1、 Programmable logic controller Cynthia Cooper From Wikipedia, the free encyclopedia A programmable logic controller or simply programmable controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose comp
2、uters, 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 control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a rea
3、l time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result. Features Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right； power supply, controller,
4、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 input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, an
5、alog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrangeme
6、nts 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 relays, cam timers, and drum sequencers. Often, a single PLC can be pr
7、ogrammed to replace thousands of 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
8、 in simple 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 demands for the existing technicians. Other early PLCs used
9、a form of 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, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and comm
10、unication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. Under the IEC 61131-3 standard, PLCs can be programmed using st
11、andards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. PLC compared with other control systems PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufact
12、uring 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 systems so the cost of a packaged
13、 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 solution。 For high volume or very
14、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 microcontroller-based design would be appropriate where hundreds or thousands of units will be
15、 produced and so the development cost (design of power supplies and input/output hardware) 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 progra
16、mming of these controllers. However, some specialty vehicles such as transit busses economically 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 al
17、gorithms and performance beyond the capability of even high-performance PLCs. Very high-speed 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, derivat
18、ive or PID controller. A PID 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 distributed control system (DCS) would instea
19、d be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut. Digital and analog signals Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, l
20、imit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or 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,
21、 values below 2VDC representing 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 ac
22、curacy depending on the device 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 us
23、e voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 4-20 mA or 0 - 10 V input would be converted into an integer value of 0 - 32767. Example As 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 the tank.
