Traditionally safety systems have required hard wiring and the use of electro-mechanical components, as required by the NFPA79, National Fire Protection Association “Electrical Standard for Industrial Machinery”. Section 9.6.3 states that a Category 0 stop shall only have hard-wired electro-mechanical components, and shall not depend on electronic components, hardware or software, or the transmission of commands over a communications network. Likewise, a Category 1 stop shall be accomplished by electro-mechanical equipments. Although this requirement is stipulated for emergency stop circuits, both redundant standard PLC and safety PLC have begun replacing the other safety-related hard-wired circuits.
For instance, there are many applications where the Programmable Logic Controller (PLC) has been used to control equipment, including the safety-related parts of the control system. Standard PLC controllers typically used in safety applications are configured in pairs. The redundant controller is used to support a safe and orderly shutdown in the event the primary controller fails. In addition to multiple controllers, designed of safety applications using standard PLCs utilize additional I/O inputs to monitor safety system output signals, and more outputs to generate test pulses for the safety system’s input modules. Also, applications designed around standard PLCs require custom software to control, monitor, and diagnose the safety system. To summarize, designing safety systems around standard controllers requires additional I/O hardware, engineering time, and wiring to support the safety portion of the application, in addition to the software and hardware required to run the application.
The packages of redundant PLC-based are available that reduce drastically engineering effort and eliminate the controller certification phase by providing complete software/hardware kits certified for use in press control applications. For example, Rockwell Automation’s Clutch/Brake control package based on redundant MicroLogix processors has been certified by TÜV as "suitable as a control and monitoring system for mechanical presses according to ANSI B11.1-1988 and EN 692-1996." Using two PLCs provides redundancy to improve the safety integrity of the system. The inputs and outputs are cross-wired to provide checking and self- monitoring.
These concepts improve the safety integrity of the system as compared to the use of a single PLC. The dual, cross-wired PLC configuration has demonstrated that electronic components can provide an acceptable integrity level for safety systems.Allen-Bradley 6556 MicroLogix Clutch/BrakeController for Mechanical Stamping Presses.