Purpose And Origins Of Plcs Computer Science Essay

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The PLC was devised by the motor industry. Many of the manufacturing processes used within this industry were automated by the use of hydraulic, electrical and Pneumatic. These methods of manufacture were originally controlled by use of a rigid control circuit. These systems proved difficult to work with as if there was ever a need to change of a piece of defective equipment the system would require a full rewire, a software upgrade and reconfiguration. This proved costly and very time consuming.

A development in micro computers lead to realisation that the computer could utilise equipment by switching on and off in response to a pattern of inputs. If a change was required a change could be carried out by use software installed within a computer. This was when the birth of the PLC was confirmed.

Rigid systems mentioned above are still used today but only to perform single control actions.

The PLC performs operations with a computer program rather than using real components by copying software uploaded.

The PLC could be described as a small computer that has been designed for specific duties within the industrial world. A list below gives you an insight of technologies that utilise the PLC.




Traffic Light Systems.

Production Processes.

Types of PLC

There are 3 types of PLC that we are going to discuss as a part of this assignment. They are listed below:

The Unitary PLC.

The modular PLC.

Rack Style PLC.

The Unitary PLC is a box which contains every feature. Being the least expensive it is also the smallest unit. Is can be utilised within a small or fixed application. Without the capability of being expanded it limits the devise to onboard I/O. Containing there own power source this technology is aged and no longer popular for use in engineering. Being old technology if it was to break it will be difficult if not impossible to repair and reprogrammed. These use either 1 or 2 input output channels. This technology utilises high speed input and pulse train output for motion control in the simplest form.

The Modular PLC uses many module ranges that slot together to create a system. The system comprising of a power supply. This main module which contains the CPU and the input and output module. The number of input and output terminals can be increased to cope with the varied hardware systems; this is very advantageous The Module PLC has a wide parameter of applications. The system designer using this system has to ability to create automated systems has flexibility to design systems to required standards. The module PLC holds many of no on board I/O. These are arranged using arrangement known as a DIN rail arrangement. This system will require the use a separate power supply. Using the DIN rail allows the use of more I/O and speciality modules are to be utilised. The use of Module PLC's are generally used in higher I/O activities.

Rack Style PLC's are more expensive and powerful than the PLC's above. The rack provides the backplane of the power communication; this greatly increases the rate of communication between the modules and the processor. This also allows some specialty modules to communicate with each other without the need of the processor. Some brands utilise multiple processors that can be placed within the same rack which also share inputs. Racks also permit redundant processors for use in systems such as fire control systems or water pumps. The modules that are able to be used within a rack system allow for a greater and more extensively organised module system. Rack systems have greater availability of I/O points

Architecture and Terminology.

The PLC utilises output terminals in order to switch things on and off. System feedback sensors are used to determine the decisions to activate; the system feedback sensors are connected via the input terminal of the PLC. Decisions to activate rely upon the programme that is stored within the memory of the PLC. The method of storing this information can be either ROM (Read only memory) or RAM (Random access memory). The system requires continuous communication with the external devises such as a display monitor and digital converters. .

Below is a basic diagram of the PLC setup.

Figure 1.

The CPU Central Processing Unit.

The CPU is the brain of the PLC. The CPU consists of numerous items; the CPU is made up of the microprocessor, memory chip as well as these items the following are also found within the CPU. Control of logic is managed by use of integrated circuits, monitoring equipment, and communication software. This system replaces the need for central relay timers and sequencers. Different operating modes are found within the CPU. The CPU is able to run using different operating modes. To execute programs and carry out the operating processes the CPU is placed in the run mode so all required activities can take place. The PLC being a dedicated controller will run a specific program repetitively. One cycle through the desired program is often called a scan time; this involves analysing the inputs from the other modules fitted within the system, also executing the outputs after analysis of the input information. Scan times can be as quick as 1/1000th of a second. The CPU memory is utilised to store the programs while also holding the status of the inputs. This cycle can be seen in Figure 2 below


Figure 2.

Input Modules.

Input modules are used to connect the input terminals to the remaining parts of the system. The internal electronics are isolated from each terminal with the use of an OPTO isolator. This is used to pass information of the status of the input whether it is on or off. This is carried out by using a diode and a piece of equipment called a phototransistor.

.The illustration of an OPTO isolator can be seen in Figure 3. An OPTO isolator is a devise the prevent voltages to high from other components in on side of the same circuit. This is done to prevent a damage or interference between components on the other side of the circuit. Regulating voltages is carried out by the OPTO isolators by converting the electricity into a beam of light. This prevents any voltage spikes impacting any more than a single circuit a overall interference is also decreased. Also noise is reduced. These are both common problems incurred in traditional communication connections.

Figure 3

Output Module.

. Switches are contained within the output modules that are activated by the CPU. This is done in order to connect two terminals. This enables current to flow in the external circuits. In doing so this will activate the devises connected such as, pneumatic and hydraulic solenoid valves, heating elements and motors. As no to overload these connections care has to be taken. The switch used within the circuit could be either a transistor or relay. Below in Figure 4 is a basic schematic of a typical output arrangement.

Figure 4


. A PLC is made up of banks of RAM and ROM. RAM comes in the form of computer data storage. A random access devise permits data stored to be utilized in nearly the same amount of time for any storage location. Data can therefore be accessed in any random order quickly. ROM is storage medium used within computer and other electronically powered equipment within the system. The data contained with ROM cannot be modifies like that stored in RAM. Its purpose is to mainly distribute. Programs when written are uploaded and stored within the RAM. Programs which are permanent such as monitoring and status updating of the connected input and outputs are stored within the ROM.


The PLC has the ability of running diagnostic, testing and monitoring activities within the system. These activities are stored within the ROM. Visual devises such as LED's provide the status of the inputs and outputs fitted within the system. setting switches to a certain state will enable the user to fix a bank of switches to the input side of the testing programme. This method of setup certifies that the appropriate outputs coincide to that of what parameters are set. This can only be carried out once the PLC is linked to a computing devise. Highlighted areas of concern will be raised once diagnostic and simulations are carried out on the system.

Programming methods.

The use of an Ethernet cable enables the PC to be connected to the Using the ladder style logic will allow alteration to be made to the software installed. Along with this diagnostic testing can be carried out using this cable, this therefore highlights any area of concern within the installed logical demands.

. Many programming panels are available of which the complexity can vary. Anything from a simple key pad to a handheld pc with full user interfaces. These computers enable the use of simulations, graphics monitoring and diagnostics. Testing and development are carried out using this software. Once complete is it only at this time the data/ software is uploaded on to the PLC being utilised. Data transfer is done using conventional methods whether it be magnetic tape, compact disc or cable. Also with ever expanding technologies you can carry this out via the internet.


.An input devise refers to a devise which takes information to the physical heart of the PLC. This also refers to the PLC hardware that connects such devises as the sensors or transducers. Decisions are made by the PLC using input information. This is based on the programs installed within the software. This will lead to the energising and de energising of the outputs.

Digital and analogue are 2 inputs used in PLC's and are most commonly known as DI and AI. Analogue inputs can be temperature, pressure and uses a wide range of values. A digital input can be as simple as a switch. These are set up in two states. Either on or off.

Analogue sensors work typically between 4-20ma or 1.5dc and use any of the following devises. Current sensors or temperature sensors.

Digital inputs are voltage independent in both AC and DC forms and use many devises. For instance a relay contact or limit switch.

The type of hardware used will depend on the inputs that have been utilised. The use of mixed input output modules allow for the use of integrated inputs.

Digital inputs of PLC's use LED's. If the LED is on the input is on. This also allows for trouble shooting on the equipment.

Wiring will have to be checked if the LED is on when it should not be. This defect can also be attributed to a defective input devise. The same will apply if it was the other way around. I.e. LED off when you expect it to be on.

Multi meters can be used to aid in trouble shooting equipment without displays.

PLC input modules would be generally discounted as the problem until all diagnostics have been carried out.

Input Devises.

An input is a devise that supplies an on/off signal to the input port of the PLC. I will expand on the most commonly used input devises.


A switch generates and ON/OFF signal. This is as a result of a mechanical input which in fact opens or closes switch.

Switches are either contacts which are open or closed

An open switch within a circuit requires a mechanical input to be used which will in turn closes the circuit allowing power to flow.

The same principle occurs when using a normally closed switch but the opposite happens when the switch is used. The contact is broken stopping the flow of power.

Proximity Switches.

Switches after making contact or near an object that provide an electrical circuit are known proximity switches. These are often found within industry within overhead cranes conveyors etc.,

.Inductive proximately switches utilise magnetic fields to sense distances between objects. This is very similar to how a metal detector works. A wire in a coil creates high frequency electromagnetic fields. An electrical circuit measure this current. If metallic parts get within a distance of the coil the current will subsequently increase and depending on setting the proximity switch will open or close. This can be seen below in figure 5.

Figure 5.

A capacitive proximity switch sense distance to object by detecting changes in capacitance around it. A capacitive proximity switch detects change in capacitance around it when nearing an object. A radio frequency oscillator is engaged to a metal plate. When the plate gets closer to an object, the detector sends a signal to switch to open or close dependent on settings. These devises are very sensitive and pick up on many objects which conduct a magnetic field. This is actually a disadvantage. This devise can be seen below in figure 6.

Figure 6


PLC output are devises or circuits controlled by the PLC A light or motor can be referred to as a PLC output devise Actuators are used the convert the electrical signal provided by the PLC to a required physical movement.

Two types of these are found. These are the on-off output and the Variable. Digital outputs are used as the on/off devises. These are often motors as they either need to be on or off. A variable speed motor is an Analogue outputs. This is because the motor can be either on or full speed.


The PLC would be useless if it did not have the capability of communicating with other devises. A requirement of this is to link programming devises to any equipment or devises that require power. Modern equipment send and receive any information in digital format and are connected via a network to the PLC. Mainframe computers control the larger systems which contain the PLC. The PLC has to be linked to both computer and computer network system.

Links can be made using cables which utilise serial or parallel data. . Ribbon like cables that link disc drives to motherboards are a type of parallel data.

Modems fitted to modern pc's are types of serial data.

Ribbon Cable

Like a ribbon a ribbon cable is flat and wide. The wires run in parallel formations along side one another. Ribbon cables are more often than not found internally with the electronics. In industry it is also known as a multi planer cable. In the ribbon formation each wire in the cable is utilised to connect corresponding contacts. This makes it imperative that the cable connects to the correct end. Visual assistance is given with the integration of colours, this prevent cross wire.

Figure 7

Fibre Optics

Fibre optic cable which are very common are thin glass strands housed within an external casing. When light is shone into the end of on of the strands Digital information is carried at high speed by pulsing light along these strands Compared to copper glass fibre carries more data and at higher speed and is not often effected by electromagnetic fields/interference. The capability of bundling strands together allows for greater transfer capabilities.

Ladder Diagrams

Ladder Diagram (LD) Traditional ladder logic is graphical programming language. The ladder diagram below shows the traditional ladder logic used in graphical programming language. Initially programmed with simple contacts that simulated the opening and closing of relays, Ladder Logic programming has been expanded to include such functions as counters, timers, shift registers, and math operations. This simulates the function of the opening and closing of relays. This can be expanded to include such things timers and lights.

Figure 8

Function Block

Function Block Diagram (FBD) - This depicts signal and data that flows through re-usable function blocks. This method is useful when you need to express the interconnections of the control system.

Figure 9

Ladder Diagrams and Schematics.




Asked to input a system to enable a drilling rig to utilise the inputted logic an explanation of the cycle is explained below.

Once the circuit become live the stop solenoid 5 will be triggered once sensors 2,3 and 5 activated. This will move the stop left. Once this action has been carried out the feed is required to place an item ready for drilling. This action (the movement of solenoid 1) will occur when sensors 1, 5 and 4 are activated. This activity will activate the drill (SOL 2). This happens when sensors 2,4 and 5 are activated. This will lead to the item being drilled. Once the drilling activity has taken place the drill will retract (SOL3) once sensors 6,2 and 4 are activated. Sol 4 will trigger 4, 2 and 5 are activated. Retracting the stop which releases the item. This ladder is written to allow this method of work to run continuous. The relays are placed within the ladder that if there was to be a reason to stop the activity the activity will fail safe. It is only when the start coil on the left hand side will the activity return to the start up configuration. This allows for safe working practices within the system. This method of relay would be seen in many PLC mechanical work modes.


As PLCS was a new subject to me, I found it quite difficult to construct this project. Sourcing the information that I felt fitted the parameters set within the assignment took times and became a little confusing. The Programming of the drilling mechanism as part of class room work was understandable but to place it within a ladder diagram and activate the PLC program was incomprehensible. I felt that more hands on examples were required or to actually watch a plc system in action would have enabled me to create a more substantial report. I would like to thank Keith Charman for his help throughout this assignment. He was able to explain and establish an understanding of PLCS when I became confused with the subject.