MONITORING A DYNAMIC CONDITION OF A ROTARY ELEMENT, PARTICULARLY A PNEUMATIC TIRE

Abstract: At least one dynamic condition of a pneumatic tire (304, 604, 804, 904, 1004) of a vehicle (802, 1002) is monitored with a transponder (302, 602, 806, 1020) mounted in the tire by sensing (520, 820) fluctuations in coupling of power provided to the transponder form an interrogator (808, 1030). The fluctuations in coupling can provide an indication of dynamic conditions such as tire angular position, rotational velocity and acceleration, steering angle, lateral acceleration, and radial runout. Alternatively, fluctuations in coupling of a signal from the transponder to the interrogator can be sensed to provide an indication of dynamic conditions associated with the pneumatic tire.

Description: MONITORING A DYNAMIC CONDITION OF A ROTARY ELEMENT, PARTICULARLY A PNEUMATIC TIRE CROSS-REFERENCE TO RELATED APPLICATIONS This application relates to PCT Patent Application entitled DISPOSITION OF TRANSPONDER COUPLING ELEMENTS IN TIRES (Attorney's docket No. DN1999124PCT) having a filing date concurrent herewith. TECHNICAL FIELD OF THE INVENTION The invention relates to monitoring a dynamic condition of a rotary (rotating) element and, more particularly, a dynamic condition such as rotational velocity of a pneumatic tire. BACKGROUND OF THE INVENTION THE ASSIGNEE'S ONGOING DEVELOPMENT EFFORTS For a century, the Goodyear Tire & Rubber Company of Akron Ohio, assignee of the present invention, has been the uncontested industry leader in tire product technology. For example, as early as 1892, a puncture-resistant tire was patented. In 1934, a year recognized as the beginning of the run-flat era, Goodyear introduced the Lifeguard (tm) safety tube, a fabric tube within the tire, used commercially by auto makers and on trucks. In 1993, Goodyear' s Eagle GS-C EMT (Extended Mobility Technology) Tire won the Discover Award for Automotive Technological Innovation. In 1996, the Goodyear Eagle FI run-flat tire was chosen as standard equipment on the 1997 Chevrolet C-5 Corvette. Other examples of the strides Goodyear has taken in the advancement of tire and related technologies include, but are not limited to, the following patented inventions: Commonly-owned U.S. Patent No. 3.665.387 (Enabnit; 1972), entitled SIGNALLING SYSTEM FOR LOW TIRE CONDITION ON A VEHICLE, incorporated in its entirety by reference herein, discloses a low tire pressure warning system adaptable for any number of wheels of a vehicle and providing dashboard indications of system operation and low pressure conditions while the vehicle is in motion. Commonly-owned U.S. Patent No. 3.831.161 (Enabnit; 1974), entitled FAIL-SAFE MONITORING APPARATUS, incorporated in its entirety by reference herein, discloses monitoring vehicle tire pressure wherein the operator is warned of an abnormal or unsafe condition of one or more of the tires. Commonly-owned U.S. Patent No. 3.872.424 (Enabnit; 1975), entitled APPARATUS AND METHOD FOR TRANSMITTING AUXILIARY SIGNALS ON EXISTING VEHICLE WIRING, incorporated in its entirety by reference herein, discloses communicating with low tire pressure monitoring circuits using power pulses carried on existing vehicle wiring (e.g., the turn signal circuits). Commonly-owned U.S. Patent No. 4.052.696 (Enabnit; 1977), entitled TIRE CONDITION MONITOR, incorporated in its entirety by reference herein, discloses a tire condition sensing circuit that includes a ferrite element that changes from a ferromagnetic to a non-ferromagnetic state in response to a temperature increase above the material Curie point. Commonly-owned U.S. Patent No. 4.099.157 (Enabnit; 1978), entitled SINGLE WIRE POWER/SIGNAL SYSTEM FOR VEHICLE AUXILIARY DEVICES, incorporated in its entirety by reference herein, discloses providing both power to and receiving detection signals from a remotely located condition monitoring device using a single wire with ground return through the vehicle frame. Commonly-owned U.S. Patent No. 4.108.701 (Stanley; 1978), entitled METHOD FOR MAKING HOSE INCORPORATING AN EMBEDDED STATIC GROUND CONDUCTOR, and related U.S. Patent No. 4.168.198 (Stanley; 1979), entitled APPARATUS FOR MAKING HOSE INCORPORATING AN EMBEDDED STATIC GROUND CONDUCTOR, both incoφorated in their entirety by reference herein. Commonly-owned U.S. Patent No. 4.911.217 (Dunn, et. al.; 1990), entitled INTEGRATED CIRCUIT TRANSPONDER IN A PNEUMATIC TIRE FOR TIRE IDENTIFICATION, incorporated in its entirety by reference herein, discloses an RF transponder in a pneumatic tire. Figure la of this patent illustrates a prior-art identification system ("reader") that can be used to interrogate and power the transponder within the tire. The identification system includes a portable hand-held module having within it an exciter and associated circuitry for indicating to a user the numerical identification of the tire/transponder in response to an interrogation signal. Commonly-owned U.S. Patent No. 5.181.975 (Pollack, et. al.; 1993), entitled INTEGRATED CIRCUIT TRANSPONDER WITH COIL ANTENNA IN A PNEUMATIC TIRE FOR USE IN TIRE IDENTIFICATION, incorporated in its entirety by reference herein, discloses a pneumatic tire having an integrated circuit (IC) transponder and pressure transducer. As described in this patent, in a tire that has already been manufactured, the transponder may be attached to an inner surface of the tire by means of a tire patch or other similar material or device. Commonly-owned U.S. Patent No. 5.218.861 (Brown, et al.; 1993), entitled PNEUMATIC TIRE HAVING AN INTEGRATED CIRCUIT TRANSPONDER AND PRESSURE TRANSDUCER, incorporated in its entirety by reference herein, discloses a pneumatic tire having an integrated circuit (IC) transponder and pressure transducer mounted within the pneumatic tire. Upon interrogation (polling) by an external RF signal provided by a "reader", the transponder transmits tire identification and tire pressure data in digitally- coded form. The transponder is "passive" in that it is not self-powered, but rather obtains its operating power from the externally-provided RF signal. The commonly-owned U.S. Patents referenced immediately hereinabove are indicative of the long-standing, far-reaching and ongoing efforts being made by the Goodyear Tire & Rubber Company in advancing tire product technology. DYNAMIC CONDITIONS Dynamic conditions such as position and angular velocity of a rotary element are readily measured. United States Patent No. 3.831.570. incoφorated in its entirety by reference herein, discloses measuring a crankshaft's rotary position using a magnetic pickup coil, a rotating toothed wheel, and a permanent magnet. The toothed wheel has a number of teeth corresponding to the number of spark plugs. United States Patent No. 3.832.640. incoφorated in its entirety by reference herein, discloses determining a large number of angular relationships (rotary positions) in a rotating element such as a crankshaft. Straightforward tire revolution counters are well known, and are disclosed for example in United States Patent Nos. 4.842.486 and 5.524.034. both of which are incoφorated in their entirety by reference herein. United States Patent No. 5.218.862. incoφorated in its entirety by reference herein, discloses a tire pressure monitor comprising wheel speed sensors located at the vehicle's wheels to convey wheel speed information to an electronic controller. This patent notes that the wheel speed discrepancy between one tire to the others indicates the relative tire pressure, but that discrepancy may also be indicative of the vehicle turning, accelerating or decelerating, going up or down steep grades, or of one wheel slipping, or of a cross wind bearing on the vehicle. United States Patent No. 5.274.355. incoφorated in its entirety by reference herein, discloses a system for monitoring pressure and temperature of rotating pneumatic tires. An elastic diaphragm is embedded in or bonded to the sidewall of the tire. Tire pressure changes cause measurable expansion and contraction of the diaphragm surface. The diaphragm comprises a pair of reflective stripes spaced apart from one another at a fixed distance to define a reference dimension, and another pair of reflective stripes which move relative to one another as the diaphragm expands or contracts in relation to the tire inflation pressure. The time interval between pulses associated with the reflective stripes, as sensed by photodetectors affixed to the vehicle indicates individual tire speed. The background infrared radiation naturally emitted from the tire indicates tire temperature. United States Patent No. 5.345.217. incoφorated in its entirety by reference herein, discloses measuring wheel speed of a motor vehicle with a multi-tooth pulse generator on each wheel (as is used on electronic Automatic Braking Systems) to produce a series of pulses spaced apart by equal increments of angular rotation of each wheel. The speed of each wheel is compared to the others, to determine if, and to what extent, a tire is deflated. United States Patent No. 5.569.848. incoφorated in its entirety by reference herein, discloses a system for monitoring tire pressure, comprising toothed ring sensors affixed to each wheel assembly, a sensor operatively associated with each respective toothed ring and producing signals that are a measure of the rotational velocity of the wheels, and a computer receiving signals from the sensors. The computer monitors the wheel speed sensors during vehicle operation, calculating and indicating vehicle speed, distance traveled by the vehicle and low tire pressure. United States Patent No. 5.721.528. incoφorated in its entirety by reference herein, discloses a low tire pressure warning system utilizing angular displacement sensors at each wheel. The system utilizes wheel displacement sensors already in place on vehicles that include ABS systems. Certain vehicle operating conditions, such as excessive or very low speeds, braking, and turns, are determined from the sensor outputs. STATIC CONDITIONS In addition to the aforementioned dynamic conditions (e.g., position, rotational velocity and acceleration), static conditions are also associated with rotating elements such as pneumatic tires -for example, pressure and temperature. Failure to correct quickly for improper tire pressure may result in excessive tire wear, blowouts, poor gasoline mileage and steering difficulties. An automatic tire deflation warning system is especially critical for "run flat" tires, where the deflated condition is barely detectable by the driver himself. Sensors for static parameters are typically located within the rotating tire, and associated circuitry can transmit data indicative of a sensed condition to an on-board receiver within the vehicle. TRANSPONDER SYSTEMS FOR PNEUMATIC TIRES A "transponder" is an electronic device capable of both receiving and transmitting radio frequency (RF) signals. Transponder systems, typically including a plurality of transponders and a single interrogator are well known and disclosed, for example, in United States Patent No. 5.339.073. incoφorated in its entirety by reference herein. It is known to put transponders (and associated sensors) in pneumatic tires of motor vehicles. These transponders transmit a RF wave, with or without variable data (e.g., tire pressure, temperature, position) and/or fixed data (e.g. , tire ID) to outside the tire, and receive RF signals, with or without data, from outside the tire. A separate transponder is typically associated with each tire of a motor vehicle to monitor and transmit tire-related data. Typically, an "interrogator" having both transmitting and receiving capabilities is used to communicate with the transponders. The interrogator may be "hand-held", or mounted "on-board" the vehicle, or positioned along or in a roadway (e.g. , "drive-over", or "drive by"). "Active" transponders have their own power supply (e.g., a battery). They transmit signals, and are typically also capable of receiving signals to control their functionality. "Passive" transponders are powered by the energy of an incoming RF signal, such as from an interrogator. Passive transponders fall into two general categories, those having only passive circuitry, and those having some active circuitry. In the main, transponders which are passive transponders having some active circuitry are discussed herein. United States Patent No. 5.612.671. incoφorated in its entirety by reference herein, discloses a low tire pressure warning system having a pressure sensor and radio transmitter in each wheel, and a vehicle-mounted receiver including a microprocessor. United States Patent No. 4.609.905. incoφorated in its entirety by reference herein, discloses a passive transponder having only passive circuitry. A RF transmitter in the vehicle interrogates the transponder, which reflects a predetermined harmonic of the RF signal back to a receiver as a function of the state of an associated pressure switch. United States Patent No. 4.067.235. incoφorated in its entirety by reference herein, discloses a passive transponder with a tire pressure sensor. Electromagnetic radiation generated by a power transmitter is received by a receiving antenna comprising an inductor and a capacitor in the tire pressure sensor. This radiation is converted by a rectifier-filter to electricity to power active components (oscillator, buffer amplifier, transmitter) of the transponder. United States Patent No. 4.724.427. incoφorated in its entirety by reference herein, discloses a passive transponder that receives a carrier signal from an interrogator. The carrier signal is rectified by a rectifying circuit connected across the transponder's antenna coil to generate electricity to power the transponder. Data is encoded and mixed with the carrier signal in a balanced modulator circuit. The output of the balanced modulator circuit is transmitted back to the interrogator unit. United States Patent No. 4.730.188. incoφorated in its entirety by reference herein, discloses a passive transponder excited by an inductive coupling from an interrogator. The transponder responds to the interrogator via the inductive coupling with a signal constituting a stream of data. The transponder comprises an induction coil serving as its antenna, and a full wave rectifier bridge and smoothing capacitor connected across the antenna to provide DC voltage (power) to active circuitry within the transponder. United States Patent No. 4.911.217. incoφorated in its entirety by reference herein, discloses an RF transponder in a pneumatic tire. Figure la illustrates a prior-art identification system ("reader") that can be used to interrogate and power the transponder within the tire. A portable hand-held module has within it an exciter and circuitry for indicating the numerical identification of the tire/transponder. United States Patent Nos. 5.181.975 and 5.218.861. incoφorated in their entirety by reference herein, disclose a pneumatic tire having an integrated circuit passive transponder located within the structure of the tire for use in tire identification and pressure data transmission. The interrogation signal is rectified by circuitry in the transponder, which then utilizes the rectified signal as its source of electrical power for use in its transmission of digitally encoded signals. United States Patent No. 4.220.907. incoφorated in its entirety by reference herein, discloses a low tire pressure alarm system for vehicles. Each wheel is provided with a transmitter, and there is a common receiver comprising a suitable antenna such as a ferrite loopstick. United States Patent No. 4.319.220. incoφorated in its entirety by reference herein, discloses a system for monitoring tire pressure, comprising wheel units in the tires and a common receiver. Each wheel unit has an antenna comprising a continuous wire loop disposed against the inner periphery of the tire for transmitting signals and for receiving power. Multiple antennas may be provided for the receiver, and may be in the form of ferrite loopsticks. United States Patent No. 5.319.354. incoφorated in its entirety by reference herein, discloses an antenna structure for communicating with an electronic tag (transponder) implanted in a pneumatic tire. This patent recognizes that the orientation of the transponder with respect to the antenna communicating with the transponder can adversely affect coupling between the interrogation antenna and the transponder antenna. A construction of an interrogation antenna is described so that, regardless of the position of the transponder in the pneumatic tire, a position of which is unknown, the coupling is always of the same quality. The following U.S. Patents are also cited as being of interest: 3.835.451 4.755.345. 5.095.309. 5.235.326. 5.537.867. 5.559.507. 5.594.448. 5.731.754. 5.790.016. 5.790.016. 5.824.891. and 5.826.207. all of which are incoφorated in their entirety by reference herein. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide method and apparatus for monitoring one or more dynamic operating conditions of a rotary element such as a pneumatic motor vehicle tire, as defined in one or more of the appended claims and, as such, having the capability of being implemented in a manner to accomplish one or more of the subsidiary objects. It is another object of the invention to provide data indicative of forces acting upon a vehicle tire by augmenting the functionality of existing tire transponder systems (e.g., pressure monitoring systems). According to the invention, fluctuations in coupling of RF energy between an onboard interrogator and a tire-mounted transponder of a tire monitoring system are detected (sensed) as being indicative of one or more dynamic conditions affecting the tire. The coupling fluctuations may be sensed as power fluctuations at the transponder, and data indicative of the fluctuations transmitted to an on-board interrogator. (The transponder may be a passive transponder which is powered by the interrogator.) Alternatively, the fluctuations may be sensed at the interrogator, based on the strength of signals transmitted by the transponder. In the case of a pneumatic motor vehicle tire, the fluctuations will typically exhibit a periodicity related to the rotation of the tire, and may include amplitude variations and phase shifts. In this manner, the instantaneous tire angular position and its time derivatives, velocity and acceleration are readily determined. Dynamic conditions of a pneumatic tire in addition to tire angular position, such as rotational velocity and acceleration, lateral acceleration, radial runout, torsional effects about one or more of the tire axes, steering angle and the like will also cause discernable coupling (power) fluctuations. Asymmetric deformations of the pneumatic tire in response to forces such as steering inputs to the front tires during cornering and other vehicle dynamic conditions will also cause discernable coupling (power) fluctuations. Displacement of the tire/wheel assembly in jounce and rebound will also cause discernable coupling fluctuations. Information regarding the dynamic conditions acting upon the tire can be utilized in any desired manner by the vehicle operator and/or by a vehicle control system. According to an aspect of the invention, fine increments of angular position can be determined by inteφolating between relatively few positions causing discernable coupling (power) fluctuations (events), or by increasing the number of discernable coupling fluctuations which can be sensed per tire revolution. In an embodiment of the invention, a transponder is located adjacent the tread of the tire, has an antenna, and is loosely coupled to a conductive hoop which is preferably disposed adjacent the tread of the tire. A number of inductive elements may be provided at spaced intervals on the hoop to induce coupling fluctuations in the form of "blips", or the like. According to an aspect of the invention, the tire is a radial ply pneumatic tire, and the coupling element is a hoop disposed beneath the tread, between the carcass structure and the belt structure, such as on the equatorial plane of the tire. The hoop may serve the additional puφose of enhancing the run-flat qualities of the tire. The term "run-flat" as used herein means that the tire structure alone is sufficiently strong to support the vehicle load when the tire is operated in the uninflated condition, the sidewall and internal surfaces of the tire not collapsing or buckling onto themselves, without requiring any internal devices to prevent the tire from collapsing. Other objects, features and advantages of the invention will become apparent from the description that follows. BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments. Certain elements in selected ones of the drawings may be illustrated not-to-scale, for illustrative clarity. Often, similar elements throughout the drawings may be referred to by similar reference numerals. For example, the element 199 in a figure (or embodiment) may be similar in many respects to the element 299 in an other figure (or embodiment). Such a relationship, if any, between similar elements in different figures or embodiments will become apparent throughout the specification, including, if applicable, in the claims and abstract. In some cases, similar elements may be referred to with similar numbers in a single drawing. For example, a plurality of elements 199 may be referred to as 199a, 199b, 199c, etc. The cross-sectional views, if any, presented herein may be in the form of "slices", or "near-sighted" cross-sectional views, omitting certain background lines which would otherwise be visible in a true cross-sectional view, for illustrative clarity. The structure, operation, and advantages of the present preferred embodiment of the invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying drawings, wherein: Figure 1 is a simplified block diagram of a tire pressure monitoring system, according to the prior art; Figure 2 is a simplified block diagram of a passive transponder, according to the prior art; Figure 3 A is a side plan view, partially sectioned, of a tire having a transponder mounted therein, according to the prior art; Figure 3B is a cross-sectional view of the tire of Figure 3 A, taken on a line 3B-3B through Figure 3A, according to the prior art; Figure 3C is a cross-sectional view of the tire of Figure 3A, rotated 180 degrees, taken on a line 3C-3C through Figure 3A, according to the prior art; Figure 4 is a graph of coupling strength between the interrogator antenna and transponder of Figure 3 A, according to the invention; Figure 5 is a simplified block diagram of a passive transponder, according to the invention; Figure 6A is a side plan view, partially sectioned, of a tire with a transponder and a hoop coupling element disposed therein, according to the invention; Figure 6B is a cross-sectional view of the tire of Figure 6A, taken on a line 6B-6B through Figure 6A, according to the invention; Figure 6C is a cross-sectional view of the tire of Figure 6A, rotated 180 degrees, taken on a line 6C-6C through Figure 6A, according to the invention; Figure 7 is a graph of coupling strength between the interrogator antenna and transponder of Figure 6A, according to the invention; Figure 8 is a block diagram of a tire pressure monitoring system, according to the present invention; Figure 9 is a side plan view of a tire with a transponder and a hoop coupling element disposed therein, with the tire shown in phantom, according to the invention; Figure 9A is a graph of coupling strength between the interrogator antenna and transponder of Figure 9, according to the invention; and Figure 10 is a block diagram of a condition monitoring and control system, according to the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a typical tire pressure monitoring system 100 of the prior art installed on a motor vehicle 102 (shown in dashed lines) having four pneumatic tires 104a..104d installed on four respective wheels (not shown). A transponder ("TAG") 106a..106d is disposed within each of the tires 104a..104d, respectively. The transponders 106a..106d are preferably passive transponders which obtain their operating power from an RF signal such as is typically generated by an on-board interrogator 108 which is mounted within the vehicle. The interrogator 108 comprises an RF transmitter 112 (e.g., for powering passive transponders), an RF receiver 114, control logic 116 which may include a microprocessor (μP), and a display device 118 such a visual display and optionally including an audible alarm. Antennas ("ANT") 110a.. ll0d are disposed on the vehicle 102, preferably adjacent the tires 104a.. l04d, respectively, such as in the wheel wells of the vehicle. The antennas 110a..1 lOd are suitably ferrite loopstick antennas. The use of multiple vehicle antennas 110a..1 lOd, each at a fixed position on the vehicle adjacent a respective tire 104a.. l04d is well known and preferred, and is disclosed in United States Patent Nos. 3.553.060: 3.810.090: 4.220.907: 5.541.574: and 5.774.047. all of which are incoφorated in their entirety by reference herein. In use, the interrogator 108 powers the transponders 106a..106d which, in turn, transmit data indicative of a measured condition (e.g., air pressure) back to the interrogator. In any such system, it is desirable to have efficient and effective coupling of signals between the fixed antennas 110a..ll0d (alternatively, one centrally-located fixed antenna) and the moving (i.e., when the vehicle is in motion) transponders (each of which has its own antenna, not shown). Figure 2 illustrates, generally, an exemplary passive RF transponder 200 (compare any of 106a..106d) of the prior art. An antenna 202, such as a coil antenna, receives a carrier signal from the interrogator 108 (via antennas 110a..ll0d). The carrier signal, of frequency F, is rectified by a rectifier circuit 204 connected to the transponder's antenna 202 in order to generate operating power for active circuitry in the transponder - in this example, for clock and control logic circuitry 206 and for sensor interface and data generation circuitry 208. Data derived from one or more condition sensor(s) 210, such as a temperature sensor and a pressure sensor, is optionally stored in memory 212, and is encoded (e.g., digitized) and mixed with the carrier signal in a modulator circuit 214. The output of the modulator circuit 214 is transmitted via the antenna 202 back to the interrogator 108. It is well understood that the clock and control logic circuitry 206 can derive a clock signal in a straightforward manner from the RF carrier signal transmitted by the interrogator 108. For example, the carrier signal illuminating (powering) the transponder may be at 125 kHz and may be passed through a "divide-by- 10" divider circuit (not shown) to generate clock pulses at 12.5 kHz. It is also well understood that clock signals can be counted by a counter (not shown), and that an accumulated count in a counter can be digitized and transmitted by a transponder. Figures 3A, 3B and 3C illustrate an RF transponder system 300 of the prior art comprising a passive transponder 302 (compare any of 106a..106d, 200) disposed within a pneumatic tire 304 (compare any of 104a..104d). In this example, the transponder 302 is a passive transponder, and is mounted in any suitable manner to an inner surface 306 of the tire 304. An antenna 320 (compare any of 110a .. HOd) is disposed on the vehicle near the tire 304 such as in a wheel well, to provide electromagnetic radiation to power the transponder 302 (in the case of a passive transponder), as well as to receive signals from the transponder 302. As illustrated in Figures 3 A, 3B and 3C, the vehicle antenna 320 is fixedly disposed at the "12 O'clock" orientation with respect to the tire 304, such as abreast of a top portion of the tire. It should be understood that the antenna 320 may be disposed at any position suitable for coupling electromagnetically with the transponder 302 without interfering with movement (e.g., rotation, steering, rebounding) of the tire 304. As the tire 304 rotates (as indicated by the arrow 330), the transponder 302 will be alternately nearer to and farther from the antenna 320. As illustrated in Figures 3 A and 3B, the tire 304 is oriented so that the transponder 302 is at the "6 O'clock" position with respect to the antenna 320. This being the furthest the transponder 302 can be from the antenna 320, the coupling of an RF signal between the antenna 320 and the transponder 302 will be relatively weak as compared with the coupling between the antenna 320 and the transponder 302 when the tire has rotated 180 degrees and the transponder 302 is in the "12 O'clock" position, as illustrated in Figure 3C. Thus, it is evident that, as the tire 304 rotates, there will be cyclical fluctuations in the RF energy coupling between the antenna 320 and the transponder 302. Figure 4 is a graph 400 illustrating fluctuations in the coupling of RF energy between an antenna (e.g., 320) of an on-board interrogator (e.g., 108) and a transponder (e.g., 302) disposed within a pneumatic tire, such as has been described hereinabove. The x-axis is the angle (in degrees) between the transponder 302 and the vehicle antenna 320 as the tire rotates. 0 degrees is where the transponder 302 is closest to the antenna 320 (as in Figure 3C), and 180 degrees is where the transponder 302 is farthest from antenna 320 (as in Figure 3B). The y-axis is coupling magnitude, in arbitrary units. Generally, the coupling strength varies cyclically, and inversely with the distance between the interrogator antenna (e.g., 320) and the transponder (e.g. , 302) as illustrated by the line 402. It can be observed that the coupling strength is maximum, when the wheel as it turns under operating conditions, so that the transponder is next to the antenna 320 and quickly decreases as the transponder moves away from the antenna. SENSING FLUCTUATIONS AT THE TRANSPONDER The aforementioned United States Patent No. 5.319.354 recognizes that there will be coupling variations such as have been described with respect to Figure 4, and teaches that such coupling strength fluctuation is detrimental to powering and communicating with the transponder, and describes a construction of an interrogation antenna so that, regardless of the position of the transponder in the pneumatic tire, the coupling is always of the same quality. It is an object of the present invention to advantageously employ these coupling variations. Figure 5 illustrates major functional components of an exemplary passive RF transponder 500 (compare 200) of the present invention. In a manner similar to that described with respect to the transponder 200, an antenna 502 (compare 202), such as a coil 5 antenna, receives a carrier signal from the interrogator 108 (via antennas 110a..ll0d). The carrier signal, of frequency F, is rectified by a rectifier circuit 504 (compare 204) to generate operating power. Clock and control logic circuitry 506 (compare 206) and sensor interface and data generation circuitry 508 (compare 208) are provided. Data derived from one or more condition sensor(s) 510 (compare 210) is optionally stored in memory 512 o (compare 212), and is impressed on the antenna 502 by a modulator circuit 514 (compare 214) for transmission back to the interrogator 108. The passive RF transponder 500 of the present invention also includes a power monitoring circuit 520 capable of monitoring the magnitude and/or phase of the signal on the antenna 502, preferably prior to rectification and regulation. 5 For detecting (sensing) fluctuations in the magnitude of the signal received by the transponder antenna 502, the power monitoring circuit 520 is suitably a combination of an envelope detector and threshold detector, such as is disclosed in United States Patent No. 4.285.236. incoφorated in its entirety by reference herein. In this manner, fluctuations in the signal powering the transponder can be detected. Attention is also directed to circuitry o for monitoring a change in voltage across a coil which is proportional to peak-to-peak variations in power consumption and power output, as is disclosed in United States Patent No. 5.559.507. incoφorated in its entirety by reference herein. In light of the teachings set forth herein, one having ordinary skill in the art to which the present invention most nearly pertains will recognize that it is relatively 5 straightforward to detect "events" such as maximums, minimums, and null points in a signal being monitored, and to develop information regarding the duration or spacing of the events. For example, a time interval between a series of events can be determined by counting clock pulses, and information regarding the time interval can be imposed on a signal for transmission, such as by converting a count to a binary number, and modulating 0 an RF transmission. The time or angular units between such "events" can be used to detect torsional effects about one or more of the tire axes. In this context, for example, the change in angular spacing or time between "events" could indicate a change in torsion acting about the tire's lateral axis of rotation. INCORPORATING A COUPLING ELEMENT As mentioned above, it is generally desirable to ensure uniform, or at least adequate coupling of RF signals between an on-board interrogator and a tire-mounted transponder, irrespective of the orientation of the wheel vis-a-vis the vehicle-mounted antenna. Similarly, when using a handheld device to read (including optionally powering) a tire-mounted transponder, effective coupling irrespective of tire orientation, sometimes referred to as "360 degree readability" is generally desirable. In a worst-case scenario, the orientation of a tire relative to a vehicle-mounted antenna could produce a "brownout" condition during a portion of the tire's revolution. The aforementioned United States Patent No. 5.319.354 proposes an antenna construction to ameliorate this "problem". The aforementioned, commonly-owned United States Patent Nos. 5.181.975 and 5.218.861. address improving coupling to a tire-mounted transponder, and disclose an annular bead (HOOP) of the tire acting as the primary winding of a transformer. A transponder is disposed near the HOOP and has a coil antenna that is loosely coupled to the HOOP and is the secondary winding of the transformer. Figures 6A, 6B and 6C illustrate an RF transponder system 600 (compare 300) comprising a passive transponder 602 (compare 302) disposed within a pneumatic tire 604 (compare 304). The transponder system 600 is similar to the previously-described transponder system 300, in the following respects. A transponder 602 is mounted in any suitable manner to an inner surface 606 (compare 306) of the tire 604. An antenna 620 (compare 320) is disposed on the vehicle, adjacent the tire 604, such as in a wheel well of the vehicle directly next to the tire 604. The antenna 620 provides electromagnetic radiation for powering the transponder 602, and receives signals from the transponder 602. This embodiment of a transponder system 600 incoφorates a hoop 630 extending circumferentially around the inner surface 606 of the tire 604. The hoop 630 is an electrically-conductive member which may function as a primary winding of a coupling transformer, with a coil antenna of the transponder functioning as a secondary winding of the coupling transformer, as described in the aforementioned United States Patent Nos. 5.181.975 and 5.218.861. Generally, the hoop 630 beneficially affects (enhances) coupling between the transponder 602 and the external antenna 620 and, in this regard, can be considered to be a "coupling element" . The hoop 630 is suitably a single turn of wire having its ends connected (shorted) to one another, but may comprise multiple turns or layers of wire or conductive material. The hoop 630 should have a low electrical resistance, preferably less than 10 ohms. The inclusion of a hoop enhances the "360-degree" readability of the transponder 602. Regarding the actual location of the hoop 630, it should be understood that the hoop 630 is illustrated extending circumferentially around the inner surface 606 of the tire 604, slightly offset from the equatorial plane (EP) of the tire 604, for illustrative puφoses only. It is within the scope of the invention, that the hoop 630 may be disposed on the axial centerline (equatorial plane) of the tire 604, passing directly underneath the transponder 602, and may also be "buried" in the body of the tire 604. Regarding the hoop 630 itself, it should be understood that the hoop is an exemplary complete, endless, short-circuit loop of any electrically-conductive material suitable to function as a coupling element. The hoop 630 is suitably brass-plated high-tensile strength steel which exhibits good mechanical strength and resistance to corrosion when placed in the environment of an interior of a pneumatic tire. The hoop 630 may comprise multiple strands of such wire, which optionally may be plated (e.g. , with nickel or gold). It is within the scope of the invention that the hoop 630 element may be formed as two or more lengths of wire joined, in any suitable manner (such as by wrapping or soldering) at their ends to form a complete hoop extending around the entire circumference of the tire 604, or may be formed as a loop having two ends, or as an elongate conductor extending partially around a circumference of the tire, including on or in the sidewall thereof, including a loop having a coupling coil incoφorated therein. It is further within the scope of the invention that the hoop 630 element may be formed as to multiple layers of tire cord material encased within conductive rubber. In addition to its electrical characteristics as a coupling element, if the hoop is suitably rigid, it may serve the additional puφose of enhancing the run-flat qualities (e.g., deflated radial rigidity) of the tire. As illustrated in Figures A, 6B and 6C, the vehicle antenna 620 is fixedly disposed at the "12 O'clock" orientation with respect to the tire 604, such as abreast of a top portion of the tire. It should be understood that the antenna 620 may be disposed at any position suitable for coupling electromagnetically with the transponder 602 without interfering with movement (e.g. , rotation, steering, rebounding) of the tire 604. In a manner similar to that described hereinabove, as the tire 604 rotates (as indicated by the arrow 632), the transponder 602 will be alternately nearer to (as in Figure 6C) and farther from (as in Figure 6B) the antenna 620. Hence, "direct" coupling between the antenna 620 and the transponder 602 will vary cyclically. In addition to the direct coupling between the transponder 602 and the antenna 620, the hoop 630 "indirectly " couples energy between the transponder 602 and the antenna 620. This indirect electromagnetic coupling, between transponder-to-hoop-to-antenna remains relatively constant, because both the distance between hoop and antenna and the distance between hoop and transponder stay the same, even as the distance between transponder and antenna vary cyclically. The tire bead in United States Patent No. 5.181.975 serves a similar puφose to hoop 630, but may suffer from its proximity to a metal tire rim. The present invention addresses this situation by locating the hoop as far as practical from the rim. Figure 7 is a graph 700 (compare 400) illustrating fluctuations in the coupling of RF energy between an antenna (e.g. , 620) of an on-board interrogator (e.g., 108) and a transponder (e.g., 602) disposed within a pneumatic tire, such as has been described hereinabove. The x-axis is the angle (in degrees) between the transponder 602 and the vehicle antenna 620 as the tire rotates. 0 degrees is where the transponder 602 is closest to the antenna 620, and 180 degrees is where the transponder 602 is farthest from antenna 620. The y-axis is coupling magnitude/phase, in arbitrary units. As will become evident, the coupling varies in both magnitude and phase, as indicated by the line 702. A number of points 704, 706, 708, 710, 712, 714, 716 and 718 are indicated on the line to aid in the following description of coupling (power) fluctuation. The transponder 602 and the interrogator antenna 620 are "indirectly" coupled by the hoop 630, and this indirect coupling is relatively constant over the entire revolution of the tire (between 0-degrees and 360-degrees). But when the transponder 602 gets close to the antenna 620, they become "directly" coupled with one another. This direct coupling is stronger than the indirect coupling, and is of opposite phase. The following can be observed. Between 0-degrees (point 704) and approximately 270-degrees (point 706), coupling of energy to the transponder 602 is indirect and relatively constant - for example, of magnitude "A" with a " + " phase. At approximately 270-degrees (point 706), direct coupling starts to manifest itself and, being of opposite phase to the indirect coupling, to cancel out the indirect coupling. At approximately 280-degrees (point 708), there is a "zero crossing" or "null dip" in the signal being received by the transponder as it transits from " + " phase to "-" phase. As the tire continues to rotate, the magnitude of the signal being received by the transponder increases to a magnitude of 2 A at approximately 290-degrees (point 710). Between approximately 290-degrees (point 710) and approximately 330-degrees (point 712), the magnitude of the signal is relatively constant at a magnitude/phase of -2 A. At approximately 330-degrees (point 712), the direct coupling starts to fade, and power decreases. At approximately 340-degrees (point 714), there is another "zero crossing" or "null dip" in the signal being received by the transponder as it transits from "- " phase to " + " phase. As the tire continues to rotate, the magnitude of the signal being received by the transponder increases to a magnitude of A at approximately 350-degrees (point 716). Between approximately 350-degrees (point 717) and 360-degrees (point 718), the magnitude of the signal is relatively constant at a magnitude/phase of + A. (The point 718 is equivalent to the point 704.) The points 708 and 714 represent "zero crossings" or "null dips", and are readily sensed by a power monitor circuit (e.g., 520) within the transponder. It is therefore, for example, a relatively straightforward matter to count clock pulses (e.g. , with clock and control logic 506) during an interval 720 between the two zero crossings 708 and 714, and to store a count in a register or other suitable memory element (e.g., 512) for transmission in a digital data stream to an on-board interrogator (e.g., 108). The duration of the interval 720 will be representative of the rotational velocity of the tire - the shorter the interval, the fewer number of clock pulses counted during the interval, and the greater the rotational velocity of the tire. A typical tire on a typical passenger vehicle traveling at 60 mph (100 kph) will make one complete revolution in approximately 60 milliseconds (ms). The phase shifts occurring at the points 708 and 714 are suitably detected by circuitry such as is disclosed in United States Patent No. 5.764.138. incoφorated in its entirety by reference herein. It should be understood that since the zero crossings (708, 714) imply that, at certain tire orientations no power is being coupled to the transponder 602, the transponder power supply (e.g. , rectifier 504) should have some storage element such as a capacitor incoφorated therein. In a similar manner, even if power is available to get "over the hump" (past the zero crossings), any signal being transmitted by the transponder to the on- board interrogator will also be subject to blackouts at certain tire positions. SENSING FLUCTUATIONS AT THE INTERROGATOR As described hereinabove, certain problems can accompany sensing power (coupling) fluctuations at the transponder. There is now described a technique for sensing power fluctuations at the on-board interrogator. Figure 8 illustrates a tire pressure monitoring system 800, comparable to that described with respect to Figure 1, in that a transponder ("TAG") 806a..806d (compare 106a..106d) is disposed within each of the tires 804a..804d (compare 104a..104d), respectively, of a motor vehicle 802 (compare 102). An on-board interrogator 808 (compare 108) comprises an RF transmitter 812 (compare 112), an RF receiver 814 (compare 114), control logic 816 (compare 116), and a display device 818 (compare 118). One or more antennas ("ANT") 810a..810d (compare 110a..l l0d), such as ferrite loopstick antennas, are disposed on the vehicle 802. In this example, the on-board interrogator 808 is provided with a power monitoring circuit 820, comparable to the power monitor 520 of the transponder 500, to detect (sense) fluctuations in the coupling of an RF signal being transmitted by a tire-transponder to the on-board interrogator. Monitoring power fluctuations at the interrogator, rather than at the transponder, allows for the use of a simple prior art transponder (e.g., 200). It also allows for the use of an active (e.g., battery-powered) transponder, or a simple transmitter rather than a transponder. If the transponder need not be powered by the interrogator, the interrogator could simply be a receiver. FINER INCREMENTS. ETC. As described hereinabove, it is possible to monitor certain events (discernable/measurable coupling fluctuations such as null points 708 and 714) and determine elapsed time during an interval (e.g., 720) between the two events. In this manner, it is possible to determine the angular velocity of the tire, rotation-by-rotation. Evidently, acceleration can also be determined in a straightforward manner by the change in velocity. The tire's angular position can also be determined. In the example set forth above, the null points 708 and 714 occur at tire angular positions of 280 degrees and 340 degrees, respectively. It is evident that the uneven spacing of the null points is beneficial in determining which null point is which. (With the tire rotating, a pattern of "blips" would be discernable, as follows: "blip, blip, pause, blip, blip, pause ... ".) If the null points 708 and 714 occurred 180 degrees apart, phase reversal would have to be tracked in order to determine which null point is which. Specific points (discernable fluctuations) in the signal powering (alternatively, in the signal coming from) the transponder are indicative of known discrete angular positions of the tire, which may change as a function of torsional effects in the tire. For example, every 180 degrees of tire rotation a 'blip' or other signal feature is detected (e.g., a maximum and minimum as discussed with respect to Figure 4). The time interval between these 'blips' can be determined (such as by counting clock pulses occurring in the transponder in the period between 'blips'). This time interval then can be used to divide the next time interval predictively into a fixed number of discrete intervals, for example, 256, each of which would then represent 1/256 (one-two-hundred-fifty-sixth) of 180 degrees, or, about 0.704 degrees of tire rotation. In other words, the time period between 'blips' every 180 degrees is assumed to remain relatively constant for the time period between the last 'blip' and the next. This assumed or predicted period is divided into equal intervals of time based on the transponder's clock frequency, but because little actual time change can occur between 'blips in only one half revolution of the tire, regardless of its acceleration, the exemplified 256 pulses or intervals of time actually represent uniform intervals of tire angular rotation. If the 'blips' are not evenly-spaced, yet their positions are known (see, e.g., Figure 7), angular positions between discernable coupling fluctuations can also be determined in a similar manner. In an embodiment of the invention, finer increments of angular position can be determined by inteφolating between the aforementioned few discernable coupling fluctuations. This reasonably presumes that the rotational velocity of the tire will be fairly constant throughout a given revolution. One having ordinary skill in the art to which the present invention most nearly pertains will appreciate how to implement inteφolation, in either software or hardware, in light of the teachings set forth herein. An example of a hardware inteφolation technique is disclosed in the aforementioned United States Patent No. 3.832.640. In an alternate embodiment of the invention, finer increments of angular position can be determined by increasing the number of discernable coupling fluctuations (events) which are sensed per tire revolution. Figure 9 is similar to Figure 6A, and illustrates a transponder 902 (compare 602) disposed on an inside surface of a tire 904 (shown in dashed lines, compare 604), and a coupling element hoop 930 (compare 630) within the tire. An antenna 920 (compare 620) for an on-board interrogator (not shown, compare 108) is illustrated. The antenna 920 is shown adjacent the "12 O'clock" position of the tire 904, and the transponder 902 is shown at the 0 degree orientation of the tire. In this embodiment of the invention, a plurality of inductive elements 910, 912 and 914 are shown incoφorated into the hoop at various points about the hoop's circumference. Such inductive elements may be ferrite rings or steel protrusions, or merely widening or narrowing of the hoop itself. These inductive elements will cause fluctuations ("blips") in the RF coupling as each inductive element passes the antenna 920. As illustrated, the inductive elements 910, 912 and 914 are not evenly-spaced about the circumference of the hoop 930. Rather, the element 910 is disposed at 0 degrees, the element 912 is disposed at 90 degrees, and the element 914 is disposed at 270 degrees. (The transponder 902 is disposed at 180 degrees.) With the tire rotating, a pattern of "blips" will be discernable, as follows: "blip, blip, pause, blip". This uneven spacing of the inductive elements, and consequent uneven blip-spacing, can be advantageous for ascertaining a particular orientation of the tire, rather than merely its rotational velocity. It is within the scope of the invention that inductive elements for causing discernable coupling fluctuations can be incoφorated into the tire, about the circumference of the tire, without there being a hoop, including with or without there being an "antenna" coupling element, as described hereinbelow. Figure 9A is a graph 950 (compare 400) illustrating fluctuations in the coupling of RF energy between the antenna 920 and the transponder 902. The x-axis is the angle (in degrees) between the transponder 902 and the vehicle antenna 920 as the tire rotates. 0 degrees is where the transponder 902 is closest to the antenna 920, and 180 degrees is where the transponder 902 is farthest from antenna 920. The y-axis is coupling magmtude, in arbitrary units. Generally, the coupling varies cyclically, due to the tire rotation, and varies in some manner, as illustrated by the line 952 where the variation is magnified for illustrative puφoses. In this figure, the "blips" 960, 962 and 964 caused by the inductive elements 910, 912 and 914, respectively, are illustrated. ALTERNATE EMBODIMENTS Various embodiments have been described hereinabove wherein a hoop (e.g., 630) is disposed in a tire (e.g., 604), along with a transponder (e.g., 602) to provide for 360- degree readability. In those embodiments, the transponder is inductively coupled to the hoop, as described in the aforementioned United States Patent Nos. 5.181.975 and 5.218.861. In an alternate embodiment of the invention, a nearly complete circle of wire (or other suitable electrically-conductive material) would be disposed in the tire in lieu of the completely-circular hoop. The nearly complete circle of wire would have two ends, and a coupling coil attached between the two ends, and would be a "loop antenna". A transponder having an internal antenna in the form of a coupling coil would be disposed closely adjacent the coupling coil of the loop antenna so as to effect transformer-type coupling between the two coupling coils. An advantage of this would be that coil-to-coil coupling will tend to be stronger than loop-to-coil coupling. In yet another embodiment of the invention, a loop antenna (a nearly complete circle of wire) could be directly connected ("hard- wired") to the transponder, such as is disclosed in the aforementioned copending US Provisional Patent Application No. 60/095.176. In either case (coupling coil or direct connection), the loop antenna functions as a coupling element in much the same manner as the aforementioned hoop(s). AN EXEMPLARY SYSTEM Figure 10 shows an embodiment of a condition monitoring and control system 1000, installed on a vehicle 1002 (shown in dashed lines), such as a typical passenger vehicle having four pneumatic tires 1004a, 1004b, 1004c and 1004d installed on four respective wheels (not shown). The vehicle 1002 is preferably equipped with an RS-485 (or equivalent) multiplexed serial data bus 1006 controlled by an on-board vehicle computer 1008 having an RS-485 interface 1010. Preferably, a central display unit 1012 is connected either directly to the computer 1008 or is operatively connected (as shown) to the computer 1008 via the data bus 1006. The data bus 1006 is suitably a twisted pair of insulated wires (labeled "A" and "B"), preferably with a minimum of one twist per inch. It is within the scope of the invention that if no data bus is provided on the vehicle, one can be added thereto. For example, in the absence of an existing vehicle data bus, a dedicated data bus may be provided, such as a bi-directional data bus conforming to RS-485 or other suitable serial communications standards. Each of the four tires 1004a..1004d is equipped with an electronic module ("TAG") 1020a..1020d, respectively, and associated sensor (not shown, well known) capable of monitoring one or more conditions such as air pressure and air temperature within the tire, and of transmitting a radio frequency (RF) signal indicative of (e.g., modulated as a function of) the monitored condition(s) within the respective vehicle tire. The tags 1020a..1020d are suitably transponders, but may alternatively simply comprise one or more condition sensors and a radio frequency transmitter, as described hereinabove. The system 1000 comprises four monitors (or "interrogation units") 1030a..1030d, each associated with a respective one of the tires 1004a..1004d and preferably located in proximity therewith, such as mounted within the wheel wells of the vehicle. Each monitor 1030a..1030d is connected to a source of power (as indicated by the lines terminating in circles and triangles) and is connected to the multiplexed serial data bus 1006 for individually communicating with the on-board computer 1008. Each monitor 1030a..1030d is generally comparable to any of the interrogators (e.g., 108, 808) described hereinabove. Each monitor 1030a..1030d comprises an antenna 1032a..1032d (compare 110a..ll0d, 810a..810d), respectively, a receiver (not shown, compare 114, 814) for receiving transmissions from the tag, and a transmitter (not shown, compare 112, 812) for transmitting to (and optionally powering) a respective one of the tags 1020a..1020d. In the case of coupling fluctuations being sensed at the interrogation unit, rather than at the transponder, each monitor 1030a..1030d is provided with a suitable power monitor 1034a..l034d (compare 820), respectively. Each monitor 1030a..1030d comprises a suitable data transceiver (such as the DS36277 Dominant Mode Multipoint Transceiver by National Semiconductor) to facilitate two-way data transmission via the data bus 1006. A monitor's transmissions to the respective tag may comprise a carrier signal for energizing a passive tag, and may comprise signals to "wake up" an active tag which is in a low-power sleep mode. It is within the scope of the invention that all components of the monitor (1030), including the antenna (1032) can be encapsulated in a single package. Alternatively, the antenna (1032) can be disposed outside of such a package. Monitored condition information carried by the RF signals from the respective tags 1020a..1020d can be decoded (e.g., demodulated) and provided to the on-board computer 1008 for subsequent display (1012) to the operator of the vehicle. It is within the scope of the invention that suitable discernable visual and/or audible warnings can be used at the option of the vehicle manufacturer. Additionally, the information regarding dynamic conditions of the tires can be utilized in controlling the vehicle, such as by providing relevant inputs to a "smart" suspension system. Dynamic conditions of a pneumatic tire in addition to tire angular position, such as rotational velocity and acceleration, lateral acceleration, radial runout, torsional effects about one or more of the tire axes, steering angle and the like will also cause discernable coupling (power) fluctuations. The power level to the transponder can be analyzed from a frequency standpoint to obtain dynamic data. Fourier analysis of the waveform will produce data containing harmonics of the base frequency. This data can be used to control suspension, steering or other frequency related characteristics of discrete vehicle systems. The vibration characteristic of these vehicle systems determine handling, subjective "feel" of the vehicle and noise, both interior and exterior. Control of these frequency related characteristics within the various vehicle systems can be used on a dynamic basis to control harmonic amplitudes, resonances and aesthetic parameters. For example, the transponder signal or power levels can be used in conjunction with a vehicle sound frequency generator (speaker) to provide active noise cancellation in response to harmonic amplitude analysis. CONCLUDING COMMENTS There have thus been described methods and apparatus for monitoring a dynamic condition of a rotary element such as a pneumatic tire. Generally, the magnitude and/or phase of RF energy coupled between a vehicle antenna and a tire-mounted transponder is monitored, and fluctuations utilized as indicative of dynamic conditions of the tire. The use of hoops and loop antennas within the tire has been described. It is within the scope of the invention that hoops, loops and the like can be embedded in the carcass of the tire, rather than disposed on an inner surface thereof. It is within the scope of the invention that hoops, loops and the like following a non- planar "seφentine" path can be disposed on the inside surface of the tire (or embedded in the carcass of the tire). It is within the scope of the invention that phenomena other than null points, dips, spikes and the like can be monitored as being indicative of dynamic tire conditions. For example, the average level of RF coupling may decrease as the tire is steered (turned laterally), thereby indicating the tire's steering angle. It is within the scope of the invention that the vehicle antenna can be disposed other than at approximately the 12 O'clock position with respect to the tire. For example, a vehicle antenna could be disposed at the 9 O'clock position with respect to the tire, in which case, steering (turning) the tire would affect signal coupling. Although the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character -it being understood that only preferred embodiments have been shown and described, and that all changes and modifications that come within the spirit of the invention are desired to be protected. Undoubtedly, many other "variations" on the "themes" set forth hereinabove will occur to one having ordinary skill in the art to which the present invention most nearly pertains, and such variations are intended to be within the scope of the invention, as disclosed herein.

Claims: What is claimed is: I. Method for monitoring a dynamic condition of a pneumatic tire (304, 604, 804, 904, 1004) of a vehicle (802, 1002), comprising: a transponder (302, 602, 806, 1020) mounted in the tire; an interrogator (808, 1030) on-board the vehicle characterized by: sensing (520, 820) coupling fluctuations in RF energy passing between the interrogator and the transponder. 2. Method, according to claim 1, characterized in that: the transponder is a passive transponder and is powered by an RF signal from the interrogator. 3. Method, according to claim 1, characterized in that: sensing is performed at the transponder. 4. Method, according to claim 3, characterized by: the transponder transmitting data related to the sensed coupling fluctuations to the interrogator. 5. Method, according to claim 1, characterized in that: sensing is performed at the interrogator. 6. Method, according to claim 1, characterized by: incoφorating a coupling element (630, 930) into the tire. 7. Method, according to claim 6, characterized in that: the coupling element is a hoop. 8. Method, according to claim 6, characterized in that: the coupling element is inductively coupled to the transponder. 9. Method, according to claim 6, characterized in that: the coupling element is a loop antenna. 10. Method, according to claim 1, characterized in that: the coupling fluctuations are indicative of dynamic conditions selected from the group consisting of tire angular position, rotational velocity and acceleration, lateral acceleration, radial runout, torsional effects about one or more of the tire axes, and steering angle. II . Method, according to claim 1, characterized in that: the power monitoring circuit (520, 820) controls vibration characteristics of vehicle (802, 1002) on a dynamic basis. 12. In a vehicle (802, 1002) having pneumatic tires (304, 604, 804, 904, 1004), a system for monitoring dynamic tire conditions comprising: an RF transponder (302, 602, 806, 1020) in each of the vehicle tires; characterized by: at least one interrogation unit (808, 1030) for communicating with the transponders via antennas (620, 810, 920, 1032) mounted adjacent each tire; and power monitoring circuit (520, 820) for sensing coupling fluctuations in RF energy passing between the antennas and the transponders. 13. System, according to claim 12, characterized in that: an interrogation unit (1030a.. l030d) and corresponding antenna (1032a..1032d) are disposed adjacent each tire. 14. System, according to claim 12, characterized in that: the dynamic conditions are selected from the group consisting of tire angular position, rotational velocity and acceleration, lateral acceleration, radial runout, torsional effects about one or more of the tire axes, and steering angle. 15. System, according to claim 12, characterized in that: each transponder comprises a power monitor (1034a..1034d). 16. System, according to claim 12, characterized by: a coupling element (630, 930) incoφorated into the tire. 17. System, according to claim 16, characterized in that: the coupling element is a hoop. 18. System, according to claim 16, characterized in that: the coupling element is inductively coupled to the transponder. 19. System, according to claim 16, characterized in that: the coupling element is a loop antenna. 20. System, according to claim 12, characterized by: inductive elements (910, 912, 914) disposed about the circumference of the tire for causing coupling fluctuations in RF energy passing between the antennas and the transponders. 21. System, according to claim 12, characterized by: a computer (1008) connected via a data bus (1006) to the interrogation units (1030a..1030d). 22. System, according to claim 12, characterized by: the power momtoring circuit (520, 820) controlling frequency related characteristics of a vehicle system (802, 1002) on a dynamic basis.

Inventors: BROWN ROBERT WALTER, BROWN, ROBERT, WALTER
Applicants: BROWN ROBERT WALTER, GOODYEAR TIRE & RUBBER, BROWN, ROBERT, WALTER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1999
Main classification: B60C23
IPC codes: B60C19, B60C23, B60T8, G01P3