In making displacement measurements on rotating targets, the speed of the target surface moving past the face of the sensor can be a source of error. A point is reached, based on the oscillator frequency of the system, beyond which the target surface appears to move away from the sensor. This phenomenon is referred to as a "disappearing target".


Delving into the physics behind why this occurs is beyond the scope of this tech note. An accepted explanation of the effect: consider the fact that the eddy current induced in the surface of the target is circular and concentric to the coil housed in the sensor. As the surface velocity increases, the electromagnetic field produced by the eddy current shifts in the direction of the target rotation. The resulting eccentricity of the sensor coil field and eddy current field results in an output change similar to what happens when the target moves farther away from the sensor. This effect increases as the surface velocity increases.


There are two major influences on the surface velocity effect that can be specified, the oscillator frequency, and the diameter of the sensor coil. To keep surface velocity effects to insignificant levels, oscillator frequency, sensor diameter and surface velocity need to be such that there is at least 1 oscillator cycle in the time it takes the target surface to move 2% of the sensor diameter. The oscillator frequency used with Kaman Instrumentation's standard sensors is typically 500 KHz or 1 MHz.


Calculating Surface Velocity
Calculating the surface velocity in inches per second is shown below. For cylindrical targets, use the outside diameter of the cylinder. For rotating discs the diameter is that of the track at the center of the sensor.


surface velocity 1 image


Sensor Selection
To determine the minimum acceptable sensor diameter calculate the following:


surface velocity 2 image


An Example
Determine the minimum sensor diameter for a 6" diameter disc is spinning at 6000 rpm with the sensor located at a 5" diameter. Assume we are considering Kaman's KDM-8206 Series product typically operating at 500 kHz.

First calculate the surface velocity:


surface velocity 3 image

Then determine the minimum sensor diameter:


surface velocity 4 image


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