Abstract:
A strain sensor apparatus for a rotatable shaft including a radiation emitter/receiver, a vibration element attached to the shaft and a reflector that is positioned to reflect radiation onto the vibration element.

Description:
BACKGROUND 
     The present invention relates to a method and apparatus associated with wireless flexural behaviour measurement and in particular, but not exclusively, steady and/or vibrational torque measurement of a shaft of a gas turbine engine for example. 
     Conventional wireless sensors find a wide range of applications in the area of instrumentation for example in engine development work, processing plants and medicine. Wireless sensors have shown potential for vibration and rotation monitoring. When applied for instrumentation on development gas turbine engines, wireless sensors have a number of advantages such as the reduction of expensive wiring, the reduction of complexity, reduced set up time for monitoring and removal of connector faults. 
     One application of wireless sensors is the measurement of torque on a rotating shaft. The majority of torque sensors employ strain gauges and use slip rings, inductive or optical links to transfer data. 
     One such torque sensor apparatus  30  is shown on  FIG. 1  and comprises an emitter/transceiver  32  directed to a shaft  34  having a metal wire  36  attached thereto. The metal wire is attached between two points on the shaft at an angle to its rotational axis  38 . If no torque is applied common in-service vibrations in the shaft excite the string and make it vibrate at its resonant frequency f 0 , determined by string&#39;s geometrical and material parameters. Application of a torque to the shaft alters the string tension, resulting in a corresponding change of the resonance frequency. The microwave transceiver  32 , directed towards the sensing wire  44 , emits a radio-frequency (RF) signal which is reflected by the wire  44 . The amplitude of the return signal  54  is modulated due to the string&#39;s vibrations. The change of amplitude between successive, once-per-revolution readings is indicative of the torque applied to the shaft  34 . 
     SUMMARY 
     However, this prior art arrangement is disadvantaged because the wire vibrates in three dimensions; producing omni-directional wave scattering that reduces the signal strength back to the transceiver. 
     Therefore it is an object of the present invention to provide new torque sensor apparatus and method of measuring torque which obviates the above mentioned problems. 
     In accordance with the present invention there is provided a strain sensor apparatus for a rotatable shaft comprising a radiation emitter/receiver, a vibration element attached to the shaft and a reflector that is positioned to reflect radiation onto the vibration element. 
     Preferably, the reflector is concave and positioned to reflect radiation onto a part of the vibrational element having the greatest amplitude. 
     Alternatively, the reflector comprises walls that channel radiation onto the vibrational element. 
     Preferably, the reflectors are positioned to reflect radiation from the vibrating element back to the radiation emitter/receiver. 
     Preferably, a radiation-reflective annulus surrounds the shaft and vibration element. 
     Preferably, a waveguide extends between the emitter/transceiver and an aperture defined in the annulus. 
     Alternatively, two vibration elements are positioned to reflect radiation from one to the other. 
     Preferably, the two vibration elements are spaced apart and angled at approximately 90 degrees to one another. 
     Preferably, the vibration element is a wire. 
     Alternatively, the vibration element is a plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic view of a prior art torque sensor apparatus applied to a shaft; 
         FIG. 2  is a schematic section of a prior art three-shaft ducted fan gas turbine engine; 
         FIG. 3  is a schematic side view of torque sensor apparatus applied to a shaft in accordance with the present invention; 
       FIGS  4   a - 4   b  are a schematic layout of embodiments of torque sensor apparatus in accordance with the present invention; 
         FIG. 5  is schematic axial view of further embodiment of a torque sensor apparatus applied to a shaft in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 2 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis X-X. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , an intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . 
     The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct (not shown) to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  16 ,  17 ,  18  respectively drive the high and intermediate pressure compressors  14 ,  13  and the fan  12  by interconnecting shafts  20 ,  21 ,  22  respectively thereby making up high, intermediate and low-pressure spools. 
     Referring to  FIG. 3 , a novel torque sensor apparatus comprises an emitter/transceiver  42  directed to a vibration element  44 , in this example a metal wire, attached to a shaft  34 , for example between two flanges  33 ,  35 . A radiation-reflective annulus  46  surrounds the shaft radially outwardly of the vibration element  44 . A waveguide  48 , itself radiation-reflective, extends between the emitter/transceiver  42  and an aperture  50  defined in the annulus  46 . 
     The emitter/transceiver  42  emits microwave radiation (solid line  52 ), which is channelled through the waveguide  48 , through the aperture  50  and into the generally annular space  41  between the shaft  34  and annulus  46 . Both the shaft  34  and importantly the annulus  46  are substantially impermeable to (microwave) radiation, such that their surfaces reflect the radiation. The microwave radiation output  52  is then guided between the rotating shaft  34  and the annulus  46  and impinges on the vibrational wire  44 . The radiation reflected by the vibrational element, or return signal, is shown as a dashed line  54 . Thus, while the shaft  34  is rotating, the vibrating element  44  will be able to ‘see’ the microwave radiation constantly rather than at a once-per-revolution interval of the prior art arrangement. This novel torque sensor apparatus  40 , therefore results in a higher average signal level being detected, which in turn is capable of giving a much improved quantity and quality data. 
     A further advantage of the invention is the continuous visibility of the signal, rather than a once-per-revolution ‘snapshot’ which enables detection of behavioural defects such as flutter. In other words vibrational characteristics that occur within a single revolution of the shaft are not capable of being detected by the prior art arrangement and indeed its results may be affected by unexplainable or not-apparent phenomena. As will be described later this is not the case with the arrangement of the present invention. 
     In  FIG. 4   a , the torque sensor apparatus  40  comprises two reflectors  56   a ,  56   b  at approximately 135° to one another with the vibration elements  44   a ,  44   b  opposite the reflectors at 45° to shaft axis in order to achieve optimum signal conditions. Note that the incoming and returning radiation paths  52 ,  54  are along the same lines.  FIG. 4   b  is an improvement because the radiation is reflected off reflector  56   a  or  56   b  respectively onto string  44   a  or  44   b  respectively. The energy loss after reflection will be less compared to the previous configuration as the reflectors are rigid. 
     In  FIG. 4   b , two reflectors  56   a ,  56   b  are arranged at approximately 90° and are adjacent a single vibration element  44 . One reflector  56   a  directs the microwaves towards the vibration element and the other reflector  56   b  directs the reflected signal from the vibration element back to the detector  42 . This again is to achieve a stronger signal at the detector because the reflector  56   a  is capable of focusing emitted radiation  52  onto the vibration element. The third improvement is described in  FIG. 4   b  where a parabolic reflector will concentrate the radiation onto the centre of a string and the reflections will be directed back to the receiver. The parabolic reflector enables maximum radiation transfer. The above description is fine for string geometries, however will provide an even greater effect to the proposed plate  60  of  FIG. 5 . In particular configuration of  FIG. 4   b  with parabolic reflectors will be able to direct the radiation onto the centre point of the plate. 
     In the embodiment described with reference to  FIG. 4   b , the reflectors may be either planar in shape or may be concave shape. In particular, the concave shape enables the radiation to be focussed onto the centre of the vibration element  44  where its amplitude is greatest and therefore its reflection response is further improved. 
       FIG. 5  shows a further embodiment of the present invention wherein a groove  60  is formed in the shaft  34  and comprises side walls  62  and an end wall  64 . A vibrating element  66 , here in the form of a plate, is located groove and is spaced apart from the end wall  64 . The side walls  62  converge towards the plate  66  and channel or focus the incoming radiation towards the vibrating element  66 . The vibrating element may be a wire. In both cases the end wall  64  is a reflector and reflects the radiation from the vibrating element  66  back to the transceiver  42 . 
     Various combinations of the embodiments may be used by the skilled artisan for any particular application and all are intended to be within the scope of the present invention. For example, the vibration plate  60  may be substituted in place of any of the vibration wires  44   a ,  44   b  in  FIGS. 4   a  and  4   b . More than one pair of vibration elements and more than one pair of reflectors may be used. 
     Although microwave radiation is a preferred wavelength other forms of radiation may be used. Typically the annulus is made from suitable material to reflect the radiation. Similarly, the vibration elements  44 ,  66  may be made from similar materials.