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DMT 353/3 Programmable Logic Controller (PLC)

DMT 353/3 Programmable Logic Controller (PLC). PLC System. Introduction. What is PLC? PLC is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices. Definition.

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DMT 353/3 Programmable Logic Controller (PLC)

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  1. DMT 353/3Programmable Logic Controller (PLC) PLC System

  2. Introduction • What is PLC? PLC is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices

  3. Definition A formal definition of a PLC comes from the National Electrical Manufacturers Association (NEMA): A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes.

  4. Gantry Robot Control System

  5. PLC Overview • A PLC has many "input" terminals, through which it interprets "high" and "low" logical states from sensors and switches. • It also has many output terminals, through which it outputs "high" and "low" signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control. • In an effort to make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams. • Thus, an industrial electrician or engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions.

  6. In a traditional system, all control devices are wired directly to each other .. Input Output In a PLC system, all control devices are wired to the PLC.. Input Output

  7. The program takes the place of much of the external wiring that would be required for control of a process. Advantages of a PLC Control System • Eliminates much of the hard wiring that was associated with conventional relay control circuits.

  8. Example • Lets say that two push buttons, PB1 and PB2, are connected to a PLC. Two pilot lights, PL1 and PL2, are also connected to the PLC. • The way these devices are connected now pressing push button PB1 turns on pilot light PL1 and pressing push button PB2 turns on pilot light PL2. • Lets say that you want to change this around so that PB1 controls PL2 and PB2 controls PL1. • In a traditional system, you would have to rewire the circuit so that the wiring from the first push button goes to the second pilot light and vice versa. • However, because these devices are connected to a PLC, making this change is simple - small change in the control program.

  9. Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error. Advantages of a PLC Control System • Increased Reliability: Once a program has been written and tested it can be downloaded to other PLCs.

  10. It is easier to create and change a program in a PLC than to wire and rewire a circuit. End-users can modify the program in the field. Advantages of a PLC Control System • More Flexibility: Original equipment manufacturers (OEMs) can provide system updates for a process by simply sending out a new program.

  11. Generally, if an application requires more than about 6 control relays, it will usually be less expensive to install a PLC. Advantages of a PLC Control System • Lower Costs: Originally PLCs were designed to replace relay control logic. The cost savings using PLCs have been so significant that relay control is becoming obsolete, except for power applications.

  12. They can be networked to perform such functions as: supervisory control, data gathering, monitoring devices and process parameters, and downloading and uploading of programs. Advantages of a PLC Control System • Communications Capability: A PLC can communicate with other controllers or computer equipment.

  13. Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick response capability. Advantages of a PLC Control System • Faster Response Time: PLCs operate in real-time which means that an event taking place in the field will result in an operation or output taking place.

  14. The control program can be watched in real-time as it executes to find and fix problems Advantages of a PLC Control System • Easier To Troubleshoot: PLCs have resident diagnostic and override functions that allows users to easily trace and correct software and hardware problems.

  15. PLC Size Classification

  16. PLC “product” Classification

  17. Others PLC Brands • Others well known PLC brands are ABB Ltd., Koyo, Honeywell, Modicon, Allen-Bradley, Tesco Controls and Panasonic (Matsushita)

  18. Q & A

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