Abstract:
A motion sensor for tire pressure monitors and other applications includes an insulative collar, a conductive coil spring mounted on the cap, a conductive connector that extend through the cap and connects to the spring, and a conductive can around and spaced from the spring. The cap closes and seals the open end of the can. Acceleration of the motion sensor causes the coil spring to make electrical contact with the can to act as a switch closure.

Description:
This application claims the benefit under 35 U.S.C. § 119(e) of the U.S. provisional patent application No. 60/595,477 filed Jul. 8, 2005. 

   FIELD OF THE INVENTION 
   This invention relates to sensing devices and more particularly to a motion sensor that is particularly suited for tire pressure monitors and systems. 
   BACKGROUND ART 
   There are a number to tire pressure monitoring devices and systems presently on the market. Generally the tire pressure monitoring devices are mounted inside the tire. Powering such a device, inside a tire on a rotating wheel, with the vehicle electrical system would be complex and expensive. Therefore, these known tire pressure monitoring devices include batteries for electrical power. 
   Since these known tire pressure monitoring devices are inside a tire, battery replacement is difficult. A motion sensor can be incorporated into the devices to reduce power consumption and extend battery life. Such a motion sensor can sense tire rotation and turn the tire pressure monitoring device on when the tire rotates above a selected speed. 
   DISCLOSURE OF THE INVENTION 
   A motion sensor includes a cap, a coil spring, a connector and a can. The cap is made of an electrically insulative material. The spring is electrically conductive and mounts on the cap. The connector electrically connects to the coil spring and extends through the cap. The can has a electrically conductive inner surface forming an interior cavity surrounding the spring. The spring is spaced a selected distance from the inner surface of the can. The can has an open first end and a spaced, closed second end. The cap fits into and seals the open end of the can. During acceleration of the motion sensor, the coil spring flexes to contact the inner surface of the can to electrically connect the can to the connector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of this invention are described in connection with the accompanying drawings that bear similar reference numerals in which: 
       FIG. 1  is a bottom view of a motion sensor embodying features of the present invention. 
       FIG. 2  is a sectional view of the sensor of  FIG. 1  taken along line  2 — 2 . 
       FIG. 3  is a sectional view of the sensor of  FIG. 1  taken along line  2 — 2  with an alternative spring and an alternative stem. 
       FIG. 4  is a perspective view of the stem of  FIG. 3 . 
       FIG. 5  is a perspective view of an alternative can for the motion sensor of  FIG. 1 . 
       FIG. 6  is a diagramic view of a tire pressure monitoring device with the motion sensor of  FIG. 1 . 
       FIG. 7  is a perspective view of modified cap for the motion sensor of  FIG. 1 . 
       FIG. 8  is a perspective view of another modified cap for the motion sensor of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 and 2 , a motion sensor  12  embodying features of present invention includes a cap  14 , a spring  15 , a connector  16  and an outer housing or can  17 . The cap  14  is made of an insulative material. The cap  14  has a substantially cylindrical base portion  19  with a first face  20  and a spaced, oppositely facing second face  21 . A circumferential wall  22  projects from the first face  20 , forming a spring well  23 . A central aperture  24  extends through the base portion  19  from the first face  20  to the second face  21 . 
   The spring  15  is made of an electrically conductive, elastic material and is a coil spring having a plurality of turns  26 . The turns  26  are formed of wire having a selected diameter. The spring  15  is generally cylindrical with spaced first and second ends  27  and  28 . The first end  27  has an inwardly projecting, transverse lip  29  formed by several turns  26  coiling inwardly. A spring aperture  30  formed by the lip  29  is sized to match the central aperture  24  of the base portion  19  of the cap  14 . 
   The connector  16  includes a stem  32  and a pin portion  33 , each made of an electrically conductive material. The stem  32  has a cylindrical inner portion  35 , a substantially cylindrical outer portion  36 , and a shoulder portion  37  between the inner and outer portions  35  and  36 . The shoulder portion  37  extends radially outwardly relative to the inner and outer portions  35  and  36 . The shoulder portion  37  is sized to fit into the spring and is larger in diameter than the spring aperture  30 . The outer portion  36  is sized to fit through the spring aperture  30 , and to fit into and seal the central aperture  24  of the base portion  19  of the cap  14 . The outer portion  36  includes a plurality of protruding sharp ridges  38 . 
   The pin portion  33  includes a coil section  40 , a transverse section  41  and a parallel section  42 . The coil section  40  has a plurality of coils  43  sized to receive the outer portion  36  of the stem  32 . The transverse section  41  extends from the coil section  40  transverse to the stem  32 . The pin portion  33  bends between the transverse and parallel sections  41  and  42  with the parallel section  42  extending from the transverse section  41  parallel to the stem  32 . 
   The can  17  includes a cylindrical portion  45 , and spaced first and second ends  46  and  47 . The first end  46  is open and the second end  47  is closed. The inner surface  48  of the can  17  is made of an electrically conductive material and forms an interior cavity  49 . The base portion  19  of the cap  14  is sized to fit into and seal the first end  46  of the can  17 . 
   The motion sensor  12  is assembled as follows. The spring  15  is placed on the stem  32  with the outer portion  35  of the stem  32  projecting through the spring aperture  30 . The outer portion  36  of the stem  32  is pressed through the central aperture  24  of the base portion  19  of the cap  14 , with the first end  27  of the spring  15  in the spring well  23  of the cap  14 . The ridges  38  of the outer portion  36  of the stem  32  seal the central aperture  24  of the base portion  19  of the cap  14 . The cap  14  is pressed into the can  17 , with the spring  15  inside the can  17  and the cylindrical portion  45  of the can  17  spaced concentrically around the spring  15 . Preferably, the base portion  19  of the cap  14  is sized to expand the sides of the can  17  to seal the interior cavity  49 . The motion sensor  12  is sealed to prevent corrosion of the inner surface  48  and the spring  15 . 
   The motion sensor  12  can be very small. By way of example, and not as a limitation, the length of the can  17  can be about 0.25 to 0.33 inches and the diameter of the can  17  can be about 0.187 inches. The motion sensor  12  can be assembled to a circuit board with the parallel section  42  of the pin portion  33  of the connector  16  extending through the circuit board by electrically connecting the parallel section  42  and the second end  47  of the can  17  to the circuit board. 
     FIG. 3  shows a motion sensor  12  with an alternative stem  52  and an alternative spring  53 . Referring to  FIG. 4 , the stem  52  has a cylindrical inner portion  55 , an outer portion  56 , and a shoulder portion  57  between the inner and outer portions  55  and  56 . The outer portion  56  has a cylindrical first section  59  extending from the shoulder portion  57 , and a second section  60 , with a smaller diameter than the first section  59 , extends from the first section  59 . The first section  59  has a cylindrical, projecting lip  61  that forms a circular groove with the second section  60 , at the connecting point of the first and second sections  59  and  60 . As shown in  FIG. 3 , after outer portion  56  of the stem  52  is assembled into the central aperture  24  of the base portion  19  of the cap  14 , the lip  61  is pressed or expanded outwardly to seal the central aperture  24 . 
   The spring  53  is made of an electrically conductive, elastic material and is a coil spring having a plurality of turns  63 . The turns  63  are formed of wire having a selected diameter. The spring  53  is generally cylindrical with spaced first and second ends  64  and  65 . The first end  64  of the spring  53  is similar to the first end  27  of the spring  15 , previously described. The has one or more turns  63  that flair or diverge outwardly, having a larger diameter than the remainder of the turns  63 . 
   The motion sensor  12  is substantially omnidirectional. When the motion sensor  12  is accelerated transverse to the axis A of the can  17 , the spring  53  bends and the second end  65  of the spring  53  contacts the inner surface  48  of the can  17 , thereby electrically connecting the can  17  to the connector  16 . The sensitivity of the motion sensor  12 , in terms of the acceleration required for the second end  65  of the spring  53  to contact the inner surface  48  of the can  17 , can be selected in several ways. The sensitivity is selected, by way of example, and not as a limitation by selection of the diameter of the wire of the turns  63  of the spring  53 , the length of the spring  53 , the height of the wall  22  of the base portion  19  of the cap  14 , and the distance from the inner surface  48  of the can  17  to the second end  65  of the spring  53 . The distance from the inner surface  48  of the can  17  to the second end  65  of the spring  53  by selecting the diameter of the inner surface  48  of the can  17  and by selecting the flair of the second end  65  of the spring  53 . 
   Referring to  FIG. 5 , an alternative can  67  includes a cylindrical portion  69 , and spaced first and second ends  70  and  71 . The first end  70  is open and the second end  71  is closed. The inner surface  72  of the can  67  is made of an electrically conductive material and forms an interior cavity  73 . The base portion  19  of the cap  14  is sized to fit into and seal the first end  70  of the can  67 . A plurality of circumferentially spaced tabs  74  project from the first end  70  of the can  67 . The can  67  is used with a connector  16  having only the stem  33  or  52 , without the pin portion  33 . The tabs  74  of the can  67  and the stem  33  or  52  can mount directly to a circuit board. 
     FIG. 6  shows a tire pressure monitoring device  77  including a circuit board  79 , an air pressure measuring device  80 , a battery  81 , an integrated circuit  82 , a transceiver  83  and the motion sensor  12 . The motion sensor  12  and integrated circuit  82  are both connected to the battery  81 , and to the air pressure measuring device  80  and transceiver  83 . The tire pressure monitoring device  77  is mounted in a tire and when the tire reaches a selected speed, centrifugal force causes the second end  65  of the spring  53  to contact the inner surface  48  of the can  17 , activating the air pressure measuring device  80  and the transceiver  83 . The integrated circuit  82  latches the power to the air pressure measuring device  80  and the transceiver  83  for a selected time, such as 3 seconds, to provide consistent power when the tire is rolling near the minimum speed. 
   Referring to  FIG. 7 , modified cap  14  includes a tab  85  that projects from the wall  22 . When the motion sensor  12  is assembled, the tab  85  projects between the spring  15  and the inner surface  48  of the can  17 . When the motion sensor  12  is assembled, the tab  85  extends substantially to the second end  28  of the spring  15 . The tab  85  prevents the second end  28  of the spring  15  from contacting the inner surface  48  of the can  17  when the motion sensor  12  is accelerated in a direction opposite the tab  85 . A stub  86  projecting from the base portion  19  opposite the wall  22  assures correct orientation of the motion sensor  12 .  FIG. 8  shows another modified cap  14  with two tabs  85 , at 180 degree relative to each other, projecting from the wall  22 . The tabs  85  make the motion sensor directional. Other arrangements of tabs  85  can be provided. 
   Although the motion sensor  12  has been described for use in a tire pressure monitoring system, the motion sensor  12  can be used in other applications where acceleration or shock must be sensed. By way of example, and not as a limitation, such applications can include an acceleration switch for safe arm devices in bombs and missiles, an anti-theft sensor for electronics boxes, and a shock sensor for packages. 
   Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in detail of structure may be without departing from the spirit thereof.