05-10-2012, 05:04 PM
PLC & DDCMIS
PLC.ppt (Size: 590 KB / Downloads: 113)
What is PLC?
A PLC is an industrially hardened computer-based unit that performs discrete or continuous control functions in a variety of processing plant and factory environments.
Originally intended as relay replacement equipment for the automotive industry, the PLC can be found in some part of virtually every type of industry imaginable.
The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems.
What is inside a PLC?
Power Supply
It converts Power Line voltages to those required by electronic components of PLC.
It may be Integral or separately mounted.
It provides the isolation necessary to protect electronic components from most high-voltage line spikes.
It is rated for heat dissipation requirements for plant floor operation.
Outputs
There are three common categories of outputs:
Discreet: Pilot lights, solenoid valves, annunciator windows (lamp box)
Register: Panel meters or displays
Analog: Variable speed drives or I/P converters (control valves)
Most I/O systems are modular in nature; that is, a system can be arranged by use of modules that contain multiples of I/O points.These modules can be composed of 1, 4, 8, or 16 points and plug into the existing bus structure.
The bus structure is really a high-speed multiplexer that carries information back and forth between the I/O modules and the CPU.
One of the important functions of I/O is its ability to isolate real-world signals from the low signal levels in the I/O bus
Input Systems
Inputs are defined as real-world signals giving the controller real-time status of process variables.
These signals can be analog or digital, low or high frequency, maintained or momentary.
They are presented as a varying voltage, current or resistance value.
Signals from thermocouples and RTDs are common examples of analog signals.
Pushbuttons, limit switches & electromechanical relay contacts are familiar examples of digital, contact closure type signals.
Memory Unit
It is the library where the application program is stored.
It is also where the PLC’s executive program is stored. An executive program functions as the operating system of the PLC. It is the program that interprets, manages and executes the user’s application program.
It is the part of PLC where process data from the input modules and control data for the output modules are temporarily stored as data tables.
Memory can be volatile or nonvolatile. Volatile memory is erased if power is removed. Battery backup is provided for most units with volatile memory to avoid loss of program in the event of power outage.
Programmer Unit
It provides an interface between the PLC and the user during program development, start-up and troubleshooting.
The instructions to be performed during each scan are coded and inserted into memory with the programmer.
Programmers vary from small hand-held units the size of a large calculator to desktop stand-alone intelligent CRT-based units.
Programmers come complete with documentation, reproduction, I/O status & on-line and off-line programming ability.
Many PLC use a PC as the programming tool using the serial interface and a Programming Software.
How a PLC Operates?
Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. It records this data into its memory to be used during the next step.
Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time using the input data obtained in previous step and decides about the states of output. It will store the execution results for use later during the next step.
Advantages of PLC
Compared with electromechanical relay systems, PLCs offer the following advantages:
Ease of programming and reprogramming in the plant.
A programming language that is based on relay wiring symbols familiar to most plant personnel
High reliability and minimal maintenance
Small physical size
Ability to communicate with computer systems in the plant
Moderate to low initial investment cost
Rugged construction
Modular design