Tire monitoring system

A tire pressure monitoring system identifies tire locations by recognizing that certain characteristics are unique to front-located tires verses rear-located tires as well as left-side tires and right-side tires. A control circuit is coupled to a temperature sensor and an accelerometer for each tire to receive information processed to discriminate between left and right and front and rear tires, respectively. With this information and a pressure signal, the monitoring system provides the operator with tire location and pressure information without reprogramming upon tire rotation. The system also alerts the operator to run-flat tire operational constraints.

BACKGROUND OF THE INVENTION
 The present invention relates to a tire pressure monitoring system for a
 vehicle and particularly one which can identify the location of a tire
 without reprogramming upon rotation of tires on the vehicle.
 The utilization of tire pressure monitors has been described in numerous
 patents in which tire pressure sensors have been mounted in the wheel,
 attached to the valve stem, or embedded in the tire itself. Such systems
 typically use a sensor and associated circuit for each tire which
 transmits a modulated radio frequency signal to a receiver in the vehicle
 for sending information indicating when the pressure of a tire has reached
 a predetermined low threshold. The receiver and associated circuitry, upon
 receipt of information indicating a low pressure condition, provides the
 driver with an alerting signal to the low tire pressure condition. Many
 systems utilize a tire pressure sensor and transmitter which uniquely
 identify each tire with an identification code also transmitted to the
 receiver such that not only is the operator made aware of the existence of
 a low pressure condition in one of the tires, the tire location is also
 identified. U.S. Pat. No. 5,661,651 discloses one such system in which the
 frequency of the transmitted signal identifies the tire. In other systems,
 a binary code identifying each tire is employed.
 A problem with such systems occurs upon the rotation of tires, which is
 recommended on a frequent basis by many tire or vehicle manufacturers.
 Thus, tires are rotated from front to rear, from side to side, or both to
 promote even wear. In such case, a tire which originally was identified to
 the operator as being located, for example, on the left front of the
 vehicle may now be located on the right rear and a system which identifies
 tire location would now indicate a tire problem at the wrong location to
 the vehicle operator. It has been suggested that this problem can be
 overcome by reprogramming the tire location as suggested by, for example,
 U.S. Pat. No. 5,463,374 where it is necessary to manually place a strong
 magnet near each of the remote wheel-mounted transmitters upon rotation of
 a tire to reidentify the location of the tire. This not only requires a
 manual step of having the operator or service personnel place a magnet
 near each tire, it also requires the tire-mounted transmitter to include a
 magnetic switch as part of the mechanism, thereby adding to the cost,
 reliability and complexity of the overall system. Other approaches also
 suggest the use of a service tool which must be manually coupled to each
 tire to reprogram the tire location upon rotation of the tires.
 Thus, with existing systems, the rotation of tires on a vehicle requires
 intervention, typically with service personnel or by an operator
 sufficiently skilled to reprogram the tire location, so that the
 monitoring system can recognize the new location of a tire. There exists a
 need, therefore, for a system which allows rotation of tires on a vehicle
 and which automatically identifies the new tire location to the system
 display and monitor.
 With the introduction of "run-flat" tires, a new tire related problem has
 also been introduced, namely, the requirement that an operator only drive
 on a "run-flat" tire for about fifty miles at a maximum speed of 55 mph.
 There is a need also, therefore, to notify a driver that a "run-flat" tire
 has lost its pressure and alert the driver to the speed and distance
 limitations upon such occurrence.
 SUMMARY OF THE PRESENT INVENTION
 The system of the present invention allows for tire location identification
 by recognizing that certain characteristics are unique to front-located
 tires verses rear-located tires as well as left-side tires and right-side
 tires. It has been discovered that, when a vehicle is in operation, front
 tires typically have a higher operating temperature than rear tires. Thus,
 by sensing tire temperature information, such information can be employed
 to discriminate between front-mounted and rear-mounted tires.
 Additionally, sensors can be provided which distinguish between left- and
 right-side mounting of wheels. One such sensor can be an
 accelerometer-type device which, when a vehicle accelerates, makes contact
 with two of three contacts and, when the vehicle decelerates, makes
 contact with a different two of three contacts. When such a sensor is
 mounted on a left-side wheel of the vehicle, upon acceleration, one set of
 contacts are closed, however, when the same type of sensor is positioned
 in the same location on a wheel on the right side, the other set of
 contacts are closed. With this information, therefore, the signal
 generated by such sensors can be employed to discriminate between left-
 and right-side mounted tires. With the information provided by these
 sensors, a control circuit can identify each tire's location and provide
 the operator with accurate pressure information for each tire. By coupling
 the control circuit to the vehicle's speed and distance signal
 information, typically available on the vehicle bus, "run-flat" warning
 signals can also be provided to the vehicle operator.
 These and other features, objects and advantages of the present invention
 will become apparent upon reading the following description thereof
 together with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring initially to FIG. 1, there is shown a tire pressure monitoring
 system 10 of the present invention which includes a plurality of vehicle
 wheel-mounted sensor control circuits 12, each of which are substantially
 identical and are mounted to each of the vehicle wheels, preferably in the
 valve stem. In a typical vehicle, at least four such sensor control
 circuits 12 are employed and, in some vehicles, an additional one may be
 employed in connection with a spare tire. Each of the circuits 12 provides
 a modulated RF tire pressure information signal 14 to the vehicle-mounted
 receiving and control circuit 16 which receives signals from each of the
 transmitter circuits 12, demodulates the information and provides a
 display to the vehicle operator of the status of each of the tires being
 monitored. Each of the circuits 12, thus, includes tire sensor circuit 20
 (shown in detail in FIGS. 2 and 3) and an RF transmitter 22 for
 transmitting an RF signal (typically at a frequency of 315 MHz or in that
 frequency band) which is modulated with a multiple bit code identifying
 not only the particular tire but also the location of the tire as
 described in greater detail below. Coupled to the output of each of the
 transmitters 22 is a transmitting antenna 24 for transmitting the
 modulated RF energy from each of the wheels being monitored to a receiving
 antenna 13 associated with the vehicle's monitor and control circuit 16.
 Circuit 16 further includes a receiver 15 coupled to antenna 13 for
 receiving the information transmitted by each of the circuits 12 for
 demodulating the RF energy and for providing a binary bit stream to a
 microprocessor 17 also coupled to the vehicle system bus 18 for receiving
 not only tire pressure information from receiver 15 but also speed and
 distance information from the vehicle system bus. Microprocessor 17 has an
 output coupled to a display 19 which can be a graphic display such as a
 plan view illustrating the position of each of the tires to the operator
 with the tire pressures displayed numerically immediately adjacent each
 tire. Alternately, it can be an alpha/numeric display displaying, for
 example, LF for left front and the tire pressure, such as 32 psi, adjacent
 the tire identification. Display 19 can be an LCD, a vacuum florescent, or
 any other suitable display commonly employed in the automotive industry.
 System 10, therefore, provides to the vehicle operator a visual display of
 the location of each of the tires on the vehicle as well as the status of
 the tire pressure and, with the interconnection to the system bus 18 with
 run-flat tires, provides a suitable display, such as a flashing LED or a
 counter which decrements a fifty-mile limit display to the operator,
 indicating the limits of use of a run-flat tire. Microprocessor 17 may
 also provide, upon the occurrence of a flat in a run-flat tire, a speed
 limiting alarm, either audio or visual, to warn the driver that the 55 mph
 speed limit has been reached. The tire sensor circuit 20, which is capable
 of discriminating between front and rear and left- and right-side mounted
 tires and, therefore, provides tire location information to circuit 16
 regardless of the rotation or changing of tires, is now described in
 connection with FIGS. 2 and 3.
 Referring now to FIGS. 2 and 3, the sensor circuit 20 includes an
 acceleration/deceleration sensor 30, which is shown schematically in FIG.
 3 and which is mounted to each of the wheels 11 of the vehicle, as is the
 circuit 20. By mounting the circuit and sensor in a valve stem of a wheel,
 it communicates with the tire pressure within the tire as well as the
 ambient temperature surrounding the tire and wheel. The valve stem-mounted
 pressure sensor microprocessor and transmitter, together with the
 batteries employed therewith are included in an ASIC (application specific
 integration circuit), which is modified as described below, but otherwise
 is a tire pressure sensor and transmitter commercially available from the
 Schrader Bridgeport Company. The accelerometer sensor 30 is added to the
 commercially available circuit board. As is schematically shown in FIG. 3,
 the accelerometer comprises a housing 32 into which a rolling conductive
 ball 34 is mounted with a floor defining a contact 36 (contact C) and a
 pair of inclined contacts 35, 37 (contacts A and B, respectively) in
 spaced relationship to one another with a gap 33 which can be selectively
 bridged by ball contact 34 when the centrifugal force of the rotation of
 wheel 11 is sufficient to cause the ball 35 to roll along the surface of
 contact 35 until it bridges gap 33 between contacts 35 and 37. As can be
 appreciated, the accelerometer and decelerometer sensor 30 is mounted to a
 vehicle wheel 11, as shown schematically in FIG. 3 such that upon initial
 acceleration (in a direction indicated by arrow A in FIG. 3), ball 34
 moves rearwardly into the position shown in FIG. 3, making contact with
 contacts 35 and 36. Upon reaching a near steady state velocity, the ball
 34 rolls up into gap 33 making contact between contacts 35 and 37. Upon
 deceleration in a direction opposite arrow A in FIG. 3, ball 34 rolls to
 the opposite side as shown in FIG. 3 and makes the contact between
 contacts 37 and 36. By mounting the sensor 30 on each of the wheels, this
 sequence of operation, which normally occurs when the car begins movement
 and subsequently slows, will provide an opposition sequence of contacts
 for left- and right-side mounted tires. This information, therefore, can
 be used to discriminate between left- and right-side tires, as described
 in greater detail below in connection with FIGS. 4 and 5.
 Contacts 35 and 37 (A and B), thus, define a switch 40 shown schematically
 in FIG. 2, which is coupled to ground through contact 35 by means of a
 resistor 42. The sensor circuit 20 includes a lithium battery 44 which
 supplies a voltage Vcc through a unijunction transistor 46 controlled by
 switch 40 by grounding the gate 47 of transistor 46, upon closure of
 switch 40 when gap 33 is bridged by a conductive ball 34, shown in FIG. 3.
 This switch closure is coupled to gate 47 of transistor switch 46 by means
 of a coupling capacitor 43 and an RC circuit including resistor 45 and
 capacitor 48 coupled from the gate 47 of transistor 46 to ground as shown.
 Power from battery 44 is continuously applied to microprocessor 50 of the
 sensor circuit 20 by means of a pair of serially coupled resistors 51 and
 53, such that the microprocessor 50 is always powered. The microprocessor
 typically includes a power-saving sleep mode and wakes up when the voltage
 Vcc is applied to the sensor circuit upon activation of the switch 40 by
 movement of the vehicle. This feature provides power saving for the
 lithium battery 44 and allows the circuit to provide temperature and
 pressure information upon application of the switched voltage Vcc to the
 sensing circuit that is shown in FIG. 3.
 Microprocessor 50 receives input signals from sensor 30 from contacts A, B
 and C (35, 37 and 36), as shown in FIGS. 2 and 3, as well as pressure
 information "P" from a pressure transducer 60 and a temperature
 transducer, such as thermistor 70 (FIG. 3). This information is digitized
 and applied to modulate the signal from an RF transmitter 22 coupled to
 Vcc for receiving operating power therefrom and to an antenna 24 for
 transmitting the tire pressure, temperature, and sequence of contacts
 between switch contacts A, B and C to the vehicle's receiving and
 monitoring circuit 16, which demodulates the information, as described in
 connection with FIGS. 4-6, to identify each of the tire's location as well
 as their pressure.
 The sensor circuit 20, as seen in FIG. 3, includes output terminals A, B
 and C associated with contacts A, B and C (35, 37 and 36, respectively)
 coupled to the input terminal of microprocessor 50, as shown in FIG. 2.
 Contact 35 is coupled to ground through a resister 38, while contact 36 is
 coupled to ground through a resistor 39. The microprocessor 50
 sequentially interrogates the status of switch 30, as graphically
 illustrated by the switches 52 and 54 in FIG. 3, to sequentially generate
 a binary code indicating the AC connection, the AB connection and the BC
 connection with the AC connection being illustrated in FIG. 3. Thus, for
 the AB connection, switch 52 would be moved to a downward position from
 that shown such that the power would be applied to contact A and, when the
 AB contact is made, a logic "1" signal would be provided on output
 terminal B in FIGS. 2 and 3. Switches 52 and 54 are internal solid state
 switches contained within the microprocessor, which is programmed to
 sequentially interrogate the status of each of the contacts 35, 37 and 36
 and their relationship to one another. Switches 52 and 54 are only graphic
 illustrations of this operation of the microprocessor, which is controlled
 by a conventional subroutine added to the commercially available program
 for digitizing pressure information from pressure sensor 60.
 Pressure sensor 60 is coupled to the switched power Vcc circuit from
 transistor 46, as shown in FIG. 3, to provide a pressure indicating signal
 "P" to the microprocessor. The thermistor 70 is coupled to Vcc through a
 voltage divider circuit including a resistor 72 to provide temperature
 signal "T" applied to input terminal T of microprocessor 50, as shown in
 FIG. 2. The microprocessor 50 provides a serial bit stream of modulated
 binary bits at an RF frequency of approximately 315 MHz, providing tire
 pressure and temperature information as well as the sequence of closure of
 contacts of switch 30, which is transmitted by antenna 24 to the vehicle's
 monitoring circuit 16 which receives the signals via antenna 13 (FIG. 1).
 The receiving circuit 16 includes an RF receiver 15 which receives
 transmission from all of the tires. Each of the transmitters 22 may be a
 different frequency with receiver 15 including a separate channel for each
 of the transmitters 22. Preferably, the bit steam from each of the sensor
 circuits 20 uniquely identifies that sensor and the location of the sensor
 and, therefore, the tire on the vehicle. The transmission format includes
 22 bits identifying the sensor and its detected pressure. The number of
 bits is relatively high so that vehicles adjacent to one another do not
 interfere with their respective transmission of tire pressure information.
 Two additional bits are included to provide temperature information and
 three additional bits are provided to provide the sequence of operation of
 switch 30 to the transmitted signal information.
 As noted in the background, it was discovered that the front tires, when
 the vehicle has been in operation for five to ten minutes, typically run
 at a higher temperature than the rear tires. This is due in part to the
 mass of the engine being located centrally between the tires and
 generating additional ambient heat as well as the fact that the turning of
 the vehicle in its operation also generates more heat in the front tires,
 typically, than in the rear tires. Thus, it is not uncharacteristic for
 the front tires to run anywhere from 10.degree. to 15.degree. higher than
 the rear tires during vehicle operation. This fact is employed by the
 system of the present invention to provide a discrimination between front
 and rear tires. As discussed above, the determination between left- and
 right-side tires is accomplished utilizing the accelerator/decelerator
 sensing switch 30 and the sequence of closure of the contacts A, B and C,
 thereof, for each of the wheels.
 Tire discrimination is shown in FIG. 4, where the change in temperature in
 the front tires is relatively large between an initial cold condition and
 while running, while the Delta (.DELTA.) "T" temperature differential in
 the rear tires is relatively small. The sequence of connections between
 contacts A, B and C discriminate between left- and right-side tires. The
 left-side tires provide a sequence AC initially, AB then BC, as seen in
 the diagram of FIG. 4. The right-side mounted tires, on the other hand,
 provide a sequence of BC, AB and AC due to the difference in the contacts
 upon acceleration and deceleration when the sensor is mounted on the left-
 or right-side of the vehicle. This information is sufficient to uniquely
 identify the location of each tire and provide such information to the
 vehicle operator. The microprocessor 17 in the vehicle control and monitor
 circuit 16 is programmed to analyze the transmitted information of signals
 14 from each of the tires to make such a determination and to display the
 results to the vehicle operator. The programming of the microprocessor to
 detect the tire location is illustrated by the program flow diagrams of
 FIGS. 5 and 6 now briefly described.
 In FIG. 5, the right and left tire distinction based upon the sequence of
 contacts of switch 30 is described beginning at start block 80. Test block
 82 determines initially whether there has been a contact made between
 contacts A and B of switch 30. If, for example, the tires are located on
 the left side, such a connection is made upon acceleration of the vehicle
 and the program moves to block 84, where it waits to detect a different
 connection and whether the different connection is a contact between A and
 B indicating that the vehicle has now reached an operational speed. If it
 does not occur, the program loops through blocks 80, 82 and 84 until it
 does, in which case the program moves to block 86, where it waits until a
 different connection is made and detects whether it is a contact between
 switch contacts B and C. If no contact between B and C has yet occurred,
 the program loops through blocks 80, 82, 84 and 86 until the sequence has
 been detected. Once detected, it increments a left tire counter as
 indicated by block 88. Once the sequence has been detected, it continues
 to determine whether the sequence has occurred and, if it has, for sixteen
 cycles as indicated by block 90, a flag is set as indicated by block 92,
 indicating that the detected tire is a left-side tire. If sixteen cycles
 have not been detected, between each cycle there is a one-minute delay, as
 indicated by block 89. This provides sufficient driving time of sixteen
 minutes for the tires to warm up and provides not only the left/right
 discrimination but also the front/rear discrimination information
 accurately to the microprocessor and subsequently provides accurate
 displayed information to a vehicle operator.
 If the connection between A and C does not first occur as indicated by
 block 82, the program moves to block 83 to determine if the connection
 exists between B and C, indicating a possible right-side tire detection.
 If a connection between B and C is detected, the program moves to block 85
 to determine whether a connection exists between A and B and, if not, the
 program cycles through blocks 80, 82, 83 and 85 until a connection between
 A and B exists, indicating that the right-side tire is likely. If a
 connection between A and B is detected by the movement of ball 34 to the
 AB position indicating the vehicle is in motion, the program awaits a
 connection between A and C, as indicated by block 87, and cycles through
 the blocks until such switch transition occurs. When this occurs, the
 right tire counter 88' is incremented and by block 90' this count is
 compared with a preset count of sixteen to determine whether sixteen
 cycles of sequence of switch operation has been detected. If not, the
 sixty-second delay 89' is inserted in the program and the program again
 loops through blocks 80 through 90'. Once sixteen loops have occurred, a
 flag is set as indicated in block 92' indicating that the tire is on the
 right side of the vehicle, and counters 88 and 88' are cleared. The
 microprocessor simultaneously looks at information from each of the four
 tires, cycling through a similar routine as that shown in FIG. 5 for each
 of the four tires of a passenger vehicle.
 Simultaneously the microprocessor program runs a subroutine, as shown in
 FIG. 6, which includes a start block 100 initiated once switch A and B is
 made and the vehicle is in operation. The program then tests at block 102
 to determine whether or not the vehicle has been in motion for five
 minutes to provide time for the tires to warm up and, if not, cycles
 through blocks 100 and 102 until the five-minute timer has expired. Upon
 expiration of the five-minute clock, the program looks at all the
 temperature information as indicated by block 104 and determines the
 difference for each tire between the initial tire temperature upon
 starting (indicated by block 100) and after five minutes of operation. The
 change for each tire is identified in the flow diagram as Delta T1 for
 tire one and Delta T2 for tire two with the Delta temperatures extending
 through Delta Tn, with the n representing the number of tires. The program
 then tests the difference between each of the tire temperatures, as
 indicated by block 106, to determine if the temperature difference from
 the initial temperature reading for each of the tires and their subsequent
 temperature and such difference between two different tires is greater
 than 5.degree. F., as indicated by block 106. Thus, as illustrated by
 block 106, the change of temperature of T1 is subtracted from the change
 of temperature in T2 to determine if the difference between the two
 changes in tire temperatures is greater than 5.degree.. If it is, it is a
 representation that a front tire is being compared with a rear tire and,
 for the example shown in FIG. 6, T1 is a front tire as determined by block
 108 and a front tire counter is incremented one count.
 The program then proceeds to block 110 to await such calculations before
 setting a tire flag as indicated by block 112. If four cycles have not
 been detected, indicating that tire one is a front tire, the program goes
 to a sixty-minute timer 114 to allow the tires to cool down to an
 equilibrium state before retesting. The four-cycle temperature
 differential is provided to reliably identify a front or rear tire. If in
 block 106 the difference in temperature differences between two adjacent
 tires being tested is not 5.degree. as indicated by block 107, the program
 reverses the test to determine whether tire two is a front tire. If the
 differences in pressure between T1 and T2 as tested in block 107 indicates
 greater than 5.degree., the increment two tire is the front tire counter
 109 is set and upon receiving four cycles of such information, as shown by
 counter 110' a set tire two is in front flag is set as indicated by block
 112'. The counters for four cycles is then reset. If four cycles have not
 been received, a sixty-minute delay (block 114) is inserted to allow the
 tires to reach an equilibrium temperature. As illustrated in FIG. 6, only
 the differences between temperatures in tire one and tire two are being
 compared, it being understood that for each of the four tires a comparison
 is made to discriminate between front and rear tires. In a passenger
 vehicle, the front tires and rear tires have distinguishable operating
 temperature differences with the differences between the differences
 reliably being greater than 5.degree. F. Thus, with the system of the
 present invention, a tire monitoring system is provided which
 discriminates between front and rear and left- and right-side tires,
 thereby providing the vehicle monitoring circuit with a signal which can
 uniquely identify the location of a tire regardless of whether it has been
 rotated on the vehicle without reprogramming the microprocessor or the
 sensor located within the tire or wheel.
 Additionally, the microprocessor 17 of the vehicle monitoring circuit is
 coupled to the system bus 18, as shown in FIG. 1, and, when run-flat tires
 are employed, the pressure signal indicating that one of the tires has
 become flat is applied by the monitor circuit 12 to the microprocessor 17
 together with an identification of the tire. Upon receipt of such a
 signal, the microprocessor processes the speed information from the
 vehicle bus as well as the odometer information from the vehicle bus and
 provides a signal to the display 19 that the speed should be limited to 55
 mph, such as by flashing a warning light or audible signal if that speed
 is exceeded. In addition, a fifty-mile counter is set and decremented to
 provide an additional alarm or alarms as the fifty-mile distance limit is
 reached to the vehicle operator, also indicating that the life of the
 run-flat tire is being reached. Thus, the tire pressure monitor and
 display system of the present invention provides additional safety and
 operational features not found in other systems and provides a reliable,
 relatively inexpensive system which does not require specialized service
 tools or personnel to reprogram the microprocessor upon rotation of the
 tires.
 It will become apparent to those skilled in the art that various
 modifications to the preferred embodiment of the invention as described
 herein can be made without departing from the spirit or scope of the
 invention as defined by the appended claims.