An Introduction To The Plc Information Technology Essay

This means of control consisted of numerous cabinets which housed the relays which acted as the interface between input devices such as switches and buttons and physical external output devices such as valves, presses actuators, motors etc..

The relay is an electromagnetic component and would operate once an input (switch) was selected, which then energised a coil and magnetically closed a mechanical switch to provide an output (control supply) to the physical hardware via the relay contacts.

Not only did this process require large amounts of storage space but the need for copious amounts of cable and hardwiring was required between the input/output components and devices.

Cost implications were also incurred due to maintenance requirements, power consumption and rewiring when circuit and automation designs altered.

Then back in the 1968 a Mr E.Moorley, a member of the automotive industry proposed to the giant American automotive company General Motors, to implement his device namely, the Modular Digital Controller otherwise known as MODICON.

Here the birth of the PLC originated.

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The original MODICON 084 (the original PLC)

What is a PLC?

The Programmable Logic Controller (PLC) is basically a computer which allows industry to perform and control automated processes using simple program logic.

There are various types of PLC’s namely Rack mounted, Unitary or modular and each have their own advantages/dis-advantages, and however these will be discussed later.

A PLC’s standard architecture consists of:

A power supply unit:

The power supply is used to convert the mains incoming AC supply to usually 24v dc which in turn supplies the PLC’s components such as the CPU, Input/output and communication cards.

A back up battery is also housed within the supply unit which is used in situations where the main power supply fails. The battery will then hold a charge large enough to prevent any data loss.

The Central Processing Unit (CPU)

The Central Processing unit within a plc performs mathematical logic and simply reads, controls and executes demands within the program and is commonly referred to as the brain.

It Interfaces and scans input signals and continuously updates their status within the program, allowing output signals to energise/de-energise hardware.

Below is a simple format of the tasks carried out by the CPU.

CPU operating cycle

The scan cycle is a very important function within the plc and is the operation which that takes place internally.

It basically has a 3 step process and can be seen below. This 3 step process is known a one scan.

Step 1: From the start of the scan cycle the process will read and determine the status of inputs from devices such as various sensors and switches.

Step 2: The ladder logic programme will know be updated within the software.

Step 3: As long as the program diagnostics are satisfied then the output devices will be turned on or off.

The scan cycle also effects the speed in which the program is executed but can also depend on the:

A: the length of the program and B: The speed of the Processor.

Nominal scan speeds are usually in microseconds.

Opto isolator.

The opto isolator is an important electronic device within the CPU and is used for protection purposes to prevent the CPU becoming damaged from external electrical sources, by means of electrical isolation.

This is achieved with the interaction of two components, namely an L.E.D and the transistor.

It operates by sending a signal to the L.E.D which is then picked up by the transistor. This allows the power supply to be switched without any electrical connection but just by means of the light source.

Memory card

A memory card is used to store program data (ladder logic) within a plc and is commonly used for memory back up in the event of power failure.

Some PLCs provide integrated memory, however the majority of plc’s today provide a memory card slot and allows the user to both read and write information to the card.

This allows the card to be changed if necessary, such as if the card becomes damaged or for to allow the card to be interchanged with other plc’s which may perform the same operations.

Input/ Output modules

These modules can be configured within a plc to suit the operating needs. An input card will be hard wired between sensors and switches for example, where the output cards will be hard wired to hardware such as motors, actuators, pistons etc..

The CPU will then monitor and update status variables and execute the output signals to external hardware in reference with the program.

Communication modules/card such as Ethernet, Profibus, Profinet

and interbus.

All the above mentioned modules are used for different networks for communication purposes and are used to interface with their respective devices for data device control, display, diagnostics, parameters etc… 

Another example of a communication module is an Analogue card which is commonly used to connect to analogue sensors to monitor and communicate values for voltage and current sensors, thermocouples, resistors and resistance thermometers as well as analogue actuators.

The PLC Requires software which allows programs to be written in various language formats such as STL (statement logic), Ladder logic and FB (Function block), again these will be discussed later.

To summarise, a PLC is used to control a wide variety of machinery by means of scanning these programs which are stored within the CPU’s RAM (Random access Memory) or ROM (Read Only Memory).

Where RAM allows stored information within the Plc to be accessed and altered by some means, such as altering lines of logic within the program file.

And ROM only allows PLC data to be accessed and read, but is not able to be changed in any way.

By reading the multiple input signals received, the PLC will scan and provide output signals with relation to its stored program to operate electromechanical hardware.

Below is a basic diagram of how the PLC interfaces and functions.

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Below is an image of How a typical rack mounted plc will be assembled.

Power Supply CPU

Input/output modules

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Memory card slot

Profi – net cable

Types of PLC’

Below is a description of the most commonly used types of PLC’s within industry today and their advantages/disadvantages.

Rack mounted:

The rack mounted PLC is more commonly used where large automation processes are required such as production lines.

This is a similar concept to the modular design but the modules are on standard cards that slot into a standard rack and are interchangeable and allows the user to alter the configuration in terms of more inputs/outputs, maybe add an analogue card or to use various types of communication cards such as Ethernet, profi-bus, interbus or profi-net for example.

This type of system is initially expensive to purchase but is a better long term solution for large processes/programmes, as it allows memory cards and process cards to be plugged in at any time when the system needs changing or updating. It also provides its own power supply therefore the scan time is extremely quicker and can cope when the design is expanded.

The CPU can also be changed to suit the systems requirements which allows the user to increase its size when the system is expanding which prevents the scan cycle time from slowing down.

Advantages of rack mounted.

Allow large extensions to the programme

Operates/runs and scans programme more quickly and efficiently due to quick scan time.

adaptability

Can easily be replaced

Contains its own power source

Slot and memory cards can be interchangeable or re programmed and easily replaced.

Integrated power supply

Can be fed with analogue inputs.

Disadvantages

Overwhelming initial cost

Modular

The modular PLC followed on from the unitary PLC and is very similar in size and design but is slightly more expensive. They consist of 3 basic modules namely, the input, output and CPU with the power source being provided separately.

Their advantage is allowing a range of modules to slot together to expand the system by usually mounting them on a DIN rail and by simply adding another unit and connecting the processor of the existing unit via a plug. However this expansion is limited due to the capability of the CPU and scan speeds.

These are also commonly used for small manufacturing process requirements but with a possibility of expansion if processes were to increase.

Advantages of modular,

The main advantage is that the number of input and output terminals can be expanded to cope with the changes to the hardware system

Cheap to purchase

Disadvantages

Requires separate power supply

Whole Module needs replacing if any component becomes damaged.

Limited expansion is allowed depending on type of brand

Unitary.

The unitary PLC is one of the earliest types used back in the 1970’s and is very compact with a simplistic design and is the least expensive of PLC types on the market. It houses all of the basic features such as, the CPU and I/O boards and are commonly used to run small manufacturing processes or fixed machines. However a separate power supply is required which can have an effect on the scan and processing cycle times.http://www.plcedge.com/image-files/plc-fixed-architecture.jpg

These PLC’s are very reliable, however their main disadvantage is that they are non-extendable and limit its potential if the requirements of the manufacturing process need to be modified or enhanced.

Advantages of unitary

Cheap to buy

Can be bought to run small or single

programmes such as operating a single piston to open or close a door.

Simple plug in/plug out system

Disadvantage.

Whole beast needs replacing if any component becomes damaged.

Can only run relatively small programmes

Unable to expand

Inputs/Outputs ( I/O)

Input/output signals are information which has been received from various types of sensors and switches, which are then processed by the PLC to provide an output signal to operate external hardware such as Motors , Actuators Pumps etc.

There are two types of input signals, Analogue and digital.

Analogue signals are used to monitor a range of values which allows the PLC to respond to pre-set values within a program.

Examples of this would be flow control, where air or fluid can be monitored and controlled by gradually closing/opening a shut valve in relation to the values set in the program and the actual flow rate value.

Digital signals however operate on a binary system which are simply on/off states and are unable to monitor a range of values. They are commonly used for Push buttons, photo cells, limit switches, Proxy switches etc.

Take a simple limit switch which is supplied with a 24v dc supply. Once the limit switch is made the 24v signal will be received at the PLC which is processed and updates the status of the related contact within the program. This can be seen below.

Where A+ B are contacts within a PLC program and resembles the status of an external input device such a proxy switch. Both switches have to be made to create an output to energise the status lamp.

It is these I/O signals that are converted from electrical to electronic values which use Boolean logic to help control output devices. This is shown below.

FIG:1:

The above logic in fig:1shows two And gates which indicate that to receive a resultant output from the second gate both signals (A+B) have to be true along with a signal from C.

Remember these signals(A,B and C) represent an input device.

Fig:2 below shows the resultant Boolean logic table.

FIG:2

Types of sensors which are used to create input signals.

Proxy switch:

The proximity switch is used to detect an object when it is moved within its range. An example of this may be with a production line which carries a product on a conveyor. Once the product reaches the proxy switch an input signal is provided and its status altered in the PLC.

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The proxy switch comes in a variety of options and are chosen in relations to the components required to be detected.

Switches such as the photoelectric sensor which operates by means of transmitting and receiving infra-red light signals or, a capacitive switch which operates when an object enters it’s the electrostatic field set up which in turn increases its amplitude signal and triggers the output signal. Both the photoelectric and capacitive sensors are ideal for use with plastic products.

The inductive proxy switch is used to sense metal products and operates when a product enters its range.

The output signal is generated when a metal object enters the magnetic field set up in the switch which causes the monitoring circuit to operate.

Limit switch.

The limit switch is an electromechanical device which is in a normally open state.

It consists of two contacts with a lever arm. When a component depresses this lever a signal is generated through the lever and across the two contacts.

These switches are used for instances such as over travel. Say a transfer bar or product was to pass a certain point which is deemed to be unsafe, therefore operating the switch and causing a relay to energise and removing power from the motor which is powering the conveyor or transfer bar.

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(a) Limit switch OFF or not triggered; (b) Limit switch ON or triggered

Light beam.

A light beam is another type of input and is known as a photoelectric sensor. It consists of a transmitter which sends an infra- red signal to a reflector which is opposing it. The signal is then reflected at a given angle so that the receiver can detect it.

If the signal was blocked from the object for example the circuit will detect this and generate an output signal.

Above is a simple diagram of the photo electric sensors operation.

Programming

Modern day PLC’s are connected via PC’s and allow programs to be entered, modified and expanded to meet the needs off the user and their process requirements.

There is numerous software on the market today which differs slightly depending on the manufacturer. Examples include PL7 (unity) which is the software designed by Schneider/Telemechanique, or S7 used by Siemens and RS Logix 500 adopted by Allen Bradley.

Basically a manufacturer will design software to be compatible with its own PLC hardware range.

However all of these software packages operate by using the same language, with three common languages being available which are described below.

Structured text language (STL).

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Above is an example of STL language used with Siemens software which is based on a Pascal language. It basically lists the core addresses of inputs and outputs and enables the program to become more proficient than other types of languages due to it using less memory. This is an important factor when large programs are required to be used.

However as can be seen from the example above, it is not the easiest of program languages to interpret or use, and is usually used by more experienced programmers.

Function Block Diagram (FBD)

FBD is another graphical programming language. The main concept is the data flow that start from inputs and passes in blocks which generate the outputs.

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Above is an example taken from a Siemens s5 software program. It can be seen that input variables ( E.0.0 , E0.1and E0.2), supply a Boolean gate, i.e. ( AND, NOT and OR). The resulting outputs feed the next block which result in either an output or a feed to another Boolean gate.

Ladder logic (LD).

Ladder logic program language contains mainly binary variables and has emanated from and basically replicates relay logic used in in early control process days .

It is the most used type of language used today as it is probably the easiest to understand and follow for writing and fault finding techniques.

It consists of command rungs which follow in a sequence indicating the status of contacts linked to input peripherals and the outputs linked to hardware.

The following page is an example of a ladder logic program along with its operating description, rung by rung. (The process requirements can be seen in the drill rig exercise included).

Rung: 1

Once the control button is turned on relay 1 is energised which in turn closes the N/O contact of R1 and supplies 24v DC to the control circuit.

The rung is also retained by a second contact of the r1 relay. Supply will now only be interrupted/stopped once a complete operating cycle has been completed which opens the N/C contact of r4, or once the off button is supressed which interrupts the control supply.

Rung: 2

P3 and p5 are normally open contacts of a proximetre switch on cylinder 3 and 2 respectively. Once made relay 2 is energised which advances cylinder 1.

Rung: 3

Once p2 and p3 are made relay 3 is energised due to the contact of r5 being normally closed. This action results in cylinder 2 being advanced to carry out the drill process.

Rung: 4

Once p6 is made via the proxy switch , r5 is energised and is retained from the N/O contact of r5.

This now opens the contact situated in rung 3 and immediately returns cylinder 2 ( Drill) and retains its position another retaining contact of r5.

Rung: 5

As the contacts of p2 and p3 are now made, relay 6 is now energised via the normally closed contact of p5. R6 remains energised through the retaining contact of r6. ( This rung interacts with Rung 8)

Rung: 6

This rung is the control relay output for cylinder 1.

Rung: 7

This rung is the control relay output for cylinder 2.

Rung: 8

This rung interacts with ruung5. As relay 6 is now energised and p5 proxy switch is made, cylinder 3 retracts and ejects the part.

As soon as cylinder 3 has fully retracted, proxy switch 4 is made which in turn energises relay 4.

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Now the N/C contact of r4 in rung 1 opens and the control circuit de-energises.

All cylinders now return to their home position and the sequence will cycle again once the start button is pressed.

As to provide a continuous production cycle , the start button may represent a proxy switch for part present.

Networks and communication.

PLC networking is the communication between the plc and its peripherals which provide a means of control for industrial automation.

There are many networking options used in the engineering industry such as ASCII, Profibus, Ethernet and Profinet and each have their own advantages and disadvantages, therefore have to be considered carefully when deciding which will best serve the operations purpose.

Below are two examples of a common networking system:

American Standard Code for Information Interchange (ASCII) Networking.

This type of network can be broken down into its various components described below.

Black cable: This cable supplies the peripherals with a power supply usually 24v dc via a transformer.

Yellow cable: This cable is supplied independently from the black cable and is used to communicate I/O status of peripherals.

Supply module.: Used to supply voltage to the whole network such as the master module, CPU and both cables.

Slave modules: These modules connect to both the yellow and black cable by a simple vampire clip. These then supply power to the peripherals and communicates the I/O status back the CPU .

The supply modules have to be addressed however which allows the cpu to interface with them.

This type of system is very common in large automation processes due its many advantages. Firstly it is a very Quick and responsive mainly due to the way it interfaces with the cpu as compared to other networks.

Most networks operate where the CPU is continuously monitoring, interfacing and exchanging data acquisition. However the ASCII system will only respond when its sees a change in state and therefore increases the scan/process cycle times .

Another advantage is that it allows peripherals to be added, removed or modified whenever necessary without the need for hard wiring back to the control panel. This is achieved with the slave modules by simply clipping modules anywhere within the network (Yellow/Black cables),and re-addressing them . These modules then interface with the CPU through Frequency values.

This eliminates the need for labour and increased costs when the line needs to be extended or modified.

However, although the above advantages are major factors, it is not necessarily the best option for small applications for example, mainly due to its initial high cost when purchased.

Profi-Bus.

The profi-bus network is an open, digital communication system with a wide range of applications, particularly in the fields of factory and process automation. It communicates basically through two types of devices, ( Master and a Slave module).

The whole network communicates and transfers data acquisition through a profi-bus cable. It operates by the master sending a request from a slave module . The slave module will then respond basically with a hand shake to the master, indicating that its diagnostics are happy and the process can be continued.

Note: Although any device connected to the bus can see a request, the master is the only one who can control the bus.

PROFIBUS is run through an individual PLC and is suitable for both fast, time-critical applications and complex communication tasks and commonly used for smaller process applications as compared to the ASCII network.

Its Downfall however is again not only its high purchasing costs but the fact that if the Master was to fail, then the network or process will stop functioning.

Common network cables.

The networks communicate between peripherals through a variety of cables which all have individual advantages and are important factors when considering their application.

Most PLC networks operate in industrial environments and therefore many considerations have to be taken into account such as protection, speed of communication and interference properties. This is to ensure the cable will perform to its highest capabilities and meet the requirements of the network processing.

Coaxial cable

Coaxial cable looks similar to the cable used to carry TV signal. A solid-core copper wire runs down the middle of the cable. Around that solid-core copper wire is a layer of insulation covered by braided wire and metal foil, which shields against electromagnetic interferences. A final layer of insulation covers the braided wire. https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcQhXBM27hXuDF7emPjEMxGgSPSBZyuI-_2DCwcp5U04-r1UTWC9

Twisted Pair Cablehttp://www.gore.com/MungoBlobs/controlled-impedance_shielded_twisted_pair_illus.jpg

Twisted-pair cable is the most common type of cabling you can see in today’s Local Area Networks (LAN) . A pair of wires form a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk which is generated by adjacent pairs. When a wire is carrying a current, the current creates a magnetic field around the wire. This field can interfere with signals on nearby wires. To eliminate this, pairs of wires carry signals in opposite directions, so that the two magnetic fields also occur in opposite directions and cancel each other out. This process is known as cancellation.

Two Types of Twisted Pairs are Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP).

Unshielded twisted-pair (UTP) cable is the most common networking media.

Unshielded twisted-pair (UTP) consists of four pairs of thin, copper wires covered in color-coded plastic insulation that are twisted together.

Fibre optic Cable

Optical Fibre cables use optical fibres that carry digital data signals in the form of modulated pulses of light. An optical fibre consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, known as the cladding. There are two fibres per cable-one to transmit and one to receive. The core also can be an optical-quality clear plastic, and the cladding can be made up of gel that reflects signals back into the fibre to reduce signal loss. http://img.tfd.com/cde/FIBER288.GIF

There are two types of fibre optic cable: Single Mode Fibre (SMF) and Multi-Mode Fibre (MMF).

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