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Most manufacturers of magnetic sensors base their design on having very small air gaps of
.005" to .015". Increasing the size of this gap produces a fall-off of signal voltage. In practical
application, such small gaps could be hazardous, especially if the object being sensed is not concentric or tends to
have a loss of concentricity with increased speed or motion. MSI's Premium line of VR sensors employ a special
"focusing tip" which enables the sensor to operate at larger gaps in excess of .060". The closer the
sensor is to the object being sensed, the greater the amplitude. MSI sensors however, allow the sensor to be farther
from the target without rapid signal deterioration as in other sensors.
Motion Sensors also manufactures a variation of variable reluctance sensors known as inductive
magnetic sensors. These differ from standard VR sensors in that the magnet is not in the unit, but in the object
being sensed. They are used when the rotating device sensed cannot be made of ferrous materials. For instance, if the
target to be sensed is made of nonmagnetic stainless steel, brass, aluminum or plastic, a centerpole magnet may be
placed in the center of the rotating device. Alternatively, magnets can be imbedded (equally spaced) into the
target's outer rim or diameter. Inductive sensors will sense the magnetic fields and produce a signal. They are also
designed to provide this capability at air gap distances substantially greater than standard VR sensors, as would be
required for centerpole magnet locations.
In summary, variable reluctance sensors convert mechanical motion to electric energy without
direct contact when positioned near a turning rotor, gear, shaft or other regularly moving device. The output signal
can be fed into a digital counter, totalizer, analog converter, tachometer, overspeed or underspeed control,
oscilloscope, or other monitoring and control device. The sensor provides a simple, reliable, inexpensive transducer
for highly sophisticated control systems. |
