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This document has been created to give an overview of what PLC Systems are and why we use them in modern day life. By writing this report I hope to give a good understanding of what I know about PLC Systems. I will start with a short history of PLCs showing how they first started off to what we currently use today including the different design types.
History of PLC's
A PLC is a Programmable Logic Controller. It is a digital device that is used to control electromechanical systems/processes. They were designed in mind of replacing systems that are controlled by relays. Up until the late 60's automation in factories would use sometimes thousands of relays and cam timers to achieve simple tasks. This was very expensive and took up a lot of space. The expense was seen when the relays had to be changed, in a situation were all the relays had to be updated and changed, an electrician would be required to individually re-wire each relay resulting in very high labour time. Digital computers started to be used in a lot of industries to control processes but were far from perfect. The computer would have to meet very strict requirements that were not yet popular. These requirements would consist of specialist programmers that at this time would be an expensive asset. The computer would have to be protected to withstand the environments in which it was being used. The computer would process bit-form input and output in order to control everything. At this stage an operator would be needed to monitor the system to keep 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 driven system. A proposal was accepted from Bedford Associates who later went on to producing the first Programmable Logic Controller. The image below shows the PLC 084 (name given to it by Bedford Associates as it was their 84th project. The unit stayed in service for almost 20 years.
As shown in the photo, PLCs were very large. Over the years PLCs have greatly reduced in size as well as their performance has greatly increased. There are now also a few different types of PLCs that have been designed and used over the years. The main types of PLCs are Unitary, Modular and Rack Mounted.
PLC Design Types
Unitary PLC's are the simplest form of a Programmable Logic Controller. These controllers are single compact units that have all the components including the processor, inputs and outputs built in to one housing. Having all components built in to one sealed unit means there is no room for expansion so you are restricted to the amount 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 you're limited to the constraints of the controller for example if a controller is built with 8 inputs and 7 outputs, that is all that controller will ever do.
Modular 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 usually
contained in the base module. Other modules are then added on as expansions of inputs and outputs and analog to digital signal converters.
This type of program is perfect for a system that may need to expand in the future, unlike the Unitary type design, this type is thought of as a more future proof design. That being said a Modular design controller doesn't have and infinite amount of expansion, there will only be room for a certain amount of expansion.
Rack Mounting PLCs are the best design for large scale use. They work in a very similar way to the Modular type of PLC where extra modules can be added for expansion only much 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 network to connect them meaning that each module is separate from one another. Using this type or system allows us to expand on a much 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 networking the same as many departments in a business today e.g. Computer networking. Using this design of PLC pretty 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 Output Devices
There 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 connected. Inputs are normally some sort of sensor or switch that feeds back to the PLC and allows the PLC to monitor and use the data to signal and operate the relevant 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 switches are a very popular form of input used with PLC's. The PLC will monitor the switch and wait for a signal to be sent from the switch. Switches normally operate in two ways; normally open or normally closed. With a normally open switch a signal is sent to the switch but doesn't return (reach the PLC) due to an open circuit. When the switch is made (pressed) the circuit 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 received 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 piston that is pressed by the piston itself when the piston reaches the relevant stroke, these are called limit switches.
Non-mechanical Digital Sources
Non mechanical digital resources refers to sensor inputs that don't 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 state 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 opposed to the old systems using the normal light spectrum so that natural light sources don't interfere.
Transducers 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.
There 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 thermostat and air conditioning unit, the thermostat being the input to the PLC allowing the PLC to know when a preset 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, turning the A/C unit off.
Automation 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 broken down in to different sections such as; Remote I/O, peer 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 controlled eliminating the need for a controller at each point and resulting in a cost 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 PLCs to control their systems with the knowledge of what is happening in all the other systems. This allows for similar programming due to having to only program each controller to operate its designated system. This type of networking allows for a safe 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 ladder 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, edited and tested (virtually) via a computer and then downloaded on to the PLC. Other forms of intelligent devices can also be used with PLC systems to receive data for monitoring purposes.
The internal architecture is made up of the CPU, storage devices, memory, opto-isolators, input and output units, flags and shift registers. All of these work together to form a very intelligent device.
The CPU (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 actually 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 depending 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 while 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.
Scanning is the process that the PLC goes through starting with the input and ending with the output. One scan cycle would go as follows:
READ INPUT ---> EXECUTE PROGRAM ---> PROCESS MESSAGES --->
EXECUTE SELF DIAGNOSTICS ---> WRITE OUTPUTS
Read Input = PLC keeps checking for input signal
Execute Program = PLC prepares program but doesn't send it
Process Program = PLC reads the program and passes it on
Execute Self Diagnostics = PLC will check the program works (theory test)
Write Output = PLC then signals relevant outputs
This is just one full 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 individual input before 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 Techniques
There are three forms of signal used with PLCs; analog, Digital and Discrete.
Analog signals are typically 0-10v DC or 4-20mA. These inputs are converted in to numerical values when they enter 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 mechanical input devices (see section 7 - non mechanical digital sources).
A discrete signal is sort of a mixture of the two above. It is a signal that can have a variable value 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 = 3x1000, the second digit = 5x100, the third digit = 6x10 and the fourth digit = 3x1. This allows 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 before 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.
The 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 1x2^7 + 0x2^6 + 0x2^5 + 1x2^4 + 1x2^3 + 1x2^2 + 0x2^1 + 0x2^0 = 156. Or displayed as 128 + 0 + 0 + 16 + 8 + 4 + 0 + 0 = 156.
Methods of Programming
PLCs can be programmed in various different ways; Ladder/logic diagrams, statement lists, functions
Ladder and logic diagrams are a very popular simple way or PLC programming. They are a sort of pictorial type of programming that allows the programmer to see exactly whats happening. From the ladder diagram you are able to use use simulation software 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.
Simulation of Ladder Diagram
When the designer is happy with the PLC simulation 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.
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