Machine and robot builders often need safety-rated monitoring and control, which can be accomplished with safety-rated relays or PLCs. "Many applications could use either a safety relay or PLC, but the choice depends on four main considerations: complexity, flexibility, availability and footprint," says George Schuster, senior industry specialist for the automotive team at Rockwell Automation.
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Safety applications become complex when more than about three inputs must be monitored and controlled, or when communication over a digital network is required. "Safety relays are hardwired to perform simple diagnostics and logic, and work well when the amount of safety points in the application are small in numbers and simple in functionality," observes John D'Silva, PE, factory automation marketing and promotions, Siemens Industry. "They're well-suited for standalone machine safety applications that have a maximum of two to three safety devices or functions, for example, single-cell robot control for E-stop and door monitoring. But any application with more than three safety devices or functions or a requirement for safety networking can be served best by a safety PLC."
Required integration also can drive complexity to PLC levels, says Deana Fu, product marketing engineer at Mitsubishi Electric Automation. "Applications that require integration between standard automation devices and safety components are better served by a safety-rated controller," Fu states. "Machine control of a robot, conveyor belt or oven chamber is generally the responsibility of a standard controller, but a safety system needs to be installed around hazardous areas to ensure safe operation."
The second factor driving the safety relay versus PLC decision is flexibility. "It's easier and more cost-effective to expand or change a safety function using a safety-rated PLC rather than a safety relay. With a safety-rated PLC, the safety and standard control systems share information, and use one development environment, which helps eliminate expensive redevelopment," Schuster notes.
Conversely, applications that are simple and fixed in function can often be served more effectively with safety relays. "Machine builders usually only have a few safety loops or safety instrumented functions, and in these instances a safety PLC can be overkill," observes Jim McConahay, senior applications engineer at Moore Industries. "One option is to use a safety trip alarm module as a single loop logic solver to monitor a position or status variable. If the input exceeds a selected high or low trip point, one or multiple relay outputs send a warning to a higher level control or safety system, handle local on/off control, and/or provide emergency shutdown. Single-loop logic solvers are easier to use than safety PLCs, and can easily meet SIL 1 and SIL 2 protection levels, and can even be wired in 2oo3 voting configurations to meet SIL 3 applications."
Availability is the third factor driving the safety relay versus PLC decision, and high availability requires status and other diagnostic information. "As the sensors move away from the controller, it becomes more difficult to communicate status of the safety I/O back with relays," notes Zachary Stank, product marketing specialist for safety at Phoenix Contact. "Safety PLCs control safety I/O through network communication protocols, allowing for both control and status to be viewed on a single controller. Safety PLCs and distributed safety logic also provide more powerful and flexible safety logic that may be necessary for larger machines and process control."
Richard Harris, manager of channel relations at Omron Automation and Safety, concurs. "Customers want more information and control over machines. Programmable solutions provide significantly greater diagnostic information, which can be transferred easily to the host controller," Harris says. "Unscheduled machine stops can be monitored and analyzed to reduce downtime."
The final consideration is footprint, typically reduced when using safety-rated PLCs. "Certain models of our programmable safety controllers can replace safety monitoring relays, and eliminate the need for safety mat controllers, muting modules, slow and stop speed-detection modules and non-contact switch controllers, Harris adds.
Not only will such a solution reduce required footprint, it can also cut costs in many instances, and also reduce OEM stocking requirements.