Inductive proximity sensors are designed for wear-free and non-contact detection of metal objects. One of the first sensing technologies developed, these sensors are still widely used today and incorporate a coil wrapped around a ferrite core (see Figure 1), an oscillator, detector circuit and a solid-state output. They generate a high frequency electro-magnetic AC field that interacts with the target.
Figure 1
Ferrous and nonferrous metals affect proximity sensors differently, and are sensed at different ranges depending upon the metal being detected. To sense different metals the sensing range must be adjusted to accommodate what is commonly referred to as a 'correction factor' (see Table 1). Sensor technology and design has evolved to a point where correction factors do not need to be applied: this is widely referred to as 'factor 1' sensing. These factor 1 sensors use multiple coils that detect all metals at the same range without adjustment, giving them a longer sensing range than regular proximity sensors. Instead of a single coil inducing and being affected by eddy currents on a target as in standard proximity sensing, factor 1 sensors use separate, independent sender and receiver coils. Because of this, ferrous and nonferrous metals have the same effect on factor 1 sensors and are rated for the same operating distance.
Not all factor 1 sensors are created equal: some operate using two coils, some three or four; some completely remove the ferrite core (see Figure 2), while others incorporate it into the design. Factor 1 sensors without ferrite cores are inherently immune to magnetic field interference that often occurs during electric welding operations, lifts and electric furnaces. The absence of the ferrite core also allows factor 1 sensors to operate at a higher switching frequency.
Figure 2
Factor 1 sensors have also been developed in multiple housing types to allow for chemical resistance and sudden temperature variations, such as those in food and beverage applications. Some of these sensors can even withstand temperatures ranging from -30°C to 85°C. Factor 1 sensors can be flush or non-flush mountable, although some manufacturers' factor 1 sensors allow only limited recessed mounting.
Figure 3
Though currently not capable of being used in intrinsically safe or analog sensors, factor 1 technology sensors can offer numerous solutions, with extended range, ferrous and nonferrous applications and stocking issues.
Typical correction factors apply to standard inductive sensors when a nonferrous target is being detected.
Figure 4
The correction factors are nominal values. Deviation may be due to variations in oscillator frequency, alloy composition, purity and target geometry.
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