Machine Design

Sensor Sense: Inductive sensors and resistance welding

Resistance welding is known to create electromagnetic interference at levels high enough to falsely trigger inductive proximity sensors.

Inductive proximity detectors used near welding applications require special coil and core combinations to prevent false triggering. Air-core coils wound on plastic spools reduce the coil's sensitivity to external magnetic fields. Low-permeability cores also reduce interference from outside magnetic forces while helping concentrate and direct the coil's own magnetic field.

An internal time-delay circuit between the level detector and the output keeps the output from responding to any signal shorter than the delay period. So the sensor output ignores any interference from short-duration spot-welding pulses.

Sensor manufacturers use three common methods to combat this problem: time-based filtering, lowpermeability core and coil systems, and air-coil systems.

Time-based filtering installs a time-delay circuit that ignores any target-present signal shorter than 75 msec. The delayed response time is perfectly acceptable in most applications. That's because the operations that most sensors monitor — such as clamp fully closed or part-in-position — typically get measured in seconds, not milliseconds.

Low-permeability core materials developed in recent years help conduct and shape the oscillator's high-frequency field while resisting the influences of external fields. One such material is carbonyl iron with a magnetic permeability one-tenth that of ferrite. Sensors using carbonyliron cores withstand external magnetic fields 10X stronger than a comparable ferrite system.

Air-core sensors eliminate magnetic core materials entirely. The coil wraps around a nonmagnetic spool usually made of plastic. Air-core sensors offer the highest immunity to both ac and dc magnetic fields because there is no core to concentrate the external magnetism.

On the surface, it appears air cores provide the ideal solution to all welding sensor applications. However, the lack of field guidance provided by a permeable core requires more intricate coil production. The extra cost of that precision along with increased calibration time make air-core models a higher-cost alternative compared to the other two methods.

Pepperl+Fuchs ( provided information for this article.

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