Rich Gibson
Certified Machinery Safety Expert (TÜV NORD)
ABB Jokab Safety Products

First-generation safety interlocks, like the mechanical door switch, were the best available solution 50 years ago ‒ but were far short of perfect. Read how the technology evolved – not always smoothly – to today’s safer, more reliable sensors.

The most fundamental machine-safety device is probably the door switch or interlock. Open a door, fence, or shield, and a switch stops the mechanical motion of the machine. The technology has been around for about 50 years, and you can still find new equipment that relies on these mechanical door switches.

Over the years, newer and better devices were adopted and then actually rejected for a time before making a resurgence. Today, newer and far better technology has emerged that provides superior machine reliability and operator safety.

Before talking about the state of the art in safety sensors, let’s look at how this technology evolved.

The first door contacts

The operating environment for the original safety switches was much different than today. Back then, relays provided the safety logic, making all the if/and/then decisions. There were no PLCs or logic or software, just panels full of relays clicking and clacking.

“Removing a key from the interlock on a machine door would actually cut the power to the contactors,” explains Rich Gibson, marketing manager for ABB’s Jokab safety products. “That would then stop the machine motion. The problem was that the relays were 115 VAC, which meant those switches had to break open higher-current AC contactors to de-energize the coils.”

Each time those relays clicked and clacked, every time they broke or made contact, there was arcing. Over time, that arcing would frequently result in the contacts welding together, which was the most common failure mode for relay-based systems.

When they failed in the open (machine stopped) position, that was a nuisance. When they welded shut in the closed (machine energized) position, it was a major safety hazard.

Mechanical safety switches

The solution they came up with to overcome contact welding was an improvement, but it came with a new weakness. They created a safety switch based on a mechanical linkage, an armature that increased the leverage on the contacts.

Now, when you removed the interlock key and opened the door, a cam mechanism would drive the armature that would force open the contacts, even if they were welded shut. Safety had been ensured, but at the cost of a more mechanically complex, and therefore inherently less-reliable, solution.

The arrival of low-voltage

“Fast forward 20 or 30 years,” says Gibson. “The technology shifted from 115 VAC to 24 VDC. These DC safety relays combined with safety PLCs and safety controllers, all operating on lower current, eliminating the possibility of welded contacts. That simply wouldn’t happen.”

In addition, the older-style mechanical switches that relied on physical contact between the door and the frame were increasingly replaced with non-contact magnetic switches. The active side of the switch, the component with wires running back to the panel, typically relied on a reed switch. In its simplest form, the reed switch has two contacts that look like metal reeds. They are contained inside a glass envelope filled with nitrogen, making parallel contact along their entire surface. These magnetic switches brought a new level of reliability to safety switches.

“The same couldn’t be said for machine reliability, though,” Gibson says. “The problem was that the gap between the magnet and the switch was critical. As gates sagged, as doors got out of alignment or as fences bent, the magnets no longer reliably actuated the switches. The natural vibration of the machine was often all it took to separate the magnet and switch enough to bring the machine to a stop and start the frustrating and time-consuming search for the problem while the machine sat idle.”

The poor reliability of these types of safety systems and the resulting loss of production became a huge issue in the 1990s. Equipment owners had to choose between compromising on safety or reliability; they couldn’t have both. So many started gravitating back to the old mechanical switches.

“I think this issue gave safety a black eye because it created the impression – accurate at the time – that adding safety to the machine meant hampering production,” Gibson says.

Coded magnetic switches

Today’s non-contact safety sensors with coded magnetic switches overcome that problem. These more-powerful magnets are far less sensitive to alignment issues. The sensors are low voltage DC and have no mechanical wear, successfully combining both high safety-system integrity and machine reliability.

One of the added advantages of these sensors is that the magnets provide an added layer of safety by reducing the ability to spoof them.

“Machine operators and maintenance techs are very adept finding workarounds to safety systems,” Gibson observes. “In the past, you could outsmart some switches with a simple fridge magnet. Today’s magnets are uniquely coded, making that more difficult and, in most cases, impossible.”

The future of sensors

The next big thing in safety sensors is radio frequency (RF) technology. Now, in addition to a non-contact, non-mechanical device, you eliminate the magnet and get a much higher tolerance for misalignment.

Each ABB Eden non-contact safety sensor includes one of about 100,000 potential codes. That makes it much less likely that someone can defeat the switch and cheat the doors. Gibson believes most safety-product manufacturers will move to these coded RF sensors in the next few years.

“Some industries, like packaging, have warmly embraced coded RF sensors,” Gibson says. “Their equipment has a lot of doors, so they were especially eager to overcome the pain of the previous generation of safety-sensor technology. In the manufacturing industry, though, they are still feeling that pain but are beginning to see a solution in coded safety sensors.”

It is also a step forward in predictive maintenance, because the sensor can transmit a warning if it is starting to get close to the threshold where it won’t make contact with its mating component. This raises the reliability considerably.

“It really has been an evolution,” Gibson says. “Those early safety switches were the best we could do at the time with the technology we had. But today, equipment makers have far superior options, and even better solutions are in the not-too-distant future.”