Patent Description:
Apparatus and associated methods relate to a system for determining rotation frequency of a rotatable member, as defined in claim <NUM>. The system includes a magnet, a Fabry-Perot cavity, a light source, a detector, and a speed calculator. The magnet generates a magnetic field proximate the rotatable member. Rotation of the rotatable member causes changes in the magnetic field generated by the magnet. The Fabry-Perot cavity is formed between a first mirror and a second mirror. The second mirror is bonded to a magneto-strictive material having a thickness dimension that changes in response to changes in the magnetic field caused by rotation of the rotatable member, thereby moving the second mirror relative to the first mirror. The light source is configured to project light into the Fabry-Perot cavity. The detector is configured to detect light reflected from the Fabry-Perot cavity. The speed calculator is configured to determine rotation frequency of the rotatable member based on a principal wavelength of light detected by the detector.

Some embodiments relate to a method for determining rotation frequency of a rotatable member, as defined in claim <NUM>. A magnetic field proximate the rotatable member is generated, via a magnet. Rotation of the rotatable member causes changes in the magnetic field generated by the magnet. The second mirror of the Fabry-Perot cavity is moved, via a magneto-strictive material, in response to the changes in the magnetic field caused by rotation of the rotatable member. Light is projected, via a light source, into Fabry-Perot cavity formed between a first mirror and the second mirror. A reflected portion of the light projected into the Fabry-Perot cavity is reflected, via the Fabry-Perot cavity. The light reflected from the Fabry-Perot cavity is detected, via a detector, Rotation frequency of the rotatable member is determined, via a speed calculator, based on a principal wavelength of light detected by the detector.

Apparatus and associated methods relate to optically determining rotation frequency of a rotatable member using a Fabry-Perot cavity formed between a first mirror and a second mirror. A cavity dimension between the first mirror and a second mirror changes in response to movement of the secomd mirror with respect to the first mirror. The second mirror is bonded to a magneto-strictive material having a thickness dimension that changes in response to changes in a magnetic field. A magnet generates the magnetic field, which changes in response to teeth of a toothed phonic wheel passing through the magnetic field. Rotation of the rotatable member, which is coupled to the toothed phonic wheel, causes the teeth to pass through the magnetic field.

<FIG> is a schematic diagram of a Fabry-Perot based speed sensor. In <FIG>, Fabry-Perot based speed sensor <NUM> is aligned with toothed phonic wheel <NUM>, which can be coupled to a rotatable member, such as, for example, a shaft of an airplane engine (not depicted). Fabry-Perot based speed sensor <NUM> includes magnet <NUM>, Fabry-Perot cavity <NUM>, light source <NUM>, and detector <NUM>. Magnet <NUM> is shown generating magnetic field <NUM> proximate toothed phonic wheel <NUM>. As teeth <NUM> of toothed phonic wheel <NUM> rotate through magnetic field <NUM>, teeth <NUM> cause magnetic field <NUM> to change as teeth <NUM> sweep through the magnetic flux lines of magnetic field. Such changes to magnetic field <NUM> are caused when a magnetic permeability of teeth <NUM> is different from the magnetic permeability of free space, for magnetic field <NUM> is preferentially directed into high-permeability materials. Thus, to increase the changes in the magnetic field caused by movement of teeth <NUM> past magnet <NUM>, phonic wheel (or at least teeth <NUM> of phonic wheel) <NUM> can be made of a high permeability material.

Fabry-Perot cavity <NUM> is situated or located between magnet <NUM> and toothed phonic wheel <NUM>, such that Fabry-Perot cavity is within a region of magnetic field <NUM> that changes in response to relative location of teeth <NUM> of toothed phonic wheel <NUM>. Such a location of Fabry-Perot cavity <NUM> is a location where changes to magnetic field <NUM>, which are caused by motion of teeth <NUM> past magnet <NUM>, are greatest. Fabry-Perot cavity <NUM> is formed between first mirror <NUM> and second mirror <NUM>. A cavity dimension of Fabry-Perot cavity changes in response to relative movement between first mirror <NUM> and second mirror <NUM>, as will be shown in more detail below. Second mirror <NUM> is bonded to magneto-strictive material <NUM> having a thickness dimension that changes in response to changes in magnetic field <NUM>. Magneto-strictive material <NUM> is also coupled to a fixed member on a side opposite to the side bonded to second mirror <NUM>, such that changes in the thickness dimension caused by changes in magnetic field <NUM> result in movement of second mirror <NUM>. The fixed member is fixed relative to first mirror <NUM>, thereby ensuring that changes to the thickness dimension of magneto-strictive material <NUM> result in relative movement between first mirror <NUM> and second mirror <NUM>.

The principal of operation of Fabry-Perot speed sensor <NUM> is that rotation of phonic wheel <NUM> proximate magnet <NUM> causes changes in magnetic field <NUM> therebetween, where Fabry-Perot cavity <NUM> is located. Changes in magnetic field <NUM> causes dimensional changes to magneto-strictive material <NUM>. The dimensional changes to magneto-strictive material <NUM> causes movement of second mirror <NUM>. Movement of second mirror <NUM> causes dimensional cavity changes of Fabry-Perot cavity <NUM>. Dimensional cavity changes of Fabry-Perot cavity <NUM> causes changes in metrics of light reflected thereby. Thus, the metrics of the light reflected by Fabry-Perot cavity <NUM> are indicative of rotational speed of phonic wheel <NUM>, and thereby indicative of speeds of any member axially connected thereto, such as, for example, a shaft of an aircraft engine.

<FIG> are schematic diagrams of a Fabry-Perot cavity in two different magnetic fields. In <FIG>, portions of Fabry-Perot based speed sensor <NUM>, which is depicted in <FIG>, are shown in more detail, and in two different magnetic field conditions. In <FIG>, Fabry-Perot cavity <NUM> has a relatively-large cavity dimension 32A, which results from a first magnetic field condition. Such a relatively-large cavity dimension 32A of Fabry-Perot cavity <NUM> results from magneto-strictive material <NUM> having a relatively-small thickness dimension 34A. Magneto-strictive material <NUM> is bonded to a fixed structure at first side <NUM> and bonded to second mirror <NUM> at second side <NUM>. The fixed structure to which magneto-strictive material <NUM> is bonded is fixed relative to first mirror <NUM>, such that cavity dimension 32A can change in response to changes in thickness dimension 34A. Cavity dimension 32A determines various wavelengths at which constructive interference of light can occur therewithin, as well as wavelengths at which destructive interference of light can occur therewithin. At such wavelengths corresponding to such constructive interference, light is well reflected from Fabry-Perot cavity <NUM>. At wavelengths corresponding to destructive interference, light is poorly reflected from Fabry-Perot cavity <NUM>. Thus, the spectral response of the light reflected by Fabry-Perot cavity <NUM> is indicative of cavity dimension 32A. Both thickness dimension 34A and cavity dimension 32A are measured in a direction orthogonal to both first mirror <NUM> and second mirror <NUM>, which are parallel to one another.

In <FIG>, Fabry-Perot cavity <NUM> has a relatively small cavity dimension 32B, which results from a second magnetic field condition. Such a relatively small cavity dimension 32B of Fabry-Perot cavity <NUM> results from magnetostrictive material <NUM> having a relatively large thickness dimension 34B. Because the fixed structure to which magneto-strictive material <NUM> is bonded is fixed relative to first mirror <NUM>, such that cavity dimension 32B can change in response to changes in thickness dimension 34B. Cavity dimension 32B determines a wavelength different from the one determined by cavity dimension 32A at which constructive interference of light can occur therewithin. At such a wavelength corresponding to such constructive interference, light is again preferentially reflected from Fabry-Perot cavity <NUM>. Thus, the wavelength that such preferential reflection occurs is indicative of cavity dimension 32B. Both thickness dimension 34B and cavity dimension 32B are again measured in a direction orthogonal to both first mirror <NUM> and second mirror <NUM>, which are parallel to one another.

In the depicted embodiment, optical fiber <NUM> both transmits the light generated by light source <NUM> (depicted in <FIG>) from light source <NUM> to Fabry-Perot cavity <NUM> and transmits the light reflected from Fabry-Perot cavity <NUM> to detector <NUM> (depicted in <FIG>). First mirror <NUM> is partially reflective so as to enable light to transmit therethrough. Incident light is transmitted by optical fiber <NUM> through first mirror <NUM> into Fabry-Perot cavity <NUM>. Then light reflected by Fabry-Perot cavity <NUM> is transmitted from within Fabry-Perot cavity through first mirror <NUM> and into optical fiber <NUM>. The reflectivity of first mirror <NUM> and second mirror <NUM> determine the finesse of Fabry-Perot cavity <NUM>. The finesse of the Fabry-Perot cavity <NUM> affects fine structure of the spectral response of light reflected from Fabry-Perot cavity <NUM>.

<FIG> is graph depicting the spectral response of light reflected from the Fabry-Perot cavity for two different cavity dimensions. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, and amplitude/wavelength relations 48A and 48B. Horizontal axis <NUM> is indicative of wavelength of light reflected from Fabry-Perot cavity <NUM> and detected by detector <NUM>. Vertical axis <NUM> is indicative of amplitude of light reflected from Fabry-Perot cavity <NUM> and detected by detector <NUM>. Spectral responses 48A and 48B correspond to optical light detected for the two magnetic field conditions that resulted in the cavity dimensions 32A and 32B depicted in <FIG>, respectively. Spectral response 48A corresponds to the reflected signal when Fabry-Perot cavity <NUM> has a relatively large cavity dimension 32A (e.g., a dimensional change that is only half a wavelength can be considered relatively large), as depicted in <FIG>. At such a relatively large cavity dimension 32A, the spectral response indicates constructive interference occurring at specific wavelengths, such as at peak-amplitude wavelengths λ<NUM>, λ<NUM>, and λ<NUM>. A wavelength difference ΔλA between adjacent peaks in spectral response 48A is indicative of cavity dimension 32A. Spectral response 48B corresponds to the detected optical spectral response when Fabry-Perot cavity <NUM> has a relatively small cavity dimension 32B, as depicted in <FIG>. At such a relatively small cavity dimension 32B, the spectral response indicates constructive interference occurring at other specific wavelengths, such as at peak-amplitude wavelengths λ<NUM>, λ<NUM>, and λ<NUM>. A wavelength difference ΔλAB between adjacent peaks in spectral response 48B is indicative of cavity dimension 32B.

In some embodiments, light source <NUM> can be a laser that projects light of a single wavelength into Fabry-Perot cavity <NUM>. Detector <NUM> will then monitor the amplitude of the reflected light of that same wavelength to determine movement of second mirror <NUM>. In such an embodiment, the spectral response is reduced to a single wavelength response, permitting a photodetector, for example, to function as detector <NUM>. In other embodiments, the light source projects a broader spectrum of light, within a band of wavelengths into Fabry-Perot cavity <NUM>. In such embodiments, the detector can determine movement of second mirror <NUM> by the detection of reflected light of more than a single wavelength. In such embodiments detector <NUM> can be a spectrum analyzer, for example.

Magnetic field <NUM>, as depicted in <FIG>, will change in a periodic fashion as teeth <NUM> of toothed phonic wheel <NUM> rotates, thereby producing a periodic shift in peak-amplitude wavelengths, such as between peak-amplitude wavelengths λ<NUM> and λ<NUM>. Such periodicity of a detected peak-amplitude wavelength is indicative of the rotation frequency of toothed phonic wheel <NUM>.

<FIG> is a block diagram of an embodiment of a Fabry-Perot based speed sensor. In <FIG>, Fabry-Perot based speed sensor <NUM> is depicted in block diagram format and includes optical coupler <NUM> which permits a single optical fiber embodiment of Fabry-Perot based speed sensor <NUM>. Optical coupler <NUM> is configured to perform two functions. First, optical coupler <NUM> directs the light generated by light source <NUM> to optical fiber <NUM>, which in turn transmits the light generated to Fabry-Perot cavity <NUM>. Second, optical coupler <NUM> directs, to spectrometer <NUM>, light reflected from Fabry-Perot cavity <NUM> as transmitted to detector <NUM> by optical fiber <NUM>. In the depicted embodiment, detector <NUM> includes spectrometer <NUM> and signal processor <NUM>. Spectrometer <NUM> detects spectral responses of such light reflected by Fabry-Perot cavity <NUM>, as indicated in FIGS. Speed calculator <NUM> can then determine peak-amplitude wavelengths from such spectral responses and determine a rotation frequency indicated by time changes of such peak-amplitude wavelengths (e.g., periodicity of such peak-amplitude wavelengths).

Various embodiments can use more of fewer components are depicted in the embodiments described above with reference to <FIG>. For example, the speed calculator shown in <FIG> can include a any of various kinds of processors as are known in the art, such as, for example, a signal processor, a microprocessor, a programmable logic array, etc. Similarly the light source and/or detector can include any of the various light sources and/or detectors as are known in the art, such as for example a laser diode, an array of laser diodes, a gas laser, a Light Emitting Diode (LED), a Super-luminescent Light Emitting Diode (SLED), etc..

Claim 1:
A system for determining rotation frequency of a rotatable member, the system comprising:
a rotatable member (<NUM>);
a magnet (<NUM>) for generating a magnetic field proximate the rotatable member, wherein the rotatable member is arranged such that rotation of the rotatable member causes changes in the magnetic field generated by the magnet, wherein the rotatable member includes a toothed phonic wheel (<NUM>) aligned proximate the magnet such that changes in position of teeth of the toothed phonic wheel relative to the magnet causes the changes in the magnetic field;
a Fabry-Perot cavity (<NUM>) formed between a first mirror (<NUM>) and a second mirror (<NUM>), the second mirror is bonded to a magneto-strictive material (<NUM>) having a thickness dimension that changes in response to changes in the magnetic field caused by rotation of the rotatable member, thereby moving the second mirror relative to the first mirror, wherein the Fabry-Perot cavity is located between the magnet and the toothed phonic wheel, and wherein the Fabry-Perot cavity is aligned such that the cavity dimension (32A) is transverse to a line segment of shortest dimension directed between the magnet and the toothed phonic wheel;
a light source (<NUM>) configured to project light into the Fabry-Perot cavity;
a detector (<NUM>) configured to detect light reflected from the Fabry-Perot cavity; and
a speed calculator (<NUM>) configured to determine rotation frequency of the rotatable member based on a principal wavelength of light detected by the detector.