Sunday, February 6, 2011

Large Relay System Modernization of PLC

The modernization of a machine control system that will be changed from hardwired relay logic to PLC programmed logic. The field devices to be used will remain the same, with the exception of those that the controller can implement such as control relays, timer, interlocks, etc. the benefits of modernizing the control of this machine are:
• Less energy consumption
• Less space required for the control panel
• A more reliable control system
• A flexible system that can accommodate future expansion

An initial review of the ladder diagram relay indicates that certain portions of the logic should be left hardwired. This will keep all emergency stop conditions independent of the controller. The hydraulic pump motor, which is energized only when the master start push button is pushed, should also be left hardwired. The SCR (Safety Control Relay) will provide power to the rest of the system if hydraulic pump motor is operating properly and no emergency push button is depressed. Furthermore, the PLC will energize the fault coil, thus closing PLC fault contact 1 and opening PLC fault contact 2.

We can now start assigning the real inputs and outputs document. We will assign internal output addresses to all control relays, as well as interlocks and timers from control relays. The inputs with multiple contacts such as Selector Switch (SS) and Limit Switch (LS) have only one connection to the controller.

The PLC programming coding illustrates several special coding techniques that must be used to implement the PLC logic. Among these techniques are the software MCR function, instantaneous contacts from timers, OFF delay timers, and the separation of rungs with multiple outputs.

An MCR internal output, specified through the program software, performs a function similar to a hardwired MCR. Referring to the relay logic diagram, if the MCR energized, its contact will close, allowing power to flow to the rest of the system. The internal MCR 1700 accomplishes this same function in the PLC software.

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