An example is the sensing of a rotary cam position. The oblong shape of the cam dictates use of a sensor able to detect different shapes and sizes. A simple prox sensor merely detects the presence or absence of a target — an obvious problem where the cam is always present. Inductive linear analog sensors offer the best alternative for this and similar applications.
Inductive linear analog sensors work on the same principle as inductive proximity sensors, but produce an analog output. Inductive sensors contain an oscillator that creates a high-frequency electromagnetic field that radiates from a coil wrapped around a ferrite core in the front of the sensor. Eddy currents induced on the surface of a target entering the high-frequency field change the oscillator output amplitude.
The difference between inductive prox sensors and inductive analog sensors is in the output. Rather than triggering an on-or-off signal as the target moves in or out of sensing range, the target remains within range and the sensor measures the position of the target within that range. The sensor generates an analog voltage or current proportional to the distance between the target and the sensor face in the axial plane.
The analog output makes inductive analog sensors good for applications that require sensing items that are odd shapes or in close proximity. They also work for measurements that need high repeatability. Analog sensors can provide two to four times the repeatability in sensing distance. Standard repeatability error of prox sensors is typically less than or equal to 2% of the rated operating distance. Inductive analog sensors have less than 1% repeatability error and, in some cases, less than 0.5% is possible.
Analog sensors generally can be found in such form factors as barrel, rectangular, ring style, and so forth, as well as in sizes from 4 to 80 mm. The most appropriate sensor type and shape depends on the target.
Typical applications that use analog sensing include those where four analog sensors mount on four legs to ensure a target is properly centered, or where two sensors sit on either side of an object to determine its thickness. Analog sensors also can detect the tension of an arm as it swings back and forth, as well as detect bearing wear and mechanical shaft run-out, position feeding over an angled target, position of odd-shaped targets including cones, and small-part identification.
Turck Inc. (turck.com) provided information for this column.