristics.

　　一）Arithmetic Functions

　　Simple PLC operation function including logic operations, timing and counting functions; ordinary PLC computing functions also include data shift, comparison, etc. functions; more complex computing functions algebra, and data transfer; there are large PLC analog PID computing and other advanced computing functions. With the advent of open systems are currently in the PLC with a communication function, and some products have communication with the next crew, some products have to communicate with peers or PC, some products also have a factory or enterprise network for data communication functions. Selection of the requirements should be designed from the practical application point of view, a reasonable selection of the desired measurement function. Most applications need only logical and timing counting function, some applications require data transfer and compare, when used in the detection and control of analog, only use algebra, numerical conversion and PID operation and so on. To display data needs decoding and encoding operations.

　　二）Control Function

　　Control functions include PID control operation, the feedforward compensation control operation, the ratio of control operations, etc., shall be determined in accordance with the control requirements. PLC is mainly used for sequential logic control, therefore, most often the case with single or multi-loop controller solutions analog control, sometimes using a dedicated intelligent input and output units to complete the necessary control functions, improve the processing speed and PLC saving memory capacity. Such as using PID control unit, high-speed counters, analog units with speed compensation, ASC code conversion unit and the like.

　　三）Communication Function

　　Medium-sized PLC system should support multiple fieldbus and standard communication protocols (such as TCP / IP), the need should be able to connect with the factory management network (TCP / IP). Communication protocol should be consistent with ISO / IEEE communication standard, should be open communication network.

　　The communication interface PLC system should include serial and parallel communication interfaces (RS2232C / 422A / 423/485), RIO communication port, Ethernet, common DCS interface; and medium-sized PLC communication bus (including interface devices and cables) should 1 : 1 redundant configuration, communication bus should be consistent with international standards, communication from the device should meet practical requirements.

　　PLC system communications network, the network communication rate higher level should be greater than 1Mbps, communication load is not greater than 60%. Communication network PLC system is mainly in the form are the following forms: 1) PC-based station, many same model PLC from the station, consisting of simple PLC network; 2) 1 PLC-based station, the other with the PLC model from the station constituting the master-slave PLC network; 3) PLC network via specific network interfaces to large DCS DCS as the subnet; and 4) Special PLC network (PLC manufacturers dedicated communications network).

　　To reduce the CPU communication tasks, according to the actual needs of the network component, should choose a different communication functions (point, fieldbus and industrial Ethernet) communications processor.

　　四）Programming features

　　Off-line programming mode: PLC and programmer utilities a CPU, the programmer in programming mode, CPU serves only programmer does not control field devices. Upon completion of programming, the programmer switches to Run mode, CPU control of field devices, can not be programmed. Off-line programming mode reduces system cost, but is not convenient to use and debug. Online programming mode: CPU and programmer has its own CPU, the host CPU is responsible for on-site control, and within a scan cycle with the programmer for data exchange, data transmission program or programmer to prepare online to the host, the next scan cycle, the host program is running according to the newly received. In this way the higher costs, but the system debugging and easy to operate, often used in medium and large PLC.

　　Five kinds of standardized programming languages: Sequential Function Chart (SFC), Ladder Diagram (LD), Function Block Diagram (FBD) three kinds of graphic language and Statement List (IL), Structured Text (ST) language both texts. Choice of programming language should comply with its standards (IEC6113123), should also support multiple languages programming forms, such as C, Basic, etc., in order to meet the control requirements of special control applications.

　　五）Diagnostic function

　　PLC diagnostics functions, including diagnostic hardware and software. Hardware diagnostics to determine the location of the fault of the hardware by a hardware logic, software diagnostics and external diagnosis since the beginning of the diagnosis. PLC via software performance and features internal diagnosis within diagnosis, through software PLC CPU and external input and output components such as the exchange of information outside the diagnostic functions of diagnosis.

　　The strength of the PLC diagnostic function, the direct impact on the operations and maintenance staff technical capacity requirements, and the impact mean time to repair.

　　六）Processing speed

　　PLC uses scanning mode. From real-time requirements, the processing speed should sooner the better, if the signal duration is less than the scan time, the PLC will not scan the signal, causing the loss of signal data.

　　Processing speed and the length of the user program, CPU processing speed, software quality and so on. Currently, fast response PLC contacts, high speed, each binary instruction execution time is about 0.2 ~ 0.4Ls, so they can adapt to the control requirements, the corresponding requirements of fast application needs. Scan cycle (processor scan) should be met: Small PLC scan time is not more than 0.5ms / K; medium-sized PLC scan time is not more than 0.2ms / K.

　　四、Choice of models

　　一）PLC Type

　　PLC is structured into integral type and module type two categories, according to the application environment is divided into two types of site installation and control room installation; CPU word length is divided into a press, 4, 8, 16, 32, 64 and so on. From the application point of view, generally can control or selection of input and output points.
　　Integral PLC's I / O points is fixed, and therefore less room for user choice for small control systems; modular PLC offers a variety of I / O cards or card, so users can be more rational selection and configuration control system the I / O points, functional expansion is convenient and flexible, generally used for medium-sized control system.

　　二）Select the input and output modules

　　Input and output modules selection should consider the unity and application requirements. For example, the input module, you should consider the signal level, the signal transmission distance, signal isolation, signal power supply and other applications. Output modules, should be considered optional output module type, usually relay output module with low prices, wide voltage range, short life, longer response time and other characteristics; SCR output modules are suitable for frequent switching, inductive low power factor load situations, but more expensive, overload and poor. Output module has DC output, AC output and analog output, etc., should be consistent with the application requirements.

　　According to the application requirements, a reasonable selection of intelligent input and output modules in order to increase the level of control and reduce application costs. Consider the need to extend the rack or remote I / O racks and so on.

　　三）Power Supply Selection

　　PLC power supply, in addition to the introduction of the device while the introduction of PLC product specification requirements should be based on the design and selection, the general PLC power supply design selection should 220VAC power, consistent with the domestic grid voltage. Critical applications should use an uninterruptible power supply or a regulated power supply.

　　If the PLC itself has to use the power, you should check whether the current supplied to meet the application requirements, or external power supply should be designed. To prevent external high voltage power supply due to misuse and the introduction of PLC, the input and output signals of isolation is necessary, and sometimes can be a simple diode or fuse tube isolation.

　　四）Choose memory

　　Since the development of integrated computer chip technology, the price of memory has dropped, therefore, in order to ensure the normal operation of the application program, the general requirements PLC memory capacity, according to 256 I / O points selected at least 8K memory options. When the need for complex control functions, should choose a larger capacity, higher grades of memory.

　　五）Select the redundancy function
　　Redundancy control unit
　　（1）Important process unit: CPU (including memory) and redundant power supply should 1B1.
　　（2）It can also be used when needed PLC Hot Standby redundant system hardware and software configuration of the Hot Standby, 2 or 3 of the heavy weight of the redundant fault-tolerant systems.
　　Redundant .I / O interface unit
　　（1）Control loop multi I / O card should be redundant configuration.
　　（2）Important detection point multi-point I / O cards can be redundant configuration. 3) the need for significant I / O signals are available with two or three of the heavy weight of the I / O interface unit.

　　六）Economic considerations

　　When selecting PLC, should be considered cost-effective. View of economy, should also consider the application of scalability, operability, input-output ratio and other factors, were compared and balanced, and ultimately elect more satisfactory products.

　　O points have a direct impact on prices. An input-output card for each additional member on the need to increase certain fees. When the number of points to a certain value, the corresponding memory capacity, chassis, motherboards, also have a corresponding increase, thus increasing the number of points on the CPU selection, memory capacity, the range of control functions and other options have an effect. In the estimation and selection should take full account of the entire control system has a more reasonable cost performance.