Abstract:
A battery powered tire pressure sensor system with a high sensitivity stretch sensor assembly having a variable resistance longitudinal displacement characteristic. The stretch sensor assembly has at least two juxtaposed stretch sensors, each with a first layer bearing the variable resistance element and a second support layer. The sensor assembly is mounted on or in the side wall of a pneumatic tire so that the assembly is displaced by the tire side wall and the resistance is a function of internal tire pressure. The assembly is coupled to a processor which samples the resistance of the stretch sensor assembly periodically. When the processor determines that the pressure is outside a safe range, an r.f. generator is activated by the processor to generate an unsafe tire pressure signal. This signal is converted by a receiver to a warning for the driver. A power saving unit controls application of electrical power to the system as a function of tire speed to prolong battery life.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to automotive tire pressure sensors. More particularly, this invention relates to a battery powered method and system for monitoring internal tire pressure of vehicle tires using a sensor system with improved sensitivity and a power saving device. 
   Tire pressure sensor systems are known and are commonly used to monitor the internal air pressure in individual pneumatic tires of a vehicle and to provide a warning signal to the driver whenever the internal air pressure in one or more of the vehicle tires is dangerously low or high. The warning signal is typically generated by an r.f. signal generator controlled by a microprocessor connected to the tire pressure sensor, the warning signal being generated whenever the internal tire pressure measured by the sensor lies outside a predetermined normal operating range, signifying either a high or a low pressure condition. This r.f. signal is transmitted to a vehicle-mounted receiver, which uses the warning signal to alert the driver either visually (by activating a warning lamp or display) or audibly (by activating an audible alarm) or both. Electrical power to the sensor circuitry is provided by a battery, which must be replaced when the available power drops below a useful level. 
   Known tire pressure systems, such as that disclosed in commonly assigned, co-pending patent application Ser. No. 10/346,490 filed Jan. 21, 2003 for “External Mount Tire Pressure Sensor System”, the disclosure of which is hereby incorporated by reference, use a mechanical strain sensor having an essentially linear variable resistance characteristic in one branch of an electrical bridge circuit to measure the internal pressure of a tire to which the sensor is attached. This type of sensor is relatively insensitive to mechanical vibrations, which are regularly encountered in an automotive environment. In addition, the configuration of the electrical circuitry (i.e., the electrical bridge circuit) is relatively simple, has well-known performance characteristics, and has been found to be reasonably reliable in operation. 
   In spite of the effectiveness of the known sensor circuitry using the strain sensor and bridge circuit, there are inherent limitations which limit the performance of such devices. Firstly, due to the fact that only a single variable resistance element (the strain gauge) is incorporated into one branch of the bridge circuit, the sensitivity of the sensor circuit is limited to the variable resistance range of the single strain gauge used. This limits the potential measurement range of the sensor system. In addition, the known sensor circuitry is susceptible to measurement inaccuracies due to different coefficients of thermal resistivity of the variable resistance strain sensor and the fixed resistances forming the bridge circuit. Secondly, since the sensor circuitry is continuously powered by the essential battery, the useful lifetime of the battery is limited by the battery energy capacity. This drawback is compounded by the need for components having relatively small physical size due to installation constraints. As a consequence, battery replacement is a major constraint to the efficacy of such known sensor systems. 
   Efforts to provide a simple yet accurate and durable tire pressure monitoring system devoid of the above-noted disadvantages have not been successful to date. 
   SUMMARY OF THE INVENTION 
   The invention comprises a method and system for monitoring internal vehicle tire pressure employing a variable resistance sensor assembly having greater sensitivity than known devices and more tolerant of temperature fluctuations; and a power saving unit providing extended useful battery life. 
   From a first apparatus aspect, the invention comprises an improvement for a tire pressure sensor system having a variable resistance displacement sensor for providing resistance values indicative of internal tire pressure when coupled to a pneumatic tire; a processor coupled to the displacement sensor element for converting resistance values corresponding to inadequate internal tire pressure to r.f. generator activation signals; and an r.f. generator circuit for transmitting an unsafe tire pressure warning signal when activated by the processor. The improvement comprises a variable resistance sensor assembly having first and second individual stretch sensors, each stretch sensor having a first flexible layer containing a variable resistance element and a second flexible support layer, with the individual stretch sensors being arranged with the first flexible layer of the first stretch sensor in facing relation with the first flexible layer of the second stretch sensor so that the variable resistance elements face each other. The variable resistance elements are inserted in an electrical bridge circuit having four branches: a first pair of the four branches have fixed resistance elements connected in series, while a second pair of the four branches have the variable resistance elements of the first and second stretch sensors connected in series. 
   In a preferred variation of this basic embodiment, the variable resistance sensor assembly further includes third and fourth individual stretch sensors, with each of the third and fourth stretch sensors having a first flexible layer containing a variable resistance element and a second flexible support layer. The third and fourth individual stretch sensors are mutually arranged with the second flexible support layer of the third stretch sensor in facing relation with the second flexible support layer of the fourth stretch sensor. Also, the first flexible layer of the third stretch sensor is arranged in facing relation with the second flexible support layer of the second stretch sensor. The variable resistance elements are inserted in an electrical bridge circuit having four branches: a first one of the branches has the variable resistance element of the first stretch sensor, a second one of the branches has the variable resistance element of the fourth stretch sensor, a third one of the branches has the variable resistance element of the second stretch sensor, and a fourth one of the branches has the variable resistance element of the third stretch sensor. The first and second branches are connected in series, and the third and fourth branches are connected in series. 
   In both of the above embodiments, the ohmic electrical connections in the bridge circuit ensure that resistance changes due to temperature changes are cancelled out by the configuration of the resistance components. 
   The tire pressure sensor system components comprising the processor, the r.f. generator circuit, the variable resistance sensor assembly, and a battery are all mounted on a common support substrate having a flexible portion underlying at least the variable resistance sensor assembly. The support substrate can be mounted on a tire side wall-either the outside wall or the inside wall; or embedded in the tire side wall during the tire formation process. In surface mount installations, a sensor guide secured to a tire side wall slidably captures a free end of the sensor assembly. The other end of the sensor assembly is secured to the tire side wall. This arrangement prevents excessive longitudinal stretching of the sensor assembly and premature failure. 
   From a second apparatus aspect the invention comprises a power saving unit for use in a tire pressure sensor system having a variable resistance sensor for providing resistance values indicative of internal tire pressure when coupled to a pneumatic tire; a processor coupled to the sensor for converting resistance values corresponding to inadequate internal tire pressure to r.f. generator activation signals; and an r.f. generator circuit for transmitting an unsafe tire pressure warning signal when activated by the processor. The power saving unit limits the application of electrical power to the variable resistance sensor in a manner related to tire speed so that power is only applied, and thus drawn from the battery, for a measurement period related to tire speed after the tire speed has reached a threshold speed value. 
   Preferably, this measurement period is related to the time required for a tire of a given size to complete a preselected number of revolutions. The power saving unit has an input terminal adapted to be coupled to a source of electrical power (the battery in a particular embodiment), an output terminal for supplying electrical power to the variable resistance sensor, and a vehicle speed sensitive switch for connecting the input terminal to the output terminal when the tire attains a first predetermined speed and for disconnecting the input terminal from the output terminal when the speed of the tire drops below the first predetermined speed. In one embodiment, the switch comprises an electrically conductive contact member, such as a spring, having a first portion connected to the output terminal and a free end, and an electrically conductive pivot member, such as a spring or a pivot arm, having a first portion connected to the input terminal and a mass member mounted on a free end. The mass member is mounted to make physical contact with the free end of the contact member when the tire attains the first predetermined speed, thus enabling the transfer of electrical power from the input terminal to the output terminal. Preferably, the mass member has opposing ends; and the switch is provided with first and second contact members connected to the output terminal, with the first contact member having a free end located in the path of one of the opposing ends of the mass member, and the second contact member having a free end located in the path of the other one of the opposing ends of the mass member. With this configuration, the positioning of the power saving unit on a vehicle tire is facilitated. 
   In an alternate embodiment, a magnetically actuatable reed switch is coupled between the input terminal and output terminal, and a magnet is mounted on the free end of the pivot arm to activate the reed switch when the tire attains the first predetermined speed. 
   In an alternate embodiment, the power saving unit further includes a control signal output terminal coupled to the processor; and the vehicle speed sensitive switch includes control signal means for connecting the power input terminal to the control signal output terminal when the tire attains a second predetermined speed different from (and preferably higher than) the first predetermined speed and for disconnecting the input terminal from the control signal output terminal when the speed of the tire drops below the second predetermined speed. When received, the control signal serves as an indication to the processor that a different smaller measurement period can now be used. This different measurement period is also related to the time required for the tire to complete a preselected number of revolutions. 
   In this embodiment, the switch configuration is essentially the same as the switch used in the first embodiment. The control signal means comprises a contact member having a first portion connected to the control signal output terminal and a free end, and the mass member in the switch is mounted to make physical contact with the free end of the contact member when the tire attains the second predetermined speed. Similar to the first embodiment, the mass member preferably has opposing ends; and the control signal means includes first and second contact members connected to the control signal output terminal, the first contact member having a free end located in the path of one of the opposing ends of the mass member, and the second contact member having a free end located in the path of the other one of the opposing ends. 
   From a process aspect, the invention comprises a method of reducing power consumption in an electrically powered tire pressure sensor system having a variable resistance sensor for providing resistance values indicative of internal tire pressure when coupled to a pneumatic tire, a processor coupled to the sensor for converting resistance values corresponding to inadequate tire pressure to r.f. generator activation signals, and an r.f. generator circuit for transmitting an unsafe tire pressure warning signal when activated by the processor, the method comprising the steps of: 
   (a) providing a source of electrical power; and 
   (b) applying the electrical power to the variable resistance sensor for a tire pressure measurement period whose duration is a related to tire speed. Step (b) of applying preferably includes the steps of (i) preventing the application of electrical power to the variable resistance sensor until the tire speed reaches a first tire speed threshold, (ii) furnishing electrical power to the variable resistance sensor for a measurement period related to the period of time required for a preselected number of tire revolutions at the first tire speed threshold when the tire speed reaches the first tire speed threshold, and (iii) terminating the application of electrical power to the variable resistance sensor when the tire speed falls below the first tire speed threshold. 
   The method may further provide for a second measurement period by modifying step (b) of applying to further include the step of changing the length of the measurement period to a different value when the tire speed reaches a second tire speed threshold, the different value being related to the period of time required for a preselected number of tire revolutions at the second tire speed threshold. 
   The invention provides a convenient solution to the problem of monitoring internal tire pressure in vehicles equipped with pneumatic tires. The system can be installed either during manufacture of a new tire, manufacture of a new vehicle or as an aftermarket item. Further, existing vehicles without tire pressure sensor systems can easily be retrofitted with a state-of-the-art system at relatively low cost. This is particularly beneficial in jurisdictions which mandate low tire pressure warning devices on all road vehicles. The sensor assembly provides substantially enhanced measurement sensitivity, and the power saving unit substantially reduces power consumption, which is particularly important in those installations which use a relatively inaccessible battery as a source of electrical power. 
   For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a prior art single tire pressure monitor circuit using a single stretch sensor in a bridge circuit; 
       FIG. 2  is a perspective view of a single tire pressure monitor circuit having four stretch sensors in a bridge circuit according to the invention; 
       FIG. 3  is a perspective view similar to  FIG. 2  showing an alternate embodiment of a single tire pressure monitor circuit having two stretch sensors arranged in series connected branches of a bridge circuit; 
       FIG. 4  shows the comparative sensitivity of the prior art bridge circuit of  FIG. 1  and the two embodiments of the invention shown in  FIGS. 2 and 3 ; 
       FIG. 5  is a schematic perspective view of a tire pressure monitoring system according to the invention showing the physical layout of the major components; 
       FIG. 6  is a perspective view showing a single tire pressure monitoring system according to the invention mounted on the outside wall of a tire; 
       FIG. 7  is a sectional view through a vehicle wheel and tire showing two possible placements of the invention; 
       FIG. 8  is a sectional view similar to  FIG. 7  showing an internal placement of the invention; 
       FIG. 9  is a schematic view of a first embodiment of a motion detector according to the invention; 
       FIG. 10  is a schematic view of an alternate embodiment of a motion detector according to the invention; 
       FIGS. 10A and 10B  are schematic views of an alternate embodiment of a motion detector according to the invention having a reed switch; 
       FIG. 11  is a schematic view of a multi-stage embodiment of a motion detector according to the invention; and 
       FIGS. 12A and 12B  are timing diagrams illustrating the operation of a multi-stage motion detector of  FIG. 11  at two different wheel speeds. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Turning now to the drawings,  FIG. 1  is a schematic view of a prior art single tire pressure monitor circuit using a single stretch sensor in a bridge circuit. As seen in this Fig., the monitor circuit, generally designated with reference numeral  10 , includes a single stretch sensor 12 ohmically connected in one branch of a bridge circuit having three additional branches each with a fixed resistance R ohmically connected as shown. Stretch sensor  12  is a known component having the property of an ohmic resistance which varies in a predictable amount with linear longitudinal displacement of the sensor body. Stretch sensor  12  has a first layer  14  on which a thin variable resistance element  15  is mounted; and a second, base layer which carries the first layer and provides additional mechanical strength for sensor  12 . The fixed resistances R are all of equal value. A reference voltage Vin from a source of D.C. electrical power (not shown) is applied to two nodes of bridge circuit  10 . Stretch sensor  12  is affixed to a vehicle tire (not shown) in such a manner that the stretch sensor  12  will flex as a function of internal pressure. The resistance value of stretch sensor  12  depends upon the amount of flexing due to the internal tire pressure and the direction in which the flexing occurs. As shown in  FIG. 1 , when sensor  12  flexes in a first direction the value of the resistance increases (R+r), where R is the at rest resistance value of sensor  12  and r is the additional resistance value due to the flexing of sensor  12 . Similarly, when sensor  12  flexes in the opposite direction, the value of the resistance decreases (R−r). As the resistance of stretch sensor varies, the measuring voltage Vout will vary accordingly, thus providing a measured value of internal tire pressure. 
     FIG. 2  is a perspective view of a single tire pressure monitor circuit having four stretch sensors  22   a – 22   d  in a bridge circuit  20  according to the invention. As seen in this Fig., each sensor assembly  22  comprises four two layer stretch sensors each having a first layer  24  bearing the variable resistance element, and a base layer  25 . Two of the stretch sensors  22   a ,  22   b  are arranged with the first layers in facing relation; while the two remaining stretch sensors  22   c ,  22   d  are arranged in back-to-back relation. The sensors  22   a  and  22   c  which face in the same direction are designated in  FIG. 2  with the annotation R+r; while the two sensors  22   b  and  22   d  which face in the same direction but opposite from the direction of sensors  22   a  and  22   c  are designated with the annotation R−r. The sensors are ohmically connected as shown in  FIG. 2  with the R+r sensors arranged in two opposite branches of the bridge circuit  20 , and the R−r sensors arranged in the other two opposite branches of the bridge circuit  20 . With this arrangement, any variations in resistance due to thermal effects are totally cancelled out electrically, so that measured resistance values are a pure function of internal tire pressure. The sensor assembly  22  is physically mounted to the tire in the manner described below. 
     FIG. 3  is a perspective view similar to  FIG. 2  showing an alternate embodiment of a single tire pressure monitor circuit having two stretch sensors arranged in series connected branches of a bridge circuit. As seen in this Fig., each sensor assembly  32  comprises two two layer stretch sensors each having a first layer  24  bearing the variable resistance element, and a base layer  25 . The two stretch sensors  32   a ,  32   b  are arranged with the first layers in facing relation in an R+r, R−r configuration. The single layer sensors are ohmically connected as shown in  FIG. 3  with the R+r sensor and the R−r sensor arranged in series connection in adjacent branches of the bridge circuit  30 . The other two branches of the bridge circuit are provided with fixed resistance elements  26  of equal value R. With this arrangement, any variations in the variable resistance elements due to thermal effects are totally cancelled out electrically, and any variations in the fixed resistance elements R due to thermal effects are totally cancelled out electrically so that measured resistance values are a pure function of internal tire pressure. The sensor assembly  32  is physically mounted to the tire in the manner described below. 
     FIG. 4  shows the comparative sensitivity of the prior art bridge circuit of  FIG. 1  and the two embodiments of the invention shown in  FIGS. 2 and 3 . As seen in this Fig., for the single sensor prior art device shown in  FIG. 1  the magnitude of the output voltage Vout is a function of r/4R. For the two sensor embodiment of  FIG. 3  the magnitude of the output voltage Vout is a function of r/2R. For the four sensor embodiment of  FIG. 2 , the magnitude of the output voltage Vout is a function of r/R. As will be appreciated by those skilled in the art, the  FIG. 2  embodiment provides an increase in sensitivity by a factor of four over the prior art arrangement; while the  FIG. 3  embodiment provides an increase in sensitivity by a factor of two. This represents a substantial improvement in measurement capability. 
     FIG. 5  is a schematic perspective view of a tire pressure monitoring system  50  according to the invention showing the physical layout of the major components. As seen in this Fig., the major components of the tire pressure monitoring system  50  include an integrated circuit  51 , a battery  52 , a stretch sensor assembly  22  or  32 , an antenna  54 , and a motion detector  55  (described below). These components are secured in any desired fashion (such as by using a suitable adhesive) to a substrate layer  56 . The integrated circuit  51  contains the active electronic components usually found in an r.f. monitoring system and will not be further described as this arrangement is well known to those skilled in the art. The antenna  54  is coupled to the r.f. section of integrated circuit  51  in the usual manner. The stretch sensor assembly  22 ,  32  is ohmically connected to a bridge circuit contained in integrated circuit  51 . Battery  52  is connected to the power input terminals of integrated circuit  51 . Substrate layer  56  is adhered to a mounting layer  57  using a suitable adhesive. At least those portions of substrate later  56  and mounting layer  57  underlying stretch sensor assembly  22  or  32  should be sufficiently flexible to allow the stretch sensor assembly to flex with the tire side wall in order to provide an accurate resistance value. For surface mount installations (described below), a generally U-shaped sensor guide  58  having anchor ends  59   a ,  59   b  slidably captures sensor assembly  22 ,  32  and the underlying portions of substrate layer  56  and mounting layer  57 . Sensor guide is dimensioned to maintain sensor assembly  22 ,  32  closely adjacent the tire side wall, while at the same time permitting sliding motion of sensor assembly  22 ,  32  within sensor guide  58 . 
   Sensor assembly  22 ,  32  is fixed at the lower end thereof to a first tire anchor point (the outer tire surface, the inner tire surface or an internal anchor point-see below) by adhering the generally rectangular lower portion of substrate layer  56  and mounting layer  57  to the first tire anchor point. The anchor ends  59   a ,  59   b  of sensor guide  58  are fixed to a second tire anchor point. When the contour of the tire side wall changes due to a change in internal tire pressure, sensor assembly  22 ,  32  will flex with the contour change due to the fact that sensor assembly  22 ,  32  is fixed to the tire anchor point at the lower end thereof and is slidably retained in close proximity to the tire side wall by sensor guide  58 . However, since only the lower end of sensor assembly  22 ,  32  is fixed to the first tire anchor point, sensor assembly  22 ,  32  cannot be stretched to the breaking point, which could occur if sensor assembly  22 ,  32  were firmly adhered along its entire length. This mounting arrangement prevents premature failure of sensor assembly  22 ,  32 . 
   As shown in  FIG. 6 , a single tire pressure monitoring system  50  according to the invention can be mounted on the outside wall of a tire  61  by attaching the mounting layer  57  ( FIG. 5 ) to the tire sidewall at an appropriate location. This can be done using a suitable adhesive, such as an epoxy adhesive. Preferably, the system  50  is adhered to the tire sidewall using a two component hook-and-loop attachment system, such as that sold under the Velcro trademark. This arrangement provides addition vibration damping to an installed tire pressure sensing system. 
     FIG. 7  is a sectional view taken through a vehicle tire and wheel assembly illustrating two alternate placements of the tire pressure sensing system  50 . As seen in this Fig., the system  50  can be attached to the outside wall  61  of the vehicle tire using the attachment mechanism described above. This placement allows for easy replacement of an exhausted battery  52  since the battery  52  is readily accessible. Alternatively, the system  50  can be attached to the inside tire wall  62  prior to mounting the tire on the wheel  63 . This arrangement provides protection for the system  50  components from mechanical abrasion and severe environmental conditions, but has the disadvantage that the tire must be removed from the wheel  63  when battery  52  needs replacement. 
     FIG. 8  is a sectional view similar to  FIG. 7  showing another alternate placement of the tire pressure sensor system. As seen in this Fig., sensor system  50  is molded into the interior of the tire between outer side wall  61  and inner side wall  62 . Since the temperatures required for the tire molding process are relatively low compared to the temperature tolerance of the components of system  50 , this internal placement is practical. The internal arrangement shown provides the maximum protection for the components of system  50  since they are entirely encased in the tire material. However, when the battery  52  is exhausted, it cannot be replaced with this arrangement. 
   The resistance measurement process used to determine internal tire pressure is very similar to that disclosed in the above-referenced pending U.S. patent application Ser. No. 10/346,490. The value of the measured resistance of stretch sensor assembly  22 ,  32  varies between a maximum R max when the pressure sensor system  50  is located a minimum distance from the pavement and subject to maximum displacement (closest to the pavement), and a minimum R min when the pressure sensor system  50  is at the maximum distance from the pavement (farthest from the pavement) and subject to minimum displacement. The parameter which is used to compute tire pressure is the difference R=(R max)−(R min). This parameter is calculated by programmed circuitry within integrated circuit  51 . When this value lies within a predetermined acceptable range defined by two predetermined threshold values, no signal is transmitted from antenna  54  since the internal tire pressure is within the permitted range. When the value of R is greater than a predetermined first threshold value-signifying a low pressure condition, integrated circuit  51  activates an internal r.f. transmitter, which causes a low pressure signal to be transmitted from antenna  54 . Similarly, when the value of R is less than a predetermined second threshold value-signifying a high pressure condition, integrated circuit  51  activates the internal r.f. transmitter, which causes a high pressure signal to be transmitted from antenna  54 . The low pressure signal or high pressure signal is received by conventional on-board receiver circuitry (not shown), which converts the low or high pressure signal to a perceivable warning signal, such as a visible indicator, an audible alarm, or both. In general, the receiver circuitry includes a decoder for decoding the low and high pressure signals to a form which can be used to operate the warning indicator. Representative examples of such receivers are illustrated and described in U.S. Pat. Nos. 5,900,808; 6,175,301; and 6,453,737. Since the receiver circuitry is conventional and well-known to those skilled in the art, further description is deemed unnecessary. 
   To conserve battery power, resistance measurements can be made periodically, rather than continuously. For example, an initial vale of R may first be calculated. If the value of R is less than the first threshold value and higher than the second threshold value (i.e. indicates that the tire pressure lies within the acceptable range), integrated circuit  51  will wait for one minute, and then proceed with another calculation of the parameter R. If any calculation results in a value of R which lies outside the range defined by the two threshold levels (i.e. higher than the first threshold or lower than the second threshold), integrated circuit  51  will wait for a shorter period (ten seconds) and then perform another calculation of the parameter R. If the result is another value of R which lies outside the range defined by the two thresholds, integrated circuit  51  activates the r.f. transmitter to generate a low or high tire pressure signal. If the result is a successive value of R which does not lie outside the range defined by the two thresholds, integrated circuit  51  will wait for one minute, and then proceed with the next calculation. 
   To further conserve battery power, power from the battery  52  to integrated circuit  51  is selectively applied under control of a motion detector  55 , a first embodiment  55 A of which is shown in  FIG. 9 . As seen in this Fig., one terminal of battery  52  (the positive terminal in this embodiment) is connected to a first terminal  91  of motion detector  55 A. Terminal  91  is ohmically connected to a pair of contact springs  92 ,  93  disposed along a pivot path  94  of a mass block  95 . Mass block  95 , which is fabricated from an electrically conductive material, is mounted to the upper end of a pivot spring  96 , also fabricated from an electrically conductive material. The lower end of pivot spring  96  is ohmically connected to a power output terminal  98 . Power output terminal  98  is connected to the power input terminal of integrated circuit  51 . 
   In operation, when the vehicle tire to which tire pressure sensor system  50  is attached is at rest, mass block  95  is positioned centrally of contact springs  92 ,  93  and maintained in this position by the action of pivot spring  96 . In this central position, mass block is out of contact with contact springs  92 ,  93  and, as a result, power from battery  52  does not flow to output terminal  98  and no power is consumed. As the vehicle tire starts to rotate, mass block  95  is deflected along pivot path  94  under the influence of centrifugal force in the direction of either contact spring  92  or contact spring  93 , depending on the orientation of motion detector  55  on the tire side wall and the direction of rotation of the tire. When the rotational speed of the tire reaches a predetermined value (e.g. 10 m.p.h.), mass block  95  is deflected a sufficient distance to make contact with one of the two contact springs  92 ,  93 . At this point, an ohmic electrical circuit is established between power input terminal  91  and power output terminal  98 , and D.C. electrical current can flow from battery  52  to integrated circuit  51 . It should be noted that the tire speed at which power is applied to integrated circuit  51  is a matter of design choice and can be set at a value deemed appropriate to one of skill in the art. Once a power connection is established between battery  52  and integrated circuit  51 , the tire pressure measurement process described above commences. 
     FIG. 10  illustrates an alternate embodiment of the motion detector  55 B. In this embodiment, pivot spring  96  is replaced by a pivot arm  101 , pivotally mounted at the bottom end thereof to a fixed reference point and having a ferro-magnetic mass block  103  mounted on the upper end thereof. A permanent magnet  105  is secured to a fixed reference point of motion detector  55 B. Operation of the embodiment of  FIG. 10  is very similar to the embodiment of  FIG. 9 , with the difference that the magnetic force between mass block  103  and permanent magnet  105  maintains mass block  103  out of contact with spring  92  or spring  93  until the magnitude of the centrifugal force due to the rotation of the tire exceeds the magnitude of the magnetic holding force between mass block  103  and permanent magnet  105 . 
     FIGS. 10A and 10B  illustrate another alternate embodiment of the motion detector  55 C.  FIG. 10A  shows motion detector  55 C in the unactuated state, while  FIG. 10B  shows motion detector  55 C in the actuated state. In this embodiment, pivot arm  101  has a magnet  106  mounted on the upper end thereof. Permanent magnet  105  is secured to a fixed reference point of motion detector  55 C. Contact springs  92 ,  93  are replaced by a magnetically actuated normally open contact reed switch  108  having a first terminal  109  ohmically connected to input terminal  91  and a second terminal  110  ohmically connected to terminal  98 . Operation of the embodiment of  FIGS. 10A and 10B  is as follows. When the magnitude of the centrifugal force due to the rotation of the tire is less than the magnitude of the magnetic holding force between magnet  106  and magnet  105 , pivot arm  101  and magnet  106  are maintained in the attitude illustrated in  FIG. 10A , in which magnet  106  is sufficiently remote from reed switch  108  that reed switch remains in the unactuated state and no electrical power is transferred between terminal  91  and terminal  98 . When the magnitude of the centrifugal force due to the rotation of the tire exceeds the magnitude of the magnetic holding force between magnet  106  and magnet  105 , pivot arm  101  and magnet  106  are rotated (counter-clock wise in  FIG. 10B ) so that magnet  106  approaches reed switch  108  and causes the contacts therein to close, thereby ohmically connecting terminals  91  and  98  and transferring electrical power from battery  52  to integrated circuit  51 . While only one reed switch  108  is shown in  FIGS. 10A and 10B , it is understood that a pair of reed switches  108  may be used in motion detector  55 C positioned at locations similar to the locations of contact springs  92 ,  93  in the embodiment of  FIG. 9 . 
   As will now be apparent, the inclusion of motion detector  55 A– 55 C in the power circuit of tire pressure sensor system  50  prolongs the useful life of battery  52  by preventing the application of D.C. electrical power to integrated circuit  51  when the vehicle to which the tire is rotatably attached is at rest or moving at a speed at which tire pressure is not a matter of concern. Even further power savings can be achieved by the multi-stage motion detector  55 D shown in  FIG. 11 . As seen in this Fig., multi-stage motion detector  55 D has the same elements  91 ,  92 ,  93 ,  95 ,  96 , and  98  incorporated therein as motion detector  55 A. In addition, motion detector  55 D includes an additional pair of power contact springs  112 ,  113  mounted along opposite ends of the pivot path  94  of mass block  95  but arranged at points along the pivot path  94  which are outboard of the inner contact faces of contact springs  92 ,  93 . Power contact springs  112 ,  113  are ohmically connected in parallel to an additional output terminal  115 , which is connected to a dedicated input port of integrated circuit  51 . The purpose of contact springs  112 ,  113  and output terminal  115  is to provide a control signal to integrated circuit  51  signifying that the tire rotation speed has achieved a predetermined higher value than that signified by contact between mass block  95  and either contact spring  92  or contact spring  93 . For example, the mechanical parameters controlling the rotational speed at which springs  92 ,  93  and mass block  95  make contact and permit the application of electrical power to integrated circuit  51  to enable the tire pressure measurement process may be set at 10 m.p.h.; while the mechanical parameters controlling the rotational speed at which springs  112 ,  113  and mass block  95  make contact and generate the control signal may be set at 50 m.p.h. The control signal on output terminal  115  is used for the following purpose. 
   During the tire pressure measurement process, a significant amount of power is consumed from battery  52  when electrical current is applied to sensor assembly  50 . Multi-stage motion detector  55 D enables integrated circuit  51  to minimize the total amount of current applied during the measurement process by limiting the measurement period to the time period required to make an accurate measurement of the tire pressure as a function of vehicle speed.  FIGS. 12A and 12B  illustrate this power tailoring technique for a 205/65R15 tire having a radius of 0.32 m. For a vehicle speed of 10 m.p.h., the time required for one revolution of this specific tire is 0.45 second. Thus, the minimum time period required to obtain a measurement of Rmin and Rmax is 0.45 second. Since the angular position of the tire at any given moment when electrical power is applied to integrated circuit  51  is indeterminate with the present system, it is prudent to enable the tire pressure measurement process for two complete revolutions of the tire after power is applied. With reference to  FIG. 12A , at a vehicle speed of 10 m.p.h. the tire pressure measurement process is enabled for 0.90 second, which is the time required for two complete revolutions of the tire after the measurement process is enabled. Thus, with motion detector  55 D, after electrical power is transferred from battery  52  to integrated circuit  51  via the conductive path terminal  91 , spring  96 , mass block  95 , contact spring  92  or  93 , and terminal  98 , the tire pressure measurement process is enabled for 0.90 second when the control signal on terminal  115  is inactive or deasserted. 
   For a vehicle speed of 50 m.p.h., the time required for one revolution of the same tire is 0.09 second; and two complete revolutions require 0.18 second. Thus, the minimum time period established to obtain a reliable measurement of Rmin and Rmax is 0.18 second. With reference to  FIG. 12B , at a vehicle speed of 50 m.p.h. the tire pressure measurement process is enabled for 0.18 second, which is the time required for two complete revolutions of the tire after the measurement process is enabled. Thus, with multi-stage motion detector  55 D, after electrical power is transferred from battery  52  to integrated circuit  51  via the conductive path terminal  91 , spring  96 , mass block  95 , contact spring  92  or  93 , and terminal  98 ; and electrical power is transferred from battery  52  to integrated circuit  51  via the conductive path terminal  91 , spring  86 , mass block  95 , contact spring  112  or  113 , and terminal  115  (thereby asserting the control signal), the tire pressure measurement process is enabled for only 0.18 second. 
   As will now be apparent, multi-stage motion detector  55 D limits power consumption during the tire pressure measurement process while still allowing an accurate measurement of tire pressure to be obtained. It is understood that, although multi-stage motion detector  55 D has been described with reference to the common elements of motion detector  55 A, detector  55 D may be configured using the common elements of motion detectors  55 B and  55 C. Also, it is understood that additional stages may be added to multi-stage motion detector  55 D to incorporate more and different speed thresholds than the two thresholds described above. For example, an additional set of contact springs may be installed at wider spacings than contact springs  112 ,  113  shown in  FIG. 11  to specify a third, higher speed threshold with a shorter power-on time period. Further, it is understood that the measurement periods can be based on a different number of revolutions of the tire than the two revolution example in the preferred embodiment, if desired. 
   While the preferred embodiments have been thus-far described as a single unit for one tire, in practice each tire of a vehicle will be equipped with a tire pressure sensor system  50 . Various encoding arrangements can be made to uniquely identify each individual sensor, and the warning indicator can be configured to identify the particular tire which is improperly inflated. 
   As will now be apparent, the invention provides a simple, low cost tire pressure sensor system which is relatively simple in construction and enjoys higher measurement sensitivity than known systems using a single stretch sensor. In addition, the tire pressure sensor system according to the invention can accommodate various modes of installation, such as being incorporated into the tire during manufacture, installed on the inside wall of the tire before mounting on the wheel, and installed on the outer side wall of the tire after mounting on the wheel. Further, the motion detector portion of the invention limits power consumption and thus prolongs battery life. Lastly, the invention provides an accurate and reliable system for monitoring tire safety on all vehicles using pneumatic tires. 
   While the invention has been described with reference to particular preferred embodiments, various modifications, alternate embodiments, and equivalents may be employed, as desired. For example, paired springs  92 ,  93  may be replaced with a single spring positioned along the pivot axis of the mass block support member, if desired. If a single spring is used, care must be taken to orient the sensor system in the proper direction on the tire to ensure that application of electrical power to integrated circuit  51  will occur upon forward motion of the vehicle. 
   Also, while the invention has been described with reference to the use of adhesives for attaching the sensor to the tire side wall, other known techniques may be used, if deemed suitable, for the purpose of attaching the sensor to the tire side wall. Therefore, the above should not be construed as limiting the invention, which is defined by the appended claims.