Abstract:
Determining a distance between stationary and moving portions of a turbo machine including generating, by a fixed gap capacitive probe, a first output signal based on a characteristic of a gas in a gas flow path of the turbo machine, wherein the fixed gap capacitive probe is located in the stationary portion and is configured to sense a characteristic of a gas flowing through the gas flow path. Also, a variable gap capacitive probe, located adjacent the fixed gap capacitive probe, is used to generate a second output signal based on a distance between the variable gap capacitive probe and the moving portion, wherein the variable gap probe is configured to capacitively couple to the moving portion of the turbo machine. Afterwards, the value of the second output signal can be adjusted based on the first output signal to produce an adjusted output signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of non-contact sensors and, more particularly to a non-contact capacitive distance measurement sensor adapted for use in a turbine engine. 
     BACKGROUND OF THE INVENTION 
     A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. It is common to monitor the position of the blades relative to a flowpath wall within the turbine, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect a gap distance between the blade tips and the flowpath wall, as well as detect blade vibrations. 
     In addition, control of blade-tip clearance in the compressor and turbine sections of gas turbine engines can improve efficiency, minimize leakage flow, and shorten engine development time. Tip clearance varies throughout different operating conditions (e.g., start-up, idle, full power, shut-down) because of different radial forces and different thermal expansion coefficients and heat transfer. A real-time clearance control system can lead to turbine designs that eliminate rubbing of the housing and minimize leakage flow for maximum engine efficiency. In particular, in a turbine design that features hydraulic clearance optimization (HCO), measurement of blade tip clearance can be especially beneficial. 
     One conventional proximity sensor includes a capacitance gap sensor that has a single sensing electrode that is energized by a voltage so as to generate an electric field in the expected path of a turbine blade. The sensor is located within a cavity of the turbine casing near where a blade will pass. The blade and casing of the turbine provide a virtual ground for the electrode such that the electrode and the blade act as a capacitor. When a turbine blade passes through the generated electric field, the capacitance between the electrode and the blade changes. A magnitude of the change in the capacitance between the electrode and the virtual ground is used as an indicator of a proximity of the turbine blade to the electrode. 
     The above approach has a number of drawbacks. In particular, the ambient conditions where the sensor is located affects the magnitude of a resulting change in the sensor&#39;s capacitance. Furthermore, the conditions within a turbine, such as near the first and second row, may reach temperatures of about 2500 C or more. Operation in such an environment can degrade the performance of a conventional capacitance gap sensor such that it may fall out of calibration in a matter of days or weeks. This is especially the case in gas turbine applications where it is critical to measure blade clearances in the turbine during the whole operation cycle. The sensor should be capable of working in environments including ambient air at atmospheric pressure during engine start up, in vitiated air that is the exhaust gas from the combustor with pressures in the 20-30 bar range and temperatures in the range of 1200 C to 1500 C at base load operation, and in hot air at quickly varying pressure and temperature during engine shut down. 
     Accordingly, there is currently an unmet need for a proximity sensor, for example a turbine blade proximity sensor, which provides accurate results in a variety of environments, over a relatively long period of time without re-calibration. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a method for determining a distance between a stationary portion of a turbo machine that defines a gas flow path and a moving portion of the turbo machine within the gas flow path. The method includes generating, by a fixed gap capacitive probe, a first output signal based on a characteristic of a gas in the gas flow path, wherein the fixed gap capacitive probe is located in the stationary portion of the turbo machine and is configured to sense a characteristic of a gas flowing through the gas flow path. Also, a variable gap capacitive probe, located adjacent the fixed gap capacitive probe, is used to generate a second output signal based on a distance between the variable gap capacitive probe and the moving portion, wherein the variable gap probe is configured to capacitively couple to the moving portion of the turbo machine. Afterwards, the value of the second output signal can be adjusted based on the first output signal to produce an adjusted output signal. 
     Another aspect of the present invention relates to a proximity sensor for determining a distance between a stationary portion of a turbo machine that defines a gas flow path and a moving portion of the turbo machine within the gas flow path. The proximity sensor includes a fixed gap capacitive probe located in the stationary portion of the turbo machine and configured to sense a characteristic of a gas flowing through the gas flow path and to generate a first output signal based on the characteristic of the gas. The sensor also includes a variable gap capacitive probe located adjacent the fixed gap capacitive probe, wherein the variable gap probe is configured to capacitively couple to the moving portion of the turbo machine and to generate a second output signal based on a distance between the variable gap capacitive probe and the moving portion. A signal processor is also included that is configured to adjust a value of the second output signal based on the first output signal to produce an adjusted output signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
         FIG. 1  is a diagrammatic view illustrating a turbine and a proximity sensor probe in accordance with the principles of the present invention; 
         FIG. 2A  is a more detailed view of the sensor probe of  FIG. 1  in accordance with the principles of the present invention; 
         FIGS. 2B and 2C  depict two different top views of the sensor probe of  FIG. 1  in accordance with the principles of the present invention; 
         FIG. 3  illustrates a block level diagram of a sensor with at least two probe heads in accordance with the principles of the present invention; and 
         FIG. 4  depicts a flowchart of an exemplary process for measuring blade tip clearance in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
       FIG. 1  diagrammatically illustrates a turbine  8  including a shrouded turbine blade row  10  in which the embodiments of the present invention can be employed in a proximity sensor for a blade tip clearance monitoring system to monitor and/or measure a distance between a blade tip and a turbine&#39;s casing. While the detailed description below may provide, by way of example, a proximity sensor located in the turbine section of a turbo machine, one of ordinary skill will readily recognize that the proximity sensor can also be used in a similar fashion in a compressor section to measure a blade tip clearance between a blade tip and the compressor casing or housing. Turbine blades  14  are connected to a rotor  16  by means of a rotor disk  18 . 
     A proximity sensor probe  22  in accordance with the principles of the present invention is also shown in  FIG. 1 . In the illustrated embodiment, one exemplary probe sensor  22  is shown. However, embodiments of the present invention also contemplate a plurality of individual probe sensor structures or probes in circumferentially spaced relation to each other for monitoring the proximity of the turbine blades  14 . Similarly, respective probe sensors  22  can be located at different turbine blade row locations spaces axially along the axis of rotation  46 . 
     Referring to  FIG. 1 , the probe  22  may be located in an opening  35  of a casing or wall portion  36  of the turbine  8 . This opening  35  can be a through-opening that passes entirely through the wall portion  36  or the opening  35  may be a cavity that only partially passes through the wall portion  36 . The probe includes a sensor  38  near a tip of the turbine blades  14 . For example, the sensor  38  may be spaced such that when a turbine blade  14  is at its closest location relative the sensor  38 , they are spaced by about 1 to 10 mm. At its furthest distance, the sensor  38  may be spaced approximately 250 mm from a tip of a turbine blade  14 . 
     The mounting of the probe sensor structure  22  through the wall portion  36  of the turbine  8  may be provided in a conventional manner. As described below, a probe control module  28  produces a signal  306  ( FIG. 2A ) that is applied to the sensor  38  through a communication medium  302  (e.g., shielded coaxial cable). As the blades  14  rotate about an axis  46 , the proximity of the sensor  38  to one of the turbine blades  14  produces a change in a signal  304  produced by a probe control module  28 . The produced signal  304  can then be used to determine a distance from the sensor  38  to a nearby turbine blade  14 . This proximity information  304  can be communicated (e.g., via a communications channel) to a vibration monitoring system, a HCO system, or other monitoring systems. 
       FIG. 2A  is a more detailed view of a sensor probe  22  in accordance with the principles of the present invention. In particular,  FIG. 2A  reveals that the probe  22  includes at least two capacitance sensors located within the opening  35 . As shown, one of the capacitance sensors is a fixed gap probe  201  and another sensor is a variable gab probe  203 . 
     While one of ordinary skill will recognize that a wide variety of non-contact capacitance sensors can be used without departing from the scope of the present invention, a particular example embodiment is described so as to make possible operational details more concrete and clear. However, the example capacitance sensors and operations are not intended to limit the scope of the present invention to only this example. If two conductive surfaces are separated by a distance (i.e., a gap) and a voltage is applied to one of the surfaces, an electric field is created because of the difference in charges stored on each of the surfaces. Capacitance refers to the ability of the surfaces to hold a charge. If a constant current is applied, the capacitance change can be monitored as a linear voltage change related to the distance between the two surfaces. In particular, capacitance is calculated according to:
 
 C =[(area of the surfaces)×(gap dielectric constant)/gap distance]  EQ. 1
 
     For the fixed gap capacitive probe  201 , there are two conductive surfaces or plates  212 ,  214  spaced apart a known distance  206 . As is known in the art, there is a non-conductive frame or structure  209  that rigidly connects the two surfaces or plates  212 ,  214  to one another at a predetermined distance in a way so as to form a relatively open gap region  207  through which an ambient medium  200  can flow. In this type of probe, the area of the surfaces  212 ,  214  is fixed and the distance  206  between the surfaces  212 ,  214  is fixed. Thus, according to EQ. 1 above, if the capacitance measured between the surfaces or plates  212 ,  214  changes, then that change is due to a change in the dielectric constant of the ambient medium  200 . 
     For the variable gap probe  203 , one of the conductive surfaces or plates  216  is located near where a turbine blade tip  14  will pass. The tip  215  of the turbine blade  14  acts as a second conductive surface, or plate, to form a capacitor with a gap distance of  208 . Assuming that the area of the surface or plate  216  remains constant and the dielectric constant of the ambient medium  200  remains constant, then a capacitance measured by the variable gap probe  203  will vary based on the distance  208  between the probe and the tip  215  of the turbine blade  14 . 
     As discussed in further detail below, the two capacitance probes  201 ,  203  may transmit and receive electrical signals via a communications medium  302 . In particular, a driving signal  306  (e.g., a constant current signal) can be supplied to each of the probes  201 ,  203 . In response, the fixed gap probe  201  can provide a voltage signal  202  that varies as the capacitance between the surfaces  212 ,  214  varies; and the variable gap probe  203  can provide a voltage signal  204  that varies as its measured capacitance value varies. 
     When the turbine is in operation, the casing wall  36  defines a gas flow path and the turbine blade  14  moves within this gas flow path. The gas that flows through the gas flow path defined by the casing wall  36  is an ambient medium  200  that exists between the two plates  212 ,  214  of the fixed gap probe  201  and between the blade tip  215  and the plate  216  of the variable gap sensor  203 . To assist the ambient medium  200  that occupies the region between the conductive surface  216  and the turbine blade tip  215  to be similar in composition to that which is between the conductive surfaces  212 ,  214  of the fixed gap probe  201 , the opening  35  may be designed sufficiently large to permit passage of gases to the probe  201 , such as by providing an edge  210  that flares outwardly from a radial line. A transition point  219  between a radial section of the opening  35  and the flared portion  210  can be such that it is approximately at the same height as the top conductive surface  214  of the fixed gap probe. For example, this height can be about 10 mm. Thus, at the bottom end of the opening  35 , its inside diameter may be larger than that of the top end of the opening  35 . A distance  218  between the probes (either one) and an adjacent sidewall of the opening  35  can vary without departing from the scope of the present invention. 
     In  FIG. 2A , the conductive surface  216  of the variable gap probe  203  is shown as being located substantially aligned with an inner surface  220  of the casing wall  36 . This positioning allows a distance measure by the variable gap probe  203  to be more easily correlated to the distance between the casing  36  and the tip  215  of turbine blade  14 . However, one of ordinary skill will recognize that such alignment is not required and that other positions of the variable gap probe  203  can be accounted for through appropriate calibration. 
       FIGS. 2B and 2C  depict two different top views of the sensor probe  22  in accordance with the principles of the present invention. In  FIG. 2B , a first probe  242  may be either the fixed gap probe  201  or the variable gap probe  203  and a second probe  246  is whatever type of probe (e.g.,  201  or  203 ) that the first probe  242  is not. The travel direction  248  of the turbine blade  14  is shown so that the fixed position of the two probes  242 ,  246  are approximately aligned in that direction  248 . However, one of ordinary skill will recognize that the fixed location of second probe  246  may be shifted along a direction  240  without departing from the scope of the present invention. 
     In  FIG. 2C , a first probe  254  may be either the fixed gap probe  201  or the variable gap probe  203  and a second probe  252  is whatever type of probe (e.g.,  201  or  203 ) that the first probe  254  is not. The travel direction  248  of the turbine blade  14  is shown so that the fixed position of the two probes  252 ,  254  are approximately aligned parallel to that direction  248 . However, one of ordinary skill will recognize that the fixed location of second probe  252  may be shifted along a direction  250  without departing from the scope of the present invention. Also, a distance between the probes  252 ,  254  may vary, to the extent that they are both exposed to the same ambient medium  200 , without departing from the scope of the present invention. 
     The dashed line  244  of  FIG. 2B  shows the inside diameter of the opening  35  caused by the flare  210  which helps or effects a flow of the ambient medium  200  which occupies the gap between the variable gap probe  203  and the turbine blade tip  215  to also be present in-between the two surfaces  212 ,  214  of the fixed gap probe  201 . 
       FIG. 3  illustrates a block level diagram of a sensor with at least two probe heads in accordance with the principles of the present invention. A probe control module  28  may include circuitry  322  for producing a driver signal  306  as well as other circuitry  320  to receive and process signals. While there could be respective circuitry (not shown) to produce two different drive signals  306 , one for each of the probes  201 ,  203 , producing a single drive signal to concurrently drive both of the probes  201 ,  203  reduces the noise floor of the system. Also, having the two probes  201 ,  203  share a common grounded plate can help reduce noise as well. 
     In response to being driven, each of the probes  201 ,  203  will produce a respective response signal: “signal  1 ”  202  and “signal  2 ”  204 . The response signals  202 ,  204  indicate a respective capacitance measured, or sensed, by each of the probes  201 ,  203 . As is known in the art, a capacitance probe may have internal filters and amplifiers to convert a measured capacitance change into an output voltage signal. The output voltage signal from a capacitance probe is what the response signals  202 ,  204  are referring to. An amount of change in an output voltage signal for a given gap distance change is commonly referred to as the sensitivity of the capacitance probe. For example, if a capacitance probe is designed such that a gap distance change of 1 mm corresponds to an output voltage change of 10 volts, then the sensitivity of that probe would be (1 mm)/(10V) or 0.1 mm/V. 
     In describing  FIG. 2A , the variable gap probe  203  was initially described for simplicity purposes as measuring a capacitance that varies based on the distance between the surface  216  and the turbine blade tip  215 . In reality, that capacitance also depends on the dielectric constant of the material which comprises the gap between the probe  203  and the blade  14  (i.e., the gas, or ambient medium  200 , flowing through the gas flow path defined by a casing wall). While aspects of the present invention are applicable to either the compressor section of a turbo machine or the turbine section of a turbo machine, certain ones of those aspects are particularly beneficial within the turbine section. 
     In the turbine section, the ambient medium  200  that occupies the gap between the sensor  203  and the blade  14  changes dramatically during operation of the turbine. In a conventional capacitance sensor, distortion, known as “flame noise”, of the measured signal can therefore occur. In contrast, a capacitive sensor in accordance with the principles of the present invention can reduce or eliminate any medium related variation of a measured capacitance. The fixed gap probe  201  has a gap distance that does not change during operation. The variable gap probe  203  is co-located with, or adjacent to, the fixed gap sensor  201 . Because both probes are next to one another they are exposed to substantially the same ambient medium  200  at all times during operation of the turbine. The signal  202  output from the fixed gap probe  201  in an operating turbine will reflect variations related to changes in the ambient medium. The signal  204  output from the variable gap probe  203  is affected by both changes in the blade tip clearance and changes to the ambient medium during operation. Subtracting the signal  202  from the signal  204  will provide a difference signal  304  that reflects only changes in the blade tip clearance. 
     In  FIG. 3 , the signals  202  and  204  can be provided to a signal processor  320  (e.g., either analog or digital) of the probe control module  28  and combined in such a way as to produce the difference signal  304 . Assuming the respective sensitivity of each of the probes  201 ,  203  is the same (e.g. 1.0 mm/V), then the two signals can be easily combined to produce the difference signal  304 . For example, if an initial output voltage signal  202  from the fixed gap probe  201  is 5V and an output signal  204  from the variable gap probe  203  is calibrated to be 8V when the blade tip  215  is a known distance from the sensor  203 , then a difference signal  304  of 3V indicates the blade tip clearance is at that known, calibrated distance. If, at some later time during operation of the turbine, the output voltage signal  202  from the fixed gap probe  203  climbs to 7V and the output voltage signal  204  from the variable gap probe  203  climbs to 12V, then the difference signal  304  is calculated to be 5V. In other words, although the variable gap probe output signal  204  changed by 4V the difference signal  304  only changes by 2V; which accurately reflects the change to the output signal  204  caused by the blade tip clearance changing. By adjusting the output signal  204  based on the fixed gap probe output signal  202 , any change due to variations in the ambient medium can be accounted for. In this example, based on the hypothetical sensitivity value, the 2V change indicates that the blade tip clearance has increased by 2 mm from the known, calibrated distance. 
     A more complicated example would be if the respective sensitivities of the two probes were different. In this instance, the signal processor  320  would scale one of the signals  202 ,  204  accordingly so that they could be combined in a meaningful manner. For example, if the fixed gap probe  201  had a sensitivity of 2 mm/V and the variable gab probe  203  had a sensitivity of 1 mm/V, then scaling one of the output signals  202 ,  204  would be beneficial before combining them. In this hypothetical example, changes in the ambient medium  200  that result in the fixed gap probe  201  increasing its output signal  202  from 5V to 6V would indicate that the fixed gap probe detects changes equivalent to a gap increase of 2 mm. Concurrently, the operating conditions might cause the variable gap probe  203  to increase its output signal from 8V to 11V. 
     Because of the different sensitivities of the two probes  201 ,  203 , the 1V increase of the output signal  202  is equivalent to a 2V increase in the output signal  204 . Thus, the output signal  202  is doubled by the signal processor  320  before it is subtracted from the output signal  204 . The result is that the difference signal  304  in this example is 1V and indicates that the blade tip clearance has increased by 1 mm during operation. 
       FIG. 4  depicts a flowchart of an exemplary process for measuring blade tip clearance in accordance with the principles of the present invention. In step  402 , the fixed gap probe  201  and the variable gap probe  203  are installed within the turbine casing and calibrated to determine a baseline output signal. Conceptually, calibration could be performed during the manufacturing process as well. Ultimately, the goal is to determine the baseline difference signal  304  that corresponds to a known distance between the variable gap probe  203  and the tip  215  of the turbine blade  14 . Once calibration is completed, then in step  404  the turbine can be operated and the two probes  201 ,  203  can be electrically driven (in step  406 ) in order to produce output signals  202 ,  204 . 
     In step  408 , the respective output signals  202 ,  204  are generated using the two probes  201 ,  203  and received by the signal generator  320 . As described in the second example above, one of the output signals  202 ,  204  may optionally be scaled, in step  410 . In step  412 , the value of the output signal  204  from the variable gap probe  203  is adjusted based on the value of the output signal  202  from the fixed gap probe  201 . For example, the output signal  202  can be subtracted from the output signal  204  to produce an adjusted signal (e.g., the difference signal  304 ). In step  414 , the adjusted signal (which is indicative of blade tip clearance) is provided to external monitoring modules that may relate to HCO control, maintenance condition logging, or other similar equipment. 
     The distance monitoring/measuring process described in the flowchart of  FIG. 4  repeats in step  416  so that the distance between a casing wall and a turbine blade tip  215  can be continually assessed during operation of the turbine. Depending on the purpose for which the blade tip clearance monitoring is being performed, the rate of sampling the value of the adjusted signal can vary. For example, as shown in step  416 , the adjusted signal can be calculated and output at a rate of every 10 seconds (i.e., 0.1 Hz) or as often as every millisecond (i.e., 1000 Hz). One of ordinary skill will recognize that this rate of repetition can vary without departing from the scope of the present invention. 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.