Abstract:
A pressure guiding bump is configured on the center of a pressure gauge to obtain a single “conductivity--pressure” curve feature which is independent from any position wherever a pressure is applied on the guiding bump. When a pressure is applied, the guiding bump guides the pressure against a fixed deformable area to be deformed, whatever the pressure is, the deformed area is nearly a same area. The pressure gauge is extraordinarily adequate to be designed in a weighing machine with parallel connection in between them.

Description:
FIELD OF THE INVENTION 
     This invention relates to a pressure gauge, especially a pressure gauge having a pressure guiding bump, to which a fixed area is pressed to deform when a pressure is applied thereon. 
     BACKGROUND 
       FIGS. 1-6  is a prior art 
       FIG. 1  shows a prior art of a pressure gauge 
     A section view of the traditional pressure gauge is shown. A piezoresistor is made of a top stack TS and a bottom stack BS. A spacer  15  is inserted in between the two stakes in the periphery to make a center space  16  in between the two stacks. 
     The top stack TS includes sequentially from top to bottom: a top substrate  10 , a top metal electrode  11 , and a top piezoresistive layer  12 . The bottom stack BS includes sequentially from top to bottom: a bottom piezoresistive layer  129 , a bottom metal electrode  11 , and a bottom substrate  109 . A spacer  15  is inserted to form a center space  16  in between the top piezoresistive layer  12  and the bottom piezoresistive layer  129 . The top metal electrode  11  electrically couples to a first electrode of the electronic system  13 , and the bottom electrode  129  electrically couples to a second electrode of the electronic system  13 . 
       FIG. 2  is an initial status of the prior art 
     When a pressure is applied to the pressure gauge  100  initially, the top piezoresistive layer  12  bends down to touch the bottom piezoresistive layer  129 . Just before touching, an initial thickness L 1  is a total thickness of the top piezoresistive layer  12  plus the bottom piezoresistive  129 . An output resistance can be calculated according to ohm&#39;s law as: R 1 =ρL 1 /A 1 . At initial status, the contact area A 1  at point P 1  approaches zero. Therefore, the output resistance R 1  is calculated to be infinite as follows:
 
 R 1→∞ when  A 1→0.
 
       FIG. 3  is a stable status under pressure of the prior art 
     The pressure gauge  100  is pressed further so that the piezoresistive layers  12 ,  129  are compressed and the total thickness L 2  of the two piezoresistive layers  12 ,  129  becomes lesser than the initial thickness L 1 . In the meanwhile, the contact area A 2  at area P 2  is larger than the initial contact area A 1 . At this moment, the output resistance is calculated as follows:
 
 R 2=ρ L 2/ A 2∘
 
       FIG. 4  is pressure tests of the prior art 
     Three different points P 1 , P 2 , and P 3  are chosen to be tested in a prior art pressure gauge  100 . Point P 1  is the center of the pressure gauge  100 , point P 2  is a little far away from the center point P 1 , and point P 3  is even farther away from the center point P 1 . Various pressures are applied to each of the three points for checking the corresponding conductance, the conductance--pressure curves are then made as shown in  FIG. 5 . 
       FIG. 5  is the conductance--pressure curves for points P 1 , P 2 , and P 3   
     The top line is for point P 1 , the middle line is for point P 2 , and the bottom line is for point P 3 . The curve for P 1  has a largest slop, the curve for P 2  has a less slop, and the curve for P 3  has a least slop. The curve slop is lesser as the test point farther away from the center. In other words, the farther a test point is away from the center, the less precision it becomes. Further in other words, different conductance can be obtained when a same pressure is applied at a different point of a traditional pressure gauge  100 . Curve P 1  has the best identification ability, curve P 2  has moderate identification ability, and curve P 3  has the worst identification ability. The position dependent curve--pressure feature makes the prior art pressure gauge  100  unreliable, unless a fixed test position is used. Take an example to see different conductance is obtained for a same pressure: a conductance of 6.5*10exp (−4)/ohm is obtained for curve P 1  at 20 psi; a conductance of 3.5*10exp (−4)/ohm is obtained for curve P 2  at 20 psi; and a conductance of 2.8*10exp (−4)/ohm is obtained for curve P 3  at 20 psi. Serious problems shall be caused if the prior art pressure gauge  100  is designed in a weighing machine with parallel connection. It becomes a big challenge as how to design a correction circuit to modify the deviation in order to obtain a linear output in order for realizing a weighing machine with the traditional pressure gauge  100 . 
       FIG. 6  is a pressure test with prior art pressure gauge. 
       FIG. 6  shows when a product Wt with a rugged bottom is put on a parallel connected prior art pressure gauges  101 ,  102 ,  103  which are configured on a substrate  209 . As shown in the figure, the rugged bottom of the product Wt touches point P 1  of the pressure gauge  101 , touches point P 2  of the pressure gauge  102 , and touches point P 3  of the pressure gauge  103 . It is difficult to obtain an accurate weight from the prior art pressure gauge  100  because of the non-consistent conductance-pressure curve for different points P 1 , P 2  and P 3 . 
     Now, please refer to  FIG. 3 . The basic principle for the prior art follows the Law of Resistance R=ρL/A, the changes of the total thickness L, and the changes of the touching area A between the two piezoresistive layers  12 ,  129  are two determinants for the output resistance R. Therefore, the prior art pressure gauge needs to consider the two factors when a pressure is applied, and especially when a pressure is applied on partial area instead of full surface of the pressure gauge  100 . Further more, an anti-pressure of the spacer  15  in the peripheral is another headache problem needs to be overcome for the prior art pressure gauge  100 . A single conductance--pressure curve for a pressure gauge independent of position with a stable and reproducible output is desired for a long time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1˜6  Prior Art 
         FIGS. 7˜9  is a first embodiment according to the present invention. 
         FIG. 10  is a perspective view of a product according to  FIG. 7   
         FIG. 11  is a modification design to  FIG. 10 . 
         FIG. 12  is a constant conductance-pressure curve for a product of either  FIG. 10  or  FIG. 11   
         FIG. 13A  is a parallel connection of pressure gauges as shown in  FIG. 11   
         FIG. 13B  is an equivalent circuit for a product of  FIG. 13A   
         FIG. 14  is a testing example for  FIG. 13A   
         FIG. 15  is a modification embodiment to  FIG. 11   
         FIG. 16  is a first application embodiment of the present invention 
         FIG. 17  is a second application embodiment of the present invention 
         FIG. 18  is a third application embodiment of the present invention 
         FIG. 19  is an explosion diagram of  FIG. 18   
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A single conductance--pressure curve for a pressure gauge independent of position with a stable and reproducible output is accordingly devised to overcome the shortcomings of the prior art. The revised pressure gauge has a fixed deformable area which eliminates output deviation of the prior art. 
     A guiding bump which can be made of rubber or something similar is configured on top center of the pressure gauge; the guiding bump is a hard piece to press a fixed area independent of the magnitude of an applied pressure and independent of the position of the applied pressure. Because the deformed area A is a constant and therefore the thickness changes L of the deformed piezoresistive layers is the only consideration for the output resistance. 
     R=ρL/A, since deformable area A becomes a constant, the resistance is simplified as follows: R=kL 
     A pressure gauge with single conductance-pressure curve independent of position is obtained according to the invention. The anti-pressure caused from the spacer can be overcome by the arrangement of the guiding bump in a position of the top center and the guiding bump does not extend to the periphery of the pressure gauge. Since the modified pressure gauge has a single conductance--pressure curve, it is suitable to be designed in parallel connection with each other or one another to form an ideal weighing machine. 
       FIGS. 7˜9  is a first embodiment according to the present invention. 
       FIG. 7  is a section view of the structure of the first embodiment. 
     A guiding bump  21  is configured on the top center of the top substrate  10  of a prior art pressure gauge  100  as shown in  FIG. 1  to form revised a pressure gauge  200  according to the present invention. The guiding bump  21  is located in an area not extending to the periphery to avoid the anti-pressure caused by the spacer  15  when a pressure is applied. 
       FIG. 8  is an initial status for  FIG. 7   
     Initially, when a pressure is applied, the guiding bump  21  is downward pressed with a fixed deformable area A 1 , the deformed area A 1  is designed to be in a center area keeping away from the spacer  15  with a clearance A 2  to avoid the anti-pressure from the spacer  15 . The initial total thickness L 3  is the sum of the thickness of both piezoresistive layers  12 , 129 . 
       FIG. 9  is a stable status under a pressure for  FIG. 7   
     A fixed deformable area A 1  is downward compressed. When stable, the total thickness L 4  becomes lesser than the initial total thickness L 3 . Thickness L 4  is a sum of the compressed thickness of the two piezoresistive layers  12 ,  129 . Since the deformable area A is a constant, the output resistance R can be calculated according to the simplified formula:
 
 R=kL.  
 
       FIG. 10  is a perspective view of a product according to  FIG. 7   
     A flat guiding bump  21  made of a hard material such as plastic, metal . . . etc is configured on the top center of the pressure gauge  200 . A fixed deformable area A 1  is downward compressed when a pressure is applied on the guiding bump  21 . The compressed area is always the same wherever the pressure is applied on the bump  21 , for example, a same conductance or resistance output is obtained if a same pressure is applied either at point P 4 , point P 5 , or point P 6 . Point P 4  is at the center of the bump  21 , point P 5  is a little far away from point P 4 , and point P 6  is even farther away from point P 4 . 
       FIG. 11  is a modification design to  FIG. 10 . 
     A convex guiding bump  21 B is configured on the top center of the pressure gauge  300 . The structure is similar to the product of  FIG. 10 . The feature and effect is the same as that of the product of  FIG. 10 . 
       FIG. 12  is a constant conductance-pressure curve for a product of either  FIG. 10  or  FIG. 11   
     A single conductance-curve independent of position is shown as  FIG. 12  for a product of either  FIG. 10  or  FIG. 11 . e.g. A same value of 6.0*10 −4 /ohm is obtained when a same pressure is applied either on point P 4 , P 5 , or P 6  to the product of  FIG. 10  or  FIG. 11 . 
       FIG. 13A  is a parallel connection of pressure gauges as shown in  FIG. 11   
     The product of  FIG. 11  can be designed in a form of parallel connection so as to form a weighing machine for measuring bigger weights. A first pressure gauge  301  is connected in parallel with a second pressure gauge  302 . A first pressure F 1  and a second pressure F 2  can be added to output through a calculation circuit when the first pressure F 1  is applied on the first pressure gauge  301  and the second pressure F 2  is applied on the second pressure gauge  302 . 
       FIG. 13B  is an equivalent circuit for a product of  FIG. 13A   
     The first pressure gauge  301  represents a first variable resistor R 1 , and the second pressure gauge  302  represents a second variable resistor R 2 . Each of the first variable resistor R 1  and the second variable resistor R 2  reveals a same conductance--pressure curve feature. The output resistance R is calculated as: 1/R=1/R 1 +1/R 2 . When an object Wt with a weight WG weighs on a weighing machine of  FIG. 13A , the weight WG is calculated as follows: 
     
       
         
           
             
               
                 
                   
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     According to Ohm&#39;s law R=V/I, the weight WG is further calculated as:
 
 WG=α*I/V∘ 
 
     Wherein, WG is the weight, F 1  is a force applied on the first gauge, F 2  is a force applied on the second gauge, and α is a constant. 
     This embodiment can be realized only when both the variable resistor R 1  and R 2  have a linear output for the resistance. The pressure gauges  200 ,  300  according to this invention well satisfies the requirement to have a linear output. 
       FIG. 14  is a testing example for  FIG. 13A   
     A weighing machine is made of three parallel connected pressure gauges  301 ,  302 ,  303  according to  FIGS. 13A ,  13 B. The three pressure gauges  301 ,  302 ,  303  are configured on a substrate  309  which can be a flexible or non-flexible one. Electric wires  308  are configured on top surface of the substrate  309 , and electrically couple each and all of the pressure gauges  301 ,  302 ,  303  to an electronic system (not shown). An object with a weight Wt has a rugged bottom surface and contacts the three pressure gauges  301 ,  302 ,  302  at points P 4 , P 5 , and P 6  individually. Point P 4  is on the center of the pressure gauge  301 , point P 5  is a little far away from the center of the pressure gauge  302 , and point P 6  is even farther away from the center of the pressure gauge  303 . A reproducible weight Wt can be obtained according to this invention; however which can not be obtained if made with traditional pressure gauges of  FIG. 1 . This is because each of the three pressure gauges  301 ,  302 ,  303  has a single conductance-pressure curve feature which is independent of position to be pressed on each of the pressure gauges  301 ,  302 ,  303 . 
       FIG. 15  is a modification embodiment to  FIG. 11   
     Two guiding bumps  21 C are configured on top surface of a pressure gauge  400 . Each of the guiding bumps  21 C is located in a position away from periphery to avoid the anti-pressure from the fringe spacer  15 . The effect for the pressure gauge  400  is similar to the pressure gauge  300  of  FIG. 11 . 
       FIG. 16  is a first application embodiment of the present invention 
     A weighing machine  30  is made of the pressure gauge  300  of  FIG. 11 . Each of four pressure gauges  300  is configured on bottom of each of the four corners of a hard plate  309 B. The four pressure gauges  300  are parallel connected and electrically coupling to a electronic system (not shown). A display  31  is configured to show the weight calculated from the electronic system. 
       FIG. 17  is a second application embodiment of the present invention 
     A flexible substrate  40  is used in this application for carrying a plurality of pressure gauges  300 . The pressure gauges  300  are arranged in a pattern of a matrix; however different pattern such as a pair of feet for standing, or boxing area for boxing games . . . etc., can be also realized. The flexible substrate weighing machine can be folded or rolled up to put away when unused. 
       FIG. 18  is a third application embodiment of the present invention 
     A flexible top stack TS, spacers  52 , and a flexible bottom stack BS are sandwiched to form a flexible piezoresistor strip. The top stack TS is composed sequentially of a top substrate, a top metal electrode, and a top piezoresistive layer. The bottom stack BS is composed sequentially of a bottom piezoresistive layer, a bottom metal electrode, and a bottom substrate. A plurality of pressure guiding bumps  51  are configured on top surface of the top substrate. Each of the guiding bumps  51  is configured in a position away from a position above spacers  52 . 
       FIG. 19  is an explosion diagram of  FIG. 18   
     Spacers  52  are sandwiched in between the top stack TS and the bottom stack BS to create a predetermined space between the two stacks. The spacers  52  are configured in between top piezoresistive layer and bottom piezoresistive layer. 
     While several embodiments have been described by way of example, it will be apparent to those skilled in the art that various modifications may be made without departing from the spirit of the present invention. Such modifications are all within the scope of the present invention, as defined by the appended claims.