Patent Publication Number: US-2022212751-A1

Title: Power assist device for bicycles, and bicycle

Description:
TECHNICAL FIELD 
     The present invention relates to an electric power assist device for bicycles, and a bicycle. In particular, the present invention relates to an electric power assist device for bicycles capable of estimating a pedaling power and controlling an electric motor for generating an assist power based on the estimated pedaling force, and a bicycle fitted with the electric power assist device. 
     BACKGROUND ART 
     Known power assisted bicycles include those configured to measure the distortion of a pedal system by using a strain gauge, calculate a pedaling force based on the measured distortion value, and control an electric motor based on the calculated pedaling force (See Patent Document 1), and those configured to detect a pedaling force by using a pedaling force sensor, the sensor including a pedaling force transmitting sleeve attached to a crankshaft driven by the pedal, and control an electric motor based on the detected pedaling force (See Patent Document 2). 
     PRIOR ART DOCUMENT(S) 
     Patent Document(s) 
     Patent Document 1: JP2007-091159A 
     Patent Document 2: US6196347B1 
     SUMMARY OF THE INVENTION 
     Task to be Accomplished by the Invention 
     Known technologies for power assisted bicycles as described above inconveniently involve complicated features for detecting a pedaling force. In particular, in order to use such technologies with existing bicycles, some modifications need to be made to the bicycles, which can be obstacles to conversion of existing bicycles to power assisted ones by attaching electric power assist devices to them. 
     The present invention has been made in view of the problem of the prior art, and a primary object of the present invention is to provide an electric power assist device for bicycles, which enables proper power assist control in response to a pedaling force without any complicated feature for detecting pedaling force and any necessary modification to the bicycle. 
     Means to Accomplish the Task 
     An aspect of the present invention provides an electric power-assist device for bicycles, the device comprising: a crankshaft ( 24 ) configured to be driven by a pedaling force transmitted from a pedal ( 30 ) via a crankarm ( 26 ,  28 ) on each side; an electric motor connected to the crankshaft or the crankarm of a bicycle in a torque transmitting relationship; a battery ( 122 ) which is provided in the bicycle ( 10 ) and serves as a power source for the electric motor; a housing ( 52 ) which can be attached to a frame structure ( 18 ) of the bicycle, wherein the electric motor is provided in the housing; a rotating member ( 64 ) rotatably supported by the housing, rotationally driven by the electric motor, and connected to the crankshaft or the crankarm of the bicycle in a torque transmitting relationship; and a control unit ( 150 ) for controlling operation of the electric motor, wherein the electric power assist device further comprises: a dynamic brake circuit ( 126 ,  170 ) connected to the electric motor; a rotational angle sensor ( 134 ) configured to detect a crank rotational angle of the crankshaft; and a current sensor ( 140 ) configured to detect a value of motor current supplied to the electric motor, wherein the control unit comprises: a dynamic brake controller ( 160 ) configured to stop supply of current from the battery to the electric motor and bring the dynamic brake circuit to an ON state when the crank rotational angle is within one or more predetermined crank rotational angle ranges; a pedaling force estimator ( 154 ) configured to estimate a pedaling force put on each pedal of the bicycle based on a value of reverse current caused by a counter electromotive force and detected by the current sensor when the dynamic brake circuit is in the ON state; and a motor drive controller ( 164 ) configured to control the electric motor based on the pedaling force estimated by the pedaling force estimator. 
     This configuration enables proper power assist control in response to a pedaling force without any complicated feature for detecting pedaling force and any necessary modification to the bicycle. 
     In this electric power assist device, preferably, the pedaling force estimator estimates the pedaling force to be greater with an increasing value of reverse current. 
     In this configuration, the estimation of pedaling force can be properly performed. 
     In this electric power assist device, preferably, the pedaling force estimator determines the pedaling force based on a value of effective reverse current, the value of effective reverse current being calculated by subtracting a value of reverse current caused by inertial rotation of the crankshaft from the value of reverse current caused by the counter electromotive force, when the dynamic brake circuit is in the ON state. 
     This configuration enables the accurate estimation of pedaling force. 
     In this electric power assist device, preferably, the motor drive controller causes the electric motor to stop operation when the value of effective reverse current is a negative value. 
     This configuration prevents the electric power assist device from performing unnecessary power assist when the bicycle is coasting with no pedaling. 
     In this electric power assist device, preferably, the dynamic brake controller brings the dynamic brake circuit to the ON state when the crank rotational angle of the crankshaft is within the one or more predetermined crank rotational angle ranges, wherein the one or more predetermined crank rotational angle ranges include (i) a crank rotational angle range including a crank rotational angle of 90 degrees from a reference crank rotational angle, the reference crank rotational angle being a crank rotational angle when the pedal of a crankarm on one side is located at its highest position, and/or (ii) a crank rotational angle range including a crank rotational angle of 270 degrees from the reference crank rotational angle. 
     In this configuration, the estimation of pedaling force can be properly performed based on a value of reverse current caused by a counter electromotive force. 
     In this electric power assist device, preferably, when the electric power-assist device is power-off, the dynamic brake controller brings the dynamic brake circuit to an always-ON state, and, after the electric power-assist device is turned on again, the dynamic brake controller brings the dynamic brake circuit to the ON state only when the crank rotational angle is within the one or more predetermined crank rotational angle ranges. 
     In this configuration, even though the dynamic brake circuit is in an always-ON state when the electric power-assist device is power-off, after the electric power-assist device is turned on again, the dynamic brake circuit returns to a normal state in which the dynamic brake circuit is in the ON state only when the crank rotational angle is within the predetermined crank rotational angle range(s). 
     In this electric power assist device, preferably, when the electric power-assist device is power-off, the dynamic brake controller brings the dynamic brake circuit to an always-ON state, and, after the electric power-assist device is turned on again and the crankshaft is manually rotated to preset crank rotational angles in a certain sequence, the dynamic brake controller brings the dynamic brake circuit to the ON state only when the crank rotational angle is within the predetermined crank rotational angle range(s). 
     In this configuration, the dynamic brake circuit can be in the always-ON state, making the bicycle harder to pedal, which serves as a theft-prevention measure. 
     In this electric power assist device, preferably, the electric power assist device further comprises a crank rotational direction determiner configured to determine whether the crankshaft is rotating in a forward direction or in a reverse direction based on crank rotational angles detected by the rotational angle sensor, wherein the motor drive controller causes the electric motor to stop operation when the rotational direction determiner determines that the crankshaft is rotating in the reverse direction. 
     This configuration prevents the electric power assist device from performing unnecessary power assist when the crankshaft rotates in the reverse direction. 
     In this electric power assist device, preferably, the electric power assist device further comprises a rotational movement sensor ( 136 ) configured to detect rotational movement of the crankshaft; and a pedaling force presence determiner ( 158 ) configured to determine whether or not a pedaling force is put on each pedal from the rotational movement of the crankshaft detected by the rotational movement sensor, wherein the motor drive controller causes the electric motor to stop operation when the pedaling force presence determiner determines that no pedaling force is put on the pedals. 
     This configuration prevents the electric power assist device from performing unnecessary power assist when there is no pedaling force on the pedal. 
     In this electric power assist device, preferably, the electric power assist device further comprises a gyro sensor ( 138 ) for detecting a tilt angle of the bicycle with respect to a direction of gravity, wherein the motor drive controller corrects a rotational output of the electric motor based on the tilt angle detected by the gyro sensor. 
     This configuration enables the power assist to be performed as needed by the rider, and improves the safety. 
     In this electric power assist device, preferably, the electric power assist device further comprises a voltage sensor ( 140 ) for detecting a voltage of the battery, wherein the motor drive controller reduces a rotational output of the electric motor in response to a voltage drop detected by the voltage sensor. 
     This configuration prevents over-discharging of the battery. 
     Another aspect of the present invention provides a bicycle fitted with the above-described electric power assist device. 
     This configuration enables proper power assist control in response to a pedaling force without any complicated feature for detecting pedaling force and any necessary modification to the bicycle. 
     Effect of the Invention 
     In an electric power assist device and a bicycle according to the present invention, proper power assist control in response to a pedaling force can be performed without any complicated feature for detecting pedaling force and any necessary modification to the bicycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an electric power assist device according to a first embodiment of the present invention and a bicycle fitted with the electric power assist device; 
         FIG. 2  is a fragmentary perspective view of the electric power assist device and the bicycle of the first embodiment; 
         FIG. 3  is a fragmentary exploded perspective view of the electric power assist device and the bicycle of the first embodiment; 
         FIG. 4  is a block diagram of a power assist control system for the electric power assist device and a bicycle fitted with the electric power assist device of the first embodiment; 
         FIG. 5  is an explanatory view showing the relationship between the crank rotational angle and the position of the pedal of the bicycle; 
         FIG. 6  is a graph showing the relationship between the crank rotational angle and the motor current in the bicycle 
         FIG. 7  is a flowchart of power assist control of the electric power assist device of the first embodiment; and 
         FIG. 8  is a block diagram of a power assist control system for the electric power assist device according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     An electric power assist device and a bicycle according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 6 . 
     As shown in  FIGS. 1 to 3 , a bicycle  10  is provided with a frame structure  18  that includes a seat tube  12  extending substantially in the vertical direction and having a saddle (not shown) attached to the upper end thereof, a down tube  14  extending substantially in the fore and aft direction, and left and right chain stays  16 . The lower end of the seat tube  12 , the rear end of the down tube  14 , and the front ends of the chain stays  16  are connected to one another by a bearing tube  20  for supporting a crankshaft and also serving as a pipe joint. 
     The bearing tube  20  rotatably supports a crankshaft  24  (see  FIG. 3 ) extending substantially horizontally in the lateral direction. The left and right shaft ends of the crankshaft  24  project out of the bearing tube  20 , and the base ends of the left and right crankarms  26  and  28  are fixed to the respective shaft ends of the crankshaft  24  with a rotational phase difference of 180 degrees. The crankshaft  24  forms the rotational center of the crankarms  26  and  28 , and the rotational center axis of the crankshaft  24  and the rotational center axis of the crankarms  26  and  28  coincide with each other. 
     A pedal  30  is attached to the free end of each of the crankarms  26 ,  28 . A drive sprocket  32  (chain wheel) is positioned between the crankarm  28  on the right side and the bearing tube  20 . The drive sprocket  32  is coaxially connected (fixed) to the crankshaft  24 . 
     The crankshaft  24  can be rotationally driven by the left and right crankarms  26  and  28 . The rotation of the crankshaft  24  is transmitted to the drive sprocket  32 , and is transmitted from the drive sprocket  32  to the rear wheel (not shown) by a chain transmission mechanism (not shown). As a result, the electric power assisted bicycle  10  travels forward. A one-way clutch (not shown) is provided between the chain transmission mechanism and the rear wheel, and the rear wheel inertially rotate when the pedaling force is not put on the pedal  30  and the crankshaft  24  does not rotate. The above described configuration is one of a typical structure of a bicycle  10 . 
     The bicycle  10  is provided with a unitized and retrofittable electric power assist device (electric power assist unit)  50 . In the following description, the various directions such as up/down, front/rear, and right/left are based on the state where the electric power assist device  50  is attached to the frame structure  18  of the electric power assisted bicycle  10  as shown in  FIGS. 1 and 2 . 
     The electric power assist device  50  is provided with a housing  52  having a hollow structure. The housing  52  includes a ring portion  54  and a tongue shaped extension portion  56  extending radially outward from the ring portion  54 . An electric motor  58  for generating an assist force is attached to the right surface of the extension portion  56 . One end of the electric motor  58  is fixed to the extension portion  56  so that the rotational axis of the rotor output shaft (not shown) is directed in the lateral direction. 
     As shown in  FIG. 3 , the ring portion  54  includes a cylindrical portion  62  centrally defining a central opening  60  that is open in the lateral direction, The cylindrical portion  62  rotatably supports an annular rotational output member  64  on the outer periphery thereof. The cylindrical portion  62 , together with the rotational output member  64 , is disposed between the frame structure  18  and the crankarm  26  in a coaxial relationship with the crankshaft  24  while the crankshaft  24  extends laterally through the central opening  60  in a loosely received state. The rotational output member  64  is connected to the electric motor  58  in a torque transmitting relationship via a gear train (not shown) provided in the housing  52 , and is thereby rotationally driven by the electric motor  58  in a coaxial relationship with the crankshaft  24 . 
     The cylindrical portion  62  and the rotational output member  64  are installed between the frame structure  18  and the crankarm  26  by the following procedure. First, the left pedal  30  which is on the side not fitted with the drive sprocket  32  is removed by using a common tool such as a spanner. Next, with the electric power assist device  50  tilted sideways (the posture in which the electric motor  58  faces upward), the free end side of the left crankarm  26  is inserted into the central opening  60 , and with the crankarm  26  passed into the central opening  60 , the electric power assist device  50  is moved toward the base end side (rotational center side) of the crankarm  26  along the extending direction of the crankarm  26 . 
     As a result, the crankarm  26  is passed through the cylindrical portion  62  and the rotational output member  64  until the cylindrical portion  62  and the rotational output member  64  are positioned near the base end of the crankarm  26 . In other words, the inner diameter of the central opening  60  is dimensioned so that the crankarm  26  may be passed through the central opening  60 . 
     If the central opening  60  has a large enough inner diameter to allow the pedal  30  attached to a crankarm  26  to be passed through, the assembly work can be performed without requiring the pedal  30  to be removed. 
     Next, the electric power assist device  50  is oriented to the normal posture (the posture shown in  FIG. 2 ) in which the electric motor  58  faces sideways, and the crankshaft  24  is loosely passed into the central opening  60  in the axial direction. As a result, the cylindrical portion  62  and the rotational output member  64  can be positioned between the frame structure  18  and the crankarm  26  with the crankshaft  24  extending laterally and loosely received in the central opening  60  simply by removing the pedal  30  or without even requiring the pedal  30  to be removed. 
     The rotational output member  64  is connected to the crankshaft  24  and the crankarm  26  by a connecting mechanism  70 . The connecting mechanism  70  includes a connecting main member  72  and two clamp piece members  74 . A crankshaft fitting hole  26 A is defined at the base end of the crankarm  26 . A crank connecting member  78  is fitted in the crankshaft fitting hole  26 A. 
     The connecting main member  72  has a substantially circular disk shape, and is fixed to the rotational output member  64  at the peripheral edge thereof by a plurality of bolts  75  and to the crank connecting member  78  at the central part thereof by a bolt  76 . As a result, the rotational output member  64  is coaxially positioned relative to the crankshaft  24  via the connecting main member  72  and the crankarm  26 . 
     The two clamp piece members  74 , each having a wedge shape, are positioned on either side of the base end part of the crankarm  26  with respect to the rotational direction thereof, such that each clamp piece member  74  is in contact with an angled edge  73  of a corresponding connecting main member  72 . A bolt  80  is provided for each clamp piece member  74 , and as the bolts  80  are tightened to connect the clamp piece members  74  to the connecting main member  72 , the clamp piece members  74  are caused to slide toward each other guided by the angled edges with interposing the crankarm  26  from both sides with respect to the rotational direction, so that the crankarm  26  and the connecting main member  72  are connected to each other in a torque transmitting relationship. 
     As a result, the rotational output member  64  is coaxially connected to the crankshaft  24  in a torque transmitting relationship, via the connecting main member  72  and the crankarm  26 , and thus the rotational output member  64  and the connecting main member  72  can rotate together with the crankshaft  24 . The rotational output member  64  and the connecting main member  72  are collectively referred to as a rotating member. 
     The clamp piece members  74  define bolt through-holes  81 , through which the bolts  80  are passed. The bolt through-holes  81  are each formed in an oval shape so that, as the bolts  80  are tightened to connect the clamp piece members  74  to the connecting main member  72 , the clamp piece members  74  can slide toward each other. 
     The extension portion  56  of the housing  52  is positioned under the down tube  14  while supporting the electric motor  58 . The extension portion  56  is supported by and suspended from the down tube  14  via a support mechanism  90 . 
     The support mechanism  90  includes a frame-side mount member  92 . The frame-side mount member  92  includes a support base member  98  fixed to the down tube  14  by a fastening band  94  and provided with a rectangular frame structure part  96  in a lower part thereof, and a support member  104 , the support member  104  having a rectangular plate-shaped part  100  fitted into the rectangular frame structure part  96  and fixed to the support base member  98 , and a depending piece  102  depending downward from the rectangular plate-shaped part  100  and extending in the fore and aft direction. 
     The depending piece  102  is a cantilever piece, and defines a through-hole  103  which extends in the axial direction of the crankshaft  24  (or in the lateral direction) and is provided with an annular shoulder. A cylindrical fixed-side bush  106  is fitted in the through-hole  103  of the depending piece  102  in a rotationally fast manner 
     A female screw  108  is formed on the inner circumferential surface of the large diameter portion  106 B. A male screw  109  formed on the outer circumferential surface of a movable-side bush  110  is threaded with the female screw  108  so that the movable-side bush  110  can be threaded into and out of the large diameter portion  106 B in the axial direction of the crankshaft  24 , or in the lateral direction. 
     The movable-side bush  110  is provided with a flange part  112  on the side remote from the fixed-side bush  106 . The flange part  112  is provided with an uneven shape similar to flower petals so that the movable-side bush  110  can be turned by hand. The flange surface of the flange part  112  is in direct contact with the laterally inwardly facing end surface of a boss part (connecting part)  66  formed on an upper part of the extension portion  56  of the housing  52 . 
     The frame-side mount member  92  fixedly supports the housing  52  with a fastening bolt  114  which is centrally passed through the fixed-side bush  106  and the movable-side bush  110  in the axial direction of the crankshaft  24 , and threaded into a screw hole  68  (not shown) of the boss part  66 . 
     In this way, the fixed-side bush  106  and the movable-side bush  110  form a screw mechanism with the female screw  108  and the male screw  109  extending in the axial direction of the crankshaft  24  between the housing  52  and the frame structure  18 , and the screw mechanism serves as an adjustment mechanism capable of adjusting (increasing or decreasing) the distance between the frame-side mount member  92  and the housing  52  along the axial direction of the crankshaft  24 . 
     By suitably adjusting the distance along the axial direction, the tilting of the rotational output member  64  with respect to the central axis (crankshaft line) of the crankshaft  24  can be corrected, and the posture of the rotational output member  64  can be adjusted so that the rotational output member  64  extends along a plane orthogonal to the crankshaft axial line. 
     Triangular marks  116  and  118  (marks [1] to [3]  118 ) that are externally visible are printed on the outer surfaces of the extension portion  56  and the connecting main member  72  of the housing  52 , respectively, and alignment of the marks indicates that the connecting main member  72 , (i.e., the rotational output member  64  integrated with the connecting main member  72 ) is rotated to a predetermined rotational position with respect to the housing  52 . As shown in  FIG. 1 , the mark  116  and the marks [1] to [3]  118  are aligned when the angle β formed by the straight line (unit center line) C and the straight line (crankarm line) E is 90 degrees, where the unit center line C passes through the rotation center point A of the crankshaft  24  (that of the rotational output member  64 ) and the rotation center point B of the electric motor  58 , and the crankarm line E passes through the rotation center point A of the crankshaft  24  and the rotation center point D of the pedal  30 . The marks [1] to [3]  118  are numbers on a dial and printed at equal intervals from each other in the direction of rotation of the connecting main member  72 . 
     The mounting angle of the housing  52  with respect to the frame structure  18  determines the angle γ formed by the vertical line V along the direction of gravity and the unit center line C, which is detected based on an output signal of a gyro sensor  138  described later. When the mark  116  and the mark [1] of the marks  118  are aligned; that is, when the crankarm  26  is rotated to the rotational angle β, the angle α formed by the vertical line V and the crankarm line E is β−γ (i.e, α=90−γ), and a rotational angle sensor  134  is calibrated based on the angle α as described later by a calibrator  152  in a control unit  150 . 
     The extension portion  56  of the housing  52  contains a control unit  150  (see  FIG. 4 ) for electric power assist therein. A battery  122  consisting of a secondary battery that serves as a power source for the electric motor  58  and the control unit  150  is attached to the seat tube  12  by a fastening band (not shown) or any other fastening means. 
     As shown in  FIG. 4 , the electric motor  58  is a three-phase AC motor having windings LU, LV, and LW of U-phase, V-phase, and W-phase, respectively, and electric current (electric power) is supplied from the battery  122  to each of the windings LU, LV, and LW via a power switch  124  and an inverter  126 . 
     The inverter  126  serves as a power converter that is PWM-controlled by the control unit  150 . In the inverter  126 , for each phase (U phase, V phase, W phase), two sets of parallel circuits, each consisting of a power transistor  128  and a diode  130 , are connected in series, so that the voltage applied to each phase of the electric motor  58  can be quantitatively adjusted by pulse width modulation. 
     When all the power transistors  128  of the respective phases are simultaneously in the ON state, allowing the windings LU, LV, and LW to be short-circuited to each other, the inverter  126  operates as a dynamic brake circuit. In other words, the inverter  126  serves both as a power converter and a dynamic brake circuit. 
     Next, the control system of the electric power assist device  50  will be described with reference to  FIG. 4 . 
     A rotational angle sensor  134 , a pulse sensor  136 , a gyro sensor  138 , a current sensor  140 , and a voltage sensor  142  are attached to the electric power assist device  50 . 
     The rotational angle sensor  134  is provided on the electric motor  58  or the housing  52 , and configured to detect the motor rotational angle or the rotational angle (absolute angle) of the rotational output member  64 . When the housing  52  of the electric power assist device  50  is attached to the frame structure  18  of the bicycle  10  and the rotational output member  64  is connected to the crankshaft  24  by the connecting mechanism  70 , the rotational angle sensor  134  detects the crank rotational angle (absolute angle) of the crankshaft  24 . 
     As shown in  FIG. 5 , the zero point (origin) of the absolute angle that is detected by the rotational angle sensor  134  is set to the crank rotational angle of the crankshaft  24  when the pedal  30  of the crankarm  26  is located at the highest position. 
     In the following description, based on the premise that the electric power assist device  50  is attached to the frame structure  18  for the sake of simplicity, the rotational angle sensor  134  is sometimes described as being configured to detect the crank rotational angle of the crankshaft  24 , which is equivalent to the motor rotational angle or the rotational angle of the rotational output member  64 . 
     The pulse sensor  136  is provided in the housing  52  and detects the rotation of the crankshaft  24  and thus the rotational output member  64 . 
     The gyro sensor  138  is provided in the housing  52  and detects the tilt angle of the rotational output member  64  (crankshaft  24 ). The gyro sensor  138  forms part of the tilt angle detector. Specifically, based on the output signal of the gyro sensor  138 , the tilt angle detector detects the tilt angle of the electric power assist device  50  in the left-right direction and the fore and aft direction with respect to the direction of gravity; that is, the vertical line V. 
     The current sensor  140  detects the current values (motor current value (Im) and reverse current value (Idb)) of the connection lines  59 U,  59 V,  59 W for the respective phases, each connection line connecting the electric motor  58  and the inverter  126 . Although the current values may be detected for all three phases, the detection of the current values of at least two of the three phases is sufficient for controlling the electric power assist device. 
     The voltage sensor  142  detects the voltage of the battery  122 . 
     The control unit  150  is an electronically controlled device including a microcomputer and other components. The control unit includes a calibrator  152 , a pedaling force estimator  154 , a crank rotational direction determiner  156 , a pedaling force presence determiner  158 , a dynamic brake controller  160 , an effective reverse current calculator  162 , a motor drive controller  164 . 
     The calibrator  152  calibrates the rotational angle sensor  134 , setting the reference absolute angle measured by the rotational angle sensor  134  on a flat ground based on the direction of gravity detected based on the output signal from the gyro sensor  138 . 
     When the rotational angle sensor  134  is calibrated by setting the reference absolute angle of the rotational angle sensor  134  (detecting the origin angular position) based on the angle α calculated by α=90−γ, where γ is determined by the mounting angle of the housing  52  with respect to the frame structure  18  when the mark  116  and the mark [1]  118  are aligned; that is, when the rotational angle β of the crankarm  26  is 90 degrees. 
     A user can align the mark  116  with the mark [1]  118  by turning the crankarm  26  in the reverse direction, or turning the crankarm  26  with the rear wheel (not shown) of the bicycle  10  away from the road surface. When the mark  116  with the mark [1]  118  are aligned for a predetermined period of time, the calibrator  152  performs the calibration of the rotational angle sensor  134  as described above. 
     As a result, even when the mounting angle of the electric power assist device  50  with respect to the bicycle  10  varies, the electric power assist device can perform proper power assist control through the proper detection of the crank rotational angles (absolute angles). 
     For the details of the calibration of the rotational angle sensor  134 , refer to the international patent publication (WO2020/217664A) of the Applicant&#39;s international patent application based on the Japanese Patent Application No. 2019-084258. 
     The pedaling force estimator  154  estimates the pedaling force of the bicycle  10  based on a value of effective reverse current (Ier) calculated by the effective reverse current calculator  162  described later. The pedaling force estimator  154  estimates the pedaling force to be greater with an increasing value of the reverse current (Ier). Specifically, the greater the value of the reverse current is, the greater the estimated pedaling force is, and the smaller the value of the reverse current is, the smatter the estimated pedaling force is. When the value of effective reverse current is a negative value, the pedaling force estimator  154  determines that the pedaling force is zero, thereby causing the electric motor  58  to stop operation. 
     When a value of reverse current (Idb) caused by a counter electromotive force, which occurs while the dynamic brake circuit is in an ON state as described later, is not stable, the pedaling force estimator  154  estimates the pedaling force of the bicycle  10  based on the values of motor current detected by the current sensor  140  at predetermined crank rotation angles, or the angular velocity or acceleration values calculated from the change in the crank rotation angle θ detected by the rotational angle sensor  134 . For the details of the estimation of the pedaling force, refer to the international patent publication (WO2020/158280A) of the Applicant&#39;s international patent application based on the Japanese Patent Application No. 2019-052798. 
     The motor drive controller  164 , which is configured to generate PWM signals, outputs a drive torque control command (PWM signal) to the inverter  126  so as to operate the electric motor  58  with an electric power according primarily to the pedaling force estimated by the pedaling force estimator  154 . 
     Under a condition that the motor drive controller  164  provides PWM signals to the inverter, the inverter  126  serves as a power converter; that is, quantitatively sets the electric power to be supplied from the battery  122  to the electric motor  58 . As a result, the electric motor  58  assists the pedaling with the drive torque according to the estimated value of the pedaling force. 
     The crank rotational direction determiner  156  determines whether the crankshaft  24  is rotating in a forward direction or in a reverse direction based on the crank rotational angles detected by the rotational angle sensor  134 . When the crank rotational direction determiner  156  determines that the crankshaft  24  rotates in the reverse direction, the motor drive controller  164  performs control to cause the electric motor  58  to stop operating. This prevents unnecessary power assist from being applied when the crankshaft  24  rotates in the reverse direction. 
     The pedaling force presence determiner  158  determines whether or not a pedaling force is put on the pedal  30  from the rotational movement of the crankshaft  24  detected by the rotational angle sensor  134 . When the pedaling force presence determiner  158  determines there is no pedaling force on the pedal, the motor drive controller  164  performs control to cause the electric motor  58  to stop operating. This prevents unnecessary power assist from being applied when there is no pedaling force on the pedal. 
     The motor drive controller  164  further performs correction control to increase or decrease the drive torque of the electric motor  58  according to the tilt angle with respect to the direction of gravity detected based on the output signal from the gyro sensor  138 ; that is, according to the tilts of the bicycle  10  in the left-right direction and the fore and aft direction with respect to the direction of gravity. This feature enables the power assist to be performed as needed by the rider, and improves the safety. For example, when the bicycle  10  is tilted to the left or right during turning, the motor drive controller decreases the assist force, and when the bicycle  10  is tilted in the fore and aft direction on an uphill road, the motor drive controller increases the assist force. 
     The motor drive controller  164  further performs control to reduce the rotational output of the electric motor  58  in response to the decrease in the battery voltage detected by the voltage sensor  142 . This feature prevents over-discharging of the battery  122 , thereby extending the life of the battery  122 . This feature can also reduce the power consumption of the battery  122 , thereby extending the power assist available distance (duration time) on one charge of the battery  122 . 
     When the crank rotational angle detected by the rotational angle sensor  134  is within one or more predetermined crank rotational angle ranges, the dynamic brake controller  160  stops supply of current from the battery  122  to the electric motor  58  (blocking the motor current) and brings all the power transistors  128  of the respective phases in the inverter  126  to the ON state at the same time so that the inverter  126  can operate as a dynamic brake circuit. When the inverter operates in this dynamic brake state (when the dynamic brake circuit is in an ON state), the electric motor  58  is rotationally driven by a pedaling force to generate a corresponding counter electromotive force. 
     The counter electromotive force is rather small when the pedaling force is decreased because the blocking of motor current results in a significant reduction in the speed in that case. The counter electromotive force is large when the assist power is small and the pedaling force is increased because the blocking of motor current does not lead to a significant drop in the speed. 
     As shown in  FIG. 5 , the dynamic brake controller  160  brings the dynamic brake circuit to an ON state while blocking the motor current when the crank rotational angle of the crankshaft is within predetermined crank rotational angle ranges, where the predetermined crank rotational angle ranges include (i) a crank rotational angle range θ 1  including a crank rotational angle of 90 degrees in the pedaling direction from a reference crank rotational angle (zero point), the reference crank rotational angle being a crank rotational angle when the pedal  30  of a crankarm on one side is located at its highest position, and (ii) a crank rotational angle range θ 2  including a crank rotational angle of 270 degrees in the pedaling direction from the reference crank rotational angle. As a result, as shown in  FIG. 6 , a counter electromotive force occurs in each of the crank rotational angle range θ 1  and the crank rotational angle range θ 2  to generate a reverse current with reverse polarity from that of the motor current supplied from the battery  122 . Each of the crank rotational angle ranges θ 1  and θ 2  may be between 2 degrees to 7 degrees, preferably about 5 degrees. 
     The effective reverse current calculator  162  receives a value of reverse current (Idb) caused by the above-described counter electromotive force from the current sensor  140 , and calculates the value of reverse current (Iig) caused by inertial rotation (see  FIG. 6 ) based on a motor reverse current i, a rotational speed of the electric motor (Nm) and a rotational speed of the crankshaft (Nc) by the following equation (1), and also calculates the value of effective reverse current (Ier) by subtracting the value of reverse current (Iig) from the value of reverse current (Idb) by the following equation ( 2 ). 
         Iig={Jm·α 1 ·Nm +( Jc+Jo ) R 2·α2 ·Nc +10 ·Kt·i }/(10· V )  (1)
 
     where 
     Jm (kg/m 2 ): moment of inertia of the electric motor  58 , 
     Jc (kg/m 2 ): moment of inertia of the crank mechanism including the crankarms  26  and  28 , 
     Jo (kg/m 2 ): moment of inertia of the rotational output part including the rotational output member  64 , 
     R: gear ratio of a gear train, 
     Kt: torque constant for the electric motor  58 , 
     α 1  (rad/sec 2 ): motor angular acceleration, 
     α 2  (rad/sec 2 ): crank angular acceleration, 
     Nm (rpm): rotational speed of the electric motor, 
     Nc (rpm): rotation speed of the crankshaft, 
     i (A): value of motor residual current, and 
     V (V): system voltage. 
         Ier=Idb−Iig   (2)
 
     In this way, the value of effective reverse current (Ier) is calculated for each of the crank rotational angle range θ 1  and the crank rotational angle range θ 2 ; that is, for every half rotation of the crankshaft  24 , as shown in  FIG. 6 . 
     The crank rotational angle range θ 1  and the crank rotational angle range θ 2  are crank rotational angle ranges in which the pedaling force put on each of the pedals  30  of the left and right sides is effectively used to drive the bicycle, the value of reverse current (Idb) caused by the counter electromotive force properly reflects the pedaling force. As a result, the value of effective reverse current (Ier) reflecting the pedaling force can be calculated for every half rotation of the crankshaft  24 . 
     This configuration enables proper power assist control in response to a pedaling force without any complicated feature for detecting pedaling force and any necessary modification to the bicycle  10 . 
     When the power switch  124  is turned off, the dynamic brake controller  160  brings the dynamic brake circuit to an always-ON state, and after the power switch  124  is turned on again and the crankshaft is manually rotated to preset crank rotational angles in a certain sequence, the dynamic brake controller  160  returns the dynamic brake circuit to a normal state in which the dynamic brake circuit is in the ON state only when the crank rotational angle is within the predetermined crank rotational angle range as described above. 
     In this configuration, the dynamic brake circuit is maintained in the always-ON state so that a pedaling force required to move the bicycle is sufficiently large to the extent that would not allow a rider to keep the bicycle moving and balanced, until the control unit  150  confirms, based on the output signal from the rotational angle sensor  134 , that the connecting main member  72  is rotated to the preset crank rotational angles in the certain sequence, for example, the connecting main member  72  is rotated such that the respective marks  118  are sequentially aligned with the mark  116  such that the numbers of the aligned marks are in a sequence of [1]-[3]-[2] (dial adjustment operation). 
     When this sequence of numbers for dial adjustment is kept secret by the user, the configuration with these marks  116  and  118  serves as a dial-lock-type lock mechanism. Specifically, as the dynamic brake circuit is maintained in the always-ON state and thus a pedaling force required to move the bicycle is sufficiently large to the extent that would not allow a rider to keep the bicycle moving and balanced, this configuration provides an anti-theft feature. 
     This dial-lock-type anti-theft feature can be disabled by providing a mode setting command to the control unit  150 . When the anti-theft feature is disabled, the dynamic brake circuit returns to a normal state in which the dynamic brake circuit is in the ON state only when the crank rotational angle is within the predetermined crank rotational angle ranges as described above. 
     Next, a power assist control routine executed by the control unit  150  will be described with reference to the flowchart shown in  FIG. 7 . 
     This control routine is started when the power switch  124  of the electric power assist device  50  is turned on. First, power is fed to each of the sensors  134 ,  136 ,  138 ,  140 , and  142 , and the control unit  150  confirms that the dial adjustment operation, in which marks  118  are sequentially aligned with the mark  116 , is performed in a proper manner (step S 10 ). The dynamic brake controller  160  maintains the dynamic brake circuit in the always-ON state, to which the dynamic brake circuit was brought when the power switch  124  was previously turned off, until the dial adjustment operation is properly performed. This control provides an anti-theft feature. 
     When the dial adjustment operation is properly performed, the dynamic brake controller  160  brings the dynamic brake circuit to an OFF state (step S 11 ), and then the calibrator  152  performs the calibration of the rotational angle sensor  134  (step S 12 ). 
     When the calibration is completed, the control unit  150  performs a standby state entry process, causing the electric power assist device  50  to enter a standby state (step S 13 ). The standby state entry process involves stopping the electric motor  58 . 
     Next, the control unit  150  determines whether or not the power switch  124  of the electric power assist device  50  is turned off (step S 14 ). When the power switch  124  is turned off, the dynamic brake controller  160  brings the dynamic brake circuit to the always-ON state (step S 15 ) and performs a power-off process (step S 16 ). The power-off process involves stopping the power supply to each of the sensors  134 ,  136 ,  138 ,  140  and  142 . 
     When the power switch  124  is not turned off, the crank rotational direction determiner  156  determines whether or not the electric motor  58  rotates in the reverse direction (step S 17 ). When the electric motor  58  is rotating in the reverse direction, the control routine returns to the standby state entry process (step S 13 ). 
     When the electric motor  58  is not rotating in the reverse direction, the control unit  150  confirms that the electric motor  58  has made one rotation (step S 18 ), and then starts the electric motor  58  (step S 19 ). 
     Next, when the crank rotational angle is within a predetermined ranges θ 1  or θ 2 , the dynamic brake controller  160  stops supply of current to the electric motor  58  (blocking the motor current) and brings the dynamic brake circuit to the ON state (step S 20 ). Under this condition that the motor current is blocked and the dynamic brake circuit is in the ON-state, a counter electromotive force occurs in the electric motor  58 , and the current sensor  140  detects a value of reverse current (Idb) caused by the counter electromotive force (step S 21 ). 
     Next, the control unit  150  determines whether or not the reverse current value (Idb) is stable (step S 22 ). When the reverse current value (Idb) is stable, the effective reverse current calculator  162  calculates a value of reverse current (Iig) caused by inertial rotation (step S 23 ). 
     Next, the control unit  150  determines whether or not the value of reverse current Idb−Iig is greater than zero (&gt;0) (step S 24 ). When the value of reverse current Idb−Iig is not greater than zero; that is, when the value of effective reverse current (Ier) is zero or a negative value, the control unit  150  determines that the vehicle is coasting, which requires no power assist, and the control routine returns to the standby state entry process (step S 13 ). 
     When the value of reverse current Idb−Iig is greater than zero (&gt;0), the pedaling force estimator  154  estimates the pedaling force of the bicycle  10  based on a value of effective reverse current (Ier) (step S 25 ), and the control unit  150  calculates a motor drive output (PWM signal) based on the estimated pedaling force (step S 26 ). 
     Next, the motor drive controller  164  performs a motor drive process based on the calculated motor drive output (step S 27 ). The motor drive process involves energizing the electric motor  58  with electric power corresponding to the calculated motor drive output. As a result, the electric motor can assist the pedaling in response to the pedaling force. 
     In the step  22  of determining whether or not the reverse current value (Idb) is stable, when determining that the reverse current value (Idb) is not stable, the pedaling force estimator  154  estimates the pedaling force based on the values of motor current, or the angular velocity or angular acceleration values calculated from the change in the crank rotation angle θ (step S 28 ), and the control unit  150  calculates a motor drive output (PWM signal) based on the estimated pedaling force. (step S 26 ). 
     This configuration enables proper power assist control in response to a pedaling force without any complicated feature for detecting pedaling force and any necessary modification to the bicycle  10 , and also enables the power assist to be performed as needed by the rider. 
     The present invention has been described in terms of specific embodiments, but is not limited by such embodiments, and can be modified in various ways without departing from the scope of the present invention. For example, in a second embodiment of the present invention shown in  FIG. 8 , instead of allowing the windings to be short-circuited to each other in the inverter  126 , a dynamic brake circuit  170  including resistors Ru, Rv, and Rw may be used, wherein each of the resistors Ru, Rv, and Rw is connected to a corresponding phase of the electric motor  58  by a normally closed relay switch  172 . The effective reverse current value (Ier) may be determined not by calculation, but by searching a data map including parameters such as remaining motor current value and rotational speed of the crankshaft. In addition, all of the components shown in the above-described embodiments are not necessarily essential for the present invention, but can be appropriately omitted and substituted as long as such omission and substitution do not deviate from the gist of the present invention. 
     GLOSSARY 
       10  bicycle 
       12  Seat tube 
       14 : down tube 
       16  chain stay 
       18  frame structure 
       20  bearing tube 
       24  crankshaft 
       26  crankarm 
       26 A crankshaft fitting hole 
       27  crankarm mounting screw 
       28  crankarm 
       30  pedal 
       32  drive sprocket 
       50  electric power assist device 
       52  housing 
       54  ring potion 
       56  extension potion 
       58  electric motor 
       60  central opening 
       62  cylindrical potion 
       64  rotational output member 
       66  boss part 
       70  connecting mechanism 
       72  connecting main member 
       73  angled edge 
       74  clamp piece member 
       75  bolt 
       76  bolt 
       78  crank connecting member 
       80  bolts 
       81  through-hole 
       90  support mechanism 
       92  mount member 
       94  fastening band 
       96  rectangular frame structure part 
       98  support base member 
       100  rectangular plate-shaped part 
       102  depending piece 
       103  through-hole 
       104  support member 
       106  fixed-side bush 
       106 B large diameter potion 
       108  female screw 
       109  male screw 
       110  movable-side bush 
       112  flange part 
       114  fastening bolt 
       116  mark 
       118  mark 
       122  battery 
       124  power switch 
       126  inverter 
       128  power transistor 
       130  diode 
       134  rotational angle sensor 
       136  pulse sensor (rotation sensor) 
       138  gyro sensor 
       140  current sensor 
       142  voltage sensor 
       150  control unit 
       152  calibrator 
       154  pedaling force estimator 
       156  crank rotational direction determiner 
       158  pedaling force presence determiner 
       160  dynamic brake controller 
       162  effective reverse current calculator 
       164  motor drive controller 
       170  dynamic brake circuit 
       172  normally closed relay switch 
     LU winding 
     LV winding 
     LW winding 
     Ru resister 
     Rv resister 
     Rw resister