Abstract:
A wear sensor ( 30, 50, 60 ) installed on a surface area ( 24 ) of a component ( 20, 21 ) subject to wear from an opposing surface ( 74, 75 ). The sensor has a proximal portion ( 32 A,  52 A,  62 A) and a distal portion ( 32 C,  52 C,  62 C) relative to a wear starting position ( 26 ). An electrical circuit ( 40 ) measures an electrical characteristic such as resistance of the sensor, which changes with progressive reduction of the sensor from the proximal portion to the distal portion during a widening reduction wear of the surface from the starting position. The measuring circuit quantifies the electrical changes to derive a wear depth based on a known geometry of the wear depth per wear width. In this manner, wear depth may be measured with a surface mounted sensor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of pending U.S. patent application Ser. No. 11/269,044 filed on Nov. 8, 2005 now U.S. Pat. No. 7,618,712, which is a continuation-in-part of pending U.S. patent application Ser. No. 11/122,566 filed May 5, 2005 now abandoned, which claims the benefit of Provisional Patent Application No. 60/581,662 filed on Jun. 21, 2004, and which is also a continuation-in-part of U.S. patent application Ser. No. 11/018,816 filed Dec. 20, 2004, now U.S. Pat. No. 7,270,890, which is a continuation-in-part of U.S. patent application Ser. No. 10/252,236 filed Sep. 23, 2002, now U.S. Pat. No. 6,838,157, all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electrical sensors for detecting surface reduction wear as it widens and deepens on a surface, particularly wear on curved components such as spring clips in combustion turbine engines. 
     BACKGROUND OF THE INVENTION 
     Components such as spring clips in engines can experience surface wear from contact with other components under operational vibrations and dynamic forces. Sensors have been designed to provide real-time monitoring of component wear during engine operation. Such monitoring improves safety and reduces operating and maintenance costs by indicating a maintenance requirement before it causes damage or unscheduled outages. 
     It is known to place multiple sensors at different depths in a coating on a component surface to sense a depth of wear in real time. However, multi-layer sensors require significant extra work and expense to embed because each layer must be laid down separately. Generally, N sensors require about N times more work to install than 1 sensor. Also, placing sensors at multiple depths at a single location is problematic because sensor material is not a good wear material, so these sensors can cause spalling and can reduce the life of the wear material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a perspective partial view of a curved component with a 2D sensor having three nested sub-elements. 
         FIG. 2  shows the component of  FIG. 1  after wear from an opposed planar contacting surface. 
         FIG. 3  shows a 2D sensor embodiment with a ladder geometry. 
         FIG. 4  shows a 2D sensor embodiment with a film geometry. 
         FIG. 5  is a sectional front view of a 2D sensor with film geometry installed on a coating on a component. 
         FIG. 6  shows a view as in  FIG. 5  with an opposed planar component causing widening wear on the component and sensor. 
         FIG. 7  is a sectional front view of a flat component and sensor being reduced by widening wear from an opposed curved component. 
         FIG. 8  is a sectional front view of a sensor installed between electrically insulating layers of a coating on a component. 
         FIG. 9  shows a conceptual graph of characteristic measurement data from the sensor geometry of  FIG. 1 . 
         FIG. 10  shows a conceptual graph of characteristic measurement data from the sensor geometry of  FIG. 3 . 
         FIG. 11  shows a conceptual graph of characteristic measurement data from the sensor geometry of  FIG. 4 , and an acceptable time series envelope. 
         FIG. 12  shows a sensor installed on a gas turbine combustor spring clip. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows part of a component  20  having a substrate  22  with a surface  24 . The surface  24  has a wear starting position  26 , which is an initial contact area, point, or line of a touching component (not shown). The component  20  and the touching component have different curvatures. This results in a wear pattern that widens  28  predictably as it deepens. For example, the component  20  may be convex as shown, and the opposed component may be planar, or vice versa. 
     A 2D sensor element  30  is installed on an area of the surface  24 . Herein, the term “2D sensor element” means an element that follows a surface geometry at a single level. This definition includes for example a single wire, plural wires, a film, a ladder, and the like, that follows either a planar surface or a curved surface at a single level. The single level may be an outer uncoated surface as shown in  FIGS. 1-4 , an outer coated surface as shown in  FIGS. 5-7 , o a subsurface level with a further outer coating as shown in  FIG. 8 . Examples of curved surfaces include cylindrical and spherical surfaces. In contrast, a “3D sensor element” has features that are distinct at different depths in a surface or coating. 
     The 2D sensor element  30  in this embodiment comprises nested electrical conductor loops in the form of rail pairs  31 A,  31 B,  31 C and respective rungs  32 A,  32 B,  32 C. The 2D sensor includes a proximal portion  32 A and a distal portion  32 C relative to the wear starting position  26 . Each rung in this embodiment may be independently connected to an electrical measuring circuit  40  that measures an electrical characteristic such as resistance, capacitance or impedance of each loop, and may also energize each loop. This circuit may include an analog to digital signal converter as known in the art. The electrical measuring circuit may be connected  41  to, or be a part of, a monitoring computer  42 , which may include a memory  43  and a clock  44 . In this example each nested loop comprises a zigzag rung between two rails. Zigzag rungs are not essential, but they may increase the sensitivity and/or coverage of each loop compared to alternates such as smoothly curved conductor loops or straight rungs. 
     The sensor elements of embodiments herein may be deposited on a substrate or within or on a wear-resistant layer such as a metal, ceramic, or cermet coating on a substrate as variously shown, using a thin film deposition process such as plasma spraying, electron beam physical vapor deposition, chemical vapor deposition, pulsed laser deposition, mini-plasma, cold spray, direct-write, mini high velocity oxy-fuel, or solution plasma spraying, for example. The substrate may be metal or another material such as a ceramic or ceramic matrix composite. An appropriate deposition process may be selected accordingly as known in the art. 
       FIG. 2  shows a wear pattern  34  that has been worn to a given depth D by an opposed planar surface. The wear pattern  34  has a predictable growth geometry over time, based on the relative curvatures of the component surface  24  and the opposed surface. The proximal rung  32 A and the middle rung  32 B of the sensor element  30  have been broken in  FIG. 2 . The monitoring computer  42  can compute the wear depth D based on the width of the wear pattern  34  relative to its depth when a given rung is broken. Wear can be quantified as a percentage of maximum acceptable wear or in other units, such as age codes or numeric levels progressing from minor wear to maximum wear, as each successive rung  32 A-C is broken. 
       FIG. 3  shows a sensor embodiment  50  with a ladder geometry, including two generally parallel rails  51  and multiple rungs  52 A- 52 E connected between the rails. This sensor produces stepwise changes in the characteristic measurement, which is shown as resistance in the embodiment of  FIG. 10 . These steps are detectable by the electrical measuring circuit  40 , and allow it to quantify the depth of the wear using only a surface-mounted sensor. 
       FIG. 4  shows a sensor embodiment  60  with a film geometry  62 , including a proximal end  62 A and a distal end  62 B relative to the wear starting position  26 . This sensor produces generally gradual changes in the characteristic measurement, as shown in  FIG. 11 . The electrical measuring circuit  40  quantifies the wear based on these changes. 
       FIG. 5  shows a sectional front view of a component with a sensor element  62  installed on a coating  70  on the substrate  22 . The coating  70  may be a wear coating, an electrical insulation coating, or a thermal insulation coating over an optional bond coat  72  on the substrate  22  as known in the art. 
       FIG. 6  shows a view as in  FIG. 5  with an opposed planar surface  74  causing wear on the coating and sensor. The film  62  has been reduced by this wear, resulting in a changed electrical characteristic of a circuit that includes sensor element  62 . 
       FIG. 7  shows a sectional front view of a flat component  21  with a sensor element  62  being reduced by wear from an opposed curved surface  75 . 
       FIG. 8  shows a sectional front view of a sensor element  62  installed between electrically insulating layers  76 ,  77  within a coating  70  on a component. 
     The sensor embodiments herein may be formed as follows:
         1. If the substrate has a high dielectric constant, as with an insulating ceramic like Al 2 O 3 , the sensor element may be deposited directly on the substrate. Otherwise, deposit an electrically insulating layer  76  on the substrate surface  24  using a material such as an oxide ceramic with high dielectric/insulating properties like Al 2 O 3 , Yttria Stabilized Zirconia, and MgAl 2 O 4 .   2. Deposit the sensor layer  62  using an electrically conducting material with oxidation resistance at the operational temperature. For example Ni—Cr is suitable for operation at about 500° F. (260° C.), which works for a gas turbine combustor spring clip operating in this range. An exemplary sensor thickness is in the range of about 5-25 microns, with 5 microns being one embodiment.   3. If an electrically conductive wear coating is to be applied over the sensor, then first deposit an electrically insulating layer  77  over the sensor using an insulating material such as described in step 1. Such insulating layer  77  may be applied over the sensor without a further wear coating.   4. Optionally deposit a wear coating  70 , such as an alloy of Cr 2 C 3 —NiCr or WC—Co, or commercial products known as Stellite 6B or T800. An exemplary thickness of the wear coating is in the range of about 0.4-0.5 mm.       

     Optionally, a trench or depression may be cut into the substrate for a sensor element, then the trench bottom surface may be coated with electrical insulation, then the sensor element may be deposited on the electrical insulation, then the sensor element may be coated with electrical insulation, then the trench may be filled with a wear resistant material or with the substrate material to achieve a smooth contact surface. 
       FIG. 9  shows a conceptual graph of characteristic measurement data from the sensor  30  of  FIG. 1 . As each rung  32 A,  32 B,  32 C is successively broken by wear, a respective resistance measurement  32 AO,  32 BO,  32 CO between respective rail pairs  31 A,  31 B,  31 C jumps from low to high resistance. 
       FIG. 10  shows a conceptual graph of characteristic measurement data from the sensor  50  of  FIG. 3 . As each rung  52 A- 52 E is successively broken by wear, a respective step  52 AO- 52 EO occurs in the resistance measurement of the sensor. 
       FIG. 11  shows a conceptual graph of characteristic measurement data from the sensor  60  of  FIG. 4 . Also illustrated is an acceptable wear envelope  84  for the measurement data curve  80 . When a maximum wear limit  84  is reached, maintenance is required. The monitoring computer  40  may predict when this limit will be reached based on the slope of the measured data curve, and can thus provide a maintenance alert with a predetermined lead time. 
       FIG. 12  illustrates a sensor herein applied to a gas turbine combustor spring clip  90  having a base  91 , a spring plate  92  with a curved wear starting position  26 , an electrical insulation layer  94 , a sensor element  62 , and a sensor lead  63 . 
     The monitoring computer  42  may store a time series of actual measured data  80  from each sensor, starting from an installation or replacement time of the sensor. Engineering data may be stored in the computer to provide an acceptable time series envelope  82  for the measured data. If a sensor does not measure an expected amount of wear after a given time interval, this may indicate a failed sensor, a bad connection, a loose component, or a manufacturing inconsistency. The clock may be configured to count operating time, real time, on-off cycles, and/or thermal cycles. 
     Each sensor  30 ,  50 ,  60  may have a proximal portion  32 A,  52 A,  62 A that touches or crosses the wear starting position  26  as variously shown. Such a sensor will indicate even a slight amount of wear of the surface, which can provide early validation of the sensor and component. The computer may issue an alert if an actual measurement of the electrical characteristic over time is not within the acceptable time series envelope, indicating that the sensor is changing substantially faster or slower than expected. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.