FAN BLADE SPEED-SENSING APPARATUS

Abstract: A speed sensor for sensing the rotational speed of an N bladed fan such as in a turbo-machine application includes a polarization-preserving optical circuit (20) positioned adjacent the locus described by the remote ends of the blades (B). First (S₁,S₂,S₃,S₄) and second (S₅,S₆,S₇,S₈) sets of sensing segments of the optical circuit (20) are spaced apart from one another by an angular spacing equal to that of the blades and with the first set of sensing segments angularly offset from the second set. The optical circuit has two orthogonal polarization modes, and the first sets are cross-coupled by a coupler (C) to transpose the polarization modes. The orthogonal polarization modes of optical energy launched into the optical circuit at an input (22) are subjected to phase shifts as the optical fibre is perturbed by pressure variations caused by the moving fan blades with the cumulative phase shift in each portion of the optical circuit subtracted as a consequence of the cross-coupling and the resulting light output at an output (24) having an intensity or other characteristics which varies as a function of the rotational speed of the fan.

Description: The invention relates to apparatus for sensing speed and, more particularly, to speed sensors for sensing the speed of fan blades and thus the speed of rotation of a fan or fan-like structure about an axis of rotation. Various electrical devices are known for sensing rotational speed, including the speed of rotation of a multi-blade fan. For example, electric sensors using magnetic fields or capacitive effects are known in which the rotating structure generates an electrical pulse signal as a function of the rotational speed. For example and in the context of a rotating fan, magnetic sensors are known in which a magnetic pick-up coil or a Hall effect sensor is positioned in close proximity to the ends of the fan blades. As the fan rotates, the ends of the blades move past the sensor to perturb the magnetic field adjacent the sensor to generate a sequence of electrical pulses having a repetition rate that is proportional to rotational speed. Capacitive-type sensors operate in a similar manner and usually employ a simple bridge circuit that detects a change in the capacitance between the sensor and the blade end to produce a pulse output. Other types of sensors use a light beam that is periodically interrupted, e.g., chopped, by the motion of the fan blades to cause a photo-sensor to produce an electrical pulse output having a pulse repetition rate that varies with the rotational speed of the fan blade. In general, the type of speed sensor systems discussed can be characterised as electrical and tend to produce EMI (electromagnetic interference) and, conversely, are EMI-sensitive. In certain environments, for example in sensing the speed of rotation in aircraft fan-jets or other turbo-machinery, shielding is required to prevent or minimize adverse EMI effects. In general, shielding adds to the capacitive impedance in any electrical system and can adversely limit the upper operational frequency of the system as well as add undesirable weight in an airborne application. Traditional photo-electric systems, in addition to requiring shielded components in many applications, are sensitive to and can be operationally compromised by the accumulation of debris and other light obscuring contaminants on their optically active surfaces or in the optical path between the light source and its receiver. One of the trends in the instrumentation field is the incorporation of optical fibres into the instrument system to take advantage of their lightweight, immunity to electric and magnetic fields, and their high bandwidth. For example, the output of electrical sensors can be converted to light pulses and transmitted via unshielded optical fibre to a receiver for further processing. In the context of prior photo-electric systems, light energy can also be directly launched into a fibre for transmission to a remote processing location. In this latter situation, the optical path is nonetheless "open" and sensitive to light obscuring contaminants in a manner analogous to the traditional photo-electric system. Ideally, any system that uses optical energy should be closed, that is, not subject to the effects of light obscuring contaminants. In view of the above, the invention is concerned with overcoming the problem of providing a speed sensor that senses speed in an optical manner and which uses a closed optical path that is immune to environmental effects. According to the invention, there is provided a apparatus for sensing fan blade speed, characterised by a fibre-optic circuit for transmitting optical energy in first and second polarisation modes and having an input port for accepting light from a light source and an output port, polarisation transposing means for transposing, at an intermediate point between the input port and the output port, the polarisation modes in the fibre-optic circuit, support means for supporting first and second selected paths of the fibre-optic circuit in serial fashion adjacent the locus described by the fan blade movement, whereby the fibre-optic circuit in the selected paths is subjected to pressure variations caused by the blade movement so as to alter a characteristic of the optical energy transmitted therethrough, the first selected path being the path between the input port and the intermediate point and the second selected path being the path between the intermediate point and the output port, and speed determining means for determining the speed of rotation from the light output at the output port. A polarimetric optical speed sensor embodying the invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIG.1 is a front elevational view, in schematic form, of a rotatably mounted fan with the speed sensor mounted around the periphery of the fan; FIG. 2 is a detail view, in perspective, of a portion of the speed sensor of FIG. 1; and FIG. 3 is an exploded isometric view of an end-to-end butt coupling of two polarization-preserving optical fibres in the speed sensor and which are coupled to transpose the orthogonal modes from one fibre to the other. The polarimetric optical speed sensor and its application to the determination of the rotational speed of a fan or fan-like structure is shown in general form in FIG. 1. As shown, a fan 10, such as the bypass fan of a turbo-fan aircraft engine, is mounted for rotation about an axis 12 in the clockwise direction at a speed w as indicated by the arrow 14. The fan 10 includes N fan blades B that are mounted at equal angular spacings. In the illustrated embodiment, the fan 10 includes eight blades B mounted about the axis 12. A circular cowling 16 or equivalent structure surrounds the fan 10 and is preferably dimensioned so that the interior surface 18 facing the fan 10 is subjected to the pressure waves generated by the remote ends of the moving blades B. In general, the variation in pressure P at any point on the interior surface 18 is a sinusoidal function of the number N of blades B and the rotational speed w. An optical circuit, designated in general form by the reference 20, is mounted on the cowling 16 to be affected, as described below, by the pressure variations caused by the moving blades B. Optical energy is introduced into the optical circuit 20 through an input port 22 by a suitable optical source (not specifically shown) and passes through the optical circuit 20 to an output port 24 and to an output-light processor 26 that provides an electrical indication of the speed of rotation of the fan 10. The optical circuit 20 is formed from a polarization-preserving fibre (not specifically illustrated) which, in general form, includes a light transmitting core surrounded by a cladding having an index of refraction less than that of the core to guide light through the core. The core is configured to support transmission of light having orthogonal polarization modes and preserve the modes as the light is guided in the core. Such cores can be formed by controlling the cross-sectional shape of the core, for example by providing an ellipsoidal cross-section in which the major and minor axes support respective orthogonal modes or by placing a circular core under compressive stresses to cause a relative change in the index of refraction in the stressed direction relative to the less-stressed orthogonal direction. Regardless of the manner by which the polarization-preserving fibre is formed, light having orthogonally polarized components introduced into the fibre will be guided in the core along the fibre with the orthogonal modes preserved. The optical circuit 20, as described more fully below, includes a first optical fibre 20A serially connected to a second optical fibre 20B through a coupler C with the end of the first optical fibre 20A cross-coupled to the second optical fibre 20B to transpose the two modes, that is, the fast axis of the first fibre is coupled to the slow axis of the second fibre and the slow axis of the first fibre is connected to the fast axis of the second fibre. This cross-coupling can be accomplished via an end-to-end butt coupling as illustrated in exploded perspective in FIG. 3. As shown in FIG. 3, the first fibre 20A includes a core 30A surrounded by a cladding 32A with the core 30A supporting first and second orthogonal polarization modes as represented in schematic fashion by the arrows M₁ and M₂. The first fibre 20A is butt-coupled to the second fibre 20B with the core 30A and the cladding 32A of the first fibre 20A co-aligned with the core 30B and cladding 32B of the second fibre 20B. The first and second fibres 20A and 20B are rotated relative to one another so that the mode M₁ of the first fibre 20A is coupled into the mode M₂ of the second fibre and the mode M₂ of the first fibre 20A is coupled into the mode M₁ of the second fibre 20B thus to effect cross-coupling or transposition of the polarization modes. In addition to the cross-coupling butt coupler C described in relation to FIG. 3, discrete axis translators (not shown) may be used to effect the desired mode transposition through the coupling C. The optical circuit 20, which includes the first fibre 20A extending between the light input port 22 and the coupling C and the second fibre 20B extending between the coupling C and the light output port 24, is divided into a plurality of sensing segments S n that are exposed to the pressure variations caused by the movement of the blades B past the sensing segment S n . As shown in FIG. 2, a sensing segment S n is formed by presenting a selected length L of the optical fibre to the pressure variations induced by the movement of the blades B. In the preferred form, holes 38 are formed in a selected angular relationship in the wall of the cowling 16 and the optical fibres 20A and 20B threaded through the holes 38 to place the selected lengths L of the fibre on the inside surface 18 of the cowling 16 adjacent to the locus described by the remote ends of the blades B as the fan 10 rotates about the axis 12. The optical fibres 20A and 20B may be held in place by adhesive or preformed attachment structures (not shown). The orientation of the optical fibres 20A and 20B is such that the optical circuit 20 is oriented with one of the polarization axes, e.g., the M₁ orientation, dominantly affected by the pressure variations. In addition to threading the optical fibres 20A and 20B through the holes 38, an equivalent structure can be provided by placing the entire optical circuit 20 on the interior surface 18 of the cowling 16 and selectively shielding or otherwise isolating alternate sections of the optical fibres 20A and 20B from the pressure variations, thus defining pressure-responsive segments S n between the shielded or isolated portions. The path length of each segment S n should be sufficiently long to cause the pressure variations consequent to fan blade B movement to affect the light guided through the core of the fibres. The placement of the various sensing segments S n and their angular relationship are varied to phase-modulate the light transmitted through the optical fibres 20A and 20B. More specifically, the first four sensing segments S₁, S₂, S₃ and S₄ (representing N/2 sensing segments in the case of the preferred embodiment) are defined from the first optical fibre 20A and positioned at an angular spacing of 360°/N, and the second four sensing segments S₅, S₆, S₇ and S₈ are formed from the optical fibre 20B and likewise positioned at an angular spacing of 360°/N but offset from the first four sensing segments by 360°/2N. In the illustrated case of N = 8, the first four sensing segments S₁, S₂, S₃ and S₄ are located at 0°, 45°, 90° and 135°, and the second four sensing segments S₅, S₆, S₇ and S₈ are located at 157.5°, 247.5° and 292.5°. The location of each of the four sensing segments S₁, S₂, S₃ and S₄ of the first set can be expressed as O₁ = (m - 1)2pi/N where m represents the identity of the particular segment (being, accordingly, 1, 2, 3 or 4). Similarly, the location of each of the four sensing segments S₅, S₆, S₇ and S₈ can be expressed as O₂ = (2m - 3)pi/N where m again represents the identity of the particular segment (being accordingly, 5, 6, 7 or 8). As stated, N represents the number of blades B. As each fan blade B passes a sensing segment Sn, the local hydrostatic pressure varies in a periodic manner as a function of the initial, ambient pressure P, the number N of blades B, and the rotational speed w. The portion of the optical fibre that defines each sensing segment Sn is subjected to a periodic pressure-induced stress which is effective to cause a change in phase of the polarized light transmitted in the core 30A or 30B, this phase change represented as d = 2piLP/f where f is a constant unique to the optical fibre for any given wavelength and L is the path length (FIG. 2) of the fibre at each sensing segment S n . For the first four sensing segments S₁, S₂, S₃ and S₄, which are effectively connected in series, the individual phase changes are additive as follows: d₁ = N/2[2piL/f][P₀ + (P₁/2)(1 + cos(Nwt))] + d₁, where w represents rotational speed, t represents time, d₁ represents the cumulative phase changes due to environmental conditions and the inherent bi-refringence of the optical fibre used to form the optical circuit 20, and P₀ is the average pressure (or stress), P₁ is the pressure (or stress) fluctuation during the passage of a fan blade past a sensing element. The second term in square brackets represent the pressure variation as a function of fan speed. The equation is a well-known equation of photo-elasticity and is discussed in detail in page 411 of "Experimental Stress Analysis" by J.W. Dally and W.F. Riley, published in 1978 by McGraw-Hill Book Company, New York, U.S.A. In a similar manner, the individual phase changes for the second set of sensing segments S₅, S₆, S₇ and S₈ are additive as follows: d₂ = N/2[2pi/f][P₀ + (P₁/2)(1 + cos(Nwt + pi))] + d₂, where d₂ represents the cumulative phase changes due to environmental conditions and the inherent bi-refringence mentioned above. Because the transmission axes for the optical fibre 20A and 20B are transposed at the coupling C (FIGS. 2 and 3), the phase shifts of the optical fibres 20A and 20B are effectively subtracted from one another as follows: d T = N/2[piLP₁/f][cos(Nwt) - cos(Nwt + pi)] +(d₁-d₂) Assuming that the environmentally induced background phase shift and the inherent bi-refringence for both the optical fibre portions 20A and 20B are substantially equal, the total phase change can be represented as follows: d T = (NpiLP₁/f)sin(Nwt) The polarimetric fibre optical sensor is thus configured to produce an optical signal output I at the output port 24 that is proportional to the sine of the total phase shift d T so that the optical signal I is determined as follows: Iαsin[NpiLP₁/f)sin(Nwt)] Where the pressure differences caused by movement of the fan blades B are small, that is, d T << 1, the following relationship holds for the optical signal I evaluated by the output signal processor 26: Iα(NpiLP₁/f)sin(Nwt) An optical output is thus provided which is directly related to the rotational speed w of the fan 10 about its axis of rotation 12 and in which environmentally induced background phase shifts and phase shifts that are consequent on the inherent bi-refringence of the optical fibre are substantially cancelled. As can be appreciated, the sensor described provides an optical speed sensor in which a closed, all-optical circuit is used to provide an accurate indication of rotational speed with relative immunity from environmental factors. Thus it will be appreciated from the above that the optical speed sensor described overcomes the stated problem. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the true scope of the present invention will be determined by reference to the appended claims.

Claims: 1. Apparatus for sensing fan blade speed, characterised by a fibre-optic circuit (20) for transmitting optical energy in first and second polarisation modes and having an input port (22) for accepting light from a light source and an output port (24), polarisation transposing means (C) for transposing, at an intermediate point between the input port (22) and the output port (24), the polarisation modes in the fibre-optic circuit (20), support means (16) for supporting first and second selected paths (20A and 20B) of the fibre-optic circuit (20) in serial fashion adjacent the locus described by the fan blade movement, whereby the fibre-optic circuit in the selected paths (20A and 20B) is subjected to pressure variations caused by the blade movement so as to alter a characteristic of the optical energy transmitted therethrough, the first selected path (20A) being the path between the input port (22) and the intermediate point (C) and the second selected path (20B) being the path between the intermediate point (C) and the output port (24), and speed determining means (26) for determining the speed of rotation from the light output at the output port (24). 2. Apparatus according to claim 1, characterised in that the fan blade movement is constituted by the movement of the multiple blades (B) of a fan or fan-like structure about an axis of rotation (12). 3. Apparatus according to claim 2, characterised in that there are N blades (B), and the support means (16) supports N/2 first selected path lengths (S₁....S₄) in the first selected path (20A) and N/2 second selected path lengths (S₅....S₈) in the second selected path (20B), and in that the said locus is the locus described by the tips of the moving blades (B). 4. Apparatus according to claim 3, characterised in that each of the first selected path lengths (S₁....S₄) is supported by the support means (16) at an angular spacing of 360°/N from the next one thereof, and in that each of the second selected path lengths (S₅....S₈) is supported by the support means (16) at an angular spacing of 360°/N from the next one thereof. 5. Apparatus according to claim 4, characterised in that the angular spacing between the particular ones of the first and second selected path lengths ((S₄, S₅) which are immediately separated by the transposing means (C) are angularly separated by an angular spacing of 360°/2N. 6. Apparatus according to any one of claims 2 to 5, characterised in that the support means comprises a cowling structure (16) within which the fan blades rotate and co-axial with the axis (12) of rotation of the blades (B). 7. Apparatus according to any preceding claim, in which the fibre-optic circuit between the input port (22) and the intermediate point (C) is constituted by a first polarisation-preserving optical fibre (20A), and the fibre-optic circuit between the intermediate point (C) and the output point (24) is constituted by another polarisation-preserving optical fibre (20B), each polarisation-preserving optical fibre incorporating respective means transmitting optical energy in first and second dominant polarised orientations, and in that the transposing means (C) cross-couples the means transmitting the first dominant polarised orientation of the fibre optical fibre to the means transmitting the second dominant polarised orientation of the second optical fibre and cross-couples the means transmitting the second dominant polarised orientation of the first optical fibre to the means transmitting the first dominant polarised orientation of the second optical fibre. 8. Apparatus according to any preceding claim, characterised in that the first and second dominant polarised orientations are orthogonal orientations.

Inventors: SPILLMAN, WILLIAM BERT, JR.
Applicants: SIMMONDS PRECISION PRODUCTS INC.
Application year: 1988
Main classification: G01P3
IPC codes: G01P3, G01P5