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'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, cm2).
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 103 Ω-cm, the semiinsulator is between 103 to 1010 Ω-cm, and the insulator exceeds 1010 Ω-cm. As the semiconductor and semiinsulator have a resistivity between 10−4 to 1010 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˜1010Ω-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*106 Ω-cm. In this embodiment, resistance of the piezoresistive layers 33 and 339 varies from 104Ω to 102Ω with the pressure increased. When no pressure is applied, the piezoresistive layers 33 and 339 respectively have a resistance of 104Ω. When an overload pressure is applied, the piezoresistive layers 33 and 339 respectively have a minimum resistance of 102Ω. The ballast resistive layer 32 has 10 um thickness and 1 cm2 area, and resistance of the ballast resistive layer 32 is determined by the following formula:
R=(3.2*106 Ω-cm*10 μm)/1 cm2=3.2*103Ω
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*103Ω, and the piezoresistive layers 33 and 339 have a variable resistance, total series resistance of the pressure sensor exceeds 3.2*103Ω. Even if an overload pressure is applied to the pressure sensor, output resistance of the pressure sensor can be kept above 3.2*103Ω.
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*106 Ω-cm. In this embodiment, resistance of the piezoresistive layers 33 and 339 varies from 104Ω to 102Ω with the pressure increased. When no pressure is applied, the piezoresistive layers 33 and 339 respectively have a resistance of 104Ω. When an overload pressure is applied, the piezoresistive layers 33 and 339 respectively have a minimum resistance of 102Ω. 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 cm2 area, and resistance of the ballast resistive layers 32 and 329 is determined by the following formula:
R=(3.2*106 Ω-cm*10 μm)/1 cm2=3.2*103Ω
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*103Ω, and total resistance of the ballast resistive layers 32 and 329 is 6.4*103Ω. Hence, total series resistance of the pressure sensor definitely exceeds 6.4*103Ω. Even if an overload pressure is applied to the pressure sensor, output resistance of the pressure sensor can be maintained above 6.4*103Ω.
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*106 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 104Ω to 102Ω with the pressure increased. When no pressure is applied, the piezoresistive layers 33 and 339 respectively have a resistance of 104Ω. When an overload pressure is applied, the piezoresistive layers 33 and 339 respectively have a minimum resistance of 102Ω. 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 cm2 area, and resistance of the ballast resistive layer 32 of carbon paste is determined by the following formula:
R=(3.2*106 Ω-cm*10 μm)/1 cm2=3.2*103Ω
Similarly, the ballast resistive layer 429 of silver paste has 10 um thickness and 1 cm2 area. Resistance of the ballast resistive layer 429 is determined by the following formula:
R=(4.5*10−3 Ω-cm*10 μm)/1 cm2=4.5*10−6Ω
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*103Ω, and the ballast resistive layer 429 has a resistance of 4.5*10−6Ω. Hence, total series resistance of the pressure sensor exceeds 3.2*103Ω. Even if an overload pressure is applied to the pressure sensor, output resistance of the pressure sensor can be maintained above 3.2*103Ω, 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*105Ω when the pressure is 4.4*10−1 KPa, and the resistance is about 1*102Ω when the pressure is 3*102 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*100 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*103Ω.
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*100 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*103Ω.
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*100 KPa, and line A-B is suitable for a device applied in a pressure range of 4.4*10−1 KPa 101 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.
Patent Grant
8210994
