Patent Publication Number: US-2006010961-A1

Title: Method and apparatus for detecting leakage rate in a tire pressure monitoring system

Description:
RELATED APPLICATIONS  
      The present invention is related to applications (Attorney Docket 201-1003) entitled “Method And System For Mitigating False Alarms In A Tire Pressure Monitoring System For An Automotive Vehicle”; (Attorney Docket 201-0718) entitled “Method And System For Resetting Tire Pressure Monitoring System For An Automotive Vehicle”; (Attorney Docket 201-0745) entitled “Method And System For Detecting The Presence Of A Spare Replacement In A Tire Pressure Monitoring System For An Automotive Vehicle”; (Attorney Docket 201-0690) entitled “Method And System For Automatically Extending A Tire Pressure Monitoring System For An Automotive Vehicle To Include Auxiliary Tires”; (Attorney Docket 201-0738) entitled “Method And System Of Notifying Of Overuse Of A Mini-Spare Tire In A Tire Pressure Monitoring System For An Automotive Vehicle”; (Attorney Docket 201-1265) entitled “Tire Pressure Monitoring System With A Signal Initiator”; (Attorney Docket 201-1389) entitled “Method And Apparatus For Automatically Identifying The Location Of Pressure Sensors In A Tire Pressure Monitoring System”; (Attorney Docket 201-1424) entitled “Method And Apparatus For Reminding The Vehicle Operator To Refill The Spare Tire In A Tire Pressure Monitoring System”. Each of these applications are incorporated by reference herein.  
     TECHNICAL FIELD  
      The present invention relates generally to a tire pressure monitoring system for an automotive vehicle, and more particularly, to a method and system for detecting a tire leakage rate in a tire pressure monitoring system.  
     BACKGROUND OF THE INVENTION  
      Various types of pressure sensing systems for monitoring the pressure within the tires of an automotive vehicle have been proposed. Such systems generate a pressure signal using an electromagnetic (EM) signal, which is transmitted to a receiver. The pressure signal corresponds to the air pressure within the tire. When the tire pressure drops below a predetermined pressure, an indicator is used to signal the vehicle operator of the low pressure. A tire is made of a porous material, and therefore naturally leaks air over time. If this leakage rate increases, e.g., because the tire integrity has been compromised by a small puncture, a leaky valve, or a defect in the tire/wheel interface a user will be presented with an increased number of warnings from his or her vehicle&#39;s tire pressure monitoring system. Usually, a user will refill a low-pressure tire when presented with such a warning, and will not take the vehicle in for service. Because of this practice, a user will not immediately have the tire checked for integrity if a small leak exists, and will do so only after a number of warnings in a short period of time. However, a tire that has an excessive leakage rate should be checked by a trained technician as soon as possible.  
      It would therefore be desirable to provide a tire pressure monitoring system that can determine when a tire has an excessive leakage rate.  
     SUMMARY OF THE INVENTION  
      The present invention provides a system and method for identifying the position of the tires relative to the vehicle.  
      In one aspect of the invention, a method for determining an excessive air leakage rate in a tire of a vehicle with a tire pressure monitoring system is disclosed. This method comprises the steps of determining a starting tire pressure and a starting tire temperature of a tire of a vehicle at a first time. The method further comprises determining a current tire pressure and a current tire temperature of the tire at a second time. The method also comprises determining a time lapse between the first and second time. The method additionally comprises the step of calculating a tire leakage rate of the tire based on the starting tire pressure, the starting tire temperature, the current tire pressure, the current tire temperature, and the time lapse.  
      In a further aspect of the invention, a system for determining an excessive air leakage rate in a tire of a vehicle in a tire pressure monitoring system is disclosed. The system comprises a tire temperature sensor that is capable of determining a starting tire temperature of a tire of a vehicle at a first time and a current tire temperature of the tire at a second time. The system further comprises a tire pressure sensor that is capable of determining a starting tire pressure of the tire at approximately the first time and a current tire pressure of the tire at approximately the second time. Additionally, the system comprises a clock timer that is capable of determining a time lapse between the first and second time. The system also comprises a processor that is capable of calculating a tire leakage rate of the tire based on the starting tire pressure, the starting tire temperature, the current tire pressure, the current tire temperature, and the time lapse.  
      One advantage of the invention is that the vehicle operator can be presented with instructions to have the vehicle&#39;s tires checked by a trained technician in situations where a tire has an excessive leakage rate. Another advantage of the invention is that the vehicle operator can be quickly alerted of a small leak that may go undetected without such a method or system.  
      Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagrammatic view of a pressure monitoring system according to the present invention.  
       FIG. 2  is a functional flowchart of the monitoring system according to the present invention.  
       FIG. 3  is a block diagrammatic view of a pressure transmitter according to the present invention.  
       FIG. 4  is a diagrammatic view of a digital word from a pressure transmitter.  
       FIG. 5  is a flow chart illustrating determining a pressure status in a first stage of pressure determination according to the present invention.  
       FIG. 6  is a flow chart illustrating determining a warning status in a second stage of pressure determination according to the present invention.  
       FIG. 7  is a state diagram of low pressure sensor status according to the present invention.  
       FIG. 8  is a state diagram of high pressure sensor status according to the present invention.  
       FIG. 9  is a state diagram of a flat pressure sensor status.  
       FIG. 11  is a state diagram of a low pressure warning status.  
       FIG. 12  is a state diagram of a high pressure warning status.  
       FIG. 13  is a state diagram of a flat pressure warning status.  
       FIG. 14  is a flowchart of the operation of the system when a tire pressure is increased by filling.  
       FIG. 15  is a flowchart of the operation of the system when a spare tire is placed into the rolling position.  
       FIG. 16  is a state diagram of the spare tire identification according to the present invention.  
       FIG. 17  is a block diagrammatic view of a trailer having pressure circuits according to the present invention.  
       FIG. 18  is an elevational view of a display according to the present invention.  
       FIG. 19  is a flow chart of a method of automatically updating the tire pressure monitoring system in the presence of additional tires.  
       FIG. 20  is a flow chart of a method for indicating the end of the recommended travel distance of a mini-spare tire.  
       FIG. 21 , a flowchart of the tire location method according to the present invention is shown.  
       FIG. 22 , a flowchart of the tire location method according to the present invention is shown.  
       FIG. 23 , a flowchart of the spare tire reminder system according to the present invention is shown.  
       FIG. 24 , a flowchart of a process for entering the tire location method according to the present invention is shown.  
       FIG. 25 , a flowchart of a process for locating the position of the tires according to the present invention is shown.  
       FIG. 26 , a flowchart of a process for determining the leakage rate of a tire according to the present invention is shown. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      In the following figures, the same reference numerals will be used to illustrate the same components. Those skilled in the art will recognize that the various components set forth herein could be changed without varying from the scope of the invention.  
      Referring now to  FIG. 1 , an automotive vehicle  10  has a pressure monitoring system  12  for monitoring the air pressure within a left front tire  14   a , a right front tire  14   b , a right rear tire  14   c , and a left rear tire  14   d . Each tire  14   a - 14   d  has a respective tire pressure sensor circuit  16   a ,  16   b ,  16   c , and  16   d , each of which has a respective antenna  18   a ,  18   b ,  18   c , and  18   d . Each tire is positioned upon a corresponding wheel.  
      A fifth tire or spare tire  14   e  is also illustrated having a tire pressure sensor circuit  16   e  and a respective antenna  18   e . Although five wheels are illustrated, the pressure of various numbers of wheels may be increased. For example, the present invention applies equally to vehicles such as pickup trucks that have dual wheels for each rear wheel. Also, various numbers of wheels may be used in a heavy duty truck application having dual wheels at a number of locations. Further, the present invention is also applicable to trailers and extra spares as will be further described below.  
      Each tire  14  may have a respective initiator  20   a - 20   e  positioned within the wheel wells adjacent to the tire  14 . Initiator  20  generates a low frequency RF signal initiator and is used to initiate a response from each wheel so that the position of each wheel may be recognized automatically by the pressure monitoring system  12 . Initiators  20   a - 20   e  are preferably coupled directly to a controller  22 . In commercial embodiments where the position programming is done manually, the initiators may be eliminated. In an alternative embodiment, batteryless high frequency initiator systems may be used.  
      Controller  22  is preferably a microprocessor based controller having a programmable CPU that may be programmed to perform various functions and processes including those set forth herein.  
      Controller  22  has a memory  26  associated therewith. Memory  26  may be various types of memory including ROM or RAM. Memory  26  is illustrated as a separate component. However, those skilled in the art will recognize controller  22  may have memory  26  therein. Memory  26  is used to store various thresholds, calibrations, tire characteristics, wheel characteristics, serial numbers, conversion factors, temperature probes, spare tire operating parameters, and other values needed in the calculation, calibration and operation of the pressure monitoring system  12 . For example, memory may contain a table that includes the sensor identification thereof. Also, the warning statuses of each of the tires may also be stored within the table.  
      Controller  22  is also coupled to a receiver  28 . Although receiver  28  is illustrated as a separate component, receiver  28  may also be included within controller  22 . Receiver  28  has an antenna  30  associated therewith. Receiver  30  is used to receive pressure and various information from tire pressure circuits  16   a - 16   e . Controller  22  is also coupled to a plurality of sensors. Such sensors may include a barometric pressure sensor  32 , an ambient temperature sensor  34 , a distance sensor  36 , a speed sensor  38 , a brake pedal sensor  41 , and an ignition sensor  42 . Of course, various other types of sensors may be used. Barometric pressure sensor  32  generates a barometric pressure signal corresponding to the ambient barometric pressure. The barometric pressure may be measured directly, calculated, or inferred from various sensor outputs. The barometric pressure compensation is preferably used but is not required in calculation for determining the pressure within each tire  14 . Temperature sensor  34  generates an ambient temperature signal corresponding to the ambient temperature and may be used to generate a temperature profile.  
      Distance sensor  36  may be one of a variety of sensors or combinations of sensors to determine the distance traveled for the automotive vehicle. The distance traveled may merely be obtained from another vehicle system either directly or by monitoring the velocity together with a timer  44  to obtain a rough idea of distance traveled. Speed sensor  38  may be a variety of speed sensing sources commonly used in automotive vehicles such as a two wheel used in anti-lock braking systems, or a transmission sensor.  
      Timer  44  may also be used to measure various times associated with the process set forth herein. The timer  44 , for example, may measure the time the spare tire is stowed, or measure a time after an initiator signal.  
      Brake pedal sensor  41  may generate a brake-on or brake-off signal indicating that the brake pedal is being depressed or not depressed, respectively. Brake pedal sensor  41  may be useful in various applications such as the programming or calibrating of the pressure monitoring system  12 .  
      Ignition sensor  42  may be one of a variety of types of sensors to determine if the ignition is powered on. When the ignition is on, a run signal may be generated. When the ignition is off, an off signal is generated. A simple ignition switch may act as an ignition sensor  42 . Of course, sensing the voltage on a particular control line may also provide an indication of whether the ignition is activated. Preferably, pressure monitoring system  12  may not be powered when the ignition is off. However, in one constructed embodiment, the system receives information about once an hour after the ignition has been turned off.  
      A telematics system  46  may be used to communicate various information to and from a central location from a vehicle. For example, the control location may keep track of service intervals and use and inform the vehicle operator service is required.  
      A counter  48  may also be included in control system  12 . Counter  48  may count, for example, the number of times a particular action is performed. For example, counter  48  may be used to count the number of key-off to key-on transitions. Of course, the counting function may be inherent in controller  22 .  
      Controller  22  may also be coupled to a button  50  or plurality of buttons  50  for inputting various information, resetting the controller  22 , or various other functions as will be evident to those skilled in the art through the following description.  
      Controller  22  may also be coupled to an indicator  52 . Indicator  52  may include an indicator light or display panel  54 , which generates a visual signal, or an audible device  56  such as a speaker or buzzer that generates an audible signal. Indicator  52  may provide some indication as to the operability of the system such as confirming receipt of a signal such as a calibration signal or other commands, warnings, and controls as will be further described below. Indicator may be an LED or LCD panel used to provide commands to the vehicle operator when manual calibrations are performed.  
      A pressure monitoring system  12  of  FIG. 1 , having various functional blocks is further illustrated in  FIG. 2 . These functional blocks may take place within receiver  28 , controller  22 , or a combination thereof from  FIG. 1 . Also, memory  26  of  FIG. 1  is used to store the various ranges. Referring to  FIG. 2 , an end-of-line (EOL) tester  58  may also be coupled to pressure monitoring system. EOL tester  58  provides test functions to EOL diagnostic block  60 . EOL tester  58  in conjunction with EOL diagnostic block  60  may be used to provide acceptable pressure ranges  62  and other diagnostic functions to determine fault within the system. The EOL tester  58  may be used in the manufacturing process to store various information in the memory such as various thresholds, tire characteristics, and to initially program the locations corresponding to the vehicle tires.  
      Vehicle speed sensor  38 , ignition switch  42 , and brake on/off switch  41  may be coupled to a manual learn mode activation input process block  64 . Together block  64  and sensors  38 ,  41 , and  42  allow an association block  66  to associate the various tire pressure sensors to the locations of the vehicles. Block  66  associates the various tire pressure sensors in the memory at block  68 . The transmissions from the various sensors are decoded in block  70 , which may be performed in receiver  28  above. The decoded information is provided to block  66  and to a block  72 , which processes the various information such as the ranges, the various sensor locations, and the current transmission process. In the processing frame  72  the sensor status pressure and transmission ID may be linked to a tire pressure monitor  74  which may be used to provide a warning status to an output block  76  which in turn may provide information to an external controller  78  and to indicator  52 .  
      An auto learn block  80  may also be used to associate the various tire pressure sensor monitors with the locations of the tires in the vehicle. This process may replace or be in addition to the manual learn block  64 . The auto learn function, however, uses initiators such as the initiator  20   b  as shown. The various features of  FIG. 2  will be described further in more detail.  
      Referring now to  FIG. 3 , a typical tire pressure sensor circuit  16   a  as first described in  FIG. 1  is illustrated. Although only one tire pressure sensor circuit  16  is shown, each may be commonly configured. Pressure monitoring system  12  has a transmitter/receiver or transceiver  90 . Transmitter/receiver  90  is coupled to antenna  18   a  for transmitting various information to receiver  28 . The receiver portion may be used to receive an activation signal for an initiator located at each wheel. The pressure sensor may have various information such as a serial number memory  92 , a pressure sensor  94  for determining the pressure within the tire, a temperature sensor  96  for determining the temperature within the tire, and a motion detector  98  which may be used to activate the system pressure sensing system. The initial message is referred to as a “wake” message, meaning the pressure sensing circuit is now activated to send its pressure transmissions and the other data.  
      Each of the transceiver  90 , serial number memory  92 , pressure sensor  94 , temperature sensor  96 , and motion sensor  98  is coupled to battery  100 . Battery  100  is preferably a long-life battery capable supplying power throughout the life of the tire.  
      A sensor function monitor  101  may also be incorporated into tire pressure sensor circuit  16 . Sensor function monitor  101  generates an error signal when various portions of the tire pressure circuit are not operating or are operating incorrectly. Also, sensor function monitor may generate a signal indicating that the circuit  16  is operating normally.  
      Referring now also to  FIG. 4 , a word  102  generated by the tire pressure sensor circuit  16  of  FIG. 3  is illustrated. Word  102  may comprise a transmitter identification serial number portion  104  followed by a data portion  106  in a predetermined format. For example, data section  106  may include a wake or initial status pressure information followed by temperature information. Motion detector  28  may initiate the transmission of the word  102  to the transmitter/receiver  90 . The word  102  is preferably such that the decode RF transmission block  70  is able to decode the information and validate the word while providing the identification number or serial number, the pressure, the temperature, and a sensor function.  
      Referring now to  FIG. 5 , a high level flow chart illustrating obtaining a sensor pressure status from the measured pressure is illustrated. The pressure status is determined in a similar manner for each of the tires on the vehicle. In block  120  the pressure is measured at the pressure sensor and transmitted to the receiver and is ultimately used in the controller. The pressure measured is compared to a low pressure threshold and a low pressure warning is generated if the measured pressure is below the low pressure threshold. In block  124  if the measured pressure is above the high pressure warning, then a high pressure warning is generated. In block  126  if the measured pressure is below a flat pressure, then a flat pressure warning is generated. In block  128  the pressure status is obtained from blocks  122 ,  124 , and  126 . The sensor pressure status is a first stage of pressure monitoring according to the present invention.  
      Referring now to  FIG. 6 , a second stage of pressure monitoring is illustrated in a high level flow chart view. Once the sensor pressure status is obtained in block  128  of  FIG. 5 , a low pressure warning status, a high pressure warning status, a flat pressure warning status, and an overall sensor status is used to form a composite warning status. In block  130  the low pressure warning status is determined. In block  132  the high pressure warning status is determined. In block  134  a flat pressure warning status is determined. As will be further described below, preferably several measurements take place during normal operation to confirm the status. Each of the low pressure warning status, high pressure warning status, and flat pressure warning status are combined together to form the composite warning status in block  136 . The low pressure warning status, the high pressure warning status, and the flat pressure warning status may have two statuses indicative of a warning state indicating the conditions are not met and a not warning state indicating the conditions are not met.  
      Referring now to  FIG. 7 , a state diagram for determining the sensor pressure status is illustrated. Block  138  corresponds to a not low sensor status. In the following example, both the front tire pressure and the rear tire pressure may have different threshold values. Also, the spare tire may also have its own threshold values. When any of the tires is below its low pressure threshold and a warning status is not low, block  140  is performed. Of course, those in the art will recognize that some hysteresis may be built into the system so that not exactly the same thresholds may be used to transition back. In block  140  the low warning status is determined in the second stage as will be described below. In block  140  when the warning status is not low and the sensor is equal to or above the threshold for the tire, then the sensor pressure status is not low and the system returns to block  138 . In block  140  when a low warning status is determined, then block  142  is performed. In block  142  when the pressure is greater than or equal to the threshold pressure of the associated tire, then block  144  is performed. In block  144  a “not low” warning status is determined as will be further described below. When the tire pressures are less than their associated low thresholds, then block  142  is executed. In block  144  when a warning status of not low is determined, block  138  is executed. Blocks  138  through  144  illustrate a continuous loop in which the sensor readings are monitored and a sensor pressure status and warning status are used to move therethrough.  
      Referring now to  FIG. 8 , a similar state diagram to that of  FIG. 7  is illustrated relative to a high pressure threshold. In block  146  the warning status is not high. To move from block  146  to  148  the pressure of the particular tire exceeds a high pressure threshold. When the pressure reading exceeds one of the high pressure thresholds for one of the tires, block  148  determines a high warning status. A high warning status is determined as will be further described below. When subsequent readings of the pressure sensor are lower than or equal to the high pressure threshold, then block  146  is again executed. In block  148  if the high warning status criteria are met, a high warning status is generated and block  150  is executed. Again, the thresholds may be offset slightly to provide hysteresis. In block  150  when the pressure reading drops below a high pressure threshold then block  152  is executed. If subsequent readings are greater than the high pressure threshold then block  150  is again executed. In block  152  when the not high warning status criteria are met, as will be further described below, a not high warning status is generated and block  146  is again executed.  
      Referring now to  FIG. 9 , a state diagram for determining the presence of a flat tire is illustrated. When the warning status is not flat and the tire pressure for each tire falls below a predetermined flat threshold, then block  156  is executed. Again, the thresholds may be offset slightly to provide hysteresis. In block  156  if a subsequent pressure reading is greater than the flat threshold, then block  154  is again executed. In block  156 , if the criteria for generating a flat warning status is met, as will be further described below, block  158  is executed. In block  158  when the pressure reading of a subsequent reading exceeds or is equal to a flat threshold, then block  160  is executed. Block  160  determines a not flat warning status in a similar manner to that of block  156 . In block  160  if the subsequent readings drop below the flat warning threshold, then block  158  is again executed. In block  160  if the criteria for not flat warning status is met, then block  154  is executed.  
      Preferably, the processes shown in  FIGS. 7, 8 , and  9  are simultaneously performed for each wheel.  
      Referring now to  FIG. 10 , the results obtained from  FIGS. 7, 8 , and  9  are shown in respective columns. True in the columns refers to that pressure threshold being crossed. Thus, the output pressure status shown in the right hand column is “in range” when each of the pressure thresholds are not met. A flat pressure status refers to the flat pressure threshold being met. A low pressure status is obtained when a low pressure threshold is crossed, and a high pressure status when a high pressure threshold is exceeded.  
      Referring now to  FIG. 11 , blocks  140  and  144  of  FIG. 7  are illustrated in further detail. In each of these blocks the qualification process for either a pressure not low warning status or a low pressure warning status is illustrated. Upon an initial status reading the system is set to a not low warning status as indicated by arrow  163  and block  162  is executed. On the initial status reading, if a low pressure status is obtained in the first reading, block  164  is executed which immediately generates a low warning status. Thus, no waiting periods or other measurements are necessary from an initial standpoint. Loop  165  back to the pressure not low block  162  signifies that the initial value was in range and the status value is not an initial value. When the pressure status signal is low from  FIG. 7 , a warning qualification process is started in block  168 . In block  168  if subsequent pressure status signals are not low, block  162  is executed. In block  168  if a predetermined number of pressure status signals are low or a certain number of pressure status signals over a fixed time period are low, for example five warning events, block  164  is executed. In block  164  when a not low pressure status is obtained a qualification timer is initiated in block  170 . If a subsequent low pressure warning is received, then block  164  is again executed. In block  170  if a low warning qualification timer expires, the low warning status if false or “not low pressure” and block  162  is executed. The warning status is initiated as represented by arrow  163  by a wake message received from a spare and the vehicle speed is greater than three miles per hour and the low warning status indicates the tire pressure is not low.  
      Referring now to  FIG. 12 , a state diagram of the qualification for generating a warning status for high pressure is illustrated. Once again, an initial step represented by arrow  177  is a default state in which the initial status is set to not high. In block  178  when the pressure sensor status is high, block  180  is executed in which the high pressure is qualified. In the transition from block  178  to  180  a high warning qualification process is initiated. As mentioned above in  FIG. 11 , the qualification may be a predetermined number of sequential pressure sensor status readings being high or a predetermined number of pressure sensor status readings being high over a predetermined time. In block  180  if a pressure status is not high before qualification, step  178  is re-executed. In block  180  if a predetermined of pressure sensor status readings are high, then a high warning status is generated in block  182 . When a high warning status is generated, if a subsequent pressure status is not high then a qualification timer again starts in block  184 . In block  184  if a subsequent pressure status is high then step  182  is executed. In step  184  the not high pressure is qualified before issuing a not high warning status. Thus, a predetermined number of not high pressure statuses must be received before qualification. When a predetermined number of not high pressures are obtained, step  178  is again executed.  
      Referring now to  FIG. 13 , a flat warning status is generated in a similar manner to the low warning status of  FIG. 11 . The difference between flat warning and low warning is the flat warning is a substantially lower pressure than the low warning. This system also begins when a wake up message is received and the speed is greater than three miles per hour. Other considerations may also initiate the process. The default is illustrated by arrow  191 . When the first pressure status reading is obtained and the pressure sensor status indicates a flat tire, a flat warning status of true is provided in block  194 . Loop  196  resets the initial value flag to false after the initial status value is received. In block  192  if a subsequent sensor pressure status is flat, a qualification timer is initiated in block  198 . In block  198  if a not flat sensor pressure status is received, block  192  is again executed. In block  198  if the qualification process has a predetermined number of flat warning events, either consecutively or during a time period, block  194  is executed. In block  194  if a not flat sensor pressure status is obtained a not flat pressure qualification process is initiated in block  200 . In block  200  if a subsequent flat warning is received, block  194  is again executed. In block  200  if a predetermined number of not flat pressure statuses are provided, the flat warning status is not false, then block  192  is again executed.  
      As mentioned above in  FIG. 6 , the output of the warning status generators of  FIGS. 11, 12 , and  13  generate a composite warning status as illustrated by the following table.  
                                                   Flat   Low   High   Composite           Warning   Warning   Warning   Warning       Sensor Status   Status   Status   Status   Status                  Don&#39;t Care   TRUE   Don&#39;t Care   Don&#39;t Care   Flat       Don&#39;t Care   False   TRUE   Don&#39;t Care   Low       Don&#39;t Care   False   False   TRUE   High       Transmitter_Fau   False   False   False   Fault       In Range   False   False   False   In Range                  
 
      Thus, the composite warning status has an independent flat warning status portion, a high warning status portion, and a low warning status portion. Also, the composite warning may also include a sensor status portion to indicate a transmitter fault on behalf of the pressure sensor. In response to the composite warning status signal, the tire pressure monitoring system may provide some indication through the indicator such as a displayed word, a series of words, an indicator light or a text message that service or adjustment of the tire pressure may be required.  
      Referring now to  FIG. 14 , a method for automatically updating the system when a pressure suddenly increases. In step  220  the tires are associated with the vehicle locations. Various methods for associating the vehicle tire locations are described herein. In step  222  the operator fills the tire and thereby increases the pressure therein. In step  224  the pressure sensor circuit preferably transmits a pressure reading when an increase of a predetermined amount is sensed. In the present example, 1.5 psi is used. Thus, when the pressure increases at least 1.5 psi the system receives a pressure warning from that tire. In step  226  the increased pressure reading is compared to a normal range. If the pressure increase still does not provide a pressure reading within the normal range the warning statuses are maintained in step  228 . In step  226  when the new pressure reading is within the normal range the warnings are automatically reset in step  230  for that particular time. The displays and the warning status memory may all be reset.  
      In step  232  new warning statuses are generated for each of the rolling locations of the vehicle. Also, a new status may also be generated for a spare tire.  
      Referring now to  FIG. 15 , the present invention preferably automatically updates the warning statuses of the system in response to increased tire pressure that indicates replacement of one of the tires with the spare tire. In step  240  each tire is associated with a rolling location in the vehicle. The spare tire is associated with the spare tire location. Various methods for associating as described above may be used. In step  242  the vehicle operator places the spare tire into a rolling position. Preferably, the spare tire is placed in the rolling tire position with a low tire pressure. However, the present invention does not rely upon proper placement. In step  244  the prior spare tire is awakened when rolling movement is provided. The system recognizes that this tire was a previous spare tire and thus now places the spare tire identification into the memory. Thus, the previously spare tire is now associated with a rolling location. When the previously spare tire is associated with a rolling location the warning statuses in the warning status memory are reset in step  246 . In step  248  the previous spare may be associated into the non-rolling location in the memory after the warning status is generated or in step  244  as mentioned above. In step  250  new warning statuses are generated for the rolling locations that include the previous spare tire.  
      The resetting of the warning statuses in step  246  may include resetting the display on which each of the warning statuses are displayed.  
      Referring now to  FIG. 16 , step  240  is illustrated in more detail. The system starts in block  281  when a message expected from a tire is missed by the control system. The missed message may, for example, be from a fourth tire in a four tire system that has been replaced with another tire such as a spare. The missed message initiates a timer represented by arrow  278 . If a message is received before a predetermined time, and the tire is a rolling tire and the timer is stopped as represented by arrow  280 . When the timer expires and the vehicle speed is indicative of the vehicle moving in block  281 , the tire status is set to a pending spare as represented by block  282 . If the vehicle stops moving the tire status is again set to rolling.  
      Referring back to block  282 , when the status is a pending spare status and any of the other tires have a pending rolling status block  284  is executed in which the tire status is set as a spare status. When the tire status is set to spare and a pressure message is received and the vehicle is moving, a counter is initiated and a timer is started as illustrated by arrow  286 . If the timer expires, the count is set to zero as represented by arrow  288  and the spare tire status is maintained. Likewise, if the vehicle is not moving the counter is reset to zero and the timer is stopped as represented by arrow  290 . In this manner the spare tire status is maintained. If the counter counts to a predetermined count indicative of a number of messages received, the tire status is set to pending rolling and the count is reset to zero as represented by block  292 . In block  292  if the vehicle stops moving the tire status is once again returned to spare status and the functions described above with respect to block  284  are executed. In block  292 , if any of the other tire statuses is a pending spare status, then the tire status is rolling and the system returns to block  281 .  
      From the above, it is evident that the vehicle speed sensor and a timer are used to distinguish the various statuses of the vehicle. Thus, when an expected transmission is missed, the system recognizes the spare tire and stores the spare tire identification within the system along with the status. Thereafter, the spare tire becomes recognized as one of the rolling tires and thus the system operates receiving normal updates from each of the tires at the rolling positions. As can be seen at least one tire must be in a pending rolling status and one in a pending spare status for the system to change the status. This indicates the movement of one tire. Also, this system presumes that the identification of the spare is known.  
      Referring to  FIG. 17 , the tire pressure monitoring system  12  described in  FIG. 1  of the present invention is preferably suitable for use with auxiliary tires in auxiliary locations. The auxiliary tires may, for example be positioned on a trailer  300 . Trailer  300  is illustrated having a plurality of auxiliary positions including trailer tires  14 F,  14 G,  14 H, and  14 I. The trailer may also carry spare tires in auxiliary locations such as tire  14 J and  14 K. Each of the auxiliary tires includes a respective transmitter  16 F- 16 J and a transmitting antenna  18 F- 18 J. The vehicle itself may also have an auxiliary location such as on top of the roof, underneath the vehicle, or attached to the rear bumper. The present invention senses the presence of an auxiliary tire associated with the vehicle and programs the auxiliary transmitter&#39;s identification into the warning status memory. Each of the vehicle transmitters  16 F- 16 J has an associated transmitter identification. The transmitter identifications are programmed into the system so that little chance of erroneous entry is provided.  
      Referring now to  FIG. 18 , indicator  52  of  FIG. 1  is illustrated as an LED display  302 . LED display  302  has LEDs  304 A,  304 B,  304 C, and  304 D corresponding to rolling locations of the vehicle. In addition, an LED  304 E corresponding to the position of the spare tire location is shown. In addition, an auxiliary LED  304 F is shown. LED  304 F corresponds to one or many of the auxiliary locations possible. Of course, those skilled in the art will recognize that several auxiliary LEDs may be incorporated into display  302 . An audible indicator  306  may also be incorporated into display  302 . Various other forms of display such as a liquid crystal display, navigation system display, or other types of displays may be incorporated into the system.  
      Referring now to  FIG. 19 , a method according to the present invention is shown. In step  310  a plurality of transmissions is received from the transmitters around the vehicle. These transmitters may include transmitters that have not yet been programmed into the vehicle warning status memory. It should be noted that the auxiliary sensors as well as other transmissions from adjacent vehicle transmitters may also be received. In step  314 , the amount of time of a transmission is also monitored. The amount of time may, for example be the cumulative time or the cumulative time over a monitored period. In step  316  when the vehicle has been in motion for a predetermined amount of time as measured by steps  312  and  314 , step  318  is executed. In step  318  if more than five sensors have been received for at least a predetermined amount of time, step  320  is executed. Step  318  used five sensors to indicate four rolling sensors and one spare tire sensor. However, the number five is used to signify the normal amount of tires typically associated with a vehicle. This number may be increased when vehicles have multiple tires in various locations. In step  320  an extended mode is entered to indicate that more than the normal amount of tires are associated with the vehicle. The pressure transmitter identifications have been transmitted for a predetermined amount of time while the vehicle has been moving and thus these transmitters are most likely associated with the vehicle rather than a nearby vehicle.  
      In step  322  a learn mode is entered. In step  324  the auxiliary transmitter identifications are added to the warning status memory. Thus, the rolling tires, the spare tires, and any auxiliary tire transmitter identification numbers are now associated with the warning status memory. In step  326  warning statuses for all the sensors may be generated as described above. Preferably, a warning status is provided when a tire is over pressure, under pressure, or flat. Referring back to step  318 , when no more than the normal number of transmitter identifications is received, a normal mode is entered in step  328  to indicate to the system that no further identifications need to be programmed into the system. In step  328  the display is used to display the various warning statuses for each of the tire locations.  
      It should be noted that adding auxiliary tires to the system requires a tire transmitter to be added to the valve stem, or attached to the wheel as well, of any additional auxiliary tires if one is not present. This addition is relatively easy. The system may automatically switch from normal mode to extended mode as described above. However, step  318  may be replaced by detection that a trailer has been electrically connected to a trailer socket. The buttons  50  above may be used to program in various pressure thresholds in the case that the auxiliary tires have different pressure thresholds for the flat tire, low tire, and high pressure settings.  
      Referring now to  FIG. 20 , a system for warning of use of a mini-spare is started in step  350 . In step  350  it is determined whether the mini-spare has replaced a rolling tire. If the mini-spare has not replaced the rolling tire then step  350  is repeated. The presence of the mini-spare is preferably determined automatically such as in the manner described above. Also, the operator of the vehicle may push a button or otherwise manually enter the presence of the mini-spare into the system. For automatic programming, the spare tire may provide a special data signal indicating that the tire is a mini-spare rather than a regular spare tire.  
      In step  351  the speed of the mini-spare is determined. The speed of the mini-spare may be determined as a function of the vehicle speed. That is, the vehicle speed may correspond exactly to the speed of the mini-spare. In step  352  the mini-spare speed is compared to a mini-spare speed threshold. The mini-spare speed threshold is typically provided by the manufacturer of the mini-spare. Oftentimes the speed threshold is about 55 miles per hour. The mini-spare speed threshold may be programmed at the factory during assembly of the vehicle or may be manually entered into the system. In step  352 , if the mini-spare speed threshold has been exceeded a warning signal is generated in step  354 . The warning signal may, for example, be an audible signal or a visual signal. The audible signal may be provided through a warning buzzer or chime. The visual signal may provide a display or LED display.  
      Referring back to step  350 , the distance may also be determined simultaneously with the speed of step  351 - 354 . In step  358 , the distance from replacement is measured as the vehicle travels. The distance measured may be activated by the replacement of the spare. That is, the distance may start to be measured when the system receives the mini-spare identification signal. Of course, in a manual system the distance may be determined from the time of manually entering the presence of a mini-spare into the system. The system may also keep track of the cumulative distance traveled if the spare has been used intermittently.  
      The system may also activate the timer noted above. By determining a time signal from the time of reset and measuring the vehicle speed at various times, the distance traveled may be generated according to the formula  
         D   i     =       ∑     n   =   1     i     ⁢       V   i     *   Δ   ⁢           ⁢     T     i   -   1     i             
          where D i  is the distance traveled from the time the mini-spare is started to be used until the ith measurement of vehicle speed, V i  is the ith measurement of vehicle speed, and ΔT i-1   i  is the amount of time between the ith and (i−1)th measurement of vehicle speed. The distance traveled may also be obtained from odometer readings placed on the communication bus of the vehicle.        

      When in step  360  the mini-spare distance threshold is not exceeded, step  358  is repeated. When the mini-spare threshold is exceeded a distance warning signal is generated in step  362 . The distance warning signal may also be stored in the warning status memory.  
      In step  364  a distance and speed warning is displayed in response to the distance and speed warning signal. The display may be displayed in a variety of manners set forth above such as on an LCD display, a navigation display, an LED display, warning chimes, or the like.  
      It should be noted that the mini-spare takes the place of spare tire  14   e  set forth in  FIG. 1 . In addition, the spare tire may also include a pressure sensing circuit such as that used in a typical rolling tire or a regular spare. The mini-spare is a lighter and more compact version of the regular spare tire.  
      Referring now to  FIG. 21 , a method for automatically determining the location of each of the tires in the vehicle is illustrated in a state diagrammatic form. In block  400  the vehicle speed is measured and the ignition status is also monitored. When the ignition status is in a run state and the vehicle speed is greater than a predetermined speed such as 20 miles per hour, a low frequency initiator is activated and a counter is set to one and a timer is started. In block  402 , a signal from the pressure sensor is expected and thus the system waits for data therefrom. Arrow  404  represents that the three second timer has expired before the signal was received. In this situation the counter is incremented and the low frequency initiator is again activated along with the reactivation of the three second timer. In block  402  when the identification signal from the pressure sensor is the same as one of the identifiers already stored in the status memory, and the sensor status in the sensor signal indicates an initial status, block  406  is executed. The initial status is generated in response to the low frequency initiator. That is, normal operating conditions such as reporting pressures do not include the initial status indication. In block  406  the existing identification is confirmed by reactivating the low frequency initiator. When another sensor identification signal not matching the previous signal is received and the status of that signal is also an initial status, the count is incremented and a three second timer is started. The status of the low frequency initiator is reset to null and step  402  is again executed. The transition from block  406  to block  402  indicates the system is confused because two conflicting sensor identifications were received. Upon conflict the system is restarted in block  402 . In block  406 , when no different sensor identification signals are received the low frequency initiator status is existing and the system continues in block  408  described below.  
      Referring back to block  402 , when the sensor identification signal is previously unstored in the memory and the sensor status is an initial status, block  410  is executed. In block  410  the low frequency initiator is again activated to confirm the newly-received sensor identification. When another sensor identification other than the newly-received sensor identification is received that has an initial status or the three second timer expires and the initiator status is still trying to confirm or the three second timer is running, the sensor status is an initial status and the sensor identification is an existing identification and the low frequency initiator status is still trying to confirm, then the count is incremented and the three second timer is started, the low frequency initiator status is reset to null and the low frequency initiator is again activated before the system returns to block  402 . In block  410  when the three second timer expires and the low frequency status is “pending new”, then the initiator status is set to confirm, the low frequency initiator is activated and a three second timer is started while setting the sensor identification to null as represented by arrow  312 .  
      In block  410  when the three second timer is running the sensor status is in initial state and the sensor identification is confirmed, block  408  is executed as will be described below.  
      Referring back to block  402 , when the count is greater than a predetermined count such as five, a pending fault is indicated and the system returns to block  408  in which the above steps  402  through  412  are again performed for each of the plurality of tire locations. In block  408  the statuses of each of the tire locations are held in memory when the ignition is in a run state. When the ignition indicates off or an “accessory” position in block  414 , the system returns to block  400 .  
      It should be noted that each of the tire position locations are determined either sequentially or simultaneously to determine the positions relative to the vehicle thereof.  
      Referring now to  FIG. 22 , a method for increasing the power of the low frequency initiator is described. This aspect of the invention allows the low frequency initiator to provide only enough power so that a response may be generated from the respective tire transmitter and reducing the potential of receiving signals from adjacent vehicles. This system is a follow on to the system described above with respect to  FIG. 21 . More specifically, this aspect of the invention may be performed each time the low frequency initiator is activated or upon the first time each low frequency initiator is activated such as in blocks  402 ,  406 , and  410  in either a primary or a confirmation mode. Preferably, this aspect of the invention is performed once during each cycle so that a power level may be stored in the memory and each subsequent cycle is maintained at that level. For example, this aspect of the invention may be performed during block  400  when the vehicle speed is above a predetermined threshold and the ignition status is a run status.  
      In step  430 , the low frequency initiator is activated so as to generate a first initiator signal from the low frequency initiator. Preferably, the first initiator signal has a first power level that is a relatively low power level.  
      In step  432 , a timer is started. In step  434 , a counter is started. The timer in step  432  corresponds to the amount of time the system waits for a signal. The counter corresponds to the number of activations before an error will be generated. If a predetermined amount of time expires on the timer, the count may be incremented as will be described below. In step  436  it is determined whether or not a signal has been received from the sensor. If a signal has been received from the sensor, the data is processed in step  438 . Processing the data may include various steps including storing the transmitter identification from the transmitter or various other processes as described above, particularly in  FIG. 21 . If no signal has been received from the sensor transmitter, step  440  determines whether or not the timer has exceeded a predetermined time limit. If the timer has not exceeded a predetermined time limit then step  436  is repeated. In step  440  if the timer has exceeded the limit, the counter is increased in step  442 . In step  444  the counter is monitored to see if the counter has exceeded a predetermined limit. When the counter has not exceeded the predetermined limit, step  446  is executed in which the power at which the low frequency initiator is operating is monitored. In step  446  if the power that the low frequency initiator is operating has not reached a maximum power limit, the power limit is increased in step  448  and the initiator is again activated in step  441 . The power is preferably increased by increasing the current to the initiator.  
      Referring back to step  446  and  444 , if the counter has exceeded the limit in step  444  or the maximum power limit has been reached in step  446 , an error signal is generated in step  450 . The error signal may be displayed through an indicator or generated through an audible warning device.  
      Referring now to  FIG. 23 , a method for generating a reminder to fill the spare tire is illustrated. In step  500  various sensors and information stored in memory is determined. For example, a timer signal timing various functions such as timing the time that the tire is stored, the time that the spare tire is used as a rolling tire, the ambient temperature and various information stored into the system such as information about the wheels and tires. Other information may include the distance the tire is used as a rolling tire, the tire material and construction which may include the tire size, speed rating, load rating, the speed used as a rolling tire, the wheel material and wheel profile, and the temperature used as a rolling tire and the temperature used as a spare tire. In step  502  the time stowed is determined from the timer. In step  504  the temperature profile is determined from a temperature sensor. The temperature profile may include a rolling temperature profile as well as a stored temperature profile corresponding to the temperature profile when the vehicle was rolling and when the vehicle is stored, respectively. The temperature profile is an overall profile over the life of the spare and thus is substantially longer than merely a “key-on” temperature profile.  
      The time used as a rolling tire is determined in step  506 . In step  506  the timer is used to provide this information. To determine if the spare tire is a rolling tire, one of the above algorithms may be used to determine the spare in a rolling position. When the velocity exceeds a predetermined velocity the tire is thus in a rolling position.  
      The tire construction also affects the deflation of the tire. The tire construction may include various information entered into the memory of the system. For example, the tire construction may include the tire size, the tire speed rating, the tire load rating, valve properties, and the material from which the tire is made.  
      In step  510 , various other factors may also be included in the deflation determination of the spare tire. For example, the speed that the vehicle traveled while the spare tire was placed in a rolling position may be determined.  
      In step  512  the tire deflation is estimated based on the above factors. In various embodiments, various factors may be included or excluded from this determination based upon the system requirements and inputs provided.  
      In step  514 , if the deflation is not greater than a predetermined value, the system repeats at step  500 . If the deflation is greater than a predetermined value, step  516  is executed. In step  516  an indication is provided to the vehicle operator that the spare tire pressure needs to be checked. Such indication may take the form of an audible or visual indication. For example, a warning bell or voice message may be generated. In addition, a warning light or display may display a “spare check” indication.  
      As can be seen, a tire deflation model may be estimated based on the various conditions measured and determined above. Each vehicle spare tire type may have different characteristics and thus must be experimentally determined for the particular type of tire. Such a model may be easily and accurately determined prior to vehicle assembly so that the controller may be programmed with an appropriate deflation model.  
      Referring now to  FIG. 24 , a method for entering a programming mode is illustrated. It should be noted that this method may also be used instead of or in addition to the method of automatically programming described above. Prior to block  600  a counter is reset to zero. Arrow  601  represents pre-existing conditions that must exist for the learn mode to be entered. That is, if the learn mode is set to a “forced exit mode” a 60 second timer expires and the vehicle speed is greater than 3 miles per hour, some of the following steps may be executed and the learn mode may be set to false. In the initialization block  600  if the ignition switch is set to run or start, the counter is set to one, a 60 second timer is set to start and a learn mode is set to false. It should be noted that the 60 second timer is an arbitrary number used in the present example and may be altered depending on the particular system requirements for the particular vehicle. In block  602 , the number of transitions from off to on (or vice-versa) must reach three as indicated by arrow  604  before ignition stage one is complete. If, however, the brake switch enters an on-state, the system is forced to exit as indicated by line  606  which continues back to block  600 . In block  602  if three transitions from off to run or start are achieved, step  608  is executed in which the brake pedal must then transition from off to on. The system recycles as indicated by arrow  610  until the system changes back the from the on-state to back to an off-state. The ignition switch is continually monitored and if the ignition switch transitions to off or accessory the learn mode is changed to “forced exit” and system follows the path indicated by arrow  612 . In block  608  if the ignition does transition from on to off, block  614  is executed in which the ignition stage again is monitored for a predetermined number of counts. As indicated by arrow  616 , if the number of counts is less than a predetermined number of counts then the system recycles in block  614 . If during the counting of ignition stages from off to on the brake switch indicates the brakes are being applied, block  600  is again executed as illustrated by block  618 . In block  614  when three transitions from off to on are found, the learn mode is entered in block  620 .  
      When the system transitions to a learn mode in block  620  above, a message is displayed in the system that indicates learn mode and indicates a tire to manually activate. The system may activate in a conventional system such as using a magnet or may activate in another manner such as deflating the tire slightly and inflating the tire which will trigger a transmission.  
      Referring now to  FIG. 25 , in the previous figure the transition from block  614  to  620  corresponds to the transition from a standby block  630  to block  632 . After block  630  the horn may be chirped to indicate the activation of a timer such as a two minute timer for which to activate the system. In block  632  when the system receives the sensor identification from the first tire such as the left front tire, the next tire such as the right front tire is performed in a similar manner. In block  634  once the right front tire message is received, block  636  performs the same method for the right rear tire. Block  638  initiates a message and receives the right rear tire. In block  640  the spare tire may also be programmed in a similar manner. The potential transmitter identifications are then stored in memory if each of the systems is not matching another identification. The system continues in block  642  in which the system status is displayed to the user. In each of steps  632  through  640  when the two minute timer expires or the vehicle speed increases below three miles per hour or the ignition transitions to off or accessory or any of the identification signals matches another identification signal already received, then an error message is generated. Such message may include “tires not learned” or other appropriate message on a digital display. Likewise, an indication such as two horn chirps separated by a predetermined time may also be generated. The system may try to activate the system again in block  642  with starting of a two minute timer without performing steps  600 - 620 . This also may occur for a predetermined time.  
      Advantageously, by performing a series of steps such as those not commonly performed together in the vehicle, the system enters the manual learn mode.  
      In addition to the above, the present invention may also use the telematics system described above to transmit and receive various information from the vehicle to a central location. For example, the present invention may generate signals that indicate the tires need to be rotated, the tire wear indicates the tires must be changed, or the tire pressure is low. The central location may transmit a signal such as an e-mail or a telephone message to the vehicle owner to let him know the condition present on the vehicle. That is, the telematics system may allow the vehicle owners to more readily have their vehicles serviced. Information such as mileage information may also be transmitted to the central location as well as the vehicle speeds and other conditions. This may assist in forming a tread wear assessment so that vehicle owners may be notified to check their tires periodically for wear so that they may be rotated and changed when necessary.  
      In addition to the above, the present invention can be used to notify a driver that his or her tires need to be refilled (or bled) even in situations that would not result in a low pressure (or high pressure) condition, i.e., the tire pressure is not below a low pressure threshold (or above a high pressure threshold).  
      This invention provides for a sliding criteria based on the duration of a low- or high-pressure measurement. Extreme pressure readings that could indicate an under- or over-inflation condition would use the fastest possible response time to alert the driver in the shortest period of time. Readings that deviate from the ideal pressure region but not significantly so would go through a more rigorous check. If the out-of-range pressure is maintained for a certain time period, the vehicle operator would be prompted to adjust the pressure. However, if the pressure returned to an acceptable range, likely the result of climatic or vehicle usage changes, the user would not be prompted to alter the pressure. The larger the deviation from the ideal pressure, the shorter period of time necessary to prompt the user of the condition. An “intelligent” system such as described here would increase effectiveness of any tire pressure system by reducing unnecessary warnings and thereby increasing customer confidence in the system.  
      This invention may also be used to alert a vehicle driver that a tire has an excessive leak. Most tire punctures produce slow leaks. In fact, slow leaks may result from a number of conditions, e.g., manufacturing defects in the tire, valve stem or wheel, ice or other debris holding the valve stem partially open, impact damage or corrosion of the wheel effecting the tire/wheel interface, or cracking of the valve stem or tire due to aging effects. A slow leak is usually detected (either by a tire pressure monitoring system or by a visual inspection by the operator) when the tire becomes significantly under-inflated. Minimal under-inflation can reduce vehicle performance, e.g., fuel economy, for long periods while undetected. Furthermore, drivers may refill a tire repeatedly, believing the under-inflation is a result of the tire&#39;s natural leakage, before suspecting the tire may have a slow leak. A tire with an excessive leakage rate should be checked as soon as possible by a trained technician.  
      Referring now to  FIG. 26 , a flowchart describing a preferred embodiment of the invention is disclosed. This algorithm is used with a tire pressure monitoring system that can determine not only the pressure but also the temperature of a tire at approximately the same time. At step  700 , the system determines the tire pressure (P tire (t)) and temperature (T tire (t)) at time t. At step  710 , the system determines whether the tire has been filled (or refilled) with air. If the tire has been filled, step  720  is performed whereby the starting pressure (P o ) and starting temperature (T o ) is set to P tire (t) and T tire (t) respectively, and the time t o  is set to t. Optional step  730  is shown wherein P o  and T o  are averaged and/or filtered to reduce the effects of system noise and unusual temperature deviations, e.g., a large temperature increase during a braking event. This filtering/averaging can be accomplished in many ways, and is preferably done by a software program. The software program could use error detection to discard aberrations and other “faulty” data. After step  730 , the method returns to step  700  and the system determines P tire (t) and T tire (t) at new time t.  
      If the tire has not been filled at step  710 , the system stores P tire (t), T tire (t) and t at step  740  for future use. Optional step is shown wherein P tire  and T tire  are averaged and/or filtered reduce the effects of system noise and unusual temperature deviations, as discussed above. At step  760  the tire leakage rate at time t (LR(t)) is calculated based upon P tire (t), T tire (t), P o /T o , and the difference between t and t o , preferably by using the equation: 
 
 LR ( t )=(( P   o   −P   tire ( t ))+( P   o   /T   o )( T   tire ( t )− T   o ))/( t−t   o ) 
 
 At step  770 , LR(t) is compared to a tire leakage rate threshold (LR max ). The tire leakage rate threshold may depend on many factors, including tire size, temperature, loading, and over- or under-inflation, however the preferred tire leakage rate threshold is approximately 2 psi per month. If LR(t) is less than LR max , the method returns to step  700  and is iterated as discussed above. If LR(t) is greater than LR max , a leakage rate alert is generated at step  780 . The leakage rate alert can be of many different types, for example, a visual display on the vehicle&#39;s instrument panel or telematics/navigation system or an audio signal, or both. Preferably, the leakage rate alert would utilize a similar medium to that utilized by the tire pressure monitoring system. 
 
      The system preferably records a number of previous readings of P tire (t) and T tire (t) at various time t&#39;s. This stored performance record could be utilized by the system in the averaging and/or filtering steps (nos.  730  and  750 ) above. This record could also be utilized by a trained technician to assist in the diagnosis of a leakage rate alert, or could be sent via the vehicle&#39;s telematics system to a distant service facility.  
      This invention could also be used in a tire pressure monitoring system without tire temperature sensing capabilities. The ambient temperature, determined for example by the vehicle&#39;s powertrain control system, could be used to approximate the measures for T o  and T tire (t). However, the system would have to wait until at least two hours after the vehicle has come to rest in order to gain an accurate approximation. The rest is required so that the tires can cool down to the ambient temperature (the temperature of a tire increases from friction when the vehicle is in motion). This delay could be accomplished by utilizing other vehicle sensor signals, for example the vehicle&#39;s speedometer, coupled to a simple timer or processor clock.  
      While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.