A crystal controlled oscillator drives a wheatstone bridge. The bridge is tuned to be near resonance so that slight changes in inductance in the sensor coil (caused by changes in the electromagnetic coupling between the sensor and the target) provide big shifts in the output of the bridge. Target movement relative to the sensor causes impedance change in the sensor coil. The impedance of the coil is detected (measured) by the RF control circuitry (synchronous demodulator circuit), then linearized by a logarithmic amplifier and amplified in the DC control stage (providing gain and offset amplification). This voltage is the system output voltage, provided to the user as an analog voltage directly proportional to target position relative to the sensor. This circuit design is utilized by both dual and single coil sensor designs. The dual coil has an active and an inactive reference coil for improved temperature stability when axial space constraints do not apply.


Kaman's KD-2306 and KDM-8206 Series products incorporate variable impedance transducer circuit designs.


Inductive bridge image




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