1、Programmable logic controllerCynthia CooperFrom Wikipedia, the free encyclopediaA programmable logic controller or simply programmable controller is a digital computer used for HYPERLINK ://wiki/Automation o Automation automation of industrial processes, such as control of machinery

2、on factory HYPERLINK ://wiki/Assembly_line o Assembly line assembly lines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to

3、 control machine operation are typically stored in battery-backed or HYPERLINK ://wiki/Non-volatile_storage o Non-volatile storage non-volatile memory. A PLC is an example of a HYPERLINK ://wiki/Real_time o Real time real time system since output results must be produ

4、ced in response to input conditions within a bounded time, otherwise unintended operation will result.FeaturesControl panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; HYPERLINK ://wiki/Power_supply o Power supply power supply,

5、controller, HYPERLINK ://wiki/Relay o Relay relay units for input and output.The main difference from other computers is that PLCs are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive HYPERLINK ://wiki/Input/output o I

6、nput/output input/output (I/O) arrangements. These connect the PLC to HYPERLINK ://wiki/Sensor o Sensor sensors and HYPERLINK ://wiki/Actuator o Actuator actuators. PLCs read limit HYPERLINK ://wiki/Switch o Switch switches, analog process variables (s

7、uch as temperature and pressure), and the positions of complex positioning systems. Some even use HYPERLINK ://wiki/Machine_vision o Machine vision machine vision. On the actuator side, PLCs operate HYPERLINK ://wiki/Electric_motor o Electric motor electric motors, HY

8、PERLINK ://wiki/Pneumatic o Pneumatic pneumatic or HYPERLINK ://wiki/Hydraulic o Hydraulic hydraulic cylinders, magnetic HYPERLINK ://wiki/Relay o Relay relays or HYPERLINK ://wiki/Solenoid o Solenoid solenoids, or analog outputs. The i

9、nput/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.PLCs were invented as replacements for automated systems that would use hundreds or thousands of HYPERLINK ://wiki/Relay o Relay re

10、lays, HYPERLINK ://wiki/Cam_timer o Cam timer cam timers, and HYPERLINK ://w/index.php?title=Drum_sequencer&action=edit&redlink=1 o Drum sequencer (page does not exist) drum sequencers. Often, a single PLC can be programmed to replace thousands of HYPERLINK :/en.wikip

11、/wiki/Relays o Relays relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed.Many of the earliest PLCs expressed all decision making logic in

12、simple HYPERLINK ://wiki/Ladder_logic o Ladder logic ladder logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training de

13、mands for the existing technicians. Other early PLCs used a form of HYPERLINK ://wiki/Instruction_list o Instruction list instruction list programming, based on a stack-based logic solver.The functionality of the PLC has evolved over the years to include sequential relay control, mot

14、ion control, HYPERLINK ://wiki/Process_control o Process control process control, HYPERLINK ://wiki/Distributed_control_system o Distributed control system distributed control systems and HYPERLINK ://wiki/Computer_network o Computer network networking

15、. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to HYPERLINK ://wiki/Desktop_computer o Desktop computer desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose d

16、esktop computer to overlap some PLCs in certain applications.Under the HYPERLINK ://wiki/IEC_61131-3 o IEC 61131-3 IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called HYPERLINK ://wiki/Seque

17、ntial_function_chart o Sequential function chart Sequential Function Charts is available on certain programmable controllers.PLC compared with other control systemsPLCs are well-adapted to a range of HYPERLINK ://wiki/Automation o Automation automation tasks. These are typically indu

18、strial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation.PLCs contain input and output devices compatible with industrial pilot devices and controls.PLC applications are typically highly customized syste

19、ms so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a generic soluti

20、on。 For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer HYPERLINK ://wiki/Dishwasher o Dishwasher dishwasher would be controlled by an electromechanical HYPERLINK ://wiki/Cam_timer o Cam timer cam timer costing

21、 only a few dollars in production quantities.A HYPERLINK ://wiki/Microcontroller o Microcontroller microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies and input/output hardware)

22、can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economi

23、cally use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-spee

24、d or precision controls may also require customized solutions; for example, aircraft flight controls.PLCs may include logic for single-variable feedback analog control loop, a proportional, integral, derivative or HYPERLINK ://wiki/PID_controller o PID controller PID controller. A PI

25、D loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a HYPERLINK ://wiki/Distributed_control_system o Distribut

26、ed control system distributed control system (DCS) would instead be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.Digital and analog signalsDigital or discrete signals behave as binary switches, yielding simply an On or Off

27、signal (1 or 0, True or False, respectively). Push buttons, limit switches, and HYPERLINK ://wiki/Photoelectric_sensor o Photoelectric sensor photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either HYPERLINK :/

28、/wiki/Voltage o Voltage voltage or HYPERLINK ://wiki/Current_%28electricity%29 o Current (electricity) current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC repres

29、enting Off, and intermediate values undefined. Initially, PLCs had only discrete I/O.Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the de

30、vice and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use HYPERLINK :/en.wikipedia

31、.org/wiki/Voltage o Voltage voltage or HYPERLINK ://wiki/Current_%28electricity%29 o Current (electricity) current with a magnitude proportional to the value of the process signal. For example, an analog HYPERLINK ://wiki/4-20_mA o 4-20 mA 4-20 mA or 0 - 10V input wou

32、ld be HYPERLINK ://wiki/A/D_converter o A/D converter converted into an integer value of 0 - 32767.ExampleAs an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in

33、 the tank.Using only digital signals, the PLC has two digital inputs from HYPERLINK ://wiki/Float_switch o Float switch float switches (tank empty and tank full). The PLC uses a digital output to open and close the inlet HYPERLINK ://wiki/Valve o Valve valve into the

34、tank.When the water level drops enough so that the tank empty float switch is off (down), the PLC will open the valve to let more water in. Once the water level raises enough so that the tank full switch is on (up), the PLC will shut the inlet to stop the water from overflowing.An analog system migh

35、t use a water HYPERLINK ://wiki/Pressure_sensor o Pressure sensor pressure sensor or a HYPERLINK ://wiki/Load_cell o Load cell load cell, and an adjustable (throttling)dripping out of the tank, the valve adjusts to slowly drip water back into the tank.In this system,

36、to avoid flutter adjustments that can wear out the valve, many PLCs incorporate HYPERLINK ://wiki/Hysteresis o Hysteresis hysteresis which essentially creates a HYPERLINK ://wiki/Deadband o Deadband deadband of activity. A technician adjusts this deadband so the valve

37、 moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear.A real system might combine both approaches, using float switches and simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates and prevent HYPERLINK

38、 ://wiki/Water_hammer o Water hammer water hammer. Backup and maintenance methods can make a real system very complicated.System scaleA small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model does not ha

39、ve enough I/O.PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often us

40、ed for HYPERLINK ://wiki/User_interface o User interface HMI (Human-Machine Interface) devices such as keypads or HYPERLINK ://wiki/Personal_computer o Personal computer PC-type workstations. Some of todays PLCs can communicate over a wide range of media including RS-

41、485, Coaxial, and even Ethernet for I/O control at network speeds up to 100 Mbit/s.ProgrammingEarly PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming HYPERLINK ://wiki/Computer_terminal o Computer terminal terminals. they w

42、ere designed to replace relay logic systems. These PLCs were programmed in HYPERLINK ://wiki/Ladder_logic o Ladder logic ladder logic, which strongly resembles a schematic diagram of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditiona

43、l programming languages such as BASIC and C. Another method is HYPERLINK ://wiki/State_Logic o State Logic State Logic, a HYPERLINK ://wiki/Very_High_Level_Programming_Language o Very High Level Programming Language Very High Level Programming Language designed to pro

44、gram PLCs based on HYPERLINK ://wiki/State_diagram o State diagram State Transition Diagrams.Recently, the International standard HYPERLINK ://wiki/IEC_61131-3 o IEC 61131-3 IEC 61131-3 has become popular. IEC 61131-3 currently defines five programming languages for p

45、rogrammable control systems: FBD ( HYPERLINK ://wiki/Function_block_diagram o Function block diagram Function block diagram), LD ( HYPERLINK ://wiki/Ladder_logic o Ladder logic Ladder diagram), ST ( HYPERLINK ://wiki/Structured_text o Structured text S

46、tructured text, similar to the HYPERLINK ://wiki/Pascal_programming_language o Pascal programming language Pascal programming language), IL ( HYPERLINK ://wiki/Instruction_list o Instruction list Instruction list, similar to HYPERLINK ://wiki/Assembly_

47、language o Assembly language assembly language) and SFC ( HYPERLINK ://wiki/Sequential_function_chart o Sequential function chart Sequential function chart). These techniques emphasize logical organization of operations.While the fundamental concepts of PLC programming are common to

48、all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.User interfacePL

49、Cs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A HYPERLINK ://wiki/User_interface o User interface Human-Machine Interface (HMI) is employed for this purpose. HMIs are also referred to as MMIs (Man Machine Interface) and GUI

50、 (Graphical User Interface).A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. Most modern PLCs can communicate over a network to some other system, such as a computer running a HYPERLINK ://wiki/SCADA

51、 o SCADA SCADA (Supervisory Control And Data Acquisition) system or web browser.CommunicationsPLCs usually have built in communications ports usually 9-Pin HYPERLINK ://wiki/RS232 o RS232 RS232, and optionally for HYPERLINK ://wiki/RS485 o RS485 RS485 and HYPERLINK :/

52、/wiki/Ethernet o Ethernet Ethernet. HYPERLINK ://wiki/Modbus o Modbus Modbus or HYPERLINK ://w/index.php?title=DF1&action=edit&redlink=1 o DF1 (page does not exist) DF1 is usually included as one of the HYPERLINK ://wiki/Communications_

53、protocols o Communications protocols communications protocols. Others options include various HYPERLINK ://wiki/Fieldbus o Fieldbus fieldbuses such as HYPERLINK ://wiki/DeviceNet o DeviceNet DeviceNet or HYPERLINK ://wiki/Profibus o Profibus Profibus.

54、HistoryThe PLC was invented in response to the needs of the American automotive industry. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using relays, timers and dedicated closed-loop controllers. The process for updating such facilitie

55、s for the yearly model change-over was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposal for an electronic replacement for hard-wired relay

56、systems.The winning proposal came from Bedford Associates of HYPERLINK ://wiki/Bedford%2C_Massachusetts o Bedford, Massachusetts Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates eighty-fourth project, was the result. Bedford Associates start

57、ed a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was HYPERLINK ://wiki/Dick_Morley o Dick Morley Dick Morley, who is considered to be the

58、 father of the PLC. The Modicon brand was sold in 1977 to HYPERLINK ://wiki/Gould_Electronics o Gould Electronics Gould Electronics, and later acquired by German Company HYPERLINK ://wiki/AEG o AEG AEG and then by French HYPERLINK ://wiki/Schneider_Ele

59、ctric o Schneider Electric Schneider Electric, the current owner.The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of its controller models. PLCs are used in many different industries and machines such as packaging and semiconductor machines. Well know

60、n PLC brands are HYPERLINK ://wiki/Toshiba o Toshiba Toshiba, HYPERLINK ://wiki/Siemens_AG o Siemens AG Siemens, HYPERLINK ://wiki/Allen-Bradley o Allen-Bradley Allen-Bradley, HYPERLINK ://wiki/ABB_Ltd. o ABB Ltd. ABB, HYPERLINK :/en.wi