Superior safety: From switches to sensors

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.”

Two technologies that simplify panel building

Berea Janzen
Product Marketing Manager – EPR, Pilot Devices, Limit Switches
ABB Electrification Products

In every assembly process, simplicity saves time and money. Two simple technologies that simplify panel making are push-in connections and compact pilot devices. Learn more about how they can help improve your assembly process.

Anyone who has been part of a Lean Six Sigma process-improvement team knows they typically rely on some very complex statistical methods. But not all of the improvements result from sophisticated tools like regression analysis, attribute agreement analysis, and t-Tests.

As someone who’s been part of a number of process-improvement projects, I know there are also a few simple but sure-fire ways to reduce errors and costs while accelerating a process. One of them is to reduce the number of process steps or activities. When it comes to panel building, there are two types of products that enable panel builders to easily simplify their process.

Connections with a simple push

Push-in terminal connections are an increasingly popular wiring method on devices like contactors. Both solid-wire and ferrule connections are as easy as inserting the wire end into the cable hole, no tool required. For stranded wire, a flat-blade screwdriver is needed to open the terminal, but no torqueing or screwing is required.

It’s easy to see how this is much faster than tightening terminal screws, typically reducing assembly time by half. It also eliminates the time needed to carefully torque each connection. The push-in connectors are engineered to ensure the required torque. As a confidence check, installers can check connections with a simple tug test.

Unlike screw-in connections, push-in connections are vibration-proof. Wires won’t vibrate loose in transit or during use. You never have to retighten them, increasing the reliability of the panel and reducing maintenance. An added benefit in high-volume panel shops is that push-in connectors enable automated assembly; the feature is very robot-friendly.

Push-in connections aren’t a new concept. They have long been embraced by electrical-equipment manufacturers in Europe. In the US, acceptance has been slower but is accelerating as manufacturers become more confident in the technology and more aware of the benefits. Many major panel shops, eager to reduce costs and production time while increasing panel reliability, have adopted push-in connections as their preferred technology.

One instead of many

Another simple process-improvement tactic is to reduce the number of components required in an assembly. Fewer parts means fewer steps in the production area. One way panel builders are reducing their component count is with all-in-one pilot devices. Rather than building up the device by attaching contacts, the pilot device installs as one unit.

The all-in-one design also eliminates the need to sort through and find the correct contacts for the device, and eliminates the possibility of assembling the wrong contacts.

The speed benefit of this design is obvious. Perhaps less obvious, but just as important in terms of cost reduction, is the inventory aspect. The all-in-one pilot device reduces the need to stock a range of contacts for your panel shop.

Some shops prefer the ability to customize or configure pilot devices on a per-panel basis. For them, the all-in-one design may not be the right choice. But for most panel shops, and particularly higher-volume shops, building up pilot devices from multiple components makes little sense.

Almost every electrical and mechanical device made today is increasingly modular, manufactured from sub-assemblies rather than individual components. This speeds and simplifies production and reduces product failures. When maintenance is required, service often consists of a simple module swap. Those same benefits come with the all-in-one pilot device.

A simple choice

Technology like push-in terminals and all-in-one pilot devices won’t revolutionize your panel-building process. Instead, like most changes that result from continuous-improvement efforts, they will create incremental but measurable benefits. Still, saving 30 seconds on each connection in a panel can add up to significant savings.

Both push-in connections and all-in-one pilot devices will absolutely simplify and speed panel assembly while increasing reliability. These technologies typically are no more, or only slightly more, expensive than the alternatives, ensuring reduced panel-production costs.

Panel builders interested in a simple way to quickly enhance their production process should consider whether products like push-in connections and all-in-one pilot devices make sense for their shops.

See power-control technology in action at ABB Customer World

ABB Customer World is a chance to learn, connect with peers, and find solutions. This year, it includes a remarkable simulation of the ABB Ability Electrical Distribution Control System, connecting devices throughout the showroom in working demo.

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