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01 May 2003

Proximity sensing gets closer

Customer needs drive changes.

By Sara Rossio

Typical construction of an inductive proximity sensor includes a sensor coil, oscillator, detector, and output.

Stagnant is one word to describe inductive proximity sensor technology, and it's been that way for decades. The technology features a coil, which forms part of an oscillator circuit (see figure above). This coil produces a magnetic field that generates from the sensor face, allowing it to sense the metallic target. The length of the coil discriminates the length of the magnetic field, along with changes to the differential transformer.

In the past year, though, engineers have added onboard electronics to see further and solve additional customer needs. Research indicates sensing range is the No. 1 improvement needed for proximity sensors. Yet users still want improvements in custom or programmable speed and communication platforms. Take a look at what top-notch proximity sensors can offer users, especially in less-than-desirable situations.

PROXIMITY SENSOR PERKS

Background metal rejection is an exclusive capability to ignore side-ground or background objects. It allows you to mount the sensor in locations where machinery, fixtures, or metal other than the target may be within the sensing field. An effective proximity sensor should be able to ignore these objects while still sensing the target.

Band sensing is the ability to sense within a specified band while ignoring objects that are too close or too far. Band sensing is important whenever spacing is critical. A user can elect to use a portion of a sensor field to achieve much higher accuracy. Band sensing is feasible with a proximity sensor designed to store information on field conditions when a target is at the correct near and far operate points.

Position sensing is the ability to teach the sensor the position of an object in multiple axes. Some traditional inductive proximity sensors can tell only whether a metal object has entered their field. They simply switch when the field is interrupted but can't differentiate positions in their field. Because the field has width and length, accuracy is limited.

For many applications, that's perfectly fine. In tight tolerance applications, you'd want better accuracy. Ultimately, you'd want a proximity sensor to determine an object's position within the field to be much more specific as to the location of a parta clamping mechanism's position, inspecting subassemblies to ensure parts are in the correct position before fastening them together, or inspecting holes to see whether they are in the correct location. You'd also want to be able to tell whether a fastener is on a part and in the correct location.

Selectable range and hysteresis permits the sensor to adapt to tight tolerance or high vibration (wide tolerance) applications. Programming is very similar to band sensing programming; however, the user sets a single operate point and release point. Once the sensor operates, it will remain in that state until the target goes past the release point. IT

Behind the byline

Sara Rossio is a marketing manager at Cutler-Hammer in Everett, Wash.