Abstract:
A tire pressure monitor method enables the reassignment of tire pressure sensors to respective tire locations on the vehicle in the event that the sensors are lost or replaced with new sensors having differing identification codes. A passenger compartment monitor, that normally monitor tire pressures through broadcast transmission from the sensors, is manually into a identification entry mode selecting a desired tire location, and when the vehicle begins travel, a motion switch in the new sensor detects vehicular motion and activates the transmission of the new identification code that is received by the monitor that then assigns the presently selected tire location to received identification code.

Description:
REFERENCE TO RELATED APPLICATION  
         [0001]    The present application is related to assignee&#39;s copending application entitled Tire Pressure Sensory and Monitoring System, Ser. No. ______, filed yy/yy/yy, by the same inventors.  
         FIELD OF THE INVENTION  
         [0002]    The invention relates to the field of to a method and apparatus for monitoring air pressure in vehicle tires. More particularly, the invention relates to an apparatus for automatically sensing tire pressure and methods for operating the apparatus.  
         BACKGROUND OF THE INVENTION  
         [0003]    Tire pressure sensors have long been used to sense the pressure of tires to indicate when the tire is below a predetermine tire pressure. These sensors use various means, typically diaphragms screwed into tire value stems and responsive to tire pressure for activating an electrical switch for generating an alarm. Various types of tire monitoring systems have been used to provide continuous vehicle tire pressure sensing and monitoring during vehicular operation. Such systems typically include a monitor located in the passenger compartment of the vehicle for receiving encoded transmitted signals for respective tires and for alerting the vehicular operator through the use of audio alarms and graphic display indicators. The ability to selectively sense the pressure of each tire is desirable so that the subject tire can then be inflated to proper air pressure levels for safety and long tire wear life. The tire sensors unidirectionally communicate with the monitor transmitting tire pressure values received and processed by the monitor.Hence, these systems typically have tire pressure sensors located on the valve stems for the respective tires each with an associated embedded transmitter for generating respective encoded signals identifying the tire. The cab mounted monitor has a receiver the graphic display for alerting the operator in the event of low tire pressures.  
           [0004]    U.S. Pat. No. 4,814,745 issued to Wang on Mar. 12, 1989 discloses a cap like signal device attached to the tire for sensing tire pressure. The cap includes an electric alarming buzzer responsive to a disk actuated by pressure disadvantageously without a cab mounted monitor. The disk activates the buzzer when the tire pressure is too low. U.S. Pat. No. 4,814,744 issued to Collins on Mar. 21, 1989 discloses a low tire pressure warning system having a mechanical side wall sensor with and dash alarm disadvantageous connected by a cable. U.S. Pat. No. 4,804,808 issued to Dal Cero on Feb. 14, 1989 discloses a pressure sensing devices that senses low tire pressure and signal low pressure using a transmitter and cab mounted receiver. U.S. Pat. No. 4,694,273 issued to Franchino on Sep. 15, 1987 disclose a tire sensing device having a movable element which activates a radio transmitter signal received by a receiver in the passenger compartment to activate visual and acoustic alarms. U.S. Pat. No. 5,289,161 issued to Huang on Feb. 22, 1994 discloses a tire pressure sensor having a diaphragm in a casing. The spring loaded diaphragm is movable between two positions. A signal producing units is activated in response to the position of the diaphragm. The signal generated is an encoded modulated RF signal for communicating an alarm signal to a receiver that determines which tire is low and activates a corresponding indication to indicate which tire has low tire pressure. Fuses in the sensors are used to generate the respective codes to match indicators of a display. U.S. Pat. No. 5,694,111 issued to Huang on Dec. 2, 1997 discloses an encoder unit and transmitter circuit for a tire pressure sensor device for generating encoded RF signals received by a cab receiver operating a display units. U.S. Pat. No. 4,734,674 issued to Thomas on Mar. 29, 1988 also discloses a tire pressure sensing device that, upon low pressure, transmits an encoded signal to a cab receiver having a plurality of display indicators on a front panel that are selectively activated to indicate the respective tire. U.S. Pat. No. 4,737,760 issued to Huang on Apr. 12, 1988 discloses another valve stem tire pressure warning device having a pressure sensitive diaphragm and spring switch for activating a transmitter signal communicated to the cab monitor. U.S. Pat. No. 4,319,220 issued to Pappas on Mar. 9, 1982 disclose a system for monitoring tire pressure of the tires having respective transmitters communicating alarm signals to a receiver in the cab monitor. U.S. Pat. No. 5,001,457 issued to Wang teaches a cab mounted monitor having displays with a graphic display for visually indicating which tire is low through the use of digitally encoded signals transmitted between respective tire sensor transmitters and the cab mounted central receiver. U.S. Pat. No. 4,970,491 issued to Saint on Nov. 13, 1990 teaches the use of specially encoded signal for a fleet of vehicle so that the receivers of one fleet of vehicle will not be activated by encoded signal from a sensor in another fleet of vehicle.  
           [0005]    Typically, these systems teach valve mounted tire pressure sensors responsive to respective tire pressures of the tires for generating respective encoded signals transmitted to receiver in a cab mounted monitor having graphic visual displays and or audio alarms for indicating which one of the tires has low tire pressure. Typically, these signal are modulated at a fix radio frequency. However, these teaching do not address the problem of interference between signals from sensor on the same vehicle, which signals are modulated at the same frequency resulting in poor reception by the receiver, which may cause a failure of the monitoring system that fails to provide the operator with expedient current tire pressure indications and alarms. Additionally, these prior systems do not enable easy methods of modifying the assignment between the tire positions and the respective code of the valve stem mounted sensors, for example, when the sensor is dysfunctional, lost or stolen. These and other disadvantages are solved or reduced using the invention.  
         SUMMARY OF THE INVENTION  
         [0006]    An object of the invention is to provide a vehicular tire pressure monitoring system that synchronizes radio transmissions from tire sensors transmitters to reduce potential interference between simultaneously transmitted tire pressure signals from the tire pressure sensor transmitters located on respective tires.  
           [0007]    Another object of the invention is to provide tire sensor motion detection switches for synchronizing tire pressure signal transmissions from respective tire pressure sensor transmitters to reduce potential interference between simultaneously transmitted tire pressure signals.  
           [0008]    Still another object of the invention is to method for reassigning the tire pressure sensor identification codes to respective tire locations.  
           [0009]    The present inventions are directed to a vehicular tire pressure monitoring system that time division synchronizes transmitted tire pressure signals from respective tire sensor modules to prevent the transmitted signals from the interfering with each other for improved tire pressure monitoring. Each tire sensor module is attached to the valve of a respective tire. the tire sensor module includes a motion detector switch that is activated when the vehicle is in motion through rotating tires that spin the sensors. Upon the detection of motion, battery power is routed to sensor electronics so that the sensor is powered during vehicular motion but remains dormant during periods of inactive vehicular motion so as to conserve sensor battery power. Each sensor module is individually configured to transmit respective tire pressure signals, including encoded identification codes, at differing non-overlapping time intervals so that the transmitted signals are staggered over time for time division synchronization. As such, the signals are transmitted at different times so as to prevent cross interference between these signals thereby improving the reception of these signals by the receiver of the monitor in the passenger compartment.  
           [0010]    In another aspect of the invention, the individual sensor modules can be activated preferably by vehicular motion to transmit the respective identification code signals received by the receiver in the monitor that is manually programmed into a code assignment mode that selects, using a display panel, the desired tire location and then assigns that tire location to the new identification code upon reception of the transmitted signal from a respective sensor module. This ability to reassign the sensor identification code enables the replacement of sensors that may have become defective, lost or stolen. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is drawing depicting a monitor display panel.  
         [0012]    [0012]FIG. 2 is schematics for the monitor display.  
         [0013]    [0013]FIG. 3 a  and  3   b  are schematics of the monitor receiver.  
         [0014]    [0014]FIG. 4 is a schematic of the monitor controller.  
         [0015]    [0015]FIG. 5 is a schematic of the tire pressure sensor module.  
         [0016]    [0016]FIG. 6 is a table of part lists indicating component values for the monitor display, monitor receiver, monitor controller and tire sensor module.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Reference is continually made throughout this description to FIG. 6 indicating component values. The vehicle tire pressure monitoring system consists of battery-powered components that are referred to as the monitor and the sensor module, shown by way of schematic representation in FIGS. 1 through 4 b,  and FIG. 5, respectively. Together, these devices monitor air pressure in the tires of a vehicle, not shown, for alerting the driver in the event that the tire pressure drops below a preprogrammed tire air pressure level. A typical four tire monitoring system consists of the monitor that is mounted inside the vehicle passenger compartment and four small tire air pressure sensor modules respectively securely attached to the valve stems of the tires. The sensor modules are encased in a small plastic housing, not shown, with suitable threads for valve stem attachment. The monitor may also be enclosed in a small plastic housing, not shown, and can be dash mounted, if desired, for receiving power from the vehicular battery, also not shown.  
         [0018]    Now, referring specifically to FIGS. 1 and 2, the monitor display panel shows a graphic representation of a vehicle through the use of light emitting diodes (LEDs) D 1 , D 2 , D 3  and D 4  indicating respective tires locations, for example, indicating back left, front left, front right and back right tire locations, respectively, for the exemplar four wheel vehicle. The display panel also includes a light emitting diode D 5  for indicating a low battery level of the sensor module. A high voltage logic one level through resistors R 3 , R 4 , R 5 , R 6  and R 7  will light up light emitting Diodes D 1 , D 2 , D 3 , D 4  and D 5 , respectively. The monitor display further includes a plurality of numerical indicators, such as displays U 1  and U 2 , which may be for example, two conventional seven segment numerical displays for indicating two digit tire pressure values. Displays U 1  and U 2  are used to display the sensed tire pressure and tire pressure trigger level when the monitor is in a programming trigger mode. Display U 1  shows the most significant digit and display U 2  shows the least significant digit. The two numeric seven segment displays U 1  and U 2  have respective activation transistors Q 2  and Q 3 , with respective base resistors R 9  and R 10 . A miniature programming switch S 1  is used for operator programming and for displaying tire pressures. A dual color LED LP 1  emits a green color indicating proper monitor operation and proper operating tire pressures, and emits a red color indicating malfunctioning of the monitor or improper tire pressures, such as a low tire pressure. The LED LP 1  has a drive transistor Q 1  with a pull resistor R 1 , base resistor R 2  and ground resistor R 8 . The monitor display is typically exposed for viewing in a passenger compartment, not shown, of the subject vehicle, also not shown, that is represented by LEDs D 1 , D 2 , D 3  and D 4 . Connector J 1  connects the display panel electronic of FIG. 2 to a connector J 3  of the monitor controller of FIG. 4.  
         [0019]    Referring to FIGS. 1 through 3 b,  and more particularly to FIGS. 3 a  and  3   b,  the monitor receiver obtains clean power from a five volt DC linear voltage regulator U 4 . The regulator U 4  receives vehicular battery twelve volts DC through connector J 2  and through a protection diode CR 1  and provides a constant five volt supply voltage VCC 5  distributed within the monitor to filter capacitors C 4 , C 5 , C 6  and C 7 , among other components. The supply voltage VCC 5  powers the receiver circuits. The receiver comprises an antenna ANT 1  coupled through an inductor L 1  and capacitor C 8  to a SAW filter U 7 . The inductor L 1  and capacitor C 8  function to set for impedance matching between the antenna ANT 1  and the SAW filter U 7  and. The SAW filter U 7  functions as front-end filtering for the received signal and is connected through impedance matching components inductor L 3  with capacitor C 13  to a low noise amplifier U 9  for demodulating the received signal. The low noise amplifier U 9  is powered by a three volt reference VCC 3  with filter capacitors C 15  and C 17  and provides an RF output signal to a frequency converter U 10  through coupling capacitor C 16 . The received signal is a 433.92 MHz RF AM signal that is picked up by the helical antenna ANT 1  and processed by the frequency converter U 10 . The frequency converter U 10  is connected to an IF tuned circuit consisting an inductor L 4  and capacitors C 21 , C 22  and C 23  for providing a tuned response while eliminating unwanted cross product signals generated within the frequency converter U 10 . The frequency converter U 10  is further connected to radio frequency (RF) and intermediate frequency (IF) bypass capacitors C 18 , C 19  and C 20 . The frequency converter U 10  uses a SAW resonator U 11  connected through resistor R 28  for generating a 423.22 MHz local oscillator signal which when mixed with the received signal at 433.92 MHz signal generates an amplitude modulated signal that is down converted to 10.7 MHz. Hence, SAW resonator U 11  provides a 423.22 MHz RF reference for mixing and down converting of the received RF signal into an IF signal at the IF output of the frequency converter U 10 .  
         [0020]    The frequency converter U 10  provides an IF output signal to a ceramic filter U 8  through resistor R 27 . The ceramic filter U 8  provides a sharp band pass response for further filtering the IF signal. An envelope detector consists of a circuit including capacitor C 11 , resistor R 25 , inductor L 2  and a detection diode U 6 . The output of the envelop detector at the diode U 6  provides a toggling digital signal that is further filtered by resistors R 22  and capacitor C 9  and is communicated to a buffering low pass filter amplifier U 5 A. The amplifier U 5 a has input capacitors C 12  and C 14 , input resistor R 26  and a feed back resistor R 24  providing a low pass filtered buffered signal. The filtered buffered toggling signal of the amplifier U 5 A is communicated to a comparator amplifier U 5 B that compares the buffered filtered toggling signal to the compare signal reference to square the filtered toggling signal into a square wave digital signal Data 3  having fast rise and fall times suitable for further digital processing. The amplifier comparator U 5 B uses resistor R 23  and capacitor C 10  to reduce noise for precise toggling of the comparator U 5 B for reducing jitter of the output digital signal Data 3 . A step down voltage regulator U 3 , with resistor R 14 , converts the VCC 5  reference into the three volt voltage reference VCC 3  that is filtered by capacitors C 2  and C 3 . Resistors R 11  and R 13  and potentiometer R 12  divide the reference VCC 3  into the compare voltage reference having a filter capacitor C 1  to provide a compare threshold to amplifiers U 5 A and U 5 B to limit standby noise. The Data 3  signal is converted by transistors Q 5  and Q 4 , using bias resistors R 15 , R 16 , R 17 , R 18 , R 19 , and R 20 , into a five volt data signal Data 5  communicated to the monitor controller. Hence, the received RF signal is converted into an IF signal that is then conditioned, filtered and detected into a VCC 3  voltage level signal and then squared and converted into a VCC 5  voltage level signal as a square wave form compatible with the conventional five volt logic levels of the monitor controller for communicating digital data for decoding and further processing by the monitor controller.  
         [0021]    Referring to FIGS.  1 - 4 , and more particularly to FIG. 4, the monitor controller includes a microprocessor U 12  having outputs RC 0 - 6  connected through respective resistor R 35 , R 34 , R 33 , R 32 , R 38 , R 37  and R 36 , through the display connectors J 3  and J 2  for respectively driving the segments f, a, b, c, , g, e, and d of the displays U 1  and U 2 . Output RC 7  drives the LED LP 1 . Output RB 7 , RB 6 , RB 5  and RB 4  respectively drive LED D 1 - 5 , and outputs RB 3  and RB 4  respectively drive transistors Q 2  and Q 3  for activating the displays U 1  and U 2 , respectively. The input RA 0  receives an input from the switch S 1 . The digital signal of Data 5  is received at the input RB 1 . The microprocessor U 12  is clocked by a oscillator consisting of a 4 MHz crystal oscillator Y 1  connected to oscillator capacitors C 25  and C 26 . A buzzer LS 1  is a piezoelectric ceramic device driven by transistor Q 6  having a base resistor R 29  connected to and controlled by the microprocessor U 12  at an RA 2  output. The buzzer LS 1  is energized by the microprocessor U 12  to alert the driver when the RF receiver picks up and decodes a low tire pressure message from a tire sensor module. A one kilo bit serial electrically erasable programmable read only memory (EEPROM) U 13  stores user-programmed low tire pressure trigger values, sensor module identification codes and other important parameters. The EEPROM U 13  is connected to the microprocessor U 12  at RA 3  and RA 4  data and clock inputs respectively using pull up resistors R 30  and R 31 . The microprocessor U 12  is powered by the reference VCC 5  having filter capacitor C 24 . The microprocessor U 13  is a high performance eight bit CMOS microprocessor with internal 2K EPROM program memory, and one hundred and twenty eight bytes of RAM data memory. On power up, resetter U 11  resets the microprocessor U 12  to start execution of an operating program stored in the microprocessor EPROM. Under program control, the microprocessor U 12  will illuminate each numeric display device by setting respective outputs to a logic one and by simultaneously turning on the transistors Q 2  and Q 3  connected to the numeric displays U 1  and U 2 . Similarly, the microprocessor U 12  can control the light emitting diodes D 1 - 4 , D 5 , and LP 1 .  
         [0022]    Referring to all of the Figures, and more particularly to FIG. 5, the tire pressure sensor module is housed in a tubular miniature enclosure, not shown, and is securely fastened to a respective tire valve stem, also not shown. The tire pressure sensor module electronics enable sensory, logic and transmitter functions. A piezoresistive pressure sensor U 18  comprises four strain resistive sensitive resistors diffused in silicon. These resistors are connected in a wheatstone bridge configuration whereby two resistors increase with positive pressure while the other two decrease in resistance. When pressure is applied to the sensor U 18  the resistors in the arms of the bridge of sensor U 18  change by an amount directly proportional to the pressure applied. When a voltage is applied to the bridge, there will be a resulting differential output voltage indicating sensed tire pressure. Sensory logic consists of a low power dual amplifier U 17 A and U 17 B connected to the sensor bridge U 18  and to resistors R 45  through R 50 , consists of a voltage regulator U 14  having filter capacitor C 28 , and consists of another high performance CMOS eight bit microprocessor U 15  with an attached EEPROM U 16  using a pull resistor R 42  on an GP 1  input. A nine VDC battery BAT 1  having filter capacitor C 27  is connected to a tire motion detection switch S 2 . The motion detection switch S 2  senses any motion of the respective vehicular tire. The switch S 2  detects motions and closes switch contacts within 100 milli seconds after the vehicle reaches fifteen miles per hour. Gravitational forces activate the motion switch S 2 . Pure hydrogen gas is contain is a small glass container, not shown, encapsulating first and second electrical contacts. Under gravitational forces, mercury disposed on the first electrical contact flows to make contact with the second electrical contact to thereby close the switch S 2 . Those skilled in the art are adept at making motion detection switches sized to a small valve stem plastic casing. The battery BAT 1  output voltage level is divided by resistor R 40  and R 41  to provide a battery voltage level reference to the microprocessor U 15  to monitor the +9V battery level. Themicroprocessor U 15  can sense the battery voltage reference through an input GP 2  coupled to the voltage divider resistors R 40  and R 41 . The amplifiers U 17 A and U 17 b condition the sensed tire pressure voltage signal for driving an analog-to-digital converter within the microprocessor U 15  to provide the microprocessor U 15  with an indication of the tire pressure sensed by the sensor U 18 . The radio frequency transmitter consists of transistor circuit Q 7 , resistors R 39  and R 44 , inductor antenna L 5 , capacitors C 29 , C 30 , and C 31  and a SAW resonator Y 2  for providing a transmitter frequency signal at 433.92 MHz that is modulated by the microprocessor U 15  through resistor R 43 . The GP0-5 terminals of the microprocessor U 15  are bidirectional inputs and outputs. The terminal GP 0  is used to receive the dataI/O signal. The terminal GP 1  is used to send or receive data to the EEPROM U 16 . The terminal GP 2  receives the battery voltage level reference and can output a digital clock signal to the EEPROM U 16 . The terminal GP 3  is connected to the output of the regulator U 14 . The terminal GP 4  is an output that is used to power the sensor U 18  and the amplifiers U 17 A and U 17 B. The terminal GP 5  outputs data to the Y 2  oscillator for amplitude modulation of the 433.92 MHz radio frequency carrier signal. Under program control, the microprocessor U 2  outputs an encoded digital message string DataI/O for pulsed amplitude modulating the carrier signal from the RF transmitter circuit.  
         [0023]    To minimize power consumption, the sensor module operates in a dormant mode and is only powered when the tire is in motion detected by motion switch S 2  that operates to connect the battery BAT 1  to the electronic components of the sensor module. The microprocessor U 15  is programmed to have an internal clock that regularly reads the air pressure in the tire from the sensor U 18  through the amplifiers U 17 A and U 17 B. If there has been a significant pressure change from the previous reading, the sensor module transmits a pulse modulated radio frequency signal to the monitor using the inductor L 5 . The RF message identifies the sensor module and contains pressure and battery data. The monitor controller receives data indicating the current tire pressure for display and comparison. Upon receiving the RF message, the monitor controller compares the tire pressure value received to a programmed value stored in the memory. If the received value is less than a predetermined low pressure trigger value, the monitor controller alerts the driver by flashing the low tire pressure value on the displays U 1  and U 2 , by flashing one of LEDs D 1 -D 4  for the respective tire, and by sounding an alarm using the buzzer LS 1 .  
         [0024]    The communication between the tire pressure sensor module and the monitor is unidirectional from the sensor module to the monitor controller. Transmitted signals are a logic one when the amplitude modulation level is high for two T and low for one T, or a logic zero when the amplitude modulation level is high for one T and low for two T, where T preferably equals ⅓ bit period, for example, T=270 microseconds. Messages are preferably transmitted with the most significant bits first. A message from a tire sensor module preferably consists of, in order, a preamble, a synch signal, three bytes of identification data, one byte of pressure data, one byte of full scale data, and one byte of battery data. The preamble is a 5.0 millisecond high level signal. The synch signal is a ten T pulse of low level signal. The three identification bytes is an identification number of the respective sensor module, and therefore of the respective tire to which the respective sensor module is attached. The full scale data is a calibration value indicating a maximum pressure read value. The battery data is a numerical value indicating the voltage level of the sensor module battery BAT 1 . The message is transmitted as an encoded RF message string.  
         [0025]    When an encoded RF message string from the tire sensor module is received by the monitor antenna ANT 1 , the RF signal is passed through the saw filter U 7  and through the low noise amplifier U 9 . The output of low noise amplifier U 9  is transmitted to the single conversion super heterodyne frequency converter U 10  that converts the received filtered signal into the modulated intermediate frequency signal at 10.7 MHz. The IF output is passed through a wide-band IF filter U 8 , and communicated to the amplitude detector U 6  for demodulating the received signal. The demodulated digital data is then further processed by the DC amplifiers U 5 A and comparator U 5 B for generating the Data 3  digital output. The digital output signal from U 5 B is passed to the voltage converter buffer of Q 5  and Q 4  for converting the voltage level from three volts to five volts. This five volt digital signal is then finally communicated to the microprocessor U 12  for decoding, verification and further processing. If the message is determined to be from a sensor module with and ID Code belonging to one of the vehicular tires, then the tire pressure value in the message is compared with a low tire pressure trigger value previously stored in the EEPROM U 13  as a low pressure comparison. If low pressure value comparison result is positive, the controller lights up the corresponding tire location LED D 1 - 4 , turns on the red color for LED LP 1 , and displays the tire pressure reading of the displays U 1  and U 2  while activating the buzzer LSI that is momentarily turned on for a predetermined period.  
         [0026]    When a tire reaches a predetermine rotational rate corresponding to a predetermined vehicular speed, for example, fifteen miles per hour, the motion switch S 2  closes to connect the battery BAT 1  that powers up the electronics in the tire pressure sensor including the piezoresistive pressure sensor U 18  that senses the tire air pressure level. The tire pressure is sensed by sensor U 18  and the amplifier U 17 A amplifies the resulting differential voltage as an amplified analog signal. The amplified analog signal is fed into the amplifier U 17 B that converts the amplified analog signal into a digital signal communicated to the microprocessor U 15 . The microprocessor U 15  compares the digital pressure value with a previous tire pressure reading, which is stored in the RAM of the microprocessor U 15 . The result of the comparison determines whether the microprocessor U 15  will encode a message and communicate that message through output GP 5  to the RF transmitter comprising resonator Y 1  for broadcasting to the monitor antenna ANT 1 . The transmitted message, if any, is transmitted by inductor L 5  and received by the antenna ANT 1 .  
         [0027]    At the end of this transmission, the microprocessor U 15  switches to a power saving mode of operation until the next wake up call. The microprocessor U 15  has an emulated internal clock that counts to awaken and interrupt the microprocessor U 15  at regular time intervals of time T 1  to take a new tire pressure reading. The microprocessor U 15  is also awakened at intervals of time T 2  to transmit an all-is-well-with-the-sensor signal to the monitor. The duration of T 1  is less than the duration of T 2 . To conserve power consumption, the tire pressure sensor module is powered down when the tire motion detection switch S 2  detects no motion in the tire when the vehicular speed drops below the predetermined vehicular speed, and is powered back up again when S 2  detects motion in the tire when the vehicle is traveling in excess of that predetermined speed. The microprocessor U 15  also monitors the battery voltage level at input GP 2  to alert the monitor controller when the battery BAT 1  has dropped below a level that may result in a inaccurate tire air pressure reading by the sensor U 18 . The tire pressure sensor U 18  can be calibrated at the factory to compensate for pressure reading imprecision between piezoresistive pressure sensor U 18  when exposed to extreme temperatures. For this purpose, the microprocessor U 15  can store pressure offsets values in the integrated non-volatile EEPROM. The values are programmed into microprocessor U 15  through inputs GP 0  receiving DATAI/O signal and a clock signal at GP 1 .  
         [0028]    The above system functions to monitor tire air pressures using respective sensor modules. The tubular sensor modules are preferably marked, for example, FL, FR, BL and BR markings respectively indicating the front left, front right, back left and back right. The sensor modules are placed on the valve stem of the tire in the respective location. The monitor is preferably preprogrammed at the factory to assign the respective identification code of the sensor modules to the respective tire location. Each sensor module type, indicated by the sensor markings, has a different preprogrammed staggered time value. Each sensor module of the same marking type is preprogrammed at the factory to transmit messages at predetermined amount of staggered time after motion is detected by the motion switch. For example, sensor module marked FL transmits messages at 0.0 seconds and repetitively every 16.0 seconds thereafter after motion detection, sensor module marked FR transmits messages at 4.0 seconds and repetitively every 16.0 seconds thereafter after motion detection, sensor module marked BL transmits messages at 8.0 seconds and repetitively every 16.0 seconds thereafter after motion detection, and sensor module marked FL transmits a message at 12.0 seconds and repetitively every 16.0 second thereafter after motion detection. In this manner, every 16.0 seconds is time divided into four non-overlapping staggered time segments. As such, cross interference between the transmitted messages is reduced.  
         [0029]    In a further aspect of the invention, the monitor system can be programmed to set the low tire pressure trigger level. The sensors have respective identification codes assigned to respective tire locations for referencing a received message to the respective tire. To enter a trigger programming mode, upon the application of power to the monitor, an operator presses the switch S 1  for a predetermined amount of time, for example, three second. After entering the trigger mode, the operator can repetitively press the switch S 1  to cycle through air pressure values, for example, between 18.0 psi and 50.0 psi, to thereby set the low pressure trigger level. In the event that any one of the pressure values of the received messages drops below this trigger level, the respective LED D 1 -D 4  is flashed as the LED LP 1  is illuminated red and the buzzer LS 1  is activated.  
         [0030]    In yet a further aspect of the inventions, the monitor system can be programmed to reassign the identification codes to different tire locations. For example, if a sensor is lost, stolen and becomes defective, the operator could order a replacement sensor module preprogrammed with the correct staggering time, but now having a new identification code so that all of the sensors may have respective unique ID codes so that monitors will only response to sensor modules on the subject vehicle. Upon the application of power, the user can hold press the switch S 1  for a longer predetermined amount of time, for example six seconds, to enter the assignment programming mode. After entering the assignment mode, the operator can repetitively push the switch S 1  to cycle through the tire location D 1 -D 4  to select the tire location for the new sensor module. After the sensor module is placed on the valve stem of the subject tire, with the other sensors removed from the remaining tires, and with the monitor in the assignment mode, the vehicle can be driven into motion to activate the motion switch of the sensor module to activate the transmission of a message containing the new ID code. Upon reception of the message, the monitor controllers assigns the received ID code to the selected tire location. In this way, new sensors can replaced if lost, stolen or defective.  
         [0031]    The above system and methods describe a preferred embodiment using exemplar devices and methods that are subject to further enhancements, improvement and modifications. However, those enhancements, improvements and modifications may nonetheless fall within the spirit and scope of the following claims.