The Plc Controlled Systems

The Plc Controlled SystemsThis document has been created to fountain an overview of what PLC Systems ar and why we apply them in modern day life. By writing this report I hope to give a good understanding of what I know about PLC Systems. I leave behind start with a short history of PLCs showing how they initiative started off to what we currently use today including the contrary innovation faces.History of PLCsA PLC is a Programmable Logic Controller. It is a formal gubbins that is utilise to control electromechanical schemes/processes. They were designed in mind of replacing carcasss that argon controlled by transcends. Up until the late 60s automation in factories would use sometimes thousands of relays and cam timers to make simple tasks. This was very expensive and took up a rush of space. The set down was seen when the relays had to be changed, in a situation were altogether the relays had to be updated and changed, an electrician would be required to unri valed after another re-wire individually(prenominal) relay resulting in very high labour time. Digital estimators started to be used in a lot of industries to control processes exactly were far from perfect. The computer would feed to meet very stiff requirements that were not yet popular. These requirements would consist of specialist schedulemers that at this time would be an expensive asset. The computer would collapse to be protected to withstand the environments in which it was being used. The computer would process bit- get input mansion and output in order to control everything. At this stage an operator would be needed to monitoring device the organisation to limit everything in check. PLC systems were first invented on request from an automotive industry General Motors. They required a system that could replace the existing relay catchn system. A proposal was accepted from Bedford Associates who later went on to producing the first Programmable Logic Controll er. The image below shows the PLC 084 (name given to it by Bedford Associates as it was their 84th project. The whole stayed in service for closely 20 years.As shown in the photo, PLCs were very large. Over the years PLCs subscribe to greatly reduced in size as well as their performance has greatly increased. There atomic number 18 now resemblingly a few different references of PLCs that have been designed and used over the years. The principal(prenominal) types of PLCs argon Unitary, standard and Rack Mounted.PLC Design TypesUnitaryUnitary PLCs are the simplest form of a Programmable Logic Controller. These controllers are wholeness compact units that have all the components including the processor, inputs and outputs built in to one ho utilise. Having all components built in to one sealed unit means there is no room for expansion so you are curtail to the make sense of input and outputs the unit has. On the other hand it does mean a small unit is produced allowing it to be used in many everyday applications such as washing machines. The photo below (obtained from google images) shows some examples of unitary PLCs. these small units would be mounted directly to the application it is controlling. The downfall to unitary controllers is that youre check to the constraints of the controller for example if a controller is built with 8 inputs and 7 outputs, that is all that controller allow ever do.ModularModular PLCs are built up of a number of different modules. These modules are linked together allowing for the controller to be customised to suit the requirements. All the core functions such as the computer processor, inputs and power regulation are usuallycontained in the base module. Other modules are then added on as expansions of inputs and outputs and running(a) to digital sign up converters.This type of program is perfect for a system that whitethorn need to expand in the future, un desire the Unitary type design, this type is thought of a s a more future proof design. That being said a Modular design controller doesnt have and infinite amount of expansion, there forget only be room for a certain amount of expansion.Rack MountedRack climbing PLCs are the best design for large scale use. They pasture in a very similar way to the Modular type of PLC where extra modules can be added for expansion only ofttimes more expansion is available. Where as the Modular design has all its main functions under one base module and expansion modules are added directly to it a Rack Mounting PLC keeps all of the modules in organised racks and uses a net operation to connect them meaning that each module is snap off from one another. Using this type or system allows us to expand on a a great deal larger scale without things getting overly complicated. This design still allows for a very neat system that allows you to remove and add modules as required without doing any harm to the system. This is a modern approach that uses lucrei ng the homogeneous as many departments in a business today e.g. Computer networking. Using this design of PLC charming much future proofs your setup for expanding as the amount of expansion is pretty much endless by adding more and more racks of modules to the network.The image above is an example of a Rack Mounting PLC (image obtained from google images).Input and end product DevicesThere are many different input and output devices that can be used with a Programmable Logic Controller. The PLC is responsible for processing all of the input and output devices committed. Inputs are normally some frame of sensor or worst that feeds back down to the PLC and allows the PLC to monitor and use the data to signboard and operate the pertinent output. An output is the process that the PLC is essentially controlling. Some examples of the different types of inputs and outputs are below.Mechanical SwitchesMechanical faultes are a very popular form of input used with PLCs. The PLC will monitor the smite and wait for a mansion to be sent from the switch. Switches normally operate in two ways normally sequent-from-the-shoulder or normally closed. With a normally open switch a signal is sent to the switch but doesnt return (reach the PLC) due to an open lap covering. When the switch is made (pressed) the move is closed and a signal returns (reaches the PLC), from this the PLC can process the data and process the relevant program. A normally closed switch operates in the opposite way where a signal is constantly being procured by the PLC and when the switch is made, the circuit is made and the PLC no longer receives the signal, from this is processes the relevant program. A typical example of a mechanical switch would be the type that would be found at the start and end of a pneumatic plunger that is pressed by the piston itself when the piston reaches the relevant stroke, these are called limit switches.Non-mechanical Digital SourcesNon mechanical digital reso urces refers to sensor inputs that dont require a direct mechanical operation to operate like the mechanical switch. Non-mechanical switches are far more complicated than mechanical switches and have no moving parts. They are also much faster than mechanical switches which is why they are used for computing. An example of a non-mechanical switch is a transistor. Transistors work by adding an electrical charge to close the switch and allow the flow of current, when the electrical charge is removed, the switch is open and the current can no longer flow. The switch uses silicone mixed with other elements as a semiconductor and when an electrical charge is added, it becomes conductive allowing the flow of current. So the change in ground would be the electrical charge that operates the transistor and the flow of current would be the signal to the PLC.Optical sensors are another form of non-mechanical source. They work by sending out an optical signal to a reflector. When the signal is interrupted the PLC will process and preform an action. Optical sources now use Infrared as conflicting to the old systems using the normal light spectrum so that natural light sources dont interfere.TransducersTransducers are a common form of sensor typically used as a measuring device. A transducer is a device that converts one form of energy into another (http//en.wikipedia.org/wiki/Transducer). They typically convert a mechanical energy in to an electrical energy, an electrical energy that can be used to report to the PLC.OutputsThere are a very wide selection of items that can be used as an output for PLC systems such as relays, lights, sirens, motor starters, solenoids, etc. These are all classed as what the PLC is essentially controlling. The PLC would use the information fed back to it from the inputs, execute a program and activate the output accordingly an example of this could be a thermoregulator and air conditioning unit, the thermostat being the input to the PLC allow ing the PLC to know when a predetermined temperature has been reached. When the temperature raises above the preset temperature required, the thermostat will send the signal to the PLC (as described in section 6), the PLC will process the signal and send a signal to the output which in this case would likely be a relay that when activated boots up the air conditioning unit. When the required temperature is reached the PLC will process and signal the relay to switch, move the A/C unit off.Communication LinksAutomation using PLC systems use networking. Networking is used for devices to communicate with each other and can come in many different forms and can be down in the mouth down in to different sections such as Remote I/O, associate to peer, host computer communications and LAN (local area network).Remote I/O is a system that has the inputs and outputs at a distance away from the PLC. This system allows a PLC to control a variety of both digital and analog points to be contro lled eliminating the need for a controller at each point and resulting in a court effective set up. The I/O configuration can connect the PLC to all sorts of plant equipment to monitor things such as cycle counts and times. Each I/O device is related to as a slave for the ones directly on the machine and the master controller that all the slave I/Os report back to. The master PLC will send a signal to the slave I/Os and which it then receives a response, the PLC then uses this response to trigger the relevant program that it then signals the remote I/O to change its outputs to suit. These signals are sent extremely fast and cycle hundreds of times per second.Peer to peer networks work slightly differently in the way that they are connected, using multiple PLCs. This type of network will connect each PLC in sequence to each other and is sometimes known as a daisy chain. This system is very clever in the way it works keeping all the PLCs in the network, up to date allowing all the PL Cs to control their systems with the knowledge of what is happening in all the other systems. This allows for similar schedule due to having to only program each controller to operate its designated system. This type of networking allows for a natural rubber working system that when set up and programmed correctly means everything will flow and work in sync far quicker than that a human could process. Unlike remote I/O, this system does not require a master PLC as they all just use each others data, however sometimes they are used as a centre control point.Host computer communications connects the PLCs on a network to a computer. Most PLCs regardless of size can normally be connected to a computer. This allows for programs to be written in foot race logic form. Ladder logic form is the programming type that is quite popular in modern programming. It allows for a sort of pectoral type of programming that personally I find easier to understand. The ladder program can be written, e dited and tested (virtually) via a computer and then downloaded on to the PLC. Other forms of sizable devices can also be used with PLC systems to receive data for monitoring purposes.Internal ArchitectureThe internal architecture is made up of the central processor, storage devices, remembrance, opto-isolators, input and output units, flags and shift registers. All of these work together to form a very intelligent device.The central processing unit (central processing unit) is where the main processing and thinking is done, this is often thought of as the brain of any intelligent device using a CPU.A PLC has to be able to store information such as programs. The programs are stored to a storage device such as a hard disk drive or solid state chip. The programs are written on an external source such as a computer and then transferred to the PLC storage device where the PLCs CPU can then run the programs.The memory in an intelligent device is often confused with the storage device but is not real(a)ly used to store information long term like the storage device is. A form of memory most commonly used is Random Access Memory (RAM), this is used in PLCs and computers as well as the vast majority of intelligent devices such as smart phones. The RAM is used as a temporary memory for programs being run, it allows the CPU to access random bits of memory as it needs it from where ever it is stored, it does this at a very fast rate. Regular storage devices such as hard disk drives cannot operate at this speed because of restrictions only allowing them to access memory in a uniform order and opineing on where the information is stored will depend on how long the CPU will take to find it.An Opto-isolator is a protection device that transfers electrical signals between the input and output spot protecting the internal circuity of the PLC. It protects against hight voltages and rapidly changing voltages that can occur in the system.Input and output ports are the ports that the input and output devices are connected to.Flags is a term given for a data type used in PLC systems, more specifically it is the term that relates to simple on/off or I/O fields.Shift registers are information from previous program cycles stored by the PLC and later used/reflected on for running other programs.Operational CharacteristicsScanning is the process that the PLC goes through starting with the input and ending with the output. whiz scan cycle would go as followsREAD INPUT answer PROGRAM PROCESS MESSAGES EXECUTE SELF DIAGNOSTICS WRITE OUTPUTSRead Input = PLC keeps checking for input signalExecute Program = PLC prepares program but doesnt send itProcess Program = PLC reads the program and passes it onExecute Self Diagnostics = PLC will check the program works (theory test)Write Output = PLC then signals relevant outputsThis is just one skilful scan cycle that happens every 5 millionths of a second, this shows just how fast PLCs operate.Continuous updating is the CPU scanning the inputs in the specified order with a build in delay. The CPU scans each various(prenominal) input forrader the program is determined. This allows the CPU to only process valid input readings but does have a negative effect on the time it takes to process when there are a lot of inputs each with the delay.Information and Communication TechniquesThere are three forms of signal used with PLCs analog, Digital and Discrete.analogue signals are typically 0-10v DC or 4-20mA. These inputs are converted in to numerical harbors when they work out the PLC so they can be processed in the program. The PLC can also convert to an analog signal on the output if required (if needed by the output device).Digital signals are different from the analog signal as they are not dynamic, instead they are normally a simple on or off signal. This signal can be processed quicker than the analog. PLCs work with digital signals internally. This type of signal comes from more non mechanic al input devices (see section 7 non mechanical digital sources).A distinct signal is sort of a mixture of the two above. It is a signal that can have a variable esteem or range that is normally voltage of current. It provides a on of signal like the digital signal but will work within set ranges. For example a PLC using 12 V DC I/O might be set that a value about 10 V DV means on and Values below 6 V DC means off.PLCs are capable of working on various numbering systems. These numbering systems can be decimal, binary, octal, hexadecimal or BCD. The most common being decimal or binary.The decimal numbering system is the linear array of digits and the placing of each digit. Depending on the order or placement of the digit will depend on their actual value, this means that you could have the same number but have a different value for each. An example would be the number 3563, the first digit = 31000, the second digit = 5100, the third digit = 610 and the fourth digit = 31. This allow s a wide range of numbers to be used as each digit can go to 0-9. It also allows for the next number to increase when the number originally exceeds 9.The binary numbering system uses a different way of translating a value. Where as with decimal number each digit can range between 0 and 9, binary systems only have 0-1. There are set numerical values that are chosen by using the 1 or 0.27=12826=6425=3224=1623=822=421=220=110011100The table above is an example of an 8 bit code. 8 bits of information (1s and 0s). When a 1 is displayed, the value above is active so the number 10011100 would actually be 127 + 0x26 + 0x25 + 124 + 123 + 122 + 0x21 + 0x20 = 156. Or displayed as 128 + 0 + 0 + 16 + 8 + 4 + 0 + 0 = 156.Methods of ProgrammingPLCs can be programmed in various different ways Ladder/logic diagrams, statement lists, functionsLadder and logic diagrams are a very popular simple way or PLC programming. They are a sort of natural type of programming that allows the programmer to see e xactly whats happening. From the ladder diagram you are able to use use simulation software system to trail run your program to check that it is working correctly before uploading it to the PLC. Within the software preset parts such as switches and relays are easily added to the program. An example of the ladder diagram is shown on the next page followed by a screenshot of the simulation. This shows the how a ladder diagram would be made and linked to a working simulation on the computer software, it is slightly different to how the actual PLC program will be written but works as a simulation before writing the real program. The diagram is drawn up with what will be required in the circuit such as sensors, switches, air supply etc. The ladder diagram is then drawn and all the parts that have been used in the circuit are linked to the ladder. Coils are also added to achieve the required cycle.Ladder DiagramSimulation of Ladder DiagramWhen the designer is happy with the PLC simulatio n program, he/she will then use it to write the actual PLC program for transferring to the PLC. This software is slightly different but still produces a ladder style diagram where operations run left to right. The PLC programming software will have preset parts that are entered to the program. Once the program reflects the tested simulation program, it is transferred straight to the PLC itself.Word count 2504 words