Abstract:
A technique for operating a sensor system is provided. The method includes exciting a first sensor with a first excitation signal at a first frequency and exciting a second sensor with a second excitation signal at a second frequency. The technique also includes combining a first measurement signal generated from the first sensor with a second measurement signal generated from the second sensor to determine a sensed parameter. The technique may be employed for reducing crosstalk between closely proximate sensors, such as capacitive probes, and may serve to determine distances within operating machines, such as turbine systems.

Description:
BACKGROUND 
   The invention relates generally to sensor systems and, more particularly, to sensor systems that are configured to reduce crosstalk between sensors in close proximity to provide an accurate measurement of distance between two objects. 
   Various types of sensors have been used to measure the distance between objects. In addition, such sensors have been used in various applications. For example, in turbine systems, the clearance between a static shroud and turbine blades is greatest when the turbine is cold, and gradually decreases as the turbine heats up and as it spins up to speed. It is desirable that a gap or clearance between the turbine blades and the shroud be maintained for safe and effective operation of the turbine. A sensor may be disposed within the turbine to measure the distance between the turbine blades and the shroud. The distance may be used to direct movement of the shroud to maintain the desired displacement between the shroud and the turbine blades. 
   Typically, two or more sensors are employed in close proximity for accurate measurement of clearance between two objects. In certain applications, capacitance probes are employed to measure the distance between two objects. The clearance measurements by these probes are affected by certain parameters such as temperature, size of the target object, crosstalk between the sensors and so forth. For accurate clearance measurements from sensors, it is desirable to reduce the crosstalk between the sensors that are positioned in a close proximity. In conventional sensor systems, a shielding mechanism is provided to reduce the crosstalk between the sensors. However, such shielding mechanisms limit the sensor dimensions and require significant design effort to achieve an optimum size of the sensor with a required shielding mechanism. 
   Moreover, in certain applications such as gas turbines, such sensor systems are typically employed to measure clearances between parts while offline testing. In such applications, it is not desirable to employ sensor systems for accurate measurements of clearance in parts during in service due to the effect of crosstalk between sensors in close proximity. Further, as the separation between the sensors decreases, the effect of crosstalk between the sensors may require design modifications and frequent calibration of the sensor systems to reduce the effect of crosstalk in the sensors and provide an accurate measurement. 
   Accordingly, there is a need to provide a sensor system that would reduce the crosstalk between sensors located in close proximity and provide an accurate measurement of the clearance between two objects. It would be also advantageous to provide a self-calibrating sensor system that could be employed for accurate clearance measurement for parts in operation for a long period of time without a need of periodic calibration. 
   BRIEF DESCRIPTION 
   Briefly, according to one embodiment a method of operating a sensor system is provided. The method includes exciting a first sensor with a first excitation signal at a first frequency, and exciting a second sensor with a second excitation signal at a second frequency. The second frequency is different than the first frequency. The method also includes combining a first measurement signal from the first sensor with a second measurement signal from the second sensor to determine a sensed parameter. 
   In another embodiment, a sensor system is provided. The sensor system includes at least two sensors, each sensor being configured to receive a respective one of a plurality of excitation signals. Each of the excitation signals has a respective frequency, with frequencies of the excitation signals being different from one another. Each of the sensors is further configured to provide a reflected signal. A combiner is configured to combine a plurality of measurement signals from respective ones of the sensors to determine a sensed parameter 

   
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  is a diagrammatical representation of a sensor system, in accordance with an exemplary embodiment of the present technique; 
       FIG. 2  is a diagrammatical representation of a sensor system in accordance with another exemplary embodiment of the present technique; 
       FIG. 3  is a diagrammatical representation of a sensor system, in accordance with yet another exemplary embodiment of the present technique; 
       FIG. 4  is a flow chart illustrating a method of operating the sensor system of  FIG. 1 ; and 
       FIG. 5  is a flow chart illustrating a method of operating the sensor system of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a sensor system  10  for measuring a clearance between two objects, such as, the clearance between a turbine blade and a shroud in a turbine of an aircraft engine. In the illustrated embodiment, the sensor system  10  includes a first sensor  12  and a second sensor  14  disposed on one of a first object  16  or a second object  18 . In the illustrated embodiment, the first and second sensors  12  and  14  are both disposed on the first object  16 . However, the first and second sensors  12  and  14  may be disposed on the second object  18 . It should be appreciated by one skilled in the art that depending upon an application and a physical configuration the term “disposed on” as described here may include secured to, mounted on, attaching with or otherwise associated with the first or second objects  16  and  18 . In this embodiment, the first and second sensors  12  and  14  are capacitive probes that are disposed in a close proximity to provide accurate capacitive measurements. It should be noted that, as used herein, the term “close proximity” refers to a separation between the first and second sensors  12  and  14  that is substantially equal to the size of the sensor tips for the first and second sensors  12  and  14 . 
   In one embodiment, the first and second objects comprise a moving object and a static object respectively. In this embodiment, the first sensor  12  and the second sensor  14  are disposed on the static object and are configured to measure the clearance between the static object and the moving object at different points in time. 
   The sensor system  10  of  FIG. 1  includes a first signal generator  20  configured to excite the first sensor  12  at a first excitation frequency. In addition, the sensor system  10  includes a second signal generator  22  configured to excite the second sensor  14  at a second excitation frequency different from the first frequency. Further, amplifiers  24  and  26  may be coupled to the first and second signal generators  20  and  22  respectively to amplify input signals generated from the first and second signal generators  20  and  22 . A capacitor  28  and a phase detection module  30  may be coupled to the first sensor  12  for measuring the capacitance through the first sensor  12 . Similarly, a capacitor  32  and a phase detection module  34  may be coupled to the second sensor  14  for measuring a capacitance through the second sensor  14 . 
   In operation, the first and second sensors  12  and  14  are excited by the first and second signal generators  20  and  22 , respectively. In this embodiment, the first and second excitation frequencies of the signals generated from the first and second signal generators  20  and  22  are harmonically unrelated. For example, if the first sensor  12  is excited at a first excitation frequency, the second sensor  14  would be excited at a second excitation frequency that is not a whole number multiple of the first excitation frequency. The first excitation frequency results in a first measurement signal from the first sensor  12 . Similarly the second excitation frequency results in a second measurement signal from the second sensor  14 . The first and second measurement signals are representative of first and second sensed parameters respectively. In this embodiment, the first and second sensed parameters are capacitances measured across the capacitors  28  and  32 , respectively. 
   The capacitance between two objects is a function of the overlap surface area (A) and the separation (S) between the two objects. The capacitance between two parallel plates is given by the following equation:
 
 C=εA/S   (1)
         where C is the capacitance;   ε is the permittivity of a medium between the parallel plates;   A is the overlap area between the parallel plates; and   S is separation of the parallel plates.       

   By sensing the capacitance (C), the sensors  12  and  14  enable the separation (S) between the probes  12  and  14  and the object  18  to be established. By manipulating equation (1) above, the following equation relates the separation (S) to the capacitance (C).
 
 S=εA/C   (2)
 
   The first measurement signal from the first sensor  12  may include a noise component from the second sensor  14  due to crosstalk between the first and second sensors  12  and  14 . Similarly, the second measurement signal from the second sensor  14  may also include a noise component from the first sensor  12 . The noise components in the first and second measurement signals may be separated through a synchronous measurement scheme, as will be described in detail below. 
   In a present embodiment, the phase detection module  30  is configured to detect a first reflected signal by using a synchronous measurement scheme based upon the first excitation frequency to generate the first measurement signal. In one embodiment, the synchronous measurement scheme includes performing multiple phase measurements to determine a phase between each of the reflected signals and the respective excitation signals. Further, each of the phases may be filtered through a filter  36  to filter any signal noise generated by the crosstalk between the first sensor  12  and the second sensor  14 . In one embodiment, the filtering is performed by averaging the measured phases. However, other techniques may be employed for the filtering of the first measured signal. Similarly, the phase detection module  34  is configured to detect a second reflected signal by using the synchronous measurement scheme based upon the second excitation frequency. Again, multiple phase measurements may be performed through the phase detection module  34  and the phases may be filtered through a second filter  38 . In one embodiment, a single phase detection module and a single filter are employed for generating the first and second measurement signals. In one embodiment, the sensor system  10  includes at least two coaxial cables that are configured to convey a respective one of excitation signals to the respective one of sensors  12  and  14 . In another embodiment, the sensor system  10  may include at least two waveguides that are configured to convey a respective one of excitation signals to the respective one of sensors  12  and  14 . 
   The first and second measurement signals from the first and second sensors  12  and  14  are combined through a combiner  40  to determine a ratiometric capacitance between the first and second objects  16  and  18 . In one embodiment, the first and second measurement signals are compared through a comparator to determine the ratiometric capacitance between the first and second objects. It should be noted that the ratiometric capacitance provides a substantially accurate crosstalk-minimized capacitance measurement between the first and second objects  16  and  18 . Further, such capacitance is used to determine the separation between the first and second objects  16  and  18  that may be used to control the clearance between the first and second objects  16  and  18  via a clearance control system  42 . As described above, the sensor system  10  employs two sensors  12  and  14  for capacitive measurements between the objects  16  and  18 . However, other configurations of the sensor system  10  having more sensors may be envisaged. 
     FIG. 2  illustrates another exemplary sensor system  44  according to another embodiment, for measuring a clearance between the first and second objects  16  and  18 . In this embodiment, the sensor system  44  employs a switched excitation technique for measuring the capacitance between the first and second objects  16  and  18  that is representative of the separation between the two objects  16  and  18 . In the illustrated embodiment, the first and second sensors  12  and  14  are disposed on one of the first or second objects  16  and  18 . In this embodiment, the first and second sensors  12  and  14  are disposed on the first object  16 . In the illustrated embodiment, the first and second sensors  12  and  14  are capacitive probes that are excited by a signal generator  46  at different points in time through switching the excitation signals between the first and second sensors  12  and  14  at a pre-determined switching interval. In this embodiment, the switching may be performed for every sample or every measurement period at a speed of at least one megahertz. In other applications sampling frequency and sampling scheme may vary. An amplifier  48  may be coupled to the signal generator  46  to boost the excitation signals for the first and second sensors  12  and  14 . 
   In a presently contemplated configuration, a switch  50  is employed to perform the switching of excitation signals between the first and second sensors  12  and  14 . Examples of the switch  50  include a radio frequency micro electromechanical system (RF MEMS) switch, a solid-state switch and so forth. In one embodiment, the switch  50  is in a first position that enables the signal generator  46  to provide a first excitation signal to the first sensor  12 . In another embodiment, the switch  50  is in a second position that enables the signal generator  46  to provide a second excitation signal to the second sensor  14 . The sensor system  44  includes a capacitor  52  and a measurement device  54  coupled to the first sensor  12  to measure capacitance through the first sensor  12 . Similarly, a capacitor  56  and a measurement device  58  may be coupled to the second sensor  14  to measure the capacitance between the first and second objects  16  and  18  through the second sensor  14 . 
   In operation, the first sensor  12  is excited with a first excitation signal. In this embodiment, the second sensor  14  is held at a pre-determined voltage to minimize the interference and noise due to the second sensor  14 . Further, the second sensor  14  may act as a return path and as a shield for sensor  12  to shield the sensor  12  from noise and interference. In one embodiment, the second sensor  14  is held at ground while exciting the first sensor  12 . The excitation of the first sensor  12  generates a first measurement signal that is measured by the measurement device  54 . In one embodiment, the measurement device  54  may include a phase detection module that is configured to measure a phase between a reflected signal and a respective excitation signal for the first sensor  12 . However, other measurement devices may be employed to measure the capacitance from the first sensor  12  to generate the first measurement signal. 
   Following the detection of the first measurement signal, the second sensor  14  is excited with a second excitation signal through switching the excitation signal from the first sensor  12  to the second sensor  14  via the switch  50 . In one embodiment, the second excitation signal has a similar frequency as that of the first excitation signal. Again, a second measurement signal is generated from the second sensor through the measurement device  58  as described above. Further, the first sensor  12  may be held at a pre-determined voltage or at ground during excitation of the second sensor  14 . The first and second measurement signals from the first and second sensors  12  and  14  are then combined through the combiner  40  to determine the capacitance between the first and second objects  16  and  18 . The measured capacitance may be used to estimate the clearance between the first and second objects  16  and  18  that may be used to control the clearance between the first and second objects  16  and  18  via the clearance control system  42 . In the illustrated embodiment, the sensor system  44  includes a single signal generator  46  and a single switch  50  for switching the excitations signals between the first and second sensors  12  and  14 . However, other configurations of the sensor system  44  with different number of sensors, switches or signal generators may be envisaged. 
     FIG. 3  illustrates an exemplary sensor system  60  for measuring clearance between two objects that employs a combination of multiple excitation and switched excitation techniques as described above with reference to  FIG. 1  and  FIG. 2 . The sensor system  60  includes four sensors that are represented by reference numerals  62 ,  64 ,  66  and  68 . Further, signal generators  70  and  72  are coupled to the sensors  62 ,  64 ,  66  and  68  to provide input excitation signals to the sensors  62 ,  64 ,  66  and  68 . In addition, the input excitation signals from the signal generators  70  and  72  may be amplified through amplifiers  74  and  76 , respectively. The excitation signals from the signal generators  70  and  72  may be switched between the sensors  62 ,  64 ,  66  and  68  through switches  78  and  80 . In operation, when switch  78  and switch  80  are in a first position the excitation signals are provided to the sensors  62  and  66 . In this embodiment, the sensors  62  and  66  are excited at different frequencies through the signal generators  70  and  72 . Alternatively, when the switch  78  and the switch  80  are in a second position the excitation signals are provided to the sensors  64  and  68 . Again, the sensors  64  and  68  are excited at different frequencies through the signal generators  70  and  72 . 
   A capacitor  82  and a measurement device  84  are coupled to the sensor  62  for measuring the capacitance between the two objects. Similarly, a capacitor  86  and a measurement device  88  are coupled to the sensor  64 . In addition, capacitors and measurement devices are coupled to the sensors  66  and  68  as represented by the reference numerals  90 ,  92 ,  94  and  96 . In one embodiment, amplifiers  98 ,  100 ,  102 , and  104  may be coupled to the sensors  62 ,  64 ,  66  and  68  respectively for amplifying the signals from the signal generators  70  and  72 . The measurement signals from the sensors  62 ,  64 ,  66  and  68  are processed to determine the clearance between the two objects. In this embodiment, the combined excitation technique enables the measurement of the capacitance via four sensors  62 ,  64 ,  66  and  68  by using only two frequencies applied at two different points in time. The technique is particularly advantageous for measurements of capacitance in applications that require a large number of sensors to cover a large area on a component. 
   Referring generally to  FIG. 4 , an exemplary method  106  of operating the sensor system  10  of  FIG. 1  is illustrated. Initially, a first sensor is excited with a first excitation signal at a first excitation frequency, as represented by step  108 . The first excitation signal may be an alternating current (AC) based signal. The first sensor provides a first measurement signal representative of a capacitance between two objects. Next, at step  110  a second sensor is excited with a second excitation signal to provide a second signal representative of the capacitance between two objects. The second sensor is excited at an excitation frequency that is different than the first excitation frequency and is harmonically unrelated to the first excitation frequency. The first and second measurement signals may be detected via a synchronous measurement scheme to eliminate any effects of crosstalk between the first and second sensors. At step  112  the first measurement signal from the first sensor is combined with the second measurement signal from the second sensor. Subsequently, the separation (S) between the sensor and the external object is established based upon the capacitance (C) sensed by the sensor, as represented by step  114 . 
     FIG. 5  illustrates an exemplary method  116  of operating the sensor system  44  of  FIG. 2 . In the illustrated method, separation (S) and/or capacitance (C) are established by employing a switched excitation technique. Initially, a first sensor is excited with a first excitation signal to provide a first measurement signal, as represented by step  118 . Next, at step  120  the excitation is switched from the first sensor to the second sensor. At step  122 , a second sensor is excited at a second excitation signal to provide a second measurement signal. Further, at step  124  the first measurement signal is combined with the second measurement signal, each signal being representative of the sensed capacitance between the sensor and an external object. At step  126  the separation (S) between the sensor and the external object is established based upon the capacitance (C) sensed by the sensor. 
   The various aspects of the method described hereinabove have utility in different applications. For example, the technique illustrated above may be used for measuring the clearance between rotating and static components in an aircraft engine. The technique may also be used in certain other applications for example for measuring clearance between objects in gas turbines, steam turbines and so forth. As noted above, even more generally, the method described here may be advantageous for providing accurate measurement of clearance between objects through sensors by reducing the crosstalk between multiple sensors. Further, the technique is particularly advantageous to provide a self-calibrating sensor system for accurate clearance measurement of parts, even in operation and over extended periods of time, enabling better clearance control in parts while in operation. 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.