Patent Publication Number: US-2021181328-A1

Title: Sensing method and sensing device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of PCT Patent Application No. PCT/JP2018/032850 filed on Sep. 5, 2018, designating the United States of America. The entire disclosure of the above-identified application, including the specification, drawings and claims is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to a sensing method and a sensing device that detect presence and a motion of an object in a specific detection area. 
     BACKGROUND 
     A sensing device that detects presence and a motion of an object in a specific detection area has been known (for example, see Patent Literature (PTL) 1). This type of sensing device performs a frequency analysis on radar signals received by a plurality of antennas, and then processes the signals to achieve long-range detection and short-range detection. 
     In the long-range detection, the presence of an object is detected by adding the results of the frequency analysis performed on received signals (radar signals) together as complex numbers. In contrast, in the short-range detection, the presence of an object is detected by adding the results of the frequency analysis performed on received signals together as amplitudes. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2016-57168 
     SUMMARY 
     Technical Problem 
     In general, high detection resolution for, for example, distance and speed, is required in radar systems to accurately detect a motion of an object. On the other hand, in order to detect the presence of an object in a relatively wide detection range, it is necessary to increase the number of sampling data during sensing, or to increase the detection distance by lowering the detection resolution. 
     However, in the conventional sensing device described above, the long-range detection and the short-range detection both use the same sampling signals. Therefore, the detection resolution is the same for the long-range detection and the short-range detection. Therefore, it is not possible to set the detection range and the detection resolution suitable for both the long-range detection and the short-range detection. 
     The present disclosure relates to a sensing method and a sensing device that detect both presence and a motion of an object accurately in a specific detection area. 
     Solution to Problem 
     A sensing method according to one aspect of the present disclosure is a sensing method for detecting presence and a motion of an object in a specific detection area using a sensor, the sensing method including: (a) performing first sensing to detect presence or absence of the object in the specific detection area using a first sensor signal received by the sensor from the specific detection area; (b) when the presence of the object in the specific detection area is detected by the first sensing in (a), continuing the first sensing and performing second sensing to detect a motion of the object using a second sensor signal transmitted from the sensor to the specific detection area, the second sensor signal having a sensing rate higher than a sensing rate of the first sensor signal; and (c) when the absence of the object in the specific detection area is detected by the first sensing in (b), stopping the second sensing and continuing the first sensing. 
     Note that these comprehensive or specific aspects of the present disclosure may be implemented as a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM, or may be implemented as any combination of a system, a method, an integrated circuit, a computer program, and a recording medium. 
     Advantageous Effects 
     With the sensing method and the sensing device according to one or more aspects of the present disclosure, both the presence and a motion of an object in a specific detection area can be detected accurately. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG. 1  is a schematic diagram for describing first sensing and second sensing to be performed by a sensing device according to Embodiment 1. 
         FIG. 2  is a block diagram illustrating the configuration of the sensing device according to Embodiment 1. 
         FIG. 3  is a flowchart illustrating a process of operations of the sensing device according to Embodiment 1. 
         FIG. 4  is a timing chart for describing a presence detection mode and a motion detection mode of the sensing device according to Embodiment 1. 
         FIG. 5A  is a graph showing an example of a chirp waveform of a first sensor signal according to Embodiment 1. 
         FIG. 5B  is a graph showing an example of a chirp waveform of a second sensor signal according to Embodiment 1. 
         FIG. 6  is a graph showing an example of a beat signal of the first sensor signal according to Embodiment 1. 
         FIG. 7  is a block diagram illustrating the configuration of a sensing device according to Embodiment 2. 
         FIG. 8A  is a graph showing an example of chirp waveforms of a first sensor signal according to Embodiment 2. 
         FIG. 8B  is a graph showing an example of chirp waveforms of a second sensor signal according to Embodiment 2. 
         FIG. 9  is a block diagram illustrating the configuration of a sensing device according to Embodiment 3. 
         FIG. 10  is a timing chart for describing a presence detection mode and a motion detection mode to be performed by the sensing device according to Embodiment 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A sensing method according to one aspect of the present disclosure is a sensing method for detecting presence and a motion of an object in a specific detection area using a sensor, the sensing method including: (a) performing first sensing to detect presence or absence of the object in the specific detection area using a first sensor signal received by the sensor from the specific detection area; (b) when the presence of the object in the specific detection area is detected by the first sensing in (a), continuing the first sensing and performing second sensing to detect a motion of the object using a second sensor signal transmitted from the sensor to the specific detection area, the second sensor signal having a sensing rate higher than a sensing rate of the first sensor signal; and (c) when the absence of the object in the specific detection area is detected by the first sensing in (b), stopping the second sensing and continuing the first sensing. 
     With this aspect, the first sensor signal is used in the first sensing, and the second sensor signal having a sensing rate higher than a sensing rate of the first sensor signal is used in the second sensing. This makes the detection resolution of the second sensor signal in the second sensing higher than the detection resolution of the first sensor signal in the first sensing. As a result, both the presence and a motion of an object in a specific detection area can be detected accurately. In addition, for example, when an object goes outside the specific detection area during which both the first sensing and the second sensing are being performed, the second sensing is stopped and the first sensing is continued. This allows the second sensing to resume smoothly if, for example, the object enters the specific detection area again. 
     For example, the sensor may be a radar sensor that transmits and receives the first sensor signal and the second sensor signal, and each of the first sensor signal and second sensor signal may be a frequency modulated continuous wave (FMCW) radar signal, the FMCW radar signal being obtained by modulating a frequency of a continuous wave radar signal. 
     With this aspect, the first sensing can be performed using the first sensor signal, which is an FMCW radar signal, and the second sensing can be performed using the second sensor signal, which is an FMCW radar signal. 
     For example, the first sensing may include transmitting the first sensor signal from the sensor per first sensing time, and the second sensing may include transmitting the second sensor signal from the sensor per second sensing time, the second sensing time being shorter than the first sensing time. 
     For example, the first sensor signal may be an FMCW radar signal that includes at least one chirp waveform in the first sensing time, and the second sensor signal may be an FMCW radar signal that includes at least one chirp waveform in the second sensing time. 
     This aspect makes it possible to set the number of chirp waveforms of the first sensor signal and the second sensor signal appropriately according to sensing contents. For example, when one of the first sensor signal and the second sensor signal includes a plurality of chirp waveforms, the speed of a motion of the object can be detected. 
     For example, the first sensing may include modulating the first sensor signal with a first modulation bandwidth and transmitting, from the sensor, the first sensor signal modulated, and the second sensing may include modulating the second sensor signal with a second modulation bandwidth and transmitting, from the sensor, the second sensor signal modulated, the second modulation bandwidth being wider than the first modulation bandwidth. 
     For example, the first sensing may include: generating a beat signal by combining the first sensor signal transmitted from the sensor and a reflected signal of the first sensor signal, the reflected signal being received by the sensor; and detecting the presence of the object in the specific detection area when a signal strength of the beat signal is greater than or equal to a threshold. 
     This aspect makes it easy to detect presence or absence of an object in a specific detection area by comparing the signal strength of the beat signal with the threshold. 
     For example, the first sensing may include detecting the absence of the object in the specific detection area when a beat frequency of the beat signal falls outside a predetermined frequency range, the predetermined frequency range being determined according to the specific detection area. 
     With this aspect, the presence of an object in a specific detection area can be detected accurately. 
     For example, the sensor may include: an infrared sensor that receives infrared radiation as the first sensor signal; and a radar sensor that transmits and receives the second sensor signal, the second sensor signal being a frequency modulated continuous wave (FMCW) radar signal. 
     With this aspect, the first sensing can be performed using the first sensor signal, which is infrared radiation, and the second sensing can be performed using the second sensor signal, which is an FMCW radar signal. 
     Moreover, a sensing device according to one aspect of the present disclosure is a sensing device that detects presence and a motion of an object in a specific detection area, the sensing device including: a sensor that receives a first sensor signal from the specific detection area and transmits a second sensor signal to the specific detection area, the second sensor signal having a sensing rate higher than a sensing rate of the first sensor signal; and a controller that controls the sensor. The controller: (a) performs first sensing to detect presence or absence of the object in the specific detection area using the first sensor signal; (b) continues the first sensing and performs second sensing to detect a motion of the object using the second sensor signal, when the presence of the object in the specific detection area is detected by the first sensing in (a); and (c) stops the second sensing and continues the first sensing, when the absence of the object in the specific detection area is detected by the first sensing in (b). 
     With this aspect, the first sensor signal is used in the first sensing, and the second sensor signal having a sensing rate higher than a sensing rate of the first sensor signal is used in the second sensing. This makes the detection resolution of the second sensor signal in the second sensing higher than the detection resolution of the first sensor signal in the first sensing. As a result, both the presence and a motion of an object in a specific detection area can be detected accurately. In addition, for example, when an object goes outside the specific detection area during which the controller performs both the first sensing and the second sensing, the controller stops the second sensing and continues the first sensing. This allows the second sensing to resume smoothly if, for example, the object enters the specific detection area again. 
     The following describes embodiments in detail with reference to the drawings. 
     Note that each embodiment described below shows a general or specific example. The numerical values, shapes, materials, structural components, the arrangement and connection of the structural components, steps and the order of the steps mentioned in the following embodiment are mere examples and not intended to limit the present disclosure. Of the structural components in the following embodiments, structural components not recited in any one of the independent claims representing broadest concepts are described as optional structural components. 
     In addition, each diagram is not necessarily a precise illustration. Moreover, throughout the figures, structural components that are essentially the same share like reference signs, and duplicate description is omitted or simplified. 
     Embodiment 1 
     [1-1. Configuration of Sensing Device] 
     First, the configuration of sensing device  2  according to Embodiment 1 will be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a schematic diagram for describing first sensing and second sensing to be performed by sensing device  2  according to Embodiment 1.  FIG. 2  is a block diagram illustrating the configuration of sensing device  2  according to Embodiment 1. 
     As illustrated in  FIG. 1 , sensing device  2  is a device for detecting presence and a motion of object  6  in specific detection area  4 . In other words, sensing device  2  performs first sensing to detect presence or absence of object  6  in specific detection area  4  using a first sensor signal (will be described later), and performs second sensing to detect motion of object  6  in specific detection area  4  using a second sensor signal (will be described later) that is different from the first sensor signal. Moreover, sensing device  2  switches between a presence detection mode and a motion detection mode. In the presence detection mode, only the first sensing is performed. In the motion detection mode, both the first sensing and the second sensing are performed. Note that as illustrated in  FIG. 1 , specific detection area  4  extends from radar sensor  8  (will be described later) of sensing device  2  to, for example, a range of distance D (for example, 5 m) in a substantially fan shape. 
     Sensing device  2  can be applied as a user interface provided in an artificial intelligence (AI) speaker, for example. In this case, object  6  is a user who operates the AI speaker, for example. The user (object  6 ) can cause a gesture-enabled AI speaker to operate by performing a gesture while the user is in specific detection area  4 . For example, the user can increase the volume of the AI speaker when the user makes a gesture of raising his/her left arm, and decrease the volume of the AI speaker when the user makes a gesture of putting his/her left arm down. 
     As illustrated in  FIG. 2 , sensing device  2  includes radar sensor  8  (an example of a sensor) and digital signal processor  10  (hereinafter referred to as “DSP  10 ”) (an example of a controller). 
     As illustrated in  FIG. 2 , radar sensor  8  is a radio frequency (RF) unit that is used to transmit and receive the first sensor signal and the second sensor signal. Here, the RF frequency of radar sensor  8  is the 60 GHz band. Note that in the present embodiment, the frequency used for radar sensor  8  is set to 60 GHz band, but the present disclosure is not limited to such configuration. Any frequency band that can be used as radar, such as 24 GHz band or 79 GHz band, may be used. 
     Radar sensor  8  includes frequency sweep circuit  12 , transmission unit  14 , and reception unit  16 . 
     Frequency sweep circuit  12  generates the first sensor signal and the second sensor signal, based on a chirp control signal from DSP  10  (will be described later). Each of the first sensor signal and the second sensor signal is a frequency modulated continuous wave (FMCW) radar signal, which is a signal obtained by modulating a frequency of a continuous wave. Frequency sweep circuit  12  outputs the generated first and second sensor signals to power amplifier  18  (will be described later) of transmission unit  14  and in-phase/quadrature (I/Q) generation circuit  26  (will be described later) of reception unit  16 . 
     Transmission unit  14  includes power amplifier  18  and transmission antenna  20 . 
     Power amplifier  18  amplifies the first sensor signal and the second sensor signal output from frequency sweep circuit  12 . 
     Transmission antenna  20  transmits the first sensor signal and the second sensor signal output from power amplifier  18  to specific detection area  4 . 
     Reception unit  16  includes reception antenna  22 , low noise amplifier  24 , I/Q generation circuit  26 , quadrature demodulator  28 , and A/D converter  30 . 
     Reception antenna  22  receives a reflected signal of the first sensor signal and a reflected signal of the second sensor signal that are reflected off one or more objects (including object  6 ) that are present in specific detection area  4 . 
     Low noise amplifier  24  amplifies the reflected signal of the first sensor signal and the reflected signal of the second sensor signal that are received by reception antenna  22  and outputs the reflected signals to quadrature demodulator  28 . 
     I/Q generation circuit  26  generates a local signal of the first sensor signal and outputs the local signal to quadrature demodulator  28 . The local signal has a 90° phase difference from the first sensor signal received from frequency sweep circuit  12 . Moreover, I/Q generation circuit  26  generates a local signal of the second sensor signal and outputs the local signal to quadrature demodulator  28 . The local signal has a 90° phase difference from the second sensor signal received from frequency sweep circuit  12 . 
     Quadrature demodulator  28  generates analog I/Q data of the first sensor signal by performing quadrature detection on the local signal of the first sensor signal output from I/Q generation circuit  26  and quadrature detection on the reflected signal of the first sensor signal amplified by low noise amplifier  24 . 
     Moreover, quadrature demodulator  28  generates analog I/Q data of the second sensor signal by performing quadrature detection on the local signal of the second sensor signal output from I/Q generation circuit  26  and quadrature detection on the reflected signal of the second sensor signal amplified by low noise amplifier  24 . 
     A/D converter  30  converts the analog I/Q data of the first sensor signal output from quadrature demodulator  28  into digital I/Q data of the first sensor signal, based on a sampling clock signal that is input. Moreover, A/D converter  30  converts the analog I/Q data of the second sensor signal from quadrature demodulator  28  into digital I/Q data of the second sensor signal, based on the sampling clock signal that is input. A/D converter  30  outputs the digital I/Q data of the first sensor signal and the digital I/Q data of the second sensor signal to signal processor  32  of DSP  10 . 
     DSP  10  is a control unit that is used to control radar sensor  8 , as illustrated in  FIG. 2 . DSP  10  controls radar sensor  8 , for example, based on a preinstalled code or a hard-wired logic circuit. DSP  10  includes signal processor  32 , presence detector  34 , motion detector  36 , and radar controller  38 . Note that each processing of signal processor  32 , presence detector  34 , motion detector  36 , and radar controller  38  may be performed by a microcomputer, for example. 
     Signal processor  32  generates a beat signal of the first sensor signal by performing fast Fourier transform (FFT) processing on the digital I/Q data of the first sensor signal output from A/D converter  30  of radar sensor  8 . Signal processor  32  outputs the generated beat signal of the first sensor signal to presence detector  34 . Moreover, signal processor  32  generates a beat signal of the second sensor signal by performing FFT processing on the digital I/Q data of the second sensor signal output from A/D converter  30  of radar sensor  8 . Signal processor  32  outputs the generated beat signal of the second sensor signal to motion detector  36 . This FFT processing makes it possible to obtain information on the distance, the relative speed, and the arrival angle of object  6 , for example. Note that signal processor  32  may change the sampling rate of the digital I/Q data using a decimation filter or the like to perform the FFT processing. 
     Presence detector  34  performs the first sensing to detect presence of object  6  in specific detection area  4  using the first sensor signal. More specifically, presence detector  34  detects presence of object  6  in specific detection area  4  by comparing the signal strength of the beat signal of the first sensor signal output from signal processor  32  with a threshold. The threshold for each frequency may be different, or may be changed to adapt to the ambient environment of sensing device  2 , such as the temperature and density of people there. Presence detector  34  outputs a first mode switching signal to radar controller  38  to cause radar controller  38  to switch from the presence detection mode to the motion detection mode, when presence detector  34  detects the presence of object  6  in specific detection area  4 . On the other hand, presence detector  34  outputs a second mode switching signal to radar controller  38  to cause radar controller  38  to switch from the motion detection mode to the presence detection mode, when presence detector  34  detects absence of object  6  in specific detection area  4 . Note that presence detector  34  continues operating in both the presence detection mode and the motion detection mode. 
     Motion detector  36  performs the second sensing to detect a motion of object  6  in specific detection area  4  using the second sensor signal. For example, motion detector  36  inputs a beat signal indicating a motion of object  6  to determine whether change in frequency or phase of the beat signal matches a predetermined change and detect the motion of object  6 . Alternatively, motion detector  36  may receive an FFT-processed signal and detect a motion of object  6  based on a result learned by machine learning. 
     Motion detector  36  detects a motion of object  6  by continuously capturing, for example, a) the distance from radar sensor  8  to object  6 , b) the angle of object  6  relative to the front direction of radar sensor  8 , and c) the speed of a motion of object  6 . More specifically, when object  6  moves his/her hand back and forth with respect to radar sensor  8 , motion detector  36  detects the motion of the hand of object  6  by detecting the distance from radar sensor  8  to object  6  being shorter or longer. Note that motion detector  36  operates in the motion detection mode, but does not operate in the presence detection mode. 
     Radar controller  38  switches from the presence detection mode to the motion detection mode based on the first mode switching signal output from presence detector  34 . Radar controller  38  generates a chirp control signal for controlling each of the chirp waveforms of the first sensor signal and the second sensor signal in motion detection mode. Moreover, radar controller  38  switches from the motion detection mode to the presence detection mode based on the second mode switching signal output from presence detector  34 . Radar controller  38  generates a chirp control signal for controlling the chirp waveform of the first sensor signal in the presence detection mode. Radar controller  38  outputs the generated chirp control signal to frequency sweep circuit  12  of radar sensor  8 . 
     Moreover, radar controller  38  may output an intermittent control signal to radar sensor  8  to enable or disable the operation of radar sensor  8  to reduce power consumption. 
     [1-2. Operations of Sensing Device] 
     Next, operations of sensing device  2  according to Embodiment 1 will be described with reference to  FIG. 3  through  FIG. 6 .  FIG. 3  is a flowchart illustrating a process of operations of sensing device  2  according to Embodiment 1.  FIG. 4  is a timing chart for describing the presence detection mode and the motion detection mode of sensing device  2  according to Embodiment 1.  FIG. 5A  is a graph showing an example of a chirp waveform of the first sensor signal according to Embodiment 1.  FIG. 5B  is a graph showing an example of a chirp waveform of a second sensor signal according to Embodiment 1.  FIG. 6  is a graph showing an example of a beat signal of the first sensor signal according to Embodiment 1. 
     As illustrated in  FIG. 3 , when an operation of sensing device  2  is started, radar controller  38  operates in the presence detection mode (S 101 ) and presence detector  34  starts the first sensing (S 102 ). 
     As illustrated in  FIG. 4 , in the presence detection mode, only the first sensing is performed by presence detector  34 . In the first sensing, the first sensor signal is transmitted from transmission antenna  20  at first sensing rate Rframe 1  per first sensing time Tc 1 . Note that first sensing rate Rframe 1  is a cycle in which the first sensor signal is transmitted from transmission antenna  20 . For example, Rframe 1 =1 Hz. First sensing time Tc 1  is time in which the first sensor signal is transmitted from transmission antenna  20  in first frame Tframe 1  (=1/Rframe 1 ), which is a reciprocal of first sensing rate Rframe 1 . For example, Tc 1 =1 ms. First frame Tframe 1  is a unit of time for processing the first sensor signal. 
     Note that during the period of first sensing time Tc 1  in first frame Tframe 1 , the operation of radar sensor  8  is enabled based on the intermittent control signal output from radar controller  38 . On the other hand, during the period other than first sensing time Tc 1  in first frame Tframe 1 , the operation of radar sensor  8  is disabled based on the intermittent control signal output from radar controller  38 . 
     In the first sensing, the signal-to-noise (S/N) ratio of the first sensor signal can be improved by setting first sensing time Tc 1  relatively long. As a result, the presence of object  6  located in a farther position can be detected. 
     As shown in  FIG. 5A , the first sensor signal includes one chirp waveform in first sensing time Tc 1 . In the example shown in  FIG. 5A , the chirp waveform of the first sensor signal is an up-chirp in which the frequency increases linearly with time. First sensing time Tc 1  of the chirp waveform is 1 ms and first modulation bandwidth BW 1  is 500 MHz. Note that the chirp waveform of the first sensor signal is an up-chirp in the present embodiment, but the present disclosure is not limited to such configuration. The chirp waveform of the first sensor signal may be a down-chirp in which the frequency decreases linearly with time, or a combination of an up-chirp and a down-chirp. 
     In the first sensing, the first sensor signal is transmitted from transmission antenna  20 , and reception antenna  22  receives a reflected signal of the first sensor signal that is reflected off one or more objects (including object  6 ) that are present in specific detection area  4 . Subsequently, as described above, signal processor  32  generates a beat signal of the first sensor signal by performing FFT processing on the digital I/Q data of the first sensor signal output from A/D converter  30 . For example, as shown in  FIG. 6 , the beat signal of the first sensor signal is a signal having a peak at beat frequency fb proportional to the distance from radar sensor  8  to object  6 . Here, when the distance from radar sensor  8  to object  6  is d, the propagation speed of the first sensor signal (speed of light) is c, the first modulation bandwidth of the first sensor signal is BW 1 , and the first sensing time is Tc 1 , beat frequency fb is expressed by the following expression 1. 
         fb =(2 d/c )*(BW1 /Tc 1)   (Expression 1)
 
     Returning to  FIG. 3 , presence detector  34  detects presence or absence of object  6  in specific detection area  4  by comparing the signal strength of the beat signal of the first sensor signal output from signal processor  32  with the threshold (S 103 ). As shown in  FIG. 6 , presence detector  34  detects the presence of object  6  in specific detection area  4  when signal strength Ib of the beat signal of the first sensor signal is greater than or equal to the threshold. On the other hand, although not shown in the figures, when signal strength Ib of the beat signal of the first sensor signal is less than the threshold, presence detector  34  detects absence of object  6  in specific detection area  4 . 
     Presence detector  34  continues the first sensing when presence detector  34  detects absence of object  6  in specific detection area  4  (No in S 103 ). 
     On the other hand, when object  6  enters specific detection area  4  and presence detector  34  detects the presence of object  6  in specific detection area  4  (YES in S 103 ), presence detector  34  outputs the first mode switching signal to radar controller  38 . Radar controller  38  switches from the presence detection mode to the motion detection mode based on the first mode switching signal output from presence detector  34  (S 104 ). In response to this, motion detector  36  starts the second sensing, and presence detector  34  continues the first sensing (S 105 ). 
     As illustrated in  FIG. 4 , in the motion detection mode, both the first sensing by presence detector  34  and the second sensing by motion detector  36  are performed. 
     The first sensing in the motion detection mode is substantially the same as the first sensing in the presence detection mode described above. Therefore, description thereof is omitted. Note that first sensing rate Rframe 1  in the motion detection mode does not necessarily need to be the same as first sensing rate Rframe 1  in the presence detection mode (for example, 1 Hz). First sensing rate Rframe 1  may be any rate lower than the rate of second sensing rate Rframe 1  (for example, 2 Hz or 5 Hz). 
     In the second sensing, the second sensor signal is transmitted from transmission antenna  20  at second sensing rate Rframe 1  per second sensing time Tc 2 . Note that second sensing rate Rframe 1  is a cycle in which the second sensor signal is transmitted from transmission antenna  20 . Second sensing rate Rframe 1  is higher than first sensing rate Rframe 1 . For example, Rframe 1 =60 Hz. Second sensing time Tc 2  is time in which the second sensor signal is transmitted from transmission antenna  20  in second frame Tframe 2  (=1/Rframe 2 ), which is a reciprocal of second sensing rate Rframe 1 . Second sensing time Tc 2  is shorter than first sensing time Tc 1 . For example, Tc 2 =0.1 ms. Second frame Tframe 2  is a unit of time for processing the second sensor signal. 
     As illustrated in  FIG. 4 , the second sensor signal is transmitted from transmission antenna  20  in the period other than first sensing time Tc 1  in first frame Tframe 1 . In other words, the first sensor signal and the second sensor signal are not simultaneously transmitted from transmission antenna  20 . 
     Note that during the period of second sensing time Tc 2  in second frame Tframe 2 , the operation of radar sensor  8  is enabled based on the intermittent control signal from radar controller  38 . On the other hand, during the period other than second sensing time Tc 2  in second frame Tframe 2 , the operation of radar sensor  8  is disabled based on the intermittent control signal output from radar controller  38 . 
     In the second sensing, a fine motion of object  6  can be detected by setting second sensing rate Rframe 2  relatively high. 
     As illustrated in  FIG. 5B , the second sensor signal includes one chirp waveform in second sensing time Tc 2 . In the example shown in  FIG. 5B , the chirp waveform of the first sensor signal is an up-chirp. Second sensing time Tc 2  of the chirp waveform is 0.1 ms and second modulation bandwidth BW 2  is 6 GHz, which is wider than first modulation bandwidth BW 1 . 
     In general, when the modulation bandwidth of the chirp waveform is BW and the light speed is c (=3×10 8  m/s), the distance resolution (detection accuracy) is expressed as c/BW/ 2 . Therefore, the distance resolution in the second sensing is approximately 2 cm, calculated by c/BW 2 / 2 =3×10 8 /6 GHz/2, which is higher than the distance resolution in the first sensing, which is approximately 30 cm, calculated by c/BW 1 / 2 =3×10 8 /500 MHz/2. This is because second modulation bandwidth BW 2  of the chirp waveform of the second sensor signal is wider than first modulation bandwidth BW 1  of the chirp waveform of the first sensor signal. 
     Note that in the present embodiment, the chirp waveform of the second sensor signal is an up-chirp, but the present disclosure is not limited to such configuration. For example, the chirp waveform of the second sensor signal may be a down-chirp, or a combination of an up-chirp and a down-chirp. 
     In the second sensing, the second sensor signal is transmitted from transmission antenna  20  and a reflected signal of the second sensor signal that is reflected off one or more objects (including object  6 ) that are present in specific detection area  4  is received by reception antenna  22 . Subsequently, as described above, signal processor  32  generates a beat signal of the second sensor signal by performing FFT processing on the digital I/Q data of the second sensor signal output from A/D converter  30 . In response to this, motion detector  36  inputs the beat signal indicating the motion of object  6  to determine whether change infrequency or phase of the beat signal matches the predetermined change and detect the motion of object  6  (S 106 ). Alternatively, motion detector  36  may receive an FFT-processed signal and detect the motion of object  6  based on a result learned by machine learning. 
     Subsequently, when presence detector  34  detects the presence of object  6  in specific detection area  4  (NO in S 107 ), step S 106  described above is performed again. 
     On the other hand, when object  6  goes outside specific detection area  4  and presence detector  34  detects absence of object  6  in specific detection area  4  (YES in S 107 ), presence detector  34  outputs the second mode switching signal to radar controller  38 . Radar controller  38  switches from the motion detection mode to the presence detection mode based on the second mode switching signal output from presence detector  34  (S 108 ). In response to this, motion detector  36  stops the second sensing and presence detector  34  continues the first sensing (S 109 ). 
     As described above, presence detector  34  continues the first sensing in both the presence detection mode and the motion detection mode. On the other hand, motion detector  36  performs second sensing only in the motion detection mode. 
     When sensing device  2  continues detecting the presence and a motion of object  6  (NO in S 110 ), sensing device  2  returns to step S 103  and repeats steps S 103  through S 109  as described above. On the other hand, when sensing device  2  ends detection of the presence and a motion of object  6  (YES in S 110 ), the process is ended. 
     [1-3. Effects] 
     As described above, in the presence detection mode, the first sensing is performed using the first sensor signal. On the other hand, in the motion detection mode, the second sensing is performed using the second sensor signal having a sensing rate higher than the sensing rate of the first sensor signal. 
     This makes the detection resolution of the second sensor signal in the second sensing higher than the detection resolution of the first sensor signal in the first sensing. As a result, both the presence and a motion of object  6  in specific detection area  4  can be detected accurately. 
     In addition, for example, when object  6  goes outside specific detection area  4  during the motion detection mode, the second sensing is stopped and the first sensing is continued by switching from the motion detection mode to the presence detection mode. This allows the second sensing in the motion detection mode to resume smoothly if, for example, object  6  enters specific detection area  4  again. 
     In addition, sensing device  2  according to the present embodiment can reduce power consumption by switching between the presence detection mode and the motion detection mode as appropriate. The following describes the reasons. 
     For example, the case where the power consumption of sensing device  2  during sensing is 250 mW, the first sensing time of the first sensor signal is 1 ms, the first sensing rate Rframe 1  is 1 Hz, the second sensing time of the second sensor signal is 0.1 ms, and the second sensing rate Rfranne 2  is 60 Hz is considered. In this case, the power consumption of sensing device  2  in the presence detection mode is 0.25 mW (=250 mW×1 ms×1 Hz). On the other hand, the power consumption of sensing device  2  in the motion detection mode is 1.75 mW, which is obtained by adding the above-described 0.25 mW and 1.5 mW (=250 mW×0.1 ms×60 Hz) together. 
     Here, when the total time of day in which operating in the presence detection mode is 23 hours and the total time in which operating in the motion detection mode is 1 hour, the average daily power consumption is approximately 0.3 mW. Accordingly, the power consumption can be kept low. This is particularly effective when sensing device  2  is driven by a battery, for example. 
     Embodiment 2 
     [2-1. Configuration of Sensing Device] 
     Next, the configuration of sensing device  2 A according to Embodiment 2 will be described with reference to  FIG. 7  through  FIG. 8B .  FIG. 7  is a block diagram illustrating the configuration of sensing device  2 A according to Embodiment 2.  FIG. 8A  is a graph showing an example of chirp waveforms of a first sensor signal according to Embodiment 2.  FIG. 8B  is a graph showing an example of chirp waveforms of a second sensor signal according to Embodiment 2. In the following embodiments, the structural components that are substantially the same as the structural components according to Embodiment 1 share like reference signs. Detailed description of such structural components will be omitted. 
     As illustrated in  FIG. 7 , sensing device  2 A according to Embodiment 2 differs from sensing device  2  according to Embodiment 1 in that radar sensor  8 A includes a plurality of reception units  16 . The configuration of each of reception units  16  is the same as the configuration of reception unit  16  described in Embodiment 1. A/D converter  30  (see  FIG. 2 ) in each of reception units  16  outputs digital I/Q data of the first sensor signal and digital I/Q data of the second sensor signal to signal processor  32 A of DSP  10 A. Note that, for convenience of description,  FIG. 7  illustrates simplified reception units  16 . In the example illustrated in  FIG. 7 , radar sensor  8 A includes two reception units  16  and one transmission unit  14 . However, radar sensor  8 A may include three or more reception units  16 , or two or more transmission units  14 . 
     Furthermore, sensing device  2 A according to Embodiment 2 differs from sensing device  2  according to Embodiment 1 in the chirp control signal generated by radar controller  38 A of DSP  10 A. More specifically, as shown in  FIG. 8A , the first sensor signal includes n chirp waveforms (n is an integer greater than or equal to two) in first sensing time n×Tc 1 . In other words, the first sensor signal includes n chirp waveforms in first sensing time Tframe 1 . Therefore, n×Tc 1  is the first sensing time in first frame Tframe 1 . In the example shown in  FIG. 8A , the chirp waveforms of the first sensor signal are up-chirps. First sensing time Tc 1  of one chirp waveform is 100 μs, and first modulation bandwidth BW 1  is 500 MHz. 
     As shown in  FIG. 8B , the second sensor signal includes k waveforms (where k is an integer greater than or equal to two) in second frame Tframe 1 . Here, k×Tc 2  is the second sensing time in second frame Tframe 1 , and is shorter than the first sensing time. Note that, in the example shown in  FIG. 8B , the chirp waveforms of the second sensor signal are up-chirps. Second sensing time Tc 2  of one chirp waveform is 50 μs and second modulation bandwidth BW 2  is 6 GHz, which is wider than first modulation bandwidth BW 1 . 
     [2-2. Effects] 
     As described above, radar sensor  8 A includes a plurality of reception units  16 . Therefore, signal processor  32 A of DSP  10 A generates a beat signal for each of reception units  16 , based on the first sensor signals received by reception units  16 . This enables signal processor  32 A to detect an arrival angle of a reflected signal of the first sensor signal reflected off object  6  or the like, based on phase differences between the beat signals. 
     Moreover, in the present embodiment, each of the first sensor signal and the second sensor signal includes a plurality of chirp signals. Therefore, signal processor  32 A can detect the speed of a motion of object  6 . 
     While in the present embodiment, the first sensor signal and the second sensor signal each include a plurality of chirp signals, the present disclosure is not limited to such configuration. For example, the first sensor signal may include one chirp signal, and the second sensor signal may include a plurality of chirp signals. Alternatively, the first sensor signal may include a plurality of chirp signals and the second sensor signal may include one chirp signal. 
     Embodiment 3 
     [3-1. Configuration and Operations of Sensing Device] 
     Next, the configuration and operations of sensing device  2 B according to Embodiment 3 will be described with reference to  FIG. 9  and  FIG. 10 .  FIG. 9  is a block diagram illustrating the configuration of sensing device  2 B according to Embodiment 3.  FIG. 10  is a timing chart for describing a presence detection mode and a motion detection mode of sensing device  2 B according to Embodiment 3. 
     As illustrated in  FIG. 9 , sensing device  2 B according to Embodiment 3 differs from sensing device  2  according to Embodiment 1 in that sensing device  2 B includes infrared sensor  40  (one example of the sensor) in addition to radar sensor  8 B. Infrared sensor  40  receives, as the first sensor signal, with a photodiode, infrared radiation emitted from object  6  such as a person (as illustrated in  FIG. 1 ) in specific sensing area  4  (as illustrated in  FIG. 1 ). 
     Reception unit  16 B of radar sensor  8 B includes reception antenna  22 , low noise amplifier  24 , I/Q generation circuit  26 , quadrature demodulator  28 , and A/D converters  30   a  and  30   b.  In addition to that, reception unit  16 B of radar sensor  8 B includes first switch  42  and second switch  44 . For convenience of description, in  FIG. 9  and  FIG. 10 , first switch  42  and second switch  44  are denoted as SW 1  and SW 2  respectively. 
     First switch  42  is an on/off switch disposed between infrared sensor  40  and A/D converter  30   b.  Second switch  44  is an on/off switch disposed between quadrature demodulator  28  and A/D converter  30   b.  Each of first switch  42  and second switch  44  is turned on and off based on an intermittent control signal output from radar controller  38 B. Note that the configurations of A/D converters  30   a  and  30   b  are the same as the configuration of A/D converter  30  in Embodiment 1. 
     Radar controller  38 B of DSP  1013  generates a chirp control signal for controlling a chirp waveform of the second sensor signal in the motion detection mode. Note that radar controller  38 B does not generate a chirp control signal for controlling a chirp waveform of the first sensor signal in the presence detection mode or the motion detection mode. Therefore, transmission antenna  20  transmits only the second sensor signal, and reception antenna  22  receives only a reflected signal of the second sensor signal. 
     As illustrated in  FIG. 10 , in the presence detection mode, only the first sensing is performed by presence detector  34 . In the first sensing, first switch  42  is turned on and off repeatedly. The first sensor signal received by infrared sensor  40  is received while first switch  42  is on. Note that in presence detection mode, second switch  44  is off, and radar sensor  8 B enables operations of A/D converter  30   b  and disables operations of A/D converter  30   a.    
     In the first sensing, infrared radiation radiated from object  6  or the like in specific detection area  4  is received as the first sensor signal by infrared sensor  40 . Subsequently, in the same manner as in Embodiment 1, signal processor  32  performs processing on digital data of the first sensor signal output from AD converter  30   b.  Subsequently, presence detector  34  detects presence or absence of object  6  in specific detection area  4  by comparing the signal strength of the first sensor signal output from signal processor  32  with a threshold. 
     As illustrated in  FIG. 10 , in the motion detection mode, both the first sensing by presence detector  34  and the second sensing by motion detector  36  are performed. In the motion detection mode, radar sensor  8 B enables both operations of A/D converters  30   a  and  30   b.    
     In the first sensing in the motion detection mode, second switch  44  is off and first switch  42  is turned on and off repeatedly. While first switch  42  is on, the first sensor signal received by infrared sensor  40  is received. 
     In the second sensing in the motion detection mode, first switch  42  is off and second switch  44  is turned on and off repeatedly. While second switch  44  is on, the second sensor signal is transmitted from transmission antenna  20 . 
     Note that in Embodiment 1, the first sensor signal and the second sensor signal are both FMCW radar signals. Therefore, there is a restriction that second sensing time Tc 2  needs to be shorter than first sensing time Tc 1 . On the other hand, since the first sensor signal is infrared radiation and the second sensor signal is an FMCW radar signal in the present embodiment, there is no such restriction as in Embodiment 1 and second sensing time Tc 2  does not necessarily need to be shorter than first sensing time Tc 1 . 
     In the second sensing, the second sensor signal is transmitted from transmission antenna  20  and a reflected signal of the second sensor signal that is reflected off object  6  or the like present in specific detection area  4  is received by reception antenna  22 . Subsequently, in the same manner as described in Embodiment 1, signal processor  32  generates a beat signal of the second sensor signal by performing FFT processing on the digital I/Q data of the second sensor signal output from A/D converter  30   a.  With this, motion detector  36  inputs a beat signal indicating the motion of object  6  to determine whether change in frequency or phase of the beat signal matches a predetermined change and detect the motion of object  6 . Alternatively, motion detector  36  may receive an FFT-processed signal and detect a motion of object  6  based on a result learned by machine learning. 
     [3-2. Effects] 
     Even when radar sensor  8 B and infrared sensor  40  are used as sensors as in the present embodiment, an effect similar to the effect of Embodiment 1 can be obtained. 
     Although infrared sensor  40  is used in the present embodiment, other types of sensors may be used instead of infrared sensor  40 . 
     Variations etc. 
     The sensing method and the sensing device according to one or more aspects of the present disclosure have been described above based on the embodiments, but the present disclosure is not limited to the embodiments. One or more aspects of the present disclosure may include, without departing from the scope of the present disclosure, one or more variations achieved by making various modifications to the present disclosure that can be conceived by those skilled in the art, or one or more embodiments obtained by combining structural components in different embodiments. 
     In each of the embodiments, sensing device  2  ( 2 A,  2 B) is provided in an AI speaker, but the present disclosure is not limited to this configuration. The sensing devices may be provided in various devices, such as a television receiver or an air conditioner. 
     Moreover, while in Embodiment 1, an example in which object  6  is a person has been described, the present disclosure is not limited to such configuration. Object  6  may be, for example, a vehicle, a bicycle, or an animal. 
     While in Embodiment 3, an example in which object  6  is a person has been described, the present disclosure is not limited to such configuration. Object  6  may be, for example, an animal that emits infrared radiation. 
     In addition, in Embodiments 1 and 2, signal processor  32  performs FFT processing on the digital I/Q data of the first sensor signal and the digital I/Q data of the second sensor signal, but the present disclosure is not limited to such configuration. For example, root means square (RMS) processing may be performed. 
     In Embodiments 1 and 2, presence detector  34  detects the presence of object  6  in specific detection area  4  when signal strength of the beat signal of the first sensor signal is greater than or equal to the threshold. However, beat frequency fb of the beat signal may be limited to a predetermined frequency range corresponding to specific detection area  4 . In other words, presence detector  34  detects absence of object  6  in specific detection area  4 , when the signal strength of the beat signal of the first sensor signal is greater than or equal to the threshold and beat frequency fb of the beat signal falls outside the predetermined frequency range that is determined according to specific detection range  4 . For example, when specific detection area  4  is within the range of distance D (see  FIG. 1 )=0 m to 5 m, first modulation bandwidth BW 1  of the chirp waveform of the first sensor signal is 0.5 GHz, and first sensing time Tc 1  is 1 ms, the upper limit of the predetermined frequency is 16.7 kHz calculated by fb=(2×5/(3×10 8 )×(500 MHz/(1×10 −3 )) from expression 1 above. Similarly, when specific detection area  4  is within the range of distance D (see  FIG. 1 )=1 m to 5 m, the upper limit of the predetermined frequency is 16.7 kHz from expression 1 above and the lower limit of the predetermined frequency is 3.4 kHz. 
     As another example, presence detector  34  may detect absence of object  6  in specific detection area  4 , when the signal strength of the beat signal of the first sensor signal is greater than or equal to the threshold and beat frequency fb of the beat signal falls outside a predetermined frequency range corresponding to a range of speed (for example, 0.5 m/s to 3 m/s) of motion of object  6 . 
     As still another example, when it is possible to detect the arrival angle of the reflected signal of the first sensor signal reflected off object  6  as in Embodiment 2, detection may be performed as follows: in other words, presence detector  34  may detect absence of object  6  in specific detection area  4 , when the signal strength of the beat signal of the first sensor signal is greater than or equal to the threshold and beat frequency fb of the beat signal falls outside a predetermined frequency range corresponding to a range of arrival angle (for example, ±60° with respect to the front direction of radar sensor  8 ). 
     In Embodiments 1 and 2, the configuration parameters of the first sensor signal and the second sensor signal are constant. However, the configuration parameters of the first sensor signal and the second sensor signal may be changed according to the signal strengths of the respective beat signals of the first sensor signal and the second sensor signal. For example, when the signal strength of the beat signal of the first sensor signal (second sensor signal) is less than a threshold, the first sensing time (second sensing time) may be increased and/or the transmission power of the first sensor signal (second sensor signal) may be increased. On the other hand, when the signal strength of the beat signal of the first sensor signal (second sensor signal) is sufficiently greater than the threshold, the first sensing time (second sensing time) may be shortened and/or the transmission power of the first sensor signal (second sensor signal) may be reduced. 
     Furthermore, in Embodiments 1 and 2, the first sensor signal is modulated with first modulation bandwidth BW 1  and transmitted from radar sensor  8  ( 8 A) per first sensing time Tc 1  in the first sensing, and the second sensor signal is modulated with second modulation bandwidth BW 2  and transmitted from radar sensor  8  ( 8 A) per second sensing time Tc 2  that is shorter than first sensing time Tc 1 , second modulation bandwidth BW 2  being wider than first modulation bandwidth BW 1 . However, the present disclosure is not limited to such configuration. It is also possible to satisfy only one of the relationship of the length between the sensing times (TC 1 &gt;TC 2 ) and the relationship of the width between the modulation bandwidths (BW 2 &gt;BW 1 ). 
     In addition, part or all of the structural components of sensing device  2  ( 2 A,  2 B) in each of the above embodiments may include a single system large scale integration (LSI). 
     A system LSI is a super-multifunction LSI manufactured with a plurality of components integrated on a single chip, and specifically is a) DSP or a microprocessor, b) a read only memory (ROM), and c) a random access memory (RAM), for example. The ROM stores a program. The system LSI will achieve its function as a result of the DSP or microprocessor operating in accordance with the program described above. 
     Note that the term system LSI has been used as an example, but depending on the degree of integration, IC, LSI, super LSI, and ultra LSI are also used. Moreover, the method of circuit integration is not limited to LSI. Integration may be realized with a dedicated circuit or a general purpose processor. A field-programmable gate array (FPGA) that can be programmed after production of LSI or a reconfigurable processor that allows reconfiguration of the connection or configuration of the inner circuit cells of the LSI circuit may be used. 
     Moreover, when advancement in semiconductor technology or derivatives of other technologies brings forth a circuit integration technology that replaces LSI, it will be appreciated that such a circuit integration technique may be used to integrate the functional blocks. Application of biotechnology is also a possibility. 
     In addition, the structural components of sensing device  2  ( 2 A,  2 B) in each of the above embodiments may be distributed among a plurality of devices connected via a communication network. 
     One or more aspects of the present disclosure may include not only such sensing device  2  ( 2 A,  2 B), but also a sensing method including the characteristic structural components included in sensing device  2  ( 2 A,  2 B) as steps. Moreover, one or more aspects of the present disclosure may be a computer program that causes a computer to execute each characteristic step included in the sensing method. Moreover, one or more aspects of the present disclosure may be a non-transitory computer-readable recording medium on which such a computer program is recorded. 
     In each embodiment, each structural component may be implemented either by dedicated hardware or by executing a software program appropriate for structural component. Each structural component may be realized as a result of a program executer such as a central processing unit (CPU) or processor reading and executing a software program stored on a storage medium such as a hard disk or semiconductor memory. 
     Although only some exemplary embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The sensing device according to one or more aspects of the present disclosure is applicable to a user interface mounted on an AI speaker, for example.