Abrasion tests on plastic and metal-faced sensors show how well they withstand metal-to-metal contact. The metal-face sensor shows wear, but still functions. The plastic-face sensor is totally destroyed. The test involved 20 hr of abrasion with a wire brush, visible at the top of the image.

Abrasion tests on plastic and metal-faced sensors show how well they withstand metal-to-metal contact. The metal-face sensor shows wear, but still functions. The plastic-face sensor is totally destroyed. The test involved 20 hr of abrasion with a wire brush, visible at the top of the image.


Traditional plastic-face and metal-barreled inductive proximity In those situations, sensors last longer if they have a metal face.

Almost a novelty just a few years ago, metal-faced products are now considered mainstream in the proximity-sensor world. They've found homes in industries such as metalforming, where metal-on-plastic abrasion quickly sends traditional sensors to the scrap. Metal-face products today come in various industrystandard housings with ac or dc outputs at a cost competitive with traditional models.

Just how much longer do metal-face proximity sensors last compared to plastic-face sensors? Abrasion testing at Pepperl+Fuchs determined that a stainless-steel sensing face that was 0.4-mm thick prolonged sensor life over 20 that of an equivalent plastic-face model.

It's wise to note that not all metal-faced sensors are created equal. The metal covering must be thick enough to protect against wear. On some sensors the metal is hardly thicker than a foil. Obviously, the protection there isn't much better than the regular plastic face. Pepperl+Fuchs recommends that a stainless-steel metal cover be at least 0.4-mm thick. That thickness of metal requires special oscillator-and-coil circuit designs to "see through" the metal cover to detect the target metal beyond.

Another problem: The addition of metal faces to inductive proximity sensors narrows the gap over which the sensors can detect their targets. Typical sensing lengths for metalface sensors range from 2 to 8 mm, the same distance for embeddable-shielded sensors. However, it's only half the usual 4 to 15-mm distance for standard plastic-face sensors.

Pepperl+Fuchs (am.pepperlfuchs.com) provided information for this column.