【正文】 trols。 little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems so the cost of a packaged PLC is low pared to the cost of a specific custombuilt controller design. On the other hand, in the case of massproduced goods, customized control systems are economic due to the lower cost of the ponents, which can be optimally chosen instead of a generic solution, and where the nonrecurring engineering charges are spread over thousands or millions of units. For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities. A microcontrollerbased design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies, input/output hardware and necessary testing and certification) can be spread over many sales, and where the enduser would not need to alter the control. Automotive applications are an example。 millions of units are built each year, and very few endusers alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLC instead of customdesigned controls, because the volumes are low and the development cost would be uneconomic. Very plex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even highperformance PLC. Very highspeed or precision controls may also require customized solutions。 for example, 4 aircraft flight controls. Programmable controllers are widely used in motion control, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that Gcode (involving a CNC machine) can be used to instruct machine movements. PLC may include logic for singlevariable feedback analog control loop, a proportional, integral, derivative or PID controller. A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLC was usually configured with only a few analog control loops。 where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLC has bee more powerful, the boundary between DCS and PLC applications has bee less distinct. PLC has similar functionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly bees more powerful and cheaper, RTU, PLC and DCS are increasingly beginning to overlap in responsibilities, and many vendors sell RTU with PLClike features and vice versa. The industry has standardized on the IEC 611313 functional block language for creating programs to run on RTU and PLC, although nearly all vendors also offer proprietary alternatives and associated development environments. 2 The structure of PLC system Structurally divides, PLC divides into the stationary type and the bined type (module type) two kinds. Stationary PLC including the CPU board, the I/O board, demonstrated the kneading board, the memory block, the power source and so on。 these elements bine not a dismountable whole. Module type PLC including the CPU module, the I/O module, the memory, the power source module, the ledger wall or the rack, these modules may defer to certain rule bination disposition. CPU constitution CPU is the PLC core, plays nerve center39。s role, wraps PLC to have CPU at least every time, it the function which entrusts with according to the PLC system program receives and stores the user program and the data, with scanning way gathering the condition or the data which sends by the scene input device, coexisting enters the stipulation in the register, 5 simultaneously, diagnoses the power source and in the PLC internal circuit active status and the programming process grammatical error and so on. After enters the movement, one by one reads the instruction from the user program memory, the duty which stipulated according to the instruction produces the corresponding control signal again after the analysis, directs the related control circuit. CPU mainly by the operator, controller, register and implementation of data link between them, control and state BUS, CPU unit also includes peripheral chips, bus interface and related circuitry. Memory is mainly used for storing programs and data, is an integral unit PLC. In the user view, unnecessarily detailed analysis of the CPU39。s internal circuitry, but the working mechanism of the various parts, or should have sufficient understanding. CPU and the control work, which will read the instructions, directives and executive orders to explain. However, the pace of work from the vibration signals. Computing devices used for digital or logic operation, under the mand of the controller work. CPU speed and memory capacity are important parameters for PLC, which determines the pace of work PLC, I/O number and software capacity, etc., thereby limiting the control of the scale. I/O modules PLC interface with the electrical circuit is through the input and output section ( I/O) pletion. I/O module integrates the PLC39。s I/O circuits。 the input register reflecting the input signal status, output point reflects the state of the output latch. Input module will transform electrical signals into digital signals into the PLC system, the output module opposite. I/O into digital input (DI), digital outputs (DO), analog input (AI), analog output (AO) modules. Common I/O as follow Switching capacity: voltage level by points, with 220VAC, 110VAC, 24VDC, by Way of isolation, there is isolation a