Abstract:
A gas turbine engine having at least a rotor shaft operatively connecting a compressor apparatus and a turbine apparatus, comprises an auxiliary gear box and a phonic wheel apparatus. The auxiliary gear box is drivingly connected to the rotor shaft and the phonic wheel apparatus includes an oil pump having toothed gear and a sensor mounted to the oil pump for sensing a rotational speed of the toothed gear.

Description:
TECHNICAL FIELD 
   The invention relates generally to gas turbine engines, and more particularly, to an improved method and apparatus for detecting the rotational speed of a gas turbine engine. 
   BACKGROUND OF THE ART 
   The rotational speed of a gas turbine engine, particularly the rotational speed of the high pressure spool shaft of the engine (sometimes referred to as the N2 speed), is a primary input variable necessary for the control logic of a gas turbine engine. In the prior art, engine speed is detected by way of a sensor positioned adjacent to a phonic wheel which is usually incorporated at a suitable location along a rotor of the engine, such as the high pressure spool shaft. A phonic wheel typically defines a number of slots extending therethrough and is mounted on a rotor shaft. A medium such as a beam of light, a magnetic field, etc. is employed such that the sensor receives the medium affected by the slots of the phonic wheel when rotating, thereby enabling it to provide data regarding the rotational speed of the rotor shaft. The phonic wheel and the associated sensor are conventionally buried within the engine, which makes access thereto for maintenance and repair very difficult. Furthermore, the conventional location of the phonic wheel and associated sensor of a gas turbine engine is in a high temperature environment inside of the engine and this can cause a high differential thermal expansion mismatch between the sensor and the tips of the phonic wheel. 
   Accordingly, there is a need to provide an improved method and apparatus for detecting the rotational speed of gas turbine engines. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide a method and apparatus for detecting a rotational speed of a rotor shaft of a gas turbine engine, overcoming the shortcomings of the prior art. 
   In one aspect, the present invention provides gas turbine engine having at least one rotor shaft operatively connecting a compressor apparatus and a turbine apparatus. The gas turbine engine further comprises an auxiliary gear box and a phonic wheel apparatus. The auxiliary gear box is drivingly connected to the rotor shaft, and the phonic wheel apparatus includes a toothed gear of the auxiliary gear box and a sensor for sensing a rotational speed of the toothed gear 
   In another aspect, the present invention provides an auxiliary gear box of a gas turbine engine which is drivingly connected to a rotor shaft of the engine. The auxiliary gear box comprises at least one pair of gears thereof to rotate in a fixed ratio with respect to a rotational speed of the rotor shaft, and a sensor adjacent to one of the gears for determining a rotational speed of the rotor shaft. 
   In another aspect, the present invention provides method for detecting a rotational speed of a rotor shaft of a gas turbine engine, which operatively connects a compressor apparatus and a turbine apparatus, and drivingly connects an auxiliary gear box. The method comprises detecting a rotational speed of one toothed gear associated with the auxiliary gear box as the toothed gear rotates in a fixed ratio with respect to the rotational speed of the rotor shaft and determining the rotational speed of the rotor shaft based on the detected rotational speed of the toothed gear and the fixed ratio. 
   Further details of these and other aspects of the present invention will be apparent from the detailed description and drawings included below. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying drawings depicting aspects of the present invention, in which: 
       FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine as an example illustrating an application of the present invention; and 
       FIG. 2  is schematic cross-sectional view of an oil gear pump affixed to an auxiliary gearbox of the engine of  FIG. 1 , incorporating an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a turbofan gas turbine engine incorporating an embodiment of the present invention is presented as an example of the application of the present invention, and includes a engine case  10 , a core casing  13 , a low pressure spool assembly seen generally at  12  which includes a fan assembly  14  and a low pressure turbine assembly  18 , and a high pressure spool assembly seen generally at  20  which includes a compressor assembly  22  and a high pressure turbine assembly  24 . The core casing  13  surrounds the low and high pressure spool assemblies  12  and  20  in order to define a main fluid path (not indicated) therethrough. An auxiliary gear box (AGB)  26  attached to the engine case  10  of the engine is drivingly connected by a tower shaft  28  to the shaft (not indicated) of the high pressure spool assembly  20  such that the speed reduction gears of the AGB  26  rotate at a fixed ratio with respect to the rotational speed of the shaft of the high pressure spool assembly  20 . The AGB  26  further includes a plurality of accessories such as an oil gear pump  30 , a generator, etc. which are affixed to the AGB  26  and are driven by the AGB  26  such that these accessories also rotate in a fixed ratio with respect to the rotational speed of the shaft of the high pressure spool assembly  20 . A starter (not shown) is also conventionally affixed to the gearbox and is drivingly connected to the shaft of the high pressure spool assembly  20  through the tower shaft  28 . 
   Therefore, in accordance with the present invention, it is preferred to determine the rotational speed of the shaft of a high pressure spool assembly  20  based on the rotational speed of one of the gears associated with the AGB  26 , such as the oil gear pump  30 , and the fixed rotation ratio of the gear with respect to the rotational speed of the shaft of the high pressure spool assembly  20 . 
   Referring to  FIGS. 1 and 2 , one embodiment of the present invention includes the oil gear pump  30  which is an AGB scavenge gear pump used in a lubricating system (not shown) of the gas turbine engine. The oil gear pump  30  is affixed to and driven by the AGB  26 . The oil gear pump  30  preferably includes a body or housing  32  defining a cavity  34  therein with an inlet  36  and outlet  38  in fluid communication with the cavity  34 , thereby allowing oil to flow through the housing  32  via the inlet  36 , the cavity  34  and the outlet  38 . A pair of preferably identical toothed gears  40 ,  42  in a gearing relationship, are operatively mounted to the housing  32  within the cavity  34  of the oil gear pump  30 . 
   The cavity  34  has a profile such that the gears  40 ,  42  can rotate in a gearing relationship without interfering with the surfaces of the cavity  34 , but will substantially block the oil flow passing through the cavity between the inlet  36  and the outlet  38  when the gears  40  and  42  are not rotating. One of the gears  40 ,  42  is driven to rotate by the AGB  26  and the other is a free gear which rotates together with but in the opposite rotational direction of the gear driven by the AGB  26 . When the gears  40 ,  42  rotate, oil contained in slots defined by adjacent teeth of the respective gears  40 ,  42  is forced to move within the cavity  34  from the inlet side to the outlet side or vice versa, depending on the rotational direction of the gears  40 ,  42 . 
   The housing  32  of the oil gear pump  30  further preferably defines a hole  44  extending thereinto and intersecting the cavity  34 , for receiving therein a sensor  46  such as a N2 speed probe of a magnetic type, such as a magnetic speed pick-up. The sensor  46  received in the hole  44  in the pump housing  32 , is adjacent to the gear  40 , preferably extending radially toward thereto with a predetermined clearance therebetween. Flanges  48 ,  50  of the sensor  46  ensure the predetermined clearance between the sensor  46  and the tips (not indicated) of the gear  40  such that the sensor  46  is enabled to detect variations in a magnetic field disturbed by the teeth and slots of the gear  40  passing thereby when the gear  40  rotates. The rotational speed of the gear  40  is calibrated from the detected variations in the magnetic field. Thus, the gear  40  and the sensor  46  in combination form a phonic wheel apparatus although the primary function of the gear  40  is one of the rotors of the oil gear pump for pressurizing an oil flow. 
   The environment of the phonic wheel is wet, as the gear and sensor are subject to oil flow in the area. An O-ring seal  52  is preferably provided between the hole  44  and the sensor  46  to prevent oil leakage from the cavity  34 . 
   The sensor  46  is in electrical contact with the electrical engine control (EEC) (not shown) of the gas turbine engine. Thus, data regarding the rotational speed of gear  40  is provided to the EEC. 
   As described, the AGB  26  is drivingly connected through the tower shaft  28  to the shaft of the high pressure spool assembly  20  and the pair of gears  40 ,  42  are driven to rotate by the AGB  26 , therefore the rotational speed of gear  40  is in a fixed ratio with respect to the rotational speed of the shaft of high pressure spool assembly  20 . This fixed ratio is known when the engine is designed and manufactured. Therefore, the instant rotational speed of the shaft of the high pressure spool assembly  20  (N2 speed) can be determined based on a calculation of the detected instant rotational speed of the gear  40  and the known fixed ratio. This is computed from time to time by the EEC and, as an output result, the instant N2 speed other than the rotational speed of gear  40  is displayed and is used as a primary input variable necessary for the control logic of the gas turbine engine. 
   The oil pump-mounted solution of the present invention is novel and has several advantages, including a novel location within the AGB which results in, among other things, a reduced tolerance stack-up and a low differential thermal expansion mismatch between the sensor  46  and the gear tips. 
   The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the invention disclosed. For example, the present invention can be applied to various types of gas turbine engines other than a turbofan gas engine which is used as an example to illustrate the application of the present invention. The oil gear pump incorporating a phonic wheel apparatus can be affixed to an AGB either outside or inside of the AGB. The sensor can be selected from any suitable types, although a magnetic speed probe is used in the embodiment of this invention. The sensor can be mounted by any suitable support structure other than the body of the oil gear pump, depending on the location of the AGB gear being selected to function as a phonic wheel. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.