Abstract:
A pressure sensor is provided, wherein a ballast resistive layer is integrated in the pressure sensor so that the resistive output curve for the pressure sensor has saturation characteristics. The pressure sensor shall be prevented from breaking down by a large current that may be caused, when an overload pressure is applied on the pressure sensor, if no ballast resistive layer is added.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Provisional Patent Application Ser. No. 61/238,559, filed on Aug. 31, 2009, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates in general to a pressure sensor and in particular to a pressure sensor having a ballast resistive element. 
     2. Description of the Related Art 
     Referring to  FIG. 1 , a conventional pressure sensor comprises two substrates  10  and  109  disposed on the top and bottom sides therefore. Two metal electrodes  11  and  119  are disposed on the substrates  10  and  109 , and two piezoresistive layers  12  and  129  are disposed on the metal electrodes  11  and  119  with a space  16  formed therebetween. Additionally, a spacer  15  is disposed between the substrates  10  and  109 , wherein a part of the spacer  15  is extended between the piezoresistive layers  12  and  129  to form the space  16 . As shown in  FIG. 1 , the metal electrodes  11  and  119  are electrically connected to a circuit system C. When no pressure is applied to the pressure sensor, the sensing circuit is open. When a pressure P is applied to the pressure sensor, as shown in  FIG. 2 , the piezoresistive layers  12  and  129  contact each other and form a closed circuit, thus enabling pressure measurement. 
     Since the piezoresistive layers  12  and  129  are made of piezoresistive material, they can have small resistance when deformed by external pressures. In the conventional pressure sensor, output resistance of the pressure sensor decreases with the increase of the pressure P. 
     According to Ohm&#39;s law (V=IR), the output resistance of the pressure sensor dominates the output current. Hence, the current I will increase when the voltage V is fixed with the decrease of the pressure P. However, when an overload pressure is applied, the linear pressure sensor may have a very small resistance that results in excessive output current. Thus, the circuit system can be damaged by the current. 
     As depicted in  FIG. 2 , when the pressure P is exerted on the pressure sensor, the substrate  10 , the metal electrode  11 , and the piezoresistive layer  12  are deformed downwardly, wherein the piezoresistive layers  12  and  129  contact each other. Output resistance of the piezoresistive layers  12  and  129  is determined by the height h thereof. 
     Resistance output of a piezoresistive sensor can be calculated by the formula R=p*L/A, wherein R is the electrical resistance output of the piezo resistive sensor (measured in ohms, Ω), L is the total thickness of the pressed piezoresistive layers (measured in centimeters, cm), and A is the pressed area applied over the piezoresistive sensor (measured in square centimeters, cm 2 ). 
     BRIEF SUMMARY OF INVENTION 
     The application provides a pressure sensor with a ballast resistive layer integrated therein, so that the resistive output curve for the pressure sensor has saturation characteristics. The pressure sensor shall be prevented from breaking down by a large current that may be caused, when an overload pressure is applied on the pressure sensor, if no ballast resistive layer is added. 
     An embodiment of the application provides a pressure sensor comprising a first ballast resistive layer, a first piezoresistive layer connected to the ballast resistive layer, a second piezoresistive layer, a first electrode layer connected to the second piezoresistive layer, and a spacer disposed between the first and second piezoresistive layers to form a space therebetween. Specifically, the pressure sensor can be applied to a boxing machine for boxing punching. The boxing machine may comprise a boxing target with the pressure sensor disposed thereon and a holder for fixing the boxing target. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS. 1 and 2  are perspective diagrams of a pressure sensor; 
         FIG. 3  is a perspective diagram of the resistivities of different materials 
         FIG. 4  is a perspective diagram of a pressure sensor according to a first embodiment of the invention; 
         FIG. 5  is a perspective diagram of a pressure sensor according to a second embodiment of the invention; 
         FIG. 6  is a perspective diagram of a pressure sensor according to a third embodiment of the invention; 
         FIG. 7  is a perspective diagram of a pressure sensor according to a fourth embodiment of the invention; 
         FIG. 8  is an embodiment of a pressure sensor based on the structure of  FIG. 4 ; 
         FIG. 9  is an embodiment of a pressure sensor based on the structure of  FIG. 6 ; 
         FIG. 10  is another embodiment of a pressure sensor based on the structure of  FIG. 6 ; 
         FIG. 11  illustrates a resistance-pressure diagram of the pressure sensor of the invention compared with the prior art; and 
         FIG. 12  is a perspective diagram of a boxing machine according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     As a ballast resistive layer is applied to a piezoresistive pressure sensor, no excessive current shall be output even if an over pressure is applied on the piezoresistive pressure sensor. 
     Referring to  FIG. 3 , resistivity of conventional conductive materials, such as metal, is between 10 −6  to 10 −4  Ω-cm. The semiconductor is between 10 −4  to 10 3  Ω-cm, the semiinsulator is between 10 3  to 10 10  Ω-cm, and the insulator exceeds 10 10  Ω-cm. As the semiconductor and semiinsulator have a resistivity between 10 −4  to 10 10  Ω-cm, they can be used as resistance materials, such as carbon paste, silver paste, metal oxide, nanoparticle, nanowire, nanosheet, nanorod, nanobelt, or carbon nanotube. 
     Referring to  FIG. 4 , a first embodiment of a pressure sensor comprises two substrates  21  and  219  disposed on the top and bottom sides thereof. Two metal electrodes  11  and  119  are respectively disposed on the substrates  21  and  219 . As shown in  FIG. 4 , a piezoresistive layer  239  is disposed on the metal electrode  219 , and a ballast resistive layer  22  is disposed between the electrode  11  and a piezoresistive layer  23 , thus forming a sandwich structure. A space  16  is formed between the piezoresistive layers  23  and  239 . A spacer  15  is disposed between the substrates  21  and  219 , wherein a part of the spacer  15  is extended between the piezoresistive layers  21  and  219  to form the space  16 . Furthermore, the metal electrodes  11  and  119  are electrically connected to a circuit system C. 
       FIG. 5  illustrates a second embodiment of a pressure sensor. Comparing  FIG. 5  with  FIG. 4 , the electrode  11  is removed from the pressure sensor. In this configuration, the ballast resistive layer  22  acts as one of the electrodes and electrically connect to the control system C. 
       FIG. 6  illustrates a third embodiment of a pressure sensor. Comparing  FIG. 6  with  FIG. 4 , the pressure sensor in  FIG. 6  comprises two ballast resistive layers  22  and  229  respectively connected to the electrodes  11  and  119 . When the ballast resistive layers  22  and  229  have a high resistance, the two ballast resistive layers  22  and  229  and the metal electrodes  11  and  119  can uniformly disperse the current density. 
       FIG. 7  illustrates a fourth embodiment of a pressure sensor. Comparing  FIG. 7  with  FIG. 4 , the electrodes  11  and  119  are removed from the pressure sensor. In this configuration, the upper ballast resistive layer  22  is electrically connected to the control system C. The lower ballast resistive layer  229  is also electrically connected to the control system C and disposed between the substrate  219  and the piezoresistive layer  239 . In some embodiments, the ballast resistive layers  22  and  229  may comprise carbon paste, carbon/silver paste, metal oxide, nanoparticle, nanowire, nanosheet, nanorod, nanobelt, or carbon nanotube, wherein the resistivity of the ballast resistive layers  22  and  229  is between 10 −4 ˜10 10 Ω-cm. 
       FIG. 8  illustrates an embodiment of a pressure sensor based on the structure of  FIG. 4 , wherein the numeral  32  represents a ballast resistive layer of carbon paste which has a resistivity of 3.2*10 6  Ω-cm. In this embodiment, resistance of the piezoresistive layers  33  and  339  varies from 10 4 Ω to 10 2 Ω with the pressure increased. When no pressure is applied, the piezoresistive layers  33  and  339  respectively have a resistance of 10 4 Ω. When an overload pressure is applied, the piezoresistive layers  33  and  339  respectively have a minimum resistance of 10 2 Ω. The ballast resistive layer  32  has 10 um thickness and 1 cm 2  area, and resistance of the ballast resistive layer  32  is determined by the following formula:
 
 R =(3.2*10 6  Ω-cm*10 μm)/1 cm 2 =3.2*10 3 Ω
 
     Total series resistance of the pressure sensor can be calculated by summing the resistances of the ballast resistive layer  32  and the piezoresistive layers  33  and  339 , wherein the resistance of the metal electrodes  11  and  119  can be ignored. The resistance of the space  16  is also ignored because the piezoresistive layers  33  and  339  contact each other and eliminate the space  16  when the pressure is applied to the pressure sensor. 
     In this embodiment, since the ballast resistive layer  32  of carbon paste has a fixed resistance 3.2*10 3 Ω, and the piezoresistive layers  33  and  339  have a variable resistance, total series resistance of the pressure sensor exceeds 3.2*10 3 Ω. Even if an overload pressure is applied to the pressure sensor, output resistance of the pressure sensor can be kept above 3.2*10 3 Ω. 
       FIG. 9  illustrates another embodiment of a pressure sensor based on the structure of  FIG. 6 , wherein the numerals  32  and  329  represent two ballast resistive layers of carbon paste which has a resistivity of 3.2*10 6  Ω-cm. In this embodiment, resistance of the piezoresistive layers  33  and  339  varies from 10 4 Ω to 10 2 Ω with the pressure increased. When no pressure is applied, the piezoresistive layers  33  and  339  respectively have a resistance of 10 4 Ω. When an overload pressure is applied, the piezoresistive layers  33  and  339  respectively have a minimum resistance of 10 2 Ω. Specifically, when the pressure is less than 6 KPa (pressure threshold), resistance of the piezoresistive layers  33  and  339  varies inversely with respect to the pressure, as the line A-B shown in  FIG. 11 . In this embodiment, the ballast resistive layer  32  has 10 um thickness and 1 cm 2  area, and resistance of the ballast resistive layers  32  and  329  is determined by the following formula:
 
 R =(3.2*10 6  Ω-cm*10 μm)/1 cm 2 =3.2*10 3 Ω
 
     Total series resistance of the pressure sensor can be calculated by summing the resistances of the ballast resistive layers  32 ,  329  and the piezoresistive layers  33  and  339 . In this embodiment, each of the ballast resistive layers  32  and  329  has a resistance of 3.2*10 3 Ω, and total resistance of the ballast resistive layers  32  and  329  is 6.4*10 3 Ω. Hence, total series resistance of the pressure sensor definitely exceeds 6.4*10 3 Ω. Even if an overload pressure is applied to the pressure sensor, output resistance of the pressure sensor can be maintained above 6.4*10 3 Ω. 
       FIG. 10  illustrates another embodiment of a pressure sensor based on the structure of  FIG. 6 , wherein the numeral  32  represents a ballast resistive layer of carbon paste which has a resistivity of 3.2*10 6  Ω-cm, and the numeral  429  represents a ballast resistive layer of silver paste which have a resistivity of 4.5*10 −3  Ω-cm. In this embodiment, resistance of the piezoresistive layers  33  and  339  varies from 10 4 Ω to 10 2 Ω with the pressure increased. When no pressure is applied, the piezoresistive layers  33  and  339  respectively have a resistance of 10 4 Ω. When an overload pressure is applied, the piezoresistive layers  33  and  339  respectively have a minimum resistance of 10 2 Ω. Specifically, when the pressure is less than 10 KPa (pressure threshold), resistance of the piezoresistive layers  33  and  339  is in linear relation to the pressure, as the line A-C shown in  FIG. 11 . In this embodiment, the ballast resistive layer  32  has 10 um thickness and 1 cm 2  area, and resistance of the ballast resistive layer  32  of carbon paste is determined by the following formula:
 
 R =(3.2*10 6  Ω-cm*10 μm)/1 cm 2 =3.2*10 3 Ω
 
     Similarly, the ballast resistive layer  429  of silver paste has 10 um thickness and 1 cm 2  area. Resistance of the ballast resistive layer  429  is determined by the following formula:
 
 R =(4.5*10 −3  Ω-cm*10 μm)/1 cm 2 =4.5*10 −6 Ω
 
     Total series resistance of the pressure sensor can be calculated by summing the resistances of the ballast resistive layers  32 ,  429  and the piezoresistive layers  33  and  339 . In this embodiment, the ballast resistive layer  32  has a resistance of 3.2*10 3 Ω, and the ballast resistive layer  429  has a resistance of 4.5*10 −6 Ω. Hence, total series resistance of the pressure sensor exceeds 3.2*10 3 Ω. Even if an overload pressure is applied to the pressure sensor, output resistance of the pressure sensor can be maintained above 3.2*10 3 Ω, as the line A-B shown in  FIG. 11 . 
     Referring to  FIG. 11 , line A-D represents variable resistance of the piezoresistive layers  19  and  129  shown in  FIG. 1 , wherein the resistance is in linear relation to the pressure. According to the line A-D in  FIG. 11 , the resistance is about 1.3*10 5 Ω when the pressure is 4.4*10 −1  KPa, and the resistance is about 1*10 2 Ω when the pressure is 3*10 2  KPa. The resistance always decreases with the increase of the pressure. 
     Comparing with the line A-D of the conventional pressure sensor, line A-C represents the output resistance of the pressure sensor in  FIGS. 8 and 10 . When the pressure is applied from 4.4*10 −1  KPa−1.1*10 0  KPa, the output resistance linearly decreases with the increase of the pressure. When the pressure exceeds 10 KPa, the output resistance is maintained above 3.2*10 3 Ω. 
     Line A-B in  FIG. 11  represents the output resistance of the pressure sensor in  FIG. 9 . When the pressure is applied from 4.4*10 −1  KPa−6*10 0  KPa, the output resistance linearly decreases with the increase of the pressure. When the pressure exceeds 6 KPa, the output resistance is limited and saturated at a fixed value about 6.4*10 3 Ω. 
     Since the output resistance of the pressure sensor has a saturation characteristic, it can be used in a boxing machine to protect the system from excessive pressures. As shown in  FIG. 12 , the boxing machine includes boxing target of a pressure sensor S mounted on a holder H and electrically connected to a circuit unit U, wherein the circuit unit U connects to a screen N to show the pressure value. When an excessive force is exerted on the pressure sensor S, the output current can be limited to prevent the circuit unit U from failure or damage. According to  FIG. 11 , line A-B is suitable for a device applied in a pressure range of 4.4*10 −1  KPa˜6*10 0  KPa, and line A-B is suitable for a device applied in a pressure range of 4.4*10 −1  KPa 10 1  KPa. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation to encompass all such modifications and similar arrangements.