Table of Contents

Introduction……………………………………...

PLCs……………………………………………...

Number Systems………………………………. ..

Terminology Used in PLC………………………..

PLC Languages……………………………………

Discrete Inputs/Outputs…………………………..

Analog Inputs and Outputs……………………

Timers……………………………………..

D O L S T A R T E

R……………………………………………

PLC elements and advantages

PLC :

A programmable logic controller (PLC), also referred to as programmable controller, is a type of computer commonly used in commercial and industrial control applications. PLCs differ from office computers in the types of tasks that they perform and the hardware and software they require to perform these tasks. While the specific applications vary widely, all PLCs monitor inputs and other variable values, make decisions based on a stored program, and control outputs to automate a process or machine. 

Basic elements of PLC :

The basic elements of a PLC include input modules or points, a central processing unit (CPU), output modules or points, and a programming device. The type of input modules or points used by a PLC depend upon the types of input devices used. Some input modules or points respond to digital inputs, also called discrete inputs, which are either on or off. Other modules or inputs respond to analog signals. These analog signals represent machine or process conditions as a range of voltage or current values. The primary function of a PLC's input circuitry is to convert the signals provided by these various switches and sensors into logic signals that can be used by the CPU. 

The CPU evaluates the status of inputs, outputs, and other variables as it executes a stored program. The CPU then sends signals to update the status of outputs.

  Output modules convert control signals from the CPU into digital or analog values that can be used to control various output devices.  The programming device is used to enter or change the PLC's program or to monitor or change stored values. Once entered, the program and associated variables are stored in the CPU.  In addition to these basic elements, a PLC system may also incorporate an operator interface device of some sort to simplify monitoring of the machine or process.

---------------------------------------- Here you can find a Simple Example of previous studied elements 

Prior to PLCs, control tasks were often performed by contactors, control relays, and other electromechanical devices with intricate interconnecting wires. This approach is often referred to as hard-wired control. Although hard-wired control solutions are capable of performing many of the same tasks as software-based solutions, they are generally more difficult to design, install, and maintain. In addition, the process of making even simple modifications to a hard-wired control solution can be difficult because the logic of the control system is determined by the interconnection of control wires and components.

 

Advantages of PLCs :

PLCs not only are capable of performing the same tasks as hard-wired control, but are also capable of many more complex applications. In addition, the PLC program and electronic communication lines replace much of the interconnecting wires required by hard-wired control. Therefore, hard-wiring, though still required to connect field devices, is less intensive. This also makes correcting errors and modifying the application easier.  Some of the additional advantages of PLCs are as follows:         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 quickly documented.Applications can be duplicated faster and less expensively.

An example of conveyer control using Programmable Logic Controllers:

 

Number Systems

Because a PLC is a computer, it stores information in the form of on or off conditions (or0), referred to as bits. Sometimes bits are used individually and sometimes they are used to represent numerical values. Understanding how these bits can be used to represent numerical values requires an understanding of the binary number system.

Decimal System:

In order to understand the binary number system, it is first useful to recall some of the basics of the decimal number system. All number systems have the same three characteristics : digits, base, weight for example, the decimal system has the following characteristics:

Ten digits 0, ,2,3,4,5,6,7,8,9Base

0Weights Powers of base 0( , 0, 00, 000,)

Binary System:

The binary system has the following characteristics:

Two digits: 0, Base 2Weights Powers of base2( ,2,4,8, 6,)

The binary system has a base of 2 and uses only two characters, and 0. Each bit is associated with a power of 2 based on its position in the number. The further to the left, the higher the power of 2. The number in the far left-hand column is referred to as the most significant bit or MSB and the number in the far right-hand column is referred to as the least significant bit or LSB A is placed in a position if that power of 2 is used in the number Otherwise, a0 is placed in a position.

The process of converting a binary number to an equal decimal value is as simple as adding the equivalent decimal value for each position in the binary number where a is shown Positions with a0 do not add to the number value.

Bits, Bytes, and Words:

Each position in a binary number is called a bit. The number of bits used to represent numbers varies with the device However, instructions and data are usually grouped in bytes and eight bits make upon a byte. Two bytes, or 6bits,make up one word.

Logic 0, Logic 1:

While PLC’s are capable of sensing and generating analog values, programmable controllers internally use signals that are on or off. These on and off conditions correspond to the binary values and 0F or example, a binary0, also called logic0, can be used to indicate that a switch is off, and a binary (logic) can be used to indicate that a switch is on.

BCD:

While it is necessary for PLC’s to use binary values, humans often need to see values represented in decimal. As a result, some input and output devices provide a decimal display where each decimal digit corresponds to four PLC binary inputs or outputs. The most common system used by input and output devices of this type is referred to as binary-coded decimal (BCD).

One example of a BCD device is a type of four-digit thumb wheels witch. Each thumb wheel digit controls four PLC inputs. This means that for a four-digit thumbwheel, 6 inputs are required. Because each thumb wheel digit only need store present decimal values from 0 through 9, only ten corresponding binary values are required for each digit.

Terminology in PLC

Developing an understanding of PLC’s requires learning some basic terminology. This section provides an overview of commonly used PLC terms, beginning with the terms sensor and actuator.

Sensors:

Sensors are devices that convert a physical condition into an electrical signal for use by a controller, such as a PLC. Sensors are connected to the input of a PLC. A pushbutton is one example of a sensor that is often connected to a PLC input. An electrical signal indicating the condition(open or closed) of the pushbutton contacts is sent from the pushbutton to the PLC.

Actuators:

Actuators are devices that convert an electrical signal from a controller, such as a PLC, into a physical condition. Actuators are connected to the PLC output. A motor starter is one example of an actuator that is often connected to a PLC output. Depending on the status of the PLC output, the motor starter either provides power to them or prevents power from flowing to the motor.

Discrete Inputs and Outputs:

Discrete inputs and outputs, also referred to as digital inputs and outputs, are either on or off. Pushbuttons, toggle switches, limit switches, proximity switches, and relay contacts are examples of devices often connected to PLC discrete inputs. Solenoids, relay and contact or coils, and indicator lamps are examples of devices often connected to PLC discrete outputs.

In the on condition, a discrete input or output is represented internal to the PLC as a logic. In the off condition, a discrete input or output is represented as a logic 0.

Motor Starter Example:

While the lamp application previously discussed is useful to explain basic PLC operation, a more practical, and only slightly more complex, application is start-stop control of an AC motor. Before examining the PLC application, first consider a hard- wired approach.

The following line diagram illustrates how a normally open and a normally closed push button might be connected to control a three-phase AC motor. In this example, a motor starter coil (M) is wired in series with a normally open, momentary Start pushbutton, a normally closed, momentary Stop pushbutton, and normally closed over load relay (OL) contacts.

Analog Inputs and Outputs: 

Many PLCs also work with analog I/O devices. Analog devices use signals that are continuously variable within a specified range such as 0 to 10 VDC or 4 to 20 mA.  Analog signals are used to represent variable values such as speed, rate of flow, temperature, weight, level, etc. In order to process an input of this type, a PLC must convert the analog signal into a digital representation.  Expansion modules are available with 4 or 8 analog inputs, 2 or 4 analog outputs, or 4 analog inputs and 1 analog output. In addition,  expansion modules are  available for use with thermocouples or RTD type sensors which sense the temperature at a specific point in a machine or process.

Analog Input Example:

 

Analog inputs can be used for a variety of purposes. In this example, a scale is connected to a load cell. A load cell is a device that generates an electrical output proportional to the force applied. The load cell in this example converts a value of weight from 0 to 50 pounds into a 0 - 10 VDC analog value. The 0 - 10 VDC load cell signal is connected to an PLC's analog input. The analog value applied to the PLC can be used in various ways. For instance, the actual weight can be compared to a desired weight for a package. Then, as the package is moved on a conveyor,  the PLC can control a gate to direct packages of varying weight. A package that weighs at or greater than a specified value is routed along one conveyor path to finished goods inventory. A package that weighs less than a specified value is routed along another conveyor path to an inspection station, where it will be checked for missing contents.

Analog Output Example :

Analog outputs from a PLC are often supplied directly or through signal converters or transmitters to control valves, instruments, electronic drives or other control devices which respond to analog signals.  For example, analog outputs from the PLC could be used to control the flow of fluid in a process by controlling AC drives. Rather than simply turning the AC drives on or off, which could be accomplished by discrete outputs, analog signals can be used to control the output of the AC drives.  This would allow the speed of the pumps to be varied dynamically in response to changes in process requirements.

CPU:

The central processor unit (CPU) is a micro processor system that contains the system memory and is the PLC’s decision- making unit. The CPU monitors inputs, outputs, and other variables and makes decisions based on instructions held in its program memory.

PLC LANGUAGES

Ladder Logic Programming:

A program consists of instructions that accomplish specific tasks. The degree of complexity of a PLC program depends upon the complexity of the application, the number and type of input and output devices, and the types of instructions used.

Ladder logic (LAD) is one programming language used with PLC’s Ladder logic incorporates programming functions that are graphically displayed to resemble symbols used in hard-wired control diagrams.

The left vertical line of a ladder logic diagram represents the power or energized conductor. The output coil instruction represents the neutral or return path of the circuit. The right vertical line, which represents the return path on a hard-wired control line diagram, is omitted Ladder logic diagrams are read from left-to-right and top-to-bottom Rungs are sometimes referred to as networks. A network may have several control elements, but only one output coil.

Statement List and Function Block Diagrams:

While ladder logics program are still common there are many other ways to program PLC’s. Two other common examples are statement list and function block diagrams.

Statement list (STL) instructions include an operation and an operand. The operation to be performed is shown on the left. The operand, the item to be operated on, is shown on the right.

Function block diagrams (FBD) include rectangular functions with inputs shown on the left side of the rectangle and outputs shown on the right side.

In the following example, the program segments perform the same function.

In addition to LAD, STL, and FBD, multiple other types of programming languages are used for PLC’s. Each type of programming has its advantages and disadvantages. Factors such as application complexity, types of programming available for a specific PLC model, and user standards and preferences determine which type of programming is used for an application.

PLC Scan:

The PLC program is executed as part of a repetitive process referred to as a scan. A PLC scan starts with the CPU reading the status of inputs. Next, the application program is executed. Then, the CPU performs internal diagnostics and communication tasks. Finally, the CPU updates the status of outputs.

This process repeats as long as the CPU in the run mode. The time required to complete as can depends on the size of the program, the number of I/O s, and the amount of communication required.

Cycle of activities in PLC Scan:

MEMORY TYPES AND SIZES

Kilo, abbreviated k, normally refers to 000 units. When talking about computer or PLC memory, however, means 024. This is because of the binary number system(2

0= 024) can refer to 024 bits,

bytes, or words, depending the context.

Random Access Memory (RAM) is memory that allows data to written to and read from any address(location) RAM is used as a temporary storage area. RAM is volatile, meaning that the data stored in RAM will be lost if power is lost. A battery backup is required to avoid losing data in the event of a power loss.

Read Only Memory (ROM) is a type of memory used were it is necessary to protect data or programs from accidental erasure. The original data stored in ROM can be read, but not changed. In addition, ROM memory is nonvolatile. This means that information will not be lost as the result of a loss of electrical power. ROM is normally used to store the programs that define the capabilities of the PLC.

Erasable Programmable Read Only Memory (EPROM) provides a level of security against unauthorized or unwanted changes in a program. EPROM’s are designed so that data stored in them can be read, but not easily altered. Changing EPROM data requires a special effort UVEPROMs(ultraviolet erasable programmable read only memory)can only be erased with an ultraviolet light. EEPROM(electronically erasable programmable read only memory), can only be erased electronically.

Basic Ladder Logic Symbols

PLC ladder logic consists of a commonly used set of symbols that represent instructions. Understanding these basic symbols is essential to understanding PLC operation.

Contacts:

One of the most confusing aspects of PLC programming for first-time users is the relationship between the device that controls a status bit and the programming function that uses a status bit. Two of the most common programming functions are the normally open (NO) contact and the normallyclosed (NC) contact. Symbolically, power flows through these contacts when they are closed. The normally open contact (NO) is closed when the input or output status bit controlling the contact is The normally closed contact (NC) is closed when the input or output status bit controlling the contact is 0.

Coils:

Coils represent relays that are energized when power flows to them. When a coil is energized, it causes a corresponding output to turn on by changing the state of the status bit controlling that output to That same output status bit maybe used to control normally open and normally closed contacts else where in the program.

Timers

In a PLC, timers are programming functions that keep track of time and allow PLC programs to provide varied responses depending on the elapsed time.

Hard-wired Timing Circuit:

Timers in a PLC program can be compared to hard-wired timing circuits, such as the one represented in the accompanying control line diagram. In this example, normally open(NO)switch (S ) is used with timer(TR ). When S closes, TR begins timing. When the timer’s preset time elapses, TR close sits associated normally open TR contact and pilot light PL turns on When S opens, TR de-energizes immediately, the TR contact opens, and PL turns off.

This type of timer is referred to as an on-delay timer. The term “on-delay” indicates that the timing begins when the timer receives a signal to turn on. In this example, that happens when S closes.

DOL STARTER

Motor Starter in a Control Circuit:

The following diagram shows the electrical relationship of acontactor and an overload relay in a typical motor control circuit.

The contactor (highlighted with the darker grey) includes an electromagnetic coil (M) and auxiliary contacts (Ma) in the control circuit and three main contacts (M) in the power circuit. The overload relay, highlighted by the lighter grey, includes three heaters contacts (OL) in the power circuit and auxillary contacts (OL) in the control circuit.

In this circuit, when the Start pushbutton is pressed, power is provided to the coil, and the M contacts close. This provides power to the motor through the OL heater contacts. At the same time, Ma contacts close so that, when the Start pusbutton is released, power is still provided to the coil. The motor will continue to run until the Stop pushbutton is pressed, unless an overload occurs. If an overload occurs, the OL heater contacts open, removing power from the motor,and OL auxiliary contacts open, removing power from the coil. Removing power from the coil is necessary to prevent the motor from automatically restarting when the overload relay cools.

This is an example of a full-voltage starter, also called an

across-the-line starter. This type of circuit starts the motor by providing the full line voltage to the motor.

COMPONENTS OF DOL STARTER

Magnetic Contactors and Starters:

Most motor applications require the use of remote control devices to start and stop the motor. Magnetic contactors (similar to the ones shown below) are commonly used to provide this function. Magnetic contactors are also used to control distribution of power in lighting and heating circuits.

Basic Contactor Operation:

Magnetic contactors operate by utilizing electromagnetic principles. A simple electromagnet can be fashioned by winding a wire around a soft iron core. When a DC voltage is applied to the wire, the iron becomes magnetic. When the DC voltage is removed from the wire, the iron returns to its nonmagnetic state.

The following illustration shows the interior of a basic contactor. There are two circuits involved in the operation of a contactor, the control circuit and the power circuit. The control circuit is connected to the coil of an electromagnet, and the power circuit is connected to the stationary contacts. The operation of this electromagnet is similar to the operation of the electromagnet we made by wrapping wire around a soft iron core. When power is supplied to the coil from the control circuit, a magnetic field is produced, magnetizing the electromagnet. The magnetic field attracts the armature to the magnet, which in turn closes the contacts. With the contacts closed, current flows through the power circuit from the line to the load. When current no longer flows through the control circuit, the electromagnet’s coil is de-energized, the magnetic field collapses, and the movable contacts open under spring pressure.

Line Line

Load Control Signal

Load

Control Signal

The following line diagram shows a contactor that provides on- off control for a three-phase motor. Note that the power to the electromagnetic coil of this contactor is controlled by SW1.

When SW1 closes, the electromagnetic coil energizes, closing the “M” contacts and applying power to the motor. When SW1 opens, the coil de-energizes, opening the “M” contacts and removing power from the motor.

Contactor De-energized Contactor Energized

Motor Starter:

Contactors provide on-off control in a variety of circuits. When used for motor control, overload protection is usually also required. When a contactor is combined with an overload relay, it is called a motor starter.

THERMAL OVERLAOD RELAYS

Bimetallic Overload Relays:

Some overload relays use a bimetallic strip to sense an overload condition. A bimetallic overload relay incorporates a small heater element wired in series with the motor and a bimetallic strip that functions as a trip lever. The bimetallic strip is made of two dissimilar metals bonded together. These metals have different thermal expansion characteristics, causing the bimetallic strip to bend when heated.

Under normal operating conditions, the heat generated by the heater element causes the bimetallic strip to bend only slightly, not enough to trip the overload relay.

If an overload condition occurs and persists long enough, the bimetallic strip bends until the overload relay’s trip mechanism is tripped. This causes the overload relay’s contacts to open, removing power from the motor.

Some bimetallic overload relays are designed to reset automatically after the bimetallic strip has cooled. Depending on the circuit, the motor may then restart automatically. In some applications, this is desirable. However, if the cause of the overload still exists, the overload relay will trip and reset repeatedly. Proper circuit design can prevent this condition, which can damage the motor.

TERMINAL BLOCKS

Figure shows the simple terminal block. Individual terminals are designed to clip onto a mounting rails. They are manufactured in a variety of sizes to accommodate any cable CSA. Green / Yellow earth are designed to make a good contact with mounting rails on tightening of the centre screw. Other colours are available but are not essential.

The terminals must be installed in the correct way round as each one is generally insulated on one on side only. An end insulator must be installed to insulate the last terminal. An end clamp must be used to prevent the terminals

from spreading apart. An Earth terminal doubles as an end clamp. Terminals with solid links or Fuse holders are also available.

PUSH BUTTONS, SWITCHES AND INDICATORS

Push buttons, switches and indicating units are manufactured in various styles. While they may differ in appearance but operate in the same way. They are available in different IP ratings to suit different environmental conditions in which they are installed. Push buttons with “build in” indicator lamps are also available.

All these units are generally known as control and signaling units. The more commonly used units are 22 mm in diameter. They are manufactured in metals for mounting on metallic doors of control panels. These doors must be earthed. Plastic version are also available and are generally used to provide double insulating controlling stations etc;

Push Buttons are colour coded as follows:

Start button : Green

Stop button: Red

Inch button : Black ( Optional excercise for faster apprentice)

Indicator units may be colour coded as follows:

Run indicator : Green

Fault indicator: Red

Power “ON” : White

Start Buttons:

A start button consists of a green coloured actuators which when pressed operates as a normally open contact. On pressing the green actuator the normally opened contact will be closed. These are fitted on panel door via a mounting collar. The actuator is fitted through a suitable hole in the panel door. A rubber washer is provided on the outside to provide ingress protection. The actuator is locked into the collar. The collar is secured to the panel board by a screw. This action compresses the rubber seal on the actuator. Finally the contact block is simply slipped onto the collar.

Stop button:

A stop button consists of a red coloured actuator, which when pressed operates a normally closed contact. On pressing the red actuator the normally closed contact will be open.

Signal lamp units:

A signal lamp unit consists of an appropriately closed lens, lamp holder and mounting arrangement similar to push button units. A 230 volts “small centered contact” SCC lamp is used in the holder. An incandescent lamp or neon lamp may be used. The phase conductor should be connected to the terminal marked X1.

CONTACTORS

Contactors are very similar to that of relays. They are specifically designed for switching load currents. They are used to supply electric motors, heaters, lamps etc. contactors are designed for high switching frequencies and long life and short switching times. The making / breaking capacity of the main contacts are matched to the possible motor starting current. High value overload currents and short circuit currents can damage the main contacts, therefore protection must be provided by fuses or MCB’s.

Generally the contactor has three main contacts and one auxillary contact. The current ratings of the main contacts depends on the load to be switched and my as low as 10amps and as high as 2000 amps. The current rating of auxillary contact is generally around 10amps.

The following figure shows a model contactor. The 3 phase supply is connected to three main contacts as shown in figure. The load is connected form the three main terminals in the bottom.

The following figure shows the main parts of an contactor:

Contactor coils:

The coils of most modern contactors can be quickly changed without the use of any special tools. These contactor coils may be AC or DC operated.

AC coil standard voltages are : 24,110,230,400 volts 50Hz.

DC coil standard voltages are : 12,24,48,230 volts.

It is most important that coil voltage and current should match the supply parameters. A 230 volts supply will not energise the 400 volts contacts. But, a 400volts contact can energise a 230 volt coil but however the coil will overheat and burnout.

Auxillary contacts: