Patent Publication Number: US-9841359-B2

Title: Extraction of tire characteristics combining direct TPMS and tire resonance analysis

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/047,343 filed on Oct. 7, 2013. 
    
    
     TECHNICAL FIELD 
     Embodiments relate generally to direct tire pressure monitoring and indirect tire pressure monitoring, and more particularly to combining direct tire pressure monitoring and tire resonance analysis for the extraction of tire characteristics to characterize other tire parameters. 
     BACKGROUND 
     There are two general approaches to monitoring the pressure in vehicle tires: direct and indirect. Direct tire pressure monitoring systems typically comprise a wheel module having one or more sensors and electronics mounted in or to the tire to directly measure the tire&#39;s pressure and wirelessly transmit measurement data to the vehicle. 
     Indirect TPMSs generally utilize information from other vehicle sensors and/or systems to indirectly estimate a tire&#39;s pressure without direct TPMS sensors or electronics being located in the tire. Indirect TPMS is attractive because it can be more cost-efficient than direct TPMS. One conventional indirect TPMS uses wheel speed signals from the anti-lock brake system (ABS). For a typical passenger vehicle having four tires, the indirect TPMS compares the four wheel speed signals to determine whether a wheel is rotating faster because of a loss of pressure and related decreased diameter. One drawback to some of these indirect systems is that the systems cannot detect whether all wheels have lost pressure over time because the values are compared. 
     One approach for overcoming this drawback is to utilize a resonance frequency method (RFM) of analysis of a single resonance frequency in the sensed data signals from the ABS. U.S. Pat. Nos. 8,207,839 and 8,347,704 describe different kinds of RFM analysis of a time series of sensed data signals that includes auto-regression analysis, Fast Fourier analysis, a Bayesian analysis, or analysis based on a linear estimation model. While different kinds of analysis are taught by these patents, the purpose of each of these known RFM approaches is to reduce the amount of computation power required in an on-board processor to do the calculations necessary to identify a single resonance frequency from which tire pressure can be indirectly estimated. While RFM analysis can represent an improvement over conventional indirect TPMS, the accuracy of the results can be impacted by the low resolution of the ABS sensed data signals and by other factors that can influence the resonance frequency beyond just the tire pressure in an individual tire. 
     Therefore, in another approach, a multidimensional resonance frequency analysis (MRFA) that includes a spectral analysis identifying at least two tire vibration modes in the wheel speed signal and isolates at least one characteristic affecting the at least two tire vibration modes, for example, that described in U.S. patent application Ser. No. 13/919,620, which is herein incorporated by reference in its entirety, can also be utilized. In MRFA approaches, the number of tire parameters that can be extracted depends on the number of different resonance modes that can be identified in the spectrum of the wheel speed signal and on a significant difference of their respective dependence on the different parameters. 
     Moreover, typical direct TPMS and indirect TPMS do not interface with each other. Therefore, there is a need for combined direct tire pressure monitoring and tire resonance analysis for the extraction of tire characteristics in order to characterize other tire parameters. 
     SUMMARY 
     In an embodiment, a tire characterization system comprises a sensor configured to provide a wheel speed signal; a direct tire pressure monitoring system (TPMS) including at least one sensor configured to provide a direct measurement of a characteristic of the tire; and a characterization engine configured to process the wheel speed signal to identify at least one tire vibration mode and use the identified at least one tire vibration mode and the direct measurement to estimate a characterized parameter of the tire. 
     In an embodiment, a method of characterizing at least one parameter of a vehicle tire, the method comprises obtaining data representative of a wheel speed of the tire; analyzing the data; obtaining a direct measurement from at least one sensor corresponding to the unknown parameter; incorporating the direct measurement into the analyzed data to remove the unknown parameter; and characterizing one or more additional parameters of the vehicle tire. 
     In an embodiment, a tire characterization system comprises at least one sensor configured to provide at least a direct measurement of a characteristic of the tire; an wheel speed sensor configured to provide a wheel speed signal; and an electronic control unit (ECU) coupled to the wheel speed sensor and configured to process the sensed wheel speed signal, to identify at least one tire vibration mode, and process the direct measurement and the identified at least one tire vibration mode in order to estimate a characterized parameter of the tire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1A  is a block diagram of a tire characterization system, according to an embodiment. 
         FIG. 1B  is a block diagram of a tire characterization system, according to an embodiment. 
         FIG. 2  is a schematic diagram of a wheel sensor system and tire, according to an embodiment. 
         FIG. 3  is a block diagram of an indirect tire pressure monitoring system (TPMS), according to an embodiment. 
         FIG. 4  is a block diagram of an indirect tire pressure monitoring system (TPMS), according to an embodiment. 
         FIG. 5  is a graphical representation of a wheel speed signal spectrum analysis, according to an embodiment. 
         FIG. 6  is a flowchart of a method, according to an embodiment. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Embodiments relate to tire characterization systems and methods for combining direct tire pressure monitoring systems (TPMS) and tire resonance analysis in indirect tire pressure monitoring systems (TPMS) for the extraction of tire characteristics to characterize other tire parameters. In embodiments, TPMS and methods that utilize anti-lock braking system (ABS) sensed signals coupled to an electronic control unit (ECU) that can comprise circuitry and/or controllers to process the sensed signals using a resonance frequency analysis (RFA) technique can be combined with direct tire pressure sensor measurements from direct TPMS systems. Because direct TPMS systems deliver a precise value of a tire characteristic (for example, tire pressure), one of the unknown parameters that influence the resonance effects can be removed. As a results, the detected resonance can be used to characterize another tire parameter that would not be accessible without knowledge of the precise value of the tire characteristic delivered by the direct TPMS system. In embodiments, the addition of one or more direct measurements from the direct TPMS system into the system of equations that describes the change of the resonance characteristic generated by the RFA technique can remove one or more unknown variables in the RFA technique. In embodiments, additional direct measurements from the direct TPMS system can be incorporated. With the incorporation of additional direct measurements, additional unknown variables can be removed and additional tire parameters can be characterized from the system of equations in the RFA technique. 
     In embodiments, a direct TPMS provides a high quality or highly accurate pressure measurement value. In contrast to indirect TPMS systems, wherein the dependence of the resonance on the pressure must be known, the direct pressure measurement value can be used to eliminate a pressure dependence on a resonance parameter. A resonance can therefore be analyzed for its dependence on another parameter. In embodiments, the resonance can be analyzed for the temperature of the tire material, which influences the friction between the tire and the driving surface. Determining the temperature of the tire material is valuable and is often not directly measurable due to mechanical assembly challenges with sensors necessarily requiring close contact with the tire material. 
     In embodiments, in the case that a sensor from the direct TPMS system also provides a temperature measured in close contact with the tire material, the direct temperature measurement value can be applied to eliminate the temperature dependence on another parameter (another influence parameter in the resonance characteristic). For example, by analyzing the resonance for additional parameter dependence, after using the one or more parameters precisely and directly measured by the direct TPMS system, the identified resonance effects that are caused by the tire can be used to extract additional information about the state of the tire. In some embodiments, information related to a state of the material of the tire can be extracted. In an embodiment, estimations of a state of material age can be determined. In another embodiment, estimations of the thickness of the tire profile can be determined. In embodiments, because the dependence of the resonance behavior on the exemplary state of material age or tire profile thickness may be relatively small, the evaluation time can be long. Therefore, in embodiments, the parameters that are accessible are typically ones that change slowly. 
     In embodiments, in the case that the temperature provided by the direct sensor is not the real tire material temperature, a correction factor from the measured temperature to the material temperature can be calculated instead of evaluating another tire parameter. 
     Referring to  FIG. 1A , a block diagram of an embodiment of a tire characterization system  10  that utilizes RFA in accordance with an embodiment is depicted. According to an embodiment, system  10  comprises characterization engine  12 , direct TPMS  14 , and indirect TPMS  16 . In an embodiment, characterization engine  12  interfaces with direct TPMS  14  and indirect TPMS  16 . Such an interface, such as a wired or wireless connection, can be provided between direct TPMS  14  and indirect TPMS  16  so as to operably couple direct TPMS  14  and indirect TPMS  16 . 
     Characterization engine  12  generally includes, in an embodiment, a processor and memory. The characterization engine processor can be any programmable device that accepts digital data as input, is configured to process the input according to instructions or algorithms, and provides results as outputs. In an embodiment, the characterization engine processor can be a central processing unit (CPU) configured to carry out the instructions of a computer program. In other embodiments, the characterization engine processor can be a digital signal processor (DSP). The processor is therefore configured to perform basic arithmetical, logical, and input/output operations. 
     The characterization engine memory can comprise volatile or non-volatile memory operably coupled to the characterization engine processor to not only provide space to execute the instructions or algorithms, but to provide the space to store the instructions themselves. In embodiments, volatile memory can include random access memory (RAM), dynamic random access memory (DRAM), or static random access memory (SRAM), for example. In embodiments, non-volatile memory can include read-only memory, flash memory, ferroelectric RAM, hard disk, floppy disk, magnetic tape, or optical disc storage, for example. The foregoing lists in no way limit the type of memory that can be used, as these embodiments are given only by way of example and are not intended to limit the scope of the claims. 
     Direct TPMS  14  can include, in an embodiment, sensor  18 . Sensor  18  can be any sensor or plurality of sensors configured to measure a value of a tire parameter. For example, sensor  18  can comprise a tire pressure sensor configured to measure the pressure of the tire. In embodiments, direct TPMS  14  can include additional or other sensors, such as one or more temperature sensors or one or more vibration sensors. Sensor  18  can be internal or external to the tire and can be mounted in or on, affixed to, embedded in, or otherwise coupled to the tire. In other embodiments, sensor  18  can be mounted proximate but not in or on the tire, such as on a rim, wheel, axle, vehicle body or other suitable place. 
     Indirect TPMS system  16  is or can include, in an embodiment, wheel speed signals for example used for an anti-lock braking system (ABS) coupled to an electronic control unit (ECU) that may comprise circuitry and/or controllers to process the sensed signals using a resonance frequency analysis (RFA) technique. The wheel speed sensor signals may for example be generated by a magnetic sensor mounted close to the wheel axis and sensing the variation of a magnetic field caused by a rotation of the wheel axis. In some embodiments, a sensor, rather than an indirect TPMS system can be configured to provide the sensed signal(s). In some embodiments, additional information from other sensed signals or stored data settings for non-tire variables and parameters can be incorporated into the RFA. In embodiments, the RFA uses a spectral analysis of tire vibrations as determined from the sensed ABS signals over different points in the spectrum that can reflect different vibration modes and different corresponding resonance frequencies. In other embodiments, the RFA approach can look for a single resonance frequency. In some embodiments, an enhanced transmission protocol for the sensed ABS signals to the ECU for analysis by the ECU can result in an increased signal to noise ratio (SNR) for the RFA technique, thereby enabling identification of potential resonance frequencies that might otherwise fall below a conventional noise threshold. 
     In various embodiments, the sensed data signals can include higher resolution data from the ABS. The different vibration modes can include radial vibration, angular vibration, and other types of tire, wheel or drive train vibrations, as well as higher order harmonics of these vibrations, and can be optimized for RFA of multiple physical variables associated with the tire in addition to tire pressure. Examples of such additional tire variables that can impact different resonance frequency modes include tire speed, temperature, thickness, size, profile, wear, age, and materials, among others. Other non-tire related variables can also be utilized in the RFA and can include ambient conditions; vehicle driving data related to acceleration, turning and braking maneuvers; and vehicle condition parameters including weight, equipment options, and feature settings such as suspension modes or traction control, among others. 
     In general, wheel speed corresponds to the first order frequency of the sensed signal from the ABS, with the wheel speed corresponding to a duration between pulses. In one embodiment, there are about 48 pulses per rotation, which provides good granularity. Changes in these characteristics can then be analyzed to determine whether any are indicative of a change in the pressure of the tire. 
     For example, an indirect TPMS can detect a change in resonance frequency of a tire. A decrease of the resonance frequency could be indicative of a lower tire pressure in the tire. Information then can be transmitted from indirect TPMS to other circuitry associated with the TPMS or vehicle ECU in several manners. In one embodiment, the sensed signals and optional non-tire variables and parameters are transmitted to and analyzed by an electronic control unit (ECU). In another embodiment, other kinds of sensed signals and optional non-tire variables and parameters can be communicated to an ECU or other vehicle microprocessor or controller system for utilization in the RFA technique. Any of these signals, variable or parameters, as well as the resonance frequencies of each tire may be communicated to other vehicle microprocessor or controller systems for utilization by the vehicle in other operations and/or display for the vehicle operator. 
     The RFA performed by the ECU, vehicle microprocessor and/or other controller circuitry can be carried out by a variety of circuit, controller, and microprocessor components that are programmed or configured to perform the RFA as described herein, either in a single component or with various portions of the RFA performed by different components in coordination with each other. The RFA can be performed based on digital data and digital techniques, including the use of a digital signal processor (DSP), analog data and analog techniques, or any combination thereof. In various embodiments, the systems and methods for performing the RFA can accomplish the RFA based on various analysis techniques that can include DSP analysis, auto-regression analysis, discrete Fourier transform, wavelet transform, Gabor transform, Fast Fourier analysis, digital or analog filter banks, a Bayesian analysis, Q factor analysis, harmonic analysis and/or analysis based on a linear estimation model, among others and alone or in combination. 
     Referring to  FIG. 2 , a schematic diagram of an embodiment of an indirect TPMS wheel sensor system  100  that utilizes RFA in accordance with an embodiment is depicted. Wheel  110  is shown with a tire  112  and an ABS system  120 . The particular relative positions of ABS system  120  and wheel  110  are merely exemplary and can vary in embodiments. Moreover, components in this and other figures herein are not necessarily drawn to scale. Because wheel  110  is a complex structure, there should be a multitude of resonances in response to complex vibrations occurring during movement. Embodiments utilize these multiple resonances to locate and isolate resonance peaks of different resonance modes, e.g., a radial vibration, r, and an angular vibration, ω. Thus, tire  112  can be modeled as a complex arrangement of mechanical resonators, δ ω  and m ω  for angular vibration, and δ r  and m r  for radial vibration. In some embodiments, the vibration of tire  112  can be modeled as a two-dimensional model of mechanical resonators, while in other embodiments, tire  112  can be modeled as a three-dimensional model of mechanical resonators. 
     In some embodiments, the information sensed from the vibrations of the tire  112 , such as by one or more sensors  114  mounted in or on, affixed to, embedded in, or otherwise coupled to tire  112 , can be analyzed for a multidimensional resonance frequency within the ABS system  120 . In other embodiments, one or more sensors  114  can be mounted proximate but not in or on tire  112 , such as on a rim, wheel, axle, vehicle body or other suitable place, though sensors so positioned may not be able to sense actual tire characteristics, such as material temperature. In some embodiments, the digitized information can be modulated onto the conventional ABS wheel speed clock signal generated by, for example, an encoder tooth wheel  122 , for transmission to and analysis by an electronic control unit (ECU). According to embodiments, additional information about multidimensional resonances of the sensed signal that can include higher-order harmonics of the wheel rotation can be provided to the ECU, which can then be utilized while reducing warning latency, thereby providing a more robust system that balances provision of early warnings with false alarms. In other embodiments, a sensor or series of sensors, rather than components of the indirect TPMS system can be configured to provide the sensed signal(s). 
     Referring to  FIG. 3 , a block diagram is depicted of an indirect TPMS wheel sensor system  200  in accordance with an embodiment. System  200  is or comprises an ABS system  220  including a speed sensor  222  in embodiments. For example, system  200  can comprise an ABS system  220  with additional circuitry and/or algorithms in order to process indirect TPMS data in one embodiment, or system  200  can comprise additional circuitry, algorithms and/or other sense and control components  240  external to ABS system  220  to carry out the processing of the indirect TPMS data. For example, one or more sensors  114  can comprise components  240 , or those sensors  114  can be considered to be part of indirect TPMS  200 , with or without additional sensors as part of components  240 . In one embodiment, the additional circuitry and/or algorithms can be part of an ECU  230  or signal processing system, though they need not be in all embodiments. Various additional data sensing and control system components  240  can also be provided in embodiments and can comprise one or more of an accelerometer sensor and/or system, an inertia sensor or sensor cluster, an ambient environment sensor and/or system and a vehicle control system, each of which can include various sensors and control arrangements known in the art to provide global vehicle parameter data, such as one or more of sensed data, operational data and/or control parameters for the vehicle. In another embodiment, characterization engine  22  can be incorporated into one or more components of wheel sensor system  200 , and for example, ECU  230 . 
     Referring to  FIG. 4 , a block diagram of ABS sensor circuitry  300  in accordance with one embodiment is depicted. Circuitry  300  includes an analog portion  302  and a digital portion  304  coupled by an analog-to-digital (ADC) converter  306 . Analog portion  302  comprises one or more Hall probes  308  or other magnetic field sensors, offset compensation circuitry  310  and gain circuitry  312 . Digital portion  304  comprises maximum/minimum detection circuitry  314 , zero-crossing detection circuitry  316 , optional frequency analysis circuitry  318 , optional analysis of harmonics circuitry  320  and pulse forming circuitry  322 . Optional frequency analysis circuitry  318  and analysis of harmonics circuitry  320  form part of an indirect TPMS  324 , in an embodiment. If none of the optional analysis or interface options are used, circuitry  300  represents one possible embodiment of a standard ABS system. The depiction of system  300  is merely exemplary, and more or fewer circuitries, sensors and other components can be implemented in other embodiments. Moreover, the block diagram of  FIG. 4  can be considered functional, such that some blocks depicted as being distinct can in fact be combined in actual implementation. Additionally, blocks depicted as being part of other blocks can, in other embodiments, be distinct therefrom or be part of other blocks, whether specifically depicted or not. 
     In one embodiment, information from indirect TPMS  200  is represented in a digital frame protocol format and is modulated onto the original ABS clock signal by adapting the pulse length to the state of the related bit of the frame. Thus, the ABS wheel speed signal is represented by the rising edge while the TPMS information is in the pulse duration of a sequence of pulses. Co-owned and co-pending U.S. application Ser. No. 13/751,335, entitled “A SIGNAL GENERATOR, A DECODER, A METHOD FOR GENERATING A TRANSMIT SIGNAL AND METHODS FOR DETERMINING SPEED DATA,” filed Jan. 28, 2013, discloses additional information regarding communicating data between a sensor and an ECU or other control or processing system and is incorporated herein by reference in its entirety. As disclosed therein, a signal generator includes a signal provider and a signal processing unit. The signal provider is configured to provide a sensor signal indicating a repeatedly detected event, occurring within differing time intervals. The signal processing unit is configured to generate a transmit signal based on the sensor signal. The transmit signal includes event information representing the temporal occurrence of the event and additional information representing additional data. The event information includes pulses or signal edges associated to detected events, wherein the pulses or signal edges are temporarily separated within the transmit signal according to the differing time intervals of detected events so that each time interval of the differing time intervals includes one pulse or one signal edge associated with a detected event. Further, the additional data includes at least one frame including a predefined number of additional data bits. The information of the additional data bits of the at least one frame is distributed over at least two time intervals of the differing time intervals. 
     In one embodiment, information from indirect TPMS  200  is represented in analog form and transmitted as analog information. The magnetic field resulting from a rotating a magnetic pole wheel or magnetic tooth wheel is measured by a speed sensor and converted to a suitable protocol and provided to an ECU as the output signal having a speed data portion and an enhanced resonance data portion. Co-owned and co-pending U.S. patent application Ser. No. 13/903,088 entitled “WHEEL SPEED SENSOR AND INTERFACE SYSTEMS AND METHODS,” filed May 28, 2013, discloses additional information regarding communicating data between a sensor and an ECU or other control or processing system and is incorporated herein by reference in its entirety. The speed sensor is herein configured to detect a magnetic field in response to speed and resonance characteristics. The speed sensor is also configured to generate a sensor output signal having speed data and enhanced resonance data which is received by the sensor output signal. The speed signal which is measured in the speed sensor and delivered as analog value can also be used as redundant information to the speed information that is encoded in the speed pulses of the classic ABS protocol. A sensor output signal may be generated having speed data and enhanced resonance data by selecting a first current level and a second current level, generating a magnetic field in response to a rotation that is influenced by tire vibrations, generating a field sensor output from the magnetic field and generating a sensor output signal from the field sensor output according to the selected first current level and the second current level, wherein the sensor output signal includes speed data and enhanced resonance data. A measurement system used in the indirect TPMS may comprise a magnetic field sensor configured to measure a magnetic field and to generate a field sensor output, a summation component configured to combine an offset with the field sensor output to provide a modified sensor output and a current modulation component configured to generate a sensor output signal from the modified sensor output, the sensor output signal having speed data and enhanced resonance data. 
     In another embodiment, a separate communication source, such as a wired or wireless connection, can be provided between indirect TPMS  200  and ECU  230 . For example, a two- or three-wire connection between sensor  222  and/or other sensors of system  200  to ECU  230  can be used, which can provide an amplified and decoupled version of the analog sensor output as it enters ADC  306  or for some other digital signal or message from the ADC  306 , FFT (e.g., circuitry  318 ) or resonance analysis. Several of these optional couplings are depicted in  FIG. 4 , and in various embodiments one or more of them can be used or omitted, and/or additional ones can be added, such that the particular depiction in  FIG. 4  is merely exemplary of one of a variety of possibilities. 
     Referring to  FIG. 5 , a graphical representation of a spectrum of a wheel speed signal according to an embodiment is shown for the sensed signal data representing wheel speed. Curve  400  represents the wheel speed spectral density based on actual average wheel speed. In this example, curve  400  two tire vibration modes are depicted. A first tire vibration mode is indicated at  410  and a second tire vibration mode is indicated at  420 . In this example, the first vibration mode  410  has a weak dependence upon tire pressure and profile thickness, but a stronger dependence upon tire temperature and state of material age. The second vibration mode  420  has a strong dependence upon tire pressure, a strong dependence upon tire temperature, a strong dependence upon profile thickness, but a weak dependence upon state of material age. 
     In an embodiment, noise floor  430  is shown in dotted line and is higher than the floor of curve  400  of the wheel spectral density. Noise floor  430  illustrates that it can be difficult to recognize the first vibration mode  410  in the spectrum because it is almost covered by the noise. However, second vibration mode  420  is readily recognizable in this spectrum and still can be dependent on pressure and another parameter of interest, as described above. Therefore, second vibration mode  420  can be used to estimate the parameter of interest if, for example, pressure is known from the direct TPMS, such as in direct TPMS  14 . 
     Here instead of using the influence of different parameters on different resonance frequencies, the absolute height of the resonance peaks or the quality factor of the resonance can be taken into account as well, which increases the diversity of the available effects, e.g., the pressure inside the tire can have a higher influence on the location of the resonance frequency while, e.g., the temperature of the rubber could have more influence on the internal friction and therefore cause a significant change of the damping and thus finally on the Q-factor of a resonance. The first resonance  410  is mainly influenced by parameters of the tire (e.g. a deformation that does not significantly change the volume) and therefore shows low pressure dependence but strong temperature dependence, for example. The second resonance is a pressure dependent effect (e.g., deformation of the tire cross-section that changes the volume) and is heavily temperature dependent as well. If at least one measurement in the frequency range of each vibration mode is available, the influencing parameters can be separated by use of equations that describe the dependence of the two measurements on each parameter, and the equations can be resolved for the independent parameters. For example, the strong temperature dependence of both modes  410  and  420  can be removed and the change in pressure isolated between modes  410  and  420 . In embodiments, then, the tire pressure measured directly by, for example, sensor  18  of direct TPMS  14  can be included such that the influence of pressure is removed from the resonance analysis. 
     In other embodiments, additional characteristics can be used to isolate the pressure or other parameters, based on their respective dependence on other characteristics. If the dependence of the measurements on the independent parameters is too complex and/or the pressure cannot be isolated, a numerical meta model of the tire can be established based on a characterization of a tire type over variations of the independent parameters. 
     Additional analysis or inclusion of additional sensed data or parameters may be used to identify or isolate further vibrational resonance modes in the wheel speed data. For example, different measurements that can be utilized to extract the different influencing parameters could be multitudes or combinations of the following: a) frequency (location) of a resonance of the wheel speed, b) spectral density of the wheel speed at a resonance (maximum), c) spectral density of the wheel speed at a minimum, d) frequency (location) of a minimum, d) spectral density of the wheel speed in a fix distance from a maximum or minimum, e) Q factor of a resonance peak, f) spectral density of the wheel speed at a fix defined frequency, g) any of the previous measurements that are selected depending on the type or dimension of the tire, g) any of the previous measurements that are selected or interpreted depending on the actual speed of the observed wheel, h) any of the previous measurements that are selected or interpreted depending on accessible vehicle parameters (e.g. speed of the car, acceleration, load and its distribution, actuation of vehicle control systems (steering, breaking, power train, vehicle stability control, active damper, global chassis control), i) any of the previous measurements that are selected depending on already acquired measurement points, j) any of the previous measurements selected depending on already evaluated parameters (e.g. tire pressure or tire temperature), and/or k) non-tire related sensed data (e.g., ambient temperature, humidity), in various embodiments. 
     Based on the new system for the spectral analysis inside the sensor using high resolution data, it can be possible to locate more than just a small pressure dependent change of the analyzed spectrum that exceeds the noise floor. Embodiments, however, are not restricted to being used with a spectral analysis of the sensor device. For example, in other embodiments the noise sensitivity of the generated pulses can be reduced, including significantly, by one or more of the following techniques: reducing a distance between the sensor and pole wheel or other target in order to have a higher anticipated magnetic field strength; providing a stronger magnetization of the pole wheel or other target itself; implementing new or different sensing techniques, such as magnetoresistive (xMR) techniques including tunneling MR, TMR; and/or using higher quality, more precise, or other improved circuitry, though such circuitry may be balanced with, e.g., increased power consumption and other factors. In general, vibrations depend on multiple physical influences, with the tire pressure being only one, and others can include the temperature of the rubber material or the thickness of the profile. Furthermore, there typically is an influence from the aging of the rubber material that changes its flexibility due to a loss of softening plasticizer depending on time and ambient conditions. Since the different resonance modes represent different deformations of the tire during the vibration, it can be assumed that the influence of each parameter on each resonance is different. If multiple resonance effects are identified, these can be used to isolate the influence of the different parameters on tire pressure. In other embodiments, it can be possible to recognize the type of tire by comparison of the multiple resonance effects with known resonance signatures of tires based on size, model, materials and/or manufacturer. Or, conversely, data can be obtained and models built for each tire and/or each tire/vehicle combination and then stored in a memory of system  100  for use during operation. 
     Referring again to system  10  as depicted in  FIG. 1A , characterization engine  12  interfaces with direct TPMS  14  and indirect TPMS  16 . In an embodiment, characterization engine  12  is configured to receive tire pressure data from direct TPMS  14 , receive data representative of wheel speed of a tire from indirect TPMS  16 , analyze the indirect TPMS  16  data to determine a plurality of resonance frequencies associated with the data, incorporate the tire pressure data to remove the influence of pressure on the plurality of resonance frequencies, and calculate one or more tire characteristics based on the plurality of resonance frequencies. In embodiments, other sensor  18  data can be received from direct TPMS  14  in order to eliminate additional influence parameters in the plurality of resonance frequencies. 
     Referring to  FIG. 1B , a block diagram of another embodiment of a tire characterization system  20  that utilizes RFA in accordance with an embodiment is depicted. Characterization system  20  generally comprises a direct TPMS  24  and an indirect TPMS  26 . Direct TPMS  24  and indirect TPMS  26  are substantially similar to direct TPMS  14  and indirect TPMS  16  as described above, except that characterization engine  22 , which is substantially similar to characterization engine  12 , can be implemented as a component of indirect TPMS  26 . In such embodiments, indirect TPMS  16  and direct TPMS  24  are configured to interface with each other in order to incorporate the direct measurements of sensor  28  into the system of equations calculated by the RFA analysis of indirect TPMS  26 . 
     Thus, in one embodiment and referring to  FIG. 6 , a method  500  of characterizing at least one parameter on a vehicle includes obtaining a wheel speed signal at  502 . At  504 , at least one vibration mode in the wheel speed signal is identified. Then, at  506 , one or more characteristics influencing the vibration mode, including at least one characteristic of interest (e.g., tire pressure), are identified and, at  508 , resolved to isolate at least one individual parameter of interest. For example, an unknown (without incorporation of direct TPMS data) parameter that influences the resonance can be isolated. In embodiments, at least two vibration modes in the wheel speed signal are identified. 
     In an optional embodiment, at  509 , a functional safety check can be performed. In an embodiment, the preliminarily-resolved at least one individual characteristic of interest can be compared to its corresponding direct measurement for validity analysis or other analysis check or confirmation in order to determine the reasonableness of the corresponding direct measurement. If the direct measurement is determined to be unreasonable compared to the preliminarily-resolved characteristic of interest, the indirect TPMS can proceed to an error state or otherwise abort the instant calculation. If the direct measurement is determined to be reasonable compared to the preliminarily-resolved characteristic of interest, the method  500  of characterizing can proceed to  510 . For example, the direct TPMS tire pressure can be verified as consistent or reasonable with an indirect TPMS tire pressure estimate and be incorporated into the method  500  of characterizing. If inconsistent or unreasonable, the direct TPMS tire pressure can be unincorporated or trigger the method to proceed to an error state, as appropriate. In an embodiment, reasonableness or unreasonableness can be determined by an absolute value difference. In other embodiments, other difference calculations can be performed. In an embodiment, therefore, the direct TPMS system provides redundancy with the indirect TPMS system. 
     At  510 , the one or more direct measurements from sensor  18  of direct TPMS  14 , for example, in system  10  of  FIG. 1A , can be incorporated into the system of equations for the isolated at least one individual parameter of interest. For example, the direct tire pressure measurement can be input for the resolved at least one characteristic of interest. At  512 , one or more additional parameters can then be characterized after the inclusion of the one or more direct measurements. The one or more additional parameters can be characterized based on, for example, analysis of the characteristics influencing the vibration modes. In one embodiment, the influence of the various characteristics can be derived by a system of equations. In other embodiments, additional sensor signals and information can be used, e.g., other signals considered at  502 . 
     In a feature and advantage of embodiments, systems and methods can estimate one or more parameters of a tire that would not be accessible in a RFA analysis without a direct TPMS measurement or measurements. For example, in an embodiment, the temperature of the tire material can be estimated, wherein it is typically difficult to directly measure such material. In other embodiments, other tire parameters can be estimated, such as state of material age or profile thickness. 
     In another feature and advantage of embodiments, systems and methods allow for the comparison of RFA-estimated parameters (from, for example, an indirect TPMS) against an actual measured parameter (from, for example, a direct TPMS). Such comparison can be useful as part of a failure analysis or functional safety analysis, for example. 
     Various embodiments of systems, devices and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, configurations and locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the invention. 
     Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. Moreover, elements described with respect to one embodiment can be implemented in other embodiments even when not described in such embodiments unless otherwise noted. Although a dependent claim may refer in the claims to a specific combination with one or more other claims, other embodiments can also include a combination of the dependent claim with the subject matter of each other dependent claim or a combination of one or more features with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended. Furthermore, it is intended also to include features of a claim in any other independent claim even if this claim is not directly made dependent to the independent claim. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in a claim.