The present invention relates to an infrared sensor device using an infrared sensor.
Home electric appliances are known that achieve power saving, or the like, by having a function that operates only when a human body is around. Also in the field of crime-prevention and security, there are products that set off an alarm when detecting invasion of a human body into a target area, and perform various operations.
In such a product, an infrared sensor (mainly, a pyroelectric sensor) is generally used as a sensor for detection of movement of a human body (See Japanese Patent Application Publication number 2009-288498). Japanese Patent Application Publication number 2009-288498 discloses a method in which an infrared attenuation filter is placed in front of a human body detection sensor (infrared sensor), sensitivity of the infrared sensor is controlled by adjusting the infrared attenuation filter, and then a human body detection area is set.
Usually, an infrared sensor generally used for human body detection is used alone. In such a usage, an area where the infrared sensor detects movement of a human body depends on a difference of temperature between a human body and a background. That is, as the human body gets further away from the infrared sensor, or as a difference of temperature between the background in the vicinity of the human body and the human body becomes smaller, a quantity of change of receiving heat quantity of the infrared sensor by the movement of the human body, that is, the sensitivity of the infrared sensor decreases. There is a problem in that an area in which detection of the movement of the human body is performed depends on the difference of temperature between the background in the vicinity of the human body and the human body.
FIG. 31 is a diagram that explains a relationship of the temperature among an infrared sensor, a human body, and a background of a detection area.
An infrared heat quantity Q that is passed to the human body from a target object is expressed by the following Expression 1.Q=σS1F12ε1ε2(T14−T24)  [Expression 1]
In the above Expression 1, σ=5.67×10−8 [W·m−2·k−4] is a Stefan-Boltzmann constant, S1 is a light-receiving area of a sensor, F12 is a configuration factor of the sensor to the target object, T1 is a surface temperature of the sensor, T2 is a surface temperature of the target object, ε1 is a radiation coefficient of the sensor, and ε2 is a radiation coefficient of the target object.
Here, as illustrated in a left diagram in FIG. 31, in a case where there is no human body 102 in an infrared-receiving area 104, an infrared sensor 101 receives an infrared heat quantity Q1 expressed by the following Expression 2 from a background 103. And additionally, as illustrated in a right diagram in FIG. 31, in a case where there is a human body 102 in the infrared-receiving area 104, the infrared sensor 101 receives an infrared heat quantity Q2 expressed by the following Expression 3 from the human body 102 and the background 103.Q1−σS1F12(00)ε1ε2(0)(T14−T34)  [Expression 2]Q2=σS1F12(1)ε1ε2(1)(T14−T24)+σS1F12(01)ε1ε2(0)(T14−T34)  [Expression 3]
In the above Expressions 2 and 3, σ=5.67×10−8 [W·m−2·k−4] is a Stefan-Boltzmann constant, S1 is a light-receiving area of a sensor, F12(00) is a configuration factor of the sensor to the background, F12(01) is a configuration factor of the sensor to the background, F12(1) is a configuration factor of the sensor to a human body, ε1 is a radiation coefficient of the sensor, ε2(0) is a radiation coefficient of a background object, ε2(1) is a radiation coefficient of the human body, T1 is a surface temperature of the sensor, T2 is a surface temperature of the human body (human body temperature), and T3 is a surface temperature of the background (background temperature).
FIG. 32 is a diagram that explains a change of the infrared heat quantity received by the infrared sensor when the human body 102 crosses a sensor detection area of the infrared sensor.
As illustrated in FIG. 32, when the human body 102 comes into a visual field of the infrared sensor 101 (infrared-receiving area 104) from outside, an infrared heat quantity received by the infrared sensor 101 changes from the infrared heat quantity Q1 expressed by the above Expression 2 to the infrared heat quantity Q2 expressed by the above Expression 3.
A value of a quantity of change of the heat quantity Q2−Q1 received by the infrared sensor 101 (a quantity of change Q2−Q1) depends on two parameters of “a distance between the human body 102 and the infrared sensor 101” and “a difference of temperature between the human body temperature T2 and the background temperature T3 (a difference of temperature T2−T3)”. Note that the shorter the distance between the human body 102 and the infrared sensor 101 is, the larger the value of the quantity of the change Q2−Q1 occurring by the movement of the human body 102 becomes. And additionally, the larger the difference of temperature T2−T3 between the human body temperature T2 and the background temperature T3 is, the larger the value of the quantity of the change Q2−Q1 becomes.
Generally, the infrared sensor 101 determines that the movement of the human body 102 is detected when the value of the quantity of the change Q2−Q1 of the infrared heat quantity exceeds a predetermined threshold value. The value of the quantity of the change Q2−Q1, as described above, is dependent on “the distance between the human body 102 and the infrared sensor 101” and “the difference of temperature between the human body temperature T2 and the background temperature T3”.
Therefore, if the threshold value of the quantity of the change Q2−Q1 is uniquely determined, it is obvious that the infrared-receiving area 104 in which the infrared sensor 101 detects the movement of the human body 102 depends on the difference of temperature T2−T3 between the human body temperature T2 and the background temperature T3. That is, when the difference of the temperature T2−T3 changes, the size of the infrared-receiving area 104 in which the infrared sensor 101 detects the movement of the human body 102 also changes.
FIG. 33 is a diagram that explains a state where the size of a sensor detection area changes due to change of the difference of temperature between the human body temperature T2 and the background temperature T3.
As illustrated in FIG. 33, for example, an infrared-receiving area 104a when the difference of the temperature T2−T3=5 degrees C. becomes larger than an infrared-receiving area 104b when the difference of temperature T2−T3=3 degrees C.
Thus, as to a conventional infrared sensor device, the size of the sensor detection area changes due to the change of the difference of temperature between the human body temperature and the background temperature. Note that modularization of a temperature sensor and the infrared sensor in order to detect the background temperature around the human body, and control of a signal amplification factor of the infrared sensor in accordance with the background temperature detected by the temperature sensor make it possible to keep the detection area of the movement of the human body constant. However, in this case, a location where the module is placed has to have the same temperature as that in the background. For example, in a case where the module is placed on a heat-generating object, there is a problem in that it is not possible to precisely detect the background temperature around the human body.
The above problem occurs not only in an infrared sensor for human body detection, but also in an infrared sensor for detection of movement of an object having a certain temperature difference between a background and the object. That is, even in a case where the movement of the object occurs outside a desired distance range from the infrared sensor, due to the size of the difference of temperature between the object and the background, the conventional infrared sensor may mistakenly determine that the movement of the object occurs in the desired distance range from the infrared sensor.
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