Fabry-perot based speed sensor
Apparatus and associated methods relate to optically determining rotation frequency of a rotatable member using a Fabry-Perot cavity formed between a mirror and a movable reflective mirror. A cavity dimension between the mirror and a movable reflective mirror changes in response to movement of the movable reflective mirror. The movable reflective 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 rotation of the rotatable member.
1 . A system for determining rotation frequency of a rotatable member, the system comprising:
a single magnet for generating a magnetic field proximate the rotatable member, rotation of the rotatable member causing changes in the magnetic field generated by the single magnet;
a Fabry-Perot cavity formed between a first mirror and a second mirror, which are parallel to one another, the Fabry-Perot cavity is located directly between the single magnet and the toothed phonic wheel, the second mirror 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;
a light source configured to project light into the Fabry-Perot cavity;
a detector configured to detect light reflected from the Fabry-Perot cavity; and
a speed calculator configured to determine rotation frequency of the rotatable member based on a principal wavelength of light detected by the detector.
2 . The system of claim 1 , wherein the rotatable member includes:
a toothed phonic wheel aligned proximate the single magnet such that changes in position of teeth of the toothed phonic wheel relative to the single magnet causes the changes in the magnetic field.
3 . The system of claim 2 , wherein the Fabry-Perot cavity is aligned such that a cavity dimension is transverse to a line segment of shortest dimension directed between the single magnet and the toothed phonic wheel.
4 . The system of claim 1 , further comprising:
an optical fiber configured to direct the light projected by the light source into the Fabry-Perot cavity.
5 . The system of claim 4 , wherein the optical fiber is further configured to direct the light reflected from the Fabry-Perot cavity to the detector.
6 . The system of claim 1 , wherein the detector can be either a photo detector or a spectrum analyzer.
7 . The system of claim 1 , wherein the Fabry-Perot cavity has a cavity dimension as measured between the first mirror and the second mirror, a metric of the light detected is determined by the cavity dimension.
8 . The system of claim 7 , wherein the cavity dimension changes in response to changes in the thickness dimension of the magneto-strictive material.
9 . The system of claim 1 , wherein the first mirror has a coefficient of reflectively between than 0.4 and 0.6.
10 . The system of claim 1 , wherein the single magnet is a permanent magnet.
11 . A method for determining rotation frequency of a rotatable member, the method comprising:
generating, via a single magnet, a magnetic field proximate the rotatable member, rotation of the rotatable member causing changes in the magnetic field generated by the single magnet;
moving, via a magneto-strictive material, a second mirror of a Fabry-Perot cavity in response to the changes in the magnetic field caused by rotation of the rotatable member;
projecting, via a light source, light into Fabry-Perot cavity formed between a first mirror and the second mirror, which are parallel to one another and located between the single magnet and the toothed phonic wheel;
reflecting, via the Fabry-Perot cavity, a reflected portion of the light projected into the Fabry-Perot cavity;
detecting, via a detector, the light reflected from the Fabry-Perot cavity; and
determining, via a speed calculator, rotation frequency of the rotatable member based on a principal wavelength of light detected by the detector.
12 . The method of claim 11 , wherein the rotatable member includes:
a toothed phonic wheel aligned proximate the single magnet such that changes in position of teeth of the toothed phonic wheel relative to the single magnet causes the changes in the magnetic field.
13 . The method of claim 11 , wherein the Fabry-Perot cavity is aligned such that a cavity dimension is transverse to a line segment of shortest dimension directed between the single magnet and the toothed phonic wheel.
14 . The method of claim 11 , further comprising:
directing, via an optical fiber, the light projected by the light source into the Fabry-Perot cavity.
15 . The method of claim 14 , further comprising:
directing, via the optical fiber, the light reflected from the Fabry-Perot cavity to the detector.
16 . The method of claim 14 , wherein the optical fiber is a first optical fiber, the method further comprising:
directing, via a second optical fiber, the light reflected from the Fabry-Perot cavity to the detector.
17 . The method of claim 11 , wherein the Fabry-Perot cavity has a cavity dimension as measured between the first mirror and the second mirror, a metric of the light detected is determined by the cavity dimension.
18 . The method of claim 17 , wherein the cavity dimension changes in response to changes in the thickness dimension of the magneto-strictive material.