The present invention relates to a load detecting device for detecting a load applied to a deformable body by measuring mechanical strain caused in the deformable body.
In the conventional load detecting device, resistance values of compressive strain resistor element 119 and tensile strain resistor element 120 vary depending on strain of deformable body 111 that has received a load, and a voltage of output electrode 118 also varies. Thus, the voltage of output electrode 118 is measured to detect the load.
The conventional load detecting device is described in Patent Literature 1.
As another conventional load detecting device, the configuration constituted by a deformable body and a strain gage attached to the deformable body is known, and is described in Patent Literature 2. A strain gage is typically covered with a resin film to ensure insulation properties, and bonded to a deformable body with a resin adhesive.
PLT 1: Unexamined Japanese Patent Publication No. 2007-127580
PLT 2: Unexamined Japanese Patent Publication No. 2008-134232
A load detecting device has a deformable body, a strain detecting element disposed on the deformable body, an adhesive layer located between the deformable body and the strain detecting element and fixing the strain detecting element to the deformable body. The adhesive layer is formed of a glass adhesive.
Load detecting device 50 has deformable body 11, strain resistor elements 12, 13, and adhesive layers 32, 35. Deformable body 11 deforms upon receiving a load. Deformable body 11 has three through-holes 17, 18. Through-hole 17 is provided in the center of deformable body 11. Two through-holes 18 are provided in both end portions of deformable body 11, respectively. Pressing member 19 for transmitting a detection load is inserted into through-hole 17. Fixing members for external mounting (not specifically shown) are inserted into two through-holes 18.
Strain resistor element 12 is a strain detection element. When strain resistor element 12 deforms, the resistance value changes. Strain resistor element 12 is disposed on a surface of deformable body 11. Adhesive layer 32 is located between deformable body 11 and strain resistor element 12 to fix strain resistor element 12 to deformable body 11.
Strain resistor element 13 is, like strain resistor element 12, also a strain detection element, and disposed on the surface of deformable body 11. Adhesive layer 35 is located between deformable body 11 and strain resistor element 13 to fix strain resistor element 13 to deformable body 11.
Load detecting device 50 further has control circuit 14, connector case 15, and wiring electrodes 16. Connector case 15 houses control circuit 14. Connector case 15 is attached to deformable body 11. Wiring electrodes 16 electrically connect strain resistor elements 12, 13 to control circuit 14.
As shown in
Strain resistor element 13, like strain resistor element 12, has supporting substrate 33, insulating layer 34, power supply terminal 20, output terminal 26, thick-film resistor patterns 27, ground terminal 23, and thick-film resistor patterns 28. Supporting substrate 33 is made of metal and has flexibility. Insulating layer 34 is provided on a surface of supporting substrate 33. Power supply terminal 20, output terminal 26, ground terminal 23, and thick-film resistor patterns 27, 28 are formed on insulating layer 34.
Strain resistor element 12 constitutes half-bridge circuit 25 by using thick-film resistor patterns 22 connected to each other in parallel between power supply terminal 20 and output terminal 21, and thick-film resistor patterns 24 connected to each other in parallel between output terminal 21 and ground terminal 23.
Strain resistor element 13 constitutes half-bridge circuit 29 by using thick-film resistor patterns 27 connected to each other in parallel between power supply terminal 20 and output terminal 26, and thick-film resistor patterns 28 connected to each other in parallel between output terminal 26 and ground terminal 23.
Respective power supply terminals 20 of strain resistor element 12 and strain resistor element 13 have the same electrical potential, whereby those terminals are considered as an identical terminal in the circuit diagram. Likewise, respective ground terminals 23 of strain resistor element 12 and strain resistor element 13 have the same electrical potential, whereby those terminals are considered as an identical terminal in the circuit diagram. Strain resistor element 12 and strain resistor element 13 are combined to constitute a full-bridge circuit.
Control circuit 14 is electrically connected to power supply terminal 20, output terminals 21, 26, and ground terminal 23 via wiring electrodes 16.
A specific manufacturing method is as follows. Firstly, a large scaled sheet of stainless steel plate 36 is prepared, and glass paste is printed on the top surface of stainless steel plate 36 to form insulating layers 31, 34. Secondly, metal glaze paste is printed on the top surface of insulating layers 31, 34 to form thick-film resistor patterns 22, 24, 27, 28. In the meanwhile, conductive paste is applied to form power supply terminal 20, output terminals 21, 26, and ground terminal 23. Thereafter, the large scaled sheet of stainless steel plate 36 is divided into pieces to produce strain resistor elements 12, 13.
In this manufacturing method, multiple strain resistor elements 12, 13 are produced from the large scaled sheet of stainless steel plate 36 by using printing process. In the printing process, many strain resistor elements 12, 13 are preferably produced in a time to achieve high production efficiency. For miniaturization, deformable body 11 is restricted due to its attachment manner or the like; however, strain resistor elements 12, 13 have no such restrictions. More strain resistor elements 12, 13 can be obtained by producing strain resistor elements 12, 13 from stainless steel plate 36 than by producing deformable body 11 having strain resistor elements 12, 13 formed thereon from stainless steel plate 36 by forming strain resistor elements 12, 13 on deformable body 11 directly.
Accordingly, bonding strain resistor elements 12, 13 to deformable body 11, like load detecting device 50 of the embodiment, is more preferable to achieve high productivity, as compared with the manner where compressive strain resistor element 119 and tensile strain resistor element 120 are directly formed on deformable body 111 like Patent Literature 1 of the conventional art.
Load detecting device 50 of the above configuration operates as follows.
Load detecting device 50, not specifically shown, is attached to a portion between a vehicle seat (not shown) and a seat rail (not shown) as an example, and is used for measuring a load of an occupant who sits on the vehicle seat. In this usage, fixing members (not shown) provided in the seat rail are inserted into through-holes 18 provided on both end sides of deformable body 11, and load detecting device 50 is fixed to the seat rail. Pressing member 19 is inserted into through-hole 17 provided in the center of deformable body 11. A tip end side of pressing member 19 is fixed to a lower portion of the vehicle seat. When a load is applied to the vehicle seat, the load is transmitted to deformable body 11 via pressing member 19, and a center portion of deformable body 11 whose both ends are supported by the fixing members (not shown) deforms downward as shown in
Hereinafter, more specified description will be made. When the detection load is applied to pressing member 19, the center portion of deformable body 11 deforms downward. At this time, compressive stress acts on thick-film resistor patterns 22, 28 disposed on a through-hole 17 side, and the resistance values of thick-film resistor patterns 22, 28 decrease. Further, tensile stress acts on thick-film resistor patterns 24, 27 disposed on a through-hole 18 side, and the resistance values of thick-film resistor patterns 24, 27 increase. Accordingly, in load detecting device 50, control circuit 14 conducts differential processing of the signals outputted from output terminals 21, 26, thereby generating detection signals according to the amplitude of the detection load.
Deformable body 11 of load detecting device 50 is made of carbon steel, and supporting substrates 30, 33 are made of stainless steel according to the embodiment. If thermal expansion coefficients of adhesive layers 32, 35 for bonding deformable body 11 and supporting substrates 30, 33 are extremely different from a thermal expansion coefficient of deformable body 11 or thermal expansion coefficients of supporting substrates 30, 33, cracks may occur in adhesive layers 32, 35 and cause delamination between deformable body 11 and supporting substrates 30, 33. Therefore, an adhesive material used for adhesive layer 32 is desired to have a similar thermal expansion coefficient to those of deformable body 11 and supporting substrates 30, 33. This ensures adhesive strength between deformable body 11 and supporting substrates 30, 33. Specifically, a glass adhesive is selected to ensure the adhesive strength because the difference between the thermal expansion coefficients of deformable body 11 and supporting substrates 30, 33 and the thermal expansion coefficients of adhesive layers 32, 35 is within 4 ppm/K. The glass adhesive is a glass-based material such as liquid glass (aqueous solution of sodium silicate).
For instance, in the case where supporting substrates 30, 33 made of stainless steel and having a thermal expansion coefficient of 11.5 ppm/K are bonded to deformable body 11 made of carbon steel and having a thermal expansion coefficient of 10.3 ppm/K, the glass adhesive is selected to have thermal expansion coefficients ranging from 6.3 ppm/k to 10.3 ppm/k as adhesive layers 32, 35, thereby securing adhesive strength.
Furthermore, it is effective that an upper limit of the thermal expansion coefficients of adhesive layers 32, 35 is determined to be larger than the smallest value of the thermal expansion coefficients of deformable body 11 and supporting substrates 30, 33 by 4 ppm/K, and a lower limit of the thermal expansion coefficients of adhesive layers 32, 35 is determined to be smaller than the largest value of the thermal expansion coefficients of deformable body 11 and supporting substrates 30, 33 by 4 ppm/K. Thus, the thermal expansion coefficients of adhesive layers 32, 35 have a difference within 4 ppm/K with respect to either of the thermal expansion coefficients of deformable body 11 and supporting substrates 30, 33. This ensures adhesive strength with respect to either of deformable body 11 and supporting substrates 30, 33. For instance, under the condition where deformable body 11 has a thermal expansion coefficient of 10.3 ppm/K and supporting substrates 30, 33 have a thermal expansion coefficient of 11.5 ppm/K, deformable body 11 and supporting substrates 30, 33 are bonded more tightly when a glass adhesive having thermal expansion coefficients in a range from 7.5 ppm/K to 14.3 ppm/K is used as adhesive layers 32, 35.
When the glass adhesive is used as adhesive layers 2, 35, Young's moduli of adhesive layers 32, 35 become larger than those of a resin-based adhesive. As a result, when a deformation is caused in deformable body 11 by the load to be detected and the deformation is transmitted to supporting substrates 30, 33, a loss of the transmission due to adhesive layers 32, 35 is reduced. This increases detection sensitivity of strain resistor elements 12, 13 with respect to the deformation of deformable body 11. As the detection sensitivity increases, resolution of the measurement is improved. When the resolution is improved, a load can be detected accurately even if a deformation of deformable body 11 with respect to the load is small. Thus, deformable body 11 can be designed to decrease a deformation with respect to a load, thereby resulting in miniaturization of load detecting device 50.
As shown in
Furthermore, in the case where deformable body 11 and supporting substrate 30 are bonded by using the epoxy-based adhesive, the detection sensitivity changes largely when temperature of use environments changes, thereby reducing detection accuracy of load detecting device 50. On the contrary, in the case where adhesive layers 32, 35 are formed by using the glass adhesive as in the embodiment, temperature dependence of Young's modulus of the glass adhesive is so small, so that a change in detection sensitivity is small even if temperature of use environments around load detecting device 50 changes. When adhesive layers 32, 35 are formed by using the glass adhesive, detection accuracy of load detecting device 50 can be improved.
A deformation of deformable body 11 is influenced by the size of deformable body 11. As the length of deformable body 11 is longer, the width is narrower, and the thickness is thinner, the deformation increases when a load is applied. As shown in
It is noted that in the embodiment, deformable body 11 is made of carbon steel and supporting substrates 30, 33 are made of stainless steel, but not limited to this example. As long as a thermal expansion coefficient of an adhesive has a difference within 4 ppm/K with respect to thermal expansion coefficient of deformable body 11 or thermal expansion coefficients of supporting substrates 30, 33, the same effect as the embodiment will be obtained. It is particularly useful to form supporting substrates 30, 33 by using metal materials because the difference between the thermal expansion coefficients of supporting substrates 30, 33 and the thermal expansion coefficient of the glass adhesive is not so large.
The load detecting device in accordance with the present invention relates to a load detecting device for detecting a load applied to a deformable body, and especially is useful in the load detecting device for measuring a load from a vehicle seat.
11 deformable body
12, 13 strain resistor element
14 control circuit
15 connector case
16 wiring electrode
17 through-hole
18 through-hole
19 pressing member
20 power supply terminal
21, 26 output terminal
22, 24, 27, 28 thick-film resistor pattern
23 ground terminal
25, 29 half-bridge circuit
30, 33 supporting substrate
31, 34 insulating layer
32, 35 adhesive layer
36 stainless steel plate
50 load detecting device
111 deformable body
112 detecting hole
116 power supply electrode
117 GND electrode
118 output electrode
119 compressive strain resistor element
120 tensile strain resistor element
121 circuit pattern
Number | Date | Country | Kind |
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2013-106876 | May 2013 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2014/002509 | 5/13/2014 | WO | 00 |