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`USOOSD83457A
`.
`5,083,457
`[11] Patent Number:
`[19]
`United States Patent
`
`Schultz
`[45] Date of Patent:
`Jan. 28, 1992
`
`[54] REMO’I‘ELY ACI'UATED mar. pREssmu;
`SENSOR
`
`[75]
`
`Inventor:
`
`4
`Thomas J. Schultz, Neenah, Wis.
`
`........................... 73/723
`2/1981 Vagoetal.
`4,250,759
`...... 73/146
`4.704.901 11/1987 Rocco etal.
`4,891,973
`1/1990 Bollweber et al.
`.
`73/146.5
`3/1990 Genesheim et a1.
`.............. 73/146.5
`4,909,074
`
`[73] Assignee:
`
`155 Development GMT-50“, Inc.,
`Necnah, Wis.
`[21] Appl. No.: 453,735
`
`Primary Examiner—Donald 0, Woodiel
`Attorney, Agent, or Firm—Foley & Lardner
`[57]
`ABSTRACT
`
`Dec. 20, 1989
`[22] Filed:
`860C 73/02
`_____
`[51]
`Int Cl 5
`[52] US. Cl. 23:21:::....i:::.'.':."73/1;15.5- 73/146.8-
`340/443. 364/558)
`[53] Field of Search ............................ 73/1465, 146.8;
`354/553; 340/445, 447, 443, 442
`
`[561
`
`Referenm Cited
`US. PATENT DOCUMENTS
`137/227
`3.592,213
`7/1971 Guy
`3,827,393
`8/1974 Winthcr ............ 116/34
`
`1/1978 Markland ct al.
`................. 73/1465
`4,067,235
`
`A tire pressure sensor is provided including a name
`ducer unit having a mechanical-to-clectrical transducer
`disP°5°d in mm” “me“ With "‘5 Pres-“ml“ “31°“
`of a tire. The transducer senses tire pressure and outputs
`en eleem'eel signal representative thereof- The "ans-
`ducer unit further includes a response signal generator
`which transmits a signal representative of tire pressure.
`A hand-held remote display unit receives the transmit»
`ted signal and converts it to visual indicia of tire pres-
`sure, for example on a ‘hreedigit digi‘e‘ display-
`
`10 Claims. 7 Drawing Sheets
`
`
`
`Pet’r Exhibit 1006
`Continental v. Wasica
`IPR2014-00295
`
`Page 000001
`
`

`

`US. Patent
`
`Jan. 28, 1992
`
`Sheet 1 of 7
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`5,083,457
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`FIG. 1
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`

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`US. Patent
`
`Jan. 23, 1992
`
`Sheet 2 of 7
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`5,083,457
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`

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`US. Patent
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`Jan. 28, 1992
`
`Sheet 3 of 7
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`5,083,457
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`Page 000004
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`

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`US. Patent
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`Jan. 28, 1992
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`US. Patent
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`US. Patent
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`Jan. 28, 1992
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`Sheet 7 of 7
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`1
`
`5,083,457
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`REMOTELY ACTUATED TIRE PRESSURE
`SENSOR
`
`TECHNICAL FIELD
`
`The present invention relates, generally, to a trans-
`mitter and receiver combination for measuring the pres-
`sure within a vehicle tire, and more particularly. to a
`transmitter including a pressure transducer and an infra-
`red generator cooperating therewith, for transmitting
`an infrared signal indicative of pressure to a hand-held
`remote receiver unit including a pressure display.
`BACKGROUND OF THE INVENTION
`
`The leading cause of premature tire failure is improp-
`erly fitted tires. Underinflated or overinflated tires can
`result in vehicle damage, inadequate traction, low gas
`mileage, premature tread wear, and blowouts, i.e., spon-
`taneous destruction of the tire.
`Presently known devices for determining tire pres-
`sure are unsatisfactory in several regards. For example,
`existing techniques for measuring tire pressure typically
`involve coupling a mechanical pressure sensor to the
`valve stem and reading a lineal gauge extending from
`the sensor. See,
`for example, Guy US. Pat. No.
`3,592,218, issued July 13, 1971. This procedure is time
`consuming, cumbersome, and wholly unsuited for use in
`inclement weather, especially when the vehicle may
`have up to eighteen or more wheels.
`Tire pressure sensors configured to be mounted di.
`rectly to the tire valve stem are also known. See, for
`example, Winther US. Pat. No. 3,827,393, issued Aug.
`6, 1974. The Winther device comprises a pressure dif-
`ferential sensor disposed within a cylindrical housing,
`which housing is threadedly attached to the valve stem.
`When the tire is properly inflated, the pressure at the
`differential pressure sensor valve is sufficient to over-
`come a spring force which acts on the valve seat. When
`the pressure within the tire drops below a predeter-
`mined threshold, the differential pressure sensor valve
`moves in the spring biased direction, porting tire pres»
`sure to a slideable piston assembly having a visibly con-
`spicuous piston rod disposed to project axially from the
`valve assembly at low tire pressures,
`Another known device involves a cap designed to
`replace existing standard valve stem caps. The color
`exhibited by the cap changes with a decrease in tire
`pressure, thus providing visual indicia of tire pressure
`loss.
`Yet another known tire pressure sensing device pro-
`duces a digital LCD readout indicative of tire pressure
`when the hand-held sensor is brought into engagement
`with the valve stem. Such a device is available from
`Leichtuug Workshops of Cleveland, Ohio, catalog No.
`93153.
`The foregoing devices are unsatisfactory in several
`regards. For example, in many of the devices, the opera
`tor must remove the valve stem cap, engage the pres-
`sure sensor with the valve stem, observe a reading, and
`thereafter replace the valve stem cap. This procedure
`must then be repeated for each tire. Other of the forego-
`ing devices are designed to replace the existing valve
`stem cap, so that the pressure sensing device need only
`be removed when it
`is necessary to inflate the tire.
`These latter devices, however, do not indicate the mag-
`nitude of the pressure level
`in the tire; rather, they
`merely indicate whether the pressure is above or below
`a predetermined threshold. Further, many prior art
`
`2
`devices rely on visual indicia at or physical connections
`to the tire stern, which is often neither readily visible
`nor accessible, e.g., the interior tires of coaxial sets of
`tires on 18 wheel trucks.
`A tire pressure sensing device is needed which over-
`comes the shortcomings of the prior art.
`SUMMARY OF THE INVENTION
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`The present invention provides a tire pressure sensor
`including a transducer unit and a remote display unit.
`The transducer unit includes a mechanical-to-electrical
`transducer disposed to sense the internal tire pressure.
`The transducer generates an electric signal representa-
`tive of the magnitude of the tire pressure and applies
`this signal to an LED driver. The LED driver modu-
`lates one or more LEDs, which directionally transmits
`an infrared (IR) signal to the hand-held display unit.
`The modulated IR signal is received by the display
`unit and applied to a processor. In response, the proces-
`sor drives a digital display, which produces visual indi-
`cia of the tire pressure.
`BRIEF DESCRIPTION OF THE DRAWING
`
`Preferred exemplary embodiments of the tire pres-
`sure sensor in accordance with the present invention
`will hereinafter be described in conjunction with the
`appended drawing, wherein like designations denote
`like elements, and:
`FIG. 1 is a schematic representation of a remote dis-
`play unit and a transmitter unit, including a transducer,
`mounted on a conventional vehicle tire;
`FIGS. 2A and 2B are schematic block diagrams of a
`preferred embodiment of the tire pressure sensor system
`in accordance with the present invention;
`FIGS. 3A, 3B and 4 are an electrical schematic cir-
`cuit diagram of an alternate preferred embodiment of
`the display unit in accordance with one aspect of the
`present invention;
`FIG. 5 is an electric schematic circuit diagram of an
`alternate preferred embodiment of a transducer unit in
`accordance with one aspect of the present invention;
`FIGS. 6—8 are flow charts of the operation of an
`exemplary tire pressure sensor system; and
`FIG. 9 is a cross-section view, taken along line 9—9
`in FIG. 1, of a tire valve stem and valve stem cap in
`accordance with an alternate preferred embodiment of
`the present invention.
`DETAILED DESCRIPTION OF PREFERRED
`EXEMPLARY EMBODIMENTS
`
`Referring now to FIG. 1, a tire pressure sensor sys—
`tem 10 in accordance with the present invention suit-
`ably comprises a display unit 12 and a transducer unit 14
`having a pressure transducer 16 in intimate contact with
`a tire 18. Pressure transducer 16 is illustratively dis-
`posed within the pressurized chamber of tire 18, be-
`tween a tread portion 20 and a rim 22 of tire 18. Those
`skilled in the art will appreciate, however, that trans-
`ducer 16 may assume any suitable disposition which
`allows it to sense the internal tire pressure. For example,
`as discussed in greater detail below, transducer 16 may
`be mounted within the valve 24 of tire 18, either in the
`stem or cap portion of the valve. Alternatively, trans-
`ducer 16 may be mounted to, embedded within or ex-
`tend from the sidewall portion of the tire, tread 20, or
`rim 22.
`
`Page 000009
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`

`

`3
`Pressure transducer 16 is configured to convert tire
`pressure into an electrical signal representative of tire
`pressure, for subsequent transmission to display unit 12.
`In response to a pressure transmission, display unit 12
`produces visual indicia, such as, for example, a digital
`readout 26, representative of tire pressure.
`Referring now to FIG. 2A, transducer unit 14 suit-
`ably includes a suitable pressure sensor (transducer 16),
`a suitable signal converter 34, a light emitting diode
`(LED) drive 36 and an LED 38.
`Pressure sensor 16 is advantageously configured to
`sense tire pressure, generate an electrical signal indica-
`tive of the sensed pressure, and apply the signal to signal
`converter 34. Signal converter 34 then converts the
`pressure signal into an encoded form suitable for use as
`a modulating signal for LED 38, i.e., LED 38 is modu-
`lated in a manner which represents the sensed pressure
`as a selected characteristic of the encoded signal, e.g.,
`frequency, pulse code, pulse width, etc. Signal con-
`verter 34 comprises a suitable encoder, such as, for
`example, a voltage-to—frequency converter, an analog-
`to—digital converter, 3 voltage-to-pulse width con-
`verter, or the like, and supporting circuitry.
`The encoded signal indicative of tire pressure pro-
`duced by signal converter 34 is applied to LED driver
`36. The output of LED driver 36 drives an LED 38,
`which emits a modulated response signal, suitably in the
`infrared frequency range, indicative of tire pressure.
`The LED emissions are preferably directional, so that
`the hand-held display unit can discriminate between
`individual tires, even where the tires are in close prox-
`imity to each other. The foregoing components associ-
`ated with schematic transducer circuit 14 are suitably
`powered by a battery circuit 40 disposed within the
`transducer unit. A specific embodiment of transducer
`unit 16 will hereinafter be described in more detail in
`conjunction with FIG. 5.
`Referring now to FIG. ZB, display unit 12 suitably
`comprises: a battery circuit 41; an appropriate sensor 42,
`e.g. an IR sensor; a processor 44; and a conventional
`display 46. The modulated signal transmitted by LED
`38 is received at display unit 12 by sensor 42. The volt-
`age from battery 41 is applied to sensor 42, and is modu-
`lated in accordance with the response signal received
`from transducer circuit 14. The modulated signal
`is
`applied to a processor 44, wherein information is ex-
`tracted from the signal and manipulated into a form
`suitable for application to display 46. Display 46 gener-
`ates visual indicia, for example a digital readout, repre-
`sentative of tire pressure. A specific embodiment of 50
`display unit 12 will hereinafter be described in more
`detail in conjunction with FIGS. 3A, 3B and 4.
`As discussed in greater detail below, the functions
`performed by the various elements comprising the fore-
`going schematic circuit diagrams may be implemented
`in a variety of ways. For example, the functions per-
`formed by the pressure sensor circuit ma be embodied
`in a unitary microchip (integrated circuit) for conve-
`nient disposition within the valve stem or valve stem
`cap of a vehicle tire. The functional elements compris-
`ing the display unit may similarly be implemented in a
`microchip or microprocessor, and incorporated into a
`hand-held remote control display device.
`Referring now to FlGS. 3A-5, an embodiment of tire
`pressure sensing system 10 employing frequency modu-
`lation for encoding will be described.
`With specific reference to FIGS. 3A, 3B and 4, pro-
`cessor 44 of display unit 12 suitably comprises a switch
`
`IO
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`5,083,457
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`4
`S], a processor U3 including a clock X1, a command
`generator circuit 56, and an amplifier circuit 58. Display
`46 suitably comprises three conventional seven segment
`displays D51, D52, and D53, and associated drivers.
`To conserve power, display unit 12 remains in a “dor-
`mant" state until “powered up" by the operator, as
`described below. In the dormant state, i.e., when switch
`51 is open, pins 21 and 22 of processor U3 terminate at
`an open circuit. When it is desired to determine the tire
`pressure, the operator depresses switch 81 to power up
`display unit 12.
`More particularly, battery 41 cooperates with switch
`81, a resistor R14, and a transistor Q3, (e.g., an IRFZAO
`field effect transistor manufactured by the Motorola
`Semiconductor Company). With switch 81 open, no
`current flows through resistor R14. When 81 is closed,
`current is applied to the base of transistor Q3 through
`resistor R14, thereby turning on transistor Q3 and al-
`lowing the output from battery 41 to be applied to pin
`22 of processor U3. Each time switch 81 is depressed,
`software resident in processor U3 initiates an active
`cycle having a predetermined duration, e.g.
`ten sec-
`onds, during which pin 22 is maintained at a high logic
`state. Closure of switch SI also drives pin 21 of proces-
`sor U3 to a low logic state (illustratively to ground).
`Processor U3 suitably comprises an HMOS-E single
`component 8-bit microcomputer, for example a Model
`87481-1 manufactured by Intel. The timing for processor
`U3 is suitably provided at pins 2 and 3 thereof by clock
`X1, suitably comprising a 3.6864 MHz crystal.
`In the powered-up condition, i.e., when switch SI is
`closed, processor U3 generates a command reference
`signal at output terminal T1 (pin 39). The command
`reference signal suitably corresponds to a tone of prede-
`termined frequency The command reference signal is
`applied to a lead T1 of command generator 56 (FIG. 4),
`thereby turning a transistor Q2 on and off in accordance
`with the frequency of the command reference signal.
`Transistor Q2 is advantageously similar to transistor
`Q3, described above.
`In response to the application of the pulsed command
`reference signal to the base of transistor Q2, VCC is
`applied across respective LEDs D2, D3 and D4. Re-
`spective LEDs D2—D4 suitably comprise respective IR
`emitters, Model No. LD271, manufactured by Seimens-
`Litronix. Thus, respective LEDs D2—D4, under the
`control of processor U3, generate emissions modulated
`with a predetermined frequency (tone). As described in
`greater detail below, the frequency modulated infrared
`signal transmitted by command generator 56 comprises
`a “wake-up" command signal CS used to activate trans-
`ducer unit 14. Also as described in greater detail below,
`transducer unit 14 responsively transmits an infrared
`signal, indicative of tire pressure, back to display unit
`12.
`
`With continued reference to FIG. 4, a response signal
`RS generated by transducer unit 14 is received by dis—
`play unit 12 at amplifier circuit 58. More particularly,
`response signal RS is sensed by a photo-sensitive transis-
`tor Ql. Transistor Q1 is suitably a photo-transistor,
`Model No. BPlO3B-3, manufactured by Seimens-
`Litronix. Upon application of response signal RS to the
`base of transistor Q1, transistor Q1 generates a signal at
`the emitter thereof indicative of response signal RS, and
`hence, indicative of the encoded sensed pressure. The
`received signal is applied to a filter comprising a capaci—
`tor Cl and a resistor R2. The filtered signal is then ap-
`plied to pin 3 of an amplifier U1, e.g., a BiMOS opera-
`
`Page 000010
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`

`5,083,457
`
`5
`tional amplifier CA3140 integrated circuit, with a gain
`of 100. The amplified signal representative of tire pres-
`sure is applied to pin 4 of a comparator U2A.
`Comparator U2A is suitably a low power, low offset
`voltage comparator, e.g., a Model No. LM339 manufac-
`tured by National Semiconductor. Comparator U2A
`advantageously cooperates with a variable resistor R6
`to adjust the sensitivity of amplifier circuit 58. Specifi—
`cally, the resistance of R6 may be selected such that a
`desired voltage level is maintained at pin 5 of compara-
`tor U2A. In this way, only those voltage levels present
`at pin 4 of comparator U2A which are above a predeter-
`mined threshold level are passed through the compara-
`tor.
`
`Comparator U2A applies an output signal to a lead
`T0 connected to pin 1 of processor U3 (FIG. 3A). Pro-
`cessor U3 converts the signal received at pin 1 thereof
`into respective first, second, and third parallel binary
`signals for subsequent application to display 46.
`More particularly, processor U3 outputs a first 4-bit
`binary signal at pins 35—38 thereof, which first 4-bit
`binary signal is applied to respective pins 1, 2, 6, and 7
`of a first display driver U4 in display 46. Driver U4
`suitably comprises a BCD-to-7-segment latch/decoder,
`for example Model No. MC54/74HC451] manufac-
`tured by Motorola. In response to the application of the
`first binary signal, driver U4 applies a first output signal,
`through a resistive network RN 1, to a first display DSl.
`Display D51 suitably comprises a 7-segment display,
`for example Model No. FNDSOO manufactured by Fair-
`child Semiconductor.
`Similarly, processor U3 applies second and third 4~bit
`binary signals to display drivers D5 and D6 which, in
`turn, drive display D82 and D83, respectively. Displays
`D51, D52, and D53 cooperate to produce a 1, 2, or 3
`digit numeric display indicative of the tire pressure
`sensed by transducer unit 14.
`Referring now to FIG. 5, transducer unit 14 suitably
`comprises a pressure transducer 60, a voltage—to-fre-
`quency converter 62, an IR sender 64, and a power
`circuit 66.
`In a preferred embodiment of the present invention,
`transducer unit 14 is configured for disposition within a
`modified valve stem cap for use in conjunction with
`conventional vehicle tire valve stems (see FIG. 9). In
`this manner, a low cost pressure sensor circuit may be
`powered by a battery which. upon depletion of power,
`may be discarded. Nonetheless, it is desirable to con-
`struct
`the transducer circuit such that a minimum
`amount of power is consumed. Transducer unit 14
`therefore preferably operates in alternative “dormant"
`and “active” states. The active state is triggered by
`reception of command signal CS from display unit 12.
`Power circuit 66 controllably provides power to the
`respective components of transducer unit 12, in accor-
`dance with the operational state. Respective parallel
`voltage outputs +BATT and +V SW are provided:
`+BATI‘, a low level, constant voltage output for sup-
`plying operating power to the various components
`comprising transducer unit 14; and +V SW, selectively
`provided through a transistor Q4, to pressure trans-
`ducer 60, converter 62, and IR sender 64. Transistor Q4
`is turned on only upon receipt of command signal CS by
`power circuit 66 from command generator 56 (FIG. 4).
`Accordingly, transducer unit 14 consumes a minimal
`amount of battery power when transistor Q4 is off.
`Command signal CS is received at a photosensitive
`transistor OS in power circuit 66. Upon reception of
`
`6
`command signal CS at the base of transistor Q5, a signal
`indicative of command signal CS is generated at the
`emitter of transistor Q5. The received signal is filtered
`(by a capacitor C16 and a resistor R31) and applied to
`an amplifier U7, suitably with a gain of 100 and equiva-
`lent to amplifier U1.
`The output of amplifier U7 is applied to a comparator
`U8, suitably equivalent to comparator U2A. Compara-
`tor U8 cooperates with a variable resistance resistor
`R33 in a manner similar to that described in connection
`with amplifier circuit 58 (FIG. 4). In this way, the sensi-
`tivity of power circuit 66 may be adjusted so that +V
`SW is applied to transducer 60 only upon the receipt by
`transistor 05 of command signals which exceed a pre-
`determined level determined by resistor 33.
`The output of comparator U8 is applied to a tone
`decoder U9, for example a Model No. LM567 decoder
`manufactured by National Semiconducter. Tone de-
`coder U9 functions as a simple decoder, producing an
`output at pin 8 thereof having a high logic state only
`when the appropriate “tone" is received by transistor
`Q5. Tone decoder U9 thus functions as a band width
`discriminator,
`rejecting input
`signals having band
`widths outside the range defined by the foregoing com-
`ponents and producing a high logic output in response
`to a tone within a predetermined band width.
`Pin 8 of decoder U9 is applied to the base of transistor
`Q4. When a high logic state signal is produced at pin 8
`of tone decoder U9, transistor Q4 is turned on, making
`+V SW available.
`Power circuit 66 thus maintains transducer circuit 14
`in a dormant state notwithstanding the receipt by
`photo-sensitive transistor Q5 of spurious input signals
`outside the predetermined band width from, for exam-
`ple, sunlight, headlights, and the like.
`Transducer 60 suitably comprises an electromechani-
`cal transducer capable of generating a low level voltage
`output, for example between 0 and 10 volts, in response
`to the application of pressures in the range typically
`exhibited by vehicle tires, i.e., up 150 psi. Piezoelectric
`materials are known to be excellent transducers. Al-
`though it is desirable to miniaturize the pressure sensing
`circuit in the preferred embodiment, a pressure sensor
`Model No. 24OPC manufactured by Microswitch has
`yielded satisfactory results in the laboratory. Those
`skilled in the art will appreciate that transducer 60 may
`comprise a suitable microsensor.
`Voltage-to-frequency converter 62 suitably com-
`prises a converter U10, for example, a voltage-to-fre-
`quency (VF) converter Model No. AD654 manufac-
`tured by Analog Devices. With +V SW applied to lead
`68 of pressure transducer 60, an output signal is pro-
`duced at lead 70 and applied to pin 4 of VF converter
`U10. In response, VF converter U10 produces an out-
`put at pin 1 thereof having a frequency which is propor-
`tional to tire pressure.
`Voltage-to—frequency converter 62 further suitably
`includes a variable resistance resistor R18 disposed in
`operative association with convertor U10. Together,
`VF converter U10 and variable resistance resistor R18
`cooperate to calibrate the output at pin 1 of conversion
`circuit 62 with respect to pressure transducer 60.
`More particularly, the resistance of resistor R18 may
`be selected such that the output at pin 1 of VP con-
`verter U10 is zero when transducer 60 is exposed to
`ambient pressure.
`Converter 62 thus generates a signal having a fre—
`quency indicative of pressure, ranging from 0 Hz at
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`5,083,457
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`7
`atmospheric pressure to a predetermined maximum
`frequency at an anticipated maximum pressure.
`The output of converter 62 is applied across a resistor
`R20 to an output (driver) transistor Q6, which may be
`equivalent to transistor QZ, described above. The fre-
`quency of the signal applied to the base of transistor Q6
`represents the modulation frequency of response signal
`RS generated by IR sender 64.
`IR sender 64 illustratively comprises a plurality of
`LEDs D5, D6, and D7, which may be equivalent to
`LEDs D2-D4 discussed above in connection with com—
`mand generator circuit 56. More particularly, LEDs
`D5-D7, in respective series connections with resistors
`R21—R23, are disposed in parallel, between +BA'IT
`from battery 40 and transistor Q6. Upon the application
`of a signal from converter 62 to the base of transistor
`Q6, a current path through each of LEDs D5-D7 is
`completed, through transistor Q6 to ground. The fre-
`quency of the signal applied at pin 1 of VF converter
`U10 represents the modulation frequency of response
`signal RS generated by respective LEDs D5—D7.
`As previously mentioned, response signal RS gener-
`ated by IR sender 64 is received by amplifier circuit 58
`(FIG. 4) of display unit 12, and is processed to derive
`pressure information for subsequent display.
`The operation of the preferred exemplary embodi~
`merit shown in FIGS. 3A-5 will be described with
`reference to FIGS. 6—8.
`
`Software resident in processor U3 governs the opera-
`tion of tire pressure sensor apparatus 10. Upon the appli-
`cation of VCC to pin 22 of processor U3, a series of
`initializing functions are performed (step 102). For ex-
`ample, a high logic input state is maintained at pin 22 for
`a predetermined cycle time, e.g. ten seconds, regardless
`of the length of time switch 81 is actually depressed by
`the operator. In contrast, a low logic state is present at
`pin 21 only while switch 51 is depressed.
`An additional initializing function involves driving
`respective displays DSl—DS3 of display circuit 46 to a
`blank condition (step 102). That is, display circuit 54
`may display, for example, three zeros, three eights (ei-
`ther constant or flashing) or, alternatively, the display
`may be literally blank with none of the segments com-
`prising the displays illuminated. Conventional BCD-to-
`7-Segment logic may be may be advantageously em—
`ployed in the control of display circuit 54.
`A further initializing function involves setting up an
`internal counter to generate the command reference
`signal at pin 39 of processor U3 (step 102), although pin
`39 is not enabled until step 110, discussed below. In a
`particularly preferred embodiment, the tone associated
`with command signal CS corresponds to a frequency of
`1209 hz. As discussed above in connection with FIGS.
`3A, 3B and 4, the command reference signal effects the
`generation of command signal CS at respective LEDs
`D2—D4 of command generator circuit 56.
`Each time switch 51 is depressed, the sequence de-
`picted in FIG. 6 is reset, and processor U3 begins exe-
`cuting at the START position. For clarity, it is pre-
`sumed that switch 51 is initially depressed once.
`When it is desired to display the tire pressure sensed
`by transducer unit 14, the operator depresses switch 81
`on hand-held display unit 12. As a result, power is ap-
`plied to pin 22 of processor U3, and the voltage level at
`pin 21 is driven low, indicating that switch 81 is de-
`pressed.
`After initialization, the state of switch 51 is checked
`(step 104) to determine if switch SI is depressed. If the
`
`8
`is determined that
`it
`voltage level at pin 21 is low,
`switch 51 is depressed; if an open circuit is detected at
`pin 21, it is determined that switch 81 is not depressed.
`If processor U3 determines that switch 81 is de-
`pressed, the output at pin 39 is enabled, thereby apply-
`ing the command reference signal to command genera-
`tor circuit 56 and transmitting command signal CS to
`transducer circuit 52 (step 110).
`If in step 104 it is determined that switch 51 is not
`depressed, a predetermined delay period of, for exam-
`ple, ten seconds, is elapsed (step 106). If after 10 seconds
`switch 51 is not depressed a second time, battery 41 is
`turned off (step 108). Thus, when the operator requires
`a “read" by closing switch 51, processor U3 executes
`the resident software and display unit 12 displays tire
`pressure for ten seconds, as described below. If switch
`81 is pressed again before ten seconds have elapsed, the
`sequence is interrupted and restarted at START to
`allow the operator to quickly monitor successive tires
`without having to undergo a ten second delay between
`readings. Moreover, battery 41 is automatically turned
`off ten seconds after the last depression of switch 51.
`As discussed above in connection with FIG. 5, trans-
`ducer circuit 14 emits response signal RS, having a
`modulation frequency indicative of tire pressure, upon
`receipt of command signal CS. Response signal RS is
`captured at pin 1 of processor U3 (step 112). Step 112
`will be explained in greater detail in connection with
`FIG. 7.
`Upon capturing response signal RS from transducer
`unit 14, command signal CS generated at command
`generator 56 is terminated, i.e., the output at pin 39 of
`processor U3 is interrupted (step 114), and the fre-
`quency of response signal RS is determined and con-
`verted by processor U3 to respective first, second and
`third 4—bit binary signals, as discussed above in connec-
`tion with FIG. 3A (step 116).
`More specifically, with reference to FIG. 7, proces-
`sor U3 monitors pin 1 to determine if the output T0 (the
`conditioned response signal RS) from amplifier circuit
`58 exhibits a high or low logic level. Ifoutput T0 is low,
`processor U3 waits for a high logic level (step 118).
`When output T0 goes high, indicating that the peak
`portion of response signal RS is present at transistor Q1,
`a pulse width counter resident in processor U3 is incre-
`mented and a l micro-second delay is triggered (step
`120). Processor U3 then checks to see if output T0 is still
`high (step 122). Steps 120 and 122 are repeated for as
`long as output T0 remains high. In this way, the dura-
`tion of the peak portion of response signal RS is re-
`flected in (is equal to) the number of increments of the
`pulse width counter, in micro-seconds (step 120).
`When output T0 goes low, the pulse width counter is
`again incremented and another micro-second delay is
`triggered (step 124). Processor U3 then checks if output
`T0 is still low (step 126). Step5124 and 126 are repeated
`until output T0 again assumes a high logic state, where-
`upon processor U3 terminates steps 112 and proceeds to
`step 114. Thus, upon completion of steps 118-126, the
`counter in processor U3 contains a count representative
`of the duration of one cycle of the response signal tone,
`i.e., the period T of one cycle, in microseconds. The
`frequency f of response signal RS, then, is equal to UT.
`After determining the frequency of the response sig-
`nal, processor U3 converts the frequency to display
`information (step 116). More specifically, the period
`count in the designated counter in processor U3 is in-
`verted to yield the frequency f of response signal R5.
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`5,083,457
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`9
`The frequency data is then convened into a form
`suitable for driving display circuit 46. The frequency
`data is within a predetermined range (span) of frequen-
`cies having a low frequency limit and a high frequency
`limit selected by the designer. More specifically, the
`low frequency value corresponds to the lowest pressure
`value in connection with which the tire pressure sensor
`apparatus will be used.
`.
`the low
`In the preferred exemplary embodiment,
`pressure value of the frequency is zero, corresponding
`to atmospheric pressure. This allows pressure trans-
`ducer 60 to be calibrated with respect to voltage-to—fre—
`quency converter 62 in a convenient manner.
`The high frequency end of the frequency span should
`be selected to correspond to the maximum pressure for
`which the device will be used. As is known in the art,
`conventional automobile tires require a maximum pres-
`sure in the range of approximately 35 to 65 psi. If the
`tire pressure sensor circuit is to be used in connection
`with automobile tires only, a suitable high limit for the
`frequency span may correspond approximately 65 to 70
`psi. However, in larger tires of the type used on semi-
`tractor trailors, a maximum pressure of approximately
`150 psi
`is desirable. Thus, if the tire pressure sensor
`apparatus is to be employed in the trucking industry, the
`upper limit of the frequency span preferably corre-
`