Sensor Sense: Advances in inductive sensing - Factor 1 sensors

June 2, 2005
Inductive proximity sensors detect metal objects without touching them.

Factor 1 inductive sensors have at least two coils in their design. One technique connects the first coil to an oscillator circuit while the other coil is part of an impedance circuit that controls the amplitude of the oscillator output. An adjustable core tunes the impedance coil so that the resonant frequency of the impedance, Zo, exactly matches the oscillator resonant frequency, o. When any metal is brought near the coils, the oscillator output shifts in both amplitude and frequency. The impedance circuit further decreases output amplitude as the oscillator frequency is no longer resonant with it. The level detector trips when the amplitude of the oscillator, as controlled by the impedance circuit, dips below the intersection point of both ferrous and nonferrous metals.

Widely used today, their basic construction consists of a coil wrapped around a ferrite core, an oscillator, an amplitude-detection circuit, and a solid-state output. Basically, they still generate a high-frequency electromagnetic field that interacts with the target.

The biggest deficiency in these sensors lies in how they interact with different metal types. Each material reacts differently to the electromagnetic field and thus affects the range at which the sensor can detect that material. The sensor must get closer to a copper part than to one made of stainless steel. These variations call for adjusting the distance between the sensor and the target to maintain optimum sensing. The amount of this adjustment is commonly referred to as the material correction factor.

A new sensor technology eliminates correction factors. The technique is known widely as Factor 1 sensing. Factor 1 sensors detect all metals, both ferrous and nonferrous, at the same range without adjustment or correction. Usually this gives them a longer sensing range for certain materials over regular proximity sensors.

Instead of a single coil creating eddy currents in the target, Factor 1 sensors use independent oscillator and impedance coils. The combined effect of both ferrous and nonferrous metals on the two coils give Factor 1 sensors the same operating distance for all metals.

Not all Factor 1 sensors are created equal. Some operate using two coils while others use three or four. Some completely remove the ferrite core while others incorporate it into the design. Factor 1 sensors without ferrite cores are inherently immune to electromagnetic interference as often arises from electric welding, electric lifts, and electronic furnaces. The absence of the ferrite core also lets Factor 1 sensors operate at higher switching frequencies thus increasing their sensitivity to smaller objects.

Though currently not usable in intrinsically safe applications or as analog sensors, Factor 1 sensors come in a variety of housings offering chemical resistance and protection against sudden temperature swings. Specific sensors can withstand temperature ranges from 30 to 85°C. While most Factor 1 sensors mount either flush or nonflush, a few allow limited recessed mounting.

Turck Inc. ( provided information for this article.

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