Eddy-current position sensors, which measure shaft displacement in rotating machinery, have been around for many years as they offer manufactur- ers high-linearity, high-speed measurements, andhigh resolution. Most eddy-current sensors on the market only measure the distance between the sensor andthe shaft—leaving room for other shaft displacements togo unnoticed. Alexei Filatov, Principal Research Engineerfor Calnetix Technologies invented a novel eddy-currentposition sensor that measures multiple displacements with a single sensor. New
Equipment Digest spoke with Alexeiand Larry Hawkins, Co-Founderand Director of Technology forMagnetic Bearings at CalnetixTechnologies to find out howthis sensor works and how itwill benefit manufacturers.
NED: What is the Eddy-Cur-
rent Position Sensor and what
does it do?
AF/LH: The Eddy-Current PositionSensor technology allows non-contactdetection of an object’s position in thespace. They are often used to measure the staticand dynamic shaft displacement in rotating machinery.
At least some part of the object (shaft) must be electrically conductive and is typically referred to as the “sensor target.” In generic terms, the operational principle ofthe sensor always includes exposing the sensor target tohigh-frequency electromagnetic fields. These fields induceeddy currents in the target, which produce their own electromagnetic field, altering the original field. The amountof the field alteration depends on the distance between thetarget and the source of the original field and is used forthe object position detection. In most designs, changes inthe inductance of the electrical coil generating the originalfield are used as a measure of the distance between theobject and the coil.
NED: Calnetix just received a patent for this technology.
Can you explain what the new technology is and why it
AF/LH: Calnetix has developed a novel eddy-current posi-
tion sensor with unique properties, which include:
• “Slide-by” position measurements–the sensor can detect
not only a normal distance between the sensor and an object
as other eddy-current position sensors do but also displace-
ments of an object when it moves tangentially to the sensor
• No coaxial cables required as in other eddy-current sensors–it uses twisted pairs, which are much easier to work withand bring through machine interfaces.
The primary motivation for this technology developmentwas the need for a better performing, more highly reliableand lower-cost alternative to the other types of non-contactposition sensors available today.
NED: Is this sensor a part of a larger product offered from
Calnetix or is it a standalone product?
AF/LH: The primary application of the new position sensoris a part of Calnetix Magnetic Bearing Systems, but it can alsobe used as a standalone product. Calnetix Magnetic Bearing Systems provide non-contact suspension of high-speedrotors in rotating machinery, including gas compressorsand expanders, motors, generators, flywheel energy storagesystems, etc. The sensor provides the feedback control input for the position control loop of the magneticbearing, allowing it to control the position of arotating shaft with a magnetic field.
NED: How is this technology different
from similar position sensors already
on the market? What specific advan-
tages does it provide to users?
AF/LH: The sensor delivers a combi-
nation of the following advantageous
properties not found collectively in
• The sensor target consists of solid steel
such that the primary shaft surface can be the
target with no added sleeves or laminations. In
contrast, other common non-contact electromagnetic
position sensors, known as “reluctance” or “inductive”, have
the target composed of electrically isolated steel laminations.
Such laminated targets are much more difficult to make, andthe sensor accuracy can be affected by the target machining—smearing and shorting between the laminations duringmachining leads to significant sensor errors.
• Position sensing along two radial and one axial axes inrotational systems can be implemented in a single assemblywith a minimal number of external wires, as often needed inMagnetic Bearing Systems.
• Excellent linearity over wide displacement range.
• High operating temperature potential (>225 °C).
• Much lower operating frequency than in other eddy-current sensors (200kHz instead of >500kHz, typically MHz)results in much simpler electronics.
• Higher operating frequency than in reluctance sensors(200kHz instead of tens of kHz) results in higher bandwidthand better rejection of electromagnetic interference withother system components switching at tens of kHz (such asswitching amplifiers and motor drives).
• Fewer components inside the sensor (no laminations,no wire-wound coils, no lamination-to winding isolation, nomagnet wire to lead wire splices).
• Well suited for high-volume production.
NED: What was the biggest challenge in developing the new
AF/LH: Achieving the necessary combination of a large gain,high linearity and large measurement range in the “slide-by”mode was the biggest challenge. This has not been achievedin any other eddy-current position sensor.
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