Principles, Standards and Implementation

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Protective Measures and Complementary Equipment

Safety Networks

Plant floor communication networks have traditionally provided manufacturers the capability to improve flexibility, increase diagnostics, increase distance, reduce installation and wiring cost, ease maintainability and generally improve the productivity of their manufacturing operations. These same motivations are also driving the implementation of industrial safety networks. These safety networks allow manufacturers to distribute safety I/O and safety devices around their machinery using a single network cable, reducing installation costs while improving diagnostics and enabling safety systems of increased complexity. They also enable safe communications between safety PLCs/controllers, allowing users to distribute their safety control among several intelligent systems.

Safety networks do not prevent communication errors from occurring. Safety networks are more capable of detecting transmission errors and then allow safety devices to take the appropriate actions. Communication errors that are detected include: message insertion, message loss, message corruption, message delay, message repeat, and incorrect message sequence.

For most applications, when an error is detected the device will go to a known de-energized state, typically called a “safety state.” The safety input or output device is responsible for detecting these communication errors and then going to the safe state if appropriate.

Early safety networks were tied to a particular media type or media access scheme, so manufacturers were required to use specific cables, network interface cards, routers, bridges, etc. that also became part of the safety function. These networks were limited in that they only supported communication between safety devices. This meant that manufacturers were required to use two or more networks for their machine control strategy (one network for standard control and another for safety related control) increasing installation, training and spare parts costs.

Modern safety networks allow a single network cable to communicate with safety and standard control devices. CIP (Common Industrial Protocol) Safety is an open standard protocol published by ODVA (Open DeviceNet Vendors Association) that allows for safety communications between safety devices on DeviceNet, ControlNet and EtherNet/IP networks. Because CIP Safety is an extension to the standard CIP protocol, safety devices and standard devices can all reside on the same network. Users can also bridge between networks containing safety devices, allowing them to subdivide safety devices to fine-tune safety response times, or to simply make distribution of safety devices easier. Because the safety protocol is solely the responsibility of the end devices (safety PLC/controller, safety I/O module, safety component), standard cables, network interface cards, bridges, and routers are used, eliminating any special networking hardware and removing these devices from the safety function.

Figure 98 shows a simplified example of a distributed I/O system. The operator opens the gate. The interlock switch, connected to the local Safety I/O block, sends its safety data over the DeviceNet network to the Safety PLC. The Safety PLC sends a signal back to the Safety I/O block to shut down the equipment inside of the gate and sends a standard output to a stack light to annunciate the gate is open. The HMI and the standard PLC monitors the safety data for display and additional control measures, like performing a cycle stop of adjacent equipment.

Figure 98: Example of a Simple Distributed Safety Network

For larger manufacturing systems, where safety information and control must be shared, Ethernet/IP can also be used. Figure 99 (shown on the next page) shows an example of communications between two safety controllers while DeviceNet is used for local distribution of I/O within a smaller subsystem.

Figure 99: Example of a Complex Distributed Safety Network