Patent Abstract:
A tire pressure monitoring system and method detects a low tire pressure condition based on an angular velocity of a wheel. When a tire is under-inflated, the radius of the tire decreases, causing the angular velocity of the wheel associated with that tire to increase. Because the angular velocity is correlated with the tire pressure, the angular velocity may be compared with a threshold to determine if the tire pressure is below a desired level.

Full Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention claims the benefit of U.S. Provisional Patent Application No. 60/524,154, filed Nov. 21, 2004. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to tire pressure monitoring system, and more particularly to a method and system for detecting low tire pressure.  
       BACKGROUND OF THE INVENTION  
       [0003]     Vehicles often incorporate tire pressure monitoring systems to alert the user when the pressure in one or more of the tires falls below a desired level. These pressure monitoring systems make it more convenient for the user to detect when a tire is in a low pressure condition because the user no longer needs to remember to check tire pressure manually. Currently-known systems include tire pressure sensors that are associated with each tire.  
         [0004]     To provide the tire pressure information to the user, however, the monitoring system must include extra hardware and calibration processes to enable the user to correlate a given sensor with a particular tire position. For example, additional hardware may be placed in the chassis near each tire to allow the sensor to transmit its pressure information to a central module via a low frequency interface. The central module detects the tire position when it receives the tire pressure information by controlling the additional hardware. The additional devices trigger the tire pressure sensor by a low range, low frequency RF signal. Thus, the central module is able to command a particular sensor associated with a given position to transmit information.  
         [0005]     A unique identification code may be assigned to each tire during a calibration process so that the central module can identify the position of a given tire. If the calibration is conducted manually by the vehicle manufacturer or dealer, no additional hardware is needed. However, the central module must be recalibrated each time the tire positions change (e.g., during rotation) or if a tire is changed, requiring the user to return to the dealer to conduct the recalibration.  
         [0006]     Another proposed system includes mounting four receiver antennas, one antenna for each tire, close to each wheel so that the power of the signal generated by one of the sensors is filtered. However, this approach also requires extra hardware for multiplexing the input of the antennas to the central module.  
         [0007]     There is a desire for a tire pressure monitoring system and method that can detect a tire pressure sensor position without extra hardware or calibration.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention is directed to a tire pressure monitoring system and method that detects a low tire pressure condition based on an angular velocity of a wheel. When a tire is under-inflated, the radius of the tire decreases, causing the angular velocity of the wheel associated with that tire to increase. Because the angular velocity is correlated with the tire pressure, the angular velocity may be compared with a threshold to determine if the tire pressure is below a desired level. A processor in a central module may correlate the angular velocity with the tire pressure.  
         [0009]     By using angular velocity to infer tire pressure, there is no need for separate tire pressure sensors in the system. The system may instead use angular velocity data from existing sensors for tire pressure detection.  
         [0010]     A system that already contains an RF tire pressure sensor may use the information provided by other systems in the car, for example an ABS system, to identify the location of the wheel with low pressure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a diagram illustrating underlying concepts used by one embodiment of the invention;  
         [0012]      FIG. 2  is a block diagram representing components in a tire pressure monitoring system according to one embodiment of the invention;  
         [0013]      FIG. 3  is a flow diagram illustrating a method used by the tire pressure monitoring system according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]      FIG. 1  is a schematic diagram that illustrates the concepts used by the invention to detect low tire pressure according to one embodiment of the invention. In this illustration, it is assumed that a rear tire  10  is inflated to a proper pressure and a front tire  12  has low pressure. The rear and front tires  10 ,  12  in a vehicle are rotatable about rear and front wheels  14 ,  16 , respectively. Because the front tire  12  has a lower pressure, the radius of the front tire  12  has a smaller radius R 2  than the radius R 1  of the properly inflated rear tire  10 .  
         [0015]     The linear velocity of a given wheel  14 ,  16  will always be known because it corresponds to the speed of the vehicle. The linear velocity of the rear wheel  14  and the front wheel  16  will always be the same. However, because the radii R 1  and R 2  are different due to the different tire pressures, the angular velocity ω of the front wheel  16  and front tire  10  will be greater than the angular velocity of the rear wheel  14  and rear tire  10 . More particularly, because w)=v/r, where is the linear velocity and r is the radius, the smaller radius R 2  of the front tire  12  will cause the front wheel  16  to rotate faster than the rear wheel  14 .  
         [0016]     Referring to  FIG. 2 , each tire  10 ,  12  has an associated sensor  20  that measures the tire pressure of the tire and transmits an RF signal indicating the tire pressure of its corresponding tire  10 ,  12  to a central module  22  via an associated antenna  24 . Note that the sensor  20  may already exist in. In one embodiment, the sensor  20  is a device that generates an RF signal output. The central module  22  itself has a receiver  28  that receives the signals from the sensors  20  and a processor  30  that evaluates the sensor signals.  
         [0017]     If all of the tires  10 ,  12  are properly inflated to equal pressures, each sensor  20  will output the same RF signal because each wheel  14 ,  16  will have the same angular velocity. If one or more of the tires  10 ,  12  is under-inflated (e.g., the front tire  12  in  FIG. 1 ), however, the reduced radius of the under-inflated tire  12  will cause the under-inflated tire  12  to rotate faster than the other tires. The sensor  20  associated with the under-inflated tire will therefore output an RF signal indicating the tire pressure has decreased to the receiver  28  of the central module  22 .  
         [0018]     The processor  30  may run a data correlation algorithm that correlates the angular speed of each tire with a specific tire pressure via, for example, a look-up table containing empirically-derived data and/or functions linking the angular speed and tire pressure. The processor  30  may also compare the angular velocity of each wheel with a maximum threshold corresponding to a minimum desirable tire pressure. Alternatively, or in addition, the processor  20  compares the angular velocities of each tire  10 ,  12  to detect if one of the tires has a faster angular velocity than the other three tires.  
         [0019]     By comparing the angular velocities of the tires, either with each other or with a threshold, the central module  22  can detect whether any of the tire has a pressure drop that is sufficient to change the radius of the tire and therefore the angular velocity of its corresponding wheel. The algorithm may take into account other factors, such as tire friction, vehicle turning and braking, etc.) in determining whether a given angular velocity indicates an undesirably low tire pressure.  
         [0020]     Detecting the existence of a low pressure tire is insufficient without a way to identify the position of the tire. To do this without the inconvenience of currently known tire position/identification systems, the central module  22  will request the information of the tire angular speed from a separate system, such as an ABS system (not shown). This system can provide the angular velocity of each wheel together with the location of each tire, thus identifying the tire that has increased the angular velocity, we can find the location of the low pressure tire.  
         [0021]     The receiver  28  of the central module  22  is positioned such that the power characteristic of the RF signal transmitted by each antenna  24  will be different. More particularly, the position of each antenna  24  will affect the power characteristic of the RF signal from that antenna  24  based on its distance from the receiver  28 . The distance between a given antenna  24  and its associated sensor  20  will also influence the power characteristic of the RF signal from that antenna  24 .  
         [0022]     Because the antennas  24  are at different distances and positions relative to the receiver  28 , the processor  30  can correlate a given power characteristic with a given tire position. Each antenna  24  will output an RF signal having a different power characteristic even when the RF signals for each tire are the same (i.e., if all four tires have the same angular velocities). Note that the power characteristics are based on the positions of the tires  10 ,  12  and not the specific tire itself. Thus, if one of the RF signals indicates an angular velocity that is higher than a desired threshold (indicating low tire pressure), it is possible to identify which tire has low tire pressure by simply checking the power characteristic of the RF signal and correlating that power characteristic with the tire position.  
         [0023]     As shown in  FIG. 3 , the processor  30  receives the RF signal from at least one of the sensors  20  via its associated antenna  24  (block  50 ) and evaluating the power characteristic of the signal (block  52 ). In one embodiment, the processor  30  compares the power characteristic of the signal with at least one reference power characteristic. The central module  22  may include a memory  32  that stores four reference power characteristics, one for each tire position in the vehicle ( FIG. 2 ). The processor  30  can therefore correlate the power characteristic of the RF signal it receives with the position of the tire  10 ,  12  associated with the signal (block  54 ), without any tire identification information.  
         [0024]     If the tires are changed or rotated, there is no need for a recalibration because the correlation between the tire pressure and the angular velocity can be detected in any of the four possible locations; the power characteristic of the RF signal does not depend on the characteristics of the tire itself. Instead, the power characteristic depends only on the position of the antenna  24  relative to the sensor  22  and to the receiver  28 . By using the correlation between the tire pressure RF signal sent to the central module and angular velocity to determine tire position, the invention eliminates the need to include extra hardware for transmitting tire location information and also eliminates extra tire identification calibration steps during the manufacturing process and updates during tire position changes.  
         [0025]     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.

Technology Classification (CPC): 1