By Phil Burgert
Occupational safety requirements, increased demands for flexibility, diagnostics and Ethernet-based safety are driving use of safety relays and programmable safety controllers while providing machine builders with options.
The safety devices solve some problems for machine builders and users but also add complexity to the task of deciding what level of safety is required, according to machine builders and system integrators.
"Our bag of tricks in the safety field is expanding constantly," says Scott Bonnet, control engineer for Rockford Systems, a safety systems integrator in Rockford, Ill. "We use a significant number of safety relays since most of our business is single-machine applications and retrofits. If we are simply adding safety equipment to a machine, we typically use some type of safety relay in the interface (Figure 1)."
The number of machines involved in Rockford's projects often dictates whether relays or more sophisticated controllers are used. "As the complexity of machines and work cells advances, midsize and larger safety PLCs have become a must," says Bonnet. "Companies find the benefits of these PLCs if they have multiple zones, multiple devices and the need for integrated controls. The percentage of our business using PLCs is increasing steadily."
Estimates of the number of safety points that should be controlled using relays vs. controllers vary, but many vendors, integrators and users put the tipping point to more sophisticated controllers at between three and 10 safety points or functions.
"Safety relays are a perfect fit to monitor a small number of safety guards or emergency stops," says Mick Garrick, lead product specialist at Phoenix Contact. "As the requirements and complexity of the safety system grows, so does the numbers of safety relays and/or safety relay expansion modules required to handle the safety control."
Some vendors, including Pilz Automation, define configurable safety relays as a safety device category for handling about five to 14 safety functions, while recommending programmable safety systems and controllers for applications with about 15 to more than 1,000 safety functions.
Safety relays and configurable safety relays are used for machine safety in at least 80% of installations by system integrators and machine builders, says Thomas Hoertig, regional sales manager for Pilz. "End users mostly expect the machine manufacturer or the system integrator to supply safety relays in the control system as part of the machine compliance to applicable industry standards," he states.
Banner Engineering terms this middle ground as safety modules, which Mike Carlson, safety products marketing manager, says can be used for four to a dozen safety functions and also have some level of configurability (Figure 2). "What they really bring to the table is the ability to interface the safeguarding solutions," he says of the increased options now available.
Keep It Simple
Another integrator view of machine safety comes from Bob Iossi, regional business manager for Maverick Technologies, Columbia, Ill. "In a six- or seven-machine automation project for stamping presses, we typically would use safety relays in connection with robots and press control," he says. "I don't do a lot of press retrofits. I do more of the automation that handles the material in and out of the press. Quite frankly, we try not to mess with the press internals because the minute you do that you have to upgrade everything to meet OSHA specs. We'll try to just interface with relay vs. doing the whole retrofit."
For most projects Maverick uses safety relays to control the press, says Iossi. "I'm not real big on programmable safety relays," he says. "With the protected logic controllers you're locking out any electrician from changing bits around and removing safeties. That's the function of it. With safety relays we have a defined standard that nobody can modify. I like that scenario better."
Dave Collins, product manager for safety products at Schneider Electric says many industrial OEMs and users want to upgrade their safety systems, but "most of them want to make sure they protect personnel without going overboard," he says. "A lot of machinery is built with a relatively small number of safety inputs, so sophisticated programmable systems would be more than what's needed and would add a level of complexity that is not needed."
However, a downside of safety relays can be the lack of diagnostics they provide, adds J.B. Titus, manager of business development and safety standards for Siemens Energy & Automation. "When the machine stops," says Titus, "you still have to go out and do some troubleshooting, find and repair the problem to restart the machine. Sometimes the diagnostics issue drives the decision."