Abstract:
A contactless detection apparatus has a first magnet ring, a second magnet ring, a first magnetic sensor, a second magnetic sensor and a controller. The two magnet rings are respectively mounted on two ends of a torsion shaft. When the torsion shaft rotates, the controller detects the magnetic fields of the two magnet rings through the two magnetic sensors. The controller calculates a twisting torque exerted on the torsion shaft and a rotational angle of the torsion shaft according to the detected magnetic fields at the same time. The detection apparatus of the invention has simple structure. The magnetic fields of both magnet rings do not interfere with each other, such that the detection result of the invention is accurate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a detection apparatus, and more particularly to a contactless detection apparatus and method for detecting a rotation direction. 
         [0003]    2. Description of Related Art 
         [0004]    In response to the concept of the environmental awareness and the exercise regimen, more and more people take a bike as a daily exercising device or a daily commuting tool. However, the riding distance and the terrain are not proper for everyone. Not everyone has enough physical strength to sustain through the riding action. As a result, an electric bike is manufactured to assist the exerciser with riding a bike. 
         [0005]    A conventional electric bike has a throttle switch and a motor installed on a bike. The motor is used to provide a pushing force to move the bike ahead. When the exerciser wants to activate the motor, the exerciser has to manually turn on the throttle switch. However, when the exerciser is riding, the exerciser needs to pay attention on the road ahead and operates the throttle switch at the same time. The complication for riding the electric bike is high. 
         [0006]    In addition, the pushing force is not provided immediately. The motor does not provide the pushing force until the exerciser steps on a crank of the bike over a half of a circular spinning movement. As a result, the exerciser still uses great effort to ride on the bike as before. The conventional throttle switch and the motor do not efficiently assist the exerciser with riding the bike. 
         [0007]    Responsive to such problems, a conventional detection device is used to detect a twisting torque and a twist angle of a shaft of a bike. The motor can be automatically activated to offer the pushing force according to the detected twisting torque and the twist angle. 
         [0008]    With reference to  FIGS. 17A and 17B , a first conventional detection device is disclosed. The detection device is adapted to be mounted on an input shaft  81  and an output shaft  82 , wherein the input shaft  81  is connected to the output shaft  82  through a coupler  83 . The detection device has a first magnet array  811 , a second magnet array  821  and a magnetic sensor  84 . 
         [0009]    The coupler  83  and the magnetic sensor  84  are mounted between the input shaft  81  and the output shaft  82 . The first magnet array  811  is mounted around the input shaft  81 . The second magnet array  821  is mounted around the output shaft  82 . Each magnet array  811 ,  821  respectively has multiple north poles (N) and south poles (S) arranged alternately. A number of the poles (N, S) of the first magnet array  811  is equal to a number of the poles (N, S) of the second magnet array  821 . The north poles (N) and the south poles (S) of the first magnet array  811  are respectively aligned with the north poles (N) and the south poles (S) of the second magnet array  821 . 
         [0010]    When the input shaft  81  rotates, the input shaft  81  turns the output shaft  82  through the coupler  83 . The coupler  83  is designed to flex when a torque is applied to either shaft, resulting in an angular displacement between the input shaft  81  and output shaft  82 . The north poles (N) of the first magnet array  811  are not exactly aligned with the north poles (N) of the second magnet array  821 , neither are the south poles (S) of the first magnet array  811  and the second magnet array  821 . As a result, the magnetic field between the first magnet array  811  and the second magnet array  821  is changed. The magnetic sensor  84  can detect the changed magnetic field. According to the changed magnetic field, a twisting torque exerted on the input shaft  81  and the output shaft  82  can be calculated. 
         [0011]    With reference to  FIG. 18 , to detect a rotational angle of a shaft  85 , a first magnet array  851  and a second magnet array  852  are mounted around the shaft  85  and are adjacent to each other, wherein a sensor  86  is mounted between the magnet arrays  851 ,  852 . The first magnet array  851  and the second magnet array  852  respectively have different number of poles (N, S). The first magnet array  851  has N pairs of poles (N, S) and the second magnet array  852  has N+1 pairs of poles (N, S). In an initial condition, the poles (N, S) of the first magnet array  851  are not actually aligned with the poles (N, S) of the second magnet array  852 . When the shaft  85  rotates, the poles (N, S) of the first magnet array  851  tend to align with the opposite poles (N, S) of the second magnet array  852 . As a result, the magnetic sensor  86  can detect the change of the magnetic field between the first magnet array  851  and the second magnet array  852 . According to the changed magnetic field, the rotational angle of the shaft  85  can be calculated. 
         [0012]    With reference to  FIG. 19 , however, the first conventional detection device needs at least three magnet arrays  811 ,  821 ,  852  to achieve the detecting action. Each magnet array  811 ,  821 ,  852  is composed of many pairs of poles (N, S). The first conventional detection device is not easy to be manufactured and causes high cost. Moreover, the magnetic fields between the three magnet arrays  811 ,  821 ,  852  may interfere with each other, such that the detection result of the first conventional detection device is not accurate. 
         [0013]    With reference to  FIG. 20 , a second conventional detection device is disclosed. A top tube  91  and a bottom tube  92  are respectively mounted on a shaft. The top tube  91  and the bottom tube  92  respectively have a disk  911 ,  921 . The disks  911 ,  921  have equal number of poles (N, S). The second conventional detection device has two Hall sensors  931 ,  932  respectively mounted beside the poles (N, S) of the two disks  911 ,  921 . The Hall sensors  931 ,  932  detect the magnetic fields of the poles (N, S) of the disks  911 ,  921 . According to the detected magnetic fields, a twisting torque exerted on the shaft can be calculated. 
         [0014]    However, such detection device is only able to detect the twisting torque. A twist angle of the shaft cannot be detected at the same time. Hence, the function of the detection device is limited. 
       SUMMARY OF THE INVENTION 
       [0015]    An objective of the present invention is to provide a contactless detection apparatus. The detection apparatus of the invention has simple structure and detects the torsion and the rotational angle at the same time. 
         [0016]    The contactless detection apparatus of the present invention comprises a first magnet ring, a second magnet ring, a first magnetic sensor, a second magnetic sensor and a controller. 
         [0017]    The first magnet ring has multiple pairs of a north pole and a south pole and is mounted around a first end of a torsion shaft, wherein a number of the pairs is even. 
         [0018]    The second magnet ring has one pair of a north pole and a south pole and is mounted around a second end of the torsion shaft. 
         [0019]    The first magnetic sensor is mounted beside the first magnet ring for detecting a magnetic field of the first magnet ring. 
         [0020]    The second magnetic sensor is mounted beside the second magnet ring for detecting a magnetic field of the second magnet ring. 
         [0021]    The controller is electrically connected to the first magnetic sensor and the second magnetic sensor and has a signal analyzer. The signal analyzer refers to a phase shift of the magnetic field of the first magnet ring to calculate a twisting torque exerted on the torsion shaft and to calculate a rotational angle of the torsion shaft according to the magnetic field of the second magnet ring. 
         [0022]    Another objective of the present invention is to provide a method for detecting a rotation direction. The method comprises the following steps of: 
         [0023]    detecting a magnetic field generated from a first magnet ring mounted around a torsion shaft by a first magnetic sensor; 
         [0024]    detecting a magnetic field generated from a second magnet ring mounted around the torsion shaft by a second magnetic sensor, wherein the first magnet ring and the second magnet ring have different numbers of pairs of poles; 
         [0025]    receiving a first potential and a second potential from the first and second magnetic sensors respectively by a controller; 
         [0026]    calculating a rotational angle of the torsion shaft according to the second potential by the controller; 
         [0027]    comparing the first potential with a reference potential to obtain a potential difference by the controller; 
         [0028]    checking a twisting torque exerted on the torsion shaft according to the potential difference by using tables set up in the controller, wherein the tables include a twist angle table and a twisting torque table. 
         [0029]    With respect to the detection apparatus of the invention, when the torsion shaft rotates, the two magnetic sensors respectively detect the magnetic fields generated by the first magnet ring and the second magnet ring. The controller then determines the twisting torque exerted on the torsion shaft and determines the rotational angle of the torsion shaft at the same time. According to the determined twisting torque and the rotational angle, the controller can automatically output an assistant torsion signal to a motor. As a result, the motor can automatically provide a pushing force according to the assistant torsion signal to help an exerciser to ride a bike to move ahead. 
         [0030]    In conclusion, the detection apparatus of the invention only has two magnet rings. The controller can determine both the twisting torque and the rotational angle by the two magnet rings. Hence, the structure of the detection apparatus is simpler than the conventional one. 
         [0031]    In addition, the two magnet rings are respectively mounted around two ends of a torsion shaft and separated from each other by a distance. Therefore, the magnetic fields of both magnet rings do not interfere with each other. The detection result of the invention is more accurate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is an operating view of a shaft sustaining a forward torque and a backward torque; 
           [0033]      FIG. 2  is a basic operational view of the present invention; 
           [0034]      FIG. 3  is a wave diagram of a magnetic field detected by the first magnetic sensor; 
           [0035]      FIG. 4  is a wave diagram of a magnetic field detected by the second magnetic sensor; 
           [0036]      FIG. 5  is an operating view of the present invention; 
           [0037]      FIG. 6A  is a wave diagram of a magnetic field detected by the first magnetic sensor; 
           [0038]      FIG. 6B  is a wave diagram of a magnetic field detected by the second magnetic sensor; 
           [0039]      FIG. 7  is a wave diagram of a potential mapping table; 
           [0040]      FIG. 8  is a wave diagram with phase shift of a magnetic field detected by the second magnetic sensor; 
           [0041]      FIG. 9  is an operating diagram for obtaining the twist angle; 
           [0042]      FIG. 10  is a flow chart for obtaining the twist angle; 
           [0043]      FIG. 11  is a block diagram of the dead zone control module; 
           [0044]      FIG. 12  is a flow chart for obtaining the assistant torsion; 
           [0045]      FIG. 13  is a perspective view of a second embodiment of the invention; 
           [0046]      FIG. 14  is an exploded perspective view of the second embodiment of the invention; 
           [0047]      FIG. 15  is a partially cross-sectional view of the second embodiment of the invention; 
           [0048]      FIG. 16  is a combination view of a gear bushing, a first bushing, a shaft and a second bushing; 
           [0049]      FIGS. 17A and 17B  are plan views of a first conventional detection device; 
           [0050]      FIG. 18  is a plan view of a first conventional detection device; 
           [0051]      FIG. 19  is a plan view of a first conventional detection device; and 
           [0052]      FIG. 20  is an operational view of a second conventional detection device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0053]    For convenience of description, a torsion shaft  11  of a bike is illustrated in  FIG. 1 . The torsion shaft  11  has a first end and a second end respectively connected to a first crank and a second crank, wherein the second end of the torsion shaft  11  is also connected to a back wheel through a gear and a chain. When an exerciser steps on the first crank to exert a forward force (Fz) on the torsion shaft  11 , the first end of the torsion shaft  11  sustains a forward torque (Tz). Meanwhile, the chain exerts a backward force (Fc) on the second end of the torsion shaft  11  based on a traction force between the back wheel and the ground, such that the second end of the torsion shaft  11  sustains a backward torque (Tc). By an interaction of the forward torque (Tz) and the backward torque (Tc), a twist angle (Δψ) between the first end and the second end of the torsion shaft  11  occurs. By measuring the twist angle (Δψ), a twisting torque exerted on the torsion shaft  11  can be calculated. 
         [0054]    The detection apparatus of the present invention can be mounted on a vehicle or a bike. With reference to  FIGS. 2 and 5 , a first embodiment of the present invention is disclosed. The detection apparatus of this invention is adapted to be mounted on a shaft unit  20  of the vehicle or the bike. The shaft unit  20  has a first portion  21  and a second portion  22  opposite to the first portion  21 . 
         [0055]    The detection apparatus of this invention mainly comprises a first magnet ring  231 , a second magnet ring  232 , a first magnetic sensor  241 , a second magnetic sensor  242  and a controller  30 . 
         [0056]    The first magnet ring  231  is mounted around the first portion  21  of the shaft unit  20  and has multiple magnet units, wherein each magnet unit has a pair of poles including a north pole  2311  and a south pole  2312 . A number of the magnet units is even. In this embodiment, the first magnet ring  231  has four magnet units. The north poles  2311 and the south poles  2312  of the four magnet units are alternately arranged along a movement trace  200 . 
         [0057]    The second magnet ring  232  is mounted around the second portion  22  of the shaft unit  20  and is adjacent to the first magnet ring  231 . The second magnet ring  232  has at least one magnet unit, wherein a number of the magnet unit(s) is odd. In this embodiment, the second magnet ring  232  has only one magnet unit with a north pole  2321  and a south pole  2322 . The north pole  2321  and the south pole  2322  are semicircular-shaped and are arranged along the movement trace  200 . 
         [0058]    The first magnetic sensor  241  is mounted beside the first magnet ring  231  and detects a magnetic field generated from the first magnet ring  231  to correspondingly generate a first potential. 
         [0059]    Similarly, the second magnetic sensor  242  is mounted beside the second magnet ring  232  and is aligned with the first magnetic sensor  241  at a particular position. The second magnetic sensor  242  detects a magnetic field generated from the second magnet ring  232  to correspondingly generate a second potential. 
         [0060]    The first magnetic sensor  241  and the second magnetic sensor  242  are aligned with each other and respectively detect the magnetic fields at the same time. Therefore, the first potential and the second potential are synchronously generated. 
         [0061]    The controller  30  is electrically connected to the first magnetic sensor  241 , the second magnetic sensor  242  and a motor  40 . The controller  30  has a signal analyzer  31 . The signal analyzer  31  receives the potentials from both sensors  241 ,  242  to calculate the twist angle (Δψ), a rotational angle (θ) and a twisting torque of the shaft unit  20 . The controller  30  activates the motor  40  according to the twist angle (Δψ), the rotational angle (θ) and the twisting torque. The rotational angle (θ) means an axially rotational position of the shaft unit  20 . For example, the rotational angle (θ) will be 0 degree if the shaft unit  20  is at an initial position. If the shaft unit  20  finishes a half of a complete circular spinning movement, the rotational angle (θ) will be 180 degrees. 
         [0062]    The motor  40  is adapted to connect to a drive system of a bike. When the motor  40  is activated, the motor  40  moves the bike ahead. By the assistance of the motor  40 , the exerciser can easily ride the bike with less effort. The following paragraphs describe how the controller  30  works. The first portion  21  of the shaft unit  20  is connected to a crank of a bike. The second portion  22  is connected to a gear as the load. When a user steps on the crank, the first portion  21  rotates to turn the gear. In other words, the first portion  21  sustains the forward torque (Tz) and the second portion  22  sustains the backward torque (Tc). 
         [0063]    With reference to  FIGS. 3 and 4 , when the exerciser steps on the crank, the shaft unit  20  sustains the forward torque (Tz) and spins. When the shaft unit  20  finishes a complete circular spinning movement (360 degrees) under a condition that the backward torque (Tc) is zero, the controller  30  traces out a first signal wave A and a second signal wave B according to the detected first potential and the second potential, wherein the first potentials in the first signal wave A are defined as reference potentials. The first signal wave A has four peaks resulted from the four magnet units of the first magnet ring  231 . The second signal wave B has one peak resulted from the one magnet unit of the second magnet ring  232 . Each rotational angle (θ) of the shaft unit  20  respectively corresponds to a particular first potential and a particular second potential. 
         [0064]    If the backward torque (Tc) is not zero, the first signal wave A 1  is plotted in broken lines in the  FIG. 3 . Apparently, a phase shift between the first signal waves A, A 1  occurs when the backward torque (Tc) is not zero, wherein a phase of the first signal wave A 1  exceeds a phase of the first signal wave A. 
         [0065]    The controller  30  has a potential mapping table  32 . With reference to  FIG. 6A and 6B , under a condition that the backward torque (Tc) is zero, the controller  30  records all of the first potentials and the second potentials corresponding to the torsion shaft&#39;s rotational angles (θ) from 0 to 360 degrees when the shaft unit  20  finishes a complete circular spinning movement. Each rotational angle (θ) of the shaft unit  20  corresponds to a particular first potential and a particular second potential. For example, when the rotational angle (θ) of the shaft unit  20  is 270 degrees, the corresponding first potential is 2.3V and the second potential is 1V. 
         [0066]    With reference to  FIG. 7 , each first potential corresponds to a particular second potential. The potential mapping table  32  in the controller  30  is set up based on the first potentials and the second potentials. 
         [0067]    With reference to  FIG. 6B , when the shaft unit  20  finishes a complete circular spinning movement, the second signal wave B runs only one out of four periods. Because each second potential in the second signal wave B is unique, each second potential corresponds to one particular rotational angle of the shaft unit  20 . The signal analyzer  31  recognizes the rotational angle (θ) of the shaft unit  20  according to the second potentials. 
         [0068]    With reference to  FIG. 8 , when the backward torque (Tc) is not zero, the phase shift as mentioned above occurs. With reference to  FIG. 9 , in the rotational angle (θ) of 270 degrees, the first potential is shifted to 2.9V as plotted at point y from the original potential of 2.3V as plotted at point x, and a potential difference (ΔV) between the points x and y is 0.6V. 
         [0069]    The controller  30  stores a twist angle table  33  established based on relationships between the potential differences (ΔV) and twist angles (Δψ). Each potential difference (ΔV) corresponds to a twist angle (Δψ). When the signal analyzer  31  gets the potential difference (ΔV), the signal analyzer  31  checks the twist angle table  33  to find the corresponding twist angle (Δψ) of the shaft unit  20 . 
         [0070]    The controller  30  further stores a twisting torque table  34  indicating relationships between multiple twist angles (Δψ) and multiple twisting torques. Each twist angle (Δψ) corresponds to a twisting torque. After the signal analyzer  31  gets the twist angle (Δψ), the signal analyzer  31  checks the twisting torque table  34  to find the corresponding twisting torque. As a result, the controller  30  correctly recognizes the twisting torque exerted on the shaft unit  20 . In conclusion, the controller  30  of the detection apparatus of the invention is able to simultaneously detect the twist angle (Δψ) and twisting torque. 
         [0071]    In brief, the signal analyzer  31  determines the twist angle (Δψ) through the steps as shown in  FIG. 10 . 
         [0072]    The signal analyzer  31  receives the first potentials and the second potentials from the first magnetic sensor  241  and the second magnetic sensor  242 . The first magnetic sensor  241  and the second magnetic sensor  242  generate the potential signals based on the magnetic fields of the first magnet ring  231  and the second magnet ring  232  (step  101 ). The first potentials are regarded as reference potentials. 
         [0073]    The signal analyzer  31  defines a reference rotational angle (θr) and a reference potential (Vr) corresponding to the reference rotational angle (θr) (step  102 ). 
         [0074]    The signal analyzer  31  compares a presently detected first potential and the reference potential (Vr) at the reference rotational angle (θr) to obtain the potential difference (ΔV) (step  103 ). 
         [0075]    After the signal analyzer  31  obtains the potential difference (ΔV), the signal analyzer  31  determines the twist angle (Δψ) of the shaft unit  20  in the twist angle table  33  (step  104 ). 
         [0076]    With reference to  FIGS. 11 and 12 , the controller  30  further has a dead zone control module  35 . The dead zone control module  35  has a dead zone setting unit  352  and a torsion assistant unit  353 . The dead zone control module  35  receives the rotational angle (θ) and the twist angle (Δψ) of the shaft unit  20  from the signal analyzer  31 . 
         [0077]    The dead zone control module  35  transforms the twist angle (Δψ) into a twisting torsion (Tdriver) based on a material stiffness coefficient (Ks). 
         [0078]    The differentiator  351  transforms the rotational angle (θ) into a rotational speed signal ({dot over (θ)}) by differentiation. The dead zone setting unit  352  transforms the rotational speed signal ({dot over (θ)}) into the dead zone range (Tdead). The dead zone range (Tdead) means a particularly rotational angle of the shaft unit  20 . 
         [0079]    After the torsion assistant unit  353  receives the information of the dead zone range (Tdead) and the twisting torsion (Tdriver), the dead zone control module  35  determines the amplitude of an assistant torsion (Tcmd) based on the received information. Finally, the dead zone control module  35  outputs an assistant torsion signal (Tmotor) to the motor  40  based on the assistant torsion (Tcmd). As a result, when the motor  40  receives the assistant torsion signal (Tmotor), the motor  40  outputs the pushing force according to the assistant torsion signal (Tmotor). 
         [0080]    In brief, the assistant torsion (Tcmd) is obtained by the steps as shown in  FIG. 12 . 
         [0081]    The dead zone control module  35  transforms the rotational angle (θ) into the rotational speed signal ({dot over (θ)}) by differentiation (step  201 ). 
         [0082]    The dead zone setting unit  352  of the dead zone control module  35  sets up the dead zone range (Tdead) according to the rotational speed signal ({dot over (θ)}) (step  202 ). 
         [0083]    The dead zone control module  35  offers the information of the dead zone range (Tdead) to the torsion assistant unit  353  (step  203 ). 
         [0084]    The dead zone control module  35  transforms the twist angle (Δψ) into a twisting torsion (Tdriver) according to the material stiffness coefficient (Ks) (step  204 ). 
         [0085]    The dead zone control module  35  determines the amplitude of the assistant torsion (Tcmd) based on the information of the twisting torsion (Tdriver) and the dead zone range (Tdead) (step  205 ). 
         [0086]    With reference to  FIGS. 13-15 , the detection apparatus of the invention is applied for a bike frame  50 . The bike frame  50  has a shaft shell  60 . A compound shaft is pivotally mounted in the shaft shell  60  and has a gear bushing  54 , a first bushing  52 , a second bushing  53 , a torsion shaft  51 , a third bushing  55 , a fixture nut  57  and a gear  58 . The first bushing  52  and the third bushing  55  act as the first portion  21  of the shaft unit  20  as illustrated in  FIG. 2 . The second bushing  53 , the gear bushing  54  and the gear  58  act as the second portion  22  of the shaft unit  20  as illustrated in  FIG. 2 . 
         [0087]    The fixture nut  57  is securely pressed in the shaft shell  60  for blocking the gear bushing  54 , the first bushing  52 , the second bushing  53 , the torsion shaft  51  and the third bushing  55  in the shaft shell  60 . 
         [0088]    The gear bushing  54  has a first end, a second end, three grooves  541 , an engagement portion  542  and a through hole  543 . The engagement portion  542  is formed on the second end of the gear bushing  54 . The grooves  541  are formed on the first end and communicate with the through hole  543 . The engagement portion  542  is adapted to connect to the gear  58 . The gear  58  as a load is adapted to connect to a chain. The second magnet ring  562  is mounted around the second end of the gear bushing  54 . The second magnetic sensor  564  is mounted in a second jack  602  of the shaft shell  60  to detect the magnetic field of the second magnet ring  562 . 
         [0089]    The first bushing  52  is mounted in the through hole  543  of the gear bushing  54  and has a body  520 , three grooves  521 , a pillar  522  and an engagement hole  523 . The body  520  has a first end with an opening  524  and a second end. The pillar  522  protrudes from the second end of the body  520  and extends out of the second end of the gear bushing  54 . The grooves  521  are formed on the first end of the body  520  and communicate with the opening  524  and respectively correspond to the grooves  541  of the gear bushing  54 . The engagement hole  523  is formed in the pillar  522  and communicates with the opening  524 . 
         [0090]    The second bushing  53  is mounted in the opening  524  of the first bushing  52  and has a body  530 , three ribs  531 , a pillar  532  and an aperture  533 . The body  530  has an exterior surface, a first end and a second end. The ribs  531  are formed on the exterior surface of the body  530  and respectively extend through the grooves  521 ,  541  of the first bushing  52  and the gear bushing  54 . The aperture  533  is formed in the second end of the body  530  and opposite to the engagement hole  523  of the first bushing  52 . The pillar  532  is formed on the first end of the body  530 . The width of the rib  531  is smaller than that of the groove  521  of the first bushing  52 . A gap is formed between the rib  531  and the first bushing  52 . 
         [0091]    The torsion shaft  51  is mounted between the first bushing  52  and the second bushing  53 . The torsion shaft  51  has a first end and a second end. The first end of the torsion shaft  51  is securely mounted in the aperture  533  of the second bushing  53 . The second end of the torsion shaft  51  is securely mounted in the engagement hole  523  of the first bushing  52 . With reference to  FIG. 16 , because the width of the rib  531  is smaller than that of the groove  521  of the first bushing  52 , the extent of twisting of the torsion shaft  51  is restricted by the groove  521  of the first bushing  52  to prevent the torsion shaft  51  from being over twisted. 
         [0092]    The third bushing  55  has a body  550 , three protrusions  551 , a pillar  552  and an engagement hole  553 . The body  550  has a first end and a second end. The engagement hole  553  is formed in the second end for pivotally holding the pillar  532  of the second bushing  53 . The protrusions  551  are formed around the second end of the body  550  and are respectively inserted into the grooves  521  of the first bushing  52 . The pillar  552  is formed on the first end of the body  550  and extends out of the fixture nut  57  for connecting to a crank. The first magnet ring  561  is mounted around the body  550  of the third bushing  55 . The first magnetic sensor  563  is mounted in a first jack  601  of the shaft shell  60  to detect the magnetic field of the first magnet ring  561 . 
         [0093]    When a user steps on the crank connected to the third bushing  55 , the compound shaft rotates. The first end of the torsion shaft  51  sustains a forward torque (Tz) and the second end of the torsion shaft  51  sustains a backward torque (Tc). 
         [0094]    When the backward torque (Tc) is not zero, the torsion shaft  51  is slightly twisted. As a result, the controller  30  detects the twisting torque and twist angle (Δψ) according to the phase shift of the signal waves A, A 1  as illustrated in  FIG. 8 . The controller  30  then automatically activates the motor  40  to provide a pushing force on the bike. The pushing force can effectively assist the user such that the user can ride the bike with ease.