Abstract:
A drop determining device includes a memory and a processor coupled to the memory. The processor executes a process including: detecting acceleration within a predetermined range; calculating acceleration outside the predetermined range, using the acceleration detected at the detecting; and determining whether drop has occurred, using the acceleration calculated at the calculating.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of International Application PCT/JP2011/063271, filed on Jun. 9, 2011 and designating the U.S., the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present invention relates to a drop determining device and a drop determining method. 
       BACKGROUND 
       [0003]    Conventionally, there has been a technique for detecting the acceleration of an object in movement and determining the state of the object based on the detection result, using an acceleration sensor. This acceleration sensor realizing the technique includes a low range sensor suitable for detection of low acceleration and a high range sensor suitable for detection of high acceleration. The low range acceleration sensor has a high resolution in detection of acceleration about ±5 G or lower, and is thus suitable for determining some state, such as walking involving the acceleration of 0.5 to 2.0 G. It is used, for example, for a pedometer for mobile terminals. On the other hand, the high range acceleration sensor has a high resolution in detection of acceleration about ±70 G, and is thus suitable for determining some state, such as dropping involving an impact of several ten to 100 G. In recent years, there has been proposed a mobile terminal which has a plurality of acceleration sensors with different ranges and switches between these sensors in accordance with the aspect.
   Patent Literature 1: Japanese Laid-open Patent Publication No. 2008-175771   
 
         [0005]    However, the acceleration range that can accurately be detected by the acceleration sensor is restricted for each sensor, and there does not exist acceleration sensor that has a high resolution in all ranges. For example, because the dropping of a mobile terminal involves a strong impact thereon, the low range acceleration sensor does not detect the accurate acceleration. In view of this circumstance, it may be considered that the drop determination may be performed by the high range acceleration sensor. However, it is difficult to adopt the high range acceleration sensor into the application requiring acceleration detection with a high resolution in a low range. That is, if a high range acceleration sensor is installed in a mobile terminal, a problem is the difficulty of realizing the function of the application, such as the pedometer. As described above, in combination of both of the low range and high range acceleration sensors, acceleration detection is possible in a wide range. However, installation of a plurality of acceleration sensors into the mobile terminal may be a factor of hindering the miniaturization or weight reduction. 
       SUMMARY 
       [0006]    To solve the above problem and attain the object, a drop determining device disclosed in this application, according to an aspect, includes a detecting unit, a calculating unit, and a determining unit. The detecting unit detects acceleration within a predetermined range. The calculating unit calculates acceleration outside the predetermined range, using the acceleration detected by the detecting unit. The determining unit determines whether drop has occurred, using the acceleration calculated by the calculating unit. 
         [0007]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a diagram illustrating a functional configuration of a mobile terminal; 
           [0010]      FIG. 2  is a diagram illustrating a data storage example in an acceleration conversion table, in a first embodiment; 
           [0011]      FIG. 3  is a diagram illustrating a hardware configuration of the mobile terminal; 
           [0012]      FIG. 4  is a flowchart for explaining an operation of the mobile terminal in the first embodiment; 
           [0013]      FIG. 5  is a diagram for explaining a procedure for calculating the impact time and the acceleration at impact, in the first embodiment; 
           [0014]      FIG. 6  is a diagram illustrating a data storage example in the acceleration conversion table, in a second embodiment; 
           [0015]      FIG. 7  is a flowchart for explaining an operation of a mobile terminal, in the second embodiment; 
           [0016]      FIG. 8  is a diagram for explaining a procedure for calculating the range of the impact time and the acceleration at impact, in the second embodiment; and 
           [0017]      FIG. 9  is a diagram illustrating a computer that executes a drop determining program. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Descriptions will now specifically be made to embodiments of a drop determining device and a drop determining method, as disclosed in the present application, with reference to the drawings. The embodiments are not intended to limit the drop determining device and drop determining method disclosed in the present application. 
       First Embodiment 
       [0019]    Descriptions will now be made to a configuration of a mobile terminal according to the embodiment, in a drop determining device disclosed in the present application.  FIG. 1  is a diagram illustrating a functional configuration of a mobile terminal  10  according to this embodiment. As illustrated in  FIG. 1 , the mobile terminal  10  has a sensor unit  11 , a sampling processing unit  12 , an impact calculating unit  13 , and an application processing unit  14 . Each of these constituent parts are connected with each other, for enabling input/output of signals or data unidirectionally or bidirectionally. 
         [0020]    The sensor unit  11  is a low range acceleration sensor unit whose range value is set to ±4 G. That is, the sensor unit  11  can detect (measure) the acceleration up to +4 G as the upper limit value, and can detect (measure) the acceleration down to −4 G as the lower limit value. The sensor unit  11  is a well-known and commonly-used sensor, and thus will not specifically be described. The sensor unit  11  is a triaxial acceleration sensor which detects acceleration in triaxial directions which are orthogonal to each other. The acceleration in the X-axis direction is a displacement value in accordance with the movement in the crosswise direction, in the exercise (walking or dropping) involving the acceleration. That is, the acceleration in the X-axis direction is the amount of movement in the crosswise direction based on the installation position of the sensor unit  11  at a predetermined point of time. The acceleration will become a positive value for an amount of movement in the left direction, and will become a negative value for an amount of movement in the right direction. The acceleration in the Y-axis direction is a displacement value in accordance with the movement in the vertical direction, in the exercise involving the acceleration. That is, the acceleration in the Y-axis direction is the amount of movement in the vertical direction based on the installation position of the sensor unit  11  at a predetermined point of time. The acceleration will become a positive value for an amount of movement in the upper direction, and will become a negative value for an amount of movement in the lower direction. The acceleration in the Z-axis direction is a displacement value in accordance with the movement in the front-back direction, in the exercise involving the acceleration. That is, the acceleration in the Z-axis direction is the amount of movement in the front-back direction based on the installation position of the sensor unit  11  at a predetermined point of time. The acceleration will become a positive value for an amount of movement in the front direction, and will become a negative value for an amount of movement in the back direction. 
         [0021]    The sampling processing unit  12  periodically samples a value of the acceleration detected by the sensor unit  11 , and outputs the value to the impact calculating unit  13 . The sampling period is preferably a short period of, for example, 1 ms, and more preferably, 0.1 ms, from the perspective of performing acceleration detection in high range and also impact calculation with high accuracy. 
         [0022]    The impact calculating unit  13  calculates the out-of-range acceleration of higher than 4 G, using the acceleration input from the sampling processing unit  12 , after detected by the sensor unit  11 . That is, the impact calculating unit  13  calculates the out-of-range acceleration, using the time at which the acceleration detected by the sensor unit  11  exceeds 4 G, the time at which the corresponding acceleration returns into the range of 4 G or lower, the slope of the acceleration before the exceeding time, and the slope of the acceleration after the returned time. The impact calculating unit  13  outputs the value of the calculated acceleration as an estimated value of the acceleration at impact, to the application processing unit  14 . 
         [0023]    During activation of the drop determining application, the application processing unit  14  converts the value (estimated value) of the acceleration at impact, calculated by the impact calculating unit  13 , into an actual measurement value, and compares the value with a threshold value. If the converted acceleration is equal to the threshold value or higher, it is determined that there is a drop. Then, the application processing unit  14  displays this drop determination result. 
         [0024]    The application processing unit  14  has an acceleration conversion table  141   a .  FIG. 2  is a diagram illustrating a data storage example in the acceleration conversion table  141   a  for converting the acceleration (estimated value) at impact into an actual measurement value, in the first embodiment. As illustrated in  FIG. 2 , the acceleration conversion table  141   a  stores the acceleration at impact that is calculated by the impact calculating unit  13  as an “M estimated value” and the acceleration at impact that is measured in advance by a high range acceleration sensor as an “actual measurement value”, in association with each other. For example, when the M estimated value is calculated to be “5.00 G”, a value “5.50 G” is referred for a comparison with the threshold value, because the actual acceleration value is set in advance to “5.50 G”. Similarly, when the M estimated value is calculated to be “80.20 G”, this value is referred for determination as to whether the drop has occurred, because the actual acceleration value set in advance is “80.68 G”. As described above, the acceleration at impact that is calculated by the impact calculating unit  13  as the M estimated value is corrected into an actual measurement value by the application processing unit  14 . 
         [0025]    The correspondence relationship between the M estimated value and the actual measurement value set in the acceleration conversion table  141   a  can be updated based on the actually measured acceleration which has been measured at impact (dropping or throwing) of the mobile terminal  10 . That is, the application processing unit  14  appropriately updates the above-described correspondence relationship in the acceleration conversion table  141   a , in accordance with the impact characteristics of the sensor unit  11  or the calculation accuracy of the M estimated value, and always maintains up-to-date status. Thus, the application processing unit  14  can perform drop determination based on the accurate acceleration value which is pretty close to the actual value, with reference to the acceleration conversion table  141   a . Therefore, the mobile terminal  10  can acquire a drop determination result with a high level of accuracy, resulting in increase in the reliability of the mobile terminal  10 . 
         [0026]    The above-described mobile terminal  10  is physically realized with, for example, a mobile phone.  FIG. 3  is a diagram illustrating a hardware configuration of a mobile phone as the mobile terminal  10 . As illustrated in  FIG. 3 , the mobile terminal  10  physically has a CPU (Central Processing Unit)  10   a , an acceleration sensor  10   b , a memory  10   c , a display unit  10   d , and a wireless unit  10   e  having an antenna A. The sensor unit  11  is realized with the acceleration sensor  10   b , as described above. The sampling processing unit  12 , the impact calculating unit  13 , and the application processing unit  14  are realized with an integrated circuit, such as the CPU  10   a . The range value, the threshold value of the drop determination, the sampling value of the acceleration, and the acceleration conversion table  141   a  are kept in the memory  10   c , such as RAM (Random Access Memory), a ROM (Read Only Memory), and a flash memory. The impact calculation result is displayed on the display unit  10   d , such as an LCD (Liquid Crystal Display). 
         [0027]    An operation of the mobile terminal  10  will now be described. The following operational descriptions will be made on the assumption that a user drops the mobile terminal  10  in the acceleration range of ±4 G on the floor surface for some reason. 
         [0028]      FIG. 4  is a flowchart for explaining an operation of the mobile terminal  10 . When the user activates a drop determining application of the mobile terminal  10  (S 1 ), the sampling processing unit  12  acquires an acceleration value in the triaxial directions input from the sensor unit  11  at a predetermined period, to start sampling of the acceleration (S 2 ). 
         [0029]    The impact calculating unit  13  always keeps the acceleration values which have been sampled at a one time-period prior to and two time-periods prior to the most recent sampling time, as sampling values (S 3 ). While the impact calculating unit  13  keeps the sampling value, it also monitors whether the latest sampling value has exceeded the range (±4 G) of the sensor unit  11  (S 4 ). As a result of the monitoring, when the sampling value has exceeded the range of the sensor unit  11  (S 4 ; Yes), it moves to the procedure of S 5 . Sampling values a 1  and a 2  corresponding to the two time periods before the excess of the range are not deleted even if the next sampling value would be acquired, and are continuously kept in the memory  10   c.    
         [0030]    In S 5 , after the excess of the range, the impact calculating unit  13  keeps a sampling value b 2  right after returned into the range and a sampling value b 3  at one time period after that. Further, the impact calculating unit  13  keeps a time t 1  of the overrange and a returned time t 2  into the range, into the memory  10   c  (S 6 ). The time t 1  can be calculated as a time at which the sensor unit  11  stops sampling the acceleration. The time t 2  can be calculated as a time at which the sensor unit  11  restarts sampling the acceleration, and can also be calculated by adding an obtained value of “the number of samples during the times t 1 t 2 *the sampling period” to the time t 1 . 
         [0031]    As a result of the above monitoring, while the sampling value remain within the range of the sensor unit  11  (S 4 ; No), the mobile terminal  10  continues to keep the sampling values corresponding to the most recent two time periods by the impact calculating unit  13  (S 3 ). The kept sampling values at this time remain in data corresponding to two time periods. Thus, the space of the memory  10   c  is not wasted, even if the sampling process is executed for a long time upon activation of the drop determining application. 
         [0032]    In S 7 , the impact calculating unit  13  calculates a slope S 1  of the acceleration before the overrange. In S 3 , the impact calculating unit  13  keeps the sampling values a 1  and a 2  right before the excess of the range. Thus, the slope S 1  can be calculated using the values and the sampling period, by |a 1 −a 2 |/c. Similarly, in S 8 , the impact calculating unit  13  calculates a slope S 2  of the acceleration after returned into the range. Because the impact calculating unit  13 , in S 5 , keeps the sampling values b 2  and b 3  right after returned into the range, the slope S 2  can be calculated using the values and the sampling period, by |b 2 −b 3 |/c. These calculation results are kept in the memory  10   c.    
         [0033]    The impact calculating unit  13  calculates a time t c  at which the mobile terminal  10  gets an impact and the acceleration M at that time, using the times t 1  and t 2  kept in S 6  and the slopes S 1  and S 2  kept in S 7  and S 8  (S 9 ). 
         [0034]    Descriptions will now specifically be made to a method for calculating the impact time t c  and the acceleration M at impact, with reference to  FIG. 5 . In  FIG. 5 , the time t [ms] is assigned to the x axis, while the acceleration [G] is assigned to the y axis. A time t 0  is the time at which the mobile terminal  10  is dropped and comes in contact with the floor surface. A time t 1  is a time at which the acceleration detected by the sensor unit  11  exceeds +4 G as the upper value of the range, while a time t 2  is a time at which the detected acceleration has returned into the range again. Further, the sampling period is set to 1 [ms], the slope (the slope between circles a 1  and a 2  in  FIG. 5 ) kept in S 7  is set as S 1 , and the slope (the slope between triangles b 2  and b 3  in  FIG. 5 ) kept in S 8  is set as S 2 . The circle a 3  is a sampling value right after (out of range) the acceleration detected by the sensor unit  11  has exceeded the upper limit of the range. Since this acceleration is not in fact detected, it is represented by a broken line, to distinguish from the circles a 1  and a 2  representing the sampling values of the detected acceleration. Similarly, the triangle b 1  is a sampling value right before (out of range) the acceleration detected by the sensor unit  11  returns into the range. Since this acceleration is not in fact detected, it is represented by a broken line to distinguish from the triangles b 2  and b 3  representing the sampling values of the detected acceleration. 
         [0035]    Under the above-described conditions, the impact time as a calculation target in S 9  is set as t c , the relationship of t c −t 1 :t 2 −t c =S 2 :S 1  is made. Thus, t c  can be calculated using the following equation (1). 
         [0000]      Time at impact  t   c =( S   1   t   1   +S   2   t   2 )/( S   1   +S   2 )  (1)
 
         [0036]    Under the above-described condition, when the acceleration at impact as a calculation target in S 9  is set as M, the relationships of M=S 1 t c +b and 4=S 1 t 1 +b (b is a constant) are made. Thus, the acceleration M of t c  can be calculated using the following equation (2). 
         [0000]      Acceleration at impact  M=S   1   S   2 ( t   2   −t   2 )/( S   1   +S   2 )+4  (2)
 
         [0037]    That is, the impact calculating unit  13  obtains two-dimensional coordinates of an intersection point D of a straight line B passing through the circles a 1  and a 2  and a straight line C passing through the triangles b 2  and b 3 . The impact calculating unit  13  assumes the x coordinate value as the time at impact t c , and also assumes the y coordinate as the acceleration at impact M. 
         [0038]    In S 10 , the application processing unit  14  converts the acceleration M calculated by the impact calculating unit  13  in S 9  into an actual measurement value. This conversion process is executed with reference to the above-described acceleration conversion table  141   a . The acceleration M is not a value detected actually by the sensor unit  11 , but is a value (estimated value) calculated using the equation just based on the actual measurement values. Thus, there is a possibility that the acceleration M does not coincide with the actual acceleration value. The application processing unit  14  performs correction to convert the estimated value into an actual measurement value, for the acceleration value for use in drop determination to approximate to the actual acceleration value, based on the correspondence relationship between the estimated value and the actual measurement value as set in the acceleration conversion table  141   a . For example, when the acceleration at impact calculated by the impact calculating unit  13  in S 9  is “80.00”, it is converted into “80.46” (see  FIG. 2 ). 
         [0039]    In S 11 , the application processing unit  14  determines whether the mobile terminal  10  has been dropped, based on the acceleration at impact (actual measurement value of  FIG. 2 ) after the conversion in S 10  and a threshold value T 1  illustrated in  FIG. 5 . That is, the application processing unit  14  compares the magnitude relationship between the acceleration at impact M and a preset threshold value T 1 . When the acceleration at impact M≧threshold value T 1 , it is determined that the drop has occurred. On the contrary, when the acceleration at impact M&lt;the threshold value T 1 , the application processing unit  14  determines that the drop has not occurred. At the time of drop, since the acceleration of several ten to 100 G occurs on the mobile terminal  10  in accordance with the material of the contact surface, the threshold value T 1  is set, for example, to 20 G. Note, however, that this set value can adequately be changed, in accordance with the specifications of the mobile terminal  10  or the calculation accuracy of the estimated value. 
         [0040]    When it is determined that the drop has occurred in S 11 , the impact time t c  calculated in S 9  as “drop time” is recorded in the memory  10   c , together with information representing that the drop has occurred. At the same time, a message is displayed on the display unit  10   d , and represents, for example, “drop has occurred, about 10:20:35, 19, May”. 
         [0041]    As described above, the mobile terminal  10  according to this embodiment has the sensor unit  11 , the impact calculating unit  13 , and the application processing unit  14 . The sensor unit  11  detects the acceleration within the above-described predetermined range. The impact calculating unit  13  calculates the acceleration outside the above-described predetermined range, using the acceleration detected by the sensor unit  11 . The application processing unit  14  determines the occurrence of the drop, using the above-described acceleration calculated by the impact calculating unit  13 . In more particular, the impact calculating unit  13  calculates the acceleration outside the above-described predetermined range, using the time t 1 , the time t 2 , the slope S 1  of the acceleration, and the slope S 2  of the acceleration. The time t 1  is the time at which the acceleration detected by the sensor unit  11  has exceeded the above-described predetermined range. The time t 2  is the time at which the acceleration has returned into the predetermined range. The slope S 1  is the slope of the acceleration before the time t 1 . The slope S 2  is the slope of the acceleration after the time t 2 . The application processing unit  14  determines the occurrence of the drop, based on whether the acceleration calculated by the impact calculating unit  13  is a predetermined value or higher. 
         [0042]    The mobile terminal  10  according to this embodiment calculates the time of applied impact and the acceleration at that time, using the acceleration detected with a low range acceleration sensor. As a result, it is possible to determine the occurrence of the drop involving a high range impact, without installing a high range acceleration sensor. In other words, the mobile terminal  10  calculates the acceleration within a range that is unmeasurable by the low range acceleration sensor due to range insufficiency, using a predetermined equation. The mobile terminal  10  assumes an acceleration value outside the range, based on the result. The mobile terminal  10  can quickly discriminate when and how much impact the drop has occurred. Thus, if the mobile terminal  10  is one to record and display the discrimination result as historical information, the user can easily recognize the occurrence of the drop and its time. Not only the user, but also a third party, such as the communication enterprise (carrier) or manufacturer, can easily and quickly know the occurrence of the drop, by referring to the above-described historical information. Even if the mobile terminal  10  is damaged or is unusable due to the impact of drop, the third party can inform the user that it is caused by the drop, based on the drop time. 
         [0043]    Specifically, the mobile terminal  10  according to this embodiment uses the slope right before the acceleration exceeds the range as the slope S 1 , of slopes of the acceleration before the time t 1 . The slope of the acceleration before the time t 1  reaches the upper limit value (4 G) of the range while increasing the accuracy, during the time since the mobile terminal  10  comes in contact with the floor surface until the acceleration exceeds the range, and resulting in the overrange. Therefore, the mobile terminal  10  uses the actual measurement value (the actual measurement value nearly outside the range as much as possible) of the acceleration right before the overrange, in calculation of the estimated acceleration, thereby enabling to calculate the acceleration with less error even outside the range. As a result, the mobile terminal  10  can estimate the acceleration with high accuracy. For the slope on the side of returning into the range, the mobile terminal  10  uses the slope right after the acceleration returns into the range as the slope S 2 , of slopes of the acceleration after the time t 2 . The slope of the acceleration after the time t 2  gets close to 0 [G], while decreasing the accuracy and reducing the acceleration value, after the mobile terminal  10  comes in contact with the floor surface, as time goes by. Therefore, the mobile terminal  10  uses the actual measurement value (the actual measurement value nearly outside the range as much as possible) of the acceleration right after returned into the range, in calculation of the estimated acceleration, thereby enabling to calculate the acceleration with less error even outside the range. As a result, the mobile terminal  10  can estimate the acceleration with high accuracy. 
       Second Embodiment 
       [0044]    Descriptions will now be made to the mobile terminal according to a second embodiment. The second embodiment differs from the first embodiment, in the method for calculating the impact time and the acceleration at impact. That is, in the first embodiment, the mobile terminal  10  has obtained the impact time and the acceleration value at impact, using the intersection point of two straight lines. However, in the second embodiment, the impact time will be calculated first, and then the range of the acceleration values at the time is obtained. 
         [0045]    The configuration of the mobile terminal according to the second embodiment is the same as that of the mobile terminal  10  of the first embodiment, except the data stored in the acceleration conversion table. Thus, the common constituent elements have the same reference numerals, and the entire configuration is not illustrated and the specific description is omitted. Hereinafter, an acceleration conversion table having a different form from that of the first embodiment will be described. 
         [0046]    The application processing unit  14  in the second embodiment has an acceleration conversion table  141   b .  FIG. 6  is a diagram illustrating a data storage example in the acceleration conversion table  141   b  for converting the acceleration (estimated value) at impact into an actual measurement value. As illustrated in  FIG. 6 , the acceleration conversion table  141   b  stores the maximum acceleration at impact that is calculated by an impact calculating unit  13 , as an “M1 estimated value”, and stores also the minimum acceleration at impact as an “M2 estimated value”. Further, the acceleration conversion table  141   b  has the acceleration at impact that has been measured in advance by a high range acceleration sensor as the “actual measurement value”, in association with these estimated values. For example, when the M1 estimated value is calculated to be “5.10 G”, and when this value is selected as the acceleration at impact, “5.59 G” is set as a corresponding actual measurement value. Thus, “5.59 G” is used for a comparison with a threshold value. Similarly, when the M2 estimated value is calculated to be “79.70 G”, and when this value is selected as the acceleration at impact, the actual measurement value set in advance is “80.57 G”. Thus, this value is used for determination as to whether the drop has occurred. As described above, the acceleration at impact (as the M1 estimated value or the M2 estimated value) calculated by the impact calculating unit  13  is corrected into an actual measurement value of the acceleration by the application processing unit  14 . Note that the M1 estimated value and the M2 estimated value corresponding to the actual measurement values and set in the acceleration conversion table  141   b  can be appropriately updated, like the first embodiment. 
         [0047]    Descriptions will now be made to an operation of a mobile terminal  10  in the second embodiment. The operation is also the same as the mobile terminal  10  in the first embodiment, except a process for calculating the impact time and the acceleration value at impact, and for converting a calculation result into an actual measurement value. Thus, the common steps have reference numerals having the common end, and will not be explained again.  FIG. 7  is a flowchart for explaining an operation of the mobile terminal  10  in the second embodiment. The operation of the mobile terminal  10  according to the second embodiment is the same as that of the mobile terminal  10  according to the first embodiment, except the steps from T 9  to T 11 . Specifically, the procedures from S 1  to S 8  and S 11  of  FIG. 4  in the first embodiment correspond to the procedures from T 1  to T 8  and T 12  of  FIG. 7  in the second embodiment, respectively. 
         [0048]    Descriptions will now be made to the procedures of T 9  to T 11  that are not included in the first embodiment, with reference to  FIG. 7  and  FIG. 8 . 
         [0049]    In  FIG. 8 , it is assumed that the time t [ms] is assigned to the x axis and the acceleration [G] is assigned the y axis, like the first embodiment. A time t 0  is the time at which the mobile terminal  10  is dropped and comes in contact with the floor surface. A time t 3  is the time at which the acceleration detected by the sensor unit  11  has exceeded +4 G as the upper limit value of the range, and a time t 4  is the time at which the detected acceleration has returned into the range again. Further, the sampling period is set to 1 [ms], the slope (the slope between circles a 4  and a 5  in  FIG. 8 ) kept in T 7  is set as S 3 , and the slope (the slope between the triangles b 5  and b 6  in  FIG. 8 ) kept in T 8  is set as S 4 . A circle t 6  is a sampling value right after (out of range) the acceleration detected by the sensor unit  11  has exceeded the upper limit value of the range. Because this acceleration is not in fact detected, it is represented by a broken line to distinguish from the circles a 4  and a 5  representing the sampling values of the detected acceleration. Similarly, the triangle b 4  is a sampling value right before (out of range) the acceleration detected by the sensor unit  11  returns into the range. Because this acceleration is not in fact detected, it is represented by a broken line to distinguish from the triangles b 5  and b 6  representing sampling values of the detected acceleration. 
         [0050]    In  FIG. 8 , a threshold value T 2  is set, and is compared with an actual measurement value in drop determination. However, this threshold value T 2  may be a different value from the threshold value T 1  in the first embodiment. 
         [0051]    Back to  FIG. 7 , in T 9 , the mobile terminal  10  calculates a time at impact t m . Because the time at impact t m  is an intermediate point between the time t 3  and the time t 4 , the following equation (3) is satisfied. 
         [0000]      Time at impact  t   m =( t   3   +t   4 )/2  (3)
 
         [0052]    In T 10 , the mobile terminal  10  calculates the maximum value and minimum value of the acceleration at the time of impact. Under the above-described condition, when the maximum acceleration at impact as a calculation target in T 10  is set as M1, the relationships of M1=S 3 t m +b and 4=S 3 t 3 +b (b is a constant) are made. Thus, the maximum acceleration M1 at the time t m  can be calculated using the following equation (4). 
         [0000]      Maximum acceleration at impact  M 1 =S   3 ( t   3   +t   4 )/2+4 −S   3   t   3   =S   3 ( t   4   −t   3 )/2+4  (4)
 
         [0053]    Similarly, under the above-described condition, when the minimum acceleration at impact as a calculation target in T 10  is set as M2, the relationships of M2=S 4 t m +b and 4=S 4 t 4 +b (b is a constant) are made. Thus, the minimum acceleration M2 at the time t m  can be calculated using the following equation (5). 
         [0000]      Minimum acceleration at impact  M 2 =S   4 ( t   3   +t   4 )/2+4 −S   4   t   4   =S   4 ( t   3   −t   4 )/2+4  (5)
 
         [0054]    That is, the impact calculating unit  13  obtains two-dimensional coordinates of an intersection point H of a straight line E passing through the circles a 4  and a 5  and a straight line G representing the time t=t m , estimates the x coordinate value as the impact time t m , and estimates the y coordinate value as the upper limit value M1 within the range of the acceleration. Similarly, the impact calculating unit  13  obtains two-dimensional coordinates of an intersection point I of a straight line F passing through the triangles b 5  and b 6  and the above-described straight line G, estimates the x coordinate value as the impact time t m , and estimates the y coordinate value as the lower limit value M2 within the range of the acceleration. 
         [0055]    In T 11 , the application processing unit  14  converts the acceleration calculated by the impact calculating unit  13  in T 10  into an actual measurement value. This conversion process is executed with reference to the above-described acceleration conversion table  141   b . The conversion of the estimated value into the actual measurement value may be performed for both of the maximum acceleration M1 and the minimum acceleration M2. However, from the perspective of enhancing the processing efficiency, the application processing unit  14  preferably performs the conversion into the actual measurement value, after calculating one value as an estimated value of a conversion target. For example, when the maximum acceleration at impact that is calculated by the impact calculating unit  13  in T 10  is “5.10”, this estimated value is converted into an actual measurement value of “5.59”. When the minimum acceleration is “79.80”, the value is converted into an actual measurement value of “80.68” (see  FIG. 6 ). 
         [0056]    As described above, the mobile terminal  10  according to the second embodiment has the sensor unit  11 , the impact calculating unit  13 , and the application processing unit  14 . The sensor unit  11  detects the acceleration within the above-described predetermined range. The impact calculating unit  13  calculates the acceleration outside the above-described predetermined range, using the acceleration detected by the sensor unit  11 . The application processing unit  14  determines whether the drop has occurred, using the acceleration calculated by the impact calculating unit  13 . In more particular, the impact calculating unit  13  calculates the maximum value and minimum value of the acceleration outside the above-described predetermined range, using the time t 3 , the time t 4 , the slope S 3  of the acceleration, and the slope S 4  of the acceleration. The time t 3  is the time at which the acceleration detected by the sensor unit  11  has exceeded the above-described predetermined range. The time t 4  is the time at which the above-described acceleration returns into the predetermined range. The slope S 3  is the slope of the acceleration before the time t 3 . The slope S 4  is the slope of the acceleration after the time t 4 . The application processing unit  14  determines the occurrence of the drop, based on whether the acceleration calculated by the impact calculating unit  13  is a predetermined value or higher. 
         [0057]    That is, the mobile terminal  10  according to the second embodiment calculates a possible range of the acceleration at impact, and further calculates the acceleration to be converted into an actual measurement value from the acceleration values within the range, as an estimated value. Thus, the acceleration at impact is estimated as a value between the upper limit value M1 and the lower limit value M2 of the calculated acceleration. There are, for example, techniques for the application processing unit  14  to select or calculate either one value within the range. The application processing unit  14  selects the M1 estimated value as the maximum value of the acceleration, as a target estimated value to be converted into an actual measurement value. As a result, there is a high possibility that “actual measurement value threshold value”. Thus, in the mobile terminal  10 , it is possible to enhance the probability of determining that the drop has occurred. On the contrary, the application processing unit  14  selects the M2 estimated value as the minimum value of the acceleration as a target estimated value to be converted into an actual measurement value. Thus, there is a rigid rule for determining the occurrence of the drop, and relatively decreasing the possibility that “actual measurement value≧threshold value”. This results in restraining the percentage of determining the occurrence of the drop, in the mobile terminal  10 , and also increasing the probability of determining that the drop has not occurred. 
         [0058]    Alternatively, the application processing unit  14  calculates an intermediate value of the M1 estimated value and the M2 estimated value, and may set the calculation result as an estimated value. The mobile terminal  10  can set the average estimated value at impact, and can use a non-biased actual measurement value as a comparison target with a threshold value, for determining whether the drop has occurred. The application processing unit  14  may set a line segment with both ends on the M1 estimated value and the M2 estimated value, and set a predetermined ratio value from the M1 estimated value as an estimated value. For example, when the predetermined ratio is 1/4, the acceleration value close to the side of the M1 estimated value will be the estimated value, and the condition for determining the drop will be mild. This causes easy determination that the drop has occurred. When the above-described predetermined ratio is set to 3/4, the acceleration value close to the side of the M2 estimated value will be the estimated value, and the condition for determining the occurrence of the drop will be rigid. This causes uneasy determination that the drop has occurred. 
         [0059]    In  FIG. 8 , the descriptions have been made to the example in which the side (straight line E) exceeding the range has the acceleration maximum value, and the side (straight line F) returning into the range has the acceleration minimum value. On the contrary, the straight line E may have the acceleration minimum value, and the straight line F may have the acceleration maximum value, depending on the slope S 3  before the exceeding of the range or the slope S 4  after returning into the range. To correspond to this case, the application processing unit  14  may set the acceleration value close to the estimated value on the side exceeding the range, as an estimated value. Specifically, when the estimated value on the side exceeding the range is the maximum estimated value M1, the application processing unit  14  sets the maximum estimated value M1 or the acceleration value at a predetermined ratio (for example, 0.1 to 0.4) from the maximum estimated value M1, as a target estimated value to be converted into an actual measurement value. On the other hand, when the estimated value on the side exceeding the range is the minimum estimated value M2, the application processing unit  14  sets the minimum estimated value M2 or the acceleration value at a predetermined ratio (for example, 0.1 to 0.4) from the minimum estimated value M2, as a target estimated value to be converted into an actual measurement value. As a result, the acceleration value which approximates to the estimated value on the side of the overrange is preferentially used as an estimated value. Thus, the mobile terminal  10  can always use the estimated value closest to the moment at which it comes in contact with the floor surface, for the conversion into an actual measurement value. This results in improving the estimation accuracy of the acceleration at the time of contact with the floor surface (at impact) and the accuracy of drop determination. 
         [0060]    In this embodiment, the impact time t m  is an intermediate point of the time t 3  and the time t 4 . However, not limited to this, the application processing unit  14  may set a line segment having both ends on the time t 3  and the time t 4 , and may set the time at a predetermined ratio (for example, 0.1 to 0.4) from the time t 3  as the impact time t m . As a result, the acceleration value at the time close to the time of exceeding the range is preferentially used as an estimated value. Therefore, the mobile terminal  10  can always use the estimated value closest to the moment that it comes in contact with the floor surface, for the conversion into an actual measurement value. This results in improving the estimation accuracy of the acceleration at the time of contact with the floor surface (at impact) and the accuracy of drop determination. 
         [0061]    The times t 1  and t 2  in the first embodiment and the times t 3  and t 4  in this embodiment are relative times based on the time t o  at the contact time. Because the mobile terminal  10  has a clock function, it can specify the actual impact time, in cooperation with this function. Therefore, the user or third party can accurately and easily know the time at which the mobile terminal  10  has been dropped, with reference to this time. 
       [Drop Determining Program] 
       [0062]    These processes described in the above embodiments can be realized by controlling a computer to execute a prepared program. Descriptions will herein be made to an example of a computer executing a drop determining program having the same function as that of the mobile terminal  10  illustrated in  FIG. 1 . 
         [0063]      FIG. 9  is a diagram illustrating a computer for executing a drop determining program. As illustrated in  FIG. 9 , a computer  100  has a CPU  110 , an input unit  120 , a monitor  130 , a voice input/output unit  140 , a wireless communication unit  150 , and an acceleration sensor  160 . Further, the computer  100  has a data memory unit, such as a RAM  170  and a hard disk unit  180 , and these are connected with each other through a bus  190 . The CPU  110  executes various arithmetic processes. The input unit  120  accepts data input from the user. The monitor  130  displays various kinds of information. The voice input/output unit  140  inputs/outputs voice. The wireless communication unit  150  transmits and receives data to and from another computer through wireless communication. The acceleration sensor  160  detects the acceleration in triaxial directions. The RAM  170  temporarily stores various kinds of information. 
         [0064]    The hard disk unit  180  stores a drop determining program  181  having the same function as that of the CPU  10   a  illustrated in  FIG. 3 . The hard disk unit  180  stores a drop determining process relevant data  182  and a determination historical file  183 , corresponding to various data (range value, threshold value for drop determination, and sampling value of acceleration) stored in the memory  10   c  illustrated in  FIG. 3 . 
         [0065]    The CPU  110  reads the drop determining program  181  from the hard disk unit  180  and expands it into the RAM  170 . As a result, the drop determining program  181  functions as a drop determining process  171 . The drop determining process  171  expands the information read from the drop determining process relevant data  182  into an area allocated to itself on the RAM  170 , and executes various data processes based on the expanded data. The drop determining process  171  outputs predetermined information to the determination historical file  183 . 
         [0066]    The above-described drop determining program  181  is not necessarily stored in the hard disk unit  180 . This program stored on a recording medium, such as a CD-ROM, may be read and executed by the computer  100 . The program may be stored in another computer (or server) connected to the computer  100  through a public line, the Internet, a LAN (Local Area Network), a WAN (Wide Area Network). In this case, the computer  100  reads and executes the program therefrom. 
         [0067]    In each of the above-described embodiments, the mobile terminal  10  is to correct the calculation result of the acceleration at impact into an actual measurement value. However, when the sampling period is a predetermined value (for example, 0.1 ms) or lower, the impact calculating unit  13  can calculate the acceleration estimated value at impact with high accuracy. Therefore, in this case, the correction process may be omitted. 
         [0068]    In each of the above embodiments, the mobile terminal  10  determines that the drop has occurred, when the acceleration in one direction reaches a threshold value or higher, of the accelerations in the triaxial directions. However, not only in the above aspect, the mobile terminal  10  estimates an out-of-range acceleration, for the accelerations in a plurality of directions. The terminal may determine that the drop has occurred, for the first time, when the estimated values in biaxial directions or triaxial directions become a predetermined threshold value or higher, of these estimated values. In this case, different values may be set for the respective three kinds of axial directions, as the threshold values. Further, the mobile terminal  10  estimates an out-of-range acceleration for the accelerations in a plurality of directions, and obtains a weighted average of the estimated values. When the resultant estimated value becomes a predetermined threshold value or higher, the mobile terminal  10  may determine that the drop has occurred. In this case, there is no need to set different values for three kinds of axis directions, as the threshold value, and one threshold value is simply set for the estimated value as a weighted average. There are some dropping manners, and the accelerations are generated in different directions in accordance with the dropping manners. Thus, at the impact on the mobile terminal  10 , the acceleration values in the axial directions vary in accordance with the dropping manner. The mobile terminal  10  can realize the drop determination based on the acceleration value close to the actual state, in consideration of not only the acceleration in one direction but also the accelerations in a plurality of axial directions, during the determination of the occurrence of the drop. This results in improving the accuracy and reliability of the drop determination of the mobile terminal  10 . 
         [0069]    Further, for calculation of the slope, it is not necessary to calculate the slope using the sampling value of the adjacent acceleration. That is, in the first embodiment, the mobile terminal  10  uses the sampling values a 1  and a 2  that are adjacent to each other, to calculate the slope right before the excess of the range. In the second embodiment also, the mobile terminal  10  uses the sampling values b 5  and b 6  that are adjacent to each other, to calculate the slope right after returning into the range. However, the mobile terminal  10  may calculate the value of the slope using the non-adjacent sampling values. For example, when the sampling period is 1 ms, the slope of the acceleration may be calculated from the sampling values at intervals of 2 ms or 3 ms. 
         [0070]    For calculation of the slope, on each exceeding of the range and returning into the range, the mobile terminal  10  may calculate the values of a plurality of slopes and obtain their average value. Now, the mobile terminal  10  can estimate the acceleration and determine the drop, based on a more accurate slope value, with less variation (fluctuation) than a case in which slope values are used respectively for right and left. This results in improving the determination accuracy and reliability of the determination on the mobile terminal  10 . Further, when calculating the average value of a plurality of slopes, the mobile terminal  10  can obtain the average value (weighted average) which has been obtained by weighting a slope value close to the outside range (4 G), based on the assumption that it is closer to the actual state. The variation in the slops can further be restrained, thus enabling to estimate the acceleration and to determine the drop based on the reliable slope value. As a result, it is possible to further improve the determination accuracy and reliability of the determination on the mobile terminal  10 . 
         [0071]    In addition, in each of the above-described embodiment, the mobile terminal  10  calculates the slope using the sampling values of at least two points. However, it is not limited to this, and the slope can be calculated using the sampling value of any one point and also 4 G as the slope value at the exceeding of or returning into the range. According to the above aspect, from the perspective of estimating the acceleration value outside the range with high accuracy, the mobile terminal  10  preferably uses the sampling value which is as close as possible to the outside of the range, for calculation of the above-described slope. For example, in the first embodiment (see  FIG. 5 ), it is preferred to use the sampling value a 2  rather than the value a 1 , in combination with the acceleration value (4 G) at the time t 1 . For example, in the second embodiment (see  FIG. 8 ), it is preferred to use the sampling value b 5  rather than the value b 6 , in combination with the acceleration value (4 G) at the time t 4 . Then, the mobile terminal  10  can estimate the acceleration value, using the slope at the moment of overrange and the slope at the moment of returning into the range. Thus, the mobile terminal  10  can determine the drop, based on the acceleration value which is as close as possible to the outside of the range. As a result, it is possible to improve the accuracy of the drop determination. It is not necessary to calculate the slope value in the above aspect for both of the exceeding side of and returning side into the range. The calculation may be performed only for either one side (for example, the exceeding side of the range). 
         [0072]    In each of the above-described embodiment, for the conversion from the acceleration estimated value at impact into the actual measurement value, the mobile terminal  10  refers to the acceleration conversion tables  141   a  and  141   b . However, it may obtain the actual measurement value using the estimated value, using a predetermined calculation formula. 
         [0073]    Further, in each of the above-described embodiments, the descriptions have been made to the mobile terminal determining the occurrence of the drop, and the dropping has been adopted as one example of movement involving high acceleration. However, the present invention is not limited to this, and may be applied to any movement other than the dropping, such as throwing onto a wall surface and collision with an object. In each of the above-described embodiments, the descriptions have been made on the assumption that the mobile terminal is a mobile phone, a smartphone, a PDA (Personal Digital Assistant). However, the present invention is not limited to the mobile terminal, and is applicable to various electronic devices having a low range acceleration sensor. 
         [0074]    Each of the constituent elements of the mobile terminal  10  illustrated in  FIG. 1  is not necessarily configured physically as illustrated in the drawings. That is, specific aspects of division and integration of the devices are not limited to those illustrated in the drawings, and the entirety or a part thereof may be physically or functionally divided or integrated in the units of arbitrary forms in accordance with various loads or the usages. For example, the sampling processing unit  12 , the impact calculating unit  13 , and the application processing unit  14  may be integrated as one constituent element. On the contrary, the impact calculating unit  13  may be divided into a part calculating the slope right before the excess of the range, a part calculating the slope right after returning into the range, and a part calculating the impact time and the acceleration at that time. The application processing unit  14  may be divided into a part converting the acceleration (estimated value) at impact into an actual measurement value, and a part determining the occurrence of the drop. The memory  10   c  may be connected through a network or a cable, as an external unit of the mobile terminal  10 . 
         [0075]    According to one aspect of the drop determining device of the present application, an advantageous effect thereof is to determine the drop using a low range acceleration sensor. 
         [0076]    All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.