(12) United States Patent
`US 6,271,748 B1
`(10) Patent N0.:
`
`Derbyshire et al. Aug. 7, 2001 (45) Date of Patent:
`
`
`US006271748B1
`
`(54) TYRE CONDITION MONITORING SYSTEM
`
`(76)
`
`Inventors: Andrew John Derbyshire, 12 Poplar
`Avenue, New Mills, Stockport,
`Cheshire, SK12 4HR; Jeremy Francis
`Siddons, 70 Nunsfield Road, Buxton,
`Derbyshire, SK17 7BN; John Kitto
`Richards, 3 Brierly Park, Buxworth,
`Whaley Bridge, Derbyshire SK12 7NW;
`Edward Charles Gibson, 40 Almond
`Place, Brimington, Chesterfield, S43
`1AG; Sean Patrick Davies, Palace
`Court, Flat 3, 9 Scarsdale Place,
`Buxton, Derbyshire, SK17 6EF, all of
`(GB)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.:
`
`08/793,586
`
`(22) PCT Filed:
`
`Aug. 31, 1995
`
`(86) PCT No.:
`
`PCT/GB95/02060
`
`§ 371 Date:
`
`Oct. 20, 1997
`
`§ 102(e) Date: Oct. 20, 1997
`
`(87) PCT Pub. No.: WO96/06747
`
`PCT Pub. Date: Mar. 7, 1996
`
`(Under 37 CFR 1.47)
`
`(30)
`
`Foreign Application Priority Data
`
`Aug. 31, 1994
`Mar. 13, 1995
`Jun. 2, 1995
`
`(GB)
`(GB)
`(GB)
`
`.................................................. 9417519
`
`.....
`9505016
`.................................................. 9511182
`
`Int. Cl.7 ............................ B60C 23/00; B60C 23/02
`(51)
`
`(52) US. Cl.
`..................
`340/442; 340/447; 73/1465
`
`(58) Field of Search
`....................... 340/442, 447,
`340/445, 444; 73/146.5, 146.3, 146.4; 200/61.22
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`
`.
`
`4/1988 Dosjoub et a1.
`4,737,761
`4/1989 Pompier .
`4,823,107
`6/1989 Pompier .
`4,837,553
`7/1989 Hebert et a1.
`4,843,872
`1/1990 Hebert .
`4,893,110
`7/1991 Dosjoub .
`5,029,468
`10/1991 Dosjoub .
`5,054,315
`2/1994 Nowicki et al.
`..................... 340/447
`5,285,189 *
`2/1994 Fiorletta ............... 340/447
`5,289,160 *
`
`...................... 340/442
`5,463,374 * 10/1995 Mendez et a1.
`5,483,827 *
`1/1996 Kulka et al.
`........................ 73/1465
`.
`..... 340/447
`5,559,484 *
`9/1996 Nowicki et al.
`
`8/1997 Coulthard ............. 340/442
`5,656,993 *
`
`..... 340/447
`5,731,754 *
`3/1998 Lee, Jr. et a1.
`5,824,891 * 10/1998 Monson .............................. 73/1465
`
`FOREIGN PATENT DOCUMENTS
`
`W0 92/14620
`
`9/1992 (W0).
`
`* cited by examiner
`
`Primary Examiner—Daniel J. Wu
`Assistant Examiner—John Tweel, Jr.
`(74) Attorney, Agent, or Firm—Sterne, Kessler, Goldstein
`& Fox, P.L.L.C.
`
`(57)
`
`ABSTRACT
`
`A tyre condition monitoring system comprises a wheel
`transmitter unit for each wheel of a vehicle. The transmitter
`unit is mountable in the wheel and has sensors for sensing
`pressure and temperature in and rotation of the wheel.
`Signals from the sensors are processed by a processor to
`produce data which is transmitted via a radio frequency
`transmitter. The data is transmitted with the data represent-
`ing a unit identity code. Transmitted data is received by a
`receiver unit where it is analyzed to determine the condition
`of the tyre. The receiver unit includes a user operable input
`for setting threshold limits for the temperature and/or pres-
`sure such that if a threshold is passed an alarm is sounded.
`Each wheel transmitter unit includes a power supply and is
`arranged so that power is only applied during the sensing
`and transmission of data. Intervals between transmissions of
`data can be varied depending on whether rotation of the
`wheel has been sensed.
`
`4,703,650
`
`11/1987 Dosjoub et a1.
`
`.
`
`24 Claims, 17 Drawing Sheets
`
`2
`
`3
`
`
`
`WHEEL
`TRANSMITTER
`
`UNN
`TX
`
`ANTENNA
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`FREQUENCY
`LINK
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`ANTENNA
`
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`
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`
`
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`
`RADIO
`DECODING
`
`
`FREQUENCY
`MICRUPRUCESSUR
`
`
`RECEIVER
`
`
`+VE GNU IGNITION
`
`SCHRADER
`
`EXH. 1003
`
`Page 1003-1
`
`Page 1003-1
`
`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 1 0f 17
`
`US 6,271,748 B1
`
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`

`US. Patent
`
`Aug. 7, 2001
`
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`US 6,271,748 B1
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`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 3 0f 17
`
`US 6,271,748 B1
`
`FIG. A
`
`START OF CALIBRATE
`
`30
`
`APPLY POWER TO
`ANALOGUE BLOCK
`
`READ PRESSURE
`TRANSDUCER
`
`READ TEMPERATURE
`SENSOR
`
`
`
`
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`
`REMOVE POWER FROM
`ANALOGUE BLOCK
`
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`VALUE IN RAM
`
`STORE HIGHEST PRESSURE
`VALUE m RAM
`
`STORE TEMERATURE
`VALUE IN RAM
`
`DELAY T SECOND
`
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`
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`
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`
`Page 1003-4
`
`Page 1003-4
`
`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 4 0f 17
`
`US 6,271,748 B1
`
`START OF REMOTE EXCLTATION MODE
`
`LO
`
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`Page 1003-5
`
`Page 1003-5
`
`

`

`US Pt
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`Aug. 7,2001
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`US6271748B1
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`
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`LAST 20 MINS?
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`
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`
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`
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`
`57
`
`52
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`
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`60 MINS? n———-———.——-
`
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`
`Page 1003-6
`
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`FIG. 6(1)
`
`Page 1003-6
`
`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 6 0f 17
`
`US 6,271,748 B1
`
`APPLY POWER TO
`ANALOGUE BLOCK
`
`READ PRESSURE
`TRANSDUCER
`
`READ TEMPERATURE
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`
`.
`
`7A
`
`REMOVE POWER FROM
`
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`
`N0
`
`
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`
`73
`
`
`
`FIG. 60:”
`
`
`
`HAS PRESSURE CHANGED
` YES
`BY MORE THAN :~2 PSI SINCE LAST
`TRANSMISSION?
`
`
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`. TRANSMISSION?
`
`
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`DATA STREAM
`
`
`
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`
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`
`DATA STEAM
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`
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`
`SLEEP MODE
`
`52
`
`Page 1003-7
`
`Page 1003-7
`
`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 7 0f 17
`
`US 6,271,748 B1
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`Page 1003-8
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`
`
`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 8 0f 17
`
`US 6,271,748 B1
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`US. Patent
`
`Aug. 7, 2001
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`US 6,271,748 B1
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`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 10 0f 17
`
`US 6,271,748 B1
`
`START
`
`133
`
`INCREASE PRES. TU PMAX
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`Page 1003-1 1
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`Page 1003-11
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`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 11 0f 17
`
`US 6,271,748 B1
`
`FIG. 13
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`11.7
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`
`Page 1003- 12
`
`Page 1003-12
`
`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 12 0f 17
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`US 6,271,748 B1
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`US 6,271,748 B1
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`Page 1003-14
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`US. Patent
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`Aug. 7, 2001
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`US. Patent
`
`Aug. 7, 2001
`
`Sheet 16 0f 17
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`US 6,271,748 B1
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`Page 1003-17
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`Page 1003-17
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`

`

`US. Patent
`
`Aug. 7, 2001
`
`Sheet 17 0f 17
`
`US 6,271,748 B1
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`Page 1003-18
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`Page 1003-18
`
`

`

`US 6,271,748 B1
`
`1
`TYRE CONDITION MONITORING SYSTEM
`
`The invention relates to a tyre condition monitoring
`system, to a sensor device, a wheel transmitter unit and a
`transducer f or use therewith, to a method of calibration, and
`to a transceiver circuit.
`
`BACKGROUND OF THE INVENTION
`
`Tyre condition monitoring systems a reused to to increase
`the safety and efficiency of the vehicle. There has been a
`great deal of interest in tyre monitoring in the past and some
`examples of recent proposals are disclosed in US. Pat. No.
`4,703,650, US. Pat. No. 4,737,761, US. Pat. No. 4,823,107,
`US. Pat. No. 4,837,553, US. Pat. No. 4,843,872, US. Pat.
`No. 4,893,110, US. Pat. No. 5,029,468 and US. Pat. No.
`5,054,315.
`In our International Patent Application No.
`PCT/GB 93/02005 published as WO-A-94/06640 we
`describe a tyre condition monitoring system comprising a
`unit mountable in a wheel of a vehicle. The unit comprises
`a sensor, a voltage controlled oscillator and a code generator
`arranged such that a coded signal is generated in a time
`period related to the value of the pressure or temperature
`sensed by the sensor. In order to conserve power the unit
`comprises a power supply which is activated by a timer from
`time to time causing the coded signal to be transmitted. Once
`the code has been transmitted the power supply is deacti-
`vated. The unit further comprises a monitor circuit which
`continuously monitors the sensor for an unacceptable pres-
`sure or temperature condition. An override circuit is respon-
`sive to the monitor circuit or to an external stimulus to
`
`activate the power supply.
`International Patent Application No. PCT/CA 92/00072
`published as WO-A-92/14620 describes a tyre monitoring
`apparatus and method in which a code representing a
`measured physical quantity, property or condition of a tyre
`is transmitted. The circuit is operable in an active mode in
`which a measurement circuit measures an instantaneous
`
`value of temperature and pressure and a transmitter circuit
`transmits a signal representing the sensed instantaneous
`values of pressure and temperature. In the low power mode
`minimal power is consumed by the measurement and trans-
`mitter circuits.
`
`STATEMENTS OF INVENTION
`
`The present invention aims to provide among other things
`an improved tyre condition monitoring system.
`According to one aspect of the invention there is provided
`a sensor device for sensing parameters associated with a
`pressurised unit, the sensor device comprising a sensor for
`sending one or more parameters associated with said pres-
`surised unit, a processor for processing signals from the
`sensor, and a transmitter for transmitting data, the processor
`being operable in plural different modes including a cali-
`bration mode in which data is recorded for known conditions
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`and in a normal operating mode in which data representing
`the one or more sensed parameters is transmitted by the
`transmitter.
`
`60
`
`According to another aspect of the invention there is
`provided a wheel
`transmitter unit for a tyre condition
`monitoring system, the wheel transmitter unit being mount-
`able to a wheel and comprising a sensor for sensing one or
`more parameters associated with said wheel, a transmitter
`for transmitting data representing the sensed one or more
`parameters, a power supply for supplying power to the
`
`65
`
`2
`sensor and the transmitter, and a condition monitor arranged
`to respond to operating conditions of the wheel transmitter
`unit
`to vary the manner in which the transmitter unit
`transmits data.
`
`According to a further aspect of the invention there is
`provided a transducer comprising: a pressure sensor for
`producing an output proportional to pressure applied thereto;
`a temperature sensor for producing an output representing
`the temperature thereof; storing means for storing calibra-
`tion data representing the behaviour of the pressure sensor in
`response to both pressure and temperature; and processing
`means for processing the pressure and temperature sensor
`outputs with reference to the stored calibration data to
`produce a calibrated output representing directly the pres-
`sure applied to the transducer.
`In another aspect the invention provides a tyre condition
`monitoring system, comprising at least one sensor device or
`wheel transmitter unit mountable in the wheel of a vehicle;
`and a receiver unit for receiving the data transmitted by the
`at least one sensor device or wheel transmitter unit and
`
`monitoring the received data, the receiver unit comprising
`user operable means for selecting one or more thresholds
`and being responsive to the one or more sensed parameters
`passing a respective user selected threshold by outputting a
`warning.
`In a further aspect the invention provides a method of
`calibrating a pressure transducer for temperature-related
`changes in the output of the transducer, the method com-
`prising: placing the transducer in a calibration chamber at a
`known temperature; varying the pressure in the chamber to
`a first pressure; recording data representing the output of the
`transducer for the first pressure; varying the pressure in the
`chamber to a second pressure; and recording data represent-
`ing the output of the transducer for the second pressure.
`The invention also provides a tyre condition monitoring
`system comprising at
`least one wheel
`transceiver unit
`mountable in a wheel of a vehicle; and a central transceiver
`unit or transmitting commands to the at least one wheel
`transceiver unit and receiving tyre condition data transmitted
`in reply to said commands from said wheel transceiver unit.
`The invention further provides a transceiver circuit com-
`prising an oscillator circuit for providing a reference fre-
`quency signal, a modulating circuit for modulating a data
`signal representing data to be transmitted with the reference
`signal and outputting the modulated signal for transmission,
`a receiver circuit for receiving a modulated data signal
`which receiver circuit is arranged to receive also signals
`from the modulating circuit derived from the reference
`signal, and a demodulating circuit for demodulating the
`received signal to extract the data therefrom.
`The above and further features of the invention are set
`
`forth with particularity in the appended claims and together
`with advantages thereof will become clearer from consid-
`eration of the following detailed description of an exemplary
`embodiments of the invention given with reference with the
`accompanying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the drawings:
`FIG. 1 is a schematic diagram or a first system embodying
`the invention;
`FIG. 2 shows in greater detail circuitry associated with a
`wheel transmitter unit;
`FIG. 3(a) shows a perspective cross sectional view
`through a capacitive pressure sensor and FIG. 3(b) shows a
`perspective view of the capacitive pressure sensor;
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`US 6,271,748 B1
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`3
`FIG. 4 is a flow diagram representing a calibration oper-
`ating mode;
`FIG. 5 is a flow diagram representing a remote excitation
`operating mode;
`FIG. 6 is a flow diagram representing a normal operating
`mode;
`FIG. 7 is a functional diagram representing convolution
`used in the wheel transmitter unit;
`FIG. 8 is a signal diagram showing an example of data
`encoded using a Manchester coding technique;
`FIG. 9 is a signal diagram showing a data stream trans-
`mitted in the normal operating mode;
`FIG. 10 is a signal diagram of a data stream transmitted
`in the remote excitation mode;
`FIG. 11 is a graph representing variations in the output of
`the pressure sensor with pressure and temperature;
`FIG. 12 is a flow diagram of a sensor calibration proce-
`dure;
`FIG. 13 is a graph representing variations in temperature
`and pressure applied to the sensor during the calibration
`procedure;
`FIG. 14 is a flow diagram of a simplified sensor calibra-
`tion procedure;
`FIG. 15 is a schematic diagram of a data signal transmit-
`ted using an alternative transmission format;
`FIG. 16 is a schematic diagram of an alternative data
`format;
`FIG. 17 is a schematic diagram of a wheel transmitter unit
`attached to the internal well of a wheel;
`FIG. 18 is a schematic diagram of an alternative arrange-
`ment for fixing the wheel unit in a wheel;
`FIG. 19 is a schematic diagram of a receiver unit;
`FIG. 20 is a view of the front cover of a display unit when
`operating in a pressure display mode;
`FIG. 21 is view of the front cover when operating in a
`temperature display mode;
`FIG. 22 is a view of the front cover when operating in a
`swap input mode;
`FIG. 23 is a view of the front cover when operating in a
`threshold input mode;
`FIG. 24 is a schematic diagram of a second system
`embodying the invention;
`FIG. 25 is a schematic diagram of circuitry associated
`with a wheel transceiver unit;
`FIG. 26 is a schematic diagram of one transceiver circuit;
`FIG. 27 is a schematic diagram of another transceiver
`unit; and
`FIG. 28 is a timing diagram of a command transmission
`to a wheel unit.
`
`DETAILED DESCRIPTION OF SYSTEMS
`EMBODYING THE INVENTION
`
`General Overview of a First System
`
`Referring now to FIG. 1 of the accompanying drawings
`there is shown a schematic diagram of a system 1 embody-
`ing the invention. The system 1 comprises a wheel trans-
`mitter unit 2 and associated transmitting antenna 3 mount-
`able in the wheel of a vehicle. It is envisaged that in practice
`the system will comprise a wheel transmitter unit for each
`wheel of the vehicle, including any spare wheels provided in
`the vehicle. The system further comprises a receiving
`
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`4
`antenna 4 which conveniently is a folded dipole printed on
`a circuit board mountable behind the dashboard of the
`
`vehicle for example. Signals from the receiving antenna 4
`are input to a radio frequency receiver which also is mount-
`able behind the dashboard and which serves to condition the
`
`signals for input to a decoding microprocessor 6.
`The decoding microprocessor 6 processes the signals
`input thereto in order to determine what information (e.g.
`temperature or pressure) has been transmitted from which
`wheel transmitter unit. The decoding microprocessor unit 6
`generates signals for driving a display unit 7 so as to provide
`on the display unit 7 an indication of the status of each of the
`wheels. Together the antenna 4, the receiver 5, the micro-
`processor 6, and the display 7 form a unit which will be
`referred to hereinafter as the receiver unit.
`
`It should be noted that whilst the radio frequency receiver
`5, the decoding microprocessor 6 and the display unit 7 are
`shown as separate functional units, they may in fact be
`combined in a single housing at a convenient
`location
`behind or in the dashboard of a vehicle. For example, it may
`be convenient to combine the radio frequency receiver 5 and
`the decoding microprocessor 6 in a single unit housed
`behind the dashboard, to provide the receiving antenna on a
`separate board and to provide the display unit mounted on
`the dashboard. Alternatively, it may be convenient to pro-
`vide a single radio unit comprising the antenna circuit board
`and the radio frequency receiver, and to provide a separate
`processing and display unit comprising the decoding micro-
`processor 6 and display unit 7. These implementation details
`are well within the scope of those possessed of the appro-
`priate skills and will not be discussed in any further detail
`herein.
`
`Wheel Transmitter Unit
`
`Turning now to FIG. 2 of the accompanying drawings
`there is shown in greater detail circuitry associated with the
`wheel
`transmitter unit 2. Each wheel
`transmitter unit 2
`
`comprises an analog circuit 8a and a digital circuit 8b. The
`analog circuit 8a comprises a pressure sensor 9, a thermistor
`10 and a reference voltage unit 11.
`Piezoresistive pressure sensors are widely available but
`are not well suited for use within a tyre because in use a
`significant current must pass through the resistive elements
`so that a pressure dependent voltage can be measured.
`Consequently, such sensors consume a relatively large
`amount of power making them unsuitable for long term use
`within a tyre. Another disadvantage of piezoresistive sensors
`is that they exhibit a large temperature coefficient. This
`results from mechanical strain caused by the difference in
`expansion coefficients of the silicon membrane and the
`supporting substrate of the sensor.
`FIGS. 3(a) and (b) show a pressure sensor comprising a
`silicon base A supporting a layer of silicon dioxide B
`defining a reference cavity C, and a silicon membrane D. A
`vacuum is formed in the cavity C. Because the silicon
`dioxide layer B is an excellent insulator, the silicon layers A
`and D form a capacitance. The upper silicon membrane
`thickness is chosen to exhibit a suitably large deflection
`upon the application of external pressure. However,
`the
`deflection must not be so large as to cause fracture or
`physical contact between the two silicon layers.
`As the silicon membrane D is deflected closer to the
`
`the capacitance between the two layers
`silicon base A,
`increases. The sensor exhibits a near-linear capacitance
`change with applied pressure with a very low intrinsic
`temperature coefficient. If the response of the sensor is
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`US 6,271,748 B1
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`5
`plotted, it can be seen that the capacitance change is super-
`imposed on an offset capacitance of around 70 pF. The
`capacitance increases by about 20 pF with an applied
`pressure of 10 Bar.
`The physical dimensions of the sensor make it particularly
`suitable for the tyre monitoring system, the sensor is smaller
`than 4 mm square before packaging.
`The thermistor is preferably a curve matched device
`because such devices are of known accuracy (102° C. is
`acceptable) thereby obviating the need to calibrate each
`wheel transmitter unit for temperature.
`The reference voltage unit 11 comprises a precision band
`gap reference device (not shown) which provides a reference
`voltage output to a tight tolerance regardless of any changes
`in the voltage supplied thereto. Signals from the pressure
`sensor 9, thermistor 10 and reference voltage unit 11 are
`output via respective amplifiers 12 to 14 to the digital circuit
`8b.
`
`Power for the wheel transmitter unit 2 is provided by a
`battery 15. A signal corresponding to the battery voltage is
`input via line 16 to the digital circuit 8b. The analog circuit
`8a, may be provided in a single ASIC. Among other things
`this offers the advantage of being able to manage simply the
`supply of power to the whole of the analog circuit 8a,
`thereby reducing the power consumed by the unit 2 as a
`whole when temperature, pressure and reference voltage
`signals are not required for processing by the digital circuit
`8b.
`
`The digital circuit 8b comprises a multiplexer 17, an
`analog to digital converter 18 and a microprocessor 19 with
`associated read only and random access memories (ROM
`and RAM) 20, 21. The pressure signal from the amplifier 12,
`the temperature signal from amplifier 13 and the battery
`voltage signal on line 16 are each input to the multiplexer 17.
`Under the control of the microprocessor 19, the multiplexer
`17 selects each of the signals in turn and outputs the selected
`signal to the analog to digital converter 18.
`The reference voltage from amplifier 14 is also input to
`the analog to digital converter 18 and provides a reference
`against which the pressure, temperature and battery signals
`are converted into digital form. (The reference voltage is
`defined to a tight tolerance in order to ensure accuracy of this
`conversion.) The reference voltage signal serves to define
`the maximum voltage which can be converted into digital
`form by the analog to digital converter 17. Thus, a reference
`voltage of 3 volts would define a voltage conversion range
`from 0 to 3 volts and thus a signal of 3 volts input from the
`multiplexer would be converted to a digital value of say 256,
`an input signal of 1.5 volts would be converted to a digital
`value of 128, and an input voltage of 0.75 volts would be
`converted to a digital value of 64. Data from the analog to
`digital converter 18 is input co the micro-processor 19 where
`it is processed.
`Wheel Unit Operating Modes
`The digital circuit 8b is preferably provided as a single
`microcontroller chip such as the PUNCHTM microcontroller
`by CSEM or the PICTM microcontroller by Arizona Micro-
`chip. Both of these proprietary devices comprises a micro-
`processor with associated ROM, RAM, multiplexer, analog
`to digital converter, etc. on a single chip. These devices are
`also operable in a standby or “sleep” mode in which power
`is removed from nearly all of the chip thereby reducing the
`power consumed when there are no temperature or pressure
`signals that require processing. As the digital circuit 8b
`enters the sleep mode a signal is generate a by the micro-
`processor 19 and output via line 22 to the analog circuit 8a
`causing power to be removed from the analog circuit 8a.
`
`6
`transmitter unit 2 also comprises a radio
`The wheel
`frequency transmitter 23 which receives encoded data from
`the microprocessor 19 for transmission. Also, a centrifugal
`detector 24 provides directly to the microprocessor 19 a
`signal indicative of centrifugal force. This signal provides
`for the microprocessor 19 an indication that the wheel is
`rotating (and therefore the vehicle to which the wheel is
`connected is in use). A mode selector 25 also provides a
`signal directly to the microprocessor for controlling the
`manner or mode in which the microprocessor functions. The
`mode selector 25 may be an induction device (for example
`similar to that described in WO-A-94/06640) which pro-
`duces control signals for the microprocessor in response to
`an electromagnetic field being applied thereto.
`The wheel transmitter unit 2 is operable in three different
`modes, namely a calibration mode, a remote excitation mode
`and a normal operating mode. Program data controlling the
`operation of the microprocessor 19 in each of these operat-
`ing modes is stored in the ROM 20.
`Calibration Mode
`
`The calibration mode is represented by the flow diagram
`in FIG. 4 of the accompanying drawings. The calibration
`mode is entered prior to installation of the wheel transmitter
`unit in the wheel of a vehicle. The transmitter unit is placed
`in a test chamber in which it
`is exposed to calibrated
`pressures. The requirements of the calibration are, naturally,
`dependent on the vehicle in which the unit is to be installed.
`However, for the majority of applications it is sufficient to
`expose the transmitter unit to two calibrated pressures in the
`test chamber, namely 0 psi and 60 psi at approximately room
`temperature.
`Referring now to FIG. 4, the calibration mode is entered
`at step 30 and at step 31 the microprocessor 19 applies
`power via line 22 to the analog circuit 8a. Next, the pressure
`signal from amplifier 12 is multiplexed into the analog to
`digital converter and the digital signal representative thereof
`is held by the microprocessor 19. This operation is repre-
`sented by the step 32 in FIG. 4. Next, at step 33 the
`temperature signal from the amplifier 13 is multiplexed in to
`the analog to digital converter, and the resulting digital value
`is held by the microprocessor. The microprocessor now has
`all the data that it requires for calibration and therefore in
`step 34 the microprocessor causes the power to be removed
`from the analog circuit.
`Depending on the pressure in the calibration chamber,
`either the low pressure value is stored in the RAM 21 in step
`35 or the high pressure value is stored in the RAM in step
`36. Also, the temperature value is stored in the RAM 21 at
`step 37. The system then pauses for one second, as repre-
`sented by step 38, and the calibration sequence is then
`re-entered by the microprocessor at step 31. The calibration
`sequence is repeated for the duration of the calibration test
`so that at
`the end of the test
`the RAM contains data
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`representing the lowest and highest pressures sensed by the
`pressure sensor 9 and the temperature sensed by the ther-
`mistor 10.
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`Data representing the output from the pressure sensor 9
`and the thermistor 10 is thus stored by the microprocessor 19
`as calibration constants in the RAM 21. This data remains in
`
`the RAM 21 for the lifetime of the battery 15. Also, the
`ROM contains a 24-bit code which identifies the wheel unit.
`
`A 24-bit code provides over 16 million different code
`combinations and therefore enables each wheel unit to have
`
`65
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`its own unique identity. The 24-bit code is programmed into
`the ROM during the manufacture of the wheel unit and
`remains with the wheel unit for the whole of its life. As will
`
`be explained in greater detail hereinafter, the identity code is
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`US 6,271,748 B1
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`7
`
`transmitted with the wheel parameter data (pressure and
`temperature) in order to enable the receiver unit to identify
`to which wheel the received parameters relate.
`Remote Excitation Mode
`
`The remote excitation or installation mode is represented
`by the flow diagram in FIG. 5 of the accompanying draw-
`ings. The remote excitation mode is a special mode that
`causes all of the calibration constants data and data repre-
`senting the present outputs from the sensor 9 and thermistor
`10 to be transmitted immediately on entry into the mode and
`then again every three seconds whilst still in this mode. The
`remote excitation mode is used during installation of a
`system or during reinstallation of part of the system follow-
`ing for example a power supply failure in a wheel unit or the
`receiver unit.
`
`Referring now to FIG. 5, following entry into the remote
`excitation or installation mode at step 40, power is applied
`to the analog circuit at step 41. The signals from the
`amplifiers 12 and 13 pertaining to the pressure and tempera-
`ture sensed by the pressure sensor 9 and thermistor 10 are
`converted into digital form and held by the microprocessor
`19 at steps 42 and 43. Then at step 44 power is removed from
`the analog circuit. Next, the microprocessor outputs data to
`the RF transmitter 23 for transmission thereby. This trans-
`mission is represented by the step 45 in FIG. 5. In step 46
`the processor waits for three seconds before returning to the
`beginning of the excitation mode process by again applying
`power to the analog block at step 41.
`The remote excitation mode is used during vehicle instal-
`lation to enable the radio frequency receiver 5 and decoding
`microprocessor 6 (see FIG. 1) to record the calibration
`constants associated with each transmitter. The calibration
`
`constants are used subsequently by the receiver to calculate
`accurate pressure values from the data transmitted from each
`wheel transmitting unit. The wheel transmitter unit 2 can be
`placed in to this remote excitation mode by way of the mode
`selector 24 at any time where an update of the data associ-
`ated with the wheel transmitter is required.
`Normal Operating Mode
`The normal operating mode is represented by the flow
`diagram in FIG. 6 of the accompanying drawings. The wheel
`transmitter unit 2 operates in the normal operating mode the
`majority of the time. In this mode the microprocessor 19
`determines whether or not the vehicle is in use by way of the
`signal from the centrifugal detector 23. When the vehicle is
`in use the pressure signal from the amplifier 12 and the
`temperature signal from the amplifier 13 are sampled every
`two seconds and depending on the values of the sampled
`data a decision is made as to whether or not the data should
`be transmitted. When the vehicle is in use the wheel trans-
`
`mitter unit 2 is arranged so that data is transmitted at least
`every ten minutes and also more frequently if there has been
`a significant change in the data since the previous transmis-
`s