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In the first time we have spoken about PLC History and Future. Today we will speak about the Programmable logic Controller (PLC) itself.

A programmable controller, as illustrated in below Figure , consists of two basic sections:
• the central processing unit
• the input/output interface system
Programmable controller block diagram

The central processing unit (CPU) governs all PLC activities. The following three components, shown in below  Figure, form the CPU:
• the processor
• the memory system
• the system power supply
Block diagram of major CPU components


The operation of a programmable controller is relatively simple. The input/ output (I/O) system is physically connected to the field devices that are encountered in the machine or that are used in the control of a process. These field devices may be discrete or analog input/output devices, such as limit switches, pressure transducers, push buttons, motor starters, solenoids, etc.
The I/O interfaces provide the connection between the CPU and the information providers (inputs) and controllable devices (outputs). During its operation, the CPU completes three processes:
(1) it reads, or accepts, the input data from the field devices via the input interfaces
(2) it executes, or performs, the control program stored in the memory system
(3) it writes, or updates, the output devices via the output interfaces.
This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.Below Figure illustrates a graphic representation of a scan.
PLC Scan Time


The input/output system forms the interface by which field devices are connected to the controller (see below Figure ).
PLC Input output interface


The main purpose of the interface is to condition the various signals received from or sent to external field devices. Incoming signals from sensors (e.g., push buttons, limit switches, analog sensors, selector switches, and thumb-wheel switches) are wired to terminals on the input interfaces. Devices that will be controlled, like motor starters, solenoid valves, pilot lights, and position valves, are connected to the terminals of the output interfaces. The system power supply provides all the voltages required for the proper operation of the various central processing unit sections.

Although not generally considered a part of the controller, the programming device, usually a personal computer or a manufacturer's mini programmer unit, is required to enter the control program into memory.
The programming device must be connected to the controller when entering or monitoring the control program.

3 comments:

räumung يقول...

شكرا لكم ..دائما موفقين ))


räumung
räumung

ali يقول...

thank you

taher ghirri يقول...

Impressed with the way the PLC was explained….. Indeed it was a gift from God almighty to the control engineer because in my days for medium size control system, the control engineer or the instrumentation engineer has to deal with lots and lots of relays of all sorts of shapes , sizes , function and different voltages( 220,110 24,12,6….. DC and the same nightmare in DC ) then converters and so on……..and the real …real nightmare is when a fault hits the system….. but when the PLC came along the control engineer can enjoy a cup of tea in peace cause he knows things are going to be all right Well don very useful stuff or should I say ………..cool stuff…man

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