Patent Publication Number: US-11664002-B2

Title: Active noise control device and vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-007125 filed on Jan. 20, 2021, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an active noise control device and a vehicle. 
     Description of the Related Art 
     JP H06-059688 A discloses an active noise canceling device. The active noise canceling device disclosed in JP H06-059688 A includes a sound generating device, a sound detection sensor, and a vibration sensor. The sound generating device is disposed in a space where noise is to be canceled. The sound detection sensor is disposed in the space where noise is to be canceled. The vibration sensor is provided for each of a plurality of vibration sources of several vibrations propagating in the space where noise is to be canceled. The active noise canceling device disclosed in JP H06-059688 A further includes a vibration signal generating means and a driving means. The vibration signal generating means generates a vibration signal having an opposite phase to the sound detected by the sound detection sensor, based on the output signals of the plurality of vibration sensors. The driving means drives the sound generating device based on the vibration signal. 
     SUMMARY OF THE INVENTION 
     However, in JP H06-059688 A, when an abnormality occurs in any of the plurality of vibration sensors, noise cannot be suitably reduced. 
     An object of the present invention is to provide an active noise control device and a vehicle that can reduce noise suitably. 
     An active noise control device according to an aspect of the present invention causes an actuator to output a canceling sound based on a control signal in order to reduce noise in a vehicle compartment of a vehicle. The active noise control device includes an adaptive filter configured to generate the control signal by performing a filtering process on a reference signal acquired by an acceleration sensor attached to the vehicle, a filter coefficient updating unit configured to update a filter coefficient of the adaptive filter based on the reference signal and an error signal acquired by detecting residual noise due to interference between the noise and the canceling sound by a microphone, a determination unit configured to determine whether an abnormality has occurred in the acceleration sensor based on a direct-current component of the reference signal, and a control unit configured to, when the determination unit determines that an abnormality has occurred in any of a plurality of the acceleration sensors, stop generation of the control signal based on the reference signal acquired by the acceleration sensor that has been determined to have the abnormality, and stop updating the filter coefficient of the adaptive filter configured to perform the filtering process on the reference signal acquired by the acceleration sensor that has been determined to have no abnormality. 
     A vehicle according to another aspect of the present invention includes the active noise control device as described above. 
     According to the present invention, it is possible to provide an active noise control device and a vehicle which can reduce noise suitably. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is illustrated by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an outline of active noise control; 
         FIG.  2    is a block diagram illustrating a part of a vehicle equipped with an active noise control device according to an embodiment; 
         FIG.  3    is a diagram illustrating an example of a configuration of a determination unit; 
         FIG.  4    is a diagram an example of a coordinate system; 
         FIG.  5    is a flowchart illustrating an example of operations of an active noise control device according to an embodiment; 
         FIG.  6    is a flowchart illustrating an example of operations of an active noise control device according to an embodiment; and 
         FIG.  7    is a flowchart illustrating an example of operations of an active noise control device according to an embodiment. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Preferred embodiments of an active noise control device and a vehicle according to the present invention will be described in detail below with reference to the accompanying drawings. 
     Embodiment 
     An active noise control device and a vehicle according to an embodiment will be described with reference to  FIGS.  1  to  7   .  FIG.  1    is a diagram illustrating an outline of active noise control. 
     An active noise control device  10  causes an actuator  16  to output a canceling sound for reducing noise (vibration noise) in a vehicle compartment  14  of a vehicle  12 . 
     The noise in the vehicle compartment  14  may include, for example, road noise. Road noise is noise that is transmitted to an occupant in the vehicle compartment  14  when a wheel vibrates due to force received from the road surface and the vibration of the wheel is transmitted to the vehicle body via a suspension. 
     The vehicle  12  is provided with a plurality of vibration sensors that detect vibration of the vehicle  12 . That is, the vehicle  12  is provided with a plurality of acceleration sensors  18 A to  18 D. The reference character  18  is used when describing the acceleration sensor in general. The reference characters  18 A to  18 D are used when describing the individual acceleration sensors. Signals r detected by the acceleration sensors  18 A to  18 D are supplied to the active noise control device  10 . That is, a signal indicating vibration is supplied to the active noise control device  10 . 
     A microphone  20  is further provided in the vehicle compartment  14 . The microphone  20  detects residual noise (cancellation error noise) due to interference between the noise and the canceling sound output from the actuator  16 . The residual noise detected by the microphone  20  is supplied to the active noise control device  10 . That is, an error signal e detected by the microphone  20  is supplied to the active noise control device  10 . 
     The active noise control device  10  generates a control signal u for outputting a canceling sound from the actuator  16 , based on the signal r detected by the acceleration sensor  18  and the error signal e detected by the microphone  20 . More specifically, the active noise control device  10  generates the control signal u such that the error signal e detected by the microphone  20  is minimized. Since the actuator  16  outputs the canceling sound based on the control signal u that minimizes the error signal e detected by the microphone  20 , the noise in the vehicle compartment  14  can be suitably canceled out by the canceling sound. In this way, the active noise control device  10  can reduce noise transmitted to an occupant in the vehicle compartment  14 . 
     Incidentally, an abnormality may occur in any of the plurality of acceleration sensors  18 . Examples of the abnormality of the acceleration sensor  18  may include the dropping off of the acceleration sensor  18 , an abnormality in the characteristics of the acceleration sensor  18 , and the like. The dropping off of the acceleration sensor  18  may occur, for example, when a portion to which a housing of the acceleration sensor  18  is attached deteriorates over time. The abnormality in the characteristics of the acceleration sensor  18  may occur, for example, when a detection unit of the acceleration sensor  18  is deteriorated due to vibration fatigue or the like. When the canceling sound is simply generated by using the signal r acquired by the acceleration sensor  18  in which the abnormality occurs, it is not always possible to suitably cancel out the noise in the vehicle compartment  14 . As a result of intensive studies, the inventors of the present application have conceived the active noise control device  10  as described below. 
       FIG.  2    is a block diagram illustrating a part of a vehicle equipped with an active noise control device according to the present embodiment. 
     As illustrated in  FIG.  2   , the active noise control device  10  includes a determination unit  26 , a control unit  28 , a storage unit  30 , an output unit  32 , filter units  34 A to  34 D, and computation units  44 . The reference character  34  is used when describing the filter unit in general. The reference characters  34 A to  34 D are used when describing the individual filter units. 
     The active noise control device  10  includes a computation device (computational processing device) (not illustrated). The computation device may be configured by a processor such as a CPU (Central Processing Unit), a DSP (Digital Signal Processor), or the like. However, the present invention is not limited to this feature. A DDS (Direct Digital Synthesizer), a DCO (Digitally Controlled Oscillator), or the like can be included in the computation device. In addition, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or the like can be included in the computation device. 
     As described above, the active noise control device  10  includes the storage unit  30 . The storage unit  30  may be configured by a volatile memory (not illustrated) and a nonvolatile memory (not illustrated). Examples of the volatile memory include, for example, a RAM or the like. Examples of the nonvolatile memory include, for example, a ROM, a flash memory, or the like. Data or the like may be stored, for example, in the volatile memory. Programs, tables, maps, and the like may be stored, for example, in the nonvolatile memory. 
     The determination unit  26 , the control unit  28 , the filter unit  34 , and the computation unit  44  can be realized by programs, which are stored in the storage unit  30 , being executed by the computation device. The output unit  32  may be configured by an output interface circuit or the like. 
     The vehicle  12  may be provided with the acceleration sensors  18 A to  18 D. Although the four acceleration sensors  18  are illustrated in  FIG.  2   , the number of acceleration sensors  18  is not limited to four. For example, a three-axis acceleration sensor can be used as the acceleration sensor  18 . The three axes are the X-axis, the Y-axis and the Z-axis. The vibration in the X-axis direction detected by the acceleration sensor  18  is supplied to the active noise control device  10  as a reference signal rx. The vibration in the Y-axis direction detected by the acceleration sensor  18  is supplied to the active noise control device  10  as a reference signal ry. The vibration in the Z-axis direction detected by the acceleration sensor  18  is supplied to the active noise control device  10  as a reference signal rz. The reference character r is used when describing the reference signal in general. The reference characters rx, ry, and rz are used when describing the individual reference signals. 
     As described above, the microphone  20  that detects the residual noise due to interference between the noise and the canceling sound is provided in the vehicle compartment  14  (see  FIG.  1   ). That is, the microphone  20  for detecting the error signal e is provided in the vehicle compartment  14 . 
     As described above, the vehicle compartment  14  (see  FIG.  1   ) is provided with the actuator  16  that outputs a canceling sound based on the control signal u. As examples of the actuator  16 , there may be cited a speaker. 
     The filter unit  34  includes adaptive filters  36 X,  36 Y, and  36 Z, acoustic characteristic filters  38 X,  38 Y, and  38 Z, filter coefficient updating units  40 X,  40 Y, and  40 Z, and computation units  42 . The reference character  36  is used when describing the adaptive filter in general. The reference characters  36 X,  36 Y, and  36 Z are used when describing the individual adaptive filters. The reference character  38  is used when describing the acoustic characteristic filter in general. The reference characters  38 X,  38 Y, and  38 Z are used when describing the individual acoustic characteristic filters. The reference character  40  is used when describing the filter coefficient updating unit in general. The reference characters  40 X,  40 Y, and  40 Z are used when describing each of the filter coefficient updating units. 
     The adaptive filter  36 X generate a control signal u 0   x  by performing a filtering process on the reference signal rx. The adaptive filter  36 Y generates a control signal u 0   y  by performing a filtering process on the reference signal ry. The adaptive filter  36 Z generates a control signal u 0   z  by performing a filtering process on the reference signal rz. The reference character u 0  is used when describing the control signal in general, whereas the reference characters u 0   x , u 0   y , and u 0   z  are used when describing the individual control signals. As the adaptive filter  36 , for example, an FIR (Finite Impulse Response) filter or the like can be used, but the present invention is not limited to this feature. The filter coefficients of the adaptive filters  36 X,  36 Y, and  36 Z are updated by filter coefficient updating units  40 X,  40 Y, and  40 Z, as described later. The FIR filter generates the control signal u 0  by performing a convolution operation on the reference signal r. 
     The acoustic characteristic filter  38 X corrects the reference signal rx by performing a filtering process on the reference signal rx according to an acoustic characteristic (transfer characteristic) from the actuator  16  to the microphone  20 . The acoustic characteristic filter  38 Y corrects the reference signal ry by performing a filtering process on the reference signal ry according to an acoustic characteristic from the actuator  16  to the microphone  20 . The acoustic characteristic filter  38 Z corrects the reference signal rz by performing a filtering process on the reference signal rz according to an acoustic characteristic from the actuator  16  to the microphone  20 . The acoustic characteristic from the actuator  16  to the microphone  20  is obtained in advance. That is, the transfer characteristic Ĉ from the actuator  16  to the microphone  20  is obtained in advance. 
     The filter coefficient updating unit  40 X updates the filter coefficient W of the adaptive filter  36 X based on the error signal e acquired by detecting the residual noise by the microphone  20  and the reference signal rx corrected by the acoustic characteristic filter  38 X. More specifically, the filter coefficient updating unit  40 X updates the filter coefficient W of the adaptive filter  36 X such that the error signal e is minimized. The filter coefficient updating unit  40 Y updates the filter coefficient W of the adaptive filter  36 Y based on the error signals e and the reference signal ry corrected by the acoustic characteristic filter  38 Y. More specifically, the filter coefficient updating unit  40 Y updates the filter coefficient W of the adaptive filter  36 Y such that the error signal e is minimized. The filter coefficient updating unit  40 Z updates the filter coefficient W of the adaptive filter  36 Z based on the error signal e and the reference signal rz corrected by the acoustic characteristic filter  38 Z. More specifically, the filter coefficient updating unit  40 Z updates the filter coefficient W of the adaptive filter  36 Z such that the error signal e is minimized. When the filter coefficient W is updated, for example, a filtered-X LMS algorithm can be used, but the present invention is not limited to this feature. 
     The filter unit  34  further includes the computation units  42 . The control signals u 0   x , u 0   y , and u 0   z  output from the adaptive filters  36 X,  36 Y, and  36 Z are input to the computation units  42 . The computation units  42  add the control signals u 0   x , u 0   y , and u 0   z  supplied from the adaptive filters  36 X,  36 Y, and  36 Z. The computation units (adders)  42  output the control signal u 0  generated by adding the plurality of control signals u 0   x , u 0   y , and u 0   z.    
     The control signals u 0  output from the filter units  34 A to  34 D are input to the computation units  44 . The computation units  44  add the control signals u 0  supplied from the respective filter units  34 A to  34 D. The computation units (adders)  44  supply a control signal u generated by adding the plurality of control signals u 0  to the actuator  16  via a power amplifier  15 . 
     The determination unit (abnormality determination unit)  26  determines whether or not an abnormality occurs in the acceleration sensor  18  based on DC (direct-current) components of the reference signals rx, ry, and rz. The reason for determining whether or not an abnormality has occurred in the acceleration sensor  18  based on the DC component of the reference signal r, is as follows. That is, since the AC component includes many vibration components generated only at the portion where the acceleration sensor  18  is attached, it is not easy to determine whether or not an abnormality has occurred in the acceleration sensor  18  based on the AC component of the reference signal r. On the other hand, since the movement of the vehicle  12  is accurately reflected in the DC component, it is relatively easy to determine whether or not an abnormality has occurred in the acceleration sensor  18  based on the DC component of the reference signal r. The gravitational acceleration (acceleration in the vertical direction), which is an important factor, is also a DC component. For this reason, in the present embodiment, whether or not an abnormality has occurred in the acceleration sensor  18  is determined based on the DC component of the reference signal r. 
       FIG.  3    is a diagram illustrating an example of a configuration of a determination unit. As illustrated in  FIG.  3   , the determination unit  26  includes a first calculating unit  46 , a second calculating unit  48 , a computation unit  62 , a detection abnormality determination unit  64 , and an attachment abnormality determination unit  70 . Although only one first calculating unit  46  is illustrated in  FIG.  3   , the first calculating unit  46  is provided for each of the plurality of acceleration sensors  18 . Although only one computation unit  62  is illustrated in  FIG.  3   , the computation unit  62  is also provided for each of the plurality of acceleration sensors  18 . Although only one detection abnormality determination unit  64  is illustrated in  FIG.  3   , the detection abnormality determination unit  64  is also provided for each of the plurality of acceleration sensors  18 . Although only one attachment abnormality determination unit  70  is illustrated in  FIG.  3   , the attachment abnormality determination unit  70  is also provided for each of the plurality of acceleration sensors  18 . 
     The first calculating unit  46  includes a DC component extraction unit  50 , a coordinate conversion unit  52 , and a jerk calculating unit  54 . 
     As described above, the vibration in the X-axis direction detected by the acceleration sensor  18  is supplied to the determination unit  26  as the reference signal rx. The DC component extraction unit  50  may extract a DC component from the reference signal rx and supply the extracted DC component to the coordinate conversion unit  52 . As described above, the vibration in the Y-axis direction detected by the acceleration sensor  18  is supplied to the determination unit  26  as the reference signal ry. The DC component extraction unit  50  extracts a DC component from the reference signal ry and supplies the extracted DC component to the coordinate conversion unit  52 . As described above, the vibration in the Z-axis direction detected by the acceleration sensor  18  is supplied to the determination unit  26  as the reference signal rz. The DC component extraction unit  50  extracts a DC component from the reference signal rz and supplies the extracted DC component to the coordinate conversion unit  52 . As described above, the DC component extraction unit  50  can extract a DC component from each of the reference signals r of the three axes (X axis, Y axis, and Z axis) and supply each of the extracted DC components to the coordinate conversion unit  52 . 
     The coordinate conversion unit  52  can perform a coordinate conversion process. Each of the X axis, the Y axis, and the Z axis of the acceleration sensor  18  does not necessarily coincide with each of the X axis, the Y axis, and the Z axis of the vehicle  12 . For this reason, the coordinate conversion unit  52  performs the coordinate conversion process such that the magnitude of the DC component of the reference signal r of each of the three axes supplied from the DC component extraction unit  50  corresponds to the coordinate system of the vehicle  12 . 
     The coordinate conversion process can be performed as follows, for example.  FIG.  4    is a diagram illustrating an example of a coordinate system. In  FIG.  4   , the characters x, y, and z indicated a coordinate system in the acceleration sensor  18 . The characters X, Y, and Z in  FIG.  4    indicate a coordinate system in the vehicle  12 . 
     The DC components of the reference signals rx, ry, and rz supplied from the acceleration sensor  18  are defined as asx, asy, and asz. That is, the accelerations in the coordinate system of the acceleration sensor  18  are defined as asx, asy, and asz. The character asx indicates an acceleration of the acceleration sensor  18  in the X-axis direction. The character asy indicated an acceleration in the Y-axis direction of the acceleration sensor  18 . The character asz indicated an acceleration of the acceleration sensor  18  in the Z-axis direction. The DC components of the reference signals rx, ry, and rz when converted into the values in the coordinate system of the vehicle  12  are denoted by avx, avy, and avz. That is, the accelerations in the coordinate system of the vehicle  12  are defined as avx, avy, and avz. The character avx indicates an acceleration of the vehicle  12  in the X-axis direction. The character avy indicates an acceleration of the vehicle  12  in the Y-axis direction. The character avz indicates an acceleration of the vehicle  12  in the Z-axis direction. When the accelerations asx, asy, and asz in the coordinate system of the acceleration sensor  18  are converted into the accelerations avx, avy, and avz in the coordinate system of the vehicle  12 , the following matrix operation can be performed. The character Av denotes an acceleration in the coordinate system of the vehicle  12 . The character As denotes an acceleration in the coordinate system of the acceleration sensor  18 . The character Rsv denotes a coordinate transformation matrix. 
     
       
         
           
             
               
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     The coordinate transformation matrix Rsv can be expressed as follows. The character Rx denotes a matrix for performing X-axis rotation. The character Ry denotes a matrix for performing Y-axis rotation. The character Rz denotes a matrix for performing Z-axis rotation. 
     
       
         
           
             
               
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     In this way, the accelerations avx, avy, and avz corresponding to the coordinate system of the vehicle  12  are calculated by the coordinate conversion unit  52 . The coordinate conversion unit  52  supplies a signal indicating the acceleration corresponding to the traveling direction of the vehicle  12  to the jerk calculating unit  54 . Here, a case where the X-axis direction in the coordinate system of the vehicle  12  is the traveling direction of the vehicle  12  will be described as an example. The coordinate conversion unit  52  supplies a signal indicating the acceleration avx in the traveling direction of the vehicle  12  to the jerk calculating unit  54 . 
     The coordinate conversion unit  52  supplies a signal indicating the acceleration of the vehicle  12  in the upper and lower directions to the attachment abnormality determination unit  70 . That is, the coordinate conversion unit  52  supplies a signal indicating the acceleration in the vertical direction to the attachment abnormality determination unit  70 . The vertical component of the DC component of the reference signal r is supplied to the attachment abnormality determination unit  70 . Here, a case where the Z-axis direction in the coordinate system of the vehicle  12  is the vertical direction of the vehicle  12  will be described as an example. The coordinate conversion unit  52  supplies a signal indicating the acceleration avz in the vertical direction of the vehicle  12  to the attachment abnormality determination unit  70 . 
     The jerk calculating unit  54  calculates a jerk jvx in the traveling direction of the vehicle  12  based on the signal supplied from the coordinate conversion unit  52 . That is, the jerk calculating unit  54  calculates the jerk jvx in the traveling direction of the vehicle  12  based on the acceleration avx in the traveling direction of the vehicle  12 . Such a jerk jvx can be acquired by calculating a change in acceleration per unit time. 
     The acceleration acquired last time is defined as avxb. The acceleration acquired this time is defined as avxn. The time from a timing at which the previous acceleration avxb was acquired to a timing at which the current acceleration avxn has been acquired is defined as Δt1. The jerk jvx is obtained by the following expression (1).
 
 Jvx =( avxn−avxb )/Δ t 1   (1)
 
     Thus, based on the signal acquired by the acceleration sensor  18 , the jerk jvx in the traveling direction of the vehicle  12  can be calculated by the first calculating unit  46 . More specifically, the jerk jvx in the traveling direction of the vehicle  12  can be calculated by the first calculating unit  46  based on the DC component of the reference signal r. 
     The second calculating unit  48  includes a speed signal acquisition unit  56 , an acceleration calculating unit  58 , and a jerk calculating unit  60 . 
     The speed signal acquisition unit  56  acquires a signal supplied from a speed sensor  19  provided at the vehicle  12 . That is, the speed signal acquisition unit  56  acquires a signal indicating the speed v. 
     The acceleration calculating unit  58  calculates an acceleration “a” of the vehicle  12  based on the signal acquired by the speed signal acquisition unit  56 . That is, the acceleration calculating unit  58  calculates the acceleration in the traveling direction of the vehicle  12  based on the signal indicating the speed v. Such an acceleration “a” can be acquired by calculating a change in speed per unit time. 
     The speed previously acquired by the speed signal acquisition unit  56  is denoted as vb. The speed currently acquired by the speed signal acquisition unit  56  is denoted as vn. The time from a timing at which the previous speed vb was acquired to a timing at which the current speed vn has been acquired is defined as Δt2. Then, the acceleration a is obtained by the following expression (2).
 
 a =( vn−vb )/Δ t 2   (2)
 
     The jerk calculating unit  60  calculates a jerk j of the vehicle  12  based on the acceleration a calculated by the acceleration calculating unit  58 . That is, the jerk calculating unit  60  calculates the jerk in the traveling direction of the vehicle  12  based on the acceleration a calculated by the acceleration calculating unit  58 . The jerk j can be acquired by calculating a change in acceleration per unit time. 
     The acceleration calculated last time by the acceleration calculating unit  58  is defined as ab. The acceleration currently calculated by the acceleration calculating unit  58  is defined as an. The time from a timing at which the previous acceleration ab was acquired to a timing at which the current acceleration an has been acquired is defined as Δt3. Then, the jerk j is obtained by the following expression (3).
 
 j =( an−ab )/Δ t 3   (3)
 
     In this manner, the jerk j in the traveling direction of the vehicle  12  is calculated by the second calculating unit  48  based on the signal acquired by the speed sensor  19  provided at the vehicle  12 . 
     The computation unit  62  calculates a difference Δj between the jerk jvx calculated by the first calculating unit  46  and the jerk j calculated by the second calculating unit  48 . The difference Δj is expressed by the following expression (4).
 
 Δj=|jvx−j|   (4)
 
     When the difference Δj between the jerk jvx calculated by the first calculating unit  46  and the jerk j calculated by the second calculating unit  48  is greater than or equal to a difference threshold value DTH, the detection abnormality determination unit  64  determines that an abnormality (detection abnormality) has occurred in the acceleration sensor  18 . More specifically, the detection abnormality determination unit  64  determines that an abnormality (detection abnormality) has occurred in the acceleration sensor  18  when the difference Δj is greater than or equal to the difference threshold value DTH, and when a state in which the difference Δj is greater than or equal to the difference threshold value DTH continues for a period of a time threshold value TTH or longer. In the present embodiment, the reason why an abnormality of the acceleration sensor  18  is determined based not on the acceleration but on the jerk, is as follows. That is, in the case where the acceleration sensor  18  whose acceleration detection accuracy is relatively low is provided at the vehicle  12 , it is not easy to determine whether or not an abnormality has occurred in the acceleration sensor  18  based on the acceleration detected by the acceleration sensor  18 . On the other hand, in the case where whether or not an abnormality has occurred in the acceleration sensor  18  is determined based on the jerk, it is possible to suitably determine whether or not an abnormality has occurred in the acceleration sensor  18 , even when the detection accuracy of the acceleration sensor  18  is relatively low. For this reason, in the present embodiment, whether or not an abnormality has occurred in the acceleration sensor  18  is determined, based on the jerk rather than the acceleration. The determination result of the detection abnormality determination unit  64  is supplied to the control unit  28 . 
     As described above, the vertical component of the DC component of the reference signal r is supplied to the attachment abnormality determination unit  70 . The positive or negative sign of the value of the vertical component of the DC component of the reference signal r differs between the case where the acceleration sensor  18  is properly attached to the vehicle  12  and the case where the acceleration sensor  18  is not properly attached to the vehicle  12 . Here, an example is described of a case where a normal condition is defined as the vertical component of the DC component of the reference signal r being negative, and an abnormal condition is defined as the vertical component of the DC component of the reference signal r being positive. 
     In a case where there is an attachment abnormality, that is, an abnormality in which the acceleration sensor  18  is attached with its front side and back side being opposite to each other, a sound having the same phase as the noise is output from the actuator  16  as a canceling sound, which may cause an increase in the noise. 
     The attachment abnormality determination unit  70  determines whether or not an abnormality (attachment abnormality) has occurred in the acceleration sensor  18  based on the positive or negative sign of the vertical component of the DC component of the reference signal r. When the vertical component of the DC component of the reference signal r is negative, the attachment abnormality determination unit  70  determines that an abnormality (attachment abnormality) has not occurred in the acceleration sensor  18 . On the other hand, when the vertical component of the DC component of the reference signal r is positive, the attachment abnormality determination unit  70  determines that an abnormality (attachment abnormality) has occurred in the acceleration sensor  18 . 
     An example has been described of a case where a normal condition is defined as the vertical component of the DC component of the reference signal r being negative, and an abnormal condition is defined as the vertical component of the DC component of the reference signal r being positive. However, the present invention is not limited thereto. On the other hand, the normal condition may be defined as the vertical component of the DC component of the reference signal r being positive, and the abnormal condition may be defined as the vertical component of the DC component of the reference signal r being negative. In such a case, when the vertical component of the DC component of the reference signal r is positive, the attachment abnormality determination unit  70  determines that an attachment abnormality has not occurred in the acceleration sensor  18 . On the other hand, when the vertical component of the DC component of the reference signal r is negative, the attachment abnormality determination unit  70  determines that an attachment abnormality has occurred in the acceleration sensor  18 . 
     When the determination unit  26  determines that an abnormality has occurred in any of the plurality of acceleration sensors  18 , the control unit  28  stops generating the control signal u 0  based on the reference signal r acquired by the acceleration sensor  18  that has been determined to have an abnormality. The reason is as follows, as to why the generation of the control signal u 0 , based on the reference signal r acquired by the acceleration sensor  18  determined to have an abnormality, is stopped. That is, when the actuator  16  is driven using the control signal u 0  based on the reference signal r acquired by the acceleration sensor  18  that has been determined to have an abnormality, the actuator  16  is driven using the inappropriate control signal u 0 . If the actuator  16  is driven with the inappropriate control signal u 0 , the noise cannot be reduced well. For this reason, in the present embodiment, generation of the control signal u 0 , based on the reference signal r acquired by the acceleration sensor  18  that has been determined to have an abnormality, is stopped. 
     When the determination unit  26  determines that an abnormality has occurred in any of the acceleration sensors  18 , the control unit  28  further performs the following control. That is, in such a case, the control unit  28  stops updating the filter coefficient W of the adaptive filter  36  that performs the filtering process on the reference signal r acquired by the acceleration sensor  18  that has been determined to have no abnormality. The reason is as follows, as to why the update of the filter coefficient W of the adaptive filter  36  that performs the filtering process on the reference signal r acquired by the acceleration sensor  18  determined to have no abnormality, is stopped. That is, in the active noise control device  10  that performs control using the signals r detected by the plurality of acceleration sensors  18 , the parameters are adjusted such that the noise reduction effect is acquired in each seat  13  in a balanced manner. For this reason, when the signal r acquired by any of the acceleration sensors  18  is missing, the balance is lost, and a phenomenon may occur in which noise is sufficiently reduced in a certain seat  13  while noise is increased in another seat  13 . For this reason, in such a case, the control unit  28  stops updating the filter coefficient W of the adaptive filter  36  that performs the filtering process on the reference signal r acquired by the acceleration sensor  18  that has been determined to have no abnormality. 
     When it is determined that an abnormality has occurred in the acceleration sensor  18 , the control unit  28  stores information indicating that an abnormality has occurred in the acceleration sensor  18  in the storage unit  30 . The information indicating that an abnormality has occurred in the acceleration sensor  18  can be used for a failure diagnosis or the like, for example. 
     The output unit  32  notifies a failure diagnosis device  66  of information indicating that an abnormality has occurred in the acceleration sensor  18 . When the failure diagnosis device  66  is connected to the vehicle  12 , the control unit  28  supplies information indicating that an abnormality has occurred in the acceleration sensor  18 , to the failure diagnosis device  66  via the output unit  32 . Since the information indicating that an abnormality has occurred in the acceleration sensor  18  is supplied to the failure diagnosis device  66 , the failure diagnosis device  66  can accurately perform a failure diagnosis. 
     When it is determined that an abnormality has occurred in the acceleration sensor  18 , the control unit  28  outputs information indicating that an abnormality has occurred in the acceleration sensor  18  to an information display device  68  provided at the vehicle  12 . The information display device  68  can display information indicating that an abnormality has occurred in the acceleration sensor  18 . Since the information indicating that an abnormality has occurred in the acceleration sensor  18  can be displayed on the information display device  68 , the user can notice that an abnormality has occurred in the acceleration sensor  18  based on the display of the information display device  68 . 
     Next, an example of operations of the active noise control device according to the present embodiment will be described with reference to  FIG.  5   .  FIG.  5    is a flowchart illustrating an example of operations of the active noise control device according to the present embodiment. 
     First, in step S 1 , the determination unit  26  determines whether or not an abnormality has occurred in the acceleration sensor  18 A. When an abnormality has occurred in the acceleration sensor  18 A (YES in step S 1 ), the process transitions to step S 5 . If no abnormality has occurred in the acceleration sensor  18 A (NO in step S 1 ), the process transitions to step S 2 . 
     In step S 2 , the determination unit  26  determines whether or not an abnormality has occurred in the acceleration sensor  18 B. When an abnormality has occurred in the acceleration sensor  18 B (YES in step S 2 ), the process transitions to step S 6 . If no abnormality has occurred in the acceleration sensor  18 B (NO in step S 2 ), the process transitions to step S 3 . 
     In step S 3 , the determination unit  26  determines whether or not an abnormality has occurred in the acceleration sensor  18 C. When an abnormality has occurred in the acceleration sensor  18 C (YES in step S 3 ), the process transitions to step S 7 . If no abnormality has occurred in the acceleration sensor  18 C (NO in step S 3 ), the process transitions to step S 4 . 
     In step S 4 , the determination unit  26  determines whether or not an abnormality has occurred in the acceleration sensor  18 D. When an abnormality has occurred in the acceleration sensor  18 D (YES in step S 4 ), the process transitions to step S 8 . If no abnormality has occurred in the acceleration sensor  18 D (NO in step S 4 ), the process illustrated in  FIG.  5    is completed. 
     In step S 5 , the control unit  28  stops generating the control signal u 0  based on the reference signal r acquired by the acceleration sensor  18 A. Thereafter, the process transitions to step S 9 . 
     In step S 6 , the control unit  28  stops generating the control signal u 0  based on the reference signal r acquired by the acceleration sensor  18 B. Thereafter, the process transitions to step S 10 . 
     In step S 7 , the control unit  28  stops generating the control signal u 0  based on the reference signal r acquired by the acceleration sensor  18 C. Thereafter, the process proceeds to step S 11 . 
     In step S 8 , the control unit  28  stops generating the control signal u 0  based on the reference signal r acquired by the acceleration sensor  18 D. Thereafter, the process proceeds to step S 12 . 
     In step S 9 , the control unit  28  stops updating the filter coefficients W of the adaptive filters  36  that perform the filtering processes on the reference signals r acquired by the acceleration sensors  18 B to  18 D. That is, the update of the filter coefficients W of the adaptive filters  36  provided in the filter units  34 B to  34 D is stopped. Upon completion of step S 9 , the process illustrated in  FIG.  5    is brought to an end. 
     In step S 10 , the control unit  28  stops updating the filter coefficients W of the adaptive filters  36  that perform the filtering processes on the reference signals r acquired by the acceleration sensors  18 A,  18 C, and  18 D. That is, the update of the filter coefficients W of the adaptive filters  36  provided in the filter units  34 A,  34 C, and  34 D is stopped. Upon completion of step S 10 , the process illustrated in  FIG.  5    is brought to an end. 
     In step S 11 , the control unit  28  stops updating the filter coefficients W of the adaptive filters  36  that perform the filtering processes on the reference signals r acquired by the acceleration sensors  18 A,  18 B, and  18 D. That is, the update of the filter coefficients W of the adaptive filters  36  provided in the filter units  34 A,  34 B, and  34 D is stopped. Upon completion of step S 11 , the process illustrated in  FIG.  5    is brought to an end. 
     In step S 12 , the control unit  28  stops updating the filter coefficients W of the adaptive filters  36  that perform the filtering processes on the reference signals r acquired by the acceleration sensors  18 A to  18 C. That is, the update of the filter coefficients W of the adaptive filters  36  provided in the filter units  34 A to  34 C is stopped. Upon completion of step S 12 , the process illustrated in  FIG.  5    is brought to an end. 
     Next, an example of operations of the active noise control device according to the present embodiment will be described with reference to  FIG.  6   .  FIG.  6    is a flowchart illustrating an example of operations of the active noise control device according to the present embodiment. 
     In step S 21 , the first calculating unit  46  calculates the jerk jvx in the traveling direction of the vehicle  12  based on the DC component of the reference signal r. 
     In step S 22 , the second calculating unit  48  calculates the jerk j in the traveling direction of the vehicle  12  based on the signal acquired by the speed sensor  19  provided at the vehicle  12 . That is, the second calculating unit  48  calculates the jerk j in the traveling direction of the vehicle  12  based on the signal indicating the speed v. 
     In step S 23 , the determination unit  26  determines whether or not the difference Δj between the jerk jvx calculated by the first calculating unit  46  and the jerk j calculated by the second calculating unit  48  is greater than or equal to the difference threshold value DTH. When the difference Δj is greater than or equal to the difference threshold value DTH (YES in step S 23 ), the process proceeds to step S 24 . When the difference Δj is less than the difference threshold value DTH (NO in step S 23 ), the process proceeds to step S 25 . 
     In step S 24 , the determination unit  26  determines that an abnormality has occurred in the acceleration sensor  18 . 
     In step S 25 , the determination unit  26  determines that no abnormality has occurred in the acceleration sensor  18 . Thus, the processing illustrated in  FIG.  6    is completed. 
     Next, an example of operations of the active noise control device according to the present embodiment will be described with reference to  FIG.  7   .  FIG.  7    is a flowchart illustrating an example of operations of the active noise control device according to the present embodiment. Here, an example is described of a case where a normal condition is defined as the vertical component of the DC component of the reference signal r being negative, and an abnormal condition is defined as the vertical component of the DC component of the reference signal r being positive. 
     In step S 31 , the determination unit  26  determines whether or not the value of the vertical component of the DC component of the reference signal r is negative. When the value of the vertical component of the DC component of the reference signal r is negative (YES in step S 31 ), the process transitions to step S 32 . When the value of the vertical component of the DC component of the reference signal r is positive (NO in step S 31 ), the process transitions to step S 33 . 
     In step S 32 , the determination unit  26  determines that no abnormality has occurred in the acceleration sensor  18 . 
     In step S 33 , the determination unit  26  determines that an abnormality has occurred in the acceleration sensor  18 . Accordingly, the process illustrated in  FIG.  7    is brought to an end. 
     As described above, in the present embodiment, when the determination unit  26  determines that an abnormality has occurred in any of the plurality of acceleration sensors  18 , generation of the control signal u 0 , based on the reference signal r acquired by the acceleration sensor  18  that has been determined to have an abnormality, is stopped. Further, in the present embodiment, the update of the filter coefficient W of the adaptive filter  36  that performs the filtering process on the reference signal r acquired by the acceleration sensor  18  that has been determined to have no abnormality, is stopped. For this reason, according to the present embodiment, even in a case where an abnormality has occurred in any of the plurality of acceleration sensors  18 , it is possible to provide the active noise control device  10  that is capable of suppressing an adverse effect due to the acceleration sensor  18  in which the abnormality has occurred, and thus it is possible to provide the active noise control device  10  that is capable of reducing noise suitably. 
     Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made thereto without departing from the essence and gist of the present invention. 
     The above-described embodiments can be summarized in the following manner. 
     The active noise control device ( 10 ) causes the actuator ( 16 ) to output the canceling sound based on the control signal (u) in order to reduce noise in the vehicle compartment ( 14 ) of the vehicle ( 12 ). The active noise control device includes the adaptive filter ( 36 X to  36 Z) configured to generate the control signal by performing the filtering process on the reference signal (rx to rz) acquired by the acceleration sensor ( 18 A to  18 D) attached to the vehicle, the filter coefficient updating unit ( 40 X to  40 Z) configured to update the filter coefficient (W) of the adaptive filter based on the reference signal and the error signal (e) acquired by detecting residual noise due to interference between the noise and the canceling sound by the microphone ( 20 ), the determination unit ( 26 ) configured to determine whether an abnormality has occurred in the acceleration sensor based on the direct-current component of the reference signal, and the control unit ( 28 ) configured to, when the determination unit determines that an abnormality has occurred in any of the plurality of the acceleration sensors, stop generation of the control signal based on the reference signal acquired by the acceleration sensor that has been determined to have the abnormality, and stop updating the filter coefficient of the adaptive filter configured to perform the filtering process on the reference signal acquired by the acceleration sensor that has been determined to have no abnormality. According to such a configuration, even when an abnormality has occurred in any of the plurality of acceleration sensors, it is possible to provide an active noise control device that is capable of suppressing an adverse effect due to the acceleration sensor in which the abnormality has occurred, and thus it is possible to provide an active noise control device that is capable of reducing noise suitably. 
     The active noise control device may further include the first calculating unit ( 46 ) configured to calculate the jerk (jvx) in a traveling direction of the vehicle based on the direct-current component of the reference signal, and the second calculating unit ( 48 ) configured to calculate the jerk (j) in the traveling direction of the vehicle based on the signal (v) acquired by the speed sensor ( 19 ) provided at the vehicle, wherein the determination unit may be configured to determine that the abnormality has occurred in the acceleration sensor when a difference (Δj) between the jerk calculated by the first calculating unit and the jerk calculated by the second calculating unit is greater than or equal to the difference threshold value (DTH). According to such a configuration, since the determination is performed using the jerk, even when an acceleration sensor having relatively low acceleration detection accuracy is provided at the vehicle, it is possible to suitably determine whether or not an abnormality has occurred in the acceleration sensor. 
     The positive or negative sign of a value of a vertical component of the direct-current component of the reference signal may differ between a case where the acceleration sensor is normally attached to the vehicle and a case where the acceleration sensor is not normally attached to the vehicle, and the determination unit may be configured to determine whether or not the abnormality has occurred in the acceleration sensor based on the positive or negative sign of the vertical component. According to such a configuration, even when an abnormality (attachment abnormality) occurs in which the acceleration sensor is attached with the front and back reversed, such an abnormality can be accurately determined. 
     In a case where it is determined that the abnormality has occurred in the acceleration sensor, the control unit may be configured to store information indicating that the abnormality has occurred in the acceleration sensor in the storage unit ( 30 ). According to such a configuration, information indicating that an abnormality has occurred in the acceleration sensor can be used for a failure diagnosis or the like. 
     The active noise control device may further includes the output unit ( 32 ) configured to notify the failure diagnosis device ( 66 ) of information indicating that the abnormality has occurred in the acceleration sensor. According to such a configuration, since information indicating that an abnormality has occurred in the acceleration sensor can be supplied to the failure diagnosis device, an accurate failure diagnosis can be performed by the failure diagnosis device. 
     In a case where it is determined that the abnormality has occurred in the acceleration sensor, the control unit may be configured to output information indicating that the abnormality has occurred in the acceleration sensor to the information display device ( 68 ) provided at the vehicle. According to such a configuration, since the information indicating that an abnormality has occurred in the acceleration sensor can be displayed on the information display device, the user can notice that an abnormality has occurred in the acceleration sensor based on the display of the information display device. 
     The acceleration sensor may be a three-axis acceleration sensor. 
     The vehicle includes the active noise control device as described above.