programmable Logic Controllers
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Joined: Dec 2009
07-12-2009, 01:56 PM
I need the seminar and presentation report of "Programmable logic controllers".
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Joined: Jul 2009
07-12-2009, 05:01 PM
Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC),its a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures,its act like a real time system since output results must be produced in response to input conditions within a bounded time
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Joined: Apr 2012
25-07-2012, 04:56 PM
Programmable Logic Controllers
Program Control Instructions
Program control instructions are used to alter the program scan from its normal sequence. Sometimes referred to as override instructions, they provide a means of executing sections of the control logic if certain conditions are met. They allow for greater program flexibility and greater efficiency in the program scan.
Hardwired Master Control Relay Circuit
Hardwired master control relays are used in relay circuitry to provide input/output power shutdown of an entire circuit.
The master control reset (MCR) instruction can be programmed to control an entire circuit or to control only selected rungs of a circuit. When the MCR instruction is false, or de-energized, all nonretentive (nonlatched) rungs below the the MCR will be de-energized even if the programmed logic for each rung is true. All retentive rungs will remain in their last state. The MCR instruction establishes a zone in the user program in which all nonretentive outputs can be turned off simultaneously. Therefore, retentive instructions should not normally be placed within an MCR zone because the MCR zone maintains retentive instructions in the last active state when the instruction goes false.
Programming MCR Instructions
If you start instructions such as timers and counters
in an MCR zone, instruction operation ceases when the
zone is disabled.
The TOF timer will activate when placed inside a false
When troubleshooting a program that contains an MCR zone you need to be aware of which rungs are within zones in order to correctly edit the circuit.
MCR controlled areas must contain only two MCR
instructions – one to define the start and one to define the end.
As in computer programming, it is sometimes desirable
to be able to jump over certain program instructions.
The jump instruction (JMP) is an output instruction used for this purpose. The advantages to the jump instruction include:
the ability to reduce the processor scan time by jumping over instructions not pertinent to the machines operation at that instant
The PLC can hold more than one program and scan only the program appropriate to operator requirements
Sections of a program can be jumped when a production fault occurs
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