Abstract:
An apparatus includes a blade clearance detection system. A probe is configured to communication detection frequencies from and gather reflected signals for the blade tip detection system. The probe has an end supported relative to the casing. A material provides a reference point. The blade tip clearance detection system is configured to generate a first detection frequency configured to pass through the material to detect the position of a target structure, generate a second detection frequency configured to reflect from and detect the reference point, and determine a position of a surface approximate to the target structure based upon the reference point.

Description:
This application is a continuation application of U.S. patent application Ser. No. 11/621,671, which was filed on Jan. 10, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of mounting a frequency probe in a turbine engine. 
     Microwave/radio frequency signals have been used to detect, for example, the position of a target component within a turbine engine. A microwave/radio generator produces a signal that is reflected by the target component and processed to detect information such as the position of the target component. 
     Current methods of instrumentation in a turbine structure require that a hole be drilled in the metal structure to allow the sensor to function. The hole is required to permit communication with a target component. A mechanical connection is required to attach the sensor to the metal structure to prevent leakage. The mechanical connections pose durability issues. 
     In one example, microwave/radio frequencies are used to detect the clearance of a turbine blade relative to an adjacent housing. The orifice used to accommodate the microwave/radio frequency instrumentation allows air and debris in the turbine gas path to collect within the sensor thereby degrading its performance. The hole also creates a potential pathway for high pressure secondary cooling air used to cool the blade outer air seal to leak through the hole and into the gas path, creating a performance loss. 
     With prior art methods it is difficult to reliably determine the proximity of the rotating turbine blades relative to the turbine case. What is needed is a method and apparatus for preventing contamination of the sensor and leakage between the cooling path and turbine gas path. What is also needed is a reliable way of establishing an absolute position of the sensor relative to the turbine blades. 
     SUMMARY OF THE INVENTION 
     An apparatus includes a blade clearance detection system. A probe is configured to communication detection frequencies from and gather reflected signals for the blade tip detection system. The probe has an end supported relative to the casing. A material provides a reference point. The blade tip clearance detection system is configured to generate a first detection frequency configured to pass through the material to detect the position of a target structure, generate a second detection frequency configured to reflect from and detect the reference point, and determine a position of a surface approximate to the target structure based upon the reference point. 
     A method of detecting blade tip clearance, in one example, is provided by generating a first detection frequency that passes through a material supported relative to a casing. The first detection frequency is reflected from a target structure. A second detection signal is generated and reflected from a reference point provided by the material. A clearance is determined between the target structure and a surface associated with the case and based upon the reference point. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially broken perspective view of a turbine section of a turbine engine. 
         FIG. 2  is and enlarged view of a portion of the cross-section shown in  FIG. 1 . 
         FIG. 3  is a schematic view of the turbine section shown in  FIG. 1  and including a position sensing system. 
         FIG. 4  is a top perspective view of a blade outer air seal. 
         FIG. 5  is one example of a port seal subassembly. 
         FIG. 6  is another example of a port seal subassembly. 
         FIG. 7  is an enlarged view of the example port seal subassembly shown in  FIGS. 2 and 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A turbine section of a gas turbine engine  10  is shown in  FIG. 1 . The engine  10  includes a hub  12  having multiple turbine blades  14  secured to the hub  12 . A housing, such as blade outer air seal (BOAS)  16 , is arranged about the turbine blades  14  near their tips. A casing  18  supports the BOAS  16 . Cooling ducts  20  are supported on the casing  18  near the BOAS  16  to control the clearance between the tips and BOAS  16  by selectively controlling cool air through the cooling duct  20 , as is known in the art. A probe  24  is supported in the casing  18  and extends to the BOAS  16 . The probe  24  is part of a position detection system, shown in  FIG. 3 , that monitors tip clearance. 
     Referring to  FIG. 3 , the tip clearance detection system includes a frequency generator  28  operable in response to commands from a controller  30 . The frequency generator  28  produces a detection frequency including microwave/radio frequencies, in one example. The detection frequency produced by the frequency generator  28  travels along a conduit  32  to the probe  24 . It is desirable for the detection frequency to travel generally uninhibited from the probe  24  to the turbine blade  14 . As the turbine blades  14  rotate about an axis A, the tip clearance detection system monitors the clearance between the tip of the turbine blades  14  and the BOAS  16 . Prior systems have simply provided an aperture in the BOAS  16 , which undesirably permits cooling air from the cooling duct  20  to enter the turbine section. A mechanical connection between the conduit  32  and the BOAS  16  was required to prevent leakage, but contributed to durability concerns. Additionally, any holes in the housing enable debris to contaminate the probe  24 . It should be understood that the above described detection system can be used to detect other information within the gas turbine engine  10  or other aircraft systems. 
     Referring to  FIGS. 2 and 4 , the probe  24  is securely retained relative to the BOAS  16  so that the clearance between the BOAS  16  and the adjacent turbine blade  14  can be detected. The BOAS  16  typically includes an impingement plate  26  that is supported between the casing  18  and the BOAS  16 . An aperture is provided in the impingement plate  26  to accommodate the probe  24 . In the example shown, the BOAS  16  includes a boss that provides a channel ring  22 . The channel ring  22  has a recess  23 , which is best shown in  FIG. 4 , to receive an end of the probe  24 . In the example, the impingement plate  26  and channel ring  22  retain the probe  24  axially and circumferentially. 
     The BOAS  16  is typically constructed from a metallic material such as an Inconel®. While Inconel® is a desirable structural material typically used in blade outer air seals, Inconel® blocks the passage of microwave/radio frequencies, which can prevent the communication between the turbine blades  14  and probe  24 . In the example, a hole  25  is provided near the end of the probe  24 . A window material  34  is supported within the hole  25 . The window material  34  is transparent to the detection frequency, permitting communication between the detection frequency and the turbine blade  14 . By “transparent” it is meant that the window material  34  permits desired passage of the detection frequency. Said another way, the window material  34  comparatively permits a better quality passage of the detection frequency relative to the housing. 
     The window material  34  is a polycrystalline, single crystalline or ceramic material, for example. In one example, the window material  34  is a metalized alumina. Other example materials include quartz, diamond, Zirconia toughened alumina, unmetalized alumina, or other materials that are transparent to the detection frequency as known by someone skilled in the art. 
     In the examples shown in  FIGS. 2 ,  4  and  7 , the window material  34  is supported by a carrier  36  that provides a subassembly  38 . The dimensions of the window material  34  are so small in some applications that it presents assembly difficulties for the turbine engine assembler. By providing a carrier arranged about the window material  34 , a larger subassembly  38  is provided that can more easily be manipulated by the assembler. 
     In one example, a shoulder  44  is provided at one end of the hole to axially locate the subassembly  38 . The subassembly  38  including the window material  34  and carrier  36  are machined to a precise height H and diameter D for the typical application. The height H can be precisely machined by polishing, for example, so that an accurate determination of tip clearance can be made. The diameter D can be achieved using an electrical discharge machining process, for example. The window material  34  acts as a reference point to enable more precise measurement of the blade tip clearance. For example, another frequency can be transmitted through the probe  24  that will not pass through the window material  34 . The signal reflected from the window material  34  can be used for reference when determining the clearance between the BOAS  16  and blade tip. The carrier  36  may extend radially beyond the channel ring  22  to include the channel ring  22  for better location of the end of the probe  24  relative to the housing  16 . Such a carrier  36  is schematically illustrated by the dashed lines in  FIG. 2 . 
     Referring to  FIG. 7 , the window material  34 , which is a metalized alumina in the example, is brazed to the carrier  36  using a brazing material  40 . In one example, the carrier  36  is an Inconel® like the BOAS  16 . The window material  34  and carrier  36  provide a subassembly  38  that is brazed to the BOAS  16  using a brazing material  40 . After securing the subassembly  38  to the BOAS  16 , the height H of the subassembly  38  can be achieved by machining. 
     Other example arrangements are shown in  FIGS. 5 and 6 . Referring to  FIG. 5 , a subassembly  38 ′ is provided by a carrier  36 ′ having a annular groove  50  machined in its inner diameter. The window material  34  is retained by the carrier  36 ′ and captured within the annular groove  50 . The outer diameter of the window material  34  and inner diameter include tapered surfaces  52  for improved retention of the window material  34 . The subassembly  38 ′ is secured to the BOAS  16  using a brazing material  40 . Referring to  FIG. 6 , the window material  34  is directly secured to the BOAS  16  using brazing material  40 . 
     Although preferred embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.