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PLC Terms and Concepts
1. PLCs resemble personal computers in internal structure, whoever, they are equipped with input-output channels ready to deal with standard industrial signals levels.
2. Although PLCs are built initial to handle sequential control as a replacement of relay logic, they are capable of performing continuous control functions ( like PID control) due to their arithmetic capabilities (They are micro processor based).
3. PLCs are noise immune. However, relays are more noise immune than PLCs due to their electrometrical nature. In fact relay themselves are noise sources!!
4. PLCs deals with different inputs and outputs signals levels (e.g. digital I/Os 12, 24 Vdc, 110, 220 Vac, analog I/Os 10 V, ±10V, 20 mA ). However, most PLCs operate internally at TTL voltage level 5 V
5. PLCs use remote I/Os to reduce cabling casts and some times to organized control signals in structured form.
6. Remote I/O units are connected to PLC unit serial communication.
7. The longest possible serial communication distance between PLCs can be achieved using fiber optics cables.
8. Coaxial cables provide good signal shielding and hence longer transmission (serial communication) than twisted pairs cables. However, fiber optics cables can carry signal longer distances.
9. Programs stored in EPROM can be erased using ultra violet rays
10. The resolution of a PLC analog input depends on number of bits for digital representation
11. Relays and Contactors are used as pre-actuators
12. An example of continuous pre-actuator is the inverter
13. Inductive proximity detectors sense metallic objects, while capacitive proximity switches sense non-metallic objects
14. Small size PLCs rarely have analog I/O points
15. Retentive PLC memory is used to save required memory status
16. PLC memory size depends on number of I/Os, future expansion, complication of problem logic & control strategy
17. PLC system is usually preferred over Relays systems because of ease of installation
ease of maintenance and operation speed.
18. Interface relays used at PLC output sometimes to protect PLC output contacts.
19. PLC size is determined by number of I/O points
20. PLC system expandability is achieved by ability to connect to remote I/O units, max. supported number of I/Os, and ability to communicate with other PLCs
21. Installed spare I/O points usually exists for the following reasons possible system expansion, maintenance and repair and due to modularity
22. Example of commonly used PLCs applications are; industrial boilers, steel industry, chemical processes, food and beverage industry, textile industry , ….
23. Emergency shut down ESD systems have to use fail safe PLC configuration
24. Fire pumps should be fault tolerant PLC systems
PLCs are not intrinsically safe for hazard areas also not all PLCs are explosion (Ex.) proof , usually PLC systems are configurable according to hazard areas
25. Over 80% of fault probability in PLC systems is due external wiring, sensors and actuators
26. PLCs communication modules are used to increase the system I/O points, to communicate with third party equipment, and to perform decentralized duties in addition to central activities.
27. PLCs are used in PID continuous control loops due to their arithmetic abilities, analog I/Os and processing speed
28. The most digital input noise immune module is the 220 volt module
The most analog noise immune module is 4-20mA module
29. To work in 50 C ambient and 90% RH in a dusty area with most PLCs we
can have properly sized IP54 enclosure
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