Sensors, Switches Supply Safety

Aug. 31, 2011
Machine Safety Systems Can Be Expensive. We Speak With Industry Suppliers and Here Are Some Recommendations to Help You Keep the Cost Down
By Dan Hebert, PE, Senior Technical Editor

Sensors and switches are the first line of defense in many machine safety systems. Many machines use lightweight guard doors, and it's important that the position of these doors be known at all times.

"Our SensaGuard switch with a magnetic latch is the first to combine a non-contact interlock switch and door latch into one integrated solution," says Steve Tambeau, product marketing specialist for safety switches at Rockwell Automation ( "It features the latest RFID technology for coding, and it uses inductive technology for sensing as well as output switching signal device (OSSD) safety outputs. It's available with a uniquely coded sensor and actuator to help protect against operators or maintenance personnel defeating the switch, and an LED on the switch provides local status."

[pullquote]The switch can be used in safety systems up to a SIL 3 and PLe, and it's rated for Category 4 safety applications, Tambeau adds. "Users can connect the switches in series and still maintain the highest level rating," he says. "It's IP69K-rated for high-pressure, high-temperature washdowns, making it ideal for food and beverage and pharmaceutical applications."

Another supplier seconds the value of non-contact switches. "In the past, many OEMs installed bulky, expensive mechanical safety switches on machine entry gates," observes Corey McAtee, safety solutions product manager at Sick ( "These switches often had a short life span due to mechanical failure, and that led to high maintenance costs. A better solution is a non-contact, magnetic switch that has a relatively small, easy-to-install form factor. Since our switches come in several sizes and shapes, users can remove unnecessary mounting hardware used with previous-generation switches, simplifying equipment build."

The sensors come in a sealed housing with an M8-style connector and have an IP67 rating. "A magnetic safety switch also has an inherently higher safety rating due to its internal redundancy and lack of any moving parts vs. a mechanical switch that is redundant electrically but not mechanically," McAtee explains.

Light curtains are used in many machine and robot safety systems, and probably would be used more if costs were lower. One way to lower the price point is to make the light curtain as simple as possible by moving logic and intelligence to external components.

"New Contrinex light curtains have minimal on-board logic and no muting or stepping, which makes them an inexpensive and practical option for safety system designs," notes Lenny Filipkowski, industrial components product manager at AutomationDirect ( "Installation and wiring are also very straightforward, as no programming is needed and no settings are required. Available mounting options allow OEMs to position the unit as needed. Each unit is made up of internal light blocks that are serialized for tracking purposes, and then assembled together and slid into the durable housing and sealed. This keeps our manufacturing costs down, and allows us to price these units very attractively."

Machine builders can customize some safety switches via a building block approach. "The modular design of our 3SE5 limit switches allows for purchase of completed units or customer assembly of custom switches comprised of plastic or metal switch bodies, contact blocks, and various operating heads for every application," says Matt Seybert, product specialist for control components and systems engineering at Siemens Industry (

"As an indirect advantage of the modular design, an OEM can replace contact blocks and operating heads easily," Seybert adds. "New features for the limit switch family include an integrated chlorinated rubber diaphragm for high functional safety in cold and aggressive environments, and optional LEDs for all enclosures. Further, tedious screw-mounting of operating heads has been replaced with an easy one-step locking mechanism, which significantly reduces installation time."