Patent Application
20100095740
The present invention relates generally to measuring the physical properties of objects or fluids and, more particularly, to methods and sensors for determining the physical properties of objects or fluids in multi-path clutter environments.
It is often necessary to measure physical properties such as temperature, strain, pressure, etc. using a wireless system. In some cases, there are a large number of multiple reflections (multi-path signal propagation environment) of the radio signals along the propagation path so that the signal to or from the sensor will be corrupted and/or modulated by the multi-path environment. When there is a number of varying coherent signal reflections along the propagation path, the result is multi-path induced variations in the phase, amplitude, and/or time domain character of the signal. This situation presents a very serious problem for sensor system design. Also, practical constraints on sensor placement, weight, size, temperature, and lifetime requirements present problems to engineers in the design of very small and light weight sensors that can operate wirelessly without a source of power.
For the purposes of describing and defining the present invention, it is noted that the term “multi-path clutter environment” refers to an environment in which electromagnetic waves are transmitted and received in the presence of reflecting structures. These reflecting structures may be stationary or may be moving within or through the environment. The reflecting structures are capable of reflecting the electromagnetic waves such that an electromagnetic wave sent through this environment may be reflected off one or more reflecting structures before reaching its intended destination. As a result of these reflections, the amplitude, phase, and/or time delay of a transmitted electromagnetic wave may be altered by the time it reaches its intended destination. Furthermore, since the reflecting structures may be moving within or through the environment, the particular reflections experienced by individual electromagnetic waves may vary in an unpredictable manner.
For the purposes of describing and defining the present invention, it is noted that the term “radio frequency signal” is an electromagnetic wave having a frequency of 1 Megahertz or higher. The term “radio frequency” may be abbreviated as “RF.”
For the purposes of describing and defining the present invention, it is noted that the term “ringdown” is utilized herein to refer to the process of the energy decay of an RF signal in a multi-path clutter environment. Similarly, “multi-path ringdown time” refers to the time required for the multiple reflections of a RF signal in a multi-path environment to decay to a low enough value to be statistically or empirically insignificant. For example, the multi-path ringdown time may be defined as the amount of time necessary for the reflected RF signals to decay to an amplitude of 10% or less of the original RF interrogation signal. Other means of defining this time may be used, as is known in the art.
For the purposes of describing and defining the present invention, the term “wireless sensor” refers generally to a sensor which is capable of measuring a physical property of an object or fluid and transmitting information relating to that measurement to another device without the use of conductors or “wires.” The wireless sensor itself, however, may have conductors or wires which are used to internally connect the individual components of the sensor. Furthermore, the wireless sensor may lack an internal power source, such as a battery. Instead, the energy necessary to perform a measurement may be wirelessly transmitted to the wireless sensor via electromagnetic waves (e.g., RF signals).
According to one embodiment of the invention, a method for determining an unknown physical property of an object or a fluid in a multi-path clutter environment comprises transmitting an RF interrogation signal to a wireless sensor physically coupled to the object or the fluid in the multi-path clutter environment, wherein the wireless sensor is operable to receive the RF interrogation signal, produce a reference signal and a measurement signal, and transmit the reference signal and the measurement signal in the multi-path clutter environment, wherein the reference signal and the measurement signal are derived from the RF interrogation signal, the reference signal is delayed by a first time delay, the measurement signal is delayed by a second time delay that is a function of the unknown physical property, and the first and second time delays are associated by a known relationship defined by the wireless sensor. The method may further comprise receiving the transmitted reference signal and the transmitted measurement signal, comparing the transmitted reference signal and the transmitted measurement signal in the time domain and, using this comparison, determining the unknown physical property of the object or the fluid.
In another embodiment, a system for determining an unknown physical property of an object or a fluid in a multi-path clutter environment comprises a transponder, a wireless sensor, and a signal processing unit. The transponder is operable to transmit a wireless RF interrogation signal to the wireless sensor in the multi-path clutter environment and receive wireless signals transmitted by the wireless sensor in the multi-path clutter environment. The wireless sensor is operable to receive the RF interrogation signal transmitted by the transponder, produce a reference signal and a measurement signal, and transmit the reference signal and the measurement signal in the multi-path clutter environment. The reference signal and measurement signal are derived from the RF interrogation signal. The reference signal is delayed by a first time delay, and the measurement signal is delayed by a second time delay that is a function of the unknown physical property. The signal processing unit is electrically coupled to the transponder and is operable to compare the reference signal and the measurement signal in the time domain and, using this comparison, determine the unknown physical property of the object or the fluid.
The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
As shown in
Continuing to refer to
The wireless sensor 40, which may be moving very rapidly relative to the transponder 20 (i.e., with peak velocities exceeding 1,000 feet per second), is operable to receive the RF interrogation signal 50 transmitted by the transponder 20, produce a reference signal and a measurement signal, and transmit the reference signal and the measurement signal to the transponder in the multi-path clutter environment. In
The wireless sensor 40 is operable to produce the reference signal and the measurement signal, both of which are derived from the RF interrogation signal 50. As will be described in detail below, the reference signal is delayed by a first time delay that is optionally a function of the physical property to be determined, and the measurement signal is delayed by a second time delay that is a function of the physical property to be determined. Since the system uses intrinsic time delay, the response from the sensor occurs after the multi-path ringdown of the RF interrogation signal is finished. The use of two delayed reflections from the SAW device permits the induced variations in the propagation environment to be cancelled, and the corresponding physical property to be derived. The time delays of the two reflected signals are detected by the transponder 20 and measured at the signal processing unit 30. This measurement may then be used to estimate the value of the physical property.
The signal processing unit 30 may be electrically coupled to the transponder 20 and is operable to compare the reference signal and the measurement signal in the time domain and, using this comparison, determine the physical property of the object. In one embodiment, the comparison may include measuring the time difference between the reference signal and the measurement signal. In another embodiment, the comparison may involve taking the ratio of the time delay of each signal. Other methods of making the comparison in the time domain may be known to those skilled in the art.
Referring again to
In wireless sensors having two or more SAW devices, each SAW device may be operable to measure either the same or a peripheral physical property. For example, a wireless sensor may have two SAW devices, each of which may be operable to measure the same physical property, such at temperature. In this case, the two temperature measurements may allow the overall system to determine the temperature gradient of the object or fluid. Furthermore, each of the SAW devices may measure a different physical property, in which case a single wireless sensor may be operable to measure, for example, temperature (e.g., a physical property) and pressure (e.g., a peripheral physical property). Other configurations which employ multiple SAW devices are contemplated.
As discussed above, because the acoustic wave is much slower than an electromagnetic wave in space, both reflected reference waves 104, 106 and both measurement waves 114, 116 are delayed by an amount of time equal to the travel time of the waves through the piezoelectric crystal structure. Furthermore, since the reference reflector 102 and the measurement reflector 112 are located at different distances from the interdigital structure 100, the amount of time required for the reference wave to travel is different than the amount of time required for the measurement wave to travel. As a result, when the waves are converted back to electromagnetic signals by the interdigital structure and transmitted by the patch antenna, the transmitted reference signal and the transmitted measurement are separated in the time domain. These two delays may be associated by a known relationship which is defined by the SAW device.
The physical property (e.g., strain or temperature) of the object or fluid to be measured may induce a variation in the SAW device that results in small changes in the time delay of the waves reflected back from the SAW device. As an illustrative example, an increase in the temperature of the SAW device (corresponding to an increase in temperature of the object or fluid being measured) may cause the propagation time of the reference waves and/or measurement waves in the SAW device to either increase or decrease. As a result, the reference signal and the measurement signal transmitted by the patch antenna of the wireless sensor will also change a corresponding amount in the time domain. This change can be captured by the transponder and measured by the signal processing unit, thus determining the temperature of the object or fluid.
Although
Similarly, a single SAW device with multiple reference/measurement reflectors may be operable to measure two or more different physical properties. For example, if a SAW device has two pairs of reference/measurement reflectors, one of the two pairs may be operable to measure temperature, while the other pair may be operable to measure mechanical strain. Thus, the temperature and strain of a turbine blade may be measured by a single wireless sensor. Furthermore, this type of sensor may be operable to measure physical properties from two or more objects or fluids. In another example, a SAW device having two pairs of reference/measurement reflectors may be mounted to a turbine blade in which one reference/measurement pair operates to measure the mechanical strain on the turbine blade itself, while the other reference/measurement pair operates to measure the oxygen content of the gas flowing by the turbine blade. Other combinations of physical properties may be measures as is known in the art.
In
In yet another embodiment, the SAW device may produce (in addition to the one or more reference/measure pairs) an identification signal for identifying the wireless sensor. The identification signal may be derived from the RF interrogation signal and may be delayed by a third time delay. The measuring system may receive the identification signal and compare it in the time domain to the reference signal. Based on this comparison, the measuring system may be able to identify the sensor. Employing an identification signal may be used when multiple wireless sensors are present in the same multi-path clutter environment and may help the measuring system determine which wireless sensor is responding to the RF interrogation signal.
With regard to the SAW device, the identification signal may be produced by placing one or more identification reflectors on the SAW device. These may be placed at a location on the SAW device such that the reference/measurement pair and the identification signal do not interfere with each other in the time domain. The identification reflectors may be placed such that the identification signal comprises a series of RF pulses which are spaced apart at a known time interval and may represent binary digits. For example, the SAW device may have four identification reflectors which produce an identification signal having four RF pulses which are spaced apart at 10 nanoseconds. The measuring system may interpret these four RF pulses as the binary number “1111,” which is the number 15 in decimal. In another example, the SAW device from the previous example may lack the third identification reflector. Thus, this arrangement may produce three RF pulses, the first two of which are spaced at 10 nanoseconds, and the last two of which are spaced at 20 nanoseconds. The measuring system may interpret these three RF pulses as the binary number 1101, which is the number 13 in decimal. In doing so, the measuring system may interpret the absence of an RF signal 10 nanoseconds after the second RF pulse as a binary zero. Many variations of producing and measuring an identification signal may be used, as is known in the art.
Because the RF interrogation signal is transmitted in a multi-path clutter environment, the RF interrogation signal may reflect off the reflecting structures. Many of these reflected signals may be returned to the transponder without having reached the wireless sensor. These reflected signals eventually decay since some energy of the signal is lost at each point of reflection. As previously discussed herein, this process is called multi-path ringdown. As an illustrative example, a 2.5 GHz interrogation signal may require approximately 15 nanoseconds to decay when the measuring system is disposed in an operating jet aircraft engine.
During the multi-path ringdown, many of these reflected RF interrogation signals may be received by the transponder. As a result, during this time, it may be more difficult for the transponder to distinguish between the decaying RF interrogation signals and the reference and measurement signals. Consequently, the wireless sensor may be designed such that the time delay introduced into the reference signal and the measurement signal may be longer than the multi-path ringdown time. In such a case, the multi-path ringdown will not interfere with the reception of the reference signal or the measurement signal.
As previously indicated, the use of two different time delays on the same SAW device permits a reference signal and a measurement signal to be transmitted to the transponder. The amount of time between the reference signal and measurement signal may be called the sensor delay. Since the sensor delay may be very short (e.g., 100 nanoseconds or less), the geometry of the propagation environment may not change significantly during that time, even in the case where the reflecting structures are moving very rapidly, such as in the compressor or turbine stage of a jet engine. Thus the sensor delay may not be affected by the multi-clutter propagation environment. The delays in the reference and measurement signals (with respect to the interrogation signal) overcome the multi-path ringdown problems of spurious reflection signals overlapping the desired data signals, and the sensor delay overcomes the problem of induced modulation of the propagating signals by the changing propagation environment due to the reflecting structures.
Referring now to
Although the signals (i.e., 120, 122, 124) shown in
Referring back to
In another embodiment, the signal processing unit 30 receives and amplifies the reference and measurement signals. The amplified signals are input to a microwave quadrature mixer. High speed switches may be used to create the RF interrogation signal and prevent it from interfering with the received signals. A microwave splitter is used to provide a reference (or local oscillator) signal to the quadrature mixer. The outputs of the quadrature mixer can be filtered, for example, with a low pass filter with a cutoff frequency of 20 MHz or less. The in-phase and the quadrature phase signals are fed to the analog-to-digital converter, which converts these signals into a digital format capable of being further processed by a computer (not shown).
These are only two exemplary embodiments of the signal processing unit. Many other variations of the signal processing unit are contemplated, as may be known to those skilled in the art.
It is contemplated that, in one embodiment, the wireless sensor may be physically mounted to the object whose physical property is being measured. For example, it may be desirable to measure a physical property of the blade of a compressor or turbine in a jet engine. The physical property measured may be the stress of the blade or the temperature of the blade. Accordingly, the wireless sensor may be mounted to the blade, using any one of a variety of mounting means, including but not limited to conventional fasteners (e.g., screws or bolts) and adhesives. Because the wireless sensor is mounted to the blade, it also may be physically coupled to the blade such that the sensor is capable of measuring a physical property of the blade.
In another embodiment, the wireless sensor may be physically mounted to one object while measuring a physical property of another object or a fluid. For example, it may be desirable to measure a physical property of the gas flowing by a blade of a compressor or turbine in a jet engine. The physical property measured may be the temperature of the gas or the chemical composition of the gas. Accordingly, the wireless sensor may be mounted to the blade as previously discussed herein. However, the location of the wireless sensor on the blade may permit the sensor to be physically coupled (i.e., fluidly coupled) to the gas flowing by the blade. As a result, the sensor may be capable of measuring the appropriate physical property of the gas. Other physical properties may be measured as can be gleaned from the technical literature or yet-to-be-discovered technology.
In still another embodiment, the wireless sensor may be mounted to an object, such as a structural member, inside a tank or a pipe. The tank or pipe may contain a fluid which is physically coupled to the wireless sensor. Accordingly, the wireless sensor may be operable to measure a physical property of the fluid in the tank or pipe. For example, the wireless sensor may be operable to measure the temperature of the fluid inside the tank or pipe. As another example, the wireless sensor may be operable to measure a chemical composition of the fluid. Other physical properties of the fluid may be also measured, as is known in the art.
In yet another embodiment, the wireless sensor may simply “float” in a fluid whose physical property is being measured. That is, one or more wireless sensors may be added to the fluid and may remain suspended in the fluid while operating. In this embodiment, the sensors are not physically mounted to anything, and their location within the fluid may depend on a number of factors, including but not limited to the weight of the sensor, whether the fluid is moving, and the specific gravity of the fluid. For example, one or more wireless sensors may be added to a fluid located in a holding tank and may sense the temperature of the fluid. As such, the sensors may float randomly within the tank. A filter, located at the outlet of the tank, may prevent the sensors from leaving the tank when the fluid is removed.
When sensing the physical property of a fluid (e.g., a gas or a liquid), the wireless sensor may be operable to measure a number of physical properties, including but not limited to a temperature of the fluid, a velocity of the fluid, a flow direction of the fluid, a pressure of the fluid, a nuclear characteristic of the fluid, an acoustic characteristic of the fluid, a chemical composition of the fluid, a pH factor of the fluid, a dielectric property of the fluid, a density of the fluid, a particle distribution of the fluid, a magnetic property of the fluid, an electrical property of the fluid, or a light absorption property of the fluid. When the wireless sensor is measuring the physical property of a gas, it may additionally determine a species of the gas or a humidity of the gas. When the wireless sensor is measuring the physical property of a liquid, it may additionally determine the viscosity of the liquid. Other physical properties of fluids (both gases and liquids) may be measured as well, as is known in the art. For example, a wireless sensor may be mounted to a blade of a compressor or turbine in a jet engine, and the blade is positioned in a gas flowing through the compressor or turbine. In this case, the comparison of the transmitted reference signal and the transmitted measurement signal (from the wireless sensor) may be used to determine an unknown physical property of the gas. In another example, a wireless sensor may be mounted to an object inside a tank or a pipe containing the fluid. In this case, the comparison of the transmitted reference signal and the transmitted measurement signal (from the wireless sensor) may be used to determine the unknown physical property of the fluid.
When sensing the physical property of an object, the wireless sensor may be operable to measure a number of physical properties, including but not limited to a temperature of the object, a stress or a strain of the object, a torque characteristic of the object, an acoustic characteristic of the object, a chemical composition of the object, a nuclear characteristic of the object, a dielectric property of the object, a magnetic property of the object, an electrical property of the object, or a light absorption property of the object. For example, the wireless sensor may be mounted to a blade of a compressor or turbine in a jet engine; a part of a helicopter blade mechanism; a shaft of a helicopter main rotor or tail rotor; a gear, a gear tooth, or a gear carrier in a transmission; or a rotating or translating machinery or a link in a kinematic mechanism. When mounted as such, the comparison of the transmitted reference signal and the transmitted measurement signal (from the wireless sensor) may be used to determine an unknown physical property of the object to which the wireless sensor is mounted, such as mechanical stress or temperature.
In still another embodiment, the wireless sensor may be mounted to an object on which the wireless sensor is operable to measure the thickness of a material placed on the object or accumulated on the object. For example, the wireless sensor may be operable to measure the thickness of ice, rust, a biological material, or a protective coating which accumulates or is placed on the object.
For the purposes of describing and defining the present invention, it is noted that reference herein to a variable being a “function” of a parameter or another variable is not intended to denote that the variable is exclusively a function of the listed parameter or variable. Rather, reference herein to a variable that is a “function” of a listed parameter is intended to be open ended such that the variable may be a function of a single parameter or a plurality of parameters.
It is further noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present invention or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.
For the purposes of describing and defining the present invention it is noted that the term “approximately” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. For example, it is stated that the multi-path ringdown time may be approximately 15 nanoseconds for a 2.5 GHz interrogation pulse. Because the ringdown may be exponential and may depend on the location and number of reflecting structures (some of which may be moving), the point in time at which the ringdown is considered complete may vary to some degree.
Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.
It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”
This continuation-in-part patent application claims priority to non-provisional patent application Ser. No. 12/330,134, filed Dec. 8, 2008, which claims priority to provisional Patent Application Ser. No. 61/012,186, filed Dec. 7, 2007.
This invention was made with government support under Contract No. FA9550-065-C-0157 awarded by Air Force Office of Scientific Research/PKC. The Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61012186 | Dec 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12330134 | Dec 2008 | US |
| Child | 12537315 | US |
