Abstract:
The invention relates generally to tire monitoring systems for collecting measured tire parameter data during vehicle operation and, more particularly, to a system and method for estimating tire wear state based upon such measurements.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to tire monitoring systems for collecting measured tire parameter data during vehicle operation and, more particularly, to a system and method for estimating tire wear state based upon such measurements. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vehicle-mounted tires may be monitored by tire pressure monitoring systems (TPMS) which measure tire parameters such as pressure and temperature during vehicle operation. Data from TPMS tire-equipped systems is used to ascertain the status of a tire based on measured tire parameters and alert the driver of conditions, such as low tire pressure or leakage, which may require remedial maintenance. Sensors within each tire are either installed at a pre-cure stage of tire manufacture or in a post-cure assembly to the tire. 
         [0003]    Other factors such as tire wear state are important considerations for vehicle operation and safety. It is accordingly further desirable to measure tire wear state and communicate wear state to vehicle systems such as braking and stability control systems in conjunction with the measured tire parameters of pressure and temperature. 
       SUMMARY OF THE INVENTION 
       [0004]    According to one aspect of the invention, a tire wear state estimation system includes a tire pressure measuring device affixed to a vehicle tire for measuring tire inflation pressure and generating tire inflation pressure data; tire vertical mode measuring means for measuring tire vertical mode frequency and generating tire vertical mode frequency data; and tire identification means for generating tire-specific frequency mode coefficients using tire-specific identification data. A tire wear estimation is made based upon the tire inflation pressure data, the vertical mode frequency data, and the tire-specific frequency mode coefficients. 
         [0005]    In another aspect, the tire-mounted pressure measuring device is operative to measure a tire cavity pressure and transmit the tire inflation pressure data from the tire cavity pressure measurement and the tire-specific identification data is stored within and accessible from the tire-mounted pressure measuring device. 
         [0006]    Pursuant to another aspect of the invention, the tire-specific frequency mode coefficients are generated by either using on-vehicle or in-tire measurement of a tire vertical mode frequency, effected respectively from either a wheel-mounted accelerometer or a tire crown-mounted accelerometer. 
         [0007]    The tire wear state estimation system, in another aspect, uses a correlation model between the tire wear state and the tire vertical mode frequency wherein the correlation model employs a recursive least squares algorithm based on a polynomial model capturing a dependency between a wear state of the tire, the tire inflation pressure data, and the tire vertical mode frequency. 
       Definitions 
       [0008]    “ANN” or “Artificial Neural Network” is an adaptive tool for non-linear statistical data modeling that changes its structure based on external or internal information that flows through a network during a learning phase. ANN neural networks are non-linear statistical data modeling tools used to model complex relationships between inputs and outputs or to find patterns in data. 
         [0009]    “Aspect ratio” of the tire means the ratio of its section height (SH) to its section width (SW) multiplied by  100  percent for expression as a percentage. 
         [0010]    “Asymmetric tread” means a tread that has a tread pattern not symmetrical about the center plane or equatorial plane EP of the tire. 
         [0011]    “Axial” and “axially” means lines or directions that are parallel to the axis of rotation of the tire. 
         [0012]    “CAN bus” is an abbreviation for controller area network. 
         [0013]    “Chafer” is a narrow strip of material placed around the outside of a tire bead to protect the cord plies from wearing and cutting against the rim and distribute the flexing above the rim. 
         [0014]    “Circumferential” means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction. 
         [0015]    “Equatorial Centerplane (CP)” means the plane perpendicular to the tire&#39;s axis of rotation and passing through the center of the tread. 
         [0016]    “Footprint” means the contact patch or area of contact created by the tire tread with a flat surface as the tire rotates or rolls. 
         [0017]    “Groove” means an elongated void area in a tire wall that may extend circumferentially or laterally about the tire wall. The “groove width” is equal to its average width over its length. A grooves is sized to accommodate an air tube as described. 
         [0018]    “Inboard side” means the side of the tire nearest the vehicle when the tire is mounted on a wheel and the wheel is mounted on the vehicle. 
         [0019]    “Kalman Filter” is a set of mathematical equations that implement a predictor-corrector type estimator that is optimal in the sense that it minimizes the estimated error covariance when some presumed conditions are met. 
         [0020]    “Lateral” means an axial direction. 
         [0021]    “Lateral edges” means a line tangent to the axially outermost tread contact patch or footprint as measured under normal load and tire inflation, the lines being parallel to the equatorial centerplane. 
         [0022]    “Luenberger Observer” is a state observer or estimation model. A “state observer” is a system that provide an estimate of the internal state of a given real system, from measurements of the input and output of the real system. It is typically computer-implemented, and provides the basis of many practical applications. 
         [0023]    “MSE” is an abbreviation for Mean square error, the error between and a measured signal and an estimated signal which the Kalman Filter minimizes. 
         [0024]    “Net contact area” means the total area of ground contacting tread elements between the lateral edges around the entire circumference of the tread divided by the gross area of the entire tread between the lateral edges. 
         [0025]    “Non-directional tread” means a tread that has no preferred direction of forward travel and is not required to be positioned on a vehicle in a specific wheel position or positions to ensure that the tread pattern is aligned with the preferred direction of travel. Conversely, a directional tread pattern has a preferred direction of travel requiring specific wheel positioning. 
         [0026]    “Outboard side” means the side of the tire farthest away from the vehicle when the tire is mounted on a wheel and the wheel is mounted on the vehicle. 
         [0027]    “Peristaltic” means operating by means of wave-like contractions that propel contained matter, such as air, along tubular pathways. 
         [0028]    “Piezoelectric Film Sensor” a device in the form of a film body that uses the piezoelectric effect actuated by a bending of the film body to measure pressure, acceleration, strain or force by converting them to an electrical charge. 
         [0029]    “PSD” is Power Spectral Density (a technical name synonymous with FFT (Fast Fourier Transform). 
         [0030]    “Radial” and “radially” means directions radially toward or away from the axis of rotation of the tire. 
         [0031]    “Rib” means a circumferentially extending strip of rubber on the tread which is defined by at least one circumferential groove and either a second such groove or a lateral edge, the strip being laterally undivided by full-depth grooves. 
         [0032]    “Sipe” means small slots molded into the tread elements of the tire that subdivide the tread surface and improve traction, sipes are generally narrow in width and close in the tires footprint as opposed to grooves that remain open in the tire&#39;s footprint. 
         [0033]    “Tread element” or “traction element” means a rib or a block element defined by having a shape adjacent grooves. 
         [0034]    “Tread Arc Width” means the arc length of the tread as measured between the lateral edges of the tread. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The invention will be described by way of example and with reference to the accompanying drawings in which: 
           [0036]      FIG. 1  is a tread wear state estimation system schematic diagram. 
           [0037]      FIG. 2A  is a frequency graph showing an amplitude vs. frequency comparison of the vertical mode at three loading levels and at a tire inflation pressure of  32  psi. 
           [0038]      FIG. 2B  is a frequency graph showing amplitude vs. frequency comparison of the vertical mode of a tire inflated to  36  psi at three loading levels. 
           [0039]      FIG. 2C  is a frequency vs. amplitude graph showing the vertical mode of a tire inflated to  40  psi at the referenced three loading levels. 
           [0040]      FIG. 3A  is a frequency vs. amplitude graph showing the vertical mode at a tire loading of  700  pounds on a tire comparing two tire cleat sizes. 
           [0041]      FIG. 3B  is a frequency vs. amplitude graph showing the vertical mode at a tire loading of  1300  pounds on a tire comparing two tire cleat sizes. 
           [0042]      FIG. 4A  is a summary frequency vs. vertical mode amplitude graph for a tire cleat of 3 mm size at three different tire loading levels. 
           [0043]      FIG. 4B  is a summary frequency vs. vertical mode amplitude graph for a tire cleat of 5 mm size at three different tire loading levels. 
           [0044]      FIG. 5  is a set of graphs showing frequency vs. vertical mode amplitude for a tire inflated to 40 psi and 32 psi, comparing the experimental results attained at four different vehicle speeds. 
           [0045]      FIG. 6  is a comparison graph of vertical mode frequency vs. amplitude for full, half and no tread conditions and showing sensitivity of the vertical mode in tabular form. 
           [0046]      FIG. 7A  is a graph similar to  FIG. 6  at a tire inflation of 32 psi and showing in table form the identified tire vertical mode frequency for the three tire wear states. 
           [0047]      FIG. 7B  is a graph and table similar to  FIG. 7A  for a tire inflated to 36 psi. 
           [0048]      FIG. 7C  is a graph and table similar to  FIG. 7A  for a tire inflated to 40 psi. 
           [0049]      FIG. 8  is a three dimensional graph showing the “goodness” of model fit, showing tire vertical mode frequency vs. tread depth and pressure. 
           [0050]      FIG. 9  is a block diagram of the tire wear state system. 
           [0051]      FIG. 10A  are graphs showing estimator performance for a new tire at a speed of 60 mph and inflation pressure of 36 psi. 
           [0052]      FIG. 10B  are graphs showing estimator performance for a half-worn tire. 
           [0053]      FIG. 10C  are the graphs showing estimator performance for a fully worn tire. 
           [0054]      FIG. 11  is a graph showing successful extraction of the tire vertical mode from the vertical accelerations signal of a hub mounted accelerometer. 
           [0055]      FIG. 12  is a surface effect graph of FFT for Unsprung Mass condition on concrete at 40 mph. 
           [0056]      FIG. 13  is a comparative graph of FFT-Fz showing amplitude vs. frequency for full tread, half tread and no tread conditions. 
           [0057]      FIG. 14  is a block level diagram of the tire wear system using On Vehicle Algorithm Implementation and Vehicle Measurement of the Tire Vertical Mode. 
           [0058]      FIG. 15  is a graph showing a second approach to tire vertical mode determination, using the vertical acceleration signal of a crown mounted accelerometer. 
           [0059]      FIG. 16A  is a graph showing the results of a load dependence study in the frequency domain showing the tire vertical mode for three load levels in power [db] vs. frequency. 
           [0060]      FIG. 16B  is a graph showing the results of a pressure dependence Study in the Frequency Domain for a tire travelling at 30, 45, 65 mph and at inflation levels of 32 and 35 psi. 
           [0061]      FIG. 17  is a block level diagram of On Vehicle Algorithm Implementation using In-tire Measurement of the Tire Vehicle Mode. 
           [0062]      FIG. 18A  shows graphs indicating the raw signal of hub vertical acceleration for a new, half worn and fully worn tire and the FFT Hub Acceleration signal in the frequency domain. 
           [0063]      FIG. 18B  are graphs showing the detrended FFT Hub Acceleration Signal for a new, half worn and fully worn tire and the peak fit graph of the acceleration signal graph. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0064]    Referring to  FIG. 1 , a tire tread wear estimation system  10  is shown based on spectral analysis of the tire vertical vibration signal. Such a system is useful in advising a vehicle owner on when to change tires and may be used to provide a driver with information on the interrelation between the state of tire tread wear and other factors such as road condition. Tire properties generally change as a function of tire wear. Accordingly, an estimate of tire tread wear level may be used as one input for tire state estimation. 
         [0065]    Tires (a representative one)  12  mounted to a vehicle  22  include ground-engaging tread regions  14  that wear over time. The tires  12  enclose a tire cavity  18  by means of a tire innerliner  16 . A tire pressure monitoring system module (TPMS)  20  may be affixed to the tire innerliner  16 . The module  20  stores tire ID information from which tire specific construction data may be identified. 
         [0066]    The system  10  employs a tire wear state estimation algorithm uses signals  26  available on a CAN bus (controller area network)  24  of vehicle  22 . The signals may include wheel speed signals, useful as an input for an ABS (anti-lock braking system) and/or a wheel hub acceleration signal, on vehicles equipped with an active suspension management system. From the wheel speed signal, mode extractions  28  are made, extracting torsional modes, and from the wheel hub acceleration signal, extracting tire vertical modes. An evaluation  30  is then conducted of the tire vertical modes to correlate the influence of the tire wear state (depth level of the tire tread  14 ) on the tire vertical mode by using spectral analysis methods. Application  32  of a correlation model is made between the tire wear state and the tire vertical mode using the tire specific models developed from TPMS-facilitated tire identification information. 
         [0067]    With reference to  FIG. 2A , the tire vertical vibration mode FFT-FZ of a vibrating tire is examined. Experimental results are illustrated graphically in graph  28  of amplitude vs. frequency (Hz) of the vibrating tire having an inflation of 32 psi for three loadings: 700, 1000 and 1,300 pounds. A wheel hop mode exits around 10 Hz (not shown in the graph) as the rim and the belt rotating the experimental tire move up and down together. The next vertical mode, the tire vertical mode, exists around 80 Hz as illustrated by peak  100 ; in this vertical belt mode the belt moves up and down but the rim does not move as much. The tire vertical tire vertical mode detected experimentally at the three loadings is shown in the associated table of  FIG. 2A ; for 700 pounds a vertical mode at 80 Hz; for 1000 pounds a vertical mode at 84 Hz., and for 1300 pounds a vertical load at 85 Hz. The wheel hop vertical mode depends mainly on suspension spring properties supporting the test wheel and tire and the overall tire stiffness. 
         [0068]      FIGS. 2B and 2C  respectively show for comparison purposes a testing of a rotating suspended tire and wheel assembly by graphs  30 ,  32  for tire inflations of 36 and 40 psi respectively. For the tire vertical mode, the following equation holds: 
         [0000]      Vertical mode frequency ω=square root of k/m
 
         [0000]    where “k” is the tire carcass, inflation pressure dependent and m is the mass of the tire belt and tread mass. The dependence of tire wear (reduction in mass of the tread) and tire vertical mode of a rotating tire forms the basis for a correlation model between the tire wear state and the tire vertical mode frequency. 
         [0069]    Apart from tire inflation pressure and tread mass, other factors were determined which influence tire vertical mode frequency. Those other factors include tire load, rolling speed and road roughness level (smooth versus rough or very rough). Inflation pressure affects the vertical stiffness of a tire; tread depth affects belt mass (m); vertical load affects impact force; rotational velocity affects impact force and stiffness; and road roughness affects input excitation. The graphs  34 ,  36 ,  38 ,  40  shown respectively in  FIGS. 3A ,  3 B,  4 A,  4 B show experimentally how different operating conditions influence the resonance frequencies of the tire. Cleated wheel tests were performed on a fixed spindle machine. Cleat inputs are known to introduce torsional and vertical excitations in a tire while the spindle machine controls tire load and rolling speed. The road roughness effects are captured by using cleats of different sizes and the inflation pressure was manually changed prior to each test. The wear dependencies were captured by using tires with different levels of non-skid depth. 
         [0070]    A hub force measurement was made on a tire using the cleated wheel test and from the hub force measurement, tire vibration modes were determined by means of an FFT (Fast Fourier Transform) analysis. The FFT analysis, conventionally used as a signal processing tool, yields tire vibration modes including the vertical mode represented in the subject graphs. As used herein, FFT is an algorithmic tool which operates by decomposing an N point time domain signal into N time domain signals each composed of a single point. The second step is to calculate the N frequency spectra corresponding to these N time domain signals. Lastly, the N spectra are synthesized into a single frequency spectrum. 
         [0071]    In  FIG. 3A , cleats of 3 mm and 5 mm were used and the vertical mode FFT-Fz determined for a load of 700 pounds. In  FIG. 3B , for the same two cleat sizes, the graph  36  at a tire loading of 1300 pounds is shown. As reflected in the graphs  34 ,  36 , results indicated that a change in the tire loading condition influences the signal amplitude but changes in the signal spectral content (mode frequency) were relatively low. 
         [0072]    Tire inflation dependencies were also tested as reflected in test results of  FIGS. 4A and 4B .  FIG. 4A  shows the test results  38  for a tire on a cleated wheel with cleat size of 3 mm for inflation pressures of 32, 36, and 40 psi. The vertical modes at the tested inflation pressure were found to be, respectively 80, 84, and 85 Hz. The results indicate that changes in signal amplitude and its spectral content are moderately high as a result of tire inflation variance. 
         [0073]      FIG. 4B  shows test results of the tire under a range of loads on a wheel having cleats of 5 mm. The graph  40  summarizes speed dependency and shows that a change in the tire rolling speed influences the signal amplitude but changes in the signal spectral content are reasonably low. A more significant change in the spectral content at higher speeds can occur where the centrifugal stiffening effect on the tire becomes a dominant factor. 
         [0074]      FIG. 5  summarizes graphically with graphs  42 ,  44  and by table the vertical mode variation under 32, 40 psi inflation levels for a range of speeds and confirms the conclusions summarized above. 
         [0075]    In  FIG. 6 , the dependency of the vertical mode frequency FFT-Fz is graphically shown in graph  46  for a tire at full tread, half tread and no tread. As seen by the differing vertical mode computations in the table, tire wear state dependencies create the highest vertical mode divergencies. The relatively high tire wear state dependencies provide the verification and basis for the development of a tire wear state estimation algorithm pursuant to the invention. 
         [0076]    Graphs  48 ,  50 ,  52  of  FIGS. 7A  through C, respectively, show test results indicative of tread wear state dependencies for a test tire at respective inflation levels of 32, 36, and 40 psi. The graphs and their tabular summaries indicate that the trends (tread wear dependencies) are consistent across all inflation pressure conditions. 
         [0077]    In  FIG. 8 , the goodness of fit of the tread wear estimation model is shown by graph  54 . The model fit is compared against experimental data and with the fit yielding a correlation coefficient (r)=0.988. Validation of the model is thus indicated. A polynomial model (second-order in pressure and first-order in tread depth) were found to give a good fit. 
         [0078]    Referring to  FIG. 9 , a tire-based flow chart model implementation is shown. The tire wear state is derived from a model  58  capturing the dependencies between the tire wear state, inflation pressure and the tire vertical mode frequency. Inflation pressure of the tire  12  and tire ID information  56  is obtained from the TPMS module  20  mounted to the tire. The Model Coefficients are tire-construction specific and are ascertained by the tire identification obtained from TPMS stored data. For a given tire construction, a recursive least squares (RLS) estimate of the tire wear state can be made using the tire inflation pressure, tire ID (to use the correct model coefficients) and the tire vertical mode frequency information. The sensors and data stored within TPMS module  20  is used to obtain pressure and tire ID information. The measurement of tire vertical mode on the vehicle is derived from either a wheel-mounted accelerometer or a tire crown-mounted accelerometer by using a spectral analysis methods. 
         [0079]    The RLS Estimation Algorithm (with forgetting factor) provides a method to iteratively update the unknown parameter at each sampling time to minimize the sum of the squares of the modeling error using the past data contained within the regression vector. The following is the model capturing the dependency between the tire wear state, inflation pressure and the tire vertical mode frequency: 
       Fitting Model 
       [0080]      Tire Vertical Mode Frequency= p 00 +p 10*pressure+ p 01*tread depth+ p 20*pressurê2+ p 11*pressure*tread depth
 
         [0081]    Model Coefficients (with 95 percent confidence bounds):
       p00=−35.94 (−171.6, 99.72)   p10=6.586 (−0.9712, 14.14)   p01=−2.31 (−5.37, 0.7512)   p20=−0.08333 (−0.1881, 0.02144)   p11=0.01786 (−0.06682, 0.1025)       
 
         [0087]    The above equation can be rewritten into a standard parameter identification form as follows: 
         [0000]      y=ψ τ θ
 
         [0088]    Where: 
         [0000]        y =(Tire Vertical Mode Frequency− p 00 −p 10*pressure− p 20*pressurê2)/( p 11*pressure+ p 01)
 
         [0000]      ψ=1
 
         [0000]      θ=Tread depth (unknown—to be estimated)
 
         [0089]    The procedure for solving the RLS problem is as follows:
       Step  0 : Initialize the unknown parameter θ( 0 ) and the covariance matrix P( 0 ); set the forgetting factor λ.   Step 1: Measure the system output y(t) and compute the regression vector φ(t).   Step 2: Calculate the identification error e(t):       
 
         [0000]        e ( t )= y ( t )−φ T ( t )·θ( t− 1)
       Step 3: Calculate the gain k(t):       
 
         [0000]        k ( t )= P ( t− 1)φ( t )[λ+φ T ( t ) P ( t− 1)φ( t )] −1  
       Step 4: Calculate the covariance matrix:       
 
         [0000]        P ( t )=(1 −k ( t )φ T ( t )λ −1   P ( t− 1)
       Step 5: Update the unknown parameter:       
 
         [0000]      θ( t )=θ( t− 1)+ k ( t ) e ( t )
       Step 6: Repeat Steps 1 through 5 for each time step.
 
Where y is the output; ψ is the regression vector; and θ is the unknown parameter. The inputs of regression vector and output are used respectively as input and output in the Recursive Least Squares (with forgetting factor) Parameter Estimation Algorithm to solve for the unknown parameter of the tire tread depth.
       
 
         [0097]    Referring to  FIGS. 10A  through C, estimation performance using the recursive least squares (RLS) estimation algorithm (with forgetting factor) is shown graphically by tire vertical mode [Hz] and corresponding tread depth (estimation) graphs.  FIG. 10A  graphs  60 ,  62  are for a new tire;  FIG. 10B  graphs  64 ,  66  for a half-worn tire; and  FIG. 10C  graphs  68 ,  70  for a fully worn tire. The tests were conducted at a tire speed of 60 kph and inflation pressure of 36 psi. As will be seen, the estimates resulting from use of the RLS algorithm are accurate. 
         [0098]    The subject method for tire wear estimation may utilize either an on-vehicle measurement of the tire vertical mode or an in-tire measurement of the tire vertical mode, or both for the purpose of cross-validation. For on-vehicle measurement, the vertical mode is extracted from the vertical acceleration signal of a hub-mounted accelerometer. Hub-mounted accelerometers are commercially available and are used as part of vehicle suspension management systems. From tests conducted on various surfaces, it was found that the tire vertical mode was successfully detected under all the test conditions. For example, graph  74  of  FIG. 12  shows the vertical mode from an accelerometer measurement conducted on a tire travelling on concrete at 40 mph. The amplitude of 9.208 and frequency [Hz] 74.38 were measured. Similar testing was conducted on an impact strip at a travel speed of 25 mph and a pothole at 10 mph. The test results all verified the extracting vertical mode from a hub-mounted accelerometer for the purpose of tread wear estimation pursuant to the subject methodology. 
         [0099]      FIG. 14  shows in block level diagram the on-vehicle algorithm implementation using vehicle measurement of the tire vertical mode. The vehicle  22  provides by CAN bus vehicle speed, load and throttle position used to detect the tire free rolling condition to a tire wear state estimation model  80 , represented graphically in  FIG. 13 . Also obtained from the vehicle is the hub vertical acceleration signal from which through spectral analysis the tire vertical mode frequency is obtained. The tire vertical mode frequency is likewise input into the tire wear state estimation model  80 . From the tire  12 , inflation pressure and tire ID data is obtained from the TPMS module  20  and input into the tire wear state estimation model.  FIG. 13  shows the model for full tread, half-tread, and no tread results in graph  76 . 
         [0100]    A second approach to measurement of the tire vertical mode envisions an in-tire measurement. An accelerometer is mounted in the crown region (tread  14 ) of the tire  12 . From the sensor, a vertical acceleration signal is obtained as represented by graph  82  of  FIG. 15 , the leading and trailing edges of which being identified. From the vertical acceleration signal, the tire vertical mode is extracted using FFT analysis technique common in the field of signal processing. Load dependence and pressure dependence studies in the frequency domain are shown graphically in  FIGS. 16A and 16B  by graphs  84 ,  86 , respectively. In  FIG. 16A , the tire vertical mode is identified for loading of 1300, 1500 and 1800 pounds. In  FIG. 16B , pressure dependence results summarized in graph  86  are form speeds of 30, 45 and 65 mph and inflation pressures of 32 and 35 psi. The results verify the efficacy of using a tire-based accelerometer sensor to identify the tire vertical mode. 
         [0101]    In  FIG. 17 , on-vehicle algorithm implementation system for in-tire measurement of the tire vertical mode is shown in block diagram. The vehicle  22  provides by CAN bus  24  the vehicle speed, load and throttle position as input into the tire wear state estimation model shown graphically in  FIG. 13 . From the tire  12 , the TPMS (including an accelerometer) module  20  provides inflation pressure, tire ID data and a radial acceleration signal from a crown mounted accelerometer. From a spectral analysis of the radial acceleration signal, as explained above, the tire vertical mode frequency is obtained and input to the tire wear state estimation model ( FIG. 13 ). An estimated tire wear state can thus be obtained. 
         [0102]      FIG. 18A  shows graphs  92 ,  94  depicting raw hub vertical acceleration signals for new, half worn and fully worn tires and the hub acceleration signal for the three tire conditions.  FIG. 18B  shows the detrended (corrected) hub acceleration signal  96  for the three tire conditions and peak fit hub acceleration signals  98  for the three tire conditions derived therefrom. The peak fit approach indicates a close fit between the graphs in all tire conditions. 
         [0103]    From the foregoing, it will be appreciated the subject tread wear estimation system utilizes a novel algorithm to estimate the tire wear state. Tire wear state is recursively estimated by using a RLS algorithm formulated based on a polynomial model which captures the dependencies between the tire wear state, inflation pressure and the tire vertical mode frequency. The model inputs for the RLS algorithm include: tire inflation pressure, tire ID (required for using the correct tire specific model coefficients) and the tire vertical mode frequency. The tire inflation pressure and tire ID information is available from a tire attached TPMS module. Information about the tire vertical mode frequency can be obtained by using one of the following methods: 
         [0104]    Approach 1: Using on-vehicle measurement of the tire vertical mode, i.e. extract the tire vertical mode frequency from the vertical acceleration signal of a hub (wheel) mounted accelerometer. 
         [0105]    Approach 2: In-tire measure of the tire vertical mode, i.e. extract the tire vertical mode frequency from the vertical acceleration signal of a crown mounted accelerometer. 
         [0106]    Both approaches may be employed for cross-validation of results. The application of the real-time RLS algorithm in achieving the desired tread wear estimation and accurate estimation results were experimentally validated. 
         [0107]    Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.