Abstract:
A system and method for sensing the periodic position of one or more objects, such as rotating blades of a turbine. The system includes a passive eddy current sensing unit having one or more magnets and first and second cores around which first and second coils are wound, respectively, which together generate first and second magnetic fields. The sensing unit is positioned so that the object periodically passes through the first and second magnetic fields in succession, and the first and second coils consequently produce first and second output signals, respectively. Each coil is individually connected to a processing circuitry that receives each of the first and second output signals. The circuitry electronically combines the first and second output signals so that common mode signals thereof electronically subtract from each other to eliminate from output of the circuitry any electromagnetic interference noise present in the first and second output signals.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electronic equipment, and more particularly to a system that utilizes passive eddy current sensors to sense rotating equipment, such as the arrival, position, and/or vibration of turbine blades and other moving objects. 
     Passive eddy current sensors and variable reluctance sensors have been employed in a wide variety of applications to sense the proximity and speed of rotating equipment, including blades (buckets) of gas turbines. Another application is to sense the positions of rotating blades within the fan, booster, compressor and turbine sections of a gas turbine engine to monitor the health of the engine. In particular, the output of a passive eddy current sensor (or other suitable position sensor) can be used to monitor blade vibrations and steady-state blade circumferential positions over the life of the engine. Changes in blade vibrations or blade static positions can indicate damage to the component and signal that an inspection may be required to prevent a catastrophic failure of an engine component. 
     Passive eddy current sensors typically contain one or more permanent magnets adjacent one or more ferromagnetic cores wound with a wire coil. The permanent magnet is typically formed of a high magnetic energy product material, notable examples of which include iron-rare earth metal alloys (for example, Nd—Fe—B) and samarium alloys (for example, Sm—Co). The core is typically formed of a magnetic steel, though other suitable magnetic materials including low carbon steels may be used depending on operating conditions. When used to monitor the vibration of blade tips, a passive eddy current sensor is mounted to maximize the electrical signal generated as each blades passes in proximity to the sensor. In particular, the sensor is oriented so that, in the absence of a blade, magnetic flux is directed through one end of the magnet and toward the rotor and its blades, then arcs back through space to the ferromagnetic core. When a blade passes through the magnetic field, eddy currents form in the blade material and the local magnetic field shifts, producing a voltage potential across the leads of the coil. Because engine casings are typically formed largely of titanium, nickel, and other nonferrous materials that exhibit low magnetic reluctance, the ends of the magnet and core are not required to be inserted entirely through the engine casing, but instead can be mounted in an external recess in the wall such that a portion of the wall separates the sensor from the hot gas path of the engine. 
     In modern gas turbine engines, the output of a passive eddy current sensor used to monitor blade vibration is delivered to the engine&#39;s FADEC (full authority digital engine control) through appropriate connectors and wiring. Passive eddy current sensors are susceptible to electromagnetic interference (EMI) noise due to the many turns of wire typically present and required in the construction of their cores, as well the long cable runs between the sensor and the engine FADEC. U.S. Pat. No. 3,932,813 to Gallant is an example of a probe design with multiple coils capable of addressing EMI noise encountered when attempting to measure the speed of turbomachinery. The Gallant sensor has an E-shaped core whose center leg is a magnet and whose outer legs are formed of a ferromagnetic material. The center magnetic establishes a symmetrical magnetic field through the two outer legs, each of which is wound with a wire coil. The coils are connected in series with a simple wire connection, with the result that EMI and other unwanted disturbances are subtracted from the output signal of the sensor. 
     The sensor taught by Gallant is disclosed as suitable for measuring the speed of a turbomachine, and not the position and vibrations of individual blades. Evaluations of passive eddy current sensors configured in accordance with Gallant have shown that the combined resistance and inductance of the wire and coils are too great for the sensor to have sufficient bandwidth to accurately sense the position and vibrations of individual airfoils. Such sensors also suffer from output wave shape limitations. Other examples of passive eddy current sensors with wire connections between coils for the purpose or having the effect of canceling noise include U.S. Pat. No. 4,967,153 to Langley, U.S. Pat. No. 5,373,234 to Kulczyk, and U.S. Pat. No. 6,483,293 to Chen. However each of these sensor designs suffers from decreased bandwidth and waveshape variations due to the combined resistance and inductance associated with having two coils wired in series. 
     More recent passive eddy current sensor designs specifically intended for blade detection are disclosed in U.S. Pat. Nos. 6,927,567 and 7,170,284 to Roeseler et al. Each of the disclosed sensors is a single-coil probe design intended or otherwise believed to improve signal bandwidth. However, neither appears to address the issue of operating in an EMI environment, and therefore these prior sensors do not appear to be capable of producing reliable measurements in a high EMI environment. 
     In view of the above, it would be desirable if a passive eddy current sensor were available that was capable of exhibiting the EMI resistance of multi-coil probe designs, while also capable of achieving the high bandwidth capability of single-coil probe designs, thereby providing the capability of sensing the position of gas turbine blades and other moving objects. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a system and method suitable for sensing the arrival, position and/or vibration of moving objects, for example, rotating blades of a turbine. 
     According to a first aspect of the invention, the system includes a passive eddy current sensing unit comprising one or more magnets and first and second cores around which first and second coils are wound, respectively, the one or more magnets, the first and second cores, and the first and second coils cooperating to generate and detect first and second magnetic fields. The sensing unit is positioned relative to the object such that the object periodically passes through the first and second magnetic fields in succession, and the first and second coils produce first and second output signals in response to the object periodically and successively passing through the first and second magnetic fields. A wire connection is not present between the first and second coils, and instead each coil is individually connected to a processing circuitry that individually receives each of the first and second output signals. The processing circuitry electronically combines the first and second output signals to produce an output corresponding to the timing of the object as it periodically and successively passes through the first and second magnetic fields. The circuitry combines the first and second output signals so that common mode signals thereof electronically subtract from each other to eliminate from the output of the processing circuitry any electromagnetic interference noise present in the first and second output signals. By avoiding a series wire connection between the first and second coils, degradation of the bandwidth and wave shape interaction associated with a series wire connection between the first and second coils is eliminated, and the output of the processing circuitry is capable of having a clean sinusoidal waveform. 
     According to a second aspect of the invention, the method includes locating a passive eddy current sensing unit in proximity to an object such that the object periodically and successively passes through first and second magnetic fields to produce separate first and second output signals, respectively. The first and second output signals are then electronically combined to produce an output corresponding to the timing of the object as it periodically and successively passes through the first and second magnetic fields. The first and second output signals are combined so that common mode signals thereof subtract from each other to eliminate from the output any electromagnetic interference noise present in the first and second output signals. 
     According to a preferred aspect of the invention, the system and method are capable of providing gain for the first and second output signals of the first and second coil-wound cores. The gain capability enables the sensing unit to have minimal size, for example, one-fifth to one-tenth of the number of coil turns that would otherwise be required to produce a comparable signal level. The smaller size and fewer number of coil turns further increase the sensor bandwidth. 
     According to another preferred aspect of the invention, the system is well suited for use as a blade position sensor system installed on a turbine, such as a gas turbine engine, in which case the object is one of multiple rotating blades of the gas turbine and the sensing unit is located in proximity to the rotating blades. In this role, the circuitry sufficiently eliminates the effect of EMI present in the operating environment of the engine to enable the sensor unit to accurately perform the task of sensing the position of the rotating blades. 
     A significant advantage of the present invention is the ability of a passive eddy current sensor to exhibit a level of EMI resistance associated with multi-coil probe designs, while also exhibiting a high bandwidth capability associated with single-coil probe designs, along with the capability of achieving greater target sensitivity than either approach. Other advantageous aspects of the invention include the ruggedness of the circuitry, which preferably can operate and survive at temperatures exceeding 125° C. For example, the sensing unit and its circuitry are capable of being subjected to the high temperatures found in the operating environment of a gas turbine engine without requiring active cooling of the circuitry. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic showing passive eddy current sensors coupled to circuitry that electronically combines separate output signals of the sensors to produce an output in which electromagnetic interference noise present in the output signals is reduced or eliminated. 
         FIG. 2  is a representation of a prototype circuit constructed in accordance with the circuitry of  FIG. 1 . 
         FIG. 3  is a graph showing the output signals of two passive eddy current sensors connected to the circuitry of  FIG. 2 . 
         FIG. 4  schematically represents the sensors and circuitry of  FIG. 1  installed in a gas turbine environment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an electrical schematic of a sensing system  10  containing two passive eddy current sensors  12 A and  12 B coupled to analog electronic processing circuitry  14  that electronically combines separate output signals  16 A and  16 B of the sensors  12 A and  12 B, respectively. The processing circuitry  14  combines the output signals  16 A-B of the sensors  12 A-B in a manner that eliminates EMI noise from the system output  18  generated by the circuitry  14  by canceling any EMI noise present in the output signals  16 A-B of the sensors  12 A-B, as well as provides gain to the output signals  16 A-B, with the result that the system  10  is also capable of exhibiting a high bandwidth capability and greater target sensitivity. The system  10  and sensors  12 A-B will be discussed as particularly well suited for sensing the position of rotating equipment, such as blades (buckets)  20  of a gas turbine engine as represented in  FIG. 4 , though other applications are foreseeable. In such an application, after being conditioned by the circuitry  14 , the output  18  generated by the circuitry  14  and processed from the sensors  12 A-B can be delivered to the FADEC or other control system (not shown) of the gas turbine engine to improve the operation of the engine through monitoring of the positions and vibrations of the blade tips  21 , as well as the rotational speed of the blades  20 . 
     As represented in  FIG. 4 , the sensors  12 A and  12 B are combined in a sensor unit  22  that resembles the multi-coil E-shaped core configuration disclosed in U.S. Pat. No. 3,932,813 to Gallant, though it should be understood that other sensor configurations are possible and within the scope of this invention. For example, two single-coil sensors similar to those disclosed in U.S. Pat. Nos. 6,927,567 and 7,170,284 to Roeseler et al. could be used. The sensor unit  22  is shown to include a permanent magnet  24  between and adjacent a pair of ferromagnetic cores  26 A and  26 B, each of which is tightly wound with a single insulated wire coil  28 A or  28 B. The permanent magnet  24  is preferably formed of a high magnetic energy product material, such as an iron-rare earth metal alloy (for example, Nd—Fe—B) or a samarium alloy (for example, Sm—Co), and the cores  26 A and  26 B are preferably formed of a magnetic steel, though the use of other magnetic materials is within the scope of the invention. To monitor the positions and vibrations of the blade tips  21 , the passive eddy current sensor unit  22  is shown mounted to maximize the electrical signal generated as each blade  20  passes in proximity to the sensor unit  22 . In particular, the sensor unit  22  is oriented so that, in the absence of a blade  20 , magnetic flux is directed through the end of the magnet  24  and toward the rotor and its blades  20 , then arcs back through space along two separate flux paths to each of the ferromagnetic cores  26 A and  26 B. When a blade  20  successively passes through the magnetic fields  30 A and  30 B defined by the flux paths, eddy currents  32  form in the blade material and shifts occur in the local magnetic field  30 A and then  30 B, successively producing the signal outputs  16 A-B in the form of a voltage potential across the leads of each coil  28 A and  28 B. 
       FIG. 4  depicts the sensor unit  22  mounted to an engine casing  34  surrounding the blades  20 . If the engine casing  34  is formed largely of titanium, nickel, and other nonferrous materials that exhibits low magnetic reluctance, the ends of the magnet  24  and cores  26 A-B are not required to be inserted entirely through the engine casing  34 , but instead can be mounted in an external recess  36  in the casing  34  such that a portion of the casing  34  separates the sensor unit  22  from the hot gas path of the engine. Other aspects of the sensors  12 A-B, including their operation, construction and installation are known in the art and will not be discussed further. 
       FIG. 4  represents the separate output signals  16 A and  16 B of the sensors  12 A and  12 B as traveling through cables to the processing circuitry  14 , such that the circuitry  14  individually receives the separate output signals  16 A and  16 B. The circuitry  14  represented in  FIG. 1  electronically combines the output signals  16 A-B from the two coils  28 A-B of the sensors  12 A-B so that the common mode signal in both coils  28 A-B subtracts from each other, eliminating EMI noise. In  FIG. 1 , signals from the negative lead of the sensor  12 A (“Sensor A”) and positive lead of the sensor  12 B (“Sensor B”) are combined and signals from the positive lead of the sensor  12 A and negative lead of the sensor  12 B are combined to serve as inputs to an amplifier. It should be noted that the circuitry  14  and the manner in which it is connected to the sensors  12 A-B in  FIG. 1  would not be compatible with the sensor configuration of Gallant, in which the cores are wired in series, because the circuitry  14  would subtract Gallant&#39;s sensor signals and cancel their outputs. In contrast, as the blade  20  passes by the sensor unit  22  of the present invention, the resulting output signals of the sensors  12 A-B are of opposite polarity due to the direction of the magnetic fields  30 A-B through their respective coils  28 A-B. As a result, subtraction of their opposite polarity signals reinforces, instead of cancels, the blade passing signal, effectively providing gain to the signal outputs  16 A-B. In  FIG. 1 , in which the resistance values of resistors R 2  and R 4  are equal and the resistance values of resistors R 1 , R 3 , R 13  and R 12  are equal, the gain is set by the ratio of the resistor values of resistor R 2  to resistor R 1 , which in  FIG. 1  is a ratio of 100/10 to yield a gain of 10. This gain capability reduces the size requirement of the passive eddy current sensor unit  22 , for example, one-fifth to one-tenth of the number of coil turns that would be required to produce a comparable signal level. The smaller size and fewer number of coil turns also have the advantage of increasing the sensor bandwidth. 
     The four amplifiers represented in  FIG. 1  are preferably implemented with silicon-on-insulator (SOI) substrates and processing technology to permit operating temperatures of up to about 260° C. (about 500° F.). As known in the art, SOI substrates typically comprises a thin epitaxial layer on an insulator. The substrate is typically formed by oxidizing one or both bonding surfaces of a pair of semiconductor (e.g., silicon) wafers prior to bonding the wafers. Most typically, a single silicon dioxide layer is grown on an epitaxial layer formed on a silicon wafer. After bonding the wafers, all but the insulator and epitaxial layer (and optionally the silicon layer of the second wafer) are etched away, such that the silicon dioxide layer forms an insulator that electrically isolates the epitaxial layer. A commercial example of solid-state amplifiers implemented on an SOI substrate using SOI processing technology is the HT1104 monolithic quad operational amplifier commercially available from Honeywell. With such high temperature capability, the circuitry  14  can be embedded into the sensor unit  22  or a housing  38  containing the sensor unit  22  (as shown in  FIG. 4 ), preferably without the need for an active cooling system dedicated to maintaining the temperature of the circuitry  14  below 125° C. as required by conventional electronics. The term “active cooling” is used herein to mean cooling systems that are in addition to the sensors  12 A-B, the circuitry  14 , and their housing  38 , and are specifically designed to transfer heat from the circuitry  14  by conduction, convection, and/or radiation. 
     The circuit  14  may further include low pass filtering and/or a differential line driving means. In  FIG. 1 , capacitors C 5  and C 6  with amplifier U 1  provide low pass filtering to the output signals  16 A-B of the sensors  12 A-B. The values of C 5  and C 6  are set equally and are adjusted to provide additional EMI filtering beyond the inherent noise cancelling capability of the dual-coil design and common mode cancellation circuitry. The differential line driving function is implemented using amplifier U 2  and resistor R 10  and amplifier U 3  and resistors R 7 , R 8 , R 9  and R 11 . Differential line driving allows the sensor signal to be transmitted to the FADEC with greater EMI immunity. 
     In an investigation leading to the invention, a prototype circuit shown in  FIG. 2  was constructed using the Honeywell HT1104 amplifier. Operationally, the prototype circuit was essentially identical to the circuitry  14  schematically represented in  FIG. 1 . Two identical passive eddy current sensors (not shown) were connected to the circuit and placed next to a source of EMI. The output of the circuit was then monitored while driving a load to simulate connection to a FADEC. The output of the circuit is shown in  FIG. 3 , and evidences that the common mode magnetic EMI noise was canceled out by the circuit. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the electrical values of the components indicated in  FIG. 1  are for reference purposes only, and are not to be interpreted as limiting the scope of the invention, the physical configuration of the sensors  12 A-B and circuitry  14  could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.