Presence / absence determination control system, presence / absence determination control method, and program

Abstract

Provided are a presence / absence determination control system, a presence / absence determination control method, and a program which are capable of appropriately controlling the execution of a presence / absence determination processing. A presence / absence determination processing execution unit (34) repeatedly executes a presence / absence determination processing for determining whether or not a subject to be measured is on a mattress placed on the bed frame of a bed used by the subject to be measured, on the basis of the result of measuring the load applied to at least one load sensor supporting the bed frame. In a case where the presence / absence determination processing is being executed, a presence / absence determination control unit (36) stops the execution of the presence / absence determination processing when the result of a comparison between a total weight calculated on the basis of the load applied to the at least one load sensor and a given bed frame weight value indicating the weight of the bed frame satisfies a given mattress removal condition.

Description

Title of Invention: PRESENCE / ABSENCE DETERMINATION CONTROL SYSTEM, PRESENCE / ABSENCE DETERMINATION CONTROL METHOD, AND PROGRAM Technical Field The present invention relates to a presence-or-absence determination control system, a presence-or-absence determination control method, and a program. Background Art Various systems for measuring biological information, such as a respiratory rate or a heart rate, of a person to be measured who is on a bed have been considered. To give an example of such systems, in Patent Literature 1, there is described a watch-over device which determines a respiratory rate and a heart rate of a care recipient on a bed in a non-contact manner through use of a microwave Doppler sensor. Citation List Patent Literature [Patent Literature 1] JP 2017-134795 A Summary of Invention Technical Problem In the technology as described in Patent Literature 1, when the respiratory rate or the heart rate of the person to be measured indicates an abnormal value, it is impossible to determine whether the cause is that an abnormality has occurred in the person to be measured or that the person to be measured is not on the bed. Thus, in order to accurately detect that an abnormality has occurred in the person to be measured, it is required to accurately perform processing for determining whether the person to be measured is on the bed (hereinafter referred to as "presence-or-absence determination processing"). Here, in a case in which the presence-or-absence determination processing based on the total weight calculated based on a load applied to a load sensor supporting the bed is being executed, when a mattress arranged on a bed frame of the bed is removed or replaced with another mattress having a different weight, it is no longer possible to appropriately determine whether the person to be measured is on the bed or not. The present invention has been made in view of the abovementioned problem, and has an object to provide a presence-or-absence determination control system, a presence-or-absence determination control method, and a program which enable appropriate control of execution of presence-or-absence determination processing. Solution to Problem (1) According to one embodiment of the present invention, there is provided a presence-or-absence determination control system including: presence-or-absence determination processing execution means for repeatedly executing presence-or-absence determination processing for determining, based on a measurement result of a load applied to at least one load sensor supporting a bed frame of a bed used by a person to be measured, whether the person to be measured is on a mattress arranged on the bed frame; and presence-or-absence determination stopping means for stopping execution of the presence-or-absence determination processing in response to a fact that a result of comparison between a total weight calculated based on the load applied to the at least one load sensor and a given bed frame weight value indicating a weight of the bed frame satisfies a given mattress removal condition in a situation in which the presence-or-absence determination processing is being executed. (2) The presence-or-absence determination control system according to Item (1) may further include presence-or-absence determination resuming means for resuming the execution of the presence-or-absence determination processing in response to a fact that a given mattress placement condition is satisfied in a situation in which the execution of the presence-or-absence determination processing is stopped. (3) In the presence-or-absence determination control system according to Item (2), the given mattress placement condition may be a condition relating to a magnitude of the load applied to the load sensor. (4) In the presence-or-absence determination control system according to Item (2) or (3), the given mattress placement condition may be a condition relating to smallness of variation in loads applied to a plurality of load sensors. (5) In the presence-or-absence determination control system according to any one of Items (2) to (4), the given mattress placement condition may be a condition relating to smallness of variation in a latest plurality of measurement results of the load applied to the load sensor. (6) In the presence-or-absence determination control system according to any one of Items (2) to (5), the presence-or-absence determination processing execution means may include: total weight data acquisition means for acquiring total weight data indicating a total weight calculated based on the load applied to the at least one load sensor; presence-or-absence information holding means for holding presence-or-absence information indicating whether the person to be measured is on the mattress; presence changing means for changing the held presence-or-absence information to the presence-or-absence information indicating that the person to be measured is on the mattress when the presence-or-absence information indicates that the person to be measured is not on the mattress and a magnitude of an increase amount, which is a value obtained by subtracting a value indicated by the total weight data based on an immediately preceding measurement result from a value indicated by the total weight data based on a latest measurement result, satisfies a given presence determination condition; and absence changing means for changing the held presence-or-absence information to the presence-or-absence information indicating that the person to be measured is not on the mattress when the presence-or-absence information indicates that the person to be measured is on the mattress and a magnitude of a decrease amount, which is a value obtained by subtracting the value indicated by the total weight data based on the latest measurement result from the value indicated by the total weight data based on the immediately preceding measurement result, satisfies a given absence determination condition, and in the presence-or-absence determination processing immediately after resumption, the total weight data indicating the total weight calculated based on the load applied to the at least one load sensor when the given mattress placement condition is satisfied may be used as the total weight data based on the immediately preceding measurement result. (7) In the presence-or-absence determination control system according to any one of Items (1) to (6), each of four corners of the bed frame may be supported by the load sensor. (8) The presence-or-absence determination control system according to any one of Items (1) to (7) may further include: Doppler data acquisition means for acquiring Doppler data indicating a measurement result obtained by a Doppler sensor provided toward the person to be measured; biological information generation means for generating biological information of the person to be measured based on the Doppler data; action execution means for executing a given action in response to a fact that the biological information satisfies a given abnormality determination condition; and suppression means for suppressing execution of the given action in response to the fact that the biological information satisfies the given abnormality determination condition when it is determined that the person to be measured is not on the mattress. (9) According to one embodiment of the present invention, there is provided a presence-or-absence determination control method including the steps of: repeatedly executing presence-or-absence determination processing for determining, based on a measurement result of a load applied to at least one load sensor supporting a bed frame of a bed used by a person to be measured, whether the person to be measured is on a mattress arranged on the bed frame; and stopping execution of the presence-or-absence determination processing in response to a fact that a result of comparison between a total weight calculated based on the load applied to the at least one load sensor and a given bed frame weight value indicating a weight of the bed frame satisfies a given mattress removal condition in a situation in which the presence-or-absence determination processing is being executed. (10) According to one embodiment of the present invention, there is provided a program for causing a computer to execute the steps of: repeatedly executing presence-or-absence determination processing for determining, based on a measurement result of a load applied to at least one load sensor supporting a bed frame of a bed used by a person to be measured, whether the person to be measured is on a mattress arranged on the bed frame; and stopping execution of the presence-or-absence determination processing in response to a fact that a result of comparison between a total weight calculated based on the load applied to the at least one load sensor and a given bed frame weight value indicating a weight of the bed frame satisfies a given mattress removal condition in a situation in which the presence-or-absence determination processing is being executed. Advantageous Effects of Invention According to the present invention, the execution of the presence-or-absence determination processing can be appropriately controlled. Brief Description of Drawings FIG. 1 is a configuration diagram of a biological information detection system in an embodiment of the present invention. FIG. 2A is a functional block diagram of a signal processing device in the embodiment of the present invention. FIG. 2B is a functional block diagram of a presence-or-absence determination processing execution module in the embodiment of the present invention. FIG. 3 is a flow chart for illustrating an example of a flow of presence-or-absence determination processing performed by the signal processing device in the embodiment of the present invention. FIG. 4 is a flow chart for illustrating an example of the flow of the presence-or-absence determination processing performed by the signal processing device in the embodiment of the present invention. FIG. 5 is a flow chart for illustrating an example of the flow of the presence-or-absence determination processing performed by the signal processing device in the embodiment of the present invention. FIG. 6 is a flow chart for illustrating an example of a flow of presence-or-absence determination control processing performed by the signal processing device in the embodiment of the present invention. FIG. 7 is a flow chart for illustrating an example of the flow of the presence-or-absence determination control processing performed by the signal processing device in the embodiment of the present invention. FIG. 8 is an explanatory diagram for illustrating an example of processing of a Doppler data acquisition module. FIG. 9 is a flow chart for illustrating an example of a flow of frequency spectrum selection processing performed by the signal processing device in the embodiment of the present invention. FIG. 10 is a diagram for schematically illustrating an example of generation of an aggregated spectrum. Description of Embodiments Now, an embodiment of the present invention is described in detail with reference to the accompanying drawings. FIG. 1 is a configuration diagram of a biological information detection system 1 in the embodiment of the present invention. As illustrated in the figure, the biological information detection system 1 includes a signal processing device 10, a Doppler sensor unit 12, and a plurality of load sensors 14. The Doppler sensor unit 12 includes a plurality of Doppler sensors. Here, it is assumed that the Doppler sensor unit 12 includes, for example, six Doppler sensors (a first Doppler sensor to a sixth Doppler sensor). In addition, as illustrated in FIG. 1, it is assumed that the biological information detection system 1 includes four load sensors 14 (14a to 14d). In this embodiment, for example, a bed 16 used by a person to be measured includes a bed frame 18 and a mattress 20. The person to be measured sleeps on the mattress 20 arranged on the bed frame 18. In addition, in this embodiment, the bed frame 18 of the bed 16 used by the person to be measured is supported by the load sensors 14. Here, as illustrated in FIG. 1, each of the four corners of the bed frame 18 may be supported by the load sensor 14. Further, the four load sensors 14 are supported by a bed base 22. Each load sensor 14 outputs load data indicating a measurement result of a load applied to the load sensor 14. The position and the number of the load sensors 14 are not limited to those illustrated in FIG. 1. For example, the biological information detection system 1 may include only one load sensor 14. As illustrated in FIG. 1, the Doppler sensor unit 12 is installed so as to face a person to be measured. In an example of FIG. 1, the Doppler sensor unit 12 is attached to a headboard of the bed frame 18. The plurality of Doppler sensors may be installed symmetrically with respect to a center line of the bed 16 (a line running through a center in a width direction of the bed 16 and extending in a lengthwise direction of the bed 16). The plurality of Doppler sensors may also be installed so as to align perpendicularly to the lengthwise direction of the bed 16 (a direction along the center line of the bed 16). The plurality of Doppler sensors may also be installed at regular intervals to form a single line. All of the Doppler sensors are installed so as to face the lengthwise direction (longitudinal direction) of the bed 16, and emit a microwave in the lengthwise direction of the bed 16. The microwave is reflected by a chest region of the person to be measured who is sleeping on the bed 16, and a reflected wave thereof is received by each of the Doppler sensors. Each of the Doppler sensors generates, from the reflected wave, a Doppler signal indicating a motion of the chest region that accompanies respiration, and outputs Doppler data obtained by digitization of this Doppler signal. Microwaves emitted from each of the Doppler sensors are shifted a little in frequency from one another, to thereby prevent cross talk with one another. With the Doppler effect, the reflected wave is shifted in frequency, and a respiratory rate of the person to be measured can be obtained by observing the frequency shift. The reflected wave is detected by quadrature detection as a Doppler signal including an I signal being an in-phase component with respect to the transmitted wave and a Q signal being a quadrature component with respect to the transmitted wave, and is output to the signal processing device 10 in a digital form. The Doppler signal input to the signal processing device 10 is time-series data, and indicates an amplitude at each time (I component and Q component). The signal processing device 10 may be formed of a publicly-known computer including, for example, a CPU, a memory, an input device, and a display. The signal processing device 10 generates the respiratory rate of the person to be measured based on the Doppler signal output from each Doppler sensor. In addition, the signal processing device 10 calculates a total weight based on a measurement result of the load applied to at least one load sensor 14, and generates total weight data indicating the calculated total weight. FIG. 2A and FIG. 2B are functional block diagrams of the signal processing device 10 in the embodiment of the present invention. As illustrated in FIG. 2A, the signal processing device 10 includes a weight data storage module 30, a load data acquisition module 32, a presence-or-absence determination processing execution module 34, a presence-or-absence determination control module 36, a Doppler data acquisition module 38, a frequency spectrum generation module 40, a frequency spectrum selection module 42, an aggregated spectrum generation module 44, a biological information generation module 46, an abnormality determination module 48, an action execution module 50, and an operation control module 52. In addition, as illustrated in FIG. 2B, the presence-or-absence determination processing execution module 34 includes a presence-or-absence information holding module 60, a body movement information holding module 62, a total weight data generation module 64, a presence-or-absence determination module 66, a presence-or-absence changing module 68, a body movement determination module 70, and a weight data management module 72. Those functional blocks are implemented when a signal processing program is executed by the signal processing device 10, which is the computer. The signal processing program may be stored in one of various computer-readable information storage media such as a semiconductor memory, and may be loaded from the medium onto the signal processing device 10. As another example, the signal processing program may be downloaded onto the signal processing device 10 through a data communication line such as the Internet. The weight data storage module 30 stores, for example, data indicating a weight. In the following description, it is assumed that the weight data storage module 30 stores personto-be-measured body weight data indicating the body weight of the person to be measured, during-absence total weight data indicating a total weight in a situation in which the mattress 20 is arranged on the bed frame 18 but the person to be measured is not on the bed 16, and bed frame weight data indicating the weight of the bed frame 18. In an initial state, given values are set for a value of the person-to-be-measured body weight data, a value of the during-absence total weight data, and a value of the bed frame weight data. For example, a value indicating a body weight of the person to be measured that has been measured in advance may be set as the value of the personto-be-measured body weight data in the initial state. In addition, a value indicating the total weight of the bed frame 18, the mattress 20, bedding, and the like measured in advance may be set as the value of the during-absence total weight data in the initial state. In addition, a value indicating the weight of the bed frame 18 measured in advance may be set as the value of the bed frame weight data in the initial state. In addition, unknown-weight data described later is also stored in the weight data storage module 30. The load data acquisition module 32 acquires, for example, load data indicating a measurement result of the load applied to the at least one load sensor 14 supporting the bed frame 18. Here, the load data output from each of the at least one load sensor 14 may be acquired. In this embodiment, for example, the presence-or-absence determination processing execution module 34 repeatedly executes the presence-or-absence determination processing for determining whether the person to be measured is on the mattress 20 arranged on the bed frame 18 based on the measurement result of the load applied to the at least one load sensor 14 supporting the bed frame 18 of the bed 16 used by the person to be measured. The presence-or-absence information holding module 60 included in the presence-or-absence determination processing execution module 34 holds, for example, presence-or-absence information indicating whether the person to be measured is on the mattress 20. Here, the presence-or-absence information may indicate that the person to be measured is on the mattress 20, that the person to be measured is not on the mattress 20, or that it is unknown whether the person to be measured is on the mattress 20. In the following description, it is assumed that the presence-or-absence information includes a one-user present flag and an unknown-state flag. The fact that a value of the one-user present flag is 1 and a value of the unknown-state flag is 0 is assumed to correspond to the fact that the presence-or-absence information indicates that the person to be measured is on the mattress 20. Further, the fact that the value of the one-user present flag and the value of the unknown-state flag are both 0 is assumed to correspond to the fact that the presence-or-absence information indicates that the person to be measured is not on the mattress 20. Further, the fact that the value of the one-user present flag is 0 and the value of the unknown-state flag is 1 is assumed to correspond to the fact that the presence-or-absence information indicates that it is unknown whether the person to be measured is on the mattress 20. The body movement information holding module 62 included in the presence-or-absence determination processing execution module 34 holds, for example, body movement information indicating whether a body movement of the person to be measured has occurred. In the following description, the fact that a value of the body movement information is 1 is assumed to correspond to the fact that the body movement information indicates that the body movement of the person to be measured has occurred, and the fact that the value of the body movement information is 0 is assumed to correspond to the fact that the body movement information indicates that the body movement of the person to be measured has not occurred. The total weight data generation module 64 included in the presence-or-absence determination processing execution module 34 calculates the total weight based on the load data acquired from each of the at least one load sensor 14, and generates the total weight data indicating the calculated total weight, for example. When the biological information detection system 1 includes one load sensor 14, the total weight data indicating the measurement result of the load applied to the load sensor 14 may be generated. Further, when the biological information detection system 1 includes a plurality of load sensors 14, the total weight data indicating a sum value of the measurement results of the respective loads applied to the plurality of load sensors 14 may be generated. The presence-or-absence determination module 66 included in the presence-or-absence determination processing execution module 34 acquires, for example, the total weight data generated by the total weight data generation module 64, and determines whether the person to be measured is on the mattress 20 based on the acquired total weight data. For example, when the presence-or-absence information indicates that the person to be measured is not on the mattress 20, the presence-or-absence determination module 66 determines whether a magnitude of a value obtained by subtracting a value indicated by the total weight data based on an immediately preceding measurement result from a value indicated by the total weight data based on a latest measurement result satisfies a given presence determination condition. In the following description, the magnitude of the value obtained by subtracting the value indicated by the total weight data based on the immediately preceding measurement result from the value indicated by the total weight data based on the latest measurement result is referred to as "increase amount." Here, the presence determination condition may be, for example, a condition that a magnitude of the increase amount is larger than a threshold value determined based on an estimated value of the body weight of the person to be measured (for example, larger than a threshold value that is 80% of the value of the person-to-be-measured body weight data). Further, for example, when the presence-or-absence information indicates that the person to be measured is on the mattress 20, the presence-or-absence determination module 66 determines whether a magnitude of a value obtained by subtracting the value indicated by the total weight data based on the latest measurement result from the value indicated by the total weight data based on the immediately preceding measurement result satisfies a given absence determination condition. In the following description, the magnitude of the value obtained by subtracting the value indicated by the total weight data based on the latest measurement result from the value indicated by the total weight data based on the immediately preceding measurement result is referred to as "decrease amount." Here, the absence determination condition may be, for example, a condition that a magnitude of the decrease amount is larger than a threshold value determined based on the estimated value of the body weight of the person to be measured (for example, larger than a threshold value that is 80% of the value of the person-to-be-measured body weight data). In this embodiment, output of the load data indicating the latest measurement result of the load applied to the load sensor 14 from the load sensor 14 to the signal processing device 10, generation of the total weight data based on the load data, and determination of whether the person to be measured is on the mattress 20 based on the total weight data are repeatedly executed (for example, at predetermined time intervals). For example, when the presence-or-absence information indicates that the person to be measured is not on the mattress 20 and the magnitude of the above-mentioned increase amount satisfies the given presence determination condition, the presence-or-absence changing module 68 included in the presence-or-absence determination processing execution module 34 changes the held presence-or-absence information to information indicating that the person to be measured is on the mattress 20. Further, for example, when the presence-or-absence information indicates that the person to be measured is on the mattress 20 and the magnitude of the above-mentioned decrease amount satisfies the given absence determination condition, the presence-or-absence changing module 68 changes the held presence-or-absence information to information indicating that the person to be measured is not on the mattress 20. The threshold value in the presence determination condition and the threshold value in the absence determination condition may be the same or different. The body movement determination module 70 included in the presence-or-absence determination processing execution module 34 determines whether the body movement of the person to be measured has occurred based on a magnitude of fluctuation in a value indicated by the total weight data, for example. Here, for example, the body movement determination module 70 may determine whether the body movement of the person to be measured has occurred every time a determination timing, which arrives at a time interval longer than the time interval at which the total weight data is generated, arrives. Here, a period between two consecutive determination timings is referred to as "body movement determination period." A maximum value or a minimum value of the value of the total weight data generated in a specific body movement determination period may be identified. Then, an average value of the values of the total weight data generated in a body movement determination period immediately preceding the specific body movement determination period may be identified. Then, when a difference between the maximum value identified for the body movement determination period and the average value identified for the body movement determination period immediately preceding the specific body movement determination period is equal to or more than a predetermined value, it may be determined that the body movement of the person to be measured has occurred in the body movement determination period. Further, when a difference between the minimum value identified for the body movement determination period and the average value identified for the body movement determination period immediately preceding the specific body movement determination period is equal to or more than a predetermined value, it may be determined that the body movement of the person to be measured has occurred in the body movement determination period. Then, when neither condition is satisfied, it may be determined that the body movement of the person to be measured has not occurred in the body movement determination period. A method of determining whether the body movement of the person to be measured has occurred is not limited to this method. Then, when it is determined that the body movement of the person to be measured has occurred, the body movement determination module 70 may set the value of the body movement information held in the body movement information holding module 62 to 1, and when it is determined that the body movement of the person to be measured has not occurred, the body movement determination module 70 may set the value of the body movement information held in the body movement information holding module 62 to 0. The weight data management module 72 included in the presence-or-absence determination processing execution module 34 determines, as the estimated value of the body weight of the person to be measured, a value obtained by subtracting a value of the total weight data in a case in which the presence-or-absence information indicates that the person to be measured is not on the mattress 20 from the value of the total weight data in a case in which the presence-or-absence information indicates that the person to be measured is on the mattress 20, for example. Then, the weight data management module 72 updates the person-to-be-measured body weight data stored in the weight data storage module 30 to data indicating the determined estimated value. The weight data management module 72 included in the presence-or-absence determination processing execution module 34 may update the estimated value of the body weight of the person to be measured to the above-mentioned increase amount in response to the held presence-or-absence information being changed from the information indicating that the person to be measured is not on the mattress 20 to the information indicating that the person to be measured is on the mattress 20. Here, the weight data management module 72 may update the person-to-be-measured body weight data stored in the weight data storage module 30 to data indicating the above-mentioned increase amount, for example. The weight data management module 72 may also update the during-absence total weight data stored in the weight data storage module 30 to data indicating the value indicated by the total weight data based on the immediately preceding measurement result. The presence-or-absence determination control module 36 controls, for example, whether the presence-or-absence determination processing is to be executed by the presence-or-absence determination processing execution module 34. In this embodiment, as described below, when the mattress 20 is removed from the bed frame 18 in a situation in which the presence-or-absence determination processing is being executed by the presence-or-absence determination processing execution module 34, the execution of the presence-or-absence determination processing is stopped. Further, when a new mattress 20 is arranged on the bed frame 18 in a situation in which the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 is stopped, the execution of the presence-or-absence determination processing is resumed. The presence-or-absence determination control module 36 stops the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 in response to the fact that a result of comparison between the total weight calculated based on the load applied to the at least one load sensor 14 and a given bed frame weight value indicating the weight of the bed frame 18 satisfies a given mattress removal condition in the situation in which the presence-or-absence determination processing is being executed by the presence-or-absence determination processing execution module 34, for example. Here, for example, the presence-or-absence determination control module 36 may stop the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 in response to the fact that a result of comparison between the total weight calculated based on the load applied to the at least one load sensor 14 and the value of the bed frame weight data stored in the weight data storage module 30 satisfies the given mattress removal condition in the situation in which the presence-or-absence determination processing is being executed by the presence-or-absence determination processing execution module 34, for example. In this embodiment, it is desired that a condition satisfied when the mattress 20 is removed from the bed frame 18 be set as the mattress removal condition. For example, the mattress removal condition may be a condition relating to smallness of a difference between the above-mentioned total weight and the value of the above-mentioned bed frame weight data. In addition, every time the total weight data is generated, it may be determined whether the difference between the value of the generated total weight data and the value of the above-mentioned bed frame weight data is equal to or smaller than a predetermined value. Then, the mattress removal condition may be a condition that it is determined consecutively for a predetermined number of times (for example, three times) or more that the difference between the value of the generated total weight data and the value of the above-mentioned bed frame weight data is equal to or less than the predetermined value. In this embodiment, the presence-or-absence determination control module 36 may acquire the total weight data generated by the total weight data generation module 64, and determine whether the given mattress removal condition is satisfied based on the acquired total weight data. In addition, in this embodiment, the presence-or-absence determination control module 36 may hold the acquired total weight data. Then, the presence-or-absence determination control module 36 may determine whether the given mattress removal condition is satisfied based on a history of the held total weight data. Further, the presence-or-absence determination control module 36 may, for example, acquire the load data acquired by the load data acquisition module 32, and determine whether the given mattress removal condition is satisfied based on the acquired load data. In addition, in this embodiment, the presence-or-absence determination control module 36 may hold the acquired load data. Then, the presence-or-absence determination control module 36 may determine whether the given mattress removal condition is satisfied based on a history of the held load data. Further, the presence-or-absence determination control module 36 resumes, for example, the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 in response to a given mattress placement condition being satisfied in the situation in which the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 is stopped. In this embodiment, it is desired that a condition satisfied when the mattress 20 is arranged on the bed frame 18 on which the mattress 20 is not arranged be set as the mattress placement condition. For example, the mattress placement condition may be a condition relating to a magnitude of the load applied to the load sensor 14. Further, the mattress placement condition may be a condition relating to smallness of variation in the loads applied to the plurality of load sensors 14. Further, the mattress placement condition may be a condition relating to smallness of variation in a latest plurality of measurement results of the load applied to the load sensor 14. In this embodiment, the presence-or-absence determination control module 36 may acquire the total weight data generated by the total weight data generation module 64, and determine whether the given mattress placement condition is satisfied based on the acquired total weight data. In addition, in this embodiment, the presence-or-absence determination control module 36 may hold the acquired total weight data. Then, the presence-or-absence determination control module 36 may determine whether the given mattress placement condition is satisfied based on the history of the held total weight data. Further, the presence-or-absence determination control module 36 may, for example, acquire the load data acquired by the load data acquisition module 32, and determine whether the given mattress placement condition is satisfied based on the acquired load data. In addition, in this embodiment, the presence-or-absence determination control module 36 may hold the acquired load data. Then, the presence-or-absence determination control module 36 may determine whether the given mattress placement condition is satisfied based on the history of the held load data. In addition, the presence-or-absence determination control module 36 may hold a mattress removal flag. Then, in the situation in which the presence-or-absence determination processing is being executed by the presence-or-absence determination processing execution module 34, a value of the mattress removal flag may be set to 0. Further, in the situation in which the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 is stopped, the value of the mattress removal flag may be set to 1. Then, when the value of the mattress removal flag is set to 0, the presence-or-absence determination control module 36 may determine whether a result of comparison with the value of the above-mentioned bed frame weight data satisfies the given mattress removal condition. Further, when the value of the mattress removal flag is set to 1, the presence-or-absence determination control module 36 may determine whether the given mattress placement condition is satisfied. In the following description, it is assumed that the mattress 20 is arranged on the bed frame 18 in an initial state, and the value of the mattress removal flag held in the presence-or-absence determination control module 36 is set to 0. In this embodiment, output of the load data indicating the latest measurement result of the load applied to the load sensor 14 from the load sensor 14 to the signal processing device 10, and the above-mentioned determination by the presence-or-absence determination control module 36 are repeatedly executed (for example, at predetermined time intervals). Now, an example of a flow of the presence-or-absence determination processing performed by the presence-or-absence determination processing execution module 34 is described with reference to flow charts illustrated as examples in FIG. 3 to FIG. 5. In the following description, the value of the personto-be-measured body weight data stored in the weight data storage module 30 is assumed to be "A". In addition, the value of the during-absence total weight data stored in the weight data storage module 30 is assumed to be "B". FIG. 3 is a diagram for illustrating an example of a flow of the presence-or-absence determination processing executed when the presence-or-absence information indicates that the person to be measured is not on the bed, that is, when the value of the one-user present flag and the value of the unknown-state flag held in the presence-or-absence information holding module 60 are both 0. In this case, the presence-or-absence determination module 66 determines whether the above-mentioned increase amount is equal to or larger than (Axr1) (Step S101) . Here, "r1" is a given value, and is, for example, 0.8. When the increase amount is not equal to or larger than (Axrl) (Step S101: N), the process illustrated in this processing example is ended. When the increase amount is equal to or larger than (Axr1) (Step S101: Y), the presence-or-absence determination module 66 determines whether the value of the total weight data based on the latest measurement result is equal to or larger than (A+B- d) and equal to or smaller than (A+B+d) (Step S102). Here, "d" is a given threshold value. When the value of the total weight data based on the latest measurement result is equal to or larger than (A+B-d) and equal to or smaller than (A+B+d) (Step S102: Y), the presence-or-absence changing module 68 sets the value of the one-user present flag held in the presence-or-absence information holding module 60 to 1 (Step S103). Then, the weight data management module 72 updates the person-to-be-measured body weight data stored in the weight data storage module 30 to data indicating the abovementioned increase amount, and updates the during-absence total weight data stored in the weight data storage module 30 to data indicating a value indicated by the total weight data based on an immediately preceding measurement result (Step S104), and the process illustrated in this processing example is ended. That is, the above-mentioned increase amount is set as the value A, and the value indicated by the total weight data based on the immediately preceding measurement result is set as the value B. When it is determined in the processing step illustrated in Step S102 that the value of the total weight data based on the latest measurement result is smaller than (A+B-d) or larger than (A+B+d) (Step S102: N), the presence-or-absence changing module 68 sets the value of the unknown-state flag to 1 (Step S105). Then, the weight data management module 72 generates unknown-weight data in which a value indicating the abovementioned increase amount is set, and stores the generated unknown-weight data in the weight data storage module 30 (Step S106). Then, the process illustrated in this processing example is ended. In the following description, a value of the unknownweight data is assumed to be "C". As described with reference to FIG. 3, the presence-or-absence changing module 68 may change the held presence-or-absence information to the information indicating that the person to be measured is on the mattress 20 when the presence-or-absence information indicates that the person to be measured is not on the mattress 20, the magnitude of the above-mentioned increase amount satisfies the presence determination condition, and a difference between a sum of the value of the person-tobe-measured body weight data and the value of the during-absence total weight data, and, the value indicated by the total weight data based on the latest measurement result, is within a given range. Then, the presence-or-absence changing module 68 may change the held presence-or-absence information to information indicating that it is unknown whether the person to be measured is on the mattress 20 when the presence-or-absence information indicates that the person to be measured is not on the mattress 20, the magnitude of the above-mentioned increase amount satisfies the presence determination condition, and the difference between the sum of the value of the person-to-be-measured body weight data and the value of the during-absence total weight data, and, the value indicated by the total weight data based on the latest measurement result, is not within the range. FIG. 4 is a diagram for illustrating an example of a flow of the presence-or-absence determination processing executed when the presence-or-absence information indicates that the person to be measured is on the bed, that is, when the value of the one-user present flag held in the presence-or-absence information holding module 60 is 1 and the value of the unknownstate flag is 0. In this case, the presence-or-absence determination module 66 determines whether the above-mentioned decrease amount is equal to or larger than (A*r2) (Step S201) . Here, "r2" is a given value, and is, for example, 0.8. The value r2 may be the same as or different from the value r1. When the decrease amount is not equal to or larger than (A*r2) (Step S201: N), the process illustrated in this processing example is ended. When the decrease amount is equal to or larger than (A*r2) (Step S201: Y), the presence-or-absence changing module 68 sets the value of the one-user present flag held in the presence-or-absence information holding module 60 to 0 (S202), and the process illustrated in this processing example is ended. FIG. 5 is a diagram for illustrating an example of a flow of the presence-or-absence determination processing executed when the presence-or-absence information indicates that it is unknown whether the person to be measured is on the bed, that is, when the value of the one-user present flag held in the presence-or-absence information holding module 60 is 0 and the value of the unknown-state flag is 1. In this case, the presence-or-absence determination module 66 determines whether the above-mentioned decrease amount is equal to or larger than (C*r3) (Step S301) . Here, "r3" is a given value, and is, for example, 0.8. The value r3 may be the same as or different from the value r1. In addition, the value r3 may be the same as or different from the value r2. When the decrease amount is not equal to or larger than (Or3) (Step S301: N), the process illustrated in this processing example is ended. When the decrease amount is equal to or larger than (C*r3) (Step S301: Y), the presence-or-absence changing module 68 sets the value of the unknown-state flag held in the presence-or-absence information holding module 60 to 0 (Step S302). Then, the weight data management module 72 deletes the unknown-weight data stored in the weight data storage module 30 (Step S303), and the process illustrated in this processing example is ended. Next, an example of a flow of presence-or-absence determination control processing performed by the presence-or-absence determination control module 36 is described with reference to flow charts illustrated as examples in FIG. 6 and FIG. 7. In the following description, the value of the bed frame weight data stored in the weight data storage module 30 is assumed to be "D". In addition, the presence-or-absence determination control module 36 is assumed to hold the abovementioned mattress removal flag. In addition, as described above, the value of the during-absence total weight data stored in the weight data storage module 30 is assumed to be "B". Further, in the following description, the presence-or-absence determination control module 36 is assumed to hold removal determination count data. In the initial state, a value of the removal determination count data held by the presence-or-absence determination control module 36 is assumed to be set to 0. In addition, the presence-or-absence determination control module 36 is assumed to hold the history of load data indicating measurement results of the loads obtained by the respective load sensors 14. FIG. 6 is a diagram for illustrating an example of a flow of the presence-or-absence determination control processing executed in a situation in which the presence-or-absence determination processing illustrated in FIG. 3 to FIG. 5 is being executed, that is, when the value of the mattress removal flag held in the presence-or-absence determination control module 36 is 0. The presence-or-absence determination control module 36 determines whether the value of the total weight data based on the latest measurement result is equal to or smaller than (D+p1) (Step S401). Here, "p1" is a given threshold value. When the value of the total weight data based on the latest measurement result is not equal to or smaller than (D+p1) (Step S401: N), the presence-or-absence determination control module 36 sets the value of the removal determination count data held by the presence-or-absence determination control module 36 to 0 (Step S402), and the process illustrated in this processing example is ended. When the value of the total weight data based on the latest measurement result is equal to or smaller than (D+p1) (Step S401: Y), the presence-or-absence determination control module 36 increases the value of the removal determination count data held by the presence-or-absence determination control module 36 by 1 (Step S403). Then, the presence-or-absence determination control module 36 confirms whether the value of the removal determination count data has reached a predetermined value X (for example, 3) (Step S404). When the value of the removal determination count data has not reached X (Step S404: N), the process illustrated in this processing example is ended. When the value of the removal determination count data has reached X (Step S404: Y), the presence-or-absence determination control module 36 sets the value of the mattress removal flag held by the presence-or-absence determination control module 36 to 1 (Step S405), stops the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 (Step S406), and the process illustrated in this processing example is ended. In the processing example illustrated in FIG. 6, a condition that a difference between the total weight and the value of the bed frame weight data is equal to or smaller than p1 for X consecutive times corresponds to the above-mentioned mattress removal condition. When the total weight is close to the weight of the bed frame 18, there is a high possibility that the mattress 20 is not arranged on the bed frame 18. In consideration of this, in the processing illustrated in FIG. 6, the condition that the difference between the total weight and the value of the bed frame weight data is equal to or smaller than p1 for X consecutive times is employed as the abovementioned mattress removal condition. The mattress removal condition is not limited to this condition. For example, the mattress removal condition may be a condition that a situation in which the difference between the total weight and the value of the bed frame weight data is equal to or smaller than p1 occurs once. FIG. 7 is a diagram for illustrating an example of a flow of the presence-or-absence determination control processing executed in the situation in which the execution of the presence-or-absence determination processing illustrated in FIG. 3 to FIG. 5 is stopped, that is, when the value of the mattress removal flag held in the presence-or-absence determination control module 36 is 1. The presence-or-absence determination control module 36 determines, for all the four load sensors 14, whether a value of the load data indicating the latest measurement result obtained by the load sensor 14 is equal to or larger than E (Step S501). Here, "E" is a given value, and may be, for example, a value obtained by dividing a sum of the above-mentioned value D and a predetermined value Y by the number of load sensors 14 (here, for example, 4). When the value of the load data indicating the latest measurement result obtained by the load sensor 14 is not equal to or larger than E for at least one load sensor 14 (Step S501: N), the process illustrated in this processing example is ended. When the value of the load data indicating the latest measurement result obtained by the load sensor 14 is equal to or larger than E for all the four load sensors 14 (Step S501: Y), the presence-or-absence determination control module 36 determines, for all the four load sensors 14, whether the value of the load data indicating the latest measurement result obtained by the load sensor 14 is equal to or larger than (F-p2) and equal to or smaller than (F+p2) (Step S502). Here, "F" is, for example, an average value of the latest measurement results of the four load sensors 14, and "p2" is a given threshold value. The value p2 may be a value dependent on F, for example, a value obtained by multiplying F by a predetermined number. For example, p2 may be F*0.1. When the value of the load data indicating the latest measurement result obtained by the load sensor 14 is smaller than (F-p2) or larger than (F+p2) for at least one load sensor 14 (Step S502: N), the process illustrated in this processing example is ended. When the value of the load data indicating the latest measurement result obtained by the load sensor 14 is equal to or larger than (F-p2) and equal to or smaller than (F+p2) for all the four load sensors 14 (Step S502: Y), the presence-or-absence determination control module 36 determines, for all the four load sensors 14, whether the variance of measurement results of the load obtained by the load sensor 14 in the latest predetermined number of times (for example, latest three times) is equal to or smaller than p3 (Step S503). Here, "p3" is a given threshold value. When the variance of the measurement results of the load obtained by the load sensor 14 in the latest predetermined number of times (for example, the latest three times) is larger than p3 for at least one load sensor 14 (Step S503: N), the process illustrated in this processing example is ended. When the variance of the measurement results of the load obtained by the load sensor 14 in the latest predetermined number of times (for example, the latest three times) is equal to or smaller than p3 for all the four load sensors 14 (Step S503: Y), the during-absence total weight data stored in the weight data storage module 30 is updated to the data indicating the value indicated by the total weight data based on the latest measurement result (Step S504), the value of the mattress removal flag held by the presence-or-absence determination control module 36 is set to 0 (Step S505), the execution of the presence-or-absence determination processing by the presence-or-absence determination processing execution module 34 is resumed (Step S506), and the process illustrated in this processing example is ended. That is, the value indicated by the total weight data based on the latest measurement result is set as the value B. In the processing example illustrated in FIG. 7, a condition that the load applied to all the load sensors 14 is equal to or larger than E, the difference between the load applied to the load sensor 14 and the average value of the loads applied to the four load sensors 14 is equal to or smaller than p2 for all the load sensors 14, and the variance of the measurement results of the load in the latest predetermined number of times is equal to or smaller than p3 for all the load sensors 14 corresponds to the above-mentioned mattress placement condition. The condition that the load applied to all the load sensors 14 is equal to or larger than E corresponds to an example of the condition relating to the magnitude of the load applied to the load sensor 14. Further, the condition that the difference between the load applied to the load sensor 14 and the average value of the loads applied to the four load sensors 14 is equal to or smaller than p2 for all the load sensors 14 corresponds to an example of the condition relating to the smallness of the variation in the loads applied to the plurality of load sensors 14. Further, the condition that the variance of the measurement results of the load in the latest predetermined number of times is equal to or smaller than p3 for all the load sensors 14 corresponds to the condition relating to the smallness of the variation in the latest plurality of measurement results of the load applied to the load sensor 14. Thus, the mattress placement condition may be a condition relating to the magnitude of the load applied to the load sensor 14, the smallness of the variation in the loads applied to the plurality of load sensors 14, and the smallness of the variation in the latest plurality of measurement results of the load applied to the load sensor 14. When the loads applied to the respective load sensors 14 are large to some extent, there is a high possibility that an object having a certain weight, such as the mattress 20, is arranged on the bed frame 18. Further, when the loads are evenly applied to the respective load sensors 14, there is a high possibility that an object having a relatively small spatial variation in the loads applied to the respective load sensors 14 at a certain point in time, such as the mattress 20, is arranged on the bed frame 18. Further, when the variation in the latest plurality of measurement results of the load applied to the load sensor 14 is small, there is a high possibility that an object having a relatively small temporal change in the load applied to each load sensor 14, such as the mattress 20, is arranged on the bed frame 18. In consideration of this, in the processing illustrated in FIG. 7, the condition relating to the magnitude of the load applied to the load sensor 14, the smallness of the variation in the loads applied to the plurality of load sensors 14, and the smallness of the variation in the latest plurality of measurement results of the load applied to the load sensor 14 is employed as the mattress placement condition. The mattress placement condition may be a condition relating to at least one of the magnitude of the load applied to the load sensor 14, the smallness of the variation in the loads applied to the plurality of load sensors 14, or the smallness of the variation in the latest plurality of measurement results of the load applied to the load sensor 14. Further, in the processing example illustrated in FIG. 7, through execution of the processing step illustrated in Step S504, the total weight data indicating the total weight calculated based on the load applied to the at least one load sensor 14 when the mattress placement condition is satisfied is used as the during-absence total weight data (that is, the total weight data based on the immediately preceding measurement result) in the presence-or-absence determination processing immediately after the resumption. In this manner, according to this embodiment, when the mattress 20 is replaced, the calibration of the value of the during-absence total weight data based on the weight of the mattress 20 newly arranged on the bed frame 18 is executed. The Doppler data acquisition module 38 acquires, for each of the plurality of Doppler sensors installed so as to face the person to be measured, Doppler data that indicates a result of measurement by the Doppler sensor in a period. For example, as illustrated in FIG. 8, the Doppler data acquisition module 38 cuts out, for each of a plurality of periods (a plurality of time windows) and for each of the plurality of Doppler sensors, a part of Doppler data indicating a result of measurement by the Doppler sensor that indicates a measurement result in the period. For example, out of first Doppler data acquired from the first Doppler sensor, pieces of Doppler data D(1, 1), D(2, 1), D(3, 1), D(4, 1) ... which indicate measurement results in a first time window, a second time window, a third time window, a fourth time window ..., respectively, are cut out. Further, out of second Doppler data acquired from the second Doppler sensor, pieces of Doppler data D(1, 2), D(2, 2), D(3, 2), D(4, 2) ... which indicate measurement results in the first time window, the second time window, the third time window, the fourth time window ..., respectively, are cut out. Similarly for each of the third Doppler sensor to the sixth Doppler sensor, Doppler data indicating a measurement result in each time window is cut out. For example, out of third Doppler data acquired from the third Doppler sensor, pieces of Doppler data D(1, 3), D(2, 3), D(3, 3), D(4, 3) ... which indicate measurement results in the first time window, the second time window, the third time window, the fourth time window ..., respectively, are cut out. Further, out of fourth Doppler data acquired from the fourth Doppler sensor, pieces of Doppler data D(1, 4), D(2, 4), D(3, 4), D(4, 4) ... which indicate measurement results in the first time window, the second time window, the third time window, the fourth time window ..., respectively, are cut out. Further, out of fifth Doppler data acquired from the fifth Doppler sensor, pieces of Doppler data D(1, 5), D(2, 5), D(3, 5), D(4, 5) ... which indicate measurement results in the first time window, the second time window, the third time window, the fourth time window ..., respectively, are cut out. Further, out of sixth Doppler data acquired from the sixth Doppler sensor, pieces of Doppler data D(1, 6), D(2, 6), D(3, 6), D(4, 6) ... which indicate measurement results in the first time window, the second time window, the third time window, the fourth time window ..., respectively, are cut out. The length of each time window is constant (for example, 60 seconds), and each time window starts at timing shifted from start timing of its preceding time window by a predetermined length of time (here, 2 seconds, for example). Applied time windows are common to every piece of Doppler data from the first Doppler data to the sixth Doppler data. That is, for each piece of Doppler data from the first Doppler data to the sixth Doppler data, a period corresponding to, for example, the first time window is the same period, and a period corresponding to the second time window is the same period as well. The frequency spectrum generation module 40 generates a frequency spectrum for, for example, each of the plurality of Doppler sensors based on the Doppler data that indicates a result of measurement by the Doppler sensor. The frequency spectrum generation module 40 executes, for example, fast Fourier transform (FFT) on Doppler data input thereto, to thereby convert the Doppler data into a frequency spectrum. Here, data of the I signal and data of the Q signal may each be converted into a frequency spectrum, for example. The frequency spectrum selection module 42 selects, for example, a plurality of frequency spectra out of the frequency spectra generated respectively for the plurality of Doppler sensors, based on peak frequencies in the frequency spectra generated respectively for the plurality of Doppler sensors. Now, an example of a flow of processing executed by the frequency spectrum selection module 42 to select frequency spectra is described with reference to a flow chart illustrated as an example in FIG. 9. The description given here is about processing of selecting a plurality of frequency spectra from, for example, frequency spectra generated for the data of the I signal and the data of the Q signal of each of six pieces of Doppler data D(1, 1), D(1, 2), D(1, 3), D(1, 4), D(1, 5), and D(1, 6) which are cut out with respect to the first time window. First, the frequency spectrum selection module 42 identifies a peak frequency that is a frequency having a largest amplitude in the frequency spectrum generated for each of the data of the I signal and the data of the Q signal of each of six pieces of Doppler data D(1, 1), D(1, 2), D(1, 3), D(1, 4), D(1, 5), and D(1, 6), as a respiratory rate corresponding to the frequency spectrum (Step S601). The frequency spectrum selection module 42 then calculates a representative value (here, an average value, for example) of the respiratory rate identified in the processing step illustrated in Step S601 (Step S602). The frequency spectrum selection module 42 then examines whether there is a frequency spectrum corresponding to a respiratory rate different from the representative value calculated in the processing step illustrated in Step S602 by a predetermined value (for example, 5) or more (that is, a respiratory rate that is an outlier) (Step S603). When there is a respiratory rate that is an outlier (Step S603: Y), the frequency spectrum selection module 42 excludes the frequency spectrum corresponding to the respiratory rate that is an outlier (Step S604), and the process returns to the processing step illustrated in Step S602. When there is no respiratory rate that is an outlier (Step S603: N), the process illustrated in this processing example is ended. In the process illustrated in this processing example, all of the frequency spectra may be selected without excluding any frequency spectrum. The frequency spectra remaining without being excluded by the processing described above correspond to the frequency spectra that are selected from frequency spectra generated for the data of the I signal and the data of the Q signal of each of the six pieces of Doppler data cut out with respect to the first time window. The processing described above is executed for each of the plurality of time windows. The aggregated spectrum generation module 44 generates an aggregated spectrum based on, for example, some or all of frequency spectra generated respectively for the plurality of Doppler sensors. As illustrated in FIG. 10, amplitudes (intensities) in a frequency spectrum of the data of the I signal of D(1, 1), a frequency spectrum of the data of the Q signal of D(1, 1), a frequency spectrum of the data of the I signal of D(1, 2), a frequency spectrum of the data of the Q signal of D(1, 2), a frequency spectrum of the data of the I signal of D(1, 3), a frequency spectrum of the data of the Q signal of D(1, 3) ... and a frequency spectrum of the data of the Q signal of D(1, 6) may be added on a frequency-by-frequency basis, to thereby generate an aggregated spectrum (1) with respect to the first time window. The aggregated spectrum (1) corresponds to an aggregated spectrum of the first time window. In this manner, the aggregated spectrum generation module 44 may generate an aggregated spectrum that is a frequency spectrum obtained by summing up some or all of frequency spectra generated respectively for the plurality of Doppler sensors. The aggregated spectrum generation module 44 may also generate an aggregated spectrum that is a frequency spectrum obtained by averaging some or all of frequency spectra generated respectively for the plurality of Doppler sensors. The aggregated spectrum generation module 44 may also generate an aggregated spectrum based on a plurality of frequency spectra selected by the frequency spectrum selection module 42. For example, a case in which the frequency spectrum of the data of the I signal of D(1, 5), the frequency spectrum of the data of the Q signal of D(1, 5), the frequency spectrum of the data of the I signal of D(1, 6), and the frequency spectrum of the data of the Q signal of D(1, 6) are excluded, and the rest of the frequency spectra are selected is discussed. The aggregated spectrum (1) generated in this case may be a frequency spectrum obtained by summing up the frequency spectrum of the data of the I signal of D(1, 1), the frequency spectrum of the data of the Q signal of D(1, 1), I signal of D(1, 2), Q signal of D(1, 2), the frequency spectrum the frequency spectrum the frequency spectrum of the data of the of the data of the of the data of the I signal of D(1, 3), the frequency spectrum of the data of the Q signal of D(1, 3) the frequency spectrum of the data of the I signal of D(1, 4), and the frequency spectrum of the data of the Q signal of D(1, 4). In this manner, influence of an outlier on an aggregated spectrum to be generated is suppressible. Although the aggregated spectrum (1) which is the aggregated spectrum of the first time window is illustrated in FIG. 10, an aggregated spectrum (2) which is an aggregated spectrum of the second time window, an aggregated spectrum (3) which is an aggregated spectrum of the third time window, and so on are generated in the same manner. The biological information generation module 46 generates biological information of the person to be measured, based on the Doppler data, for example. The biological information generation module 46 may generate biological information of the person to be measured in a period of interest based on the aggregated spectrum. The biological information generation module 46 may generate the respiratory rate of the person to be measured in the period of interest based on a peak frequency in the aggregated spectrum. For example, a frequency having the largest amplitude in the aggregated spectrum may be identified as the peak frequency. The peak frequency in the aggregated spectrum may then be generated as the respiratory rate. The biological information generation module 46 may also identify the largest amplitude and the second-largest amplitude in the aggregated spectrum. The biological information generation module 46 may then generate an amplitude ratio obtained by dividing the largest amplitude by the second-largest amplitude. The biological information generation module 46 may also generate the amplitude ratio for each of aggregated spectra calculated respectively for a plurality of time windows (for example, from the first time window to the thirtieth time window). The biological information generation module 46 may then extract aggregated spectra that have amplitude ratios equal to or more than a predetermined threshold value (for example, 1.5). The biological information generation module 46 may then generate an average value of respiratory rates generated for the respective aggregated spectra that have amplitude ratios equal to or more than the predetermined threshold value (for example, 1.5), as a respiratory rate in a period (for example, 1 minute) associated with the plurality of time windows. The biological information generation module 46 may also generate, as a reliability level, a proportion of the number of aggregated spectra that have amplitude ratios equal to or more than a predetermined threshold value (for example, 1.5) to the number of aggregated spectra calculated with respect to a plurality of time windows. In this embodiment, selection of frequency spectra by the frequency spectrum selection module 42 may be omitted. Then, the aggregated spectrum generation module 44 may use all frequency spectra in a period that are generated by the frequency spectrum generation module 40 to generate an aggregated spectrum in the period. The frequency spectrum selection module 42 may select frequency spectra on a group of Doppler sensors by a group of Doppler sensors basis. That is, the frequency spectrum selection module 42 may select, based on peak frequencies in frequency spectra generated respectively for the plurality of Doppler sensors, two or more of the plurality of Doppler sensors. Then, the aggregated spectrum generation module 44 may generate an aggregated spectrum based on frequency spectra generated respectively for the selected two or more Doppler sensors. It is not required to convert the data of the I signal and the data of the Q signal separately into frequency spectra. For example, data of a complex signal having the I component as a real part and the Q component as an imaginary part may be converted into a frequency spectrum. An aggregated spectrum may then be generated based on the frequency spectrum. In the case of measuring biological information of a person to be measured through use of the plurality of Doppler sensors, some of measurement results acquired respectively from the plurality of Doppler sensors may be low in accuracy, depending on a position, a posture, or the like of the person to be measured. Accordingly, when biological information is measured at each of the plurality of Doppler sensors and a representative value such as an average value of the measured biological information is used as a value of the biological information of the person to be measured, the thus obtained value of the biological information may be an inappropriate value. In this embodiment, biological information is generated based on an aggregated spectrum as described above. Accordingly, this embodiment enables more accurate measurement of biological information of a person to be measured than in a case in which a representative value such as an average value of measured biological information is used as the value of biological information of a person to be measured. Further, the Doppler sensors differ from one another in angle to the person to be measured and, accordingly, may slightly differ from one another in, for example, peak frequency of a frequency spectrum based on a measurement result. With regards to this, when arrangement such as arrangement in which the plurality of Doppler sensors are installed so as to align perpendicularly to the lengthwise direction of the bed 16, arrangement in which the plurality of Doppler sensors are installed symmetrically with respect to the center line of the bed 16, or arrangement in which the plurality of Doppler sensors are installed at regular intervals to form a single line is to be employed, an aggregated spectrum in which frequency spectra corresponding to the respective Doppler sensors are appropriately evened out is generated and, as a result, more accurate measurement of biological information of a person to be measured is achieved. In the following description, it is assumed that the abovementioned respiratory rate is generated at intervals of one minute. The abnormality determination module 48 determines, for example, whether the biological information of the person to be measured satisfies a given abnormality determination condition. Here, the abnormality determination condition may be, for example, a condition that the fact that the respiratory rate is 8 or less or 25 or more applies to all the five latest generated respiratory rates. The action execution module 50 executes a given action, for example, in response to the fact that the biological information of the person to be measured satisfies the abovementioned abnormality determination condition. Here, the action execution module 50 may execute a first action when the presence-or-absence information indicates that the person to be measured is on the mattress 20, and may execute a second action when the presence-or-absence information indicates that it is unknown whether the person to be measured is on the mattress 20. For example, when the value of the one-user present flag held in the presence-or-absence information holding module 60 is 1, an emergency call action (for example, requesting an ambulance) may be executed in response to the fact that the biological information of the person to be measured satisfies the abnormality determination condition. Then, when the value of the unknown-state flag held in the presence-or-absence information holding module 60 is 1, an abnormality notification action (for example, output of an alarm sound) may be executed in response to the fact that the biological information of the person to be measured satisfies the abnormality determination condition. The operation control module 52 suppresses the execution of the action in response to the fact that the biological information satisfies the abnormality determination condition when the presence-or-absence information indicates that the person to be measured is not on the mattress 20, for example. Here, the operation control module 52 may monitor the value of the one-user present flag and the value of the unknown-state flag held in the presence-or-absence information holding module 60. Then, the operation control module 52 may stop the Doppler sensor in response to detection of a change from a situation in which the value of the one-user present flag or the value of the unknown-state flag is 1 to a situation in which both the value of the one-user present flag and the value of the unknown-state flag are 0. The operation control module 52 may also activate the Doppler sensor in response to detection of a change from the situation in which both the value of the one-user present flag and the value of the unknown-state flag are 0 to the situation in which the value of the one-user present flag or the value of the unknown-state flag is 1. Then, the generation of the respiratory rate may be resumed. In this manner, the execution of the action in response to the fact that the biological information satisfies the abnormality determination condition may be suppressed by stopping the operation of the Doppler sensor. Further, the operation control module 52 may suppress the execution of the action in response to the fact that the biological information satisfies the abnormality determination condition when the presence-or-absence information indicates that the person to be measured is on the mattress 20 and it is determined that a body movement of the person to be measured has occurred. For example, it is assumed that the held value of the one-user present flag is 1. In this case, the operation control module 52 may monitor the value of the body movement information held in the body movement information holding module 62. Then, the operation control module 52 may stop the Doppler sensor in response to detection of a change from a situation in which the value of the body movement information is 1 to a situation in which the value of the body movement information is 0. Further, the operation control module 52 may activate the Doppler sensor in response to detection of a change from the situation in which the value of the body movement information is 0 to the situation in which the value of the body movement information is 1. Then, the generation of the respiratory rate may be resumed. Instead of the operation control module 52, the action execution module 50 may suppress the execution of the action in response to the fact that the biological information satisfies the abnormality determination condition when the presence-or-absence information indicates that the person to be measured is not on the mattress 20. For example, it is assumed that the Doppler sensor is operating, and the above-mentioned respiratory rate is generated at intervals of one minute. In this situation, the action execution module 50 is not required to execute the above-mentioned given action when both the value of the one-user present flag and the value of the unknown-state flag held in the presence-or-absence information holding module 60 are 0 even in a case in which the biological information of the person to be measured satisfies the given abnormality determination condition. Further, the action execution module 50 is not required to execute the above-mentioned given action when the held value of the body movement information is 1 even in a case in which the biological information of the person to be measured satisfies the abnormality determination condition. When the respiratory rate of the person to be measured indicates an abnormal value, it is impossible to determine whether the cause is that an abnormality has occurred in the person to be measured or that the person to be measured is not on the mattress 20. In this embodiment, whether the person to be measured is on the bed or not is accurately determined in the above-mentioned manner. Accordingly, when the respiratory rate of the person to be measured indicates an abnormal value, it is possible to determine whether the cause is that an abnormality has occurred in the person to be measured or that the person to be measured is not on the mattress 20. Further, as described above, the execution of the action in response to the fact that the biological information satisfies the abnormality determination condition may be suppressed when the presence-or-absence information indicates that the person to be measured is not on the mattress 20. In this manner, it is possible to prevent an action such as requesting an ambulance or outputting an alarm sound from being erroneously executed when the person to be measured is not on the mattress 20, for example. Further, when a body movement occurs, there is a high possibility that the respiratory rate cannot be accurately detected. In consideration of this, as described above, the execution of the action in response to the fact that the biological information satisfies the abnormality determination condition may be suppressed when it is determined that the body movement of the person to be measured has occurred. In this manner, it is possible to prevent the action from being erroneously executed based on the respiratory rate having a high possibility of being inaccurate. Further, as described above, the Doppler sensor may be stopped when the presence-or-absence information indicates that the person to be measured is not on the mattress 20. In this manner, it is possible to reduce power consumption of the Doppler sensor. Further, as described above, when a person or an animal (such as a pet or a child of the person to be measured) on the mattress 20 is too heavy or too light compared to the person to be measured, it may be determined that it is unknown whether the person to be measured is on the mattress 20. In this case, the emergency call action (for example, requesting an ambulance) is not executed, but the abnormality notification action (for example, output of an alarm sound) is executed in response to the fact that the biological information satisfies the abnormality determination condition. In this manner, the number of cases of failure to report can be reduced. It is also possible to prevent the emergency call action from being executed unnecessarily. Further, in this embodiment, the estimated value (the above-mentioned value "A") of the body weight of the person to be measured may be output (by, for example, displaying the estimated value or outputting the estimated value as sound). In this manner, it is possible to measure the body weight, which varies slightly from day to day, without a burden only through sleep of the person to be measured. Further, in this embodiment, when the mattress 20 arranged on the bed frame 18 is removed or replaced by another mattress 20 having a different weight, it is no longer possible to appropriately determine whether the person to be measured is on the bed 16 or not by the presence-or-absence determination processing as described above. In particular, the value of the during-absence total weight data can no longer be said to be a reliable value. In this embodiment, as described above, the execution of the presence-or-absence determination processing is stopped when the mattress removal condition is satisfied in a situation in which the presence-or-absence determination processing is being executed. Accordingly, it is possible to prevent unnecessary presence-or-absence determination processing from being executed in a situation in which it is no longer possible to appropriately determine whether the person to be measured is on the bed 16 or not. In this manner, according to this embodiment, the execution of the presence-or-absence determination processing can be appropriately controlled. Further, in this embodiment, the execution of the presence-or-absence determination processing is stopped when the mattress removal condition is satisfied in the situation in which the presence-or-absence determination processing is being executed, and hence it is possible to prevent erroneous determination that the person to be measured is on the bed 16 even though the person to be measured is not on the bed 16 when the mattress 20 is replaced and a new mattress 20 is arranged on the bed frame 18. The present invention is not limited to the above-mentioned embodiment, and various modifications can be made thereto. For example, in the above description, the respiratory rate is generated as the biological information of the person to be measured, but the heart rate can also be generated in a similar manner. In this case, a given action may be executed in response to the fact that the heart rate satisfies a given abnormality determination condition.

Claims

1. A presence-or-absence determination control system, comprising:presence-or-absence determination processing execution means for repeatedly executing presence-or-absence determination processing for determining, based on a measurement result of a load applied to at least one load sensor supporting a bed frame of a bed used by a person to be measured, whether the person to be measured is on a mattress arranged on the bed frame; andpresence-or-absence determination stopping means for stopping execution of the presence-or-absence determination processing in response to a fact that a result of comparison between a total weight calculated based on the load applied to the at least one load sensor and a given bed frame weight value indicating a weight of the bed frame satisfies a given mattress removal condition in a situation in which the presence-or-absence determination processing is being executed.

2. The presence-or-absence determination control system according to claim 1, further comprising presence-or-absence determination resuming means for resuming the execution of the presence-or-absence determination processing in response to a fact that a given mattress placement condition is satisfied in a situation in which the execution of the presence-or-absence determination processing is stopped.

3. The presence-or-absence determination control system according to claim 2, wherein the given mattress placement condition comprises a condition relating to a magnitude of the load applied to the load sensor.

4. The presence-or-absence determination control system according to claim 2, wherein the given mattress placement condition comprises a condition relating to smallness of variation in loads applied to a plurality of load sensors.

5. The presence-or-absence determination control system according to claim 2, wherein the given mattress placement condition comprises a condition relating to smallness of variation in a latest plurality of measurement results of the load applied to the load sensor.

6. The presence-or-absence determination control system according to claim 2,wherein the presence-or-absence determination processing execution means includes:total weight data acquisition means for acquiring total weight data indicating a total weight calculated based on the load applied to the at least one load sensor;presence-or-absence information holding means for holding presence-or-absence information indicating whether the person to be measured is on the mattress;presence changing means for changing the heldpresence-or-absence information to the presence-or-absence information indicating that the person to be measured is on the mattress when the presence-or-absence information indicates that the person to be measured is not on the mattress and a magnitude of an increase amount, which is a value obtained by subtracting a value indicated by the total weight data based on an immediately preceding measurement result from a value indicated by the total weight data based on a latest measurement result, satisfies a given presence determination condition; andabsence changing means for changing the held presence-or-absence information to the presence-or-absence information indicating that the person to be measured is not on the mattress when the presence-or-absence information indicates that the person to be measured is on the mattress and a magnitude of a decrease amount, which is a value obtained by subtracting the value indicated by the total weight data based on the latest measurement result from the value indicated by the total weight data based on the immediately preceding measurement result, satisfies a given absence determination condition, andwherein, in the presence-or-absence determination processing immediately after resumption, the total weight data indicating the total weight calculated based on the load applied to the at least one load sensor when the given mattress placement condition is satisfied is used as the total weight data based on the immediately preceding measurement result.

7. The presence-or-absence determination control system according to claim 1, wherein each of four corners of the bed frame is supported by the load sensor.

8. The presence-or-absence determination control system according to claim 1, further comprising:Doppler data acquisition means for acquiring Doppler data indicating a measurement result obtained by a Doppler sensor provided toward the person to be measured;biological information generation means for generating biological information of the person to be measured based on the Doppler data;action execution means for executing a given action in response to a fact that the biological information satisfies a given abnormality determination condition; andsuppression means for suppressing execution of the given action in response to the fact that the biological information satisfies the given abnormality determination condition when it is determined that the person to be measured is not on the mattress.

9. A presence-or-absence determination control method, comprising the steps of:repeatedly executing presence-or-absence determination processing for determining, based on a measurement result of a load applied to at least one load sensor supporting a bed frame of a bed used by a person to be measured, whether the person tobe measured is on a mattress arranged on the bed frame; andstopping execution of the presence-or-absence determination processing in response to a fact that a result of comparison between a total weight calculated based on the load applied to the at least one load sensor and a given bed frame weight value indicating a weight of the bed frame satisfies a given mattress removal condition in a situation in which the presence-or-absence determination processing is being executed.

10. A program for causing a computer to execute the steps of:repeatedly executing presence-or-absence determination processing for determining, based on a measurement result of a load applied to at least one load sensor supporting a bed frame of a bed used by a person to be measured, whether the person to be measured is on a mattress arranged on the bed frame; andstopping execution of the presence-or-absence determination processing in response to a fact that a result of comparison between a total weight calculated based on the load applied to the at least one load sensor and a given bed frame weight value indicating a weight of the bed frame satisfies a given mattress removal condition in a situation in which the presence-or-absence determination processing is being executed.

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