Abstract:
A liquid level detecting apparatus is provided. A resistance plate has a plurality of elongated conductive segments arranged therein. A float is configured to be moved upward and downward depending on a displacement of a liquid level to be measured. A floating arm has one end attached to the float and the other end rotatably supported to be rotated in response to upward and downward movements of the float. A contact is configured to be slid on the plurality of conductive segments in association with a rotation of the float arm depending on the liquid level. The plurality of conductive segments comprise a plurality of first segments made of a first metal material containing at least silver (Ag) and palladium (Pd), and a plurality of second segments made of a second metal material essentially containing gold (Au).

Description:
The disclosure of Japanese Patent Application No. 2011-012180 filed on Jan. 24, 2011 and Japanese Patent Application No. 2011-095301 filed on Apr. 21, 2011, including specifications, drawings and claims is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The invention relates to a liquid level detecting apparatus, and more particularly to a liquid level detecting apparatus for automatically detecting a residual amount of a liquid stored in a fuel tank for transport means, such as automobiles and airplanes, by detecting a liquid level thereof. 
     There are previously known a liquid level detecting apparatus for detecting a liquid level in a fuel tank, for example, of automobiles, in which a float moves upward and downward depending on the liquid level such that a floating arm is slid on a resistance plate, and thus the liquid level is converted into an electrical potential difference, thereby detecting the liquid level. 
     Herein, an example of the liquid level detecting apparatus will be described.  FIG. 1  is an electric block diagram illustrating a configuration of a sensor used in the liquid level detecting apparatus according to the invention and the related art.  FIG. 2  is a diagram illustrating a configuration of the liquid level detecting apparatus according to the invention and the related art.  FIG. 3  is a diagram illustrating a configuration of a variable resistor in the sensor according to the invention and the related art. 
     The sensor  2  of the liquid level detecting apparatus  1  includes a variable resistor  3  which changes a resistance value by allowing contacts  19  and  20 , as described below, to move in association with a change in liquid level within a hermetically sealed vessel inside T. The variable resistor  3  is connected in series to a fixed resistor  7 , and in turn connected to a power supply circuit  4 , which applies a predetermined voltage to the variable resistor  3  and the fixed resistor  7   
     The sensor  2 , as shown  FIGS. 2 and 3 , includes a resistance plate  13  attached to a body frame  12 , and a sliding contact element  14  connected to a proximal end of a floating arm  11 , which has also a distal end attached to a float  10  configured to float on a surface of a liquid by buoyancy relative to the liquid. The resistance plate  13  of the sensor  2  is provided with a first conductive pattern  15  and a second conductive pattern  16 . These first and second conductive patterns  15  and  16  are arranged in parallel to each other in an arc-like shape about a rotational axis  21  of the floating arm  11 . An input/output conductive portion  17  is connected to one end of the first conductive pattern  15 , and an input/output conductive portion  18  is connected to one end of the second conductive pattern  16 . 
     The first conductive pattern  15  consists of a plurality of elongated conductive segments  15   a  arranged in a circumferential direction of the arc-like shape at a predetermined interval and a resistance element  15   b  electrically connecting the conductive segments  15   a  to each other. Also, the second conductive pattern  16  consists of a plurality of elongated conductive segments  16   a  arranged in a circumferential direction of the arc-like shape at a predetermined interval and a connecting element  16   b  electrically connecting the conductive segments  16   a  to each other. 
     The sliding contact element  14  is provided with the contacts  19  and  20  electrically connected to each other. Also, the rotational axis  21  located on the proximal end of the floating arm  11  is connected to the sliding contact element  14 . The floating arm  11  pivots about the rotational axis  21  as a supporting point in a arrow Y direction in  FIG. 3  by allowing the float  10 , which floats on the surface of the liquid, to downwardly move according to an amount of the liquid consumed from a liquid level in full tank condition. In response to such a pivoting of the floating arm  11 , the sliding contact element  14  also rotates in the arrow Y direction in  FIG. 3 . By such a rotation of the sliding contact element  14 , each of the contacts  19  and  20  slide on and contact electrically with each of the conductive segments  15   a  and  16   a  respectively disposed on the first conductive pattern  15  and the second conductive pattern  16 . As a result, a length of the resistance element  15   b  interposed in a circuit between the input/output conductive portion  17  connected to the first conductive pattern  15  and the input/output conductive portion  18  connected to the second conductive pattern  16  is changed, and thus a resistance value of the circuit is changed (i.e., the resistance value of the variable resistor  3  in  FIG. 1  is changed). As described above, the variable resistor  3  consists of the first conductive pattern  15 , the second conductive pattern  16 , and the sliding contact element  14 . 
     An electrical potential difference between the input/output conductive portions  17  and  18  caused when a voltage is applied to the variable resistor  3  is detected by the sensor  2 , and an output signal of the sensor  2  is sent to a processing circuit  5 . Then, the processing circuit  5  displays a residual amount of the liquid based on the output signal of the sensor  2  on an indication device, such as a gauge  6 , in an analogue or bar graph manner. Meanwhile, a fixed resistor may be disposed in the gauge  6  on a wire for connecting the gauge  6  to the processing circuit  5 . 
     In such a liquid level detecting apparatus, silver-palladium (AgPd) alloy, silver-copper (AgCu) alloy, silver-nickel (AgNi) alloy, and the like are generally used as the material of the contacts. The conductive segments are made of a mixture of silver-palladium (AgPd) powder and glass, for example, and are manufactured by mixing silver powder, palladium powder, and glass powder to form a paste, printing the obtained paste on the resistance plate, drying, and then sintering. 
     However, the liquid level detecting apparatus can be used in a fuel tank of an automobile using as a fuel an electrolyte (alcohol) itself, such as ethanol and methanol, or gasoline containing such an electrolyte. Silver (Ag) has a lower electrical resistance and an excellent conductivity, but the contacts and the conductive segments containing such silver can be deteriorated or eroded by a sulfur component, water, an alcohol component and the like in the fuel, thereby causing an impediment in which measuring cannot be performed or an incorrect value can be created, etc., due to a poor electrical conduction. Also, due to a present world fuel situation, a possibility of using various mixed fuels is being increased, and thus, it is necessary to provide a reliable fuel system which can prevent such an impediment. Therefore, to prevent the deterioration and erosion of the conductive segments and the contacts, there are known technologies in which portions of the conductive segments, on which the contacts are slid, are coated with an alloy containing gold (Au)(e.g., see Patent Documents 1 and 2). 
     Patent Document 1: JP-A-2003-287456 
     Patent Document 2: JP-A-2009-162694 
     The technologies according to the Patent Documents 1 and 2 has an effect to deterioration and erosion resistances of the conductive segments, but tend not to provide an sufficient effectiveness of the effect because the coated layer can be thinned over time. In addition, to obtain a sufficient deterioration and erosion resistances, it is necessary to contain a large amount of gold (e.g., approximately 40% mass or more in case of the conductive segments), thereby causing a problem of increasing costs. 
     SUMMARY 
     It is therefore an object of the present invention to provide a liquid level detecting apparatus in which, even when used in the presence of a sulfur component, such as gasoline, as well as in a normal environment, the deterioration and erosion resistances can be obtained, and manufacturing costs thereof can be inhibited. 
     That is to say, the object of the invention is achieved by the following (1) to (6). 
     (1) A liquid level detecting apparatus, comprising: a resistance plate having a plurality of elongated conductive segments arranged therein; a float configured to be moved upward and downward depending on a displacement of a liquid level to be measured; a floating arm having one end attached to the float and the other end rotatably supported to be rotated in response to upward and downward movements of the float; and a contact configured to be slid on the plurality of conductive segments in association with a rotation of the float arm depending on the liquid level, wherein the plurality of conductive segments comprise a plurality of first segments made of a first metal material containing at least silver (Ag) and palladium (Pd), and a plurality of second segments made of a second metal material essentially containing gold (Au). 
     (2) The liquid level detecting apparatus according to (1), wherein a first segment group comprising at least one first segment and a second segment group comprising at least one second segment are alternatively arranged to each other. 
     (3) The liquid level detecting apparatus according to (1) or (2), wherein the first segments include base portions contacted to the resistance plate, and top portions respectively stacked over the base portions, and wherein the base portions are made of the first metal material, and the top portions are made of the second metal material. 
     (4) The liquid level detecting apparatus according to any one of (1) to (3), wherein the second metal material contains gold (Au) in a range of 95% by mass or more. 
     (5) The liquid level detecting apparatus according to any one of (1) to (4), wherein the second metal material contains a glass component. 
     (6) The liquid level detecting apparatus according to any one of (1) to (5), wherein the first metal material contains gold (Au). 
     According to the invention, the plurality of conductive segments constituting the resistance plate include a plurality of first segments made of a first metal material containing at least silver (Ag) and palladium (Pd), and a plurality of second segments made of a second metal material essentially containing gold (Au). Therefore, the liquid level detecting apparatus having sufficient deterioration and erosion resistances even when used in gasoline containing a large amount of a sulfur component or in a fuel having various components mixed therein can be provided. Specifically, the conductive segments and the contacts are subjected to a very small abrasion by sliding to each other, and thus the first metal material can be just dropped down, while gold (Au) separated by an abrasion of the second metal material is transferred to sliding surfaces. Namely, gold (Au) separated by an abrasion of the second segments is transferred to sliding surfaces of the first segments and the contacts. The transferring of gold (Au) is occurred between the conductive segments and the contacts. Because gold (Au) is transferred to the sliding surfaces, a sulfide deterioration by a sulfur component, an erosion, an oxidation, and the like can be prevented. Therefore, contact conductivity between the conductive segments and the contacts can be better remained, such that a contacting impediment can be prevented. Furthermore, gold (Au) is not necessary used for the whole of the conductive segments, thereby reducing an amount of gold used relative to that of the related art. Thus, material costs can be decreased, thereby decreasing the manufacturing costs of the liquid level detecting apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is an electric block diagram illustrating a configuration of a sensor used in a liquid level detecting apparatus according to the invention and the related art; 
         FIG. 2  is a diagram illustrating a configuration of the liquid level detecting apparatus according to the invention and the related art; 
         FIG. 3  is a diagram illustrating a configuration of a variable resistor in the sensor according to the invention and the related art; 
         FIGS. 4A and 4B  are diagrams illustrating a configuration of conductive segments according to a first embodiment of the liquid level detecting apparatus of the invention, wherein  FIG. 4A  is a partially enlarged view of the conductive segments, and  FIG. 4B  is a cross-sectional view taken along a line IVa-IVa in  FIG. 4A ; and 
         FIGS. 5A and 5B  are diagrams illustrating a configuration of conductive segments according to a second embodiment of the liquid level detecting apparatus of the invention, wherein  FIG. 5A  is a partially enlarged view of the conductive segments, and  FIG. 5B  is a cross-sectional view taken along a line Va-Va in  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, embodiments of the invention will be described in detail. A basic structure of a liquid level detecting apparatus of the invention was described in detail in the related art section with reference to  FIGS. 1 ,  2 , and  3 , but will be again described. 
     As shown in  FIG. 1 , a sensor  2  of the liquid level detecting apparatus  1  includes a variable resistor  3  which changes a resistance value by allowing contacts  19  and  20 , as described below, to move in association with a change in liquid level within a hermetically sealed vessel inside T. The variable resistor  3  is connected in series to a fixed resistor  7 , and in turn connected to a power supply circuit  4 , which applies a predetermined voltage to the variable resistor  3  and the fixed resistor  7 . 
     The sensor  2 , as shown  FIGS. 2 and 3 , includes a body frame  12 , a resistance plate  13  attached to the body frame  12 , and a sliding contact element  14 . The sliding contact element  14  is connected to a proximal end of a floating arm  11 , which has also a distal end attached to a float  10  configured to float on a surface of a liquid by buoyancy relative to the liquid. The resistance plate  13  of the sensor  2  is provided with a first conductive pattern  15  and a second conductive pattern  16 . These first and second conductive patterns  15  and  16  are arranged in parallel to each other in an arc-like shape about a rotational axis  21  of the floating arm  11 . An input/output conductive portion  17  is connected to one end of the first conductive pattern  15  on one side, and an input/output conductive portion  18  is connected to one end of the second conductive pattern  16  on other side. 
     The first conductive pattern  15  consists of a plurality of elongated conductive segments  15   a  arranged in a circumferential direction of the arc-like shape at a predetermined interval and a resistance element  15   b  electrically connecting the conductive segments  15   a  to each other. In addition, the second conductive pattern  16  consists of a plurality of elongated conductive segments  16   a  arranged in a circumferential direction of the arc-like shape at a predetermined interval and a connecting element  16   b  electrically connecting the conductive segments  16   a  to each other. The first conductive pattern  15  and the second conductive pattern  16  are spaced to each other. 
     The sliding contact element  14  has two frames concentrically arranged about the proximal end of the floating arm  11 . These two frames are respectively provided with the contacts  19  and  20  electrically connected to each other. Also, the rotational axis  21  located on the proximal end of the floating arm  11  is connected to the sliding contact element  14 . 
     The floating arm  11  pivots about the rotational axis  21  as a supporting point in a arrow Y direction in  FIG. 3  by allowing the float  10 , which floats on the surface of the liquid, to downwardly move according to an amount of the liquid consumed from a liquid level in full tank condition. In response to such a pivoting of the floating arm  11 , the sliding contact element  14  also rotates in the arrow Y direction in  FIG. 3 . By such a rotation of the sliding contact element  14 , the contact  19  slides on and contacts electrically with the conductive segments  15   a  disposed on the first conductive pattern  15 , the contact  20  slides on and contacts electrically with the conductive segments  16   a  disposed on the second conductive pattern  16 . As a result, a length of the resistance element  15   b  interposed in a circuit between the input/output conductive portion  17  connected to the first conductive pattern  15  and the input/output conductive portion  18  connected to the second conductive pattern  16  is changed, and thus a resistance value of the circuit is changed (i.e., the resistance value of the variable resistor  3  in  FIG. 1  is changed). As described above, the variable resistor  3  consists of the first conductive pattern  15 , the second conductive pattern  16 , and the sliding contact element  14 . 
     An electrical potential difference between the input/output conductive portions  17  and  18  caused when a voltage is applied to the variable resistor  3  is detected by the sensor  2 , and an output signal of the sensor  2  is sent to a processing circuit  5 . Then, the processing circuit  5  displays a residual amount of the liquid based on the output signal of the sensor  2  on an indication device, such as a gauge  6 , in an analogue or bar graph manner. Meanwhile, a fixed resistor may be disposed in the gauge  6  on a wire for connecting the gauge  6  to the processing circuit  5 . 
     According to the invention, the plurality of conductive segments  15   a  and  16   a  arranged as described above include a plurality of first segments made of a first metal material containing at least silver (Ag) and palladium (Pd), and a plurality of second segments made of a second metal material essentially containing gold (Au). 
       FIGS. 4A and 4B  are diagrams illustrating a configuration of conductive segments according to a first embodiment of the liquid level detecting apparatus of the invention, wherein  FIG. 4A  is a partially enlarged view of the conductive segments, and  FIG. 4B  is a cross-sectional view taken along a line IVa-IVa in  FIG. 4A . 
     According to the first embodiment of the invention, the plurality of conductive segments  15   a  forming the first conductive pattern  15  provided on the resistance plate  13  include a plurality of first segments  152  made of a first metal material containing at least silver (Ag) and palladium (Pd), and a plurality of second segments  154  made of a second metal material essentially containing gold (Au). As shown in  FIGS. 4A and 4B , the first segments  152  and the second segments  154  are arranged generally in parallel and alternatively to each other, and form an arc-like shape as a whole (see  FIG. 3 ). 
     When the first metal material contains at least silver (Ag) and palladium (Pd), a hardness of the first segments  152  can be increased to obtain a wear resistance, and can form a conductive pattern having an excellent conductivity. 
     According to the invention, gold (Au) may be added to the first metal material. By such an addition of gold (Au), the liquid level detecting apparatus having higher deterioration and erosion resistances against a fuel, such as gasoline, can be achieved. 
     Meanwhile, if the effects of the invention are not adversely affected, other metal materials may be added to the first metal material. The other metal materials includes, for example, cobalt (Co), nickel (Ni), ruthenium (Ru), copper (Cu), and platinum (Pt). The other metal materials may be used singly or as a combination of two or more metal materials. 
     The second metal material forming the second segments  154  essentially contains gold (Au). Gold (Au) is preferably contained in a range of 95% by mass or more, and more preferably in range of 98% by mass or more. 
     When gold (Au) is contained in a range of 95% by mass or more in the second metal material, the deterioration and erosion resistances can be significantly enhanced. 
     Preferably, the second metal material additionally contains a glass component. Because of the presences of the glass component, an effect of enhancing a hardness of the conductive segments can be achieved. The glass component includes, for example, borosilicate-lead glass and bismuth oxide glass. 
     According to the invention, if the effects of the invention are not adversely affected, other metal materials may be added to the second metal material. The other metal materials includes, for example, cobalt (Co), nickel (Ni), ruthenium (Ru), copper (Cu), and platinum (Pt). The other metal materials may be used singly or as a combination of two or more metal materials. The other metal materials are preferably contained in range of 5% by mass or less in the second metal materials. 
     Now, a method for forming the conductive pattern will be described. 
     Firstly, powders of the first metal material are mixed with a binder in a solvent to form a paste thereof. The paste is printed on the resistance plate  13  by any means, such as screen printing, and then drying. In this case, the first metal material prepared in the paste form is printed while remaining spaces at locations where the second segments will be formed. 
     Next, powders of the second metal material are mixed with a binder in a solvent to form a paste thereof. Similarly, the paste is printed on the locations in the resistance plate  13 , where the second segments will be formed, by any means, such as screen printing. Then, a dry processing is performed. 
     Finally, the whole of the resistance plate is sintered, and thus the conductive pattern having the first segments and the second segments arranged therein is formed. 
     According to the first embodiment of the invention, as described above, gold (Au) separated by an abrasion of the second segments  154  is transferred to sliding surfaces of the first segments  152 , such that a sulfide deterioration by a sulfur component, an erosion, an oxidation, and the like can be prevented. Therefore, contact conductivity between the conductive segments  15   a  and the contact  19  can be better remained, such that a contacting impediment can be prevented. In addition, the separated gold (Au) is also transferred to the contact  19  abraded by sliding, thereby also contributing to the preventing of the contacting impediment. Also, because the first segments  152  is made of the first metal material containing silver (Ag) and palladium (Pd), gold (Au) is not necessary used for the whole of the conductive segments  15   a , thereby reducing an amount of used gold relative to that of the related art. 
     Meanwhile, according to the invention, the conductive segments  16   a  forming the second conductive pattern  16  may similarly include a plurality of first segments made of a first metal material containing at least silver (Ag) and palladium (Pd), and a plurality of second segments made of a second metal material essentially containing gold (Au). In this case, the deterioration and erosion resistances of the liquid level detecting apparatus may be further enhanced, thereby inhibiting manufacturing costs thereof. 
       FIGS. 5A and 5B  are diagrams illustrating a configuration of conductive segments according to a second embodiment of the liquid level detecting apparatus of the invention, wherein  FIG. 5A  is a partially enlarged view of the conductive segments, and  FIG. 5B  is a cross-sectional view taken along a line Va-Va in  FIG. 5A . 
     Similarly, according to the second embodiment of the invention, the plurality of conductive segments  15   a  forming the first conductive pattern  15  provided on the resistance plate  13  include a plurality of first segments  152  and a plurality of second segments  154 . The first segments  152  and the second segments  154  are arranged generally in parallel and alternatively to each other, and form an arc-like shape as a whole (see  FIG. 3 ). 
     According to the second embodiment, as shown in  FIGS. 5A and 5B , the first segments  152  include base portions  157  contacted to the resistance plate  13 , and top portions  155  respectively stacked over the base portions  157 , when seen in a cross-sectional view perpendicular to a longitudinal direction of the first segments  152 . The base portions  157  are made of a first metal material containing at least silver (Ag) and palladium (Pd), and the top portions  155 , which can also contact with the contact, are made of a second metal material essentially containing gold (Au). 
     According to the second embodiment, for the first metal material and the second metal material, the same materials as those of the first metal material and the second metal material according to the first embodiment can be used. 
     According to a method for forming the conductive pattern of the second embodiment, firstly, powders of the first metal material are mixed with a binder in a solvent to form a paste thereof. The paste is printed on the locations in the resistance plate  13 , where the first segments  152  will be formed, by any means, such as screen printing. Then, a dry processing is performed. In this case, the first metal material is printed at an application thickness lower than a height of the first segments  152  when practically completed. 
     Next, powders of the second metal material are mixed with a binder in a solvent to form a paste thereof. Similarly, the paste is printed over top portions of the first metal material and on the locations, where the second segments  154  will be formed, by any means, such as screen printing. In this case, as shown in  FIG. 5B , the second metal material is preferably printed such that the first segments  152  and the second segments  154  each have an identical height a, when seen in the cross-sectional view perpendicular to the longitudinal direction of the first segments  152 . 
     Then, a dry processing is performed, and the whole of the resistance plate is sintered. As a result, the conductive pattern having the first segments and the second segments arranged therein is formed. 
     According to the second embodiment of the invention, as described above, gold (Au) separated by an abrasion of the second metal material forming the top portions  155  is transferred to sliding surfaces of the conductive segments  15   a  and the contact  19 , such that a sulfide deterioration by a sulfur component, an erosion, an oxidation, and the like can be prevented. Therefore, contact conductivity between the conductive segments  15   a  and the contact  19  can be better remained, such that a contacting impediment can be prevented. In addition, the separated gold (Au) is also transferred to the contact  19  abraded by sliding, thereby also contributing to the preventing of the contacting impediment. Also, because the base portions  157  are made of the first metal material, a wear resistance of the first segments  152  can be enhanced as a whole. Furthermore, gold (Au) is not necessary used for the whole of the conductive segments  15   a , thereby reducing an amount of gold used relative to that of the related art. Meanwhile, because the first segments  152  and the second segments  154  each have the approximately identical height a, a sulfidation resistance and a contacting stability to the contact can be enhanced, thereby sufficiently achieving the above effects. 
     According to the invention, the contacts  19  and  20  can be made of silver-palladium (AgPd) alloy, silver-copper (AgCu) alloy, silver-nickel (AgNi) alloy, silver-copper-nickel (AgCuNi) alloy, and the like. 
     The contacts  19 ,  20  are manufactured by preparing a melted alloy (ingot), forming a bar or a rod from the ingot, wire-drawing the obtained bar or rod by a die, and then heading the obtained wire having a predetermined diameter. 
     According to the invention, gold (Au) separated by an abrasion of the second segments is also attached to the contacts  19  and  20  abraded by sliding. By such an action, a contacting impediment can be prevented. 
     Also, because an amount of valuable gold used can be reduced, material costs for each of members can be decreased, thereby inhibiting an increase of the manufacturing costs of the liquid level detecting apparatus. 
     Meanwhile, the invention is not limited to the above embodiments, but the first segments and the second segments can be arranged in any manner such that a first segment group including at least one first segment and a second segment group including at least one second segment can be alternatively arranged to each other. For example, one row of the second segment may be arranged adjacently to a plurality of rows of the first segments, or a plurality of rows of the second segments may be arranged adjacently to a plurality of rows of the first segments.