Patent Publication Number: US-2023137143-A1

Title: Work machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Patent Application No. 
     2021-176675 filed on Oct. 28, 2021 with the Japan Patent Office, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a work machine. 
     U.S. Patent Application Publication No. 2019/0275657 discloses an electric power tool to which active noise control (ANC) is applied. ANC is a technique for canceling noise by using sound collected by a microphone to generate a sound having an inverted phase of the noise from a speaker at a location where the noise is desired to be canceled. 
     SUMMARY 
     There is still room for improvement in the noise reduction effect of the existing ANC. In a work machine where mechanical operating noise propagates from inside the housing to outside through an opening, the technique for effectively inhibiting the operating noise propagating outside is not yet known. 
     Therefore, one aspect of the present disclosure is to provide a technique for effectively reducing the operating noise that propagates outside, in a work machine where the operating noise propagates from inside a housing to outside through an opening. 
     According to one aspect of the present disclosure, a work machine is provided. The work machine includes a machine. The work machine includes a housing. The housing at least partly houses the machine. The housing has an opening. 
     The work machine further has a path leading into the housing from the opening. The work machine further includes a controller and a speaker. The controller is configured to cause the speaker to output a control sound for reducing operating noise. The operating noise is generated in the housing by motion of the machine and propagates in the path from a source of the operating noise to the opening. 
     The speaker is arranged so that the control sound propagates in the path with its wavefront parallel to a wavefront of the operating noise that propagates in the path. The above arrangement of the speaker can effectively reduce the operating noise that propagates in the path by the control sound with the wavefront parallel to the wavefront of the operating noise. 
     As a result, according to one aspect of the present disclosure, it is possible to effectively reduce the operating noise that propagates outside the housing from the opening through the path, and provide a work machine in which the operating noise audible to an operator is low. 
     According to another aspect of the present disclosure, in order to effectively reduce the operating noise that propagates outside the housing in a work machine where the operating noise propagates outside through the opening, a work machine according to the following items 1 and 2 may be provided. 
     [Item 1] 
     A work machine comprising: 
     a machine; 
     a housing that at least partly houses the machine, the machine having an opening; 
     a speaker; 
     a microphone configured to collect sound and output a sound signal that is an electrical signal corresponding to the sound collected; and 
     a controller configured to cause the speaker to output a control sound for reducing operating noise based on the sound signal from the microphone, the operating noise being generated in the housing by motion of the machine and propagating outside the housing through the opening, wherein the opening is an open end of the housing, wherein the speaker is arranged to have a vibrating surface along a boundary plane between inside and outside the housing defined by the open end, and is configured to output the control sound from both sides of the vibrating surface in a direction normal to the vibrating surface, and wherein the microphone is arranged within a specific distance centered on the vibrating surface in the direction normal to the vibrating surface, and the specific distance corresponds to a quarter wavelength of the operating noise. 
     [Item 2] 
     The work machine according to item 1, wherein the microphone is arranged within a distance corresponding to half a quarter wavelength or one-sixth wavelength of the operating noise as the specific distance. 
     According to the work machine according to item 1, the operating noise that propagates outside the housing through the opening can be effectively reduced by the arrangement of the speaker and the microphone. The work machine according to item 2 improves noise reduction effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the present disclosure will be described hereinafter with reference to the accompanying drawings, in which: 
         FIG.  1    is a perspective view showing the appearance of a dust collector according to one example embodiment; 
         FIG.  2    is a bottom view of a dust collector main body; 
         FIG.  3    is a perspective view of a rear housing with its internal components removed, as seen from a joining surface between a front housing and the rear housing; 
         FIG.  4    is a perspective view showing an interior of the dust collector with the rear housing removed from the collector main body; 
         FIG.  5    is a perspective view of the front housing with its internal components removed, as seen from the joining surface between the front housing and the rear housing; 
         FIG.  6    is a partly enlarged plan view of the front housing, as seen from the joining surface between the front housing and the rear housing; 
         FIG.  7    is a diagram conceptually illustrating the relationship between wavefronts of target noise and wavefronts of control sound; 
         FIG.  8    is a block diagram showing an electrical configuration of the dust collector; 
         FIG.  9    is a block diagram showing a feed-forward ANC model; 
         FIG.  10    is a perspective view of the front housing of the dust collector of a variation, with its internal components removed, as seen from a joining surface between the front housing and the rear housing; 
         FIG.  11    is a partly enlarged plan view of the front housing of the dust collector of the variation, as seen from the joining surface between the front housing and the rear housing; 
         FIG.  12    is a diagram conceptually illustrating the relationship between wavefronts of target noise and wavefronts of control sound in the dust collector of the variation; 
         FIG.  13    is a side view of a blower; 
         FIG.  14    is a perspective view of the blower; 
         FIG.  15    is a perspective sectional view of the blower; and 
         FIG.  16    is a block diagram illustrating a feedback ANC model. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     1. Overview of Embodiments 
     1.1. First Embodiment 
     A work machine in one embodiment may include a machine. The machine may be configured to work for a specific operation. Additionally or alternatively, the work machine may include a housing. The housing may at least partly house the machine. Additionally or alternatively, the housing may have an opening. 
     Additionally or alternatively, the work machine may include a path leading into the housing from the opening. Additionally or alternatively, the work machine may include a speaker. Additionally or alternatively, the work machine may include a controller. The controller may be configured to cause the speaker to output a control sound. 
     The control sound may be a sound that reduces sound that propagates in the path to the opening. For example, the control sound may reduce operating noise that is generated in the housing by motion of the machine and propagates in the path from a source of the operating noise to the opening. 
     The speaker may be arranged so that the control sound propagates in the path with its wavefront parallel to a wavefront of the operating noise. The control sound propagating with its wavefront parallel to the wavefront of the operating noise makes it possible to effectively reduce the operating noise that propagates in the path. 
     As a result, the work machine can effectively reduce the operating noise that propagates outside the housing from the opening. Specifically, it is possible to provide a work machine in which the operating noise audible to an operator is low. 
     In one embodiment, the path may be designed so that the operating noise that propagates in the path forms a plane wave with a wavefront orthogonal to an axial direction of the path. The speaker may output the control sound in the axial direction of the path so that the control sound propagates as a plane wave with a wavefront orthogonal to the axial direction of the path. Such design of the path and output of the control sound can effectively reduce the operating noise that propagates in the path. 
     In one embodiment, the path may include a first path and a second path that is coupled to the first path. The first path may be a first linear path. The second path may be a second linear path. The second linear path may be coupled to the first linear path at an angle to the first linear path. For example, the second linear path may be coupled to the first linear path at right angles. 
     In one embodiment, when the first path is coupled to the second path, the speaker may be arranged on a side wall of a coupling portion between the first path and the second path. The speaker may be arranged so as to face the first path or the second path on the side wall of the coupling portion, and output the control sound in an axial direction of the first path or the second path. 
     In one embodiment, the speaker may be arranged on a side wall of the coupling portion between the first linear path and the second linear path in the path. The speaker may be arranged so as to face the first linear path or the second linear path on the side wall of the coupling portion, and output the control sound in an axial direction of the first linear path or the second linear path. Such use of the coupling portion of the path allows the speaker to be arranged so that the wavefront of the control sound are parallel to a wavefront of the operating noise. 
     In one embodiment, the speaker may be arranged so as to output the control sound in the same direction as a propagation direction of the operating noise that propagates to the opening. The speaker may be arranged so as to output a control sound with its wavefront parallel to a wavefront of the operating noise as the control sound that propagates in the same direction as the propagation direction of the operating noise. This arrangement can effectively reduce the operating noise that is about to propagate outside the housing. 
     In one embodiment, the speaker may be arranged so as to output the control sound in a direction opposite to a propagation direction of the operating noise that propagates to the opening. The speaker may be arranged so as to output a control sound with a wavefront parallel to a wavefront of the operating noise as the control sound that propagates in the direction opposite to the propagation direction of the operating noise. This arrangement can effectively reduce the operating noise that is about to propagate outside the housing. 
     1.2. Second Embodiment 
     A work machine in one embodiment may include a machine. The machine may be configured to work for a specific operation. Additionally or alternatively, the work machine may include a housing. The housing may at least partly house the machine. Additionally or alternatively, the housing may have an opening. 
     In one embodiment, the work machine may include a speaker and a microphone. The microphone may collect sound, and output a sound signal that is an electrical signal corresponding to the collected sound. In one embodiment, the work machine may include a controller. 
     The controller may be configured to cause the speaker to output a control sound for reducing sound that propagates outside the housing through the opening based on the sound signal from the microphone. For example, the controller may be configured to cause the speaker to output a control sound for reducing operating noise. The operating noise is generated in the housing by motion of the machine and propagates outside the housing through the opening. 
     In one embodiment, the opening may be an open end of the housing. The speaker may be arranged to have a vibrating surface at the open end. Alternatively, the speaker may be arranged to have a vibrating surface along a boundary plane between inside and outside the housing defined by the open end. The speaker may be arranged so that the vibrating surface coincides with the boundary plane or is in front of or behind the boundary plane. The speaker may be configured to output the control sound from both sides of the vibrating surface in a direction normal to the vibrating surface. 
     In one embodiment, the microphone may be arranged within a specific distance centered on the vibrating surface or the boundary plane in the direction normal to the vibrating surface or the boundary plane. The specific distance may correspond to a quarter wavelength of the operating noise. Arranging the speaker and the microphone within the above distance makes it possible to effectively reduce the sound that propagates outside the housing from the open end. 
     In one embodiment, the microphone may be arranged within a distance corresponding to one-sixth wavelength of the operating noise as the specific distance. In one embodiment, the microphone may be arranged within a distance corresponding to half a quarter wavelength of the operating noise as the specific distance. Arranging the speaker and the microphone within the above distance makes it possible to further effectively reduce the sound that propagates outside the housing from the open end. 
     With respect to the aforementioned first and second embodiments, one or more of the components of the aforementioned work machine may be removed as desired. The work machine may include an additional component as desired. One or more of the components of the work machine may be replaced with another component or components as desired. 
     2. Specific Exemplary Embodiments 
     2.1. First Embodiment 
     2.1.1. Configuration of Dust Collector 
     A configuration of a dust collector  1  will be described hereinafter, as an example of a work machine. For convenience of description, the direction (front, rear, up, down, left and right) relative to the dust collector  1  is defined as shown in  FIGS.  1  to  6    in the present embodiment. 
     As shown in  FIG.  1   , the dust collector  1  of the present embodiment includes a main body  3 , an operation device  6 , and attachments  7 . The attachments  7  include shoulder belts  71 A,  71 B, and a waist belt  72 . The shoulder belts  71 A,  71 B and the waist belt  72  are attached to the rear surface of the main body  3 . 
     The shoulder belt  71 A extends from near the upper left end of the main body  3 . The shoulder belt  71 B extends from near the upper right end of the main body  3 . The waist belt  72  extends from near the bottom end of the main body  3 . The attachments  7  are used for an operator of the dust collector  1  to carry the main body  3  on its back. 
     The operation device  6  includes a switch to start or stop the dust collector  1 . The operation device  6  is manipulated by the operator. The operation device  6  is connected, via a cable  61 , to the main body  3  near the center of the bottom end of the main body  3 . 
     The main body  3  includes a housing  30  for housing major electrical and/or mechanical components of the dust collector  1 . The housing  30  includes a rear housing  301 , a front housing  302 , and a plate  303 .  FIGS.  2  and  3    show a configuration of the rear housing  301 .  FIGS.  4  and  5    show a configuration of the front housing  302 . 
     The rear housing  301  is a bottomed box-shaped member having an inner surface facing the front. The front housing  302  is a frame-shaped member with an opening. The plate  303  is a plate-shaped member that closes the opening of the front housing  302  from the front. The housing  30  is, for example, mold by injecting a resin material. 
     As shown in  FIGS.  3 ,  4 , and  5   , the housing  30  includes a suction port  31 , a dust collecting chamber  32 , a first flow path  33 , a motor chamber  34 , a second flow path  35 , a third flow path  36 , a first battery compartment  38 A, a second battery compartment  38 B, and a component placement portion  39 . 
     The suction port  31  is provided in the central portion of the top end of the housing  30 . The suction port  31  is connected to a first end of a flexible hose (not shown). A second end of the hose is connected to a nozzle having a suction port (not shown). 
     As shown in  FIG.  4   , the dust collecting chamber  32  is a rectangular internal chamber provided on the upper side of the housing  30 . The dust collecting chamber  32  stores a dust bag  41  that is connected to the suction port  31 . The dust bag  41  is made of, for example, paper. The dust bag  41  traps and collects dust sucked from the suction port  31 . 
     The first flow path  33  is provided along the right side of the dust collecting chamber  32 . The bottom end of the first flow path  33  is connected to the motor chamber  34 . A filter  42  is arranged at the boundary between the first flow path  33  and the dust collecting chamber  32 . Examples of the filter  42  may include a high efficiency particulate air filter (HEPA). 
     The motor chamber  34  is an internal chamber provided below the dust collecting chamber  32 . As shown in  FIGS.  3  to  6   , the motor chamber  34  includes an inlet port  341  in the central portion of the right end of the motor chamber  34 . The inlet port  341  is connected to the first flow path  33 . The motor chamber  34  further includes an outlet port  342  in the upper portion of the left end of the motor chamber  34 . The outlet port  342  is connected to the second flow path  35 . The motor chamber  34  houses a drive machine  43 . A thick dotted arrow shown in  FIG.  6    conceptually represents an airflow. 
     The drive machine  43  includes a fan  431 , a motor  432 , and a damper  433 . The fan  431  is connected to a rotation shaft of the motor  432 . The fan  431  receives power from the motor  432  and is rotationally driven. As a result, an airflow that travels from the inlet port  341  toward the outlet port  342  of the motor chamber  34  is generated. 
     The damper  433  is an annular member that covers the motor  432 . The damper  433  absorbs noise generated by the motor  432 . In  FIG.  4   , the motor  432  is arranged in the center of the damper  433  although not shown because the motor  432  is covered with the damper  433 . 
     A microphone  53  is also installed in the motor chamber  34 . The microphone  53  is used as the reference microphone  53  in active noise control (ANC). 
     The reference microphone  53  is provided so as to collect operating noise of the drive machine  43  generated in the housing  30 . The operating noise includes noise from the motor  432  and the fan  431  due to motion of the drive machine  43 . The reference microphone  53  outputs a sound signal that is an electrical signal corresponding to the collected noise. 
     The second flow path  35  is a linear exhaust path provided on the upper side of the motor chamber  34  and extending leftward from the motor chamber  34 . The second flow path  35  connects the outlet port  342  of the motor chamber  34  with the third flow path  36 . 
     The third flow path  36  is a linear exhaust path provided to the left of the motor chamber  34  and extending downward. The third flow path  36  includes an exhaust port  361  in its downstream portion. The third flow path  36  is coupled at an angle to the second flow path  35 . Specifically, the third flow path  36  is coupled to the second flow path  35  at right angles. As shown in  FIGS.  2  and  3   , the exhaust port  361  has the form of a group of slits that are formed on the rear surface of the housing  30 . 
     The second flow path  35  and the third flow path  36  form an L-shaped exhaust path, and controls the airflow from the motor chamber  34  to the exhaust port  361 . Specifically, the second flow path  35  and the third flow path  36  guide the airflow from the motor chamber  34  out of the housing  30  through the exhaust port  361 . 
     In the main body  3  configured as above, when airflow is generated by the motion of the drive machine  43 , external air is sucked into the internal space of the housing  30  through the suction port  31 . The sucked external air first enters the dust collecting chamber  32 , and passes through the dust bag  41  attached to the suction port  31 . This passing through allows the dust contained in the external air to be trapped. 
     The air that has passed through the dust bag  41  reaches the first flow path  33  via the filter  42 . The air that has reached the first flow path  33  passes through the motor chamber  34  and the second flow path  35  to the third flow path  36 , and is discharged to outside the housing  30  through the exhaust port  361 . 
     The operating noise from the drive machine  43  propagates through the exhaust path to the exhaust port  361 , and from the exhaust port  361  to outside the housing  30 . This operating noise is the noise to be inhibited from propagating outside the housing  30  by ANC (hereinafter, referred to as target noise). 
     In order to inhibit the target noise from propagating outside the housing  30  through the exhaust port  361 , a control speaker  54  is further provided in the exhaust path. The control speaker  54  is provided at a location on the upper side wall  35 A of the second flow path  35 , the location intersecting with the axis extending in up and down directions of the third flow path  36 . The control speaker  54  is arranged so that a vibrating surface of the control speaker  54  faces the third flow path  36 . 
     Specifically, the control speaker  54  is configured so that its vibrating surface is orthogonal to the axial direction of the third flow path  36 . This arranges the control speaker  54  to output the control sound in the axial direction of the third flow path  36  so that the control sound propagates as a plane wave with wavefronts orthogonal to the axial direction of the third flow path  36 , as shown in  FIG.  7   . 
     The solid arrow shown in  FIG.  7    conceptually represents a propagation direction of the control sound, and the line segments (solid lines) perpendicular to the solid arrow conceptually represent wavefronts of the control sound. The dashed arrow shown in  FIG.  7    conceptually represents a propagation direction of the target noise, and the line segments (dashed lines) perpendicular to the dashed arrow conceptually represent wavefronts of the target noise. The control sound is output from the control speaker  54  to cancel the target noise. 
     A mounting hole  35 B for the control speaker  54  is provided on the side wall  35 A of the second flow path  35 . The control speaker  54  is fitted into the mounting hole  35 B to be fixed to the side wall  35 A. The control speaker  54  is configured to output the control sound from the front side facing the third flow path  36 , and not to output the control sound from the back side. 
     The target noise that propagates outside the housing  30  from the exhaust port  361  through the third flow path  36  is a plane wave with wavefronts orthogonal to the axial direction of the third flow path  36 , as shown by the dashed lines in  FIG.  7   . 
     The target noise propagates in such a plane wave because a width in a direction perpendicular to the axis of the third flow path  36  is sufficiently narrow relative to the wavelength of the target noise. Sound that human hears as noise is a sound in the 2 kHz band, and the wavelength of the sound in 2 kHz band is approximately 170 mm. 
     The second flow path  35  and the third flow path  36  are designed to have a width of 50 mm to 100 mm which is sufficiently smaller than 170 mm. Accordingly, the target noise that propagates in the third flow path  36  forms a plane wave with wavefronts orthogonal to the axial direction of the third flow path  36 . 
     The control speaker  54  outputs the control sound that propagates in the same direction as the target noise that propagates in the third flow path  36  in such a plane wave. The control sound has a plane wave with wavefronts parallel to the wavefronts of the target noise. Since the directions of the wavefronts are aligned between the target noise and the control sound, the target noise is almost uniformly canceled by the control sound in the third flow path  36 . In the present embodiment, this largely lowers the level of the target noise that leaks outside from the exhaust port  361 . 
     The first battery compartment  38 A of the housing  30  defines a space that houses the first battery pack  45 A. The first battery compartment  38 A is provided near the bottom end of the housing  30 . The first battery compartment  38 A includes a first battery mounting port  381 A that is open near the lower left end of the housing  30 . 
     The second battery compartment  38 B defines a space that houses the second battery pack  45 B. The second battery compartment  38 B is provided near the bottom end of the housing  30 . The second battery compartment  38 B includes a second battery mounting port  381 B that is open near the lower right end of the housing  30 . The first and second battery packs  45 A,  45 B are respectively inserted from the first and second battery mounting ports  381 A,  381 B to the first and second battery compartments  38 A,  38 B. 
     The component placement portion  39  is an internal space located between the motor chamber  34 , the second flow path  35 , the third flow path  36 , and the first and second battery compartments  38 A,  38 B. Various electrical components are arranged in this internal space. 
     The component placement portion  39  includes a vertical portion  391  and a horizontal portion  392  that communicates with the vertical portion  391 . The vertical portion  391  corresponds to a portion surrounded on three sides by walls of the motor chamber  34 , the second flow path  35 , and the third flow path  36 . The horizontal portion  392  corresponds to a portion that is placed between the motor chamber  34  and the first and second battery compartments  38 A,  38 B. 
     A connector  52  is arranged in the horizontal portion  392 . The connector  52  is arranged between the first battery compartment  38 A and the second battery compartment  38 B. The connector  52  is provided to connect a cable  61  of the operation device  6  with an internal circuit. 
     A drive controller  44 , and an error microphone  55  used in ANC are arranged in the vertical portion  391 . The error microphone  55  is mounted to be exposed outside the housing  30  through a mounting hole formed on the bottom surface of the rear housing  301  and to be directional toward the outside of the housing  30 . 
     As shown in  FIG.  4   , the drive controller  44  is attached to a wall that defines a boundary between the vertical portion  391  and the motor chamber  34 . The drive controller  44  is a circuit board that performs power supply control, motor control, noise control, and so on. 
     The error microphone  55  is arranged at a location corresponding to a noise canceling point and not directly hit by the airflow generated by the drive machine  43 . The location corresponding to the noise canceling point is where the error microphone  55  can be assumed to be at the noise canceling point. The location corresponding to the noise canceling point is specifically in the vicinity of the exhaust port  361 . In ANC, the control sound is controlled so that the target noise and the control sound cancel each other out at the noise canceling point. The error microphone  55  collects a combined sound of the target noise discharged from the exhaust port  361  and the control sound. 
     The reference microphone  53 , the control speaker  54 , and the error microphone  55  are arranged so that time for the control sound emitted from the control speaker  54  to reach the noise canceling point is shorter than time for the target noise to directly reach the noise canceling point. During the time difference, a process to generate the control sound is executed. 
     [2.1.2. Drive Controller] 
     As shown in  FIG.  8   , the drive controller  44  includes a control circuit  441 , a dust collection circuit group  442 , a signal processing circuit group  443 , and a power-supply circuit  447 . 
     The power-supply circuit  447  delivers electric power supplied from the first and second battery packs  45 A,  45 B to each part of the dust collector  1  at an appropriate voltage. The control circuit  441  is configured as a microcomputer. The control circuit  441  includes a CPU  441 A and a memory  441 B. 
     As another example, the control circuit  441  may include, in place of or in addition to the microcomputer, a combination of electronic components such as, for example, discrete devices. The control circuit  441  may include a digital signal processor (DSP) and/or an application specific IC (ASIC). The control circuit  441  may include an application specific standard product (ASSP). The control circuit  441  may include a programmable logic device. 
     The dust collection circuit group  442  includes circuits necessary to perform the function as the dust collector  1 . Specifically, the dust collection circuit group  442  includes a motor drive circuit and a battery switching circuit. The motor drive circuit drives the motor  432 . The battery switching circuit appropriately switches a supply source of electric power between the first and second battery packs  45 A,  45 B depending on the remaining energies of the first and second battery packs  45 A,  45 B. 
     The signal processing circuit group  443  includes various types of circuits necessary to perform the function as a noise controller. The signal processing circuit group  443  includes first and second analog/digital (A/D) converters  444 ,  445  and a digital/analog (D/A) converter  446 . 
     The first A/D converter  444  converts a sound signal from the reference microphone  53  to a digital signal and supplies the digital signal to the control circuit  441 . The second A/D converter  445  converts a sound signal from the error microphone  55  to a digital signal and supplies the digital signal to the control circuit  441 . The D/A converter  446  converts control data from the control circuit  441  to analog data in order to generate a control signal to be supplied to the control speaker  54 . 
     The control circuit  441  controls the dust collection circuit group  442 . As a result, a process for achieving the function as the dust collector  1  is executed. The control circuit  441  also executes a noise reduction process for reducing the target noise. 
     The control circuit  441  executes a noise control process. As a result, feed-forward active noise control (ANC) is achieved. By ANC, the control sound for inhibiting the operating noise from propagating outside the housing  30 , in other words, the control sound for canceling the target noise, is output from the control speaker  54 . 
     [2.1.3. ANC Model] 
     Referring to  FIG.  9   , the feed-forward ANC model applied to the dust collector  1  will be described. The feed-forward ANC model includes a reference sensor M 1 , a control sound source M 2 , an error sensor M 3 , a noise control filter M 4 , a secondary system filter M 5 , and a coefficient updater M 6 . 
     The reference sensor M 1  corresponds to the reference microphone  53  and the first A/D converter  444 . The control sound source M 2  corresponds to the D/A converter  446  and the control speaker  54 . The error sensor M 3  corresponds to the error microphone  55  and the second A/D converter  445 . 
     All of the noise control filter M 4 , the secondary system filter M 5 , and the coefficient updater M 6  may be implemented by processes of the control circuit  441 , that is, software. Alternatively, some or all of the noise control filter M 4 , the secondary system filter M 5 , and the coefficient updater M 6  may be implemented by hardware. 
     The reference sensor M 1  generates a reference signal x n  by collecting the target noise. The reference signal x n  corresponds to a digital signal generated by sampling the sound signal from the reference microphone  53  at a specific sampling cycle. The subscript “n” represents discrete time, and indicates that the corresponding reference signal x n  is the n th  sampling data. 
     The noise control filter M 4  is a finite impulse response (FIR) filter including L taps. “L” is a positive integer. The noise control filter M 4  generates a control signal u n  from an L-dimensional reference vector x(n) having L reference signals {x n , x n−1 , x n−L+1 } that are most recently detected. 
     The control sound source M 2  produces a control sound in accordance with the control signal u n . The error sensor M 3  generates an error signal e n  by collecting the combined sound of the target noise and the control sound. The error signal e n  corresponds to a digital signal generated by sampling the sound signal from the error microphone  55  at a specific sampling cycle. 
     Hereinafter, a sound propagation path from the reference sensor M 1  to the error sensor M 3  is referred to as a primary system, and a sound propagation path from the control sound source M 2  to the error sensor M 3  is referred to as a secondary system. The secondary system filter M 5  is a FIR filter including N taps. “N” is a positive integer. The secondary system filter M 5  generates a filtered reference signal r n  from an N-dimensional reference vector x(n) having N reference signals {x n , x n−1 , . . . , x n−N+1 } that are most recently detected. 
     The secondary system filter M 5  is a filter modeled on the transfer characteristics of the secondary system. A fixed value is used for a coefficient of each tap. The filtered reference signal r n  is a signal obtained by adding the influence of the secondary system, which is added to the control sound when the control sound reaches the error sensor M 3 , to the reference signal x n . 
     The coefficient updater M 6  updates the coefficients {w 1 , w 2 , . . . , w L } of L taps included in the noise control filter M 4  based on the filtered reference signal r n  and the error signal e n . The coefficients {w 1 , w 2 , . . . , w L } are updated so that the target noise and the control sound cancel each other out and the error signal e n  becomes the smallest at the position of the error sensor M 3  (that is, the noise canceling point). 
     The coefficients of the noise control filter M 4  may be updated with, for example, the Filtered-x NLMS algorithm that is one of adaptive algorithms. 
     Due to the coefficient update, the target noise attenuates to be canceled by the control sound in the exhaust path. 
     [2.1.4. Effect of Dust Collector] 
     The dust collector  1  of the present embodiment described in the above achieves the following effects. 
     (Effect 1) The operating noise from the drive machine  43  generated in the housing  30  is collected by the reference microphone  53  provided adjacent to the drive machine  43 . The control sound for canceling the operating noise that is about to propagate outside the housing  30  through the third flow path  36  is output from the control speaker  54  provided in the boundary between the second flow path  35  and the third flow path  36 . Accordingly, it is possible to effectively inhibit the operating noise of the dust collector  1  from spreading to the surroundings as unpleasant noise. 
     (Effect 2) Since the width of the exhaust path in which the operating noise propagates is sufficiently small relative to the wavelength of the operating noise, the operating noise propagates in the exhaust path as a plane wave with wavefronts perpendicular to the axis of the exhaust path. The control speaker  54  is arranged so that the direction of the wavefronts of the control sound and the direction of the wavefronts of the operating noise to be canceled are aligned. Therefore, it is possible to use the control sound to effectively reduce the operating noise in the exhaust path. 
     (Effect 3) According to the present embodiment, the L-shaped exhaust path is used to arrange the control speaker  54  at a bent portion of the exhaust path, in other words, at the coupling portion between the second flow path  35  and the third flow path  36  that are coupled at right angles to each other. Such use of the bent portion or the coupling portion makes it possible to arrange the control speaker  54  to satisfy the above wavefront alignment condition without crossing the flow path. 
     [2.1.5. Variation on Speaker Arrangement] 
     In the aforementioned dust collector  1 , arrangement of the control speaker  54  is not limited to the example shown in  FIGS.  4  to  7   . For example, the control speaker  54  may be arranged as shown in  FIGS.  10  to  12   . 
     In a variation of the dust collector  1  shown in  FIGS.  10  to  12   , the front housing  302  shown in  FIGS.  4  to  6    is replaced with a front housing  302 A shown in  FIGS.  10  to  11   , and the control speaker  54  is arranged at a different location than the location in the embodiment shown in  FIGS.  4  to  6   . 
     As shown in  FIGS.  10  and  11   , the front housing  302 A has a mounting hole  36 B, into which the control speaker  54  is fitted, in a left side wall  36 A of the third flow path  36 . The front housing  302 A does not have the mounting hole  35 B in the side wall  35 A of the second flow path  35 , unlike the aforementioned front housing  302 . 
     When mounted to the mounting hole  36 B, the control speaker  54  is arranged at a location on the side wall  36 A of the third flow path  36  that intersects with the axis extending to the left and right of the second flow path  35 , so that the vibrating surface of the control speaker  54  faces the second flow path  35 . 
     The vibrating surface of the control speaker  54  mounted to the mounting hole  36 B is orthogonal to the axial direction of the second flow path  35 . This makes the control speaker  54  to output the control sound in the axial direction of the second flow path  35  so that the control sound propagates as a plane wave with wavefronts orthogonal to the axial direction of the second flow path  35 , as shown in  FIG.  12   . 
     The solid arrow shown in  FIG.  12    conceptually represents the propagation direction of the control sound, and the line segments (solid lines) perpendicular to the solid arrow conceptually represent wavefronts of the control sound. The dashed arrow shown in  FIG.  12    conceptually represents the propagation direction of the target noise, and the line segments (dashed lines) perpendicular to the dashed arrow conceptually represent wavefronts of the target noise. 
     As shown in  FIG.  12   , the target noise that propagates in the second flow path  35  is a plane wave with wavefronts orthogonal to the axial direction of the second flow path  35 . The control speaker  54  outputs the control sound from the front side facing the second flow path  35 , and does not output the control sound from the back side. 
     To the target noise that propagates in the second flow path  35  with such a plane wave, the control speaker  54  outputs a plane wave control sound with wavefronts parallel to the wavefronts of the target noise. The propagation direction of the control sound is a direction opposite to the propagation direction of the target noise. The control sound propagates in the direction opposite to the propagation direction of the target noise that propagates to the exhaust port  361 , and is reduced to cancel out the target noise. 
     In the present variation as well, the directions of the wavefronts are aligned between the target noise and the control sound. Thus, the target noise is almost uniformly canceled in the exhaust path by the control sound and reduced. This largely lowers the level of the target noise leaking outside from the exhaust port  361 . 
     2.2. Second Embodiment 
     [2.2.1. Configuration of Blower] 
     A configuration of a blower  8  will be described hereinafter, as an example of a work machine. For convenience of description, the direction (front, rear, up, down, left and right) relative to the blower  8  is defined as shown in  FIGS.  13 ,  14  and  15    in the present embodiment. 
     As shown in  FIGS.  13 ,  14  and  15   , the blower  8  includes a main housing  80 . The main housing  80  includes a suction port  81 , a discharge port  82 , and a hold part  83 . A drive machine  90  and a control circuit board  91  are further provided in the main housing  80 . 
     The suction port  81  is provided in the rear portion of the main housing  80 . The discharge port  82  is a cylindrical portion provided in the front portion of the main housing  80 . The air sucked from the suction port  81  receives energy by the motion of the drive machine  90  in the main housing  80 , and is discharged from the discharge port  82  at high speed. 
     The hold part  83  is a portion to be gripped by an operator, and is provided in the upper portion of the main housing  80 . A trigger switch  84 , which the operator can manipulate while holding the hold part  83 , is provided to the hold part  83 . 
     A coupling portion  85  for coupling a power cord is provided at the rear portion of the hold part  83 . Through the coupling portion  85 , electric power is supplied to the electrical components in the main housing  80  which include the drive machine  90  and the control circuit board  91 . 
     The drive machine  90  is provided between the suction port  81  and the discharge port  82  in the main housing  80 . The drive machine  90  includes a motor and a fan. The drive machine  90  takes in external air from the suction port  81  by the rotation of the fan, gives energy to the air taken in, and sends out the air at high speed toward the discharge port  82 . 
     In order to inhibit the operating noise generated by the motion of the drive machine  90  from being heard by the operator as noise, the blower  8  is further provided with a control speaker  87  and an error microphone  89 . 
     The control speaker  87  is arranged to have a vibrating surface perpendicular to a center axis C of the suction port  81 , and have the center of the vibrating surface on the center axis C of the suction port  81 . The control speaker  87  is configured to output a control sound from both sides of the vibrating surface, that is, front and rear sides, by vibration on the vibrating surface. 
     The control sound generated on the vibrating surface of the control speaker  87  propagates inward, which is a direction toward the inside of the main housing  80 , and outward, which is the opposite direction, along the center axis C of the suction port  81 . 
     The control speaker  87  is covered by a cover  86  outside the main housing  80 . A slit-shaped lid  810  is attached to the suction port  81 , and the cover  86  is arranged in the center of the lid  810 . 
     The error microphone  89  includes microphones  89 A,  89 B. The microphones  89 A,  89 B are arranged on concentric circles equidistant from the center axis C of the suction port  81 . Mounting portions  88  for the microphones  89 A,  89 B are provided around the slit-shaped lid  810 . The microphones  89 A,  89 B are mounted to the mounting portion  88  so as to be arranged on the concentric circles equidistant from the center axis C of the suction port  81 . 
     Sound signals from the microphones  89 A,  89 B are synthesized. The synthesized signal is used for ANC in the control circuit board  91  as a sound signal from the error microphone  89 . The sound signals as analog signals output from the microphones  89 A,  89 B may be synthesized, or may be first converted to digital signals and synthesized thereafter. According to one example, the error microphone  89  may be configured by a single microphone. Variations of the blower  8  may include an example in which the error microphone  89  includes only one of the microphones  89 A,  89 B. 
     The control circuit board  91  includes a motor controller  911  and a noise controller  912 . The motor controller  911  is configured to control the motor of the drive machine  90  in accordance with the operator&#39;s manipulation of the trigger switch  84 . The noise controller  912  is configured to reduce the operating noise generated by the motion of the drive machine  90  as target noise. 
     The noise controller  912  has a configuration corresponding to those of the A/D converter  445  and the D/A converter  446  shown in  FIG.  8   , and the control circuit  441  The noise controller  912  implements feedback ANC together with the control speaker  87  and the error microphone  89 . 
     [2.2.2. ANC Model] 
     Referring to  FIG.  16   , a feedback ANC model applied to the blower  8  will be described. The feedback ANC model implemented by the noise controller  912  merely partly differs from the feed-forward ANC model described by way of  FIG.  9   . Accordingly, the same reference numerals are affixed to components of the feedback ANC model configured in the same manner as those of the feed-forward ANC model, and descriptions thereof are not repeated. 
     As shown in  FIG.  16   , the feedback ANC model differs from the feed-forward ANC model in that the feedback ANC model does not have the reference sensor M 1  but an arrival filter M 7  and an adder M 8 . The control sound source M 2  corresponds to the control speaker  87  and the D/A converter  446 , and the error sensor M 3  corresponds to the error microphone  89  and the A/D converter  445 . 
     The noise control filter M 4 , the secondary system filter M 5 , the coefficient updater M 6 , the arrival filter M 7 , and the adder M 8  may be implemented by processing of a microcomputer when the noise controller  912  includes the microcomputer, or may be partly or entirely implemented by hardware. 
     The newly added arrival filter M 7  has the same configuration as that of the secondary system filter M 5 , and estimates an arrival signal a n  from the N control signals u n  that are most recently calculated. The arrival signal a n  represents pseudo-noise that has arrived to the error sensor M 3  from the control sound source M 2 . According to one example, the same fixed value as that for the secondary system filter M 5  may be used as the coefficient of each tap in the arrival filter M 7 . 
     The adder M 8  subtracts the error signal e n  from the arrival signal a n  to estimate a reference signal x n  that represents target noise. In other words, in the feedback ANC model, instead of the result of the detection by the reference sensor M 1 , the estimated result based on the control signals u n  and the error signal e n  is used as the reference signal x n . 
     The coefficient updater M 6  updates coefficients {w 1 , w 2 , . . . , w L } of L taps included in the noise control filter M 4  based on the filtered reference signal r n  obtained from the estimated reference signal x n  and the error signal e n , so that the target noise and the control sound cancel each other out at the position of the error sensor M 3  (that is, noise canceling point), and the error signal e n  becomes the smallest. This updating of the coefficient causes the target noise to be canceled by the control sound outside the main housing  80  from the suction port  81 . 
     [2.2.3. Arrangement of Microphone and Speaker] 
     In order to cancel the target noise, which is generated in the drive machine  90  and propagates outside the main housing  80  from the suction port  81 , with the control sound, and reduce propagation of the target noise outside the main housing  80 , the control speaker  87  and the error microphone  89  are arranged as below. 
     The control speaker  87  is arranged to have a vibrating surface parallel to the boundary plane P 0  between inside and outside the main housing  80  defined by the suction port  81  which is the open end of the main housing  80 . 
     The boundary plane P 0  is a virtual plane perpendicular to the center axis C of the suction port  81 , specifically a plane passing through the circular edge perpendicular to the center axis C of the suction port  81 . The control speaker  87  is arranged so that the vibrating surface is on the boundary plane P 0 , or the vibrating surface is located at a distance from the boundary plane P 0  in the forward and rearward direction. 
     The error microphone  89  is arranged so that a sound collection point is positioned in a section centered on the vibrating surface of the control speaker  87 , the section where the width in a direction normal to the vibrating surface is less than a specific distance D. Specifically, the error microphone  89  is arranged so that the sound collection point is positioned in a space between a plane P 1  at half the specific distance D away from the vibrating surface toward the inside of the main housing  80  and a plane P 2  at half the specific distance D away from the vibrating surface toward the outside of the main housing  80 .  FIG.  13    shows the planes P 1 , P 2 , assuming that the vibrating surface of the control speaker  87  is on the boundary plane P 0 . 
     The specific distance D is a quarter wavelength or one-sixth wavelength of the operating noise to be reduced, that is, the target noise. The operating noise generally includes the operating noise in the 1 kHz band or higher. When the speed of sound is 340 m/s, the wavelength of the sound is 340 mm. Accordingly, the error microphone  89  is arranged so that the sound collection point is in a section with a width of approximately 85 mm or 56.6 mm centered on the vibrating surface. 
     When the error microphone  89  is arranged so that the sound collection point is within one-sixth wavelength width centered on the vibrating surface of the control speaker  87 , it is possible to more effectively reduce the target noise, as compared to a case where the error microphone  89  is arranged so that the sound collection point is within a quarter wavelength width centered on the vibrating surface of the control speaker  87 . Furthermore, when the vibrating surface of the control speaker  87  and the sound collection point of the error microphone  89  are on the same plane, especially on the boundary plane P 0 , the target noise is more effectively reduced. 
     Here, why the arrangement of the control speaker  87  and the error microphone  89  under the above conditions is meaningful will be explained by theory. Generally, the sound pressure distribution of sound propagating in an acoustic tube can be expressed by the following formula. “k” is a wavenumber and k=2π/λ. “λ” corresponds to the wavelength of sound. 
         p   1 ( t,x )= A   1   e   j(ωt−kx)    
     The above corresponds to a sound pressure distribution of the target noise that propagates from the drive machine  90  to the suction port  81  in the tubular path in the main housing  80 . The forward direction of a variable x corresponds to the travelling direction of the target noise, and also corresponds to the direction from inside to outside the main housing  80 . 
     The sound pressure distribution of the control sound from the control speaker  87  can be expressed as below. The point where the variable x is zero is the point where there is the vibrating surface of the control speaker  87 . 
     
       
         
           
             
               
                 p 
                 2 
               
               ( 
               
                 t 
                 , 
                 x 
               
               ) 
             
             = 
             
               { 
               
                 
                   
                     
                       
                         A 
                         2 
                       
                       ⁢ 
                       
                         e 
                         
                           j 
                           ⁡ 
                           ( 
                           
                             
                               ω 
                               ⁢ 
                               t 
                             
                             - 
                             kx 
                             + 
                             φ 
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       ( 
                       
                         x 
                         ≥ 
                         0 
                       
                       ) 
                     
                   
                 
                 
                   
                     
                       
                         A 
                         2 
                       
                       ⁢ 
                       
                         e 
                         
                           j 
                           ⁡ 
                           ( 
                           
                             
                               ω 
                               ⁢ 
                               t 
                             
                             + 
                             kx 
                             + 
                             φ 
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       ( 
                       
                         x 
                         &lt; 
                         0 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     Accordingly, a combined sound at a point x=d (d&lt;0), which is at a distance d away from the vibrating surface toward the inside of the main housing  80 , will be expressed by the following formula. 
         p   1 ( t,d )+ p   2 ( t,d )= A   1   e   j(ωt−kd)   +A   2   e   j(ωt+kdφ)    
     The condition where the combined sound at the point x=d becomes zero is as follows. 
     
       
      
       A 
       1 
       =A 
       2  
      
     
       ω t−kd+π=ωt+kd+φ 
 
     In case that the aforementioned condition is satisfied, the sound pressure distribution on the outside of the main housing  80  from the vibrating surface is expressed as follows. 
         p   1 ( t,x )+ p   2 ( t,x )= A   1   e   j(ωt−kx) (1+ e   j(π−2kd) ) 
     Accordingly, when an inequation −π/2&lt;2kd&lt;π/2 is satisfied, the target noise is reduced outside the main housing  80 , as compared to a case where ANC is not performed. 
     The above inequation is satisfied when the distance d between the vibrating surface of the control speaker  87  and the sound collection point of the error microphone  89  is d&lt;λ/8, that is, less than one-eighth wavelength of the target noise, specifically when the error microphone  89  is in a section narrower than a width corresponding to a quarter wavelength of the target noise centered on the vibrating surface of the control speaker  87 . 
     Theoretically, the target noise is reduced to less than half, as compared to when ANC is not performed, when the distance d is less than one-twelfth wavelength of the target noise, that is, when the error microphone  89  is in a section narrower than a width corresponding to one-sixth wavelength of the target noise centered on the vibrating surface of the control speaker  87 . 
     [2.2.4. Effect of Blower] 
     The blower  8  of the present embodiment described in the above achieves the following effects. 
     (Effect 1) The operating noise from the drive machine  90  generated in the main housing  80  is canceled by a control sound from the control speaker  87  installed at the suction port  81  which is the end of the path extending from the drive machine  90 . Accordingly, it is possible to effectively reduce the operating noise of the blower  8  from being audible to the operator behind the blower  8  as unpleasant noise. 
     (Effect 2) Since the error microphone  89  is arranged within a quarter wavelength of the operating noise centered on the vibrating surface of the control speaker  87 , the operating noise can be effectively reduced. Especially, when the error microphone  89  is arranged within one-sixth wavelength of the operating noise centered on the vibrating surface, the operating noise can be all the more effectively reduced. 
     3. Others 
     (3.1) The technique of the present disclosure is not limited to application to the dust collector  1  and the blower  8 . The technique of the present disclosure may be applied to a work machine used in, for example, home carpentry, manufacturing, gardening, and/or construction work sites, in particular to a work machine that uses airflow from a fan. The technique of the present disclosure may be applied to a working machine for gardening, and/or a work machine that prepares a work site environment. For example, the technique of the present disclosure may be applied to various electric work machines such as electric lawn mower, electric grass trimmer, electric grass cutter, electric cleaner, electric blower, electric sprayer, electric spreader, electric dust collector, etc. 
     (3.2) A plurality of functions performed by a single element in the above-described embodiments may be achieved by a plurality of elements, or a function performed by a single element may be achieved by a plurality of elements. Also, a plurality of functions performed by a plurality of elements may be achieved by a single element, or a function performed by a plurality of elements may be achieved by a single element. Further, a part of a configuration in the above-described embodiments may be omitted. At least a part of a configuration in the above-described embodiments may be added to, or may replace, another configuration in the above-described embodiments.