July 2008

More than just a 'smart bolt'

By Marcel Ulrich

There is nothing like a little "friendly rivalry" in the workplace to make the day go by a little quicker.

My "nemesis" is the photoelectric manager. Recently I was giving him a good-natured ribbing regarding his "glorified infrared pen lights," when he shot back with a, "That's pretty funny coming from an engineer peddling 'smart bolts!' "


Although I would never admit it to the guy, the term "smart bolt" is a pretty good descriptor of metal face proximity sensors. They are a dead-ringer for threaded bolts and as durable as a bolt. The intelligence embedded within their metal exterior certainly qualifies as "smarts."

To understand where metal face inductive sensor technology is today, one first needs to travel back to the mid-1990s, when the metal-face inductive sensing revolution began.

Inductive proximity sensors, from their inception in the late 1950s through today, have traditionally utilized plastic sensing faces. Since the typical sensor's detection range is relatively short, between 1.5mm and 15mm, those plastic faces absorb a lot of abrasion/impact damage from the comparatively hard metal targets.

For abusive applications, replacing battered plastic-face sensors became tiresome and expensive. Industry called for a more durable sensing face. The solution? Stainless steel.

The circuitry required to detect a metal target through a metal wall while maintaining the reliability expected from a traditional proximity sensor proved a very difficult engineering task. It went against the physics principles used for years in plastic-faced products.

Traditional (plastic-face) proximity sensors detect metal by inducing a high frequency signal on an internal copper coil wound around a ferrite. The resulting electrical field travels through the plastic face undeterred, just as it would free air.

When a conductive metal object penetrates the field, the oscillator becomes loaded down. Once the loading drops the oscillations below a preset threshold, the switch output is triggered.

Manufacturers solved the sensing-metals-through-metal dilemma by utilizing a low frequency oscillator. While high frequencies conduct well on a metal objects' surface, properly calculated low frequencies can penetrate and even pass through non-ferrous metals such as stainless steel.

When a sensor is "tuned" to have its field pass through a stainless steel face, it is then capable of detecting metals on the other side.

So it is simple, right? Just lower the oscillator frequency and you have revolutionized the sensing world. … Unfortunately, it was not that easy. Field losses inherent to the stainless face required some sophisticated circuitry.

Sensors come of age, maturity

Some early metal face models had some shortcomings.

Some designs elected to use very thin stainless steel walls. These products dented easily and offered little more mechanical protection than the plastic-face models they replaced. Other designs used 2-piece housings. Unfortunately, when these designs sustained an impact, the stainless steel end cap could loosen.

There were designs that used a low oscillator frequency within the 60Hz "line frequency" spectrum. Thus when motor drives or heavy machinery were initiated, the resulting line noise would artificially collapse the oscillator and create false-on signals.

Others promised outlandish sensing ranges that looked impressive on paper, but those products exhibited hypersensitivity when installed. Sensitivity drifts from surrounding metals and ambient temperature changes created inaccuracies.

However, when properly designed, metal face proximity sensors did deliver performance as promised. Comparative testing of quality metal face sensors vs. traditional sensors revealed life increases of 20X or more. In addition, since the products typically cost only 30-40% more than a plastic face counterpart, they have proven to pay for themselves many times over.

Just as the industry call for metal-face sensors came to the forefront in the mid-1990s, the call for extended range metal face products resonated in the mid-2000s. Impact withstandability is great, but the ability to both withstand and, through greater target-to-sensor separation, avoid contact, is certainly the best.

Now, targets with unwieldy positional tolerances or undulating contours are readily detectable. Moreover, where long-range, plastic-face sensors were previously the only available option, there are now long-lasting metal face alternatives. 

The application of metal face inductive sensors continues to evolve.

Manufactures are now coating the products with spatter-resistant surfaces to enable their use in welding environments (the ferrous weld spatter would otherwise bond to the stainless steel surface).

Additionally, the seamless, one-piece housing has proven an excellent solution for high-pressure cylinder and valve position sensing applications. SAE and ISO high pressure port threading is an easy accommodation. 

Branching into the process control realm, intrinsically safe versions are also emerging.

Source: Marcel Ulrich is a manager at Pepperl+Fuchs (www.pepperl-fuchs.com) in Ohio.