Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-08-19
2001-01-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C356S370000
Reexamination Certificate
active
06177997
ABSTRACT:
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United States Government. The invention may be manufactured and used by or for the Government for governmental purposes without the payment of royalties thereon or therefor.
FIELD OF THE INVENTION
The present invention relates to magnetic bearings and more particularly to optical position sensing for magnetic bearings.
BACKGROUND OF THE INVENTION
A magnetic bearing, which includes a rotor and a stator concentrically located with respect to each other, typically controls the radial or axial distance between the rotating rotor and the stationary stator. More specifically, adjustable electromagnetic forces generated by current flowing through coils wrapped around the stator, as controlled by a control circuit adjusts distance between the stator and rotor. U.S. Pat. Nos. 4,387,935 and 4,082,376 describe details of the magnetic bearing.
To measure the position of the magnetic bearing rotor and to provide input signals to the control circuit to adjust the forces by adjusting the current through the coils, non-contacting sensors have been used including inductive sensor and capacitance type sensors.
The inductive sensor measures the eddy current generated by sensor coils. U.S. Pat. No. 4,082,376 describes the inductive sensors used in magnetic bearings. In capacitive sensors, a variable capacitor is used to sense gap distance and the resulting capacitance change is converted into an electrical signal indicative of the gap. U.S. Pat. No. 5,864,303 describes a capacitive sensor system for magnetic bearings. A drawback of both types of sensors, however, is that they are prone to electromagnetic interference (EMI) generated by the electro-magnetic coils of the magnetic bearing. Thus they cannot be placed too close to electro-magnetic coils of the magnetic bearing. Further, they tend to drift as operating temperature changes. Therefore, these types of sensors cannot guarantee long term stability due to their sensitivity to temperature and EMI.
Optical position sensors provide position information for magnetic bearings, overcoming the drawbacks mentioned above. In other words, the optical position sensor is insensitive to the EMI generated by the magnetic bearing and can be placed within or very near the electromagnet of the magnetic bearing. Further, the optical position sensor possesses characteristics of long term stability and high resolution.
U.S. Pat. No. 4,456,378 describes an optical sensor for sensing the radial position of a shaft when a portion of a light beam incident on the shaft sensor-target is blocked. The referenced patent proposes the light beam to have a relatively small diameter and be well collimated in order to pass sufficient energy to the detector and in order to provide a high sensitivity to shaft position. When using an optical position sensor, scattering effects must be taken into account as well as calibrating the intensity of the light beam. However, the referenced patent does not recognize design features needed for dealing with the effects of scattered light reflected by the sensor target and the interior of the sensor housing. It also includes a rather complex feedback system requiring shaft motion and sophisticated electronics for calibrating intensity of the light beam.
Thus, it is an object of this invention to provide optical position information for magnetic bearings.
It is a further object of this invention to provide an optical position sensor that possesses long term stability by compensating for degradation of a light beam source or temperature variation.
It is still another object of this invention to provide an optical position sensor that can perform well when scattered light is present either by generation from the light source or by reflection by the shaft sensor-target or the interior of the sensor housing.
SUMMARY
The proposed invention is an optical sensor that senses the movement of a shaft. To better appreciate the significance of this invention, a discussion regarding prior art optical position behavior is in order. Optical position sensing of a shaft is possible by arranging an LED and photodetector pair to be opposing one another and on the same axis. A highly non-linear behavior exists as the shaft sensor-target is displaced so that it obstructs the light energy reaching the photodetector. Initial displacement from the nominal shaft position will reduce the energy at a lesser rate because a significant amount of scattered light can find its way around the shaft sensor-target and reflect off the shaft sensor-target and interior surfaces of the sensor housing. Also, the energy pattern emitting from the LED is such that the energy drops off as viewed at increasing angles from the LED axis. The sensitivity of the photodetector has a similar behavior with highest sensitivity along the photodetector axis, and decreasing sensitivity for increasing detection angles with respect to the photodetector axis. As the shaft is displaced further, the photodetector output will drop off more rapidly, partly as a result of the LED/photodetector energy pattern/sensitivity and partly due to lesser energy reflected around the shaft sensor-target, thus resulting in highly non-linear behavior.
This invention uses an improved method for reducing the light energy reaching the photodetector so that the behavior between shaft displacement is such that linearity is much improved. The sensor housing is configured so the shaft sensor-target approaches a fixed surface or boss in the sensor housing. The boss, a means for permanently partially blocking the light beam, is disposed between the LED and photodetector. As viewed by the photodetector, the space formed by the shaft sensor-target and the boss appears to be a slit. The apparent slit will change area linearly with shaft displacement. It is possible to align the LED/photodetector axis such that it passes through the center of the apparent slit when the shaft is in the nominal position, utilizing the highest energy and most uniform part of the light emitted. Since the energy is essentially uniform, and the apparent slit changes area linearly with position, the system linearity is vastly improved over the prior art.
The optical sensor can be configured to measure radial or axial movement of a shaft. Detection of radial movement is made when a portion of light incident on the shaft sensor-target is blocked. For detection of axial movement, a disk is mounted on a shaft and its flat surface is used to block a portion of light. The variation in the amount of light allowed to pass through is a measure of the position of the shaft. To eliminate possible drift of system performance due to LED degradation or temperature variation, a feedback feature is added to the system.
Radial sensing is accomplished by detecting two axes of motion in a plane orthogonal to the shaft axis. In the preferred embodiment, LED/photodetector pairs(LPP) are oriented such that their axes are aligned, and they face one another, so that maximal light energy reaches the photodetectors. Looking along the shaft axis, the arrangement of LPP's are such that their axes are orthogonal and near tangent to the shaft or shaft sensor-target in four locations including above, below, and each side of the sensor-target cross-section. A sensor-target, which is mounted on the shaft, may be used as the optical target rather than the shaft itself. If the beams of light were visible, they would form a box around the circular cross-section of the sensor-target. A sensor housing is used as a holder of all the components so that the LPP's are held into an assembly. The sensor housing, which is opaque to light emitted by the LED's provides a number of bores to guide the light to the photodetectors. The bore is interrupted in the area of the shaft sensor-target so that the shaft sensor-target may partially obscure the light beam as a function of shaft position.
The bore is also partially interrupted by a fixed boss built into the sensor housing. A boss is a protuberance di
Blumenstock Kenneth A.
Hakun Claef F.
Johnson Clarence S.
Font Frank G.
Natividad Phil
The United States of America as represented by the Administrator
LandOfFree
Shaft position optical sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Shaft position optical sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Shaft position optical sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2496836