Abstract:
The purpose of the present invention is to reduce a load of preventing dew condensation, and to provide a highly reliable humidity detection device. In order to achieve the purpose, this humidity detection device is provided with: a humidity sensor having a humidity detection unit and a temperature detection unit; a heating resistor that heats the humidity sensor; and a heating control unit that controls a heating temperature of the heating resistor. The humidity detection device is characterized in having a target temperature storage unit that stores target temperatures of the heating resistor, said target temperatures having been determined corresponding to temperatures and humidities.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a humidity detector attached to an intake system of an internal combustion engine of an automobile. 
       BACKGROUND ART 
       [0002]    Conventionally, automobiles using electronic control fuel injection systems have been widely used. In engine compartments of such automobiles, various sensors and control devices are disposed. As one of them, there is a humidity detector. Although humidity detectors have recently come into use for the purpose of fuel control, humidity detectors have conventionally been used mainly for air conditioning management in automobile cabins. When humidity detectors are used in automobile cabins, there is no requirement for durability or the like which presupposes a severe environment. For example, when a humidity detector is used integrally with the intake air flow measuring device or other sensors for the purpose of controlling an engine, the humidity detector is required to have environment-resistant performance equivalent to that of the intake air flow measuring device. 
         [0003]    An environment particularly unfavorable to the humidity detector is adhesion of water droplets to a humidity detecting unit which originates from dew condensation of a detecting element unit, or the like. When moisture in air reaches a saturation point (relative humidity 100%), dew condensation occurs. A temperature at which dew condensation occurs is called a dew point. When dew condensation occurs on a humidity sensor, a significant delay is generated in detection responsiveness to changes in humidity and humidity measurement accuracy itself is negatively affected. In addition, there may be a case where the humidity detector loses a function thereof. When dew condensation occurs on the humidity detecting unit, a signal value indicating maximum or minimum humidity is output, and the humidity detector temporarily loses the function as a humidity detector until the detecting element unit dries. As a result, an engine control system is negatively affected while the function of the humidity detector is lost. This is a salient problem particularly when a polymer capacitance humidity sensor is used in an environment where an atmosphere rapidly changes such as inside of a vehicle intake pipe. A clear technical object is necessary to deal with the above problems. 
         [0004]    PTL 1 is an example of a humidity detector including a countermeasure against dew condensation described above. PTL 1 discloses that dew condensation on a humidity detecting unit is prevented by obtaining a dew point temperature, and then heating, by a heating unit included therein, the humidity detecting unit to a temperature at which there is a certain difference between a sensor temperature and the dew point temperature. 
       CITATION LIST 
     Patent Literature 
       [0005]    PTL 1: JP 2001-281182 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    Since a formula for obtaining a dew point temperature is a polynomial, a processing load of a microprocessor increases when a dew point temperature is calculated each time. An intake system of an internal combustion engine of an automobile is affected by heat from the internal combustion engine, thereby being exposed to high temperatures. Therefore, reduction in a processing load of a microprocessor of a humidity detector is required. In addition, processes performed by the microprocessor tend to increase with improvement in performance and multifunctionality thereof, and reduction in a processing load for controlling prevention of dew condensation in the microprocessor is desired. PTL 1 leaves room for consideration of the above problems. 
         [0007]    An object of the present invention is to provide a highly reliable humidity detector which reduces a processing load for preventing dew condensation 
       Solution to Problem 
       [0008]    In order to attain the object, the humidity detector of the present invention includes a humidity sensor provided with a humidity detecting unit and a temperature detecting unit, a heat generator which heats the humidity detecting unit, and a heating temperature control unit which controls a heating temperature of the heat generator. The humidity detector Includes a target temperature storing unit which stores a plurality of target temperatures of the heat generator predetermined in accordance with temperature and humidity each of which is information detected by the humidity sensor. 
       Advantageous Effects of Invention 
       [0009]    According to the present invention, it is possible to provide a highly reliable humidity detector which reduces a processing load for preventing dew condensation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a perspective view of a humidity sensor according to an embodiment of the present invention. 
           [0011]      FIG. 2  is a configuration view of the humidity sensor according to the embodiment of the present invention. 
           [0012]      FIG. 3  is a block diagram of a heating control device of the humidity sensor according to the embodiment of the present invention. 
           [0013]      FIG. 4  is a block diagram of a unit for retrieving a heating target temperature value of a heating resistor, which according to the embodiment of the present invention. 
           [0014]      FIG. 5  is a flowchart of the heating control device of the humidity sensor according to the embodiment of the present invention. 
           [0015]      FIG. 6  is a block diagram illustrating a border line between energization and non-energization of a heating resistor according to an embodiment of the present invention. 
           [0016]      FIG. 7  is a block diagram illustrating a control target area of temperature and humidity according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]    Hereinbelow, embodiments of the present invention will be described based on the drawings. 
         [0018]    A first embodiment of the present invention will be described by using  FIGS. 1 to 5 . 
         [0019]    As illustrated in  FIGS. 1 and 2 , a humidity detector attached to an intake pipe or a dedicated body (not illustrated) includes a housing  3 , a cover  2 , and an electronic circuit board  23  mounted on the housing  3 . A microprocessor  26  a humidity sensor  24 , and a heating resistor  24  are mounted on the circuit board  23 . 
         [0020]    The housing  3  includes a connector  1 , a housing support  4 , and a metal bush  5 . The connector  1  is used for fitting to a harness of an engine control apparatus. The housing support  4  is fixed to the intake pipe or the dedicated body. The metal bush  5  is used for reinforcement when the intake pipe or the dedicated body and the housing support  4  are fixed with a metal screw or the like. 
         [0021]    The housing  3  includes a subpassage constituting groove. The subpassage constituting groove constitutes a subpassage which takes in a part of air flowing in the intake pipe. The housing  3  constitutes the subpassage by the cooperation with the cover  2 . The subpassage includes a first subpassage  21  which takes in a part of air flowing in the intake pipe, an inflow passage  20 A to a second subpassage  22  from the first subpassage, and an inflow passage  20 B to the first subpassage  21  from the second subpassage  22 . The humidity sensor  24  and the heating resistor  25  are disposed in the second subpassage  22 , and humidity of air flowing in the intake pipe is detected by the humidity sensor  24 . The humidity sensor  24  is disposed in the second subpassage  22  provided so as to bypass the first subpassage  21 , in other words, provided so as to diverge from the first subpassage  21 , and thereby a configuration is obtained in which a pollutant hardly enters the second subpassage. Therefore, it is possible to prevent a pollutant from being carried to the humidity sensor. 
         [0022]    Next, a countermeasure against dew condensation on the humidity sensor  24  will be described by using  FIGS. 3 to 5 .  FIG. 3  is a block diagram of a heating temperature control device  30  of the heating resistor  26  which heats the humidity sensor  24 . 
         [0023]    As illustrated in  FIG. 3 , the heating temperature control device  30  includes the humidity sensor  24 , the microprocessor  26 , and the heating resistor  25 . The humidity sensor  24  has a temperature detecting function in addition to a humidity detecting function, and temperature information and humidity information detected by the humidity sensor  24  are transmitted to the microprocessor  26  by using a signal line  31 B for controlling the humidity sensor. The microprocessor  26  includes a humidity sensor signal processing circuit  43 , a target temperature retrieving unit  40 , and a heating element control unit  26 . The humidity sensor signal processing circuit  43  processes a detection signal from the humidity sensor  24 . The target temperature retrieving unit  40  retrieves a target temperature TT from the detection signal from the humidity sensor  24 . The heating element control unit  26  controls a heating temperature of the heating resistor  25  based on the target temperature TT retrieved by the target temperature retrieving unit  40 . The heating resistor control unit controls a heating temperature of the heating resistor to be the retrieved target temperature TT. Here, the heating temperature of the heating resistor is controlled by controlling a voltage supplied from a power supply unit not illustrated by ON/OFF control of a switching circuit not illustrated. The power supply unit and the switching circuit may be configured separately from the microprocessor  26 , or may be configured integrally therewith. The microprocessor  26  controls the heating resistor by using a signal line  31 A for controlling the heating resistor based on the temperature information and the humidity information. 
         [0024]    As illustrated in  FIG. 4 , the target temperature TT corresponding to temperature ts and humidity Hs is stored as map data in a target temperature storing unit  41  of the microprocessor  26 . In other words, the humidity detector includes a memory which stores the target temperature TT as map data. Measurement conditions, in other words, the temperature ts and the humidity Hs detected by the humidity sensor  24 , which are retrieving conditions, are transmitted to the microprocessor  26  by using the signal line  31 B for controlling the humidity sensor, and the target temperature retrieving unit retrieves a target temperature corresponding to the retrieving conditions from the map data. Here, the target temperature TT is a value calculated in advance. 
         [0025]    A calculation example of the target temperature TT is as follows. 
         [0026]    A calculation formula of relative humidity is represented by formula 1, where e denotes saturation vapor pressure [Pa] at a dew point temperature, es denotes saturation vapor pressure [Pa], ln denotes natural logarithm (logarithm to the base natural exponent e, loge x), and T denotes the absolute temperature [k]=(t° C.+273.15). 
         [0000]        U [ RH  %]= e/es× 100   (formula 1)
 
         [0027]    A calculation formula for obtaining saturation vapor pressure for water (ew) is represented by formula 2. 
         [0000]        ln ( ew ) [ Pa]− 6096.9385× T   −1 +21.2409642−2.711193×10 −2   ×T+ 1.67952×10 −5×   T   2 +2.433502× ln ( T )   (formula 2)
 
         [0028]    A calculation formula for calculating a dew point temperature from saturation vapor pressure is as follows. 
         [0000]        y=ln ( e/ 611.213)   (formula 3)
 
         [0029]    When y≧0, 
         [0000]        TDP [° C.]=13.715× y+ 8.4262×10 −1×   y   2 +1.9048×10 −2×   y   3 +7.8158×10 −3   ×y   4    (formula 4)
 
         [0030]    When y&lt;0, 
         [0000]        TDP [° C.]=13.7204× y+ 7.36631×10 −1   ×y   2 +3.32136×10 −2×   y   3 +7.78591×10 −4×   y   4    (formula 5)
 
         [0031]    A flow for obtaining a heating target temperature value of a heating resistor TT is indicated as follows. 
         [0032]    As an example of a flow of calculation for obtaining a dew point temperature T DP  [° C.], measured temperature and measured humidity, which have been measured by the humidity sensor, are denoted by ts [° C.] and He [rh %], respectively. 
         [0033]    Saturation vapor pressure es is obtained from measured temperature ts. 
         [0034]    Formula 2 gives: 
         [0000]        es=EXP ̂(−6096.9385×( ts+ 273.15) −1 +21.2409642−2.711193×10 −2 ×( ts+ 273.15)+1.673952×10 −5 ×( ts+ 273.15) 2 +2.433502× ln ( ts+ 273.15))   (formula 6)
 
         [0035]    Measured humidity Hs=U, accordingly, formulas 1 and 6 give: 
         [0000]        e=U/ 100× es=Hs/ 100× EXP ̂(−6096.9385×( ts+ 273.15) −1 +21.2409642−2.711193×10 −2 ×( ts+ 273.15)+1.673952×10 −5 ×( ts+ 273.15) 2 +2.433502× ln ( ts+ 273.15))   (formula 7)
 
         [0036]    Formula 3 gives: 
         [0000]        y=ln ( e/ 611.213)= ln ( Hs/ 100× EXP ̂(−6096.9385×( ts+ 273.15) −1 21.2409642−2.711193×10 −2 ×( ts+ 273.15)+ 1 . 673952 × 10   −5 ×( ts+ 273.15) 2 +2.433502× ln ( ts+ 273.15)/   611 . 23   )   (formula 8)
 
         [0037]    Depending on a y value obtained from formula 8, the y value is substituted into 
         [0000]    formula 4 when y≧0, or
 
formula 5 when y&lt;0,
 
and thereby the dew point temperature T DP [° C.] is calculated.
 
         [0038]    A calculation to obtain the target temperature TT DP [° C.] is as follows. First, a margin temperature TT DP [° C.] to the dew point temperature is set in advance. Determination is performed in the following manner. 
         [0039]    When ts−T DP ≧TT DP , the heating resistor is not controlled, and 
         [0040]    when ts−T DP &lt;TT DP  the heating resistor is controlled. 
         [0041]    The heating target temperature value of the heating resistor is derived from the measured values of temperature ts and humidity Hs measured by the humidity sensor  24  illustrated in  FIG. 4 , by calculating TT=TT DP +T DP . 
         [0042]    In the first embodiment of the present invention, the microprocessor  26  is configured to store in advance, in the target temperature storing unit  41 , the heating target temperature value of a heating resistor TT obtained by the above calculation formula for each temperature and each humidity, as map data, and to retrieve, by the target temperature retrieving unit  40 , a target temperature in a measurement condition from the stored target temperature values. 
         [0043]    Conventionally, a microprocessor has been caused to perform the calculation process for a countermeasure against dew condensation for each time, with the result that there has been a problem of an increase in a processing load of the microprocessor. In the embodiment, however, a configuration is adopted in which a process for obtaining a dew point temperature is not performed by the microprocessor  26 , and the heating target temperature of a heating resistor TT stored in advance is retrieved based on the measurement condition. Consequently, it is possible to reduce the processing load of the microprocessor  26 . 
         [0044]    In addition, in the first embodiment of the present invention, a temperature sensor  27  included in the humidity sensor  24  measures a temperature of an atmosphere to be measured for retrieving the target temperature. In other words, a configuration is adopted in which the humidity sensor  24  including a temperature detecting unit measures a temperature of an atmosphere to be measured for retrieving the target temperature. According to the first embodiment of the present invention, since the humidity sensor  24  which measures humidity of an atmosphere to be measured also detects a temperature of the atmosphere to be measured, the temperature of the atmosphere to be measured in the vicinity of the humidity sensor  24  which needs to avoid new condensation can be measured with good accuracy and good responsiveness. 
         [0045]    Particularly in a case of a humidity measuring device which detects humidity of intake air in an internal combustion engine of an automobile, it is necessary to perform humidity detection and control to prevent dew condensation at the same time under the circumstances where an intake air condition. changes every moment. According to the first embodiment of the present invention, since the condition of the atmosphere to be measured in the vicinity of the humidity sensor  24  can be measured with good accuracy and good responsiveness, it is possible to perform humidity detection and control to prevent dew condensation at the same time even in the case described above. 
         [0046]    In order to obtain a dew point temperature for each time as in conventional cases, it is necessary to measure a precise temperature of air before heating the humidity detecting unit. Therefore, separately from the temperature sensor provided at the humidity detecting unit, a dedicated air temperature sensor needs to be provided at a place which suffers less influence of heating of the humidity detecting unit. In other words, the air temperature sensor has to be disposed in a sensor housing. As a result, the size of the sensor housing increases to cause an increase in pressure loss in the intake pipe, limitation for circuit spaces, or the like, and thereby deterioration of performance and an increase in cost may be caused. According to the embodiment, deterioration of measurement accuracy resulting from dew condensation can be achieved by controlling the heating resistor with a temperature of the humidity sensor without using an additional temperature sensor, with lower cost than that in a case of a conventional humidity sensor used in the same environment. 
         [0047]    A second embodiment of the present invention will be described by using  FIG. 6 . Regarding the same configuration as that in the first embodiment, a description thereof will be omitted. 
         [0048]    The second embodiment of the present invention adopts a configuration in which a heating resistor is not controlled when a difference between a measured temperature ts and a dew point temperature TT DP  is greater than or equal to a margin temperature TT DP , in other words, the measured temperature ts is higher than or equal to a target temperature TT. Specifically, when a measurement condition is a low temperature and high humidity condition (ts 1 , Hs 1 ), target temperature TT 1 ≧measured temperature ts 1  is satisfied, and the heating resistor is energized until the measured temperature ts 1  reaches a heating target temperature value TT 1  corresponding to the measurement condition. When the temperature increases and the humidity decreases by the heating control, the measurement condition exceeds a heating resistor heating border line  50  to be changed into, for example, a measurement condition (ts 2 , Hs 2 ) and target temperature TT 2 ≦measured temperature ts 2  is satisfied. In that case, the heating resistor is not energized and control thereof is not performed. Here, the target temperature TT which exceeds the heating resistance heating border line  50  is set to a temperature lower than or equal to a heat resistant temperature of an object to be heated. 
         [0049]    Dew condensation can be prevented by heating the humidity detecting unit to a temperature higher than a dew point temperature. However, just simple energization of the resistor leads to further heating of the humidity detecting unit in a case of a high temperature environment, which may result in heating of the object to be heated to a temperature higher than or equal to a heat resistant temperature thereof, overheating thereof, and an extremely low humidity condition caused by high temperature. Consequently, there occurs a problem of deterioration of the measurement accuracy. 
         [0050]    According to the second embodiment of the present invention, the heating temperature of the heating resistor is controlled to be lower than the heat resistant temperature of the object to be heated, and therefore, it is possible to avoid overheating and the extremely low humidity condition to suppress the deterioration of the measurement accuracy. 
         [0051]    A third embodiment of the present invention will be described by using  FIG. 7 . Regarding the same configuration as that in the first embodiment, a description thereof will be omitted. 
         [0052]    In the third embodiment of the present invention, a margin temperature TT DP  is set to a large value and a heating amount is increased in a low temperature and high humidity area where there is a high risk of dew condensation, and thereby the risk of dew condensation is decreased. On the other hand, in an area where there is a low risk of dew condensation, the margin temperature TT DP , is set to a small value, thereby decreasing power consumption. 
         [0053]    A fourth embodiment of the present invention will be described by using  FIG. 7 . Regarding the same configuration as that in the first embodiment, a description thereof will be omitted. 
         [0054]    In addition to the above (description, by setting a heating target temperature to a value obtained from a mixing ratio and target relative humidity, it is possible to predict a temperature and relative humidity to be reached, and to shift the temperature and the relative humidity into a temperature and humidity area existing specific to the humidity sensor where good accuracy and good responsiveness are exhibited. Consequently, the same result as that of conventional methods can be obtained at low cost. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  connector 
           2  cover 
           3  housing 
           4  housing support 
           5  metal bush 
           20 A inflow passage 
           20 B inflow passage 
           21  first subpassage 
           22  second subpassage 
           23  electronic circuit board 
           24  humidity sensor 
           25  heating resistor 
           26  microprocessor 
           27  temperature sensor 
           30  heating control device 
           31 A signal line 
           31 B signal line 
           40  target temperature retrieving unit 
           41  target temperature storing unit 
           42  heating resistor control unit. 
           43  temperature sensor signal processing circuit 
           50  border line between energization and non-energization of heating resistor 
           51  target temperature data 
         TT DP  margin temperature 
         T DP  dew point temperature 
         TT target temperature 
         ts measured temperature 
         Hs measured humidity 
         TT 1  target temperature 
         ts 1  measured temperature 
         Hs 1  measured humidity 
         TT 2  target temperature 
         ts 2  measured temperature 
         Hs 2  measured humidity