Fiberoptic velocity transducer

Abstract: A fiberoptic velocity transducer (10) for use in determining the rotational speed of an object and comprising a magneto-optic material (12) coated with a dielectric material (22) that operates to selectively pass light energy (26) having a first range of wavelengths, and to reflect light energy (28) having a second range of wavelengths, so as to make the light energy with the second range of wavelengths available as a reference signal, the magneto-optic material (12) functioning to cause a Faraday rotation in the light energy having the first range of wavelengths that is characteristic of the speed of the rotating object.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to transducers for use in determining the velocity of an object and, more particularly, to a fiberoptic velocity transducer utilized in the measurement of rotational speed of an aircraft braking system component. DESCRIPTION OF THE RELATED ART Since the incorporation of rotating members into machinery, there has been a need to observe and control the velocity of the member in order to meet system efficiency and responding requirements. Such requirements have become increasingly stringent for high performance vehicular systems, particularly those connected with braking system of aircraft and other high speed vehicles. For such systems, it is critical to know the rate at which the rotational speed of the wheels is decreasing in response to application of the braking system. One means utilized in the past to determine the rotational speed of an object is a form of electromechanic tachometer. The electromechanic tachometers typically functioned to generate signals proportional to the speed of the rotating members but were limited by, among other things, electronic noise existing in the environment in which they were used. While some improvements to the early electromechanic tachometers were helpful in reducing the signal noise and improving the resolution of the tachometer, the electromechanic instruments still left much to be desired as a primary instrument for measuring velocity of a braking system component such as the brake disk or wheel. Later developments in rotational speed sensors included the use of light pulse signals, where the number of pulses observed by an electronic control unit correlated to the speed of the disk. Due to their EMI insensitivity, light weight and high resolution systems utilizing light to determine rotational speed were particularly desirable. These systems, however, had a nonhermetically sealed optical cavity which can be detrimental in applications (such as aircraft wheelspeed sensing) that are required to go through significant pressure changes in a dirty environment. Further improvements included velocity transducers utilizing magneto-optic material configured to cooperate with fiberoptic light energy which were incorporated into systems for determining rotational speed. Such systems not only embodied improved means for transmitting light pulses to electronic controllers, but were able to take advantage of Faraday rotation properties of the magneto-optic material to hermetically seal the optical cavity. Generally speaking, when subjected to a magnetic field, a particular magnetic flux density is created in the magneto-optic material which functions to cause polarized light passing through the material to undergo a Faraday rotation. Coupling then occurs between the electric field vectors of photons comprising the polarized light energy passing through the magneto-optic material and electric field vectors of the magnetically aligned atoms of the magneto-optic material. When such coupling occurs, the polarization vectors of the light energy are rotated, with the amount of Faraday rotation being determined by the magnetic flux density of the magneto-optic material. By configuring a rotating member with means to affect the magnetic flux density of the magneto-optic material to varying degrees based upon its speed and by noting the relationship between the resulting amount of rotation of the polarization vectors and the speed of the member, it was possible to determine the speed of the rotating member by monitoring the resulting polarization vectors. In this configuration, the light is modulated by the varying magnetic flux, and since an encoder disc is not necessary, the optical cavity that contains elements through which the light propagates can be hermetically sealed. In conventional applications of the velocity transducers employing magneto-optic material, the magneto-optic material is used to facilitate the development of light energy signals used to determine the speed of a rotating member. The magneto-optic material was carefully chosen so that it caused only a narrow bandwidth of wavelengths of polarized light to rotate. Accordingly, a single light source with a broad spectrum of wavelengths was conventionally propagated through the magneto-optic material, the magneto-optic material functioning to rotate the light energy having a particular range of wavelengths within the bandwidth and leave unaffected the light energy with wavelengths outside the bandwidth. In this way, the unaffected light energy was used as a reference in the calculation of rotational speed, or as a reference for an integrity check, and the rotated light energy as an indication of rotational speed. In the alternative, two light sources were simultaneously utilized and propagated through the magneto-optic material, one having a wavelength outside the bandwidth, and thus utilized as a reference, and the other being rotated by the magneto-optic material and used as an indicator of rotational speed. In either case, the conventional velocity transducer typically comprised a mirror operating to reflect the light waves, irrespective of their wavelengths, back through the magneto-optic material. These conventional velocity transducers incorporating magneto-optic material were limited, however, since they required relatively expensive parts and materials. The magneto-optic material was expensive since, in order to facilitate the development of a light energy reference, it was required to effect only a narrow bandwidth of wavelengths of light. In addition, the light source light detectors and optical fibers were expensive since the light wavelength which is not affected by the magneto-optic material is normally a longer wavelength than the ones used for standard communication fiber-optic components. EP-A-0,568,992 shows a magneto-optic sensor head for measuring the speed of rotation of an object, having a light source, a light inputting/outputting path, a polarizer, a Faraday rotator and a reflecting film. The light source in D1 operates over a single narrow range of wavelengths and the reference light is separated from the signal light, used to measure object rotation speed, such that the reference light only travels along a small part of the optical path travelled by the signal light. WO 89/06364 shows a speed sensor for sensing the speed of rotation of a rotating object having two light sources, a light inputting/outputting path, a polarizer, a Faraday rotator and a reflective surface. The light from both light sources passes through the Faraday rotator. To circumvent or overcome the problems and limitations associated with prior art velocity transducers, a velocity transducer comprised of relatively inexpensive parts and materials and that generates signals which accurately correspond to the rotational speed of a member is highly desirable. According to a first aspect of the present invention, there is provided a fiberoptic velocity transducer for determining the rotation speed of an object, comprising: means for propagating a plurality of light energy wavelengths; means for polarizing light energy; means for selectively passing a first pre-determined range of wavelengths of light energy and for selectively reflecting a second pre-determined range of wavelengths of light energy; means for causing a Faraday rotation in light energy; and means for reflecting light energy passing through said Faraday rotation mans; characterized in that said selectively passing and reflecting means is a dielectric coating affixed to said Faraday rotation means. According to a second aspect of the present invention, there is provided a method of determining the rotational speed of an object, comprising propagating a plurality of light energy wavelengths in a fiberoptic velocity transducer; polarizing the plurality of light energy wavelength; selecting passing a first predetermining range of wavelengths of light energy and selectively reflecting a second predetermined range of wavelengths of light energy; providing Faraday rotation means to cause Faraday rotation in light energy, and reflecting light energy passing through the Faraday rotation means; characterized by: affixing a dielectric coating to the Faraday rotation means such that the first predetermined range of wavelengths is passed and the second predetermined range of wavelengths is reflected thereby. Embodiments of the invention provide a fiberoptic velocity transducer functioning to transmit signals characteristic of the velocity of a rotating object. To accomplish the transmission of such signals, preferred embodiments incorporate novel structure cooperating with magneto-optic material possessing optical Faraday rotation properties. Embodiments of the invention may be applied to a variety of problems requiring the accurate measurement of rotational velocity, but has been found to be particularly beneficial to high performance braking systems such as those on aircraft or other high velocity vehicles. A fiberoptic velocity transducer embodying the present invention operates to condition a plurality of light energy sources so that they may be utilized in determination of the speed of a rotating object. More specifically, preferred embodiments include means for selectively passing a first range of wavelengths of light energy through a magneto-optic material, thereby causing a Faraday rotation in the passed light energy that is characteristic of the speed of an object being monitored, and for selectively reflecting a second range of wavelength of light energy so as to make the reflected light energy available as a reference. In one presently preferred embodiment of the present invention, the fiberoptic velocity transducer embodies a 2X1 coupler cooperating with an input/output fiber, each of which operates to emit as well as receive light energy. In addition, the fiberoptic velocity transducer comprises a lens, a polarizer and a mirror as well as a magneto-optic material having a dielectric coating affixed to a proximal side, the dielectric coating functioning to selectively pass certain wavelengths of light. In operation, light energy having a first wavelength passes through the dielectric coating of the magneto-optic material, whereas the dielectric coating reflects light energy having a second wavelength back to a signal processing system. Not until after passing through the magneto-optic material is the first wavelength light reflected back to the signal processing system by a mirror. Thus, the magneto-optic material causes a Faraday rotation, in the first wavelength light, but does not do so to the second wavelength light. In this way, the first wavelength light may be used as an indicator of the velocity of the object, and the second wavelength light may be used as a reference. In addition, since the fiberoptic transducer functions to pass only the first wavelength light, less expensive parts and materials, that is, standard off the shelf items may be used in the transducer. That is, less expensive magneto-optic material and mirrors may be utilized in the fiberoptic transducer, since they are only required to cooperate with the first wavelength of light. The magneto-optic material and the mirror need not be configured to cooperate with the second wavelength of light. In another preferred embodiment of the present invention, the transducer lacks the 2x1 coupler and further includes a GRIN (graded index) lens which is configured to collimate the light energy and an imaging lens for imaging the calibrated light on the magneto-optic material. In yet another preferred embodiment, the transducer lacks structure for collimating light energy entirely, yet contemplates the use of a sensitive detection circuit therewith. Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principals of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic representation of a preferred embodiment of the present invention, illustrating a fiberoptic velocity transducer. FIGURE 2 is another embodiment of the fiberoptic velocity transducer depicted in Figure 1. FIGURE 3 is yet another embodiment of the fiberoptic velocity transducer depicted in Figure 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As is shown in the drawings, which are included for purposes of illustration and not by way of limitation, the invention is embodied in a fiberoptic velocity transducer for use in determining the rotational speed of an object. Conventional means to determine the rotational velocity of an object has been found to either be limited in resolution or accuracy, be EMI sensitive, or comprise complex structure and systems that are expensive to manufacture. In particular, velocity transducers found in the art which employ magneto-optic material, have been constrained to use magneto-optic material that passes narrow bandwidths of light energy and fiber-optic components which are required to cooperate with light energy having non-standard wavelengths. The fiberoptic velocity transducer of the present invention is highly accurate and has optimum resolution and, comprises relatively inexpensive parts and materials. Thus, the described transducer provides a superior means for determining the rotational speed of an object. In a preferred embodiment shown in Fig. 1 there is provided a fiberoptic velocity transducer 10 functioning to transmit signals characteristic of the velocity of a rotating object. To accomplish the transmission of such signals, the present invention incorporates novel structure cooperating with magneto-optic material 12 that possesses optical Faraday rotation properties. In particular, it is contemplated in one presently preferred embodiment that the fiberoptics transducer 10 embody a 2X1 coupler 14 cooperating with an input/output fiber 16, each of which operate to emit as well as receive light energy. In addition, the fiberoptic transducer 10 comprises a lens 18, a polarizer 20, and a mirror 21. Further, the magneto-optic material 12 has a dielectric coating 22 affixed to a proximal side 24 which operates to selectively pass certain wavelengths of light. In the preferred embodiment, the fiberoptic velocity transducer 10 receives light energy from two sources 26, 28, each having a different known wavelength. The light energy having the first wavelength 26 may be utilized to generate a signal characteristic of the velocity of a particular object, whereas the light energy having the second wavelength 28 may be utilized to generate a reference signal. It is further contemplated that each of the light sources are combined into a single exciting fiberoptic line 30 by a first wave division multiplexer 32 and that the exciting fiberoptic line 30 propagates the light energy to the fiberoptic velocity transducer 10. At the fiberoptics velocity transducer 10, the light energy propagated by the exciting fiberoptics line 30 is coupled through the 2X1 coupler 14 into the input/output fiber 16. The light energy emitting from the input/output fiber 16 is then collimated by the lens 18 and thereafter, polarized by the polarizer 20. The light energy next encounters the magneto-optic material 12, the dielectric coating 22 of which functions to selectively pass the light energy having the first wavelength 26, and reflects the light energy having the second wavelength 28. The linearly polarized light energy that passes through the dielectric coating 22 is rotated by the magneto-optic material 12 to an extent corresponding to the magnetic flux density flowing through the material. It is contemplated that the object being measured be adapted with an excitor ring or similar mechanisms so that a cycling magnetic field, parallel to the direction of light propagation through the magneto-optic material 12, is created by the object which, in turn, produces a cycling magnetic flux density within the magneto-optic material 12. In this way, the magnetic field created by the moving object affects the magneto-optic material 12 to a degree which corresponds speed of the object. Moreover, the degree to which the magneto-optic material 12 is affected by the object determines the extent to which the polarized light is rotated. After the light energy having the first wavelength 26 is rotated by the magneto-optic material 12, it propagates to the mirror 21 which reflects the light back through the magneto-optic material 12. The magneto-optic material 12 again rotates the polarized light to an extent corresponding to the magnetic flux density flowing through the material. The first wavelength light energy 26 is then propagated back through the polarizer and is consequently attenuated, the amount of which is characteristic of the extent of rotation or angular position of the light energy caused by the magneto-optic material 12 and more importantly, the resulting signal can be directly related to the velocity of the object. The attenuated light is then focused by the lens 18 back into the input/output fiber 16, through the coupler 14 and to a detecting fiberoptic line 34. Thereafter, the attenuated light energy passes through a second wave division multiplexer 36 and is propagated to a first wavelength photodetector 38 which operates to measure the strength of the light energy signal. The polarized light that is reflected back by the dielectric material 22 of the magneto-optic material 12 passes unattenuated through the polarizer 20 and through the lens 18 and back into the coupler 14. From the coupler 14, the unattenuated light energy is propagated through the detecting fiberoptic line 34 to the second wave division multiplexer 36 and to a second photodetector 40 which operates to measure the strength of the unattenuated light. Finally, a signal processing unit compares the attenuated and unattenuated light energy signals, each of which having known original energies and known and different original wavelengths. Upon comparison of these signals, the velocity of the rotating object may be determined. In a second presently preferred embodiment of the present invention (Fig. 2), the fiberoptic velocity transducer includes a GRIN (graded index) lens 42 which cooperates with an imaging lens 44 to image collimated light energy through the polarizer 20 and upon the proximal side 24 of the magneto-optic material 12. In this embodiment, the exciting and detecting lines 30, 34 are held together and positioned at the center of the GRIN lens 42, so that its center line passes exactly between the two fibers. Using this scheme, the light energy emitted from the exciting fibers 30 is collimated by the GRIN lens 42 which in turn propagates the light energy at a certain angle relative to its own axis depending upon the optical parameters of the GRIN lens 42 and the radial position of the exciting fiber 30 to the lens axis. In this second preferred embodiment, once the light energy is imaged upon the magneto-optic material 12, the operation of the transducer is similar to that of the first embodiment. In this design, however, first and second wavelengths 26, 28 of light energy must have wavelengths preferably not more than 100nm apart. Therefore, in this embodiment, it is possible to use a single broad spectrum LED that would generate light energy at both first and second wavelengths 26, 28 rather than two distinct light sources. In a third presently preferred embodiment (FIG. 3), the magneto-optic transducer lacks means for collimating light energy. Instead, the light energy is directly propagated through the polarizer 20 and upon the opto-magnetic material 12. Significantly, this embodiment does not include fragile optical components needing alignment which, therefore, significantly reduces the cost of the magneto-optic transducer 10. Further, since this preferred embodiment is designed for manipulating low levels of light energy, it is contemplated that this embodiment be used in conjunction with a particularly sensitive detection circuit. From the foregoing, it will be appreciated that the fiberoptic velocity transducer described provides an accurate and inexpensive means for determining the rotational velocity of an object. Thus, the described transducer provides a superior means for determining rotational speed. While a particular form of the invention has been illustrated and described, it will also be apparent that various modifications can be made without departing from the scope of the invention. Thus, it should be understood that various changes in form, detail, and application of the present invention may be made without departing from the scope of this invention.

Claims: A fiberoptic velocity transducer (10) for determining the rotational speed of an object, comprising: means for propagating a plurality of light energy wavelengths (26); means for polarizing light energy; means for selectively passing a first predetermined range of wavelengths of light energy (26) and for selectively reflecting a second predetermined range of wavelengths of light energy (28) ; means (12) for causing a Faraday rotation in light energy; and means (21) for reflecting light energy passing through said Faraday rotation means; characterized in that said selectively passing and reflecting means is a dielectric coating (22) affixed to said Faraday rotation means (12). The fiberoptic velocity transducer of Claim 1, wherein said Faraday rotation means is magneto-optic material; and means for reflecting polarized light energy passing through said magneto-optic material. The fiberoptic velocity transducer of Claim 1 or 2, wherein each of said plurality of light energy wavelengths are provided by fiberoptic light energy sources (30, 34). The fiberoptic velocity transducer of any preceding claim, including means for coupling (14) said plurality of light energy wavelengths, and wherein said coupling means is a 2X1 coupler. A method of determining the rotational speed of an object, comprising: propagating a plurality of light energy wavelengths in a fiberoptic velocity transducer (10); polarizing the plurality of light energy wavelengths; selectively passing a first predetermining range of wavelengths of light energy (26) and selectively reflecting a second predetermined range of wavelengths of light energy (28); providing Faraday rotation means (12) to cause Faraday rotation in light energy, and reflecting light energy passing through the Faraday rotation means (12); characterized by: affixing a dielectric coating (22) to the Faraday rotation means (12) such that the first predetermined range of wavelengths (26) is passed and the second predetermined range of wavelengths (28) is reflected thereby. A method as claimed in Claim 5, in which the Faraday rotation means (12) is a magneto-optic material, the method further comprising: configuring the rotating object with means to create a magnetic flux density in the magneto-optic material which is representative of its rotational speed, and comparing a first wavelength within the first predetermined range of wavelengths (26), which has passed through the magneto-optic material, with a second wavelength within the second predetermined range of wavelengths (28) which has been reflected by the dielectric coating (22), to determine the rotational speed of the object.

Inventors: BARUCH EZRA, BARUCH, EZRA
Applicants: HYDRO AIRE DIVISION OF CRANE C, HYDRO-AIRE DIVISION OF CRANE COMPANY
Application year: 1996
Main classification: G01P3
IPC codes: G01P3, G02F1