Patent Publication Number: US-2005134446-A1

Title: Determination of wheel sensor position using radio frequency detectors in an automotive remote tire monitor system

Description:
PRIORITY CLAIM  
      This application is a continuation of application Ser. No. 10/021,284 filed Oct. 29, 2001, which is hereby incorporated by reference.  
     CROSS REFERENCE TO RELATED PATENT  
      This application is related to U.S. Pat. No. 6,518,876, filed Apr. 25, 2000 and patented Feb. 11, 2003, in the names of Emmanuel Marguet and William David Stewart and commonly assigned to the owner of the present application. The U.S. Pat. No. 6,518,876 is incorporated herein in its entirety by this reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates generally to a remote tire monitoring system. More particularly, the present invention relates to a method and apparatus for automatically updating position information for tire monitors in such a system.  
      Systems have been developed to monitor a characteristic such as tire pressure of a vehicle and to report the characteristic to a receiver at a central monitoring station using radio transmissions. A monitor is located at each tire and periodically takes a measurement of the tire characteristic. The monitor then transmits the results of the measurement in a radio frequency transmission to the central monitoring station which produces an alarm or a display in response to the measurement.  
      One problem with such systems has been the need to program the location of the transmitters at the central station. To be fully useful, the tire characteristic data is preferably associated with the tire which originated the measurement when presenting a display or alarm. Each monitor includes identification information which can be transmitted with the measurement. The tire monitor is preferably activated to produce this information and the information is then conveyed to the central station and associated with the position of the tire.  
      In the technique of U.S. Pat. No. 5,600,301, the tire monitors each include a reed switch or other magnetic device. A magnet is passed near the reed switch, causing the monitor to transmit a radio frequency transmission that includes identification data. A service technician repeats this process at each wheel and then loads the identification and position information into the central monitoring station. Another method provides a printed bar code on each tire monitor which contains the identification information and which may be read with a suitable bar code reader.  
      In U.S. Pat. No. 5,880,363, an activation signal is provided from the central controller to a low frequency transmitter at each wheel well. The transmitter generates a low frequency signal to activate the tire monitor. The tire pressure monitor responds by generating a long wave identification signal and transmitting that signal with tire pressure and identification data directly to the control unit. The long wave identification signal is used to identify the position of the tire by distinguishing this transmission from other transmissions received by the controller.  
      U.S. Pat. No. 5,883,305 discloses two-way communication of data by radio signals. A tire pressure monitor is activated by a radio frequency signal transmitted by an antenna in the wheel well adjacent the tire. The tire pressure monitor transmits a second radio frequency signal which is detected by the wheel well antenna. The second signal is demodulated to detect that tire pressure data.  
      These previous techniques have been limited in effectiveness. The magnetic programming technique may be subject to interference and crosstalk, for example in a factory where many such tire monitors are being assembled with tires and vehicles. The bar code label system requires a label at each tire which can be lost or become dirty or illegible. The apparatus for transmitting a long wave activation signal and generating a long wave identification signal therefrom is too expensive for some applications. The two-way data communication techniques requires demodulation of the received radio signals at the wheel well and coaxial cabling back to the central controller, both of which add to the cost of the system.  
      A further limitation of some of these prior techniques is the manual operation requiring activation by a service technician. A system is desired which automatically conveys wheel position data to the receiver. Such a system would be particularly useful after any change in tire position, such as tire rotation or replacement of a tire.  
      U.S. patent application Ser. No. 09/557,682, commonly assigned with the present application, discloses a system and method in which tire monitors are located at each wheel of the vehicle and periodically transmit tire data along with a tire monitor identifier. Four small, inexpensive RF detectors are located near each wheel. Each RF detector is connected to the central control unit by a power line and a ground line. When a tire monitor transmits data by emitting an RF transmission, the RF detector that is closest to the transmitter will detect the burst of RF energy. The RF detector responds to the RF energy by modulating the power line to the control unit with the envelope of the transmitted data. The control unit detects this modulation on one of its power lines. Also, the RF receiver of the control unit receives and demodulates the data transmitted by the tire monitor. The control unit associates the received data with the position indication provided by the modulation on the power line. When the positions of the wheels on the vehicle are changed, the control unit can determine the new position using the modulated power line in association with the tire monitor identifier in the transmitted data.  
      While this system has been very successful in application, a system featuring reduced cost and weight is desired. The cables that must be run from the control unit to all four RF detectors add substantially to the cost and weight of an installation. Accordingly, there is a need for a system and method which provide the operational advantages of the earlier system in a system offering reduced complexity, parts count, weight and cost.  
     SUMMARY  
      By way of introduction only, a remote tire monitor method and apparatus provide a central control unit in the cockpit or trunk of a vehicle. The control unit includes a radio frequency (RF) receiver and a controller. Tire monitors are located at each wheel of the vehicle and periodically transmit tire data along with a tire monitor identifier. Two small radio frequency (RF) detectors are positioned in proximity to two wheels on the vehicle. The RF detectors give an indication of the location of a transmitting tire monitor to the controller.  
      The present embodiments of a tire monitor system and method rely on only two RF detectors mounted on the vehicle, one in close proximity to one of the front wheels and the other in close proximity to one of the rear wheels. An RF detector can distinguish between its local transmitter and the other transmitter on the same axle by the amount of signal received. For example, if an RF detector is positioned in close proximity to the left rear wheel, then it can ideally expect to receive 100 percent reception from the transmitter on the left rear wheel and ideally around fifty percent reception from the right rear wheel&#39;s transmitter. The amount of false triggering from the front wheel transmitters is substantially zero. Every time the controller decodes transmitted RF data, the controller looks to see if and which RF detector has detected the transmission. The controller can then determine over a short period of time which sensor identifier belongs to which corner of the vehicle.  
      A method in accordance with one embodiment provides for detecting transmissions from two or more tire monitors at a detector. Then, based on a signal parameter associated with the transmission, the method provides for identifying a source tire monitor position. In one embodiment, the method further includes determining the amount of signal received from the two or more tire monitors and determining the signal parameter based thereon. In one example, a signal strength may be used as the signal parameter or to determine the signal parameter. In another, the number of transmissions received at a detector from two or more tire monitors may be used as the signal parameter or to determine the signal parameter. In another example, signal to noise ratio may be used as the signal parameter or to determine the signal parameter. In still another example, instead or in addition to the number of transmissions received, the rate of transmission reception may be used as the signal parameter or to determine the signal parameter.  
      In other embodiments, the signal parameter may be determined for a first tire monitor and for a second tire monitor. The respective signal parameters may then be compared to determine the source tire monitor position.  
      In another embodiment, transmissions are received at a central control unit of the vehicle and at the same time are detected either by a front RF detector positioned near one of the front wheels or a rear RF detector positioned near one of the rear wheels. The number of transmissions received from each tire monitor are counted and counts are maintained. After receipt of a sufficient number of transmissions, the counts are compared to determine the position of each tire monitor in the system.  
      An RF detector will detect almost all the transmissions from an adjacent tire monitor (e.g., in the same wheel well) and some of the transmissions from a tire monitor at the same end (front or rear) of the vehicle, and substantially none of the transmissions from tire monitors at the other end of the car. Therefore, based on the number of received transmissions after a time and using the tire monitor identification embedded in the transmissions, the tire monitor positions can be deduced from the counts of the received transmissions.  
      The foregoing discussion of the preferred embodiments has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a block diagram of one embodiment of a remote tire monitor system shown in conjunction with portions of a vehicle;  
       FIG. 2  is a flow diagram illustrating one embodiment of an auto learn method for the remote tire monitor system of  FIG. 1 ;  
       FIG. 3  is a flow diagram illustrating one embodiment of an auto learn method for the remote tire monitor system of  FIG. 1 ;  
       FIG. 4  is a block diagram of a vehicle with a remote tire monitor system; and  
       FIGS. 5 and 6  and are a flow diagram illustrating one embodiment of a remote tire monitor system. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS  
      Referring now to the drawing, it is a block diagram of a remote tire monitor system  100  shown in conjunction with portions of a vehicle  102 . The vehicle  102  includes in this example four tires  104 . Other numbers of tires may be included, such as a fifth tire as a spare or additional tires if the vehicle is a truck, trailer or other multi-wheeled vehicle.  
      Associated with each of the tires  104  is a transmitter or tire monitor  106 . Each of the tire monitors  106  includes a battery powered, radio frequency (RF) transmitter. Any suitable tire monitor may be used. U.S. patent application Ser. No. 09/245,938, entitled “Method And Apparatus For A Remote Tire Pressure Monitor System,” filed Feb. 5, 1999 in the name of McClelland et al., and commonly assigned with the present application is incorporated herein by reference and illustrates one suitable tire monitor for use in the remote tire pressure monitor system  100 . Each tire monitor  106  includes a sensor such as a pressure sensor for measuring a tire characteristic. The tire monitor  106  converts the measured tire characteristic to tire data. The tire data is encoded for transmission from the tire monitor  106 .  
      The tire monitor further includes a transmitter configured to transmit RF signals including the tire data. In some embodiments, the transmissions are encoded or randomized to minimize clashes at a receiver. For example, U.S. patent application Ser. No. 09/245,577, entitled “Method For Communicating Data In A Remote Tire Pressure Monitoring System,” filed Feb. 5, 1999 in the name of Bailie, et al., and commonly assigned with the present application is incorporated herein by reference. This application shows a technique in which data words are transmitted separated by a time delay. The time delay for each respective data word is defined according to a repeating pattern common to the tires so that data words are transmitted during a plurality of aperiodic time windows. Transmission parameters such as modulation techniques, transmission frequency and transmission power are chosen according to local regulations and to assure reliable reception of the RF signals.  
      The tire monitor  106  includes a motion switch  139 . The motion switch  139  closes upon detection of movement of the vehicle  100 . The motion switch  139  provides a signal to the processor  124  indicating closure of the switch  139  and motion of the vehicle. In response to closure of the switch, the tire monitor system  100  begins operating, for example, by transmitting tire data. In the illustrated embodiment, during normal operation, the tire monitor  106  transmits supervisory tire pressure information once every minute. Any suitable motion switch may be used for the switch  139 .  
      The remote tire monitor system  100  includes a control unit  110  and a plurality of radio frequency (RF) detectors  112 . In alternative embodiment, the remote tire monitor system  100  additionally includes a user display for providing user information such as tire pressure information and low tire pressure alarms. In the illustrated embodiment, each RF detector  112  is mounted on the vehicle  102  proximate an associated tire monitor  106  to detect the RF signals from the associated tire monitor  106  and produce a transmission indication in response to detected RF signals. Each of the RF detectors  112  is electrically coupled by a conductor  114  to the control unit  110 . Structure and operation of the RF detectors  112  will be described in greater detail below.  
      The control unit  110  includes an RF receiver  120 , an RF decoder  122 , and a controller  124 . The RF receiver  120  is configured to receive RF signals conveying tire data from at least one transmitting tire monitor  106  of the plurality of tire monitors  106  associated with the wheels or tires  104  of the vehicle  102 . Any suitable RF receiver circuit may be used. The design and implementation of the RF receiver  120  will depend on the type of modulation used for the RF signals, transmission frequency for the RF signals, and physical limitations such as permitted size, weight and power dissipation.  
      The RF decoder  122  is configured to receive a transmission indication from at least one receiving RF detector  112  of a plurality of RF detectors  112  associated with wheels or tires  104  of the vehicle  102 . Thus, a tire monitor  106  will transmit RF signals which are detected by the RF detector  112  associated with the transmitting tire monitor  106 . The receiving RF detector  112  signals its detection of the RF signals by providing the transmission indication on its associated conductor  114 .  
      The RF decoder  122  is further configured to identify a position of a transmitting tire monitor on the vehicle in response to the transmission indication received from an RF detector. Accordingly, the RF decoder  122  includes a plurality of input circuits  123  coupled to the conductors  114  which are in turn coupled to the RF detectors  112 . A transmission indication impressed on a conductor  114  is detected by an associated input circuit  123 . In the illustrated embodiment, there is a one-to-one relationship between input circuits  123  and RF detectors  112 . In this manner, the RF detector  112  which originated the transmission indication may be identified by the RF decoder determining which input circuit  123  detects the transmission indication. In alternative embodiments, the RF decoder  122  may include fewer than four input circuits  123  which are multiplexed in some manner among the plurality of RF detectors  112 . For example, a single input circuit  123  may be time shared among the plurality of RF detectors  112  to reduce the cost and complexity of the RF decoder  122 .  
      The RF decoder  122  is electrically coupled with the RF circuit  120 . Upon receipt of RF signals at the RF circuit  120 , the RF signals are demodulated to extract the tire data contained within the RF signals. In some applications, additional data decoding may be required after demodulation. The tire data in one exemplary embodiment includes a tire monitor identifier, or unique identification code which uniquely identifies the tire monitor  106  which transmitted the RF signals. In addition, in this exemplary embodiment, the tire data also includes tire pressure data related to a sensed tire pressure of the tire  104  at which the transmitting tire monitor  106  is located. Alternative tire data may be included or substituted for the tire pressure data, such as a number of tire revolutions, tire temperature, and so forth.  
      After extracting the tire data from the RF signals, the tire data is conveyed from the RF receiver  120  to the RF decoder  122 . The RF decoder  122  associates the tire data with a position of the transmitting tire monitor  106  on the vehicle  102 . Position information is determined using the input circuit  123  and a transmission indication received over a conductor  114  from RF detector  112 . The tire data and associated tire position are conveyed from the RF decoder  122  to the controller  124 .  
      The controller  124  controls the operation of the remote tire monitor system  100 . The controller  124  is preferably a microcontroller including a processor  128  and a memory  126 . The processor  128  operates in response to data and instructions stored in the memory  126  to control overall operation of the system  100 .  
      In the illustrated embodiment, the processor  128  stores position data for a plurality of tire monitors  106  of the remote tire monitor system  100 . The controller  124  is electrically coupled to the RF decoder  122  to receive tire data and position data from the RF decoder  122 . In the illustrated embodiment, when tire data and position data are received at the microcontroller  124 , the processor  128  retrieves stored position data from the memory  126 . In one embodiment, the position data are stored in association with a position on the vehicle, such as left front, left rear, right front or right rear. The received position data is compared with the stored position data. If there is no change, the position data is not updated and further processing may occur using the received tire data. However, the processor  128  updates the position data for the transmitting tire monitor  106  when the position of the transmitting tire monitor  106  varies from the stored position data for the transmitting tire monitor. Thus, the controller  124  includes a memory  126  and a processor configured to store in the memory  126  position of the plurality of tire monitors  106  including the position of the transmitting tire monitor which originated the received position data.  
      In an alternative embodiment, the memory  126  is not used for storage of position data. Rather, the received tire data is associated by the control unit  110  with the position information provided by the transmission indication from a RF detector  112 . The tire data and the position information from the input circuit  123  are used together to produce a display or alarm, if appropriate, by the system  100 . Additionally, in still another embodiment, the tire data omits any identifying information for the transmitting tire monitor  106  and again, the tire data and the position information from the input circuit  123  are used together to produce the appropriate display or alarm.  
      Completing the identification of the elements in  FIG. 1 , the vehicle  102  further includes a CAN driver  130 , a voltage regulator  132 , power line noise suppressor  134 , and a battery  136 . The battery  136  provides operating power for electrical systems of the vehicle  102  including the remote tire monitor system  100 . The battery  136  is a portion of the electrical power system of the vehicle, which typically also includes an alternator and other components. Such electrical power systems for vehicles are well known. The power line suppressor  134  reduces noise on the power line from the battery  136 . Noise may originate in other electrical components of the vehicle  102 , such as the ignition system. The voltage regulator  132  receives the battery voltage or other operating voltage from the power line suppressor  134  and produces a well regulated voltage for components such as the control unit  110  and CAN driver  130 . The CAN driver  130  provides electrical interface with other elements of a Controlled Area Network. Controlled Area Network or CAN is a serial communication protocol for data commonly used in automotive and other applications. The CAN bus  138  accessed by the CAN driver  130  is used to interconnect a network of electronic nodes or modules. The CAN bus operates according to an adopted standard. In conjunction with a remote tire pressure monitor system  100 , the CAN bus  138  may be used to convey tire monitor data to other locations in the vehicle  102 . For example, an alarm or a display (not shown) may be controlled to provide a visual or audible indication to an operator of the vehicle  102  that the tire data indicates an out-of-range condition, such as low tire pressure.  
      In  FIG. 1 , the RF decoder  122  and the controller  124  are shown as separate elements of the control unit  110 . In alternative embodiments, they may be combined in a single processor or logic block or circuit. Any other illustrated elements or additional elements included to enhance the functionality of the system  100  may be integrated or combined with other components of the system  100 .  
      Further, the system  100  should not be restricted to use in conjunction with a CAN bus. In alternative embodiments, any other communications medium may be employed for interconnecting the system  100  with other elements of the vehicle  102 . For example, communication buses in accordance with the J-1850 or USB standards may be substituted, or the control unit  110  may be directly hard wired with other elements of the vehicle  102 . Still further, external communications may be omitted entirely so that the system  100  is completely self-contained.  
       FIG. 1  further shows a detailed view of one embodiment of an RF detector  112  for use in the remote tire monitor system  100 . The RF detector  112  includes an antenna  140  to sense radio frequency (RF) signals transmitted from the tire monitor  106 , an amplifier  142 , an envelope detector coupled to the antenna  140  through the amplifier  142  and an output circuit  146  coupled to the envelope detector  144 . The envelope detector  144  includes a filter  149 , a diode  150 , a capacitor  152  coupled to ground and an amplifier  154 . The RF detector  112  is powered from a power line  156  and a ground line  158  provided on the conductor  114  which couples the RF detector  112  to the input circuit  123  of the RF decoder  122 . To isolate the operational circuitry of the RF detector  112  from noise on the power line  156 , the RF detector  112  further includes a resistor  160  and a capacitor  162  to ground.  
      The envelope detector  144  responds to the input signals received at the antenna and amplified by the amplifier  142  to produce at the output circuit  146  data corresponding to the envelope of the RF signals transmitted by the tire monitors  106 . Thus, the filter  148 , diode  150  and capacitor  152  together form a circuit coupled with the antenna  140  to detect an envelope of electrical signals produced by the antenna in response to the RF signals. The envelope is itself an electrical signal which is amplified in the amplifier  154 . The output signal from the amplifier  154  is applied to the base of a transistor  164 . In response to this signal at its base, the transistor  164  modulates a wire line signal on the conductor  114  in response to the envelope of the RF signals received at the RF detector  112 . That is, the signals applied at the base of the transistor  164  control turn-on of the transistor  164 , conducting current from its collector at the power node of the conductor  114  to its emitter at the ground node of the conductor  114 . As a result, the current in the conductor  114  will be modulated in response to the RF signals received at the antenna  140  of the RF detector  112 .  
      In one embodiment, to detect the modulated current, the input circuits  123  of the RF decoder in the illustrated embodiment may include a current mirror which duplicates the current drawn from the input stage of the input circuit  123 , coupled to the conductor  114 . The output current from the current mirror in the input circuit  123  is provided to a resistor which converts the current signal into a voltage signal which can be read by the microcontroller  124 . Suitable current mirror circuits are within the purview of those ordinarily skilled in the art of circuit design.  
      In this manner, then, the signal provided on the conductor  114  forms a transmission indication indicating that the tire monitor  106  associated with the RF detector  112  has transmitted an RF signal which was detected by the RF detector  112 . Producing the transmission indication includes detecting the envelope of the RF signals transmitted by the tire monitor  106  and producing a wireline signal on the conductor  114  in response to the envelope of the RF signals. In particular, in the illustrated embodiment, the wireline signal is produced by modulating a current in a conductor  114  coupled with the control unit  110 . The control unit  110  detects the modulation of the current to locate the transmitting tire monitor  106 .  
      Significantly, the RF detector  112  does not demodulate the data transmitted by the tire monitor  106 . Only the RF circuit  120  of the control unit  110  demodulates the data to extract the contents of the RF signal  106 . The RF detector only senses the presence of the transmitted RF signals. This reduces the cost of the RF detectors  112  and the overall cost of the remote tire monitor system  100 .  
      Also, by modulating the current in the conductor  114 , the RF detector&#39;s sensitivity to noise is reduced. Noise will occur in the form of voltage spikes or pulses on the conductor  114 . However, this noise will have little effect on the operation of the RF detector  112  and will have little effect on the current levels in the conductor. As a result, the conductor  114  can be, for example, a twisted pair of wire or any other inexpensive two-wire cable. Coaxial cable or other shielded cable is not necessary for implementing the system  100  using RF detector  112 .  
      In alternative embodiments, the RF circuit  120  may be omitted. In such an embodiment, the RF detectors  112  are used to detect the variations in the radio frequency signals and modulate a wire line signal on the conductors  114 . The RF decoder  122  in such an embodiment is configured to demodulate the data in conjunction with the microcontroller  124 . Current pulses on the conductor  114  are detected by the RF decoder  122  and converted to voltage pulses. The voltage pulses can be read by the microcontroller  124 . In this manner, microcontroller  124  obtains the data from the RF detectors and the RF decoder, without use of an RF circuit  120 . This has the advantage of eliminating the relatively expensive RF circuit. Further, this permits reduction in the transmit power used by the tire monitors  106  to transmit the radio frequency signals conveying the entire data. In some jurisdictions, substantially attenuated transmit power is required for applications such as tire monitors. These low transmit power requirements may be satisfied while still providing reliable performance in the remote tire monitoring system  100  by use of the RF detectors  112 .  
      In still other embodiments, the functionality described herein may be implemented using a programmed computer or other processor operating in response to data and instructions stored in memory. The processor may operate in conjunction with some or all of the hardware elements described in the embodiments shown herein.  
      The disclosed tire monitor system may be used to provide an improved auto learn or auto train method for automatically identifying positions of a plurality of tire monitors on a vehicle. As noted above, previously devices such as a transponder or magnetic activation tools were used in the car plant to train the control unit of the remote tire monitor system with identifiers for the wheel sensors or tire monitors. With the vehicle located in a training booth or activation area at the factory, the wheel sensors were activated in sequence and the control unit, expecting activated pressure transmissions in a certain order, learned the identification and position on the vehicle of the wheel sensors. So as to prevent cross talk from other training booths, each activation area is required to be RF shielded. Another method of training the receivers was to use bar code readers to scan the identifiers of the wheel sensors and input this data into the receiver. All of these methods required an additional operation either manually or by automatic readers. These operations add cost and potential for downtime.  
      In the illustrated embodiment of  FIG. 1 , no such tools are required. In the car plant at the end of the production line, a standard one to two minute dynamic test is used to test and calibrate steering, brakes etc. of the vehicle. For the illustrated embodiment, positions and identities of the four tire pressure monitor wheel sensors are automatically learned during this dynamic test.  
      This is achieved by placing the control unit or receiver in a “learn state” at a dynamic test booth. The wheel sensors transmit either once a minute as in the normal mode, or in a special initial mode corresponding to a brand new, right out of the box state, transmitting more often, for example every 30 seconds, or every 10 seconds.  
      For example, when the wheel sensors leave the manufacturer&#39;s production line, they are placed in off mode. This mode means that each wheel sensor is dormant until it is activated by the closing of its motion switch. Closing the motion switch is only achievable through centrifugal force caused by spinning the tire monitor on a rotating wheel. During normal operation, the wheel sensor, while driving, transmits tire information including supervisory tire pressure once every minute. However, in the illustrated embodiment, for the driving periods during the first 16 activations of the motion switch, the wheel sensor will transmit the supervisory pressure data once every 30 seconds (to conform to United States regulatory requirements) or 10 seconds outside the United States. Other time intervals may be used. After the initial 16 transmissions, or any other suitable number, the transmission interval is changed to its normal mode value, such as one minute. This initial mode is known as factory test mode.  
      At the time of the dynamic vehicle test, the vehicle is accelerated, causing the wheel sensors to activate with the rotation of the wheels and associated closure of their motion switches. When the wheel sensors begin transmitting tire pressure, say once every thirty seconds, each sensor&#39;s identifier is transmitted by the sensor and is received up by the RF circuit of the control unit. In this initial unlearned state, the receiver loads the new identifier into memory, associating the transmission with one of the four RF detectors. Only data received which also is synchronized to activity on one of the RF detector conductors is regarded as valid. Over the one to two minute duration of the dynamic test, each wheel sensor will transmit numerous times and the control unit can verify the tire information, such as each wheel sensor identifier, and associated wheel position. The control unit can then load this data into non-volatile memory for subsequent normal use.  
      Key advantages of this auto-learn technique is the lack of any additional labor or equipment at the vehicle assembly plant, and the lack of a need for a transponder component or magnetic switch in the wheel sensor. Also there is no possibility of learning the wrong wheels, from other vehicles due to cross talk or of getting the wrong position. Thus, cost is reduced, operation is simplified and reliability is increased. Using the illustrated embodiment of the tire monitor system, no additional activation or learning tools are required to train the control unit with the wheel sensors&#39; position on the vehicle. The only device required to train the control unit is the standard dynamic vehicle test at the end of line test in the vehicle assembly plant. Because the training procedure can be carried out in parallel with the steering and braking tests on the rolling road, and because of the factory test mode feature, no extra time or cost is required to ‘auto learn’ the tire monitor system.  
      The illustrated embodiment further provides for automatic update of tire monitor position information in the control unit upon replacement of one of the tire monitors of the system. This would occur, for example, if one of the wheels or tires of the vehicle is replaced. Due to the nature of the current embodiment, where the RF detectors are continuously indicating the position of the wheel sensors, a wheel sensor may be replaced and detected by the control unit without the need for user intervention. In this case, where a new wheel sensor is put on a wheel, the control unit initially realizes it is receiving a wrong identifier for the tire monitor, but still getting RF detector pulses from a particular wheel position. In addition, the control unit detects that the previously stored identifier for that position is no longer being received. Over a period of time, say ten minutes driving, the receiver verifies it has stopped receiving a stored identifier and is now receiving a new ID for that position. After verification, the new identifier is stored for that position and operation continues as normal.  
      The big advantage of this is the lack of need for user intervention and elimination of the need for a service tool at each service location. Tire monitor position and identification is updated automatically.  
       FIG. 2  is a flow diagram illustrating an auto learn method for the remote tire monitor system of  FIG. 1 . The method begins at block  200 . At block  202 , one or more tires with new tire monitors are mounted on a vehicle which includes a remote tire monitor system. In this embodiment, the tire monitors are in unused, out of the box condition from the manufacturer. The installation of block  202  may occur as part of the final assembly of the vehicle at the factory. Alternatively, the installation may occur when new tires are installed on the vehicle or when a remote tire monitor system is added to the vehicle.  
      At block  204 , the dynamic vehicle test is initiated and, in response, at block  206 , the tire monitors begin transmitting radio frequency (RF) signals. The dynamic vehicle test is a test to check proper functionality of the systems of the vehicle, including drive train and brakes. Alternatively, any activity which causes the tire monitors to begin transmitting may be substituted at block  204  to initiate transmission at block  206 . For example, the process of driving the vehicle from the end of the assembly line to a storage area or a final checkout area in block  204  may be adequate to begin transmission at block  206 . It is contemplated that the tire monitors each include a motion switch which activates the tire monitor in response to motion of the tire monitor on the wheel of the vehicle.  
      Further, at block  206 , the tire monitor begins transmitting at a test mode interval, such as once every 30 or 60 seconds. This aspect may be omitted but adds convenience for initializing the tire monitor system. After initialization, the interval may be reduced to reduce power drain from the battery which powers the tire monitor.  
      After transmission of the RF signals at block  206 , the RF signals are received by a receiver of the remote tire monitor system at block  208 . The RF signals are demodulated, decoded and otherwise processed to extract the data conveyed on the RF signals. For example, the tire monitor may modulate a carrier signal using data corresponding to pressure of the tire or a tire monitor identifier. The receiver of the remote tire monitor system demodulates the received RF signals to receive the data. At block  212 , the data including a tire monitor identifier, if any, is provided to a control unit of the remote tire monitor system.  
      Meanwhile, the same RF signals received and demodulated at blocks  208 ,  210  are detected at block  214 . In the preferred embodiment, the RF signals are received without demodulation, for example, using a detector of the type illustrated above in conjunction with  FIG. 1 . Other suitable RF detectors may be used. At block  216 , in response to the detected RF signals, a transmission indication is provided to the control unit. The transmission indication indicates to the control unit which RF detector of the vehicle detected the RF signals transmitted by the tire monitor and received by the receiver at block  208 .  
      At block  218 , identification information associated with the tire monitor is stored. In one embodiment, the data forming the identifier transmitted by the tire monitor and received by the receiver of the remote tire monitor system is stored in memory. Other types and formats of identification information may be stored. For example, the control unit may store an RF detector indicator which indicates which RF detector detected the received RF signals.  
      In this manner, the described method provides automatic learn capability in a remote tire monitor system. No manual intervention is necessary for the control unit to identify and store the identities and locations of individual tire monitors on the vehicle. This reduces time and cost associated with initiating operation of the remote tire monitor system.  
       FIG. 3  is a flow diagram illustrating an auto learn method for the remote tire monitor system of  FIG. 1 . The method of  FIG. 3  starts at block  300 .  
      At block  302 , RF signals transmitted by a tire monitor associated with a wheel of a vehicle are received by a receiver of the remote tire monitor system. At block  304 , the RF signals are demodulated, decoded and otherwise processed to extract the data conveyed on the RF signals. For example, the tire monitor may modulate a carrier signal using data corresponding to pressure of the tire or a tire monitor identifier. The tire monitor identifier may be a serial number or other unique or nearly-unique data associated with the tire monitor. For example, the tire monitor identifier may be multiple bit data stored in the tire monitor at the time of manufacture of the tire monitor. The receiver of the remote tire monitor system demodulates the received RF signals to receive the data. At block  306 , the data including a tire monitor identifier, if any, is provided to a control unit of the remote tire monitor system.  
      Meanwhile, the same RF signals received and demodulated at blocks  302 ,  304  are detected at block  308 . In the preferred embodiment, the RF signals are received without demodulation, for example, using a detector of the type illustrated above in conjunction with  FIG. 1 . Other suitable RF detectors may be used. At block  310 , in response to the detected RF signals, a transmission indication is provided to the control unit. The transmission indication indicates to the control unit which RF detector of the vehicle detected the RF signals transmitted by the tire monitor and received by the receiver at block  302 .  
      At block  312 , stored identification information is retrieved from memory at the control unit. In the illustrated embodiment, the identification information is stored at a memory location associated with the transmission indication or RF detector. Thus, the control unit receives a wireline indication from a receiving RF detector that a transmission has been received. Using the wireline indication, the control unit selects the memory location from which previous identification information is retrieved.  
      At block  314 , the control unit determines if the identifier received from the transmitting tire monitor matches the stored identification information. In this application, a match may mean a bit-by-bit match of received and stored data or some other level or association between the received data and the stored data. If the data match, at block  316 , the tire information such as pressure data are updated. For example, in one embodiment, tire pressure data are stored along with the identification information for the tire monitor. If the received tire pressure data varies by a predetermined amount from the stored tire pressure data, the received tire pressure data is stored and an alarm or other user indication is generated.  
      At block  318 , if there is no match between the received identifier and the stored identification information, the method waits for receipt of an additional transmission associated with this RF detector. Preferably, the tire monitor transmits pressure data and a tire monitor identifier periodically, such as once per minute. Upon receipt of a subsequent transmission, at block  320 , the method attempts to verify the previously received tire monitor identifier. This is done by comparing the newly received tire monitor identifier and the previously received tire monitor identifier to determine if there was an error in communication of the previously received tire monitor identifier. In some embodiments, multiple subsequent transmissions may be received for comparison. If there is no verification, at block  322 , the mismatched transmission received at block  302  is discarded. This condition indicates that the same tire monitor continues to transmit, and the mismatched transmission was received with an error.  
      If at block  320  the newly received data verify the previously received data, the identification information stored for this RF detector is updated with the tire monitor identifier from the received transmission. This condition indicates that the tire monitor has been changed and is communicating reliably. In this manner, the illustrated system and method provide automatic update capability after a tire monitor has been changed. This may occur if the tires of the vehicle are rotated or if one or more tires is replaced. There is thus no need to manually intervene for the remote tire monitor system to update the identities and locations of the tire monitors on the vehicle.  
       FIG. 4  is a block diagram of a vehicle  400  with a remote tire monitor system  402 . In the exemplary embodiment of  FIG. 4 , the vehicle  402  includes wheels  404 ,  406 ,  408 ,  410 . Each wheel includes a tire mounted on a rim. In other embodiments, the vehicle  400  may have other numbers of wheels. For example, in one particular embodiment, a truck has 18 wheels.  
      The remote tire monitor system  402  includes a control unit  412 , a front detector  414  and a rear detector  416 . The front detector  414  is electrically coupled to the control unit  412  by a cable  418 . Similarly, the rear detector  416  is electrically coupled to the control unit  412  by a cable  420 .  
      The remote tire monitor system  402  further includes a tire monitor associated with each wheel of the vehicle  400 . Thus, a tire monitor  424  is associated with wheel  404 ; tire monitor  426  is associated with wheel  406 ; tire monitor  428  is associated with wheel  408 ; and tire monitor  430  is associated with wheel  410 . The tire monitors are generally of the type described herein and are configured to detect a tire condition such as tire pressure and to occasionally transmit a transmission including tire data, such as tire pressure data and identification information uniquely identifying the respective tire monitor.  
      In the illustrated embodiment, the front detector  414  is positioned proximate the left front wheel  404 . For example, the front detector  414  may be mounted in the wheel well adjacent the wheel  404 . Similarly, the rear detector  416  is positioned near the left rear wheel  408 , such as in the wheel well adjacent the wheel  408 . With this mounting configuration, the front detector  414  is positioned to detect transmissions from the pair of tire monitors  424 ,  426  associated with the front wheels  404 ,  406 . The front detector  414  is proximate the left front tire monitor  424  and distal the right front tire monitor  426 . Similarly, the rear detector  416  is positioned to detect transmissions from the left rear tire monitor  428  and the right rear tire monitor  430 . The rear detector  416  is positioned proximate the left rear tire monitor  428  and distal the right rear tire monitor  430 .  
      The illustrated embodiment is exemplary only. In  FIG. 4 , the detectors  414 ,  416  are designated for detecting radio frequency transmissions from the front wheels  404 ,  406  and the rear wheels  408 ,  410 , respectively. In alternate embodiments, the RF detectors  414 ,  416  may be positioned to detect RF transmissions from the left side wheels  404 ,  408  and the right side wheels  406 ,  410  respectively. Similarly, while in  FIG. 4  the front detector  414  is positioned in proximity to the left front wheel  404 , away from the right front wheel  406 , this positioning may be reversed so that the front detector  414  is positioned near the right front wheel  406 , such as in the left front wheel well. In the same way, the rear detector  416 , shown in  FIG. 4  in proximity to the left rear wheel  408 , may be positioned in proximity to the right rear wheel  410 . Actual positioning of the RF detectors  414 ,  416  is not important. Rather, the relative signal strength or frequency of reception of RF transmissions from tire monitors is what is measured by the detectors  414 ,  416  in conjunction with the control unit  412 . It is important that each RF detector be positioned on one side or end of the car, away from the centerline, so that the relative signal strength or number of transmissions received by the RF detector from each of its associated pair of tire monitors can be determined.  
      The control unit  412  includes a receiver to receive radio frequency transmissions from tire monitors of the tire monitor system  402 , a controller  432  and a memory device  434 . The controller  432  forms a processing means and may be any suitable control device such as a microprocessor, microcontroller, application specific integrated circuit (ASIC) or logic device coupled together to perform the necessary functions described herein.  
      The memory device  434  forms a memory means for storing data and preferably is formed of semiconductor memory. In the illustrated embodiment, the memory device of the control unit  412  includes persistent memory or nonvolatile memory such as an E 2 PROM, and working memory such as random access memory (RAM). For example, the persistent memory may be used to stored tire identifiers and pressure data over extended periods of time, such as when the vehicle  400  is parked. The RAM may be organized as an array which stores counter values associated with tire monitor identifiers and tire monitor positions, as will be described in greater detail below.  
       FIG. 5  is a flow diagram illustrating operation of one embodiment of a remote tire monitor system. The method illustrated in  FIG. 5  may be used in conjunction with a remote tire monitor system of the type illustrated in  FIG. 4 . The method embodiment in  FIG. 5  allows a control unit of such a system to automatically learn the positions of the tire monitors of the system on the vehicle, referred to as a learn method or learn routine. This determination is made after receiving several transmitted frames of tire data from the respective tire monitors of the system. The control unit establishes an array of data in working memory and uses the data of the array to determine the position information for each tire monitor in the system. An example array of data is illustrated below.  
                                                       FrontRFD   Rear RFD   TotalRF_FrameCounter                                                            id1   22    2   22           id2   12    4   23           id3    2   20   20           id4    1   10   20                        
      In this example, rows of the array are defined by the identification information for each tire monitor from which data are received. In the example above, the identification information is listed as “id1,” “id2,” etc. However, in a more typical example, the identification information will be a numeric value forming a unique identifier or identification code of a transmitting tire monitor. The identification code is typically transmitted along with the tire pressure or other tire data by the tire monitor in a transmission frame. The exemplary array is shown with four rows, one for each tire monitor of the vehicle in this example. The array may also be formatted with additional rows to record data for additional transmitting tire monitors whose transmissions are received by the controller.  
      In the example array above, the columns of the array correspond to frame counter values which count the number of frames received at the respective RF detector of the system. Thus, in this example, a frame labeled with tire monitor identifier id1 has been received at the front RF detector 22 times. A frame with the same identifier id1 has been received at the rear RF detector two times, and so on. The count label TotalRF_FrameCounter is a count of the total number of frames received by the receiver of the controller from the identified tire monitor. The total frame counts recorded in this column is always greater than or equal to an RFD frame counter because the receiver has greater sensitivity than the RF detectors and detects transmissions that are missed by the RF detectors.  
      The method of  FIG. 5  begins at block  500 . The method of  FIG. 5  shows the learn routine on the production line, when the tires of the vehicle are first assembled with the tire monitors and added to the remote tire monitor system. At block  502 , it is determined if tire identifiers are already stored in electrically erasable (E 2 ) memory. This memory is nonvolatile or persistent memory which retains data stored therein even when power is removed from the memory. In the illustrated system, after installation on a vehicle, the persistent memory is empty. As soon as tire identifiers are received and verified according to the procedure of  FIG. 5 , the tire monitors are stored in the persistent memory. Thus, block  502  determines if this is the first time the tire monitor system has been operated after installation on a vehicle. If so, no tire monitor identifiers will be stored in the persistent memory and the “no” path will follow to block  504 . If tire identifiers are already stored in the persistent memory, the “yes” path is followed to block  602 .  
      At block  504 , it is determined if a frame of data has been received. If not, control remains in a loop including block  504  until a frame of data have been received. As indicated above, each frame of data transmitted by a tire monitor typically includes data corresponding to the tire identifier which uniquely identifies the transmitting tire monitor and tire data, such as data corresponding to the measured tire pressure of the tire. Other information, such as a header or synchronization data may be transmitted as well.  
      Once a frame of data has been received at block  504 , the tire monitor identifier contained in the frame of data is extracted and compared with other already-received identifiers stored in the list in working memory. If the extracted tire identifier is not present in the list, block  506 , it is added to the list, block  508 . Control then proceeds to block  510 , where the relevant wheel position counters are incremented. As noted above, each identifier has three associated counters. One counter each is associated with each RF detector of the system and stores data corresponding to the number of transmissions detected by that respective RF detector. The third counter counts the total frames received from an identified tire monitor, and is incremented after a frame is received at the receiver of the controller. Thus, the relevant wheel position counters that are incremented at block  510  include the total RF frame counter and the frame counter corresponding to the front RF detector or the rear RF detector.  
      At block  512 , a test is performed to determine if the specified criteria have been fulfilled. First, it is determined if four tire identifiers in the list have Total RF Frame counter values that are greater than a predetermined number, 20 in this example. That is, before applying the pass criteria, at least four tire identifier counters must have a value of 20 or greater. This test is implemented to ensure that there is a strong signal from a tire monitor and to eliminate any wrong or incorrect tire identifiers being added to the system. If the received signal from a tire monitor is weak, it will likely be received only a few times, rather than 20 or more times. Any other suitable number may be substituted for the predetermined number 20. Reducing the number will increase the speed at which the tire monitor positioning is learned by the system, but may increase the likelihood of incorrect tire monitor position learning.  
      According to the second criterion of the illustrated embodiment, the counter for the front RF detector must be larger than the counter for the rear RF detector for two different tire identifiers out of the four. According to the third criterion, it is determined if the the frame counter for the rear RF detector stores a value larger than the front RF detector frame counter for the two remaining tire identifiers in the list. If these criteria are not fulfilled using the tire identifiers in the list, control returns to the block  504  to await receipt of additional frame of data.  
      If these three criteria are fulfilled, however, at block  514 , two tire identifiers are selected from the list for the front axle of the vehicle, according to the second criterion above, and two tire identifiers are selected from the list for the rear axle, according to the third criterion above. Thus, at block  518 , the method has chosen four tire identifiers with a total RF frame counter value higher than 20 and has distinguished the selected tire identifiers between the front of the vehicle and the rear of the vehicle by using the front frame counter value and rear frame counter value. For example, using the values shown in the example list above, the tire identifiers corresponding to the tire monitors positioned at the front of a vehicle are tire identifiers id1 and id2. The tire identifiers corresponding to tire monitors positioned at the rear of the vehicle are id3 and id4.  
      Beginning at block  516 , the method identifies the right and left tire monitor for each axle. First it is determined if, among the identified tire identifiers from the list for each of the front and rear axles, one RF detector counter value has a higher frame counter value than the other. If not, the method cannot distinguish the two tire monitors on the axle. Control returns to block  504  to await receipt of additional frames of data. If the criterion of block  516  is met, at block  518  the tire indicator with the higher RF detector frame counter value is selected to be on the same side of the vehicle as the RF detector for that end of the vehicle. Thus, in  FIG. 4 , among the front wheels  404 ,  406 , the tire identifier associated with the larger valued RF detector counter is selected to correspond to tire monitor  426 . Similarly, the tire identifier having the lower valued RF detector counter value is selected to be associated with the tire monitor  424 . Alternatively, if, as is suggested in  FIG. 5 , those RF detectors  414  and  416  are positioned on the left side of the vehicle  400 , then of the tires of tire identifier selected at block  514 , the larger valued RF detector frame counter is associated with the left-hand side tire monitor for both axles. In the illustration of  FIG. 4 , if the RF detector  414  were instead mounted on the left-hand side of the vehicle  400 , the larger valued tire identifier would be selected to be associated with tire monitor  424  and the larger valued RF detector frame counter would be selected to be associated with tire monitor  428 . Using the example list of data above, and assuming that both tire monitors are on the left-hand side of the vehicle, the method would select id1 for the left front tire monitor and id2 for the right front tire monitor. Similarly, the method would select id3 for the left rear tire monitor and id4 for the right rear tire monitor.  
      At block  520 , the four selected tire identifiers are stored in non-volatile memory such as the E 2 PROM or other persistent memory described above. During subsequent operation of the tire monitor system, as new frames of tire data are received, the tire identification information contained in the frame will be compared with one of the selected in store for tire identifiers. If there is a match, the tire pressure information or other tire data contained in the frame will be used to update the current tire pressure information. At block  522 , the learn routine illustrated in  FIG. 5  is exited and the method of  FIG. 5  terminates.  
       FIG. 6  illustrates a method for the remote tire monitor system to learn the positioning of tire monitors on a vehicle during a normal driving operation. The method begins at block  602 , which is accessed after determining at block  502  ( FIG. 5 ) that tire monitor identifiers have already been stored in the persistent memory of the system.  
      At block  602 , the tire monitor values stored in the persistent memory are inserted into the list or array in working memory. The Total RF Frame Counter, the front RF detector counter value (for identifiers which were in the front) and the rear RF detector counter value (for identifierswhich were in the rear) for each of these array entries is preloaded with a predetermined value, such as 5. Storing preloaded values such as this gives a weighting to the tire identifiers already stored in the persistent memory and copied into the working memory array. The benefit of weighting the preloaded tire monitor values in the array in this manner is to reduce the likelihood that a tire monitor on an adjacent vehicle will be detected and selected as one of the four tire monitors of the vehicle. This could occur, for example, if more than one vehicle with comparable systems are parked adjacent each other, such as the end of an assembly line or in another location. Further, weighting the preloaded tire monitor values reduces the time required for the learn process so that reliable information can be given to the driver sooner. This process happens every time the vehicle is started and a new journey is begun.  
      At block  604 , it is determined if a frame of data has been received. If not, control remains in a loop including block  604  until a frame of data is received. Once a frame of data has been received, control proceeds to block  606 .  
      At block  606  it is determined if the tire monitor identifier contained in the received frame is already stored in the persistent memory or E 2 PROM. If not, at block  608  the received tire monitor identifier is added to the working list of tire identifiers in working memory. Control proceeds to block  610 .  
      At block  610 , the relevant wheel position counters are incremented. Operation here is similar to the operation at block  510 ,  FIG. 5 . The working list of data includes columns for each of the front and rear RF detector counters and a total RF frame counter. At block  610 , the total RF frame counter corresponding to the received tire identifier is incremented. Also at block  610 , the counter corresponding to the front or rear RF detector is incremented, depending on which RF detector sensed or detected the transmission from the transmitting tire monitor.  
      At block  612 , three criteria are tested to determine if sufficient frames of data have been received to reliably distinguish front from rear tire monitor positions. Operation of block  612  is similar to the operation of block  512 ,  FIG. 5 . At block  614 , two tire identifiers are selected to correspond to the front end of the vehicle and two tire identifiers are selected to correspond to the rear end of the vehicle. At block  612 , if all three criteria are not fulfilled, control returns to block  604  to await the receipt of additional frames of data.  
      At block  616 , it is determined if, for each of the front and rear sets of tire monitors, one tire monitor has a higher RF detector counter value. If not, control returns to block  604  to await the receipt of additional data. If so, at block  618 , the front and rear selected tire monitor pairs are each sorted among right and left tire monitors, selecting a left front, right front, left rear and right rear tire monitor. At block  620 , the four tire monitor identifiers are stored in non-volatile or persistent memory, along with position information for the tire monitor. The learn routine of  FIG. 6  is then exited at  622 .  
      From the foregoing, it can be seen that the present embodiments provide a method and apparatus which automatically conveys wheel position and data to a receiver in a vehicle. Even after changes in tire position due to tire rotation or replacement of a tire, the system automatically re-learns the position of the tires on the vehicle. No external actuation is required. Interference and cross talk are minimized by locating RF detectors in close proximity to the tire monitors. By sharing one RF detector between the front wheels and one RF detector between the rear wheels, the required number of RF detectors is reduced along with the required cabling and the concomitant cost, weight and difficulty of installation of the system. Further, the system provides automatic learn capability for learning and updating the identities of tire monitors on the vehicle without manual intervention.  
      While a particular embodiment of the present invention has been shown and described, modifications may be made. For example, while the exemplary embodiment counts received transmissions from tire monitors of the system, other embodiments may use alternate methods or detect other signal parameters to identify tire monitor positions in the system. Also, while the two learn methods of  FIGS. 5 and 6  are generally similarly for both the learn method in the production line and the learn method during normal driving, other method steps or test criteria may be substituted to change the two methods, accounting for the differing environments in which each method is practiced. It is therefore intended in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.