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Programmable Logic Controllers

Programmable Logic Controllers. LO1: Understand the design and operational characteristics of a PLC system. Unit 22: Learning Outcomes. LO1: Understand the design and operational characteristics of a PLC system LO2: Understand PLC information and communication techniques

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Programmable Logic Controllers

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  1. Programmable Logic Controllers LO1: Understand the design and operational characteristics of a PLC system

  2. Unit 22:Learning Outcomes • LO1: Understand the design and operational characteristics of a PLC system • LO2: Understand PLC information and communication techniques • LO3: Be able to apply programmable logic programming techniques • LO4: Understand alternative implementations of programmable control

  3. Learning Outcome 1 • LO1: Understand the design and operational characteristics of a PLC system • 1.1 evaluate the design characteristics of typical programmable logic devices • 1.2 describe different types of input and output device • 1.3 evaluate the different types of communication link used in programmable logic control systems • 1.4 describe the internal architecture and operational characteristics of the CPU of a typical programmable logic device

  4. Learning Outcome 1 1.1 Evaluate the design characteristics of typical programmable logic devices • unitary; modular; rack-mounted 1.2 Describe different types of input and output device • mechanical switches; non-mechanical digital sources; transducers; relays

  5. Learning Outcome 1 1.3 Evaluate the different types of communication link used in programmable logic control systems • twisted pair; coaxial; fibre-optic; networks 1.4 Describe the internal architecture and operational characteristics of the CPU of a typical programmable logic device • central processor unit (CPU); arithmetic logic unit (ALU); storage devices; memory; opto-isolators; input and output units; flags; shift; registers • scanning; performing logic operations; continuous updating; mass input/output (I/O) copying

  6. What do we meanby Controller? • A controller might be something you want… • …to control a sequence of events, • …to maintain some variable constant, or • …to follow some prescribed change

  7. Controlling aSequence of Events • Drilling a hole in an engine block • Start lowering the drill when the work-piece is in position • Start drilling when the drill reaches the work-piece • Stop drilling when the required depth is reached • Retract the drill • Switch off and wait for the next workpiece

  8. Maintaining aVariable Constant • Controlling the number of items to be packaged • Items are counted using some sensor • After a defined constant level has been reached, the controller is required to do something such as rotate the next packaging container into position

  9. Follow some prescribed change • A controller might be used to follow a prescribed change for example… • …tracking an object as it moves from point A to point B, or • …tracking the level of water as a tank fills from one level to another

  10. Types of Controller • Essentially three types… • Hard wire controlled systems • Microprocessor-controlled systems • Programmable Logic Controller

  11. Hard Wire Systems Motor Switch Relay to switch on large current to motor Low Voltage

  12. Deficiencies withHardwired Systems • Hardwired (traditional) control systems suffer from the fact that the ‘rules’ governing the control system is determined by the wiring itself. • Thus when the ‘rules’ i.e. the control needs to be changed, the wiring needs to be changed!

  13. MicroprocessorControlled Systems • Instead of hardwiring each control circuit for each control situation, the same basic arrangement of relays and sensors can be used, but we can write a programme to instruct the microprocessor how to react to each input signal. • By changing the instructions in the programme we can use the microprocessor to control a variety of different situations.

  14. A Washing Machine microprocessor control card

  15. ProgrammableLogic Controllers (PLCs) • A Programmable Logic Controller (PLC) is a special form of microprocessor based controller that uses programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes.

  16. The ProgrammableLogic Controller PROGRAMME PLC INPUTS OUTPUTS

  17. The ProgrammableLogic Controller (PLC) • A PLC is designed to be used by engineers – who may have limited knowledge of computers or computing languages • Programming is primarily concerned with implementing logic and switching operations. • Input and output devices to be controlled in the system are connected to the PLC. • The controller then monitors the inputs and outputs according to the programme and carries out the rules for which it has been programmed.

  18. The ProgrammableLogic Controller (PLC) • PLCs are similar to computers: computers being optimised for calculation and display tasks; PLCs optimised for control tasks in the industrial environment. Thus PLCs are… • Rugged, being designed to with stand vibrations, temperate, humidity and noise. • Have interfacing for inputs and outputs inside the controller • Are easily programmed and easily understood programming language that is primarily concerned with switching and logic operations

  19. Different Types PLC • PLC Systems: • There are essentially three types of mechanical design for a PLC system: a single box (Unitiary), the modular and the rack-mounted type.

  20. The Unitary Type • A unitary type (sometimes called a brick) is commonly used for small programmable controllers and is supplied as an integral compact package complete with power supply, processor, memory and input / output units • Typically such a PLC might have 6, 8, 12 or 24 inputs and 4, 8, or 16 outputs and a memory that can store some 300 to 1000 instructions

  21. The Single Box(Compact) PLC • The Mitsubishi MELSEC FX3U

  22. The MitsubishiCompact PLC Range

  23. Extending BrickPLCs to add capacity • Some brick systems have the capacity to be extended to cope with more inputs and outputs by linking input / output boxes to them OMRON CPM1A PLC

  24. The Modular Type • Systems with larger numbers of inputs and outputs are likely to be modular. • The modular type consists of separate modules for power supply, processor and input and output cards (interfaces). • You can ‘pick-&-mix’ modules to fit the requirements of the job. • Thus the mix of modules required for a particular purpose is decided by the user and the appropriate ones then plugged in to the rack.

  25. The Modular Type • To add further I/O connections, modules can simply be added into the rack. • To add additional memory, a memory module can be clipped to the PLC directly.

  26. The Rack Type • The Rack type PLC consists of individual elements (Power supply, CPU, I/O modules etc)that are clipped into the rack the engage into copper conductors in the backplane of the rack (a bus). • To add further I/O connections, modules can simply be added into the rack. • To add additional memory, a memory module can be added to the rack. • When the modules are clipped into the rack the engage into copper conductors in the backplane of the rack (a bus).

  27. Allen-Bradley PLC-5 1771 I/O Back-panel mounted 1771 I/O rack mounted

  28. Possible Modules for theAllen-Bradley PLC-5 • Power Supply Module • Processor Module • Communications Module for communication to computers, I/O adaptors and other PLC processors • I/O adaptor Module for connecting the backplane to the processor at another location • I/O modules to provide the means to convert input signals to backplane levels and backplane signals to output circuit levels.

  29. The criteria behind theselection of a particular type of PLC • Safety: • The PLC must be safe to operate in the environment for which it is to be designed – i.e. can it function effectively in an explosive environment? (i.e. dust!) • Functionality: • It probably goes with out saying the PLC needs to fit for the purpose i.e. it needs to perform the function required to meet the specification of the particular job task in question. (i.e. the number of I/O required, comms cards, ADC, etc)

  30. The criteria behind theselection of a particular type of PLC • Communication: • How will the PLC communicate with other devices in the system such as computers, HMIs, etc. (Clearly all equipment must be compatible.)

  31. The criteria behind theselection of a particular type of PLC • Compatibility • A particular company when ordering a PLC system may specify particular manufacturer’s equipment – this might be for reasons such as that employees may already be familiar with the kit and so this may reduce the need for training, etc. • Additional Programming equipment need not necessarily be purchased. • Contrast the needs of the procuring company and that of the PLC system builder!!!!!

  32. The criteria behind theselection of a particular type of PLC • Environment • The PLC must be suitable for the working environment – heat, dust, vibration, moisture, electrical inference, magnetic fields, etc) • Reliability • Consider the use in a nuclear power plant. Back up systems? What would be the cost of a breakdown? • Cost

  33. Exercises • Handout 1 • Handout 2

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