Abstract:
The fact that a movable object exits a specific area is judged with high accuracy. A first movement sensor detects the movement of the movable object in a first area. A second movement sensor detects the movement of the movable object in a second area including the first area. A time judgment unit judges whether or not a time difference from a time point when a measurement value obtained by the first movement sensor becomes a predetermined value or less to a time point when a measurement value obtained by the second movement sensor becomes the predetermined value or less is a first threshold value or more. An existence judgment unit judges that the movable object does not exist in the first area in the case where the time judgment unit has judged that the time difference is the first threshold value or more.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an apparatus for deciding whether or not a mobile object is present in a specific area. 
       BACKGROUND ART 
       [0002]    There have been proposed several apparatuses for deciding the presence of a mobile object in a specific area using a motion detection sensor such as a pyroelectric sensor. 
         [0003]    For example, PTL 1 discloses an apparatus for deciding the intrusion and leaving of a mobile object by providing different detection areas with respective corresponding detector elements, and deciding the intrusion and leaving from the temporal relations of detected signals output by the detector elements. PTL 2 discloses an apparatus that starts a timer counter when motion is no longer detected by a human body sensor, and turns off an electric device after a certain period of time has passed. 
       CITATION LIST 
     Patent Literature 
       [0004]    PTL 1: JP P8-146149A 
         [0005]    PTL 2: JP P2004-309051A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    The apparatus in PTL 1, however, has a problem that it cannot detect the presence of a mobile object in a case that the mobile object stays at an interstice between detection areas because the detection areas are disposed adjacent to each other. The apparatus in PTL 2 turns off an electric device after a certain period of time has passed since motion was last detected by a sensor; such timeout processing, however, gives rise to a long delay from when the mobile object was actually no longer in the area to when the system is turned off. 
         [0007]    The present invention has been made in view of such a problem, and its objective is to provide presence deciding apparatus, system, method and program capable of deciding that a mobile object has left a specific area with high precision. 
       Solution to Problem 
       [0008]    The present invention provides a presence deciding apparatus comprising: a time deciding section for deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a measured value indicating motion of a mobile object in a first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a second area including said first area reaches down to or below a predetermined value; and a presence deciding section for deciding that no mobile object is present in said first area in a case that it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value. 
         [0009]    The present invention provides a presence deciding system comprising: a first motion sensor for detecting motion of a mobile object in a first area; a second motion sensor for detecting motion of a mobile object in a second area including said first area; a time deciding section for deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a value measured by the first motion sensor reaches down to or below a predetermined value to when a value measured by the second motion sensor reaches down to or below a predetermined value; and a presence deciding section for deciding that no mobile object is present in said first area in a case that it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value. 
         [0010]    The present invention provides a presence deciding method comprising: deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a measured value indicating motion of a mobile object in a first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a second area including said first area reaches down to or below a predetermined value; and deciding that no mobile object is present in said first area in a case that it is decided that said time lag is equal to or greater than said first threshold value. 
         [0011]    The present invention provides a program for causing a computer connected with a first motion sensor for detecting motion of a mobile object in a first area and a second motion sensor for detecting motion of a mobile object in a second area including said first area to implement: a time deciding function for deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a value measured by the first motion sensor reaches down to or below a predetermined value to when a value measured by the second motion sensor reaches down to or below a predetermined value; and a presence deciding function for deciding that no mobile object is present in said first area in a case that it is decided by said time deciding function that said time lag is equal to or greater than said first threshold value. 
       Advantageous Effects of Invention 
       [0012]    According to the present invention, it is possible to decide that a mobile object has left a specific area with high precision. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a block diagram of a presence deciding apparatus in accordance with a first embodiment of the present invention. 
           [0014]      FIG. 2  is a diagram for explaining a scene in which a mobile object is leaving a first area. 
           [0015]      FIG. 3  is a flow chart for explaining an operation of the presence deciding apparatus in the first embodiment. 
           [0016]      FIG. 4  is a block diagram of the presence deciding apparatus in a variation of the first embodiment. 
           [0017]      FIG. 5  is a block diagram of a presence deciding apparatus in accordance with a second embodiment of the present invention. 
           [0018]      FIG. 6  is a flow chart for explaining an operation of the presence deciding apparatus in the second embodiment. 
           [0019]      FIG. 7  is a block diagram of a presence deciding apparatus in accordance with a third embodiment of the present invention. 
           [0020]      FIG. 8  is a flow chart for explaining an operation of the presence deciding apparatus in the third embodiment. 
           [0021]      FIG. 9  is a block diagram of a presence deciding apparatus in accordance with a fourth embodiment of the present invention. 
           [0022]      FIG. 10  is a flow chart for explaining an operation of the presence deciding apparatus in the fourth embodiment. 
           [0023]      FIG. 11  is a diagram of a configuration of a presence deciding apparatus in accordance with a fifth embodiment of the present invention. 
           [0024]      FIG. 12  is a diagram illustrating output values acquired by first and second motion sensors. 
           [0025]      FIG. 13  is a flow chart for explaining an operation of the presence deciding apparatus in the fifth embodiment. 
           [0026]      FIG. 14  is a flow chart for explaining an operation of the presence deciding apparatus in the fifth embodiment. 
           [0027]      FIG. 15  is a diagram for explaining an operation of the presence deciding apparatus in the fifth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    Now several embodiments of the present invention will be described with reference to the accompanying drawings. The presence deciding apparatus in accordance with the present invention is for deciding whether or not a mobile object has left a specific area. 
       First Embodiment 
       [0029]    A first embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG. 1  is a block diagram of a presence deciding apparatus in accordance with the first embodiment of the present invention. The presence deciding apparatus in accordance with the first embodiment comprises a first motion sensor  11 , a second motion sensor  12 , a time deciding section  13 , and a presence deciding section  14 . 
         [0030]    The first motion sensor  11  detects motion of a mobile object in a specific area (referred to as a first area hereinbelow). 
         [0031]    The second motion sensor  12  detects motion of a mobile object in a second area that includes the first area and is larger than the first area. 
         [0032]    For the first and second motion sensors  11 ,  12 , a pyroelectric sensor, an indoor compact radar, and a vibration sensor may be employed, for example. It should be noted that the locations to install the first and second motion sensors  11 ,  12  are not limited to positions within the respective areas insofar as the second area covered for detection by the second motion sensor  12  includes the first area covered for detection by the first motion sensor  11 . 
         [0033]    The time deciding section  13  decides whether or not a time lag D 1  is equal to or greater than a threshold time T 2 , the time lag D 1  being a period of time from when a value measured by the first motion sensor  11  reaches down to or below a predetermined value to when a value measured by the second motion sensor  12  reaches down to or below a predetermined value. By considering when the value measured by each sensor reaches down to or below a predetermined value as when a mobile object is no longer detected in the area corresponding to the sensor, it is possible to decide whether or not the mobile object has left the first area by deciding whether or not a time T 1   b  is behind a time T 1   a  by a threshold time T 2  or more, where T 1   b  represents a time at which a mobile object is no longer detected in the second area, and T 1   a  represents a time at which a mobile object is no longer detected in the first area. This is based on the concept that when a mobile object O is leaving the first area, as shown in  FIG. 2 , the time at which motion of the mobile object is no longer detected by the second motion sensor  12  is always behind the time at which motion is no longer detected by the first motion sensor  11  because it is always after the mobile object O left the first area that the mobile object O passes through the second area and leaves there. 
         [0034]    In a case that a result of the decision by the time deciding section  13  indicates that the time lag D 1  is equal to or greater than the threshold time T 2 , the presence deciding section  14  decides that no mobile object is present in the first area. 
         [0035]    The threshold time T 2  may be a predefined fixed value, or a value varying depending upon the strength of motion detected by the sensor. By changing the threshold time T 2  to a lower value for stronger motion detected by the first or second motion sensor  11 ,  12 , and changing it to a higher value for weaker motion, the threshold time T 2  can be set depending upon the moving velocity of the mobile object O, which improves precision of decision about leaving from the first area. 
         [0036]    Next, an operation of the presence deciding apparatus in the first embodiment will be described with reference to a flow chart in  FIG. 3 . 
         [0037]    The first and second motion sensors  11 ,  12  periodically perform measurement to detect motion (Step S 11 ). Results of the measurement by the first and second motion sensors  11 ,  12  are stored in a memory area by the time deciding section  13 . 
         [0038]    The time deciding section  13  decides whether or not a time lag D 1  is equal to or greater than a threshold time T 2  based on the results of the measurement by the sensors, the time lag D 1  being obtained by subtracting a time T 1   a  at which a value measured by the first motion sensor  11  reaches down to or below a predetermined value from a time T 1   b  at which a value measured by the second motion sensor  12  reaches down to or below a predetermined value (Step S 12 ). 
         [0039]    The presence deciding section  14  decides whether a mobile object is present in the first area based on a result of the decision by the time deciding section  13  (Step S 13 ). In particular, in a case that the result of the decision by the time deciding section  13  indicates that the time lag D 1  is equal to or greater than the threshold time T 2 , it decides that no mobile object is present in the first area; otherwise, it decides that a mobile object is present in the first area. 
         [0040]    The presence deciding section  14  outputs the result of the decision at Step  13  (Step S 14 ). 
         [0041]    Then, in a case that the decision about the presence of a mobile object is to be terminated, the processing is terminated (Step S 15 : YES), or in a case that the decision is to be continued, the flow goes back to Step S 11  (Step S 15 : NO). 
         [0042]    As described above, according to the present embodiment, it is possible to decide that no mobile object is present in a specific area with high precision by deciding whether or not a time lag D 1  is equal to or greater than a threshold time T 2 , the time lag D 1  being a period of time from when motion is no longer detected in the specific area to when motion is no longer detected in the second area including the specific area. 
         [0043]    It should be noted that in a case that the result of the decision by the time deciding section  13  indicates that the time lag D 1  is equal to or greater than the threshold time T 2 , the presence deciding section  14  may decide that no mobile object is present in the first area when the values measured by the first and second motion sensors  11 ,  12  are equal to or smaller than a predetermined value over a preset certain period of time. 
         [0044]    In a variation of the first embodiment, the presence decision processing for deciding whether or not a mobile object is present in a specific area may be combined with timeout processing. A block diagram of the presence deciding apparatus in this case is shown in  FIG. 4 . The presence deciding section  14  in the variation decides that no mobile object is present in the first area in a case that a predetermined timeout period T 3  has elapsed from the time at which the value measured by the first motion sensor  11  reaches down to or below a predetermined value, and in a case that it is decided by the time deciding section  13  that the time lag D 1  is equal to or greater than the threshold time T 2  before the timeout period T 3  has elapsed. Thus, it is possible to decide that a mobile object has left a specific area more accurately. 
       Second Embodiment 
       [0045]    Next, a presence deciding apparatus in accordance with a second embodiment will be described below.  FIG. 5  is a block diagram of the presence deciding apparatus in accordance with the second embodiment of the present invention. In this embodiment, portions having similar functions to those in the first embodiment are designated by similar symbols and explanation thereof will be omitted. 
         [0046]    A third motion sensor  21  detects motion of a mobile object in a third area that further includes the second area and is larger than the second area. For the third motion sensor  21 , a pyroelectric sensor, an indoor compact radar, and a vibration sensor may be employed, for example. The location to install the third motion sensor  21  is not limited to a position within the third area insofar as the third area covered for detection by the third motion sensor  21  includes the second area covered for detection by the second motion sensor  12 . 
         [0047]    The time deciding section  23  decides whether or not a time lag D 1  is equal to or greater than a threshold time T 2 , and decides whether or not a time lag D 2  is equal to or greater than a threshold time T 4  (second threshold value), the time lag D 1  being a period of time from when a value measured by the first motion sensor  11  reaches down to or below a predetermined value to when a value measured by the second motion sensor  12  reaches down to or below a predetermined value, and the time lag D 2  being a period of time from when a value measured by the first motion sensor  11  reaches down to or below a predetermined value to when a value measured by the third motion sensor  21  reaches down to or below a predetermined value. 
         [0048]    The presence deciding section  24  decides that no mobile object is present in the first area in a case that a predetermined timeout period T 3  has elapsed from a time at which the value measured by the first motion sensor  11  reaches down to or below a predetermined value, and in a case that the result of the decision by the time deciding section  23  indicates that the time lag D 1  is equal to or greater than the threshold time T 2  and the time lag D 2  is equal to or greater than the threshold time T 4  before the timeout period T 3  has elapsed. It should be noted that the threshold time T 4  may be a predefined fixed value, or a value varying depending upon the strength of motion detected by the sensor. By changing the threshold time T 4  to a lower value for stronger motion detected by the first or third motion sensor  11 ,  21 , and changing it to a higher value for weaker motion, the threshold time T 4  can be set depending upon the moving velocity of the mobile object O, which improves precision of decision about leaving from the first area. 
         [0049]    Next, an operation of the presence deciding apparatus in the second embodiment will be described with reference to a flow chart in  FIG. 6 . It should be noted that similar operations to those in the first embodiment are designated by similar symbols and explanation thereof will be omitted. 
         [0050]    The first, second, and third motion sensors  11 ,  12 ,  21  periodically perform measurement to detect motion (Step S 21 ). Results of the measurement by the first, second, and third motion sensors  11 ,  12 ,  21  are supplied to the time deciding section  23 . The time deciding section  23  stores the results of the measurements from the sensors in a memory area. The result of the measurement by the first motion sensor  11  is also supplied to the presence deciding section  24 . 
         [0051]    Next, the time deciding section  23  decides whether or not a time lag D 1  is equal to or greater than a threshold time T 2  and whether or not a time lag D 2  is equal to or greater than a threshold time T 4  based on the results of the measurement by the sensors, the time lag D 1  being obtained by subtracting a time T 1   a  at which a value measured by the first motion sensor  11  reaches down to or below a predetermined value from a time T 1   b  at which a value measured by the second motion sensor  12  reaches down to or below a predetermined value, and the time lag D 2  being obtained by subtracting the time T 1   a  at which a value measured by the first motion sensor  11  reaches down to or below a predetermined value from a time T 1   c  at which a value measured by the third motion sensor  21  reaches down to or below a predetermined value (Step S 22 ). 
         [0052]    The presence deciding section  24  decides whether a mobile object is present in the first area based on the result of the measurement by the first motion sensor  11 , and on a result of the decision by the time deciding section  23  (Step S 23 ). In particular, in a case that a timeout period T 3  has elapsed from the time at which the value measured by the first motion sensor  11  reaches down to or below a predetermined value, the section  24  decides that no mobile object is present in the first area. When the timeout period T 3  has not passed yet, the section  24  decides that no mobile object is present in the first area in a case that the result of the decision from the time deciding section  23  indicates that the time lag D 1  is equal to or greater than the threshold time T 2  and the time lag D 2  is equal to or greater than the threshold time T 4 ; otherwise, the section  24  decides that a mobile object is present in the first area. 
         [0053]    As described above, according to the present embodiment, a result of detection by the third motion sensor  21  covering for detection a third area is additionally used in presence decision, which further improves precision of the result of decision. 
         [0054]    While the presence deciding section  24  is configured to decide that no mobile object is present in the first area when satisfying both the conditions, one that the time lag D 1  is equal to or greater than the threshold time T 2 , and the other that the time lag D 2  is equal to or greater than the threshold time T 5  in the preceding description, the present invention is not limited thereto, and the section  24  may decide that no mobile object is present in the first area when at least one of these conditions is satisfied. 
         [0055]    Moreover, in a case that the aforementioned conditions for decision about the time lag D 1  and time lag D 2  are satisfied, the presence deciding section  24  may decide that no mobile object is present in the first area when the values measured by the first, second, and third motion sensors  11 ,  12 ,  21  are equal to or smaller than a predetermined value over a preset certain period of time. 
         [0056]    Furthermore, decision about the presence of a mobile object in the first area may be performed using four or more motion sensors in a manner similar to the above. 
       Third Embodiment 
       [0057]    Next, a presence deciding apparatus in accordance with a third embodiment will be described below.  FIG. 7  is a block diagram of the presence deciding apparatus in accordance with the third embodiment of the present invention. In this embodiment, portions having similar functions to those in the first embodiment are designated by similar symbols and explanation thereof will be omitted. 
         [0058]    An environmental sensor  31  performs measurement for correcting sensitivity variation of the first and second motion sensors  11 ,  12  due to environmental factors. In a case that a pyroelectric sensor is used for the first and second motion sensors  11 ,  12 , a temperature sensor or an illuminance sensor is employed as the environmental sensor  31 . This is because sensitivity of a pyroelectric sensor varies depending upon environmental factors such as temperature, illuminance, and the like. 
         [0059]    A sensitivity correcting section  32  applies sensitivity correction to the first and second motion sensors  11 ,  12  based on a result of the measurement by the environmental sensor  31 . For the environmental sensor  31  employing a temperature sensor, sensitivity of the first and second motion sensors  11 ,  12  is set higher when the temperature measured by the environmental sensor  31  is higher than a predetermined upper limit value; or sensitivity of the first and second motion sensors  11 ,  12  is set lower when the temperature measured by the environmental sensor  31  is lower than a predetermined lower limit value. Alternatively, for the environmental sensor  31  employing an illuminance sensor, sensitivity of the first and second motion sensors  11 ,  12  is set higher when the illuminance measured by the environmental sensor  31  is higher than a predetermined upper limit value; or sensitivity of the first and second motion sensors  11 ,  12  is set lower when the illuminance measured by the environmental sensor  31  is lower than a predetermined lower limit value. 
         [0060]    Next, an operation of the presence deciding apparatus in the third embodiment will be described with reference to a flow chart in  FIG. 8 . It should be noted that similar operations to those in the first embodiment are designated by similar symbols and explanation thereof will be omitted. 
         [0061]    The environmental sensor  31  periodically performs measurement (Step S 31 ). A result of the measurement by the environmental sensor  31  is supplied to the sensitivity correcting section  32 . 
         [0062]    The sensitivity correcting section  32  applies necessary sensitivity correction to the first and second motion sensors  11 ,  12  based on the result of the measurement by the environmental sensor  31  (Step S 32 ). In particular, the sensitivity correcting section  32  decides whether or not a measured value acquired from the environmental sensor  31  falls within a predetermined range, and in a case that it falls within the predetermined range, no sensitivity correction will be made. When the value measured by the environmental sensor  31  is higher than a preset upper limit value, sensitivity of the first and second motion sensors  11 ,  12  is set higher; or when the measured value is lower than a preset lower limit value, sensitivity of the first and second motion sensors  11 ,  12  is set lower. 
         [0063]    As described above, according to the present embodiment, sensitivity variation of the first and second motion sensors  11 ,  12  due to environmental factors can be corrected to thereby decide that no mobile object is present in a specific area with high precision. 
       Fourth Embodiment 
       [0064]    Next, a presence deciding apparatus in accordance with a fourth embodiment will be described below.  FIG. 9  is a block diagram of the presence deciding apparatus in accordance with the fourth embodiment of the present invention. In this embodiment, portions having similar functions to those in the first embodiment are designated by similar symbols and explanation thereof will be omitted. 
         [0065]    Once a result of decision that no mobile object is present in the first area has been supplied from the presence deciding section  44 , a power source control section  41  disconnects a power source(s) for a motion sensor(s) (second motion sensor  12 ) except the first motion sensor  11 . 
         [0066]    The presence deciding section  44  decides whether or not a mobile object is present in the first area as in the first embodiment, and outputs a result of the decision and supplies it to the power source control section  41 . 
         [0067]    Next, an operation of the presence deciding apparatus in the fourth embodiment will be described with reference to a flow chart in  FIG. 10 . It should be noted that similar operations to those in the first embodiment are designated by similar symbols and explanation thereof will be omitted. 
         [0068]    The presence deciding section  24  inputs a result of the decision at Step S 13  to the power source control section. The power source control section  41  performs necessary disconnection of a power source(s) for a motion sensor(s) except the first motion sensor based on the result of the decision (Step S 41 ). In particular, in a case that the result of the decision indicates that no mobile object is present in the first area, the section  41  disconnects a power source for the second motion sensor  12 ; or in a case that the result of the decision indicates that a mobile object is present in the first area, the section  41  does not perform power source disconnection. 
         [0069]    Thus, in a case that it is decided that no mobile object is present in the first area, motion detection by the second motion sensor  12  will be needed no longer, so that a power source(s) for an unnecessary sensor(s) is disconnected to reduce power consumption. 
         [0070]    In a case that a mobile object re-enters the first area after leaving there once, the first motion sensor  11  detects this. The power source control section  24  may turn on the power source for the second motion sensor  12  again in response to the detection. 
       Fifth Embodiment 
       [0071]    Next, a fifth embodiment will be described with reference to the accompanying drawings. The fifth embodiment is an implementation of the presence deciding apparatus in the first embodiment provided above by a computer. The presence deciding apparatus in accordance with the present embodiment is for deciding whether or not a user is present in front of a computer. 
         [0072]      FIG. 11  is a diagram of a configuration of the presence deciding apparatus in accordance with the present embodiment. The presence deciding apparatus comprises a computer  51 , a display device  52  such as a liquid crystal display, a first motion sensor  11 , and a second motion sensor  12 . 
         [0073]    The computer  51  comprises a control section  511  including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc., and a storage section  512  including a hard disk device, etc. The control section  511  in the computer  51  logically implements the time deciding section  13  and presence deciding section  14  by loading an operating program etc. stored beforehand in the storage section  512  and executing it. 
         [0074]    The first motion sensor  11  is disposed to have its coverage for detection around a front zone of the computer  51  (first area). The second motion sensor  12  is disposed to have its coverage for detection in an area (second area) including the coverage for detection of the first motion sensor  11 . The first and second motion sensors  11 ,  12  are implemented by, for example, a pyroelectric sensor that measures motion of a heat source at a measurement frequency of 30 Hz and outputs a result. 
         [0075]      FIG. 12  shows output values actually acquired by the first and second motion sensors  11 ,  12 , where a user leaves the vicinity of a front zone of a computer to a farther location, and comes back again to the front zone of the computer. It can also be seen from  FIG. 12  that leaving of a mobile object from a specific area can be decided based on a time lag in reaction between the first and second motion sensors  11 ,  12  having different coverage for detection. 
         [0076]    Next, an operation of the presence deciding apparatus in the fifth embodiment will be described with reference to  FIGS. 13-15 . 
         [0077]    A flow chart in acquiring a value measured by the second motion sensor  12  is shown in  FIG. 13 . 
         [0078]    Once the time deciding section  13  has acquired a value measured by the second motion sensor  12  (Step S 51 ), it decides whether or not the acquired measured value is greater than a threshold value TH 1  for motion detection, and in a case that the value is greater, the section  13  decides that motion is detected by the second motion sensor  12  (Step S 52 ). The measured value for use in the decision here is, for example, a standard deviation of the values measured by the second motion sensor  12  within a predetermined period of time (such as 0.5 seconds). 
         [0079]    In a case that motion is detected by the second motion sensor  12  (Step S 52 : YES), the time deciding section  13  updates a latest motion-detected time TM 1   b  for the second motion sensor  12  with the time, and stores the time (Step S 53 ). In a case that no motion is detected by the second motion sensor  12  (Step S 52 : NO), the processing is then terminated. 
         [0080]    A flow chart in acquiring a value measured by the first motion sensor  11  is shown in  FIG. 14 . 
         [0081]    Once the time deciding section  13  has acquired a value measured by the first motion sensor  11  (Step S 61 ), it compares the acquired measured value with a threshold value TH 1  to decide about the presence of motion detected by the first motion sensor  11  (Step S 62 ). The measured value for use in the decision here is, for example, a standard deviation of the values measured by the first motion sensor  11  within a predetermined period of time (such as 0.5 seconds). 
         [0082]    In a case that motion is detected by the first motion sensor  11  (Step S 62 : YES), the time deciding section  13  updates a latest motion-detected time TM 1   a  for the first motion sensor  11  with the time, stores the time (Step S 63 ), and stores in the storage section  512  and displays on the display device  52  a result of presence decision, “present” (Step S 67 ). 
         [0083]    In a case that no motion is detected by the first motion sensor  11  (Step S 62 : NO), the presence deciding section  14  decides whether a result of the preceding presence decision in this processing is “present” (Step S 64 ). 
         [0084]    In a case that the result of the preceding presence decision is “absent” (Step S 64 : NO), the presence deciding section  14  stores in the storage section  512  and displays on the display device  52  a result of presence decision, “absent” (Step S 67 ). 
         [0085]    In a case that the result of the preceding presence decision is “present” (Step S 64 : YES), the presence deciding section  14  calculates an elapsed time T 5  which is a period of time from the latest motion-detected time TM 1   a  by the first motion sensor  11  to the time point of the processing, and decides whether or not the elapsed time T 5  exceeds a preset timeout period T 3  (Step S 65 ). 
         [0086]    In a case that the elapsed time T 5  exceeds the timeout period T 3  (Step S 65 : YES), the presence deciding section  14  stores in the storage section  512  and displays on the display device  52  a result of presence decision, “absent” (Step S 67 ). 
         [0087]    In a case that the elapsed time T 5  does not exceed the timeout period T 3  (Step S 65 : NO), the time deciding section  13  decides whether or not a time lag is equal to or greater than the threshold time T 2 , the time lag being obtained by subtracting the latest motion-detected time TM 1   a  for the first motion sensor  11  from the latest motion-detected time TM 1   b  for the second motion sensor  12  (Step S 66 ). The presence deciding section  14  stores in the storage section  512  and displays on the display device  52  the result of the presence decision at Step S 57  (Step S 67 ). 
         [0088]    As described above, according to the present embodiment, it is possible to decide whether or not a user is present around the front zone of the computer  51  with high precision. 
         [0089]    It is also possible to implement the second through fourth embodiments by a computer program. In this case, a processor operated by a program stored in a program memory implements functions and operations similar to those in the embodiments described above. It is also possible to implement only part of the functions of the embodiments described above by a computer program. 
         [0090]    Moreover, the first through fifth embodiments described above may be combined. 
         [0091]    While the present invention has been described with reference to the preferred embodiments hereinabove, the present invention is not necessarily limited to these embodiments and may be practiced with several modifications within a scope of its technical concept. 
         [0092]    The present application claims priority based on Japanese Patent Application No. 2009-287111 filed on Dec. 18, 2009, the disclosure of which is incorporated herein in its entirety. 
         [0093]    Part or all of the embodiments above may be described as in Supplementary notes below, although not being limited thereto. 
         [0094]    (Supplementary note 1) A presence deciding system characterized in comprising: a first motion sensor for detecting motion of a mobile object in a first area; a second motion sensor for detecting motion of a mobile object in a second area including said first area; a time deciding section for deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a value measured by the first motion sensor reaches down to or below a predetermined value to when a value measured by the second motion sensor reaches down to or below a predetermined value; and a presence deciding section for deciding that no mobile object is present in said first area in a case that it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value. 
         [0095]    (Supplementary note 2) The presence deciding system as recited in Supplementary note 1, characterized in that: said presence deciding section decides that no mobile object is present in said first area in a case that it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value before a predetermined timeout period has elapsed from a time at which the value measured by said first motion sensor reaches down to or below a predetermined value, and in a case that said timeout period has elapsed from a time at which the value measured by said first motion sensor reaches down to or below a predetermined value. 
         [0096]    (Supplementary note 3) The presence deciding system as recited in Supplementary note 1 or 2, characterized in that: said presence deciding section decides that no mobile object is present in said first area in a case that the values measured by said first and second motion sensors are equal to or smaller than a predetermined value over a certain period of time after it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value. 
         [0097]    (Supplementary note 4) The presence deciding system as recited in any one of Supplementary notes 1 through 3, characterized in further comprising: a third motion sensor for detecting motion of a mobile object in a third area including said second area, and characterized in that: said time deciding section further decides whether or not a time lag is equal to or greater than a second threshold value, the time lag being a period of time from when a value measured by said first motion sensor reaches down to or below a predetermined value to when a value measured by said third motion sensor reaches down to or below a predetermined value, and said presence deciding section decides that no mobile object is present in said first area in a case that a result of the decision by said time deciding section indicates at least one of conditions that said time lag for said first and second areas is equal to or greater than said first threshold value and that said time lag for said first and third areas is equal to or greater than said second threshold value. 
         [0098]    (Supplementary note 5) The presence deciding system as recited in any one of Supplementary notes 1 through 4, characterized in further comprising: an environmental sensor for performing measurement for correcting sensitivity variation of said motion sensors due to environmental factors; and a sensitivity correcting section for applying sensitivity correction to said motion sensors based on a result of the measurement by said environmental sensor. 
         [0099]    (Supplementary note 6) The presence deciding system as recited in any one of Supplementary notes 1 through 5, characterized further comprising: a power source control section for disconnecting power source for at least one motion sensor except said first motion sensor in a case that it is decided by said presence deciding section that no mobile object is present in said first area. 
         [0100]    (Supplementary note 7) A presence deciding apparatus characterized in comprising: a time deciding section for deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a measured value indicating motion of a mobile object in a first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a second area including said first area reaches down to or below a predetermined value; and a presence deciding section for deciding that no mobile object is present in said first area in a case that it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value. 
         [0101]    (Supplementary note 8) The presence deciding apparatus as recited in Supplementary note 7, characterized in that: said presence deciding section decides that no mobile object is present in said first area in a case that it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value before a predetermined timeout period has elapsed from a time at which the measured value indicating motion in said first area reaches down to or below a predetermined value, and in a case said timeout period has elapsed from a time at which the measured value indicating motion in said first area reaches down to or below a predetermined value. 
         [0102]    (Supplementary note 9) The presence deciding apparatus as recited in Supplementary notes 7 or 8, characterized in that: said presence deciding section decides that no mobile object is present in said first area in a case that the measured values for said first and second areas are equal to or smaller than a predetermined value over a certain period of time after it is decided by said time deciding section that said time lag is equal to or greater than said first threshold value. 
         [0103]    (Supplementary note 10) The presence deciding apparatus as recited in any one of Supplementary notes 7 through 9, characterized in that: said time deciding section further decides whether or not a time lag is equal to or greater than a second threshold value, the time lag being a period of time from when the measured value indicating motion in said first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a third area including said second area reaches down to or below a predetermined value, and said presence deciding section decides that no mobile object is present in said first area in a case that a result of the decision by said time deciding section indicates at least one of conditions that said time lag for said first and second areas is equal to or greater than said first threshold value and that said time lag for said first and third areas is equal to or greater than said second threshold value. 
         [0104]    (Supplementary note 11) The presence deciding apparatus as recited in any one of Supplementary notes 7 through 10, characterized in further comprising: a sensitivity correcting section for applying sensitivity correction to motion sensors for detecting motion in said areas based on a result of measurement for correcting sensitivity variation of the motion sensors due to environmental factors. 
         [0105]    (Supplementary note 12) The presence deciding apparatus as recited in any one of Supplementary notes 7 through 11, characterized in further comprising: a power source control section for disconnecting power source for at least one motion sensor except a motion sensor for detecting motion in said first area in a case that it is decided by said presence deciding section that no mobile object is present in said first area. 
         [0106]    (Supplementary note 13) A presence deciding method characterized in comprising: deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a measured value indicating motion of a mobile object in a first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a second area including said first area reaches down to or below a predetermined value; and deciding that no mobile object is present in said first area in a case that it is decided that said time lag is equal to or greater than said first threshold value. 
         [0107]    (Supplementary note 14) The presence deciding method as recited in Supplementary note 13, characterized in comprising: deciding that no mobile object is present in said first area in a case that it is decided that said time lag is equal to or greater than said first threshold value before a predetermined timeout period has elapsed from a time at which the measured value indicating motion in said first area reaches down to or below a predetermined value, and in a case that said timeout period has elapsed from a time at which the measured value indicating motion in said first area reaches down to or below a predetermined value. 
         [0108]    (Supplementary note 15) The presence deciding method as recited in Supplementary note 13 or 14, characterized in comprising: deciding that no mobile object is present in said first area in a case that the measured values for said first and second areas are equal to or smaller than a predetermined value over a certain period of time after it is decided that said time lag is equal to or greater than said first threshold value. 
         [0109]    (Supplementary note 16) The presence deciding method as recited in any one of Supplementary notes 13 through 15, characterized in further comprising: deciding whether or not a time lag is equal to or greater than a second threshold value, the time lag being a period of time from when the measured value indicating motion in said first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a third area including said second area reaches down to or below a predetermined value; and deciding that no mobile object is present in said first area in a case that a result of the decision is obtained indicating at least one of conditions that said time lag for said first and second areas is equal to or greater than said first threshold value and that said time lag for said first and third areas is equal to or greater than said second threshold value. 
         [0110]    (Supplementary note 17) The presence deciding method as recited in any one of Supplementary notes 13 through 16, characterized in comprising: applying sensitivity correction to motion sensors for detecting motion in said areas based on a result of measurement for correcting sensitivity variation of the motion sensors due to environmental factors. 
         [0111]    (Supplementary note 18) The presence deciding method as recited in any one of Supplementary notes 13 through 17, characterized in comprising: disconnecting power source for at least one motion sensor except a motion sensor for detecting motion in said first area in a case that it is decided that no mobile object is present in said first area. 
         [0112]    (Supplementary note 19) A program for causing a computer connected with a first motion sensor for detecting motion of a mobile object in a first area and a second motion sensor for detecting motion of a mobile object in a second area including said first area to implement: a time deciding function for deciding whether or not a time lag is equal to or greater than a first threshold value, the time lag being a period of time from when a value measured by the first motion sensor reaches down to or below a predetermined value to when a value measured by the second motion sensor reaches down to or below a predetermined value; and a presence deciding function for deciding that no mobile object is present in said first area in a case that it is decided by said time deciding function that said time lag is equal to or greater than said first threshold value. 
         [0113]    (Supplementary note 20) The program as recited in Supplementary note 19, characterized in that: said presence deciding function decides that no mobile object is present in said first area in a case that it is decided by said time deciding function that said time lag is equal to or greater than said first threshold value before a predetermined timeout period has elapsed from a time at which the measured value indicating motion in said first area reaches down to or below a predetermined value, and in a case that said timeout period has elapsed from a time at which the measured value indicating motion in said first area reaches down to or below a predetermined value. 
         [0114]    (Supplementary note 21) The program as recited in Supplementary note 19 or 20, characterized in that: said presence deciding function decides that no mobile object is present in said first area in a case that the measured values for said first and second areas are equal to or smaller than a predetermined value over a certain period of time after it is decided by said time deciding function that said time lag is equal to or greater than said first threshold value. 
         [0115]    (Supplementary note 22) The program as recited in any one of Supplementary notes 19 through 21, characterized in that: said time deciding function further decides whether or not a time lag is equal to or greater than a second threshold value, the time lag being a period of time from when the measured value indicating motion in said first area reaches down to or below a predetermined value to when a measured value indicating motion of a mobile object in a third area including said second area reaches down to or below a predetermined value, and said presence deciding function decides that no mobile object is present in said first area in a case that a result of the decision by said time deciding function indicates at least one of conditions that said time lag for said first and second areas is equal to or greater than said first threshold value and that said time lag for said first and third areas is equal to or greater than said second threshold value. 
         [0116]    (Supplementary note 23) The program as recited in any one of Supplementary notes 19 through 22, for further implementing: a sensitivity correcting function for applying sensitivity correction to motion sensors for detecting motion in said areas based on a result of measurement for correcting sensitivity variation of the motion sensors due to environmental factors. 
         [0117]    (Supplementary note 24) The program as recited in any one of Supplementary notes 19 through 23, for further implementing: a power source control function for disconnecting power source for at least one motion sensor except a motion sensor for detecting motion in said first area in a case that it is decided by said presence deciding function that no mobile object is present in said first area. 
       REFERENCE SIGNS LIST 
       [0118]      11  First motion sensor 
         [0119]      12  Second motion sensor 
         [0120]      13  Time deciding section 
         [0121]      14  Presence deciding section 
         [0122]      21  Third motion sensor 
         [0123]      31  Environmental sensor 
         [0124]      32  Sensitivity correcting section 
         [0125]      41  Power source control section 
         [0126]      44  Presence deciding section 
         [0127]      51  Computer 
         [0128]      52  Display device 
         [0129]      511  Storage section 
         [0130]      512  Control section