Machine Design

Sensor Sense: Inductive position-measuring sensors

Inductive position-measuring systems (IPMSs) operate on the same principle as an inductive proximity sensor. But instead of a single sensing coil, position measuring uses a multiple-coil system. The outputs from each coil go to a microprocessor that analyzes and compares the sensingcoil output and determines the position of a nonmagnetic ferrous target based on the relative signal strengths from each coil. The microprocessor then outputs the position of the target as either a distance-proportional current of 4-to-20 mA, or a voltage signal from 0-to-10 V. Note that the sensing coils may mount in either a linear or circular pattern, depending on the type of location sensing desired.

Many noncontact position- measurement systems use magnetic or inductivemagnetic principles of operation. Their target must be a permanent magnet, as the sensor detects the target’s magnetic field. The magnetized target can create some problems depending on the application.

For example, magnetized targets can attract metallic particles that build up between the target and the sensor. As that gap is typically 1 mm or less, the buildup of particles may easily exceed that spacing, producing drag and wear on the sensor as the particles are pulled across the surface.

In addition, permanent magnets lose their strength over time, especially when exposed to high temperatures. If the application involves high-temperature work, the target magnet may need constant replacement.

Though IPMS targets are made of ferrous materials, they are not magnetized. So one immediate advantage is that they don’t attract metallic chips and there’s no worries of high temperatures demagnetizing the material. In addition, the air gap between target and sensor can approach 6 mm when used with appropriately sized targets.

Typical resolution for this type of sensor is 0.125 mm in linear travel or 0.4° if used in a rotary sensor. Both versions provide nearly limitless cycles of operation.

Besides the analog output, the internal microprocessor can be programmed to give switched output setpoints. In other words, the output can turn full-on or full-off when the target reaches a specific setpoint. Setpoints and travel spans can be changed with no recalibration.

Pepperl+Fuchs supplied information for this column.

Edited by Robert Repas

© 2012 Penton Media, Inc.

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