Abstract:
To reduce a signal variation of a bridge circuit connected with a temperature sensing resistor that is caused by a strain even when the strain is generated at a diaphragm portion of a substrate installed with a heater resistor and the temperature sensing resistor. In a thermal type flow rate resistor including a substrate, a diaphragm  13  formed at the substrate, and a heat generating resistor  2  and temperature detecting resistors  7  through  10  formed on the diaphragm for detecting a flow rate of a measured fluid by heating the heat generating resistor, strain detecting resistors  11  and  12  are formed on an upstream side and on a downstream side of a flow of the measured fluid relative to the heat generating resistor on the diaphragm, an amount of a strain generated on the diaphragm is detected by the strain detecting resistors, and a flow rate signal detected by the heat generating resistor and the temperature detecting resistors is compensated for the strain based on the detected amount of the strain.

Description:
TECHNICAL FIELD 
     The present invention relates to a flow rate sensor which measures a flow rate by using a heat generating resistor. For example, the present invention relates to a flow rate measuring device which detects an amount of air that is sucked to an automobile engine. 
     BACKGROUND ART 
     In a background art, as a flow rate sensor which is installed at a suction air passage of an internal combustion engine of an automobile or the like for measuring a suction air amount, a thermal type one is mainly used. This is because a thermal type flow rate sensor can directly measure a mass flow rate. 
     In recent years, low fuel consumption and exhaust gas regulation are becoming severe from a view point of global environment protection. Therefore, in a flow rate sensor which measures a suction air amount, there are manifested needs for high accuracy, backflow detection, dynamic range expansion, and suchlike. 
     In a thermal type flow rate sensor which deals with such needs, in recent years, attention is paid to fabricate a sensing element which measures a flow rate on a semiconductor substrate of silicon or the like by using a semiconductor microfabrication technology. Because a sensing element of this kind can comparatively easily be mass-produced, and therefore, the sensing element is excellent in economy, can be downsized and can be driven by low power consumption. As such a flow rate sensor, there is a flow rate sensor described in Patent Literature 1. 
     In the case of the flow rate sensor described in Patent Literature 1, the sensing element is formed with a sensing resistor on a silicon substrate via an insulating layer, and formed with a thin film portion (diaphragm portion) by removing a portion of the silicon substrate in order to thermally insulate the resistor. A heat generating resistor can be formed by arranging a resistor which is driven as a heater at the diaphragm portion. In detecting a flow rate, there is adopted a temperature difference system in which temperature sensing resistors are formed on an upstream side and on a downstream side of an air flow by interposing the heat generating resistor, and a flow rate and a direction are detected based on a difference between temperatures of the temperature sensing resistors that are arranged on the upstream side and on the downstream side. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2002-48616 
     SUMMARY OF INVENTION 
     Technical Problem 
     In Patent Literature 1, polysilicon is adopted for the resistor. A semiconductor material such as polysilicon has a piezoresistive effect in which a resistance value of the material is changed owing to a strain that is generated by deforming a shape of the material. An amount of influence of the piezoresistive effect is determined by a gauge factor of the material and the influence is effected even on a metal material of platinum or the like. Therefore, the semiconductor type flow rate sensor poses a problem that an abnormal output is easy to be brought about owing to the piezoresistive effect. 
     Hence, it is an object of the present invention that even when a strain is produced at a diaphragm portion of a substrate that is installed with a heater resistor and a temperature sensing resistor, a variation in a signal of a bridge circuit connected with the temperature sensing resistor that is caused by the strain is reduced. 
     Solution to Problem 
     The object described is achieved by an invention described in claims. 
     For example, the object described above can be achieved by providing a structure as follows to a thermal type flow rate sensor including a substrate, a diaphragm formed at the substrate, and a heat generating resistor and a temperature detecting resistor formed on the diaphragm, and detecting a flow rate of a measured fluid by heating the heat generating resistor. Strain detecting resistors are formed on an upstream side and on a downstream side of a flow of the measured fluid relative to the heat generating resistor on the diaphragm. An amount of a strain generated on the diaphragm is detected by the strain detecting resistors, and a flow rate signal detected by the heat generating resistor and the temperature detecting resistor is compensated for the strain based on the amount of the strain detected. An amount of a strain effect can be removed and an abnormal output can be made difficult to be brought about by the compensation. 
     Advantageous Effects of Invention 
     According to the present invention, even when a strain is generated at a diaphragm portion of a substrate installed with a heater resistor and a temperature sensing resistor, a signal variation of a bridge circuit connected with a temperature sensing resistor caused by the strain can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of a pattern of a sensing element of a flow rate sensor according to a first embodiment of the present invention. 
         FIG. 2  is a diagram of a configuration of a circuit of the flow rate sensor according to the first embodiment of the present invention. 
         FIG. 3  is a diagram showing an operation processing at inside of the flow rate sensor according to the first embodiment of the present invention. 
         FIG. 4  is a diagram of a pattern of a sensing element of a flow rate sensor according to a second embodiment of the present invention. 
         FIG. 5  is a diagram of a configuration of a circuit of a flow rate sensor according to the second embodiment of the present invention. 
         FIG. 6  is a diagram of a pattern of a sensing element sensor of a flow rate sensor according to a third embodiment of the present invention. 
         FIG. 7  is a diagram of a configuration of a circuit of the flow rate sensor according to the third embodiment of the present invention. 
         FIG. 8  is a diagram of a pattern of a sensing element of a flow rate sensor according to a fourth embodiment of the present invention. 
         FIG. 9  is a view showing an influence of a stress in mounting which is effected on a flow rate sensor according to an embodiment of the present invention. 
         FIG. 10  is a schematic sectional view of mounting a flow rate sensor according to an embodiment of the present invention in an actually used state. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As described above, the semiconductor flow rate sensor poses a problem that an abnormal output owing to the piezoresistive effect is easy to be brought about. The reason is that a film thickness of the diaphragm portion is only about 1 through 2 μm, and therefore, the diaphragm portion undergoes (1) a stress when the sensing element is adhered to a supporter, (2) a stress when the supporter is adhered to be mounted on a casing or the like, (3) a stress which is generated from a difference of linear expansion coefficients of mounting members that is generated by a change in an environmental temperature, and (4) various deformations of a thermal deformation and the like that are brought about by making a heater generate heat. 
     Particularly, it is preferable that the temperature sensing resistors which are formed by interposing a heater have high resistance values in view of performances thereof. Therefore, it is necessary to form the resistor such that a width of the resistor is slender and a length thereof is prolonged as a shape thereof. Therefore, the temperature sensing resistors are easy to undergo the piezoresistive effect by the stresses described above. As a result thereof, there is a possibility that the temperature sensing resistors formed by interposing the heater have resistance values respectively different from each other and an abnormal output is generated. Further, also the heater temperature detecting resistor needs to be arranged at a vicinity of the heater resistor, and it is necessary to form the heater temperature detecting resistor such that a width thereof is slender and a length thereof is prolonged similar to the temperature sensing resistor described above. As a result thereof, a resistance value of the heater temperature detecting resistor is changed, and a temperature of the heater cannot correctly be detected. As a result thereof, there is a possibility that the temperature of the heater cannot be controlled to a prescribed temperature and the abnormal output is brought about. 
     An explanation will be given of embodiments of the present invention in reference to  FIG. 1  through  FIG. 10  as follows. 
       FIG. 1  is a diagram showing a plane structure of a flow rate detecting element of a flow rate sensor according to a first embodiment of the present invention. 
     In  FIG. 1 , a detecting element  1  is formed with a cavity portion at a back face of a substrate which is configured by a material that is excellent in a heat conductivity of silicon, ceramic or the like and the cavity portion is formed with a diaphragm  13  for detecting a flow rate of air. The cavity portion is formed by etching from the back face side of the substrate by an alkaline solvent or the like. A heater resistor  2  which is a resistor for setting a flow rate is arranged on the diaphragm  13 , and a heater temperature detecting resistor  3  is arranged to surround a surrounding of the heater resistor  2 . Upstream side temperature sensing resistors  7  and  8  are arranged on an upstream side of a flow and downstream side temperature sensing resistors  9  and  10  are arranged on a downstream side thereof relative to the heater resistor  2 . Strain detecting resistors  11  and  12  are arranged on the diaphragm  13  among bonding terminals  14  through  32  which are used for connecting the temperature sensing resistors  7  through  10  and external terminals. 
     Fixed resistors  5  and  6  and a temperature measuring resistor  4  are formed on the substrate at a surrounding of the diaphragm  13 . The resistors configured on the detecting element  1  are made by a semiconductor film of polysilicon or the like and a metal film of platinum or the like resistance values of which are changed by a temperature. These elements are connected to outside by the bonding terminals  14  through  32 . Thereby, an influence of a strain that is generated at the resistor on the diaphragm  13  can be excluded, or there can be provided a flow rate output in which a stress that is generated by the strain is alleviated. 
     Incidentally, all of the heater resistor  2  and the heater temperature detecting resistor  3 , the upstream side temperature sensing resistors  7  and  8 , the downstream side temperature sensing resistors  9  and  10 , the strain detecting resistors  11  and  12  which are resistors for detecting strain amounts, the fixed resistors  5  and  6 , and the temperature measuring resistor  4  are configured by the same material. Thereby, there can be configured a flow rate sensor which can provide a flow rate output excluding an influence of a strain that is generated at the resistor on the diaphragm  13  in mounting the diaphragm or the like without increasing cost. 
       FIG. 2  is a diagram showing a configuration of a circuit of the flow rate sensor according to the first embodiment of the present invention. 
     In  FIG. 2 , the flow rate sensor includes the detecting element  1  which detects an air flow rate, an air temperature, and a strain amount that is generated at the diaphragm  13 , and an ASIC circuit  35  for converting the air flow rate and the strain amount into electric signals and adjusting the air flow rate excluding the strain amount to a prescribed characteristic. 
     A power source  38  is connected to a bridge circuit formed by the heater temperature detecting resistor  3 , the temperature measuring resistor  4 , and the fixed resistors  5  and  6 . The bonding terminal  25  showing a potential of a connection point of the heater temperature detecting resistor  3  and the fixed resistor  6 , and the bonding terminal  31  showing a potential of a connection point of the temperature measuring resistor  4  and the fixed resistor  5  are connected to an input terminal of an operational amplifier  37 . The operational amplifier  47  controls a heating current which is supplied to the heater resistor  2  by a feedback control such that these potentials become the same. Here, the heating current is supplied by a transistor  36  which is controlled by the operational amplifier  37 . 
     The power source  38  is connected to a bridge circuit which is arranged on an upstream side of a flow direction of air relative to the heater resistor  2 . The bridge circuit is formed by the temperature sensing resistors  7  and  8 , and the temperature sensing resistors  9  and  10  which are arranged on a downstream side of a flow direction of air relative to the heater resistor  2 , resistance values of which are changed by an influence of heat from the heater resistor  2 . 
     The power source  38  is also connected to a bridge circuit which is formed by the strain detecting resistors  11  and  12  which detect amounts of strains generated at the diaphragm  13 , and fixed resistors  33  and  34 . 
     With regard to a differential signal in correspondence with an air flow rate, the bonding terminal  16  (or  28 ) indicating a potential at a connection point of the temperature sensing resistors  7  and  10 , and the bonding terminal  30  (or  17 ) indicating a potential at a connection point of the temperature sensing resistors  8  and  9  are connected to an A-D converter  39 . Also, with regard to a differential signal in correspondence with an amount of a strain generated at the diaphragm  13 , the bonding terminal  20  indicating a potential at a connection point of the strain detecting resistor  11  and the fixed resistor  33 , and the bonding terminal  27  indicating a potential at a connection point of the strain detecting resistor  12  and the fixed resistor  34  are connected to an A-D converter  40 . Outputs of the A-D converters  39  and  40  are inputted to DSP  42 . At DSP  42 , the amount of the strain generated at the diaphragm  13  is compensated for, adjusted to a prescribed characteristic, and outputted by using adjustment information stored at ROM  41 . The adjusted flow rate signal is inputted to a D-A converter  43  or a frequency output converter (FRC)  44 , and is converted into a voltage signal or a frequency signal. Finally, the voltage signal or the frequency signal is outputted as a flow rate output by a multiplexer (MPX)  45  which is a change-over switch based on information stored in ROM  41 . 
     As shown in  FIG. 2 , the ASIC circuit  35  is configured by the operational amplifier  37 , the power source  38 , the transistor  36 , the A-D converters  39  and  40 , ROM  41 , DSP  42 , the D-A converter  43 , and the multiplexer (MPX)  45 . 
     By the ASIC circuit  35 , there is obtained an output in which an output of the bridge circuit which is formed by the upstream side temperature sensing resistors  7  and  8  which are arranged on the upstream side in an air flow direction relative to the heat resistor  2 , and the downstream side temperature sensing resistors  9  and  10  which are arranged on the downstream side of the air flow direction relative to the heater resistor  2 , resistance values of which are changed by an effect of heat from the heater resistor  2  and the output is compensated for a strain effect. In this way, an abnormal output is made difficult to be brought about by removing an amount of the strain effect from the output of the bridge circuit. 
       FIG. 3  is a diagram indicating an operation processing executed at inside of DSP  42  at inside of the ASIC circuit  35  of the flow rate sensor according to the first embodiment of the present invention. 
     In  FIG. 3 , a signal detected at a strain detecting unit  47  is very small, and therefore, the signal is multiplied by a prescribed gain at an operator  48 . Thereafter, a strain detecting signal as amplified, and a strain amount at an initial state which is previously written to ROM  41  are inputted to an operator  49 , where a difference therebetween is calculated. A net strain amount is calculated by the calculation. An output from a flow rate detecting unit  46  and a strain detecting signal which is calculated by the operator  49  are inputted to an operator  50 , where a prescribed operation is executed and a true flow rate detecting signal is outputted. 
       FIG. 4  is a diagram showing a plane structure of a flow rate detecting element of a flow rate sensor according to a second embodiment of the present invention. Also,  FIG. 5  is a diagram showing a circuit configuration of the flow rate sensor according to the second embodiment of the present invention. According to the second embodiment, the power source  48  is connected to a bridge circuit which is formed by the heater temperature detecting resistor  3 , the temperature measuring resistor  4 , the fixed resistors  5  and  6 , and the strain detecting resistors  11  and  12 . Also, the bonding terminal  25  indicating the potential at the connection point of the heater temperature detecting resistor  3  and the fixed resistor  6 , and the bonding terminal  31  indicating the potential at the connection point of the temperature measuring resistor  4  and the fixed resistor  5  are inputted to the input terminals of the operational amplifier  37 . The operational amplifier  37  controls the heating temperature supplied to the heater resistor  2  by a feedback control such that these potentials are equal to each other. Here, the heating temperature is supplied by the transistor  36  controlled by the operational amplifier  37 . 
     Here, an explanation will be given of a method of excluding an influence of the strain from the flow rate output when the strain is generated at the diaphragm  13 . As described above, the operational amplifier  37  controls the heating current by the feedback control such that the potential at the input terminal stays the same. The following relationship can be derived from the feedback control. When it is designated that the heater temperature detecting resistor  3 : Rht, the temperature measuring resistor  4 : Rc, the fixed resistor  5 : R7, the fixed resistor  5 : R1, the strain detecting resistor  11 : Rp1, and the strain detecting resistor  12 : Rp2,
 
 R 1· [Rc +( Rp 1+ Rp 2)]= Rht·R 7
 
When the relationship is developed with regard to Rht by putting Rp=Rp1+Rp2,
 
 Rht=R 1/ R 7·( Rc+Rp )
 
Here, when a resistance of the resistor on the diaphragm is changed by a mounting stress or the like, it can be derived as follows.
 
                     Rht   +     Δ   ⁢           ⁢   Rht       =       ⁢     R   ⁢           ⁢     1   /   R     ⁢           ⁢     7   ·     (     Rc   +   Rp   +     Δ   ⁢           ⁢   Rp       )                     =       ⁢       R   ⁢           ⁢     1   /   R     ⁢           ⁢     7   ·     (     Rc   +   Rp     )         +     R   ⁢           ⁢     1   /   R     ⁢           ⁢     7   ·   Δ     ⁢           ⁢   Rp                   
Here, a change in a resistance by a strain is generally represented by the following equation.
 
Δ R/R=K ·ε( K:  gage factor, ε: strain)
 
Δ R=K·ε·R  
 
     Thereby, the strain detecting resistor Rp may be set as follows.
 
Δ Rht=R 1/ R 7·Δ Rp  
 
 K·ε·Rht=R 1/ R 7· K·ε·Rp  
 
 Rht=R 1/ R 7· Rp  
 
 Rp=R 7/ R 1· Rht  
 
 Rp 1+ Rp 2= R 7/ R 1· Rht  
 
     The influence by the strain of the heater temperature control bridge can be excluded when the strain is generated at the diaphragm  13  by setting the strain detecting resistor as described above. 
     With regard to the differential signal in correspondence with the air flow rate, the bonding terminal  16  (or  28 ) indicating the potential at the connection point of the temperature sensing resistors  7  and  10 , and the bonding terminal  30  (or  17 ) indicating the potential at the connection point of the temperature sensing resistors  8  and  9  are connected to the A-D converter  39 . The output of the A-D converter  39  is inputted to DSP  42 . At DSP  42 , the output is adjusted to a prescribed characteristic and outputted by using the adjustment information stored to ROM  41 . The flow rate signal as adjusted is inputted to the D-A converter  43  or the frequency output converter (FRC)  44 , and converted into the voltage signal or the frequency signal. Finally, the voltage signal or the frequency signal is outputted as the flow rate output by the multiplexer (MPX)  45  which is the change-over switch based on the information stored to ROM  41 . 
     The compensation of the stress effect can be provided to the bridge circuit controlling the heater temperature detecting resistor  3  at a constant temperature by configuring the bridge circuit by the heater temperature detecting resistor  3 , the temperature measuring resistor  4 , the fixed resistors  5  and  6 , and the strain detecting resistors  11  and  12  in this way. 
       FIG. 6  is a diagram showing a plane structure of a flow rate detecting element of a flow rate sensor according to a third embodiment of the present invention. Also,  FIG. 7  is a diagram showing a circuit configuration of the flow rate sensor according to the third embodiment of the present invention. According to the third embodiment, the arrangement of the plane detecting resistors  11  and  12  according to the first embodiment is changed. Although according to the first embodiment, the strain detecting resistors  11  and  12  are arranged among the bonding terminals  14  through  32  which are used for connecting the temperature sensing resistors  7  through  10  to the external terminals, according to the third embodiment, the strain detecting resistors  11  and  12  are arranged on the diaphragm  13  on a side opposed to the bonding terminals  14  through  18 ,  20  through  25 , and  27  through  32  with the temperature sensing resistors  7  through  10  as references. The third embodiment is similar to the first embodiment in the circuit operation. The present embodiment achieves an effect in a case where also the side opposed to the side of the bonding terminals is adhered to mount when the detecting element  1  is mounted to a supporter  60  ( FIG. 9 ). 
       FIG. 8  is a diagram showing a plane structure of a flow rate detecting element of a flow rate sensor according to a fourth embodiment of the present invention. According to the fourth embodiment, the arrangement of the plane detecting resistors  11  and  12  according to the second embodiment is changed. Although according to the second embodiment, the strain detecting resistors  11  and  12  are arranged among the bonding terminals  14  through  32  which are used for connecting the temperature sensing resistors  7  through  10  to the external terminals, according to the fourth embodiment, the strain detecting resistors  11  and  12  are arranged on the diaphragm  13  on the side opposed to the bonding terminals  14  through  18 ,  20  through  25 , and  27  through  32  with the temperature sensing resistors  7  through  10  as references. The present embodiment also achieves an effect in a situation similar to that of the third embodiment. 
       FIG. 9  is a diagram showing a stress effect when the detecting element  1  of a flow rate sensor according to an embodiment of the present invention is mounted on the supporter  60 . The detecting element  1  is adhered to a concave portion  61  (cavity) which is formed at the supporter  60  by an adhesive agent  62 . Here, it is shown that when a stress is applied to the detecting element  1  from the supporter  60  and the adhesive agent  62  by a change in a environment temperature or the like, the diaphragm  13  on the detecting element  1  is deformed into a concave or convex shape, and the deformation is detected by the strain detecting resistors  11  and  12 . 
       FIG. 10  is a schematic sectional view of mounting a flow rate sensor according to the present invention in an actually used state. 
     In  FIG. 10 , a flow rate sensor  51  is mounted in the form of being inserted into an intake pipe  59 , and is fixed to the intake pipe  59  by a flange  27 . A housing  52  is attached with the supporter  60  which is mounted with the detecting element  1  and the ASIC circuit  35 . 
     An air flow  56  flowing in the intake pipe  59  is shunted into the flow rate sensor  51  by an air inlet port  53 , detours above the detecting element  1  bypassing a bypass passage  55  and is returned into the main intake pipe  59  from a passage outlet  54 . 
     LIST OF REFERENCE SIGNS 
     
         
           1  detecting element 
           2  heater resistor 
           3  heater temperature detecting resistor 
           4  temperature measuring resistor 
           5 ,  6 ,  33 ,  34  fixed resistors 
           7 ,  8  upstream side temperature sensing resistors 
           9 ,  10  downstream side temperature sensing resistors 
           11 ,  12  strain detecting resistors 
           13  diaphragm 
           14  to  32  bonding terminals 
           35  ASIC circuit 
           36  transistor 
           37  operational amplifier 
           38  power source 
           39 ,  40  A-D converters 
           41  ROM 
           42  DSP 
           43  D-A converter 
           44  frequency output converting circuit (FRC) 
           45  multiplexer (MPX) 
           46  flow rate detecting unit 
           47  strain detecting unit 
           48  to  50  operators 
           51  flow rate sensor 
           52  housing 
           53  air inlet port 
           54  passage outlet 
           55  bypass passage 
           56  air flow 
           57  flange 
           58  base 
           59  intake pipe 
           60  supporter 
           61  cavity 
           62  adhesive agent