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3.0 INTRODUCTION TO PLC SYSTEM

1. Basic Characteristics Of PLC

Introduction

Programmable Logic Controllers (PLCs), also referred to as programmable controllers, are in the computer family. They are used in commercial and industrial applications. A PLC monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine.

The National Electrical Manufacturers Association (NEMA) as defines a programmable logic controller:

A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes.

In essence, the programmable logic controller consists of computer hardware, which is programmed to simulate the operation of the individual logic and sequence elements that might be contained in a bank of relays, timers, counters, and other hard-wired components.

1. PLC Terminologies

a. Sensor

A sensor is a device that converts a physical condition into an electrical signal for use by the PLC. Sensors are connected to the input of a PLC.

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b. Actuator

Actuators convert an electrical signal from the PLC into a physical condition. Actuators are connected to the PLC output.

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c. Digital Input

A discrete input, also referred to as a digital input, is an input that is either in an ON or OFF condition. Pushbuttons, toggle switches, limit switches, proximity switches, and contact closures are examples of discrete sensors which are connected to the PLCs discrete or digital inputs. In the ON condition a discrete input may be referred to as a logic 1 or a logic high. In the OFF condition a discrete input may be referred to as a logic 0 or a logic low.

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d. Analog Input

An analog input is an input signal that has a continuous signal. Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts. In the following example, a level transmitter monitors the level of liquid in a tank. Depending on the level transmitter, the signal to the PLC can either increase or decrease as the level increases or decreases.

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e. Digital Output

A discrete output is an output that is either in an ON or OFF condition. Solenoids, contactor coils, and lamps are examples of actuator devices connected to discrete outputs. Discrete outputs may also be referred to as digital outputs.

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f. Analog Output

An analog output is an output signal that has a continuous signal. The output may be as simple as a 0-10 VDC level that drives an analog meter. Examples of analog meter outputs are speed, weight, and temperature.

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g. CPU

The central processor unit (CPU) is a microprocessor system that contains the system memory and is the PLC decisionmaking unit. The CPU monitors the inputs and makes decisions based on instructions held in the program memory. The CPU performs relay, counting, timing, data comparison, and sequential operations.

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h. PLC Programming

A program consists of one or more instructions that accomplish a task. Programming a PLC is simply constructing a set of instructions. There are several ways to look at a program such as ladder logic, statement lists, or function block diagrams.

2. PLC Background

The first Programmable Logic Controller (PLC) was developed by a group of engineers at General Motors in 1968, when the company were looking for an alternative to replace complex relay control systems.

The new control system had to meet the following requirements:

❖ Simple programming

❖ Program changes without system intervention (no internal rewiring)

❖ Smaller, cheaper and more reliable than corresponding relay control systems

❖ Simple, low cost maintenance

Since then, three decades have passed, during which the enormous progress made in the development of micro electronics did not stop short of programmable logic controllers. For instance, even if program optimization and thus a reduction of required memory capacity initially still represented an important key task for the programmer, nowadays this is hardly of any significance.

Moreover, the range of functions has grown considerably. 15 years ago, process visualisation, analogue processing or even the use of a PLC as a controller, were considered as Utopian. Nowadays, the support of these functions forms an integral part of many PLCs.

3. PLC Functions

The Functions of PLC can be divided into 3 main functions which is, sequential control, advanced control, and supervisory control.

Table 3.1: PLC Functions

|Type of Control |Functions |

|Sequential |a. Replace of relay control system |

| |b. Timer/Counter |

| |c. Replace of PCB card control |

| |d. Controller of Automatic, semi automatic and manual system |

|Advanced |a. Solve mathematical problems |

| |b. Information management |

| |c. Analog control |

| |d. Servo motor control |

| |e. Stepper motor control |

| |f. PID control |

|Supervisory |a. Alarm and display process |

| |b. Faulty display and diagnosis |

| |c. Computer interface |

| |d. Printer interface |

| |e. Factory automation network |

| |f. Local area network (LAN) |

| |g. Wide area network (WAN) |

| |h. Factory automation (FA), Flexible Manufacturing System (FMS), Computer |

| |integrated manufacturing (CIM). Etc. |

4. Types of PLC

The PLCs currently on offer in the market place have been adapted to customer requirements to such an extent that it has become possible to purchase an eminently suitable PLC for virtually any application. As such, miniature PLCs are now available with a minimum number of inputs/outputs starting from just a few hundred Pounds. Also available are larger PLCs with 28 or 256 inputs/outputs.

Depending on how the central control unit is connected to the input and output modules, differentiation can be made between compact PLCs (input module, central control unit and output module in one housing) or modular PLCs. Fig. 3.2 shows the FX0 controller by Mitsubishi representing a compact PLC as an example.

Modular PLCs may be configured individually. The modules required for the practical application – apart from digital input/output modules which can, for instance, include analogue, positioning and communication modules – are inserted in a rack, where individual modules are linked via a bus system. This type of design is also known as series technology. The examples of modular PLCs are shown in fig 3.1. These represent the familiar modular PLC series by the new S7-300 series by Siemens.

The card format PLC is a special type of modular PLC, developed during the last few years. With this type, individual or a number of printed circuit board modules are in a standardised housing. The Festo FPC 405 is representative of this type of design (Fig. 3.1).

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Fig. 3.1: Compact PLC (Mitsubishi FX0),

modular PLC(Siemens S7-300),

PLC plug-in cards (Festo FPC 405)

5. Advantages of PLC

As well as more complex tasks, can be done with a PLC. Wiring between devices and relay contacts is done in the PLC program. Hard-wiring, though still required to connect field devices, is less intensive. Modifying the application and correcting errors are easier to handle. It is easier to create and change a program in a PLC than it is to wire and rewire a circuit.

Following are just a few of the advantages of PLCs:

• Smaller physical size than hard-wire solutions.

• Easier and faster to make changes.

• PLCs have integrated diagnostics and override functions.

• Diagnostics are centrally available.

• Applications can be immediately documented.

• Applications can be duplicated faster and less expensively.

A PLC system provides many benefits to control solutions, from reliability and repeatability to programmability. The benefits achieved with programmable controllers will grow with the individual using them—the more you learn about PLCs, the more you will be able to solve other control problems.

Table 3.2 lists some of the many features and benefits obtained with a programmable controller.

Table 3.2: Typical programmable controller features and benefits.

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2. Building Structure of a PLC

With computer systems, differentiation is generally made between hardware, firmware and software. The same applies for a PLC, which is essentially based on a micro computer.

The hardware consists of the actual device technology, i.e. the printed circuit boards, integrated modules, wires, battery, housing, etc. This includes fundamental system routines, used for starting the processor after the power has been switched on. Additionally, there is the operating system in the case of programmable logic controllers, which is generally stored in a ROM, a read-only memory, or in the EPROM.

Firmware is the software part, which is permanently installed and supplied by the PLC manufacturer.

The software, which is the user program written by the PLC user. User programs are usually installed in the RAM, a random access memory, where they can be easily modified.

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Fig. 3.2: Fundamental design of a microcomputer

1. Central Processing Unit

The central processing unit, or CPU, is the most important element of a PLC. The CPU forms what can be considered to be the “brain” of the system. The three components of the CPU are:

• the processor

• the memory system

• the power supply

Fig 3.3 illustrates a simplified block diagram of a CPU. CPU architecture may differ from one manufacturer to another, but in general, most CPUs follow this typical three-component organization.

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Fig 3.3: CPU block diagram

A microprocessor consists in the main of an arithmetic unit, control unit and a small number of internal memory units, so-called registers.

The task of the arithmetic unit – the ALU (arithmetic logic unit) – is to execute arithmetic and logic operations with the data transmitted.

The control unit regulates and controls the entire logic sequence of the operations required for the execution of a command.

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Fig 3.4: Design of a microprocessor

2. Memory Unit

They are several memory elements in a PLC system:

1. System read-only-memory (ROM) to give permanent storage for the operating system and fixed data used by CPU

2. Random-access-memory (RAM) for the users 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.

4. Possibly, as a bold-on extra module, erasable and programmable read-only-memory (EPROM) for ROMs that can be programmed and then the program made permanent.

The program 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 for a period of time. After a program has been developed in RAM it may be loaded into an EPROM memory chip, often a bold-on module to PLC, and so made permanent.

3. Display & Indicator Unit

The CPU status indicators reflect the current mode of CPU operation. If, for example, the mode switch is set to the RUN position, the green RUN indicator is lit. When the mode switch is set to the STOP position, the yellow STOP indicator is lit.

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The I/O status indicators represent the On or Off status of corresponding inputs and outputs. When the CPU senses an input is on, the corresponding green indicator is lit.

4. Input Interface Unit

The input module of a PLC is the module, which sensors or other input devices are connected to. The sensor signals are to be passed on to the central control unit. The important functions of an input module (for the application) are as follows:

▪ Reliable signal detection

▪ Voltage adjustment of control voltage to logic voltage

▪ Protection of sensitive electronics from external voltages

▪ Screening of signals

The main component of today’s input modules which meets these requirements is the optocoupler.

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Fig. 3.5: Block diagram of an input module

Input devices, such as switches, pushbuttons, and other sensor devices are connected to the terminal strip under the bottom cover of the PLC.

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5. Output Interface Unit

The output module of a PLC is the module, which output devices are connected to. Output modules conduct the signals of the central control unit to final control elements, which are actuated according to the task. In the main, the function of an output – as seen from the application of the PLC – therefore includes the following:

❖ Voltage adjustment of logic voltage to control voltage

❖ Protection of sensitive electronics from spurious voltages from the controller

❖ Power amplification sufficient for the actuation of major final control elements

❖ Short-circuit and overload protection of output modules

In the case of output modules, two fundamentally different methods are available to achieve the above: Either the use of a relay or power electronics.

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Fig. 3.6: Block diagram of an output module

The optocoupler once again forms the basis for power electronics and ensures the protection of the electronics and possibly also the voltage adjustment.

If relays are used for the outputs, then the relay can assume practically all the functions of an output module: The relay contact and relay coil are electrically isolated from one another.

Relay outputs have the advantage that they can be used for different output voltages. By contrast, electronic outputs have considerably higher switching speeds and a longer service life than relays.

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3. PLC Hardware Unit

Typically a PLC system has the basic function components of processor unit, memory, power supply unit, input/output interface section, communications interface and the programming devices. Fig. 3.7 shows the basic arrangement.

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Fig. 3.7: PLC system

1. Housing Unit

Housing unit provides protection to the circuits and components - the internal components of the PLC.

2. Programming Unit

The programming unit is used to enter the required program into the memory of the processor. The program is developed in the device and then transferred to the memory unit of the PLC. Programming devices can be a hand-held device (Program console) a desktop console, or a computer.

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3. Secondary Storage

Secondary storage (also known as external memory or auxiliary storage) (well known as external hard disk), differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile.

4. Power Supply Unit

The power supply unit is needed to convert the mains a.c. voltage to the low d.c. voltage (5V) necessary for the processor and the circuits in the input and output interface modules.

5. Printer Unit

Printer unit is used to print the program of control system controlled by a PLC either graphics or text.

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4. Input and Output Device

The input devices considered include digital and analogue devices such as mechanical switches for positioning detection, proximity switches, photoelectric switches, encoders, temperature and pressure switches, potentiometers, linear variable differential transformers, strain gauges, thermistors, thermotransistors and thermocouples. Output devices considered include relays, contactors, solenoid valves and motor.

1. Input Device

The term sensor is used for input device that provides a usable output in response to a specified physical input. For example, a thermocouple is a sensor which converts a temperature difference into an electrical output.

The term transducer is generally used for a device that convert signal from one form to a difference physical form.

The following are examples of some of the commonly used PLC input devices and their sensors.

a. Mechanical Switch

A mechanical switch generates an on-off signal or signals as a result of some mechanical input causing the switch to open or close.

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Fig.3.8: Switch Sensor

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Fig. 3.9 Limit switch

b. Proximity switch

Proximity switches are used to detect the presence of an item without making contact with it.

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Fig 3.10: Proximity sensor

c. Photoelectric sensor and switch

Photoelectric switch devices can operate as transmissive types where the object being detected breaks a beam of light, usually infrared radiation, and stop it reaching the detector (fig. 3.11)(a)) or reflective types where the object being detected reflects a beam of light onto the detector (fig. 3.11 (b)).

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Fig. 3.11: Photoelectric sensor

d. Encoder

The term encoder is used for a device that provides a digital output as a result of angular or linear displacement.

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Fig 3.12: Encoder

e. Temperature Sensor

A temperature sensor is a device that gathers data concerning the temperature from a source and converts it to a form that can be understood either by an observer or another device. Several types of temperature sensor as follows:

❖ Bimetallic strip

❖ Resistive temperature detector (RTD)

❖ Thermodiodes and thermotransistor

❖ Digital Temparature sensor LM3911, LM35 etc

❖ Thermocouple

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Fig 3.13: Bimetal thermostat sensor

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Fig 3.14: RTD sensor

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Fig3.15: LM 32 and LM3911 temperature sensor

f. Position/ Displacement Sensor

The term of position sensor is used for a sensor that gives a measure of the distance between a reference point and the current location of the target, a displacement sensor being one that gives a measure of he distance between the present position of the target and previously recoded position. Two types mostly used are, resistive and angular position sensor and linear variable differential transformer (LVDT).

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Fig 3.16: LVDT

2. Output Device

The output of the ports of a PLC are of the relay type or optoisolator with transistor or triac types depending on the devices connected to them which are to be switched on or off. Generally, the digital signal from an output channel of the PLC is used to control an actuator which in turn control some process. The term actuator is used for the device which transform the electrical signal into some more powerful action which then results in the control of the process. The following are some of examples:

a. Relay

One example of output device is relay, the term contactor being used when large current are involved. When the output from the PLC is switched on, the solenoid magnetic field is produces and pulls on the contacts and so closed a switch or switches (fig. 3.17). the result is that much larger currents can be switched on. Thus relay might be used to switch on the current to a motor.

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Fig. 3.17: Relay used as an output device

b. Directional Control Valves

Another example of an actuator is a solenoid operated valve. The valve may be used to control the directions of flow of pressurised air or oil and so used to operate other devices such as a piston moving in a cylinder.

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Fig 3.18: Directional control valve

c. Motors

Motors are common actuators, but for logical control applications their properties are not that important. Typically logical control of motors consists of switching low current motors directly with a PLC, or for more powerful motors using a relay or motor starter. Many industrial processes only required the PLC to switch a d.c motor on and off.

References:

1. R. Bliesener, F. Ebel, C. Löffler, B. Plagemann,H. Regber, E. v. Terzi, A. Winter. Programmable logic controllers: Basic level TP301Textbook. FESTO DEDATIC.

2. Basic Of PLC’s. SIEMENS (.)

3. L.A Bryan, E.A Bryan. (1997). Programmable Controller : Theory & Implementations 2nd Edtion.Industrial Text.

4. W. Bolton. (2006). Programmable Logic Controller. Newnes

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Programmming devices

Program & data memory

Communications

Interface

Input Interface

Processor

Output Interface

Power supply

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