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降至低于1V以下）。当输入信号停止，LED二极管熄灭，晶体管停止导通，集电极电压增大，CPU接收到逻辑1的信息。 1.7 PLC控制器的输出
自动化系统如果不与某些输出设备相连接是不完整的。一些最常用的设备如电机，电磁阀，继电器，指示灯，和类似的声音信号。通过启动电机，或继电器，PLC可管理或控制一个单一的系统，如产品分类系统一直到复杂的系统，数控机床的定位头服务系统。输出类型可以是模拟量或数字。数字输出信号可以作为一个开关，它连接和断开连接线。模拟输出是用来产生模拟信号（例如，电机速度是由对应速度所需的电压控制）。 1.8 输出接口调整
输出接口类似于输入接口，如我们看到的图8-3。 CPU带来了一个信号使LED二极管发光。光煽动光电晶体管开始导电，从而集电极和发射极之间的电压下降到0.7V，并连接到输出装置，将此视为一个逻辑零。相反地，它意味着一个信号在输出端存在，并且被解释为逻辑1。光电晶体管没有直接连接到PLC控制器的输出端。光电晶体管和输出端之间通常有一个继电器或一个大的晶体管以能确保能够打断更强的信号。 1.9 扩展线
每一个PLC控制器的输入/输出线数量都是有限的。需要时可以由系统扩展通过扩展线连接的某些额外模块来增加，每个模块可以包含扩展的输入线和输出线。当然，在PLC控制器的扩展模块上也可以有不同性质的输入和输出(例如继电器输出控制器和晶体管输出可以在同一个扩展模块中)。 2.1 PLC运行过程
PLC的工作就是不断的扫描程序，我们可以把这个扫描周期视为三个重要的步骤，如图8-4所示。（PLC的工作过程）通常不止这三个步骤，但我们可以专注这三个重要的部分而不必考虑其它的。一般其他的部分是用来检查系统以及更新当前内部计数器和定时器的值。
关键词：可编程控制器，PLC，控制器
摘自：王伟，张艳丽主编《电气工程与自动化专业英语》，机械工业出版社，第八章（P84-P88）,Chapter 8 Programmable Logic Controller Technology
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Chapter 8 Programmable Logic Controller Technology
Introduction
PLC( programmable logic controller ) is actually an industrial microcontroller system (in more recent times we meet processors instead of microcontrollers) where you have hardware and software specifically adapted to industrial environment. It is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/of its outputs. The user enters a program, usually via software or programmer that gives the desired result.
Black schema with typical components, which PLC consists of, is found in Fig .8-1.Special attention need to be given to input an output, because in these blocks you find protection needed in isolating a CPU blocks from damaging influences that industrial environment can bring to a CPU via input lines. Program unit is usually a computer used for writing a program (often in ladder diagram). 1.1 CPU-Central Processing Unit
Central Processing Unit (CPU) is the brain of a PLC controller. CPU itself is usually one of the microcontrollers. Aforetime these were 8-bit microcontrollers such as 8051, and now these are 16-bit and 32-bit microcontrollers. Unspoken rule is that you’ll find mostly Hitachi and Fujicu microcontrollers in PLC controllers by Japanese makers, Siemens in European controllers, and Motorola microcontrollers in American ones. CPU also takes care of communication, interconnectedness among other parts of PLC controller, program execution, memory operation, overseeing input and setting up of an output. PLC controllers have complex routines for memory checkup in order to ensure that PLC memory was not damaged (memory checkup is done for safety reasons). Generally speaking, CPU unit makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early. You can simply look at any PLC controller and see that there several indicators in the form of light diodes for error signalization. 1.2 Memory
System memory (today mostly implemented in FLASH technology) is used by a PLC for an process control system. Aside from this operation system it also contains a user program translated from a ladder diagram to a binary form. FLASH memory contents can be changed only in case where user program is being changed. PLC controllers were used earlier instead of FLASH memory and have had EPROM memory instead of FLASH memory which had to be erased with UV lamp and programmed on programmers. With the use of FLASH technology this process was greatly shortened. Reprogramming a program memory is done through a serial cable in a program for application development. Use memory is divide into blocks having special functions. Some parts of a memory are used for storing input and output status. The real status of an input is stored either as “1” or as “0” in a specific memory bit. Each input or output has one corresponding bit in memory. Other parts of memory are used to store variable contents for variables used in user program. For example, timer value, or counter value would be stored in this part of the memory. 1.3 Programming a PLC Controller
PLC controller can be reprogrammed through a computer (usual way), but also through
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manual programmers (consoles). This practically means that each PLC controller can programmed through a computer if you have the software needed for programming. Today’s transmission computers are ideal for reprogramming a PLC controller in factory itself. This is of great importance to industry. Once the system is corrected, it is also important to read the right program into a PLC again. It is also good to check from time to time whether program in a PLC has not changed. This helps to avoid hazardous situation in factory rooms (some automakers have established communication networks which regularly check programs in PLC controllers to ensure execution only of good programs ). Almost every program for programming a PLC controller possesses various useful options such as: forced switching on and off the system inputs/outputs(I/O lines) , program follow up in real time as well as documenting a diagram. This documenting is necessary to understand and define failures and malfunctions. Programmer can add remarks, names of input or output devices, and comments that can be useful when finding errors,or with system maintenance. Adding comments and remarks enables any technician（and not just a person who developed the system）to understand a ladder diagram right away. Comments and remarks can even quote precisely part number if replacements would be needed. This would speed up a repair of any problems that come up due to bad parts. The old way was such that a person who developed a system had protection on the program. So nobody aside from this person could understand how it was done. Correctly documented ladder diagram allows any technician to understand thoroughly how system functions.
1.4 Power Supply
Electrical supply is used in bringing electrical energy to central processing unit. Most PLC controllers work either as 24V DC or 220V AC. On some PLC controllers you’ll find electrical supply as a separate module. Those are usually bigger PLC controllers, while small and medium series already contain the supply module. User has to determine how much current to take from I/O module to ensure that electrical supply provides appropriate amount of current. Different types of module use different amounts of electrical current. This electrical supply is usually not used to start external inputs or outputs. User has to provide separate supplies in starting PLC controller inputs or outputs because then you can ensure so called “pure” supply for the PLC controller. With pure supply we mean supply where industrial environment can not affect it damagingly. Some of the smaller PLC controllers supply their inputs with voltage from a small supply source already incorporated into a PLC. 1.5 PLC Controller Inputs
Intelligence of an automated system depends largely on the ability of a PLC controller to read signal from different types of sensors and input devices. Keys, keyboards and by functional switches are a basis for man versus machine relationship. On the other hand, in order to detect a working piece, view a mechanism in motion, check pressure or fluid level you need specific automatic devices such as proximity sensors, marginal switches, photoelectric sensors, level sensors,etc. Thus, input signals can be logical (on/off) or analogue. Smaller PLC controllers usually have only digital input lines while larger also accept analogue inputs through special units attached to PLC controller. One of the most frequent analogue signals are a current signal of 4 to 20 mA and millivolt voltage signal generated by various sensors.
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Sensors are usually used as inputs for PLCs. You can obtain sensors for different purposes. They can sense presence of some parts, measure temperature, pressure,or some other physical dimension, etc.(ex. inductive sensors can register metal object ). Other devices also can serve as inputs to PLC controller. Intelligent devices such as robots, video system, etc. often are capable of sending signals to PLC controller input modules (robot, for instance, can send a signal to PLC controller inputs as information when it has finished moving an object from one place to the other).
1.6 Input Adjustment Interface
Adjustment interface also called an interface is placed between input lines and a CPU unit. The purpose of adjustment is to protect a CPU from disproportionate signals from an outside world. Input adjustment module turns a level of real logic to a level that suits CPU unit (ex. Input from a sensor which works on 24V DC must be converted to a signal of 5V DC in order for a CPU to be able to process it ). This is typically done through opto-isolation, and this function you can view in Fig.8-2. Opto-isolation means that there is no electrical connection between external world and CPU unit. They are “optically” separated, or in other words ,signal is transmitted through light. The way this works is simple. External device brings a signal which turns LED on, whose light in turn incites photos transistor which in turn starts conducting, and a CPU sees this as logic zero (supply between collector and transmitter falls under 1V). When input signal stops LED diode turns off, transistor stops conducting, collector voltage increases,and CPU receives logic 1 as information. 1.7 PLC Controller Output
Automated system is incomplete if it is not connected with some output devices. Some of the most frequently used devices are motors, solenoids, relays, indicators, sound signalization and similar. By starting a motor, or a relay, PLC can manage or control a simple system such as system for sorting products all the way up to complex systems such as service system for positioning head of CNC machine. Output can be of analogue or digital type. Digital output signal works as a switch and it connects and disconnects line. Analogue output is used to generate the analogue signal (ex. Motor whose speed is controlled by a voltage that corresponds to a desired speed). 1.8 Output Adjustment Interface
Output interface is similar to input interface, as we see in Fig.8-3. CPU brings a signal to LED diode and turns it on. Light incites a photo transistor which begins to conduct electricity, and thus the voltage between collector and emitter falls to 0.7V, and a device attached to this output sees this as a logic zero. Inversely it means that a signal at the output exists and is interpreted as logic one. Photo transistor is not directly connected to a PLC controller output. Between photo transistor and an output usually there is a relay or a stronger transistor capable of interrupting stronger signals. 1.9 Extension Lines
Every PLC controller has a limited number of input/output line. If needed this number can be increased through certain additional modules by system extension through extension line. Each module can contain extension both of input and output lines. Also, extension modules can have inputs and outputs of a different nature from those on the PLC controller (ex. In case relay
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