Sunday, December 12, 2010

The Fundamental of Safety PLC

The systems of safety have traditionally required hard wiring and the use of electro-mechanical components, as required by the National Fire Protection Association “Electrical Standard for Industrial Machinery” (NFPA79). It states that a category 0 stop shall only have hard wired electro-mechanical components and shall not depend on electronic components (software and hardware) or the transmission of commands over a communications network. Both redundant standard PLCs and safety PLCs have begun replacing the other safety related hard wired circuits, although this requirement is stipulated for emergency stop circuits.

There are many applications where the PLC (Programmable Logic Controller) has been used to control equipment including the safety related parts of the system control. Standard PLC controllers typically used to support a safe and orderly shutdown in the event the primary controller fails. Safety applications designed using standard PLC utilize additional I/O inputs to monitor system of safety output signals, and more outputs to generate test pulses for the safety system’s input modules. Applications designed around standard PLCs require custom software to control, monitor, and diagnose the system. Designing the systems of safety around standard controllers require additional I/O hardware, engineering time, and wiring to support the safety portion of the application. In addition the software and hardware required to run the application.

Redundant PLC based packages are available that reduce drastically engineering effort and eliminate the certification of controller phase by providing complete hardware and software kits certified for use in press control application. For instance, Rockwell Automation’s Clutch or brake control package based on redundant MicroLogix processors has been certified by TUV as “suitable as a monitoring and control system for mechanical presses according to EN 692-1996 and ANSI B11.1-1998.” To use two PLCs provides redundancy to improve the safety integrity of the systems. The inputs and outputs are cross wired to provide self checking and monitoring of the operation.

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