Abstract:
A preloaded pressure sensor module (PPSM) is disclosed, where the PPSM outputs a linear Conductivity Response versus Pressure Force input. The PPSM has a convex or concave profile which is prepared by pressing a flat pressure sensor device onto to a convex or concave base respectively so that the pressure sensitive layer inside the sensor module is bent and displays a preloaded effect.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a pressure sensor, especially to a preloaded pressure sensor module (PPSM) and its applications. 
     2. Description of Related Art 
       FIG. 1  shows a prior art flat piezoresistor device 
       FIG. 1  shows a flat piezoresistor device  10  which has a top substrate  111 , a top electrode layer  112  configured under the top substrate  111 , a top piezoresistive layer  113  configured under the top electrode layer  112 ; and a bottom substrate  121 , a bottom electrode layer  122  configured on the bottom substrate  121 , a bottom piezoresistive layer  123  configured on the bottom electrode layer  122 . Further, a pair of spacers  114  is configured in between the top electrode layer  112  and the bottom electrode layer  122  such that a space  13  is formed in between the top piezoresistive layer  113  and the bottom piezoresistive layer  123 . The top electrode layer  112  electrically couples to a first electrode, for example, positive (+) end of a control unit  15 ; and the bottom electrode layer  122  electrically couples to a second electrode, for example, negative (−) end of the control circuit  15 . 
       FIG. 2  shows the flat piezoresistor device of  FIG. 1  being pressed. 
       FIG. 2  shows that the center of the flat piezoresistor of  FIG. 1  is pressed by a user. The circuit passes through the top electrode layer  112  and the bottom electrode layer  122  is closed when the top piezoresistive layer  113  touches the bottom piezoresistive layer  123 . The signal is, however, unstable in the initial stage when the two piezoresistive layers  113 ,  123  touch each other due to initial unstable surface contact as illustrated in  FIG. 3 . 
       FIG. 3  shows the electrical characteristic for the prior art of  FIG. 2   
     Y-axis shows output conductivity, X-axis shows the pressure force exerted against the flat piezoresistor device of  FIG. 1 . Line L 1  shows the electrical characteristics for Output Conductivity versus Pressure. The output conductivity is zero during the pressure ranges between P 00 ˜P 10 , where the two piezoresistive layers  113 ,  123  are not in contact. 
     The output conductivity is unstable when the pressure force at a point ranges between P 10  and P 11 . This is because of the unstable initial contact of the two piezoresistive layers  113  and  123 . Line L 1  shows linearity for pressure measurement at the points after P 11  as the characteristic becomes adequate for application of pressure measurement. For a real case, a detection threshold DT is usually set on a point after P 11 , for example at P 12  to assure measurement quality of the sensor. The pressure force at P 12  is the minimum detectable limit for the flat pressure sensor  10 . The pressure force at P 12  is also known as an activation force for the sensor  10 . 
     The disadvantage for the prior art flat pressure sensor of  FIG. 1  is that the activation force can vary among the sensors in mass production. To produce the prior art sensor illustrated in  FIG. 1 , the manufacturer faces a tradeoff between detection limit and production yield, as aggressively setting a relatively lower detection limit responsive to a relatively lighter applied pressure may reduce production yield. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art flat piezoresistor device 
         FIG. 2  shows the flat piezoresistor device of  FIG. 1  being pressed. 
         FIG. 3  shows the electrical characteristic for the prior art of  FIG. 2   
         FIG. 4A  shows a piece of piezoresistive material. 
         FIG. 4B  shows piezoresistive material of  FIG. 4A  being bent. 
         FIG. 5A  shows a process to prepare a first PPSM according to the present invention. 
         FIG. 5B  shows a perspective view of the base used in  FIG. 5A   
         FIG. 6A  shows a first PPSM according to the present invention. 
         FIG. 6B  shows a perspective view of the PPSM of  FIG. 6A . 
         FIG. 7  shows the electrical characteristic of the first PPSM of  FIG. 6A . 
         FIG. 8  shows a comparison of the electrical characteristic between the present invention and the prior art. 
         FIG. 9  shows a process to prepare a second PPSM according to the present invention. 
         FIG. 10  shows a second PPSM prepared according to  FIG. 9 . 
         FIG. 11A  shows a modified PPSM to the PPSM of  FIG. 10 . 
         FIG. 11B  shows a further modified PPSM to the PPSM of  FIG. 10 . 
         FIG. 12  shows a top view of a mobile electronic device. 
         FIG. 13  shows a first application of the PPSM according to the present invention. 
         FIG. 14  shows a second application embodiment of the PPSM according to the present invention. 
         FIG. 15  shows a third application of the PPSM according to the present 
         FIG. 16  shows a fourth application of the PPSM according to the present invention. 
         FIG. 17  shows a fifth application of the PPSM according to the present invention. 
         FIG. 18  shows a sixth application of the PPSM according to the present invention. 
         FIG. 19  shows a seventh application of the PPSM according to the present invention. 
         FIG. 20  shows an eighth application of the PPSM according to the present 
         FIG. 21  shows a ninth application of the PPSM according to the present invention. 
         FIG. 22  shows a process to prepare a third PPSM according to the present invention. 
         FIG. 23A  shows the third PPSM prepared according to  FIG. 22 . 
         FIG. 23B  shows a perspective view of  FIG. 23A . 
         FIG. 24  shows a process to prepare a fourth PPSM according to the present invention. 
         FIG. 25A  shows a concave PPSM prepared according to  FIG. 24 . 
         FIG. 25B  shows a perspective view of the concave PPSM of  FIG. 25A . 
         FIG. 26  shows a tenth application equipped with a concave PPSM according to the present invention. 
         FIG. 27  shows a second electrical characteristic for the PPSM according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention discloses a preloaded pressure sensor module (PPSM) which has a relatively lower detectable pressure limit. Multiple pressure levels can be set to trigger different functions. 
       FIG. 4A  shows a piece of piezoresistive material. 
       FIG. 4A  shows a piece of uncurved piezoresistive material PZ, which has an even thickness R 1  with a first resistance in the vertical electrical path. 
       FIG. 4B  shows piezoresistive material of  FIG. 4A  being bent. 
       FIG. 4B  shows the piezoresistive material PZ of  FIG. 4A  is bent. The center portion of the curved piezoresistive material PZ is compressed to have a thinner thickness R 2 , and the bent area poses as a preloaded effect in contrast to the unbent piece of  FIG. 4A . The compressed center of  FIG. 4B  shows a thinner thickness which has a second resistance R 2  in a vertical electrical path, since R 2  is smaller than R 1 . The thinner thickness shows a lesser resistance in vertical direction of the piezoresistive material PZ for a vertical electrical path. A lesser resistance means a higher electrical conductivity. The curved piezoresistive material PZ of  FIG. 4B  has a higher electrical conductivity in the center than the unbent one of  FIG. 4A . 
       FIG. 5A  shows a process to prepare a first PPSM according to the present invention. 
       FIG. 5A  shows a flat piezoresistive device  10  is going to be mounted onto a base  30 . The base  30  has a convex surface  301 ; the flat piezoresistive device  10  is configured on and bent along the convex surface  301  such that the flat piezoresistive device  10  is bent to form a convex sensor module  10 B. 
       FIG. 5B  shows a perspective view of the base used in  FIG. 5A   
       FIG. 5B  shows the base  30  can be one of an elongated convex block. 
       FIG. 6A  shows a first PPSM according to the present invention. 
       FIG. 6A  shows a PPSM prepared according to  FIG. 5A , the flat piezoresistive sensor device  10  is configured on and bent along the curved surface  301  of the convex base  30  to form a first preloaded pressure sensor module (PPSM)  10 B. The top piezoresistive layer  113  and the bottom piezoresistive layer  123  of the PPSM  10 B touch at least in the center portion in the section view of  FIG. 6A , and the piezoresistive layers  113 ,  123  are compressed in center portion. The PPSM  10 B provides an advantage of a preloaded pressure sensor module having a lower minimum detectable limit. 
       FIG. 6B  shows a perspective view of the PPSM of  FIG. 6A . 
       FIG. 7  shows the electrical characteristic of the first PPSM of  FIG. 6A . 
     Line L 2  shows an electrical characteristic for the PPSM of  FIG. 6B . The Y-axis is Output Conductivity and the X-axis is Pressure. L 2  shows a linear relationship for the Output Conductivity versus Pressure for the PPSM  10 B of  FIG. 6A  or  6 B. The initial portion between a pressure force at P 21  and P 22  is reserved and not to be used for measurement for quality assurance. Detection Threshold (DT) is set as shown in the figure, such that the pressure force at point P 22  is the minimum detectable limit for the PPSM  10 B. 
       FIG. 8  shows a comparison of the electrical characteristic between the present invention and the prior art. 
     Line L 2  shows the electrical characteristic for PPSM  10 B and line L 1  shows that for a conventional one as disclosed in  FIG. 1 . The preloaded feature of PPSM  10 B eliminates the initial unstable portion of the prior art sensor device, and which shifts the linear portion leftwards. A real test result for the comparison is shown in  FIG. 8 , the Detection Threshold DT is set at 1E-6/ohm to both the PPSM and the prior art device. Line L 2  shows that the PPSM  10 B can be triggered starting from only a 5 grams pressure, while the prior art sensor of  FIG. 1  can be triggered from 20 grams as shown in line L 1 . The minimum activation force of 5 grams for the PPSM is significant lower than the 20 grams for the prior art of  FIG. 1 . The lesser activation three of PPSM  10 B makes it suitable to be used as a function key. A plurality functions can be triggered depending on different output conductivity levels output from a PPSM  10 B. 
       FIG. 9  shows a process to prepare a second PPSM according to the present invention. 
       FIG. 9  shows, firstly, preparing a piezoresistive sensor device  40  which has a stack of five layers of materials, sequentially from top to bottom, a top substrate  411 , a top electrode layer  412 , a single piece of piezoresistive material  413 , a bottom electrode layer  422  and a bottom substrate  421 ; secondly, preparing a base  30 , the base  30  has a convex surface  301  on top and flat on bottom; and thirdly, configuring the piezoresistive sensor  40  onto the surface  301  of the base  30 . The piezoresistive sensor device  40  is bent and fixed on and along the curved surface  301  of the base  30 . 
       FIG. 10  shows a second PPSM prepared according to  FIG. 9 . 
       FIG. 10  shows the second PPSM  40 B comprises a stack top-down of a bent top substrate  411 , a bent top electrode layer  412 , a single piece of bent piezoresistive material  413 , a bent bottom electrode layer  422 , a bent bottom substrate  421 , and a convex base  30 . The center portion of the bent piezoresistive layer  413  is compressed in the section view. 
       FIG. 11A  shows a modified PPSM to the PPSM of  FIG. 10 . 
       FIG. 11A  shows a gap or an air space  431  is reserved in between the piezoresistive layer  413  and the bottom electrode layer  422 . The electrical characteristic for the modified PPSM of  FIG. 11A  is similar to the one as shown in  FIG. 7   
       FIG. 11B  shows a further modified PPSM to the PPSM of  FIG. 10 . 
       FIG. 11B  shows a gap or an air space  432  is reserved in between the piezoresistive layer  413  and the top electrode layer  412 . The electrical characteristic for the modified PPSM of  FIG. 11B  is similar to the one as shown in  FIG. 7 . 
       FIG. 12  shows a top view of a mobile electronic device. 
       FIG. 12  shows a top view of a mobile electronic device such as a mobile phone, a portable media player, a tablet, or a personal digital assistant (PDA) . . . , etc. that has a cover glass  601  on top. With the PPSB  10 B,  40 B mounted under the cover glass  601 , the cover glass  601  of the mobile electronic device poses a depressible function key. 
       FIG. 13  shows a first application of the PPSM according to the present invention. 
       FIG. 13  shows four PPSMs  10 B, 40 B is configured under the cover glass  601  of an electronic mobile device  60 . Each of the PPSMs  10 B,  409  is located at one of the four corners of the cover glass  601 . Each of the PPSM  10 B,  40 B electrically couples to a control unit for triggering functions of the mobile electronic device  60 . The PPSM  10 B,  40 B senses the pressure force applied on the cover glass  601  by a user and transmits a corresponding signal to the control unit when the cover glass  601  is pressed. 
       FIG. 14  shows a second application embodiment of the PPSM according to the present invention. 
       FIG. 14  shows four pieces of concave bump  622  which can be one made of rubber or plastic is provided and each is inserted in between the cover glass  601  and one of the PPSMs  10 B, 40 B as an interface complementary material there between. 
       FIG. 15  shows a third application of the PPSM according to the present invention. 
       FIG. 15  shows four concave indentations  602  made on bottom surface of the cover glass  601 ; each of the concave indentations matches one of the PPSMs  10 B,  40 B underside. 
       FIG. 16  shows a fourth application of the PPSM according to the present invention. 
       FIG. 16  shows a first strip of the PPSM  40 C is configured on the left side under the cover glass  601 , and a second strip of the PPSM  40 C is configured on the right side under the cover glass  601 . Each of the PPSMs  40 C is configured on a piece of frame  63  of the mobile electronic device  60 . An alternative is that the first strip of the PPSM is configured on the top side under the cover glass  601 , and the second strip of the PPSM is configure of bottom side under the cover glass  601 . 
       FIG. 17  shows a fifth application of the PPSM according to the present invention. 
       FIG. 17  shows a first concave indentation  602 , made on the bottom surface of the cover glass  601  for matching the first strip of the convex PPSM  40 C, and a second concave indentation  602 , made on the bottom surface of the cover glass  601  for matching the second strip of convex PPSM  40 C. 
       FIG. 18  shows a sixth application of the PPSM according to the present invention. 
       FIG. 18  shows a display module  61  is configured under a cover glass  601  within a housing frame  62  of a mobile electronic device  60 ; and two PPSMs  10 B, 40 B, 40 C are configured under the display module  61  in the section view. The PPSMs  10 B, 40 B, 40 C senses forces pressed by a user against the cover glass  601  of the mobile electronic device  60 . 
       FIG. 19  shows a seventh application of the PPSM according to the present invention. 
       FIG. 19  shows a portable electronic device having a cover glass  601 , a display module  61  configured under the cover glass  601 , a space  633  reserved between the cover glass  601  and the display module  61 ; and two PPSMs  10 B, 40 B, 40 C, one configured on the left side under the cover glass  601  in a section view, and the other one configured on the right side under the cover glass  601  in a section view. The display module  61  is configured in between the two PPSMs  10 B, 40 B,  40 C in a section view. The PPSM senses pressure forces pressed by a user against to the cover glass  601  of the mobile electronic device  60 . 
       FIG. 20  shows an eighth application of the PPSM according to the present 
       FIG. 20  is similar to  FIG. 19  only that the two PPSMs  10 B, 40 B, 40 C, are upside down configured in the section view. 
       FIG. 21  shows a ninth application of the PPSM according to the present invention. 
       FIG. 21  shows a mobile electronic device having a U shaped frame  62 , a compound display module  70  configured in the open top area of the frame  62 , the compound display module  70  has touch sensors integrated inside; a first PPSM  10 B, 40 B, 40 C configured under the module  70  in the left side, and a second PPSM  10 B, 40 B,  40 C configured under the module  70  in the right side in the section view of  FIG. 20 . The PPSM senses pressure forces pressed by a user against the module  70  of the mobile electronic device  60 . 
       FIG. 22  shows a process to prepare a third PPSM according to the present invention. 
       FIG. 22  shows a flat piezoresistive sensor device  10  is prepared, and then the flat piezoresistive sensor device  10  is pressed and mounted onto a concave base  80 , the base  80  has a concave surface  801 . The flat piezoresistive sensor device  10  is bent and attached onto the base  80  along the profile of the curved surface  801 . 
       FIG. 23A  shows the third PPSM prepared according to  801 . 
       FIG. 23A  shows a concave PPSM consisting of a piezoresistive sensor device configured on and along the concave surface  801  such that the piezoresistive sensor device is bent to form a concave PPSM  80 B. 
       FIG. 23B  shows a perspective view of the PPSM of  FIG. 23A . 
       FIG. 24  shows a process to prepare a fourth PPSM according to the present invention. 
       FIG. 24  shows, firstly, preparing a flat pressure sensor  40  which has a stack of five layers of materials, sequentially from top to bottom, a top substrate  411 , a top electrode layer  412 , a single piece of piezoresistive material  413 , a bottom electrode layer  422  and a bottom substrate  421 ; secondly, preparing a base  80 , the base  30  has a concave surface  801  on top and flat on bottom; and thirdly, configuring the pressure sensor  40  onto the surface of the base  30 . The pressure sensor  40  is bent and attached onto the base  80  along the profile of the curved surface  801  to form a concave PPSM  80 C. 
       FIG. 25A  shows a concave PPSM prepared according to  FIG. 24 . 
       FIG. 25B  shows a perspective view of the concave PPSM of  FIG. 25A . 
       FIG. 26  shows a tenth application equipped with a concave PPSM according to the present invention. 
       FIG. 26  shows a cover glass  601  of a mobile electronic device  60 , a first convex bump  603  configured under the cover glass  601  on the left side; a second convex bump  603  configured under the cover glass  601  on the right side; a first concave PPSM  80 C configured under the first convex bump  603 ; and a second concave PPSM  80 C configured under the second convex bump  603 . Each of the concave PPSMs  80 C electrically couples to a control unit to sense a force pressed by a user against the cover glass  601  for trigging a predetermined function. 
       FIG. 27  shows a second electrical characteristic for the PPSM according to the present invention. 
       FIG. 27  shows linear relationship for “Output Capacitance vs. Pressure” for the PPSM as disclosed in this invention. The output capacitance can also be used to trigger different functions with different output capacitance level. Referring to  FIG. 6A , the top electrode layer  112 , the top piezoresistive layer  113 , the bottom piezoresistive layer  123 , and the bottom electrode layer  122  are akin to a capacitor where the two piezoresistive layers  113 ,  123  could be replaced by dielectric layers to be sandwiched in between the two electrode layers  112 ,  122 . The pressure-sensitive capacitor in a state described for the PPSM  10 B ( FIG. 6A ) can also be applied to the remaining PPSM as disclosed in this invention such as PPSM  40 B ( FIG. 10 ), the PPSM of  FIGS. 11A ,  11 B, and the PPSM  40 C of  FIG. 16 . 
     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.