Abstract:
It is the objective of this invention to reduce deviation in contact resistance variation between electrodes among pressure-sensitive resistor sensors. The sensor is composed of a pair of films. One film has a plurality of electrode portions extending in a certain direction and the other film has at least one electrode portion that is arranged in a transverse relation with a plural of the electrode portions on the other film. Contact points generated by applied pressure are limited to crossing points between the electrode portions of the pair of films. As a result, variation in contact resistance due to applied pressure shift becomes more constant. Thus, it is possible to reduce deviation of contact resistance between the electrodes among sensors.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-329895 filed on October 30, 2000. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a pressure-sensitive resistor sensor for detecting pressure applied thereto. 
     Pressure-sensitive resistor sensors are proposed as shown in FIGS. 19,  20 A and  20 B. These types of sensors have a bottom side film  110  formed with a bottom side electrode  100  and a top side film  210  formed with a top side electrode  200 . The bottom side film  110  and the top side film  210  facing each other are spaced apart from each other by a predetermined distance by a spacer film  300  interposed between the bottom side film  110  and the top side film  210 . When pressure is applied to the top side film  210 , the central part of the top side film  210  is deformed downwardly as shown in FIG. 3 so that the top side electrode  200  contacts the bottom side electrode  100 . As the applied pressure increases, the number of contact points and total contact area between the electrodes  100  and  200  increase, thereby changing a contact resistance between the electrodes  100  and  200 . 
     The electrodes  100  and  200  of a proposed pressure-sensitive resistor sensor are both shaped in a planar disk as shown in FIG.  19 . Alternatively, the electrodes  100  and  200  are shaped in comb teeth and a planar disk, respectively, as shown in FIGS. 20A and 20B. With this structure, the contact points between the electrodes  100  and  200  depend on the surface roughness of the electrodes  100  and  200 . It is therefore difficult to control the number of contact points and total contact area uniformly among sensors. As a result, sensor output characteristics vary from sensor to sensor. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce deviation in contact resistance variation between electrodes among pressure-sensitive resistor sensors. 
     According to the present invention, one of a pair of films has a plurality of electrode portions and the other of the pair has at least one electrode portion that crosses the plurality of electrode portions on the one of the pair. Contact points generated by applied pressure are limited to crossing points between the electrode portions of the pair of films. Thus, the number of contact points and total contact area increase uniformly among sensors as the applied pressure increases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
     FIG. 1A is a plan view showing a first embodiment of a pressure-sensitive resistor sensor of the present invention, and FIG. 1B is a cross-sectional view of an electrode portion of the first embodiment taken along line IB—IB in FIG. 1A; 
     FIGS. 2A and 2B are cross-sectional views of the first embodiment taken along lines IIA—IIA and IIB—IIB in FIG. 1A, respectively; 
     FIG. 3 is a cross-sectional view showing operation of the first embodiment; 
     FIG. 4 is a plan view showing a seat sensor using the first embodiment; 
     FIG. 5 is a plan view showing a first modification of the first embodiment; 
     FIG. 6 is a plan view showing a second modification of the first embodiment; 
     FIG. 7A is a plan view showing a second embodiment of a pressure-sensitive resistor sensor of the present invention, and FIG. 7B is a cross-sectional view of an electrode portion of the second embodiment taken along line VIIB—VIIB in FIG. 7A; 
     FIG. 8 is a plan view showing a first modification of the second embodiment; 
     FIG. 9 is a plan view showing a second modification of the second embodiment; 
     FIG. 10 is a plan view showing a third modification of the second embodiment; 
     FIG. 11 is a plan view showing a fourth modification of the second embodiment; 
     FIG. 12 is a plan view showing a fifth modification of the second embodiment; 
     FIG. 13A is a plan view showing a sixth modification of the second embodiment, and 
     FIG. 13B is a cross-sectional view of an electrode portion of the sixth modification of the second embodiment taken along line XIIIB—XIIIB in FIG. 13A; 
     FIG. 14 is a plan view showing a seventh modification of the second embodiment; 
     FIG. 15 is a plan view showing a third embodiment of a pressure-sensitive resistor sensor of the present invention; 
     FIGS. 16A and 16B are cross-sectional views of the third embodiment taken along lines XVIA—XVIA and XVIB—XVIB in FIG. 15, respectively; 
     FIG. 17 is a plan view showing a fourth embodiment of a pressure-sensitive resistor sensor of the present invention; 
     FIGS. 18A and 18B are cross-sectional views of the fourth embodiment taken along lines XVIIIA—XVIIIA and XVIIIB—XVIIIB in FIG. 17, respectively; 
     FIG. 19 is a cross-sectional view showing a pressure-sensitive resistor sensor according to a related art; and 
     FIG. 20A is a plan view showing a pressure-sensitive resistor sensor according to another related art, and FIG. 20B is a cross-sectional view of the related art taken along line XXB—XXB in FIG.  20 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described in detail with reference to various embodiments and modifications. 
     (First Embodiment) 
     A pressure-sensitive resistor sensor according to a first embodiment is shown in FIGS. 1A,  1 B to FIG.  6  and indicated with reference numeral  1 . A plurality of sensors  1  may be connected to each other and provided in a vehicle seat  2  as shown in FIG. 4 to operate as a seat sensor that detects the size of a passenger (adult or child) on the seat  2 . 
     As shown in FIGS. 2A and 2B, each sensor  1  has a bottom side film  3  and a top side film  4  (for instance PEN films). The films  3  and  4  are attached to each other by an adhesive  6  with a spacer film  5  interposed between the films  3  and  4  to provide a predetermined clearance. The films  3  and  4  have bottom side and top side electrode layers, respectively, that face each other and are spaced apart from each other in the central part of the films  3  and  4 . 
     As shown in FIG. 1A, the bottom side electrode layer comprises a plurality of linear (straight) electrodes  7  and arc-shaped electric leads  8  that are connected to the electrodes  7 . The electrodes  7  comprise two positive side electrodes  7 A and two negative side electrodes  7 B that are both in a comb teeth shape. The number of positive side electrodes  7 A and negative side electrodes  7 B may be one, three or more. The electric leads  8  comprise positive side lead  8 A and negative side lead  8 B that face each other on the same circumference of a circle. The positive side electrodes  7 A and the negative side electrodes  7 B are aligned in parallel with each other with a predetermined clearance. 
     The electrodes  7  and the electric leads  8  are formed by means of forming base layers  7   a  and  8   a  on the bottom side film  3  and forming thereon resistive layers  7   b  and  8   b . A method to form the base layers  7   a  and  8   a  is screen-printing with Ag paste onto the surface of the bottom side film  3 . A method to form the resistive layers  7   b  and  8   b  is screen-printing with a compound of conductive particles and resin or resistive resin on the top of the base layers  7   a  and  8   a . As shown in FIG. 1A, The positive side lead  8 A and negative side lead  8 B are connected to each other with a resistive layer  9  comprising the resistive layer  8   b  ( 7   b ) to provide a function to detect an open circuit. As shown in FIG. 1B, the cross-sectional structure of the electrodes  7  and  10  is a twin-peak-mountain shape in the case that both electrodes  7  and  10  are formed with screen-printing and the widths of the electrode  7  and  10  are 1.5 mm, respectively. 
     The top side electrode layer shown in FIG. 1A comprises three linear (straight) electrodes  10 . The electrodes  10  shown in FIG.  2 A and FIG. 2B comprise base layers  10   a  on the top side film  4  and resistive layers  10   b  on the top of the base layers  10   a  to provide the same line width, e.g., 1.5 mm, as that of the bottom side electrodes  7 . The base layer  10   a  and the resistive layer  10   b  are formed in the same way to form the bottom side electrode layer, i.e., a method to form the base layer  10   a  is screen-printing with Ag paste onto the surface of the top side film  4 . A method to form the resistive layer  10   b  is screen-printing with a compound of conductive particles and resin or resistive resin on the top of the base layer  10   a.    
     The electrodes  10  are aligned in parallel with each other in a predetermined clearance. Each end of the electrodes  10  is connected to other ends of adjacent electrodes in an arc shape to provide a closed circuit. The number of the electrodes  10  may be one, two, four or more. In modifications shown in FIG.  5  and FIG. 6, electrodes  10  have two and four electrodes, respectively. As shown in FIG. 1A, the bottom side and the top side electrode layers face each other in such way that each of the bottom side electrodes  7 A and  7 B is arranged in a transverse relation with the top side electrodes  10 . 
     The operation and the advantage of the pressure-sensitive resistor sensor  1  are described as follows. As shown in FIG. 3, the central part of the top side film  4  is distorted downward and the top side electrodes  10  contact the bottom side electrodes  7  when the top side film  4  is pressed by pressure P. As shown in FIG. 1A, contact points between the bottom side electrodes  7  and the top side electrodes  10  are limited to a plurality of crossing points  11  between the electrodes  7  and  10 . 
     Therefore, the number of contact points and total contact area in crossing points  11  increases as applied pressure increases. Applied pressure is measured by contact resistance shift due to changes in both the number of contact points and total contact area. In the case that this sensor structure is employed, contact points between the bottom side electrodes  7  and the top side electrodes  10  are limited to a plurality of crossing points  11  between the electrodes  7  and  10  so that variation in contact resistance due to applied pressure shift becomes more constant. As a result, it is possible to reduce deviation of contact resistance between the electrodes  7  and  10  among sensors and allow stable detection of applied pressure. 
     (Second Embodiment) 
     In this embodiment of pressure-sensitive resistor sensor  1 , each of the electrodes  7  and  10  have a different width from that in the first embodiment. In FIGS. 7 to  12 , each of the electrodes  7  and  10  have 0.9 mm width. With this width, the cross-sectional structure of the electrodes  7  and  10  becomes a single-peak-mountain shape as shown in FIG.  7 B. In a modification shown in FIG. 12, the top side electrode layer has a plurality of circular electrodes  10  that are different from each other in diameter and arranged in a concentric shape. In this modification, contact points between the bottom side electrodes  7  and the top side electrodes  10  also are limited to a plurality of crossing points  11  between the electrodes  7  and  10  so that the same effect as that in the first embodiment is provided. 
     In a modification shown in FIGS. 13A,  13 B and  14 , at the positive side electrodes  7 A and the negative side electrodes  7 B of the bottom electrode layer, a common resistive layer  7   b  is formed to cover a plurality of base layer  7   a  in the case that the width of the electrodes  7  becomes narrower, e.g., 0.5 mm. With this width, the cross-sectional structure of the electrodes  7 A and  7 B becomes a quintuple-peak-mountain shape as shown in FIG.  13 B. 
     (Third Embodiment) 
     In a third embodiment of the pressure-sensitive resistor sensor shown in FIGS. 15,  16 A and  16 B, the base layer  10   a  of the top side electrodes  10  in both the first embodiment and the second embodiment is removed and the top side electrodes  10  comprise only the resistive layer  10   b . In this embodiment, surface roughness of the top side electrodes  10  (the resistive layer  10   b ) is homogenized so that contact resistance with the bottom side electrodes  7  becomes more stable. As a result, the deviation of contact resistance between the electrodes  7  and the electrodes  10  among sensors can be more reduced. 
     (Fourth Embodiment) 
     In a fourth embodiment shown in FIGS. 17,  18 A and  18 B, both the base layer  10   a  of the top side electrodes  10  and the base layer  7   a  of the bottom side electrodes  7  in both the first embodiment and the second embodiment are removed and both the topside electrodes  10  and the bottom side electrodes  7  comprise only resistive layer  7   b  and  10   b , respectively. In this embodiment, surface roughness of both the top side electrodes  10  (the resistive layer  10   b ) and the bottom side electrodes  7  (the resistive layer  7   b ) is homogenized so that contact resistance between both the electrodes  7  and  10  becomes furthermore stable. As a result, the deviation of contact resistance between the electrodes  7  and  10  among sensors can be furthermore reduced.