Programmable Logic Controller 英文原版第四版教程

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Programmable Logic Controller (PLC) 入门教材 Programmable Logic Controller (PLC) 是一种基于微处理器的自动控制系统，广泛应用于工业自动化、过程控制、机器人控制等领域。下面是关于 PLC 的详细知识点： 1. PLC 的定义和历史发展 Programmable Logic Controller (PLC) 是一种基于微处理器的自动控制系统，能够根据用户编写的程序来控制外部设备。PLC 的历史可以追溯到 1960年代，当时的 PLC 主要用于替代传统的继电器控制系统。随着技术的发展，PLC 的功能和性能不断提高，目前已经广泛应用于工业自动化、过程控制、机器人控制等领域。 2. PLC 的组成部分 PLC 主要由以下几个部分组成： * 微处理器（CPU）：负责执行用户编写的程序，控制外部设备的操作。 * 输入/输出模块（I/O）：负责与外部设备交互，接收和发送信号。 * 存储器（Memory）：用于存储用户编写的程序和数据。 * 电源供应模块（Power Supply）：提供电源给 PLC 的各个部分。 * 编程设备（Programming Device）：用于编写和下载用户程序。 3. PLC 的工作原理 PLC 的工作原理可以概括为以下几个步骤： * 输入信号处理：PLC 从输入模块接收外部设备的信号。 * 程序执行：PLC 根据用户编写的程序，执行相应的操作。 * 输出信号处理：PLC 向外部设备发送信号，控制其操作。 * 循环执行：PLC 不断执行上述步骤，实现对外部设备的实时控制。 4. PLC 的应用领域 PLC 广泛应用于工业自动化、过程控制、机器人控制等领域，例如： * 工业自动化：PLC 可以用于控制生产线、机器人、输送带等设备。 * 过程控制：PLC 可以用于控制化学反应、油气处理、水处理等过程。 * 机器人控制：PLC 可以用于控制机器人的运动和操作。 5. PLC 的优点和缺点 PLC 的优点包括： * 高度灵活性：PLC 可以根据用户的需求编写不同的程序，实现不同的控制功能。 * 高度可靠性：PLC 的设计和制造都遵循严格的标准，确保其在恶劣环境下的可靠操作。 * 高度灵活性：PLC 可以与不同的设备和系统集成，实现自动化控制。 PLC 的缺点包括： * 高度复杂性：PLC 的编程和操作需要具有专业知识和经验。 * 高度依赖性：PLC 的操作需要依赖于电源供应和外部设备的稳定性。 PLC 是一种功能强大、应用广泛的自动控制系统，对于工业自动化、过程控制、机器人控制等领域具有重要的作用。

interfaces are discussed in Chapter 2. Input and output devices can be

classified as giving signals which are discrete, digital or analogue

(Figure 1.5). Devices giving discrete or digital signals are ones where

the signals are either off or on. Thus a switch is a device giving a

discrete signal, either no voltage or a voltage. Digital devices can be

considered to be essentially discrete devices which give a sequence of

on−off signals. Analogue devices give signals whose size is

proportional to the size of the variable being monitored. For example,

a temperature sensor may give a voltage proportional to the

temperature.

Time

Voltage

(a)

Time

Voltage

(b)

Time

Voltage

(c)

Figure 1.5 Signals: (a) discrete, (b) digital, (c) analogue

6The communications interface is used to receive and transmit data on

communication networks from or to other remote PLCs (Figure 1.6).

It is concerned with such actions as device verification, data

acquisition, synchronisation between user applications and

connection management.

Supervisory

system

PLC 1 PLC 2

Communications

network

Machine/

plant

Machine/

plant

Figure 1.6 Basic communications model

1.3 Internal architecture Figure 1.7 shows the basic internal architecture of a PLC. It consists of a

central processing unit (CPU) containing the system microprocessor,

memory, and input/output circuitry. The CPU controls and processes all

the operations within the PLC. It is supplied with a clock with a frequency

of typically between 1 and 8 MHz. This frequency determines the

operating speed of the PLC and provides the timing and synchronisation

for all elements in the system. The information within the PLC is carried

by means of digital signals. The internal paths along which digital signals

flow are called buses. In the physical sense, a bus is just a number of

Programmable logic controllers 5

being a binary digit of 1 or 0, i.e. on/off states. The term word is used for

the group of bits constituting some information. Thus an 8-bit word might

be the binary number 00100110. Each of the bits is communicated

simultaneously along its own parallel wire. The system has four buses:

1The data bus carries the data used in the processing carried out by the

CPU. A microprocessor termed as being 8-bit has an internal data bus

which can handle 8-bit numbers. It can thus perform operations

between 8-bit numbers and deliver results as 8-bit values.

2The address bus is used to carry the addresses of memory locations.

So that each word can be located in the memory, every memory

location is given a unique address. Just like houses in a town are each

given a distinct address so that they can be located, so each word

location is given an address so that data stored at a particular location

can be accessed by the CPU either to read data located there or put,

i.e. write, data there. It is the address bus which carries the

information indicating which address is to be accessed. If the address

bus consists of 8 lines, the number of 8-bit words, and hence number

of distinct addresses, is 2

= 256. With 16 address lines, 65 536

addresses are possible.

3The control bus carries the signals used by the CPU for control, e.g.

to inform memory devices whether they are to receive data from an

input or output data and to carry timing signals used to synchronise

actions.

4The system bus is used for communications between the input/output

ports and the input/output unit.

1.3.3 Memory

There are several memory elements in a PLC system:

1System read-only-memory (ROM) to give permanent storage for the

operating system and fixed data used by the CPU.

2 Random-access memory (RAM) for the user’s program.

3 Random-access memory (RAM) for data. This is where information is

stored on the status of input and output devices and the values of

timers and counters and other internal devices. The data RAM is

sometimes referred to as a data table or register table. Part of this

memory, i.e. a block of addresses, will be set aside for input and

output addresses and the states of those inputs and outputs. Part will

be set aside for preset data and part for storing counter values, timer

values, etc.

4 Possibly, as a bolt-on extra module, erasable and programmable

read-only-memory (EPROM) for ROMs that can be programmed and

then the program made permanent.

The programs and data in RAM can be changed by the user. All PLCs

will have some amount of RAM to store programs that have been

developed by the user and program data. However, to prevent the loss of

programs when the power supply is switched off, a battery is used in the

PLC to maintain the RAM contents for a period of time. After a program

Programmable logic controllers 7

has been developed in RAM it may be loaded into an EPROM memory

chip, often a bolt-on module to the PLC, and so made permanent. In

addition there are temporary buffer stores for the input/output channels.

The storage capacity of a memory unit is determined by the number of

binary words that it can store. Thus, if a memory size is 256 words then it

can store 256 × 8 = 2048 bits if 8-bit words are used and 256 × 16 = 4096

bits if 16-bit words are used. Memory sizes are often specified in terms of

the number of storage locations available with 1K representing the

number 2

, i.e. 1024. Manufacturers supply memory chips with the

storage locations grouped in groups of 1, 4 and 8 bits. A 4K % 1 memory

has 4 % 1 % 1024 bit locations. A 4K % 8 memory has 4 % 8 % 1024 bit

locations. The term byte is used for a word of length 8 bits. Thus the 4K %

8 memory can store 4096 bytes. With a 16-bit address bus we can have 2

different addresses and so, with 8-bit words stored at each address, we can

have 2

% 8 storage locations and so use a memory of size 2

% 8/2

64K % 8 which we might be as four 16K % 8 bit memory chips.

1.3.4 Input/output unit

The input/output unit provides the interface between the system and the

outside world, allowing for connections to be made through input/output

channels to input devices such as sensors and output devices such as

motors and solenoids. It is also through the input/output unit that

programs are entered from a program panel. Every input/output point has

a unique address which can be used by the CPU. It is like a row of houses

along a road, number 10 might be the ‘house’ to be used for an input from

a particular sensor while number ‘45’ might be the ‘house’ to be used for

the output to a particular motor.

The input/output channels provide isolation and signal conditioning

functions so that sensors and actuators can often be directly connected to

them without the need for other circuitry. Electrical isolation from the

external world is usually by means of optoisolators (the term optocoupler

is also often used). Figure 1.8 shows the principle of an optoisolator.

When a digital pulse passes through the light-emitting diode, a pulse of

infrared radiation is produced. This pulse is detected by the

phototransistor and gives rise to a voltage in that circuit. The gap between

the light-emitting diode and the phototransistor gives electrical isolation

but the arrangement still allows for a digital pulse in one circuit to give

rise to a digital pulse in another circuit.

Photo-

transistor

Light-

emitting

diode

Infrared radiation

Figure 1.8 Optoisolator

The digital signal that is generally compatible with the microprocessor

in the PLC is 5 V d.c. However, signal conditioning in the input channel,

8 Programmable Logic Controllers

with isolation, enables a wide range of input signals to be supplied to it

(see Chapter 3 for more details). A range of inputs might be available with

a larger PLC, e.g. 5 V, 24 V, 110 V and 240 V digital/discrete, i.e.

on−off, signals (Figure 1.9). A small PLC is likely to have just one form

of input, e.g. 24 V.

Input

channel

5 V

24 V

110 V

240 V

5 V

Inputs:

digital signal levels

To input/

output unit

Digital

signal level

Figure 1.9 Input levels

The output from the input/output unit will be digital with a level of 5 V.

However, after signal conditioning with relays, transistors or triacs, the

output from the output channel might be a 24 V, 100 mA switching signal,

a d.c. voltage of 110 V, 1 A or perhaps 240 V, 1 A a.c., or 240 V, 2 A

a.c., from a triac output channel (Figure 1.10). With a small PLC, all the

outputs might be of one type, e.g. 240 V a.c., 1 A. With modular PLCs,

however, a range of outputs can be accommodated by selection of the

modules to be used.

channel

24 V, 100 mA

110 V, 1 A, d.c.

240 V, 1 A, a.c.

240 V, 2 A, a.c.

Switching

Outputs

Output

From

input/

output

unit

5 V

digital

Figure 1.10 Output levels

Outputs are specified as being of relay type, transistor type or triac type

(see Chapter 3 for more details):

1 With the relay type, the signal from the PLC output is used to operate

a relay and is able to switch currents of the order of a few amperes in

an external circuit. The relay not only allows small currents to switch

much larger currents but also isolates the PLC from the external

circuit. Relays are, however, relatively slow to operate. Relay outputs

are suitable for a.c. and d.c. switching. They can withstand high surge

currents and voltage transients.

2The transistor type of output uses a transistor to switch current

through the external circuit. This gives a considerably faster

switching action. It is, however, strictly for d.c. switching and is

destroyed by overcurrent and high reverse voltage. As a protection,

either a fuse or built-in electronic protection are used. Optoisolators

are used to provide isolation.

Programmable logic controllers 9

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