Abstract:
A bicycle control device is provided that uses power from a rotating member to assist the operation of a bicycle mechanism, wherein the control device includes an input member that requests assistance of the rotating member and an output member that is assisted by the rotating member, and wherein the input member electrically moves from a first position to a second position and then to a third position. A method of operating the control device comprises the steps of providing an input signal for moving the input member from the first position to the second position and then to the third position; sensing a position of the input member with an input position sensor; and determining whether the input position sensor indicates the input member is in the third position.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to bicycle transmissions and, more particularly, to features in an apparatus for assisting a speed change operation in the bicycle transmission. 
     Various devices have been developed to help reduce the effort needed to operate bicycle transmissions such as derailleurs and internal hub transmissions. Examples of such devices particularly suited to assist the operation of derailleur transmissions are shown in U.S. Pat. No. 5,358,451. The devices shown therein for assisting the operation of a rear derailleur employ multiple moving parts that are in constant motion, thus increasing the amount of moving mass as well as the possibility of premature wear on the components. Devices shown therein for assisting the operation of a front derailleur accommodate only two front sprockets. However, many bicycles have more than two front sprockets. Thus, there is a desire for an assist device that can be used with more than two sprockets. 
     Some assisting devices use electric motors or solenoids to control the assisting operation. The electric motor or solenoid may operate for the entire shifting operation or for only a part of the shifting operation, and it is often necessary to provide cams or other mechanical control structures to control the amount of involvement of the motor or solenoid. Such control structures often have an intricate structure or require complicated cooperation between the structures. 
     Furthermore, such motors or solenoids often are placed in a location where they will encounter large operating forces. This requires the motors and solenoids to have a heavy-duty construction, thus increasing the size, weight and cost of the device. However, even heavy-duty motors and solenoids may operate improperly, and it is desirable to know when such faulty operation occurs. Thus, there is a need for an assist mechanism wherein electronic components can be manufactured to function reliably at a reasonable cost. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to various features of an apparatus for assisting an operation in a bicycle mechanism. One inventive feature is directed to a bicycle control device that uses power from a rotating member to assist the operation of a bicycle mechanism, wherein the control device includes an input member that requests assistance of the rotating member and an output member that is assisted by the rotating member, and wherein the input member electrically moves from a first position to a second position and then to a third position. A method of operating the control device comprises the steps of providing an input signal for moving the input member from the first position to the second position and then to the third position; sensing a position of the input member with an input position sensor; and determining whether the input position sensor indicates the input member is in the third position. Additional inventive features may be combined to provide additional benefits, as will become readily apparent when reading the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a particular embodiment of a bicycle that incorporates an apparatus for assisting a speed change operation in a bicycle transmission; 
     FIG. 2 is a more detailed view of the shift control device; 
     FIG. 3 is an exploded view of the shift control device shown in FIG. 2; 
     FIGS.  4 (A)-(C) are schematic views showing the operation of the shift control device; 
     FIG. 5 is a closer view of the assist mechanism shown in FIG. 1; 
     FIG. 6 is an exploded view of a particular embodiment of an input unit; 
     FIG. 7 is a view of the assist mechanism showing a particular embodiment of a rotating member engaging unit; 
     FIG. 8 is a rear cross sectional view of the assist mechanism; 
     FIGS.  9 (A)- 9 (D) illustrate the operation of the rotating member engaging member; 
     FIG. 10 is an enlarged cross sectional view of the internal components of the positioning unit shown in FIG. 8; 
     FIG. 11 is a side view of a particular embodiment of a motion transmitting member; 
     FIG. 12 is a side view of a particular embodiment of an input transmission member; 
     FIG. 13 is a side view of a particular embodiment of a middle plate; 
     FIG. 14 is a side view of a particular embodiment of a positioning member; 
     FIG. 15 is a perspective view of a particular embodiment of a motion transmitting pawl; 
     FIGS.  16 (A)-(E) are views illustrating the operation of the assist mechanism in an upshifting direction; 
     FIGS.  17 (A)-(F) are views illustrating the operation of the assist mechanism in a downshifting direction; 
     FIGS.  18 (A) and  18 (B) are views illustrating the cooperation of the motion transmitting pawl with the middle plate during a downshifting operation; 
     FIGS.  19 (A) and  19 (B) are views of an alternative embodiment of a drive control mechanism; 
     FIG. 20 is a side view of an alternative embodiment of a release mechanism; 
     FIG.  21 (A) is an outer side view of a housing for an alternative embodiment of an input unit; 
     FIG.  21 (B) is an inner side view of the housing; 
     FIGS.  22 (A)- 22 (C) are views showing movement of the output transmission member when coupled to a position sensor coupling member; 
     FIGS.  23 (A)- 23 (C) are views showing movement of an output transmission member position sensor that is coupled to the output transmission member; 
     FIGS.  24 (A)- 24 (C) are views showing movement of an input transmission drive member coupled to an input drive member position sensor; 
     FIG. 25 is a view taken along line XXV—XXV in FIG.  24 (C); 
     FIG. 26 is a view of a circuit board showing conductive traces used with the input drive member position sensor and the output transmission member position sensor; 
     FIG. 27 is a block diagram of electrical components used for controlling the operation of the assist apparatus; and 
     FIG. 28 is a flow chart showing the operation of the control unit shown in FIG.  27 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a side view of a bicycle  10  that incorporates a particular embodiment of an assist mechanism  14  according to the invention for assisting a change speed operation in a bicycle transmission. Bicycle  10  may be any type of bicycle, and in this embodiment bicycle  10  includes a typical frame  18  comprising a top tube  22 , a head tube  24 , a down tube  26  extending downwardly from head tube  24 , a seat tube  30  extending downwardly from top tube  22 , a bottom bracket  32  disposed at the junction of down tube  26  and seat tube  30 , a pair of seatstays  34  extending rearwardly and downwardly from top tube  22 , and a pair of chainstays  38  extending rearwardly from bottom bracket  32 . A fork  42  is rotatably supported within head tube  24 , and a front wheel  46  is rotatably supported to the lower end of fork  42 . The rotational direction of fork  42  and wheel  46  is controlled by a handlebar  50  in a well known manner. A rear wheel  54  having a plurality of coaxially mounted freewheel sprockets (not shown) is rotatably supported at the junction of seatstays  34  and chainstays  38 , and a pedal assembly  58  supporting a plurality of front (chainwheel) sprockets  62  is rotatably supported within bottom bracket  32 . In this embodiment, three front sprockets  62  rotate coaxially and integrally with pedal assembly  58 . A chain  66  engages one of the plurality of front sprockets  62  and one of the plurality of freewheel sprockets mounted to rear wheel  54 . A front derailleur  70  moves chain  66  from one front sprocket  62  to another, and a rear derailleur  74  moves chain  66  from one freewheel sprocket to another. Both operations are well known. In this embodiment, front derailleur  70  is controlled by pulling and releasing an output control wire  78  coupled to assist mechanism  14 , and assist mechanism  14  is controlled by an inner wire  80  of a Bowden-type control cable  82  connected to a shift control device  84  mounted to the left side of handlebar  50 . Rear derailleur  74  is controlled by a Bowden-type control cable  86  in a conventional manner. 
     FIG. 2 is a view of the left side of handlebar  50  showing shift control device  84  in more detail, and FIG. 3 is an exploded view of shift control device  84 . In this embodiment, shift control device  84  is mounted between a stationary handgrip  92  and a conventional brake lever bracket  94  that supports a brake lever  98 . Shift control device  84  comprises a base member  102 , a clamping band  106 , a biasing component in the form of a spring  110 , an intermediate member  114 , an actuating component  118 , and a retainer  122 . Base member  102  comprises a tubular portion  126  and a flange portion  130 . Tubular portion  126  surrounds handlebar  50 , and flange portion  130  extends radially outwardly from an inner end of tubular portion  126 . Clamping band  106  has a locking projection  134  and mounting ears  138  and  142 , and the structure fits within an annular recess (not shown) with a locking groove formed at the inner peripheral surface of flange portion  130 . A screw  144  extends through an opening  148  in flange portion  130  and through mounting ears  138  and  142  and screws into a nut  152  disposed in another opening  153  in flange portion  130  to tighten mounting ears  138  and  142  toward each other and thereby tighten clamping band  106  and fasten base member  102  to handlebar  50 . A conventional screw-type adjustable control cable coupler  156  is disposed on flange portion  130  for receiving the outer casing  81  of control cable  82  in a conventional manner. Diametrically opposed recesses  160  (only one is visible in FIG. 3) having abutments  160   a  and  160   b  are formed at the junction of tubular portion  126  and flange portion  130 , and a base member bias engaging component  164  in the form of a spring hole is formed in flange portion  130 . An end  168  of spring  110  is fitted within spring hole  164 . 
     Intermediate member  114  is rotatably supported on tubular portion  126  of base member  102  such that spring  110  is disposed between intermediate member  114  and flange portion  130  of base member  102 . Diametrically opposed projections or stoppers  172  (only one is visible in FIG. 3) forming abutments  172   a  and  172   b  extend axially from the inner end of intermediate member  114 , and a pair of diametrically opposed intermediate member projections or stoppers  188  forming abutments  188   a  and  188   b  extend radially outwardly from an outer peripheral surface  184  of intermediate member  114 . An end  192  of spring  110  is fitted within a spring opening  194  (which functions as an intermediate member bias engaging component) formed in one of the stoppers  188  for biasing intermediate member  114  clockwise. As a result, abutments  172   a  of stoppers  172  engage abutments  160   a  (which function as base member stoppers) to limit the rotation of intermediate member  114  relative to base member  102 . 
     Actuating component  118  is rotatably supported by intermediate member  114  which, as noted above, is rotatably supported by the tubular portion  126  of base member  102 . Thus, actuating component  118  rotates coaxially around intermediate member  114 , tubular portion  126  of base member  102 , and handlebar  50 . Actuating component  118  comprises a tubular member  200 , first and second finger projections or levers  204  and  208  extending radially outwardly from tubular member  200 , a transmission control member coupling component in the form of an opening  212  for receiving a cable end bead (not shown) attached to the end of inner wire  80  so that inner wire  80  moves integrally with actuating component  114 , and diametrically opposed recesses  216  forming abutments  216   a  and  216   b . In the assembled state, intermediate member stoppers  188  are fitted within the corresponding recesses  216  between abutments  216   a  and  216   b  so that abutments  216   a  and  216   b  function as actuating member stoppers. In this embodiment, inner wire  80  of control cable  82  is under tension as a result of a biasing component disposed in assist apparatus  14 . Thus, actuating component  118  is biased in the counterclockwise direction such that abutments  188   a  of intermediate member stoppers  188  engage abutments  216   a  to limit the rotation of actuating component  118  relative to intermediate member  114  and base member  102 . 
     Retainer  122  is fitted around the outer end of tubular member  126  of base member  102 . Retainer  122  includes four recesses  220  that are evenly formed on a side surface  224  for engaging four locking tabs  228  that extend radially outwardly from the outer end of tubular portion  126  of base member  102 . Thus, retainer  122  axially fixes actuating component  118  and intermediate member  114  in place on base member  102 . 
     FIGS.  4 (A)- 4 (C) schematically illustrate the operation of shift control device  84 . FIG.  4 (A) shows actuating component  118  in an actuating component neutral position. In this position, spring  110  biases intermediate member  114  clockwise (to the right in FIG.  4 (A)) so that abutments  172   a  of stoppers  172  contact abutments  160   a  of recesses  160  on base member  102 , and a biasing component (spring) in assist mechanism  14 , indicated by reference number  232 , biases actuating component  118  counterclockwise so that abutments  216   a  of recesses  216  contact abutments  188   a  of intermediate member stoppers  188 . Thus, abutments  160   a ,  172   a ,  188   a  and  216   a  (and to some extent springs  110  and  232 ) function as neutral positioning components. Since inner wire  80  is directly coupled to actuating component  118 , inner wire  80  likewise is in a transmission control member neutral position at this time. 
     Rotating actuating component  118  clockwise from the position shown in FIG.  4 (A) against the biasing force of the biasing component  232  in assist mechanism  14  causes abutments  216   b  on actuating component  118  to contact abutments  188   b  on intermediate member stopper  188  as shown in FIG.  4 (B). Intermediate member  114  remains stationary at this time. In FIG.  4 (B), actuating component  118  is in an actuating component downshift position, and inner wire  80  is pulled into a transmission control member downshift position. 
     Rotating actuating component  118  counterclockwise from the position shown in FIG.  4 (A) causes intermediate member  114  to rotate counterclockwise (to the left in FIG.  4 (C)) against the biasing force of spring  110 , since abutments  216   a  contact abutments  188   a  of intermediate member stoppers  188  and spring  110  is ultimately coupled between actuating component  118  and base member  102 . As a result, actuating component  118  is in an actuating component upshift position, and inner wire  80  is released into a transmission control member upshift position. 
     FIG. 5 is a more detailed view of assist mechanism  14 . As shown in FIG. 5, assist mechanism  14  is mounted to bottom bracket  32 , and it includes an input unit  250 , a positioning unit  254 , and a rotating member engaging unit  258  with a cover  262 . In this embodiment, assist mechanism  14  is used in conjunction with a crank arm  266  that includes an axle mounting boss  270  having a plurality of crank arm splines  274  that nonrotatably engage a plurality of axle splines  278  formed on the end of an axle  282  that is rotatably supported by bottom bracket  32  in a well known manner. A drive flange  286  extends radially outwardly from axle mounting boss  270  and supports a pair of diametrically opposed drive members  290 . Drive members  290  have the shape of circular tubes that extend perpendicularly from the side surface  294  of drive flange  286 . 
     FIG. 6 is an exploded view of a particular embodiment of input unit  250 . Input unit  250  includes an input unit mounting member  298 , a wire coupling member  302 , spring  232 , and an input link  306 . Input unit mounting member  298  has a guide channel  310  for inner wire  80 , a central axle opening  314  for receiving an axle  318  (FIG. 10) of positioning unit  254  therethrough, and a pair of diametrically opposed openings  322  (only one opening is visible in FIG.  6 ). Wire coupling member  302  includes a wire winding groove  326  for winding and unwinding inner wire  80 , a conventional wire coupler  330  in the form of a screw  334 , a wire retainer  338  and a nut  342  for fixing inner wire  80  to wire coupling member  302 , and an axle opening  346  for receiving axle  318  of positioning unit  254 . Input link  306  functions to communicate the rotational position of wire coupling member  302  to positioning unit  254 , and it includes an axle mounting portion  350  with an axle receiving opening  352 , coupling tabs  354 , a radially extending portion  358 , and an axially extending coupling portion  362 . Coupling tabs  354  extend axially from axle mounting portion  350 , through openings  322  in input unit mounting member  298 , and into corresponding openings (not shown) in wire coupling member  302  so that wire coupling member  302  and input link  306  rotate as a unit. Thus, both wire coupling member  302  and input link  306  will assume neutral, upshift and downshift positions corresponding to the positions of actuating component  118  of shift control device  84 . Spring  232  has one end  233  mounted to wire coupling member  302  and another end  234  mounted to input unit mounting member  298  so that wire coupling member  302  and input link  306  are biased in the clockwise (wire winding) direction. 
     FIG. 7 is an oblique view of assist mechanism  14  with cover  262  of rotating member engaging unit  258  removed, FIG. 8 is a rear cross sectional view of assist mechanism  14 , and FIGS.  9 (A)- 9 (D) illustrate the operation of rotating member engaging unit  258 . As shown in FIGS. 7,  8  and  9 (A), rotating member engaging unit  258  includes a bottom bracket mounting member  370  with an opening  374  for receiving axle  282  therethrough, an axially extending side wall  378 , a cam plate  382  with a control cam slot  386  attached to side wall  378 , and an opening  390  for supporting a lower pivot shaft  392 . One end of a rotating member engaging member  394  has an arcuate rotating member engaging surface  398  for engaging drive members  290  on crank arm  266 . The other end of rotating member engaging member  394  is pivotably connected between a positioning unit interface plate  402  and a support plate  406  by a pivot shaft  410 . A cam follower  414  that engages a control cam surface  418  formed by cam slot  386  is mounted to rotating member engaging member  394  in close proximity to pivot shaft  410 . A spring  420  biases positioning unit interface plate  402  and support plate  406  in a counterclockwise direction. 
     FIG.  9 (A) shows rotating member engaging member  394  in a rotating member disengaging position, wherein drive members  290  rotate with crank arm  266  without causing any effect on assist mechanism  14 . In general, when actuating component  118  of shift control unit  84  is rotated to either the upshift position or the downshift position, then positioning unit interface plate  402  and support plate  406  pivot counterclockwise as shown in FIG.  9 (B). This causes rotating member engaging member  394  to pivot clockwise around pivot shaft  410 , since cam follower  414  is retained within cam slot  386 , to the rotating member engaging position shown in FIG.  9 (B). In this position, rotating member engaging surface  398  is disposed in the path of drive members  290 , so one of the drive members  290  will contact rotating member engaging surface  398  as shown in FIG.  9 (B) and cause rotating member engaging member  394  to rotate positioning unit interface plate  402  and support plate  406  clockwise against the biasing force of spring  420  as shown in FIG.  9 (C). As crank arm  266  continues to rotate, the engaged drive member  290  will disengage from rotating member engaging member  394 , rotating member engaging member  394  will pivot counterclockwise as shown in FIG.  9 (D) back to the rotating member disengaging position, and spring  420  will cause positioning unit interface plate  402  and support plate  406  to pivot counterclockwise back to the position shown in FIG.  9 (A). 
     FIG. 10 is an enlarged rear cross sectional view of the internal components of positioning unit  254 . As shown in FIG. 10, positioning unit  254  includes a base plate  450  supporting one end of a pawl shaft  470 ; an output transmission member in the form of a rotating member  454  rotatably supported on axle  318  and having a wire winding groove  455  for winding and unwinding output control wire  78  to a plurality of output positions; a biasing component in the form of a spring  456  for biasing rotating member  454  in a wire unwinding direction; a positioning member in the form of a positioning ratchet  458  coupled for integral rotation with rotating member  454 ; a middle plate  466  supporting the other end of pawl shaft  470 ; a position maintaining member in the form of a positioning pawl  474  supported by pawl shaft  470  for rotation between a position maintaining position and a position release position and having positioning teeth  475  and  476  (FIG.  15 (A)); a pivot shaft  477  mounted to positioning tooth  475 ; a cam follower in the form of a cam roller  478  rotatably supported by pivot shaft  477 ; and a pawl spring  482  connected between positioning pawl  474  and base plate  450  for biasing positioning pawl  474  toward the position maintaining position (counterclockwise in FIG.  15 (A)). 
     Positioning unit  254  further includes a release plate  486  rotatably supported on axle  318  and having a pivot shaft  490  supporting a cam member in the form of a cam plate  494 ; a motion transmitting member  498  rotatably supported on axle  318 ; a pawl shaft  502  mounted to motion transmitting member  498 ; a motion transmitting pawl  506  pivotably supported on pawl shaft  502 ; a spring  509  for biasing motion transmitting pawl  506  in the counterclockwise direction in FIG.  15 (A); another pawl shaft  510  mounted to motion transmitting member  498 ; a mode change pawl  514  pivotably supported on pawl shaft  510 ; an input transmission member in the form of a control plate  518  rotatably supported on axle  318 ; a base plate  522 ; a pawl shaft  526  mounted to base plate  522  and supporting a switch-off drive control member in the form of a drive control pawl  530 ; a spring  531  for biasing drive control pawl  530  in the counterclockwise direction in FIG.  15 (A); a pawl shaft  534  (FIG.  15 (A)) mounted to base plate  522  and supporting a switch-on drive control member in the form of a drive control pawl  538 ; a spring  539  for biasing drive control pawl  538  in the counterclockwise direction in FIG.  15 (A); a spring retainer  541 ; a spring  499  connected between spring retainer  541  and motion transmitting member  498  for biasing motion transmitting member  498  in the clockwise direction in FIG.  15 (A), and a retaining nut  542  for axially retaining the components on axle  318 . Base plate  450 , base plate  522  and axle  318  function as mounting units for the various components. 
     FIG. 11 is a side view of motion transmitting member  498 . Motion transmitting member  498  includes a base portion  550 , a pawl mounting ear  554  and a motion transmitting arm  558 . Base portion  550  includes an opening  562  for receiving axle  318  therethrough, a radially outwardly extending projection  566  forming an abutment  570  for contacting drive control pawl  530 , and a radially outwardly extending projection  574  forming an abutment  578  for contacting drive control pawl  538 . Pawl mounting ear  554  includes an opening  582  for mounting pawl shaft  510  (which supports mode change pawl  514 ), and motion transmitting arm  558  likewise includes an opening  586  for mounting pawl shaft  502  (which supports motion transmitting pawl  506 ). Motion transmitting arm  558  also includes an abutment  588  for contacting drive control pawl  538 , and an axially extending rotating member engaging unit interface plate  590  that attaches to positioning unit interface plate  402  through screws  594  as shown in FIGS. 8 and 10. 
     FIG. 12 is a side view of a particular embodiment of control plate  518 . Control plate  518  includes an input control member in the form of a base portion  598 , a lever arm portion  602 , and an input unit interface plate  604 . Input unit interface plate  604  includes an opening  605  for receiving coupling portion  362  (FIG. 6) of input link  306 . Base portion  598  includes input control members in the form of radially extending drive control cam surfaces or lobes  606 ,  610 ,  614  and  618 . Drive control cam lobe  606  includes an upper surface  606   a  and inclined ramps  606   b  and  606   c . Similarly, cam lobe  610  includes an upper surface  610   a  and inclined ramps  610   b  and  610   c . Cam lobe  614  includes an upper surface  614   a , an inclined ramp  614   b  and a transition surface  614   c  extending from upper surface  614   a  to an upper surface  618   a  of cam lobe  618 . Cam lobe  618  further includes a transition surface  618   b  extending from upper surface  618   a  to the outer peripheral surface  598   a  of base portion  598 . It will become apparent from the description below that cam lobes  606 ,  610  and  614 , drive control pawl  538  and motion transmitting member  498  with projection  578  comprise a switching mechanism to control the movement of rotating member engaging member  394  between the rotating member engaging position and the rotating member disengaging position. 
     FIG. 13 is a side view of a particular embodiment of middle plate  466 . Middle plate  466  includes a base portion  630 , a pawl coupling arm  634 , a downshift control plate  638 , and a pawl coupling portion  642  extending from downshift control plate  638 . Pawl coupling arm  634  includes an opening  646  for receiving a fastener (not shown) used to attach the assembly to the housing, and pawl coupling portion  642  includes an opening  650  for attaching pawl shaft  470  (which supports positioning pawl  474 ). Downshift control plate  638  defines a recess  656  having a pawl control surface  660  that functions in a manner described below. 
     FIG. 14 is a side view of positioning ratchet  458 . Positioning ratchet  458  comprises a generally annular body  670  having an inner peripheral surface  672  forming a plurality of female splines  674  that nonrotatably engage a corresponding plurality of male splines (not shown) formed on rotating member  454  so that positioning ratchet  458  and rotating member  454  rotate as a unit. An outer peripheral surface  678  forms three positioning teeth  682 ,  686  and  690  and two drive teeth  694  and  698  defining drive surfaces  694   a  and  698   a , respectively. With this structure, rotating member  454  can be set in three positions to accommodate three front sprockets  62 . Such sprockets usually comprise a small diameter sprocket, an intermediate diameter sprocket, and a large diameter sprocket. 
     FIG. 15 is a perspective view of motion transmitting pawl  506 . Motion transmitting pawl  506  includes a base portion  506   a  with an opening  506   b  for receiving pawl shaft  502 , a downshift control surface  506   c  for contacting pawl control surface  660  of middle plate  466  in a manner described below, a positioning ratchet drive surface  506   d , a release plate drive surface  506   e , and mode change pawl contact surfaces  506   f  and  506   g.    
     FIGS.  16 (A)-(E) are views illustrating the operation of positioning unit  254  in an upshifting direction. In FIG.  16 (A), positioning unit  254  is in a position such that front derailleur  70  is aligned with the small diameter front sprocket, and it is desired to move front derailleur  70  to the intermediate diameter front sprocket. In the position shown in FIG.  16 (A), the tip of drive control pawl  530  is supported by the upper surface  606   a  of cam lobe  606 , and the tip of drive control pawl  538  is located at the bottom of ramp  610   c  of cam lobe  610  such that drive control pawl  538  contacts abutment  578  on motion transmitting member  498  and holds motion transmitting member  498  in a “switch off” position. Thus, drive control pawl  538  and cam lobe  610  comprise a drive control mechanism that ordinarily maintains motion transmitting member  498  in the switch off position. Motion transmitting pawl  506  rests on the upper surface of drive tooth  694  on positioning ratchet  458 . 
     The rider then rotates actuating component  118  counterclockwise (in FIG. 3) to the upshift position so that inner wire  80  is released by actuating component  118 . This causes wire coupling member  302  to rotate clockwise in FIG. 6, and this motion is communicated via input link  306  to control plate  518  to rotate control plate  518  clockwise to the upshift position shown in FIG.  16 (B). Clockwise rotation of control plate  518  causes drive control pawl  530  to slide down ramp  606   c  of cam lobe  606  and rotate counterclockwise to the position shown in FIG.  16 (B). At the same time, drive control pawl  538  slides up ramp  614   b  of cam lobe  614  until drive control pawl  538  disengages from abutment  578  on motion transmitting member  498  and rests on upper surface  614   a  of cam lobe  614 . Since drive control pawl  538  no longer contacts abutment  578 , motion transmitting member  498  rotates clockwise until drive control pawl  538  contacts abutment  588 , and motion transmitting member  498  is in a “switch on” position as shown in FIG.  16 (B). Motion transmitting pawl  506 , no longer being held by drive tooth  694  on positioning ratchet  458 , rotates counterclockwise and rests on the outer peripheral surface  678  of positioning ratchet  458 . The clockwise motion of motion transmitting member  498  is communicated to positioning unit interface plate  402  and support plate  406  in rotating member engaging unit  258  so that rotating member engaging member  394  pivots to the position shown in FIG.  9 (B). 
     When drive member  290  on crank arm  266  engages rotating member engaging member  394  and pivots positioning unit interface plate  402  and support plate  406  to the position shown in FIG.  9 (C), the movement is communicated to motion transmitting member  498 . Positioning ratchet drive surface  506   d  of motion transmitting pawl  506  engages drive tooth  694  on positioning ratchet  458  and rotates positioning ratchet  458  and rotating member  454  to wind output control wire  78 . During that time, positioning tooth  682  presses against pawl tooth  475  of positioning pawl  474  and rotates positioning pawl  474  clockwise until pawl tooth  475  clears the tip of positioning tooth  682 . Then, positioning pawl  474  rotates counterclockwise so that pawl tooth  475  is located between positioning teeth  682  and  686  shown in FIG.  16 (C). 
     When drive member  290  on crank arm  266  disengages from rotating member engaging member  394 , positioning unit interface plate  402  and support plate  406  rotate back toward the position shown in FIG.  9 (A), and this movement is communicated to motion transmitting member  498 . Motion transmitting pawl  506  disengages from drive tooth  694  on positioning ratchet  458 , and positioning ratchet  458  and rotating member  454  rotate clockwise in accordance with the biasing force of spring  456  until positioning tooth  682  abuts against pawl tooth  475 . At this time, the front derailleur  70  is aligned with the intermediate diameter front sprocket as desired. 
     Assume, however, that at this time the rider has not yet rotated actuating component  118  back to the neutral position. In such a case, control plate  518  still would be in the upshift position with drive control pawl  538  resting on upper surface  614   a  of cam lobe  614 . In this position, drive control pawl  538  would not be able to engage abutment  578  to stop the rotation of motion transmitting member  498 . Thus, instead of returning to the switch off position shown in FIG.  16 (A), motion transmitting member  498  would continue rotating to the switch on position shown in FIG.  16 (B), rotating member engaging member  394  would return to the rotating member engaging position shown in FIG.  9 (B), and another shift would result. Such an operation may be desirable in some applications and is within the scope of the present invention. However, in this embodiment drive control pawl  530  is provided to prevent such double shifts. More specifically, drive control pawl  530 , having rotated counterclockwise as noted above, is now in the position to contact abutment  570  on motion transmitting member  498  and temporarily stop further rotation of motion transmitting member  498  so that motion transmitting member  498  is in the position shown in FIG.  16 (D). Thus, drive control pawl  530  and cam lobe  606  comprise a drive control mechanism that inhibits rotation of motion transmitting member  498  back to the switch on position after the motion transmitting mechanism transmits motion from the rotating member engaging member  394  to rotating member  454 . 
     When the rider returns actuating component  118  to the neutral position, control plate  518  likewise rotates back to the neutral position shown in FIG.  16 (E). At that time, drive control pawl  530  slides up ramp  606   c  on cam lobe  606  and rotates clockwise until control pawl  530  disengages from abutment  570  on motion transmitting member  498  and the tip of control pawl  530  rests upon the upper surface  606   a  of cam lobe  606 . Also, drive control pawl  538  slides down ramp  614   b  of cam lobe  614  and rotates counterclockwise so that the tip of drive control pawl  538  contacts abutment  578  on motion transmitting member  498  as shown in FIG.  16 (E). Motion transmitting member  498  is now in the switch off position as shown originally in FIG.  16 (A), but with positioning ratchet  458  and rotating member  454  in the position to align front derailleur  70  with the intermediate diameter front sprocket. The operation to shift from the intermediate diameter front sprocket to the large diameter front sprocket is the same. 
     FIGS.  17 (A)-(E) are views illustrating the operation of positioning unit  254  in a downshifting direction. Some components are shown in transparent view to facilitate understanding of the operation of the components that play an important role in the downshift operation. Assume rotating member  454  is in a position such that front derailleur  70  is aligned with the intermediate diameter front sprocket (the same position shown in FIG.  16 (E)), and it is desired to move front derailleur  70  to the small diameter sprocket. Accordingly, in the position shown in FIG.  17 (A), the tip of drive control pawl  530  again is supported by the upper surface  606   a  of cam lobe  606 , and the tip of drive control pawl  538  is located at the bottom of ramp  610   c  of cam lobe  610  such that drive control pawl  538  contacts abutment  578  on motion transmitting member  498 . Motion transmitting pawl  506  rests on the upper surface of drive tooth  698  on positioning ratchet  458 . Cam plate  494 , which has the overall shape of a rounded and elongated isosceles triangle, includes an axially extending positioning tab  495  that abuts against a side surface  487  of release plate  486  to hold cam plate  494  in the position shown in FIG.  17 (A). 
     The rider then rotates actuating component  118  clockwise (in FIG. 3) to the downshifted position so that inner wire  80  is pulled by actuating component  118 . This causes wire coupling member  302  to rotate counterclockwise in FIG. 6, and this motion is communicated via input link  306  to control plate  518  to rotate control plate  518  counterclockwise as show in FIG.  17 (B). Counterclockwise rotation of control plate  518  causes drive control pawl  530  to slide down ramp  606   b  of cam lobe  606  and rotate counterclockwise. At the same time, drive control pawl  538  slides up ramp  610   c  of cam lobe  610  and rotates clockwise until drive control pawl  538  disengages from abutment  578  on motion transmitting member  498  and rests on upper surface  610   a  of cam lobe  610 . Since drive control pawl  538  no longer contacts abutment  578 , motion transmitting member  498  rotates clockwise until drive control pawl  538  contacts abutment  588  and motion transmitting member  498  is in the switch on position shown in FIG.  17 (B). This time, motion transmitting pawl  506  rotates clockwise by transition surface  618   b  of cam lobe  618 , and mode change pawl  514  rotates clockwise to engage mode change pawl contact surface  506   f  on motion transmitting pawl  506  to temporarily hold motion transmitting pawl  506  in the position shown in FIG.  17 (B). The movement of motion transmitting member  498  is communicated to positioning unit interface plate  402  and support plate  406  in rotating member engaging unit  258  so that rotating member engaging member  394  pivots to the position shown in FIG.  9 (B). 
     When drive member  290  on crank arm  266  engages rotating member engaging member  394  and pivots positioning unit interface plate  402  and support plate  406  to the position shown in FIG.  9 (C), the movement again is communicated to motion transmitting member  498 , but this time release plate drive surface  506   e  of motion transmitting pawl  506  engages an abutment  487  on release plate  486  (which is currently in a first release member position), and release plate  486  rotates counterclockwise as shown in FIG.  17 (C). Thus, motion transmitting member  498  functions as a release drive member for release plate  486  in this mode. As release plate  486  rotates, a base surface  496  of cam plate  494  contacts cam roller  478  attached to positioning pawl  474  and causes positioning pawl  474  to rotate in the clockwise direction. When the tip of pawl tooth  475  clears the tip of positioning tooth  682 , positioning ratchet  458  and rotating member  454  rotate in the clockwise direction in accordance with the biasing force of spring  456  until positioning tooth  686  abuts against pawl tooth  476  to prevent uncontrolled rotation of positioning ratchet  458  and rotating member  454 . 
     As release plate  486  continues to rotate counterclockwise toward a second release member position (the end of the range of motion of release plate  486 ), cam roller  478  reaches the rounded corner or cam lobe  497  of cam plate  494 , thus causing cam plate  494  to rotate in the counterclockwise direction as shown in FIG.  17 (C). This, in turn, allows positioning pawl  474  to rotate in the counterclockwise direction so that pawl tooth  476  moves away from positioning tooth  686  to allow positioning ratchet  458  and rotating member  454  to continue rotating in the clockwise direction until rotating member  454  is positioned so that front derailleur  70  is aligned with the smaller diameter sprocket. 
     If this system operated according to known systems which use a positioning pawl and positioning ratchet to control the shifting operation, the pawl tooth  476  would remain engaged with positioning tooth  686  until release plate  486  reversed direction (i.e., rotated in the clockwise direction) to complete the shifting operation. This is not necessary with a shift control mechanism constructed according to the present invention, since the rotatable cam plate  494  allows the positioning pawl  474  to immediately complete the shifting operation even when release plate  486  is still rotating in the counterclockwise direction. Thus, release plate  486  and cam plate  494  can be considered a release control mechanism that moves positioning pawl  474  to the position release position as release plate  486  moves toward the second release member position and allows positioning pawl  474  to return to the position maintaining position as release plate  486  continues to move toward the second release member position. 
     Another advantageous feature of the preferred embodiment is the manner in which the release plate  486  is allowed to reverse direction even when motion transmitting member  498  is still rotating in the counterclockwise direction. According to the preferred embodiment, when the motion transmitting member  498  is located in the position shown in FIGS.  17 (C) and  18 (A), downshift control surface  506   c  of motion transmitting pawl  506  begins to contact the pawl control surface  660  of middle plate  466  as shown in FIG.  18 (A). Further rotation of motion transmitting member  498  causes motion transmitting pawl  506  to rotate counterclockwise as shown in FIGS.  17 (D) and  18 (B) which, in turn, causes motion transmitting pawl  506  to disengage from release plate  486 . Mode change pawl  514  also disengages from mode change pawl contact surface  506   f  of motion transmitting pawl  506  and rests on mode change pawl contact surface  506   g . Consequently, release plate  486  is allowed to return immediately to the position shown in FIG.  17 (D), even when motion transmitting member  498  is still in the counterclockwise position shown in FIG.  17 (D). 
     When drive member  290  on crank arm  266  disengages from rotating member engaging member  394 , positioning unit interface plate  402  and support plate  406  again rotate back toward the position shown in FIG.  9 (A), and this movement is communicated to motion transmitting member  498 . Once again, assume that the rider has not yet rotated actuating component  118  back to the neutral position. In such a case, control plate  518  is still in the downshift position with drive control pawl  538  resting on upper surface  610   a  of cam lobe  610 , but drive control pawl  530  contacts abutment  570  on motion transmitting member  498  so that motion transmitting member  498  is in the pause position shown in FIG.  17 (E). 
     When the rider returns actuating component  118  to the neutral position, control plate  518  likewise rotates clockwise back to the neutral position shown in FIG.  17 (F). At that time, drive control pawl  530  slides up ramp  606   b  of cam lobe  606  and rotates clockwise until drive control pawl  530  disengages from abutment  570  on motion transmitting member  498  and the tip of drive control pawl  530  rests upon upper surface  606   a  of cam lobe  606 . At the same time, drive control pawl  538  slides down ramp  610   c  of cam lobe  610  and rotates counterclockwise so that the tip of drive control pawl  538  contacts abutment  578  on motion transmitting member  498  as shown in FIG.  17 (F). Motion transmitting member  498  is now in the switch off position originally shown in FIG.  17 (A), but positioning ratchet  458  and rotating member  454  are in the position to align front derailleur  70  with the small diameter front sprocket. 
     The operation to shift from the large diameter front sprocket to the intermediate diameter front sprocket is the same. However, in this case positioning ratchet  458  would be positioned initially such that pawl tooth  475  abuts against positioning tooth  686 . As positioning pawl  474  rotates clockwise in response to pressure from cam plate  494 , pawl tooth  475  clears positioning tooth  686 , and positioning ratchet  458  rotates counterclockwise until positioning tooth  690  contacts pawl tooth  476 . When positioning pawl  474  rotates counterclockwise as the cam lobe  497  of cam plate  494  reaches cam roller  478 , pawl tooth  475  enters the space between positioning teeth  682  and  686 , and pawl tooth  476  releases positioning tooth  690  so that positioning ratchet  458  and rotating member  454  rotate clockwise until positioning tooth  682  contacts pawl tooth  475 , thus maintaining positioning ratchet  458  and rotatable member  454  in the position shown in FIG.  17 (A). 
     While the above is a description of various embodiments of the present invention, further modifications may be employed without departing from the spirit and scope of the present invention. For example, while separately operated drive control pawls  530  and  538  were provided in the preferred embodiment, the embodiment shown in FIGS.  19 (A) and  19 (B) show a single drive control pawl  700  with pawl teeth  704  and  708 . Pawl tooth  704  contacts abutment  578  on motion transmitting member  498  when motion transmitting member  498  is in the home position as shown in FIG.  19 (A). Pawl tooth  708  contacts abutment  570  on motion transmitting member  498  when motion transmitting member  498  is rotating clockwise to the switch off position and control plate  486  has not yet rotated to the neutral position as shown in FIG.  19 (B). 
     While a cam plate  494  was used to control positioning pawl  474  in a downshifting operation in the preferred embodiment, FIG. 20 shows an embodiment wherein a cam wheel  750  controls the operation of positioning pawl  474 . In this embodiment, cam wheel  750  is coaxially and rotatably mounted relative to positioning ratchet  458 . Cam wheel  750  includes a plurality of circumferentially disposed cam teeth  754  and a plurality of circumferentially disposed cam drive teeth  758 . A cam drive pawl  762  is pivotably mounted to a release plate  486 ′ through a pivot shaft  766  and biased in a counterclockwise direction by a spring  770 . When release plate  486 ′ rotates in the counterclockwise direction during a downshift operation, cam drive pawl  762  engages one of the plurality of cam drive teeth  758  and rotates cam wheel  750  in the counterclockwise direction. One of the plurality of cam teeth  754  presses against cam roller  478  and causes positioning pawl  474  to rotate in the clockwise direction in the same manner as in the preferred embodiment. When the cam tooth  754  passes cam roller  478 , positioning pawl  474  rotates in the counterclockwise direction to complete the downshift operation. Cam drive pawl  762  disengages from the corresponding cam drive tooth  758  when release plate  486 ′ rotates in the clockwise direction. 
     While a manually operated input unit  250  was described in the foregoing embodiments, an electrically operated input unit may be used instead. The following describes such an input unit. FIGS.  21 (A) and  21 (B) are laterally outer and inner side views, respectively, of a mounting unit such as a housing  800  that may be operatively coupled directly or indirectly to positioning unit  254 . The outer side of a wall  802  of housing  800  supports a motor  804 , a gear reduction unit  812 , an input brush unit  816  and an output brush unit  820 . 
     Motor  804  includes a motor drive shaft  808  that meshes with a larger diameter gear portion  824  of a gear  828 . A smaller diameter gear portion of  832  of gear  828  meshes with a larger diameter portion  836  of a gear  840 , and a smaller diameter gear portion  844  of gear  840  meshes with a larger diameter gear portion  848  of a gear  852 . A smaller diameter gear portion  856  of gear  852  meshes with a gear  860  supported by an axle  862  that passes through wall  802  to the inner side of housing  800 . 
     Input brush unit  816  rotates coaxially together with gear  860 , and it includes a conductive brush  864  that functions in a manner described below. Axle  862  supports a drive cam  865  FIG.  21 (B) with a drive projection  866  on the inner side of housing  800 . Output brush unit  820  is rotatably supported to housing  800  by an axle  867  that passes through wall  802  to the other side of housing  800 . Output brush unit  820  is disposed within a chamber  868  defined by a wall  872 , and it also includes a conductive brush  876  that functions in a manner described below. Electrical connectors  880  and  884  are attached to housing  800  to provide electrical communication with the various electrical components used in this embodiment. 
     As shown in FIG.  21 (B), axle  867  includes male coupling splines  888  that project into a recess  892  formed on the inner side of housing  800 . Male coupling splines are  888  used to couple output brush unit  820  to rotating member  454  in positioning unit  254  so that rotating member  454  and output brush unit  820  rotate coaxially as a unit. To accomplish, a coupling member  896  (FIGS.  22 (A)- 22 (C)) is mounted to rotating member  454  and is ordinarily disposed in recess  892 . In this embodiment, axle  318  of positioning unit  254  terminates in a central opening  900  formed in the inner side of boss  904  of coupling member  896 , and female coupling splines  908  are formed on the outer side of boss  904  for engaging the male coupling splines  888  on axle  862 . Coupling ears  912  and  916  are formed on a radially outer portion of rotating member  454 , and a coupling projection  920  extends laterally from a radially outer portion of coupling member  896 . Thus, coupling member  896  rotates integrally with rotating member  454  as a result of the locking engagement of coupling projection  920  with coupling ears  912  and  916 , and output brush unit  820  rotates integrally with coupling member  896  and rotating member  454  as a result of the locking engagement of splines  888  and  908 . Rotating member  454  and output brush unit  820  move between a downshifted (e.g., low) position shown in FIGS.  22 (A) and  23 (A), a neutral (e.g., middle) position shown in FIGS.  22 (B) and  23 (B), and an upshifted (e.g., top) position shown in FIGS.  22 (C) and  23 (C). 
     In the embodiments described above, wire coupling member  302  rotated input link  306  which, in turn, rotated control plate  518  to the upshift, neutral and downshift positions to produce the desired operation of assist mechanism  14 . FIGS.  24 (A)- 24 (C) and  25  show the structures that rotate control plate  518  in this embodiment. More specifically, drive cam  865  rotates an input transmission member drive member in the form of an input transmission drive link  924  that is rotatably supported to base plate  450  between a downshift position shown in FIG.  24 (A), a neutral position shown in FIG.  24 (B), and an upshift position shown in FIG.  24 (C). Input brush unit  816  is shown superimposed on drive cam  865  to facilitate a discussion of the electronic controls associated with this embodiment later on. 
     As shown in FIGS.  24 (C) and  25 , input transmission drive link  924  includes a first end such as an axle mounting portion  928  with an axle receiving opening  932  for receiving axle  318  therein (so that input transmission drive link  924  rotates coaxially with rotating member  454  and output brush unit  820 ), spring abutments  936  and  938 , a radially extending portion  940 , and an axially extending coupling portion  944  with a coupling tab  948  that fits into opening  605  in control plate  518 . First and second drive ears  952  and  956  extend radially outwardly and form first and second drive surfaces  960  and  962 , respectively. Coupling portion  944  and drive ears  952  and  956  are disposed at a radially extending second end  958  of input transmission drive link  924 . Drive projection  866  is disposed between first and second drive surfaces  960  and  962 , and the spacing of first and second drive surfaces  960  and  962  are such that drive projection  866  is spaced apart from first and second drive surfaces  960  and  962  when input transmission drive link  924  is in the neutral position as shown in FIG.  24 (B). Of course, input transmission drive link  924  can take many different forms, and many structures could be used to rotate input transmission drive link  924  to the various positions, such as various link assemblies, rotating eccentric cams, rotating intermittent contact cams, and so on. 
     A biasing mechanism in the form of a spring  968  has a coiled section  972  and a pair of spring legs  976  and  980  for biasing input transmission drive link  924  to the neutral position. More specifically, coiled section  972  surrounds axle  318 , and spring legs  976  and  980  contact spring abutments  982  and  986  formed on base plate  450  when input transmission drive link  924  is in the neutral position shown in FIG.  24 (B). When input transmission drive link  924  rotates counterclockwise to the position shown in FIG.  24 (A), spring abutment  936  presses against spring leg  976  so that spring  968  biases input transmission drive link  924  in the clockwise direction. On the other hand, when input transmission drive link  924  rotates clockwise to the position shown in FIG.  24 (C), spring abutment  938  presses against spring leg  980  so that spring  968  biases input transmission drive link  924  in the counterclockwise direction. 
     FIG. 26 is a view of a circuit board  990  that is mounted to the outer side of housing  800 . Circuit board  990  includes input position conductive traces  996  and output position conductive traces  998  (as well as other circuit elements that are not shown for easier understanding). Input conductive traces  996  include a common trace  996   a , a downshift position trace  996   b , a neutral position trace  996   c , and an upshift position trace  996   d . Input brush unit  816  is shown superimposed with input position conductive traces  996  to show the cooperation between the structures. These structures can be considered parts of an overall input drive member position sensor  1002  (FIG. 27) with a downshift position sensor  1002   a , a neutral position sensor  1002   b , and an upshift position sensor  1002   c . Control unit  1000  uses the resulting signal to determine the position of drive cam  865  and therefore input transmission drive link  924 . In the position shown in FIG. 26, input brush unit  816  is in the neutral position, wherein brush  864  connects neutral position trace  996   c  to common trace  996   a.    
     Output conductive traces  998  include a common trace  998   a , a downshifted (e.g., low) position trace  998   b , a neutral (e.g., middle) position trace  998   c , and an upshifted (e.g., top) position trace  998   d . Output brush unit  820  is shown superimposed with output position conductive traces  998  to show the cooperation between the structures. These structures can be considered parts of an overall output transmission member position sensor  1004  (FIG. 27) with a downshift position sensor  1004   a , a neutral position sensor  1004   b , and an upshift position sensor  1004   c . Control unit  1000  uses the resulting signal to determine the position of rotating member  454 . In the position shown in FIG. 26, output brush unit  820  is in the neutral position, wherein brush  876  connects neutral position trace  998   c  to common trace  998   a.    
     FIG. 27 is a block diagram of electrical components used for controlling the operation of assist mechanism  14 . In this embodiment, control unit  1000  receives signals from input drive member position sensor  1002 , output transmission member position sensor  1004 , a manually operated upshift switch  1008 , a manually operated downshift switch  1012 , a speed sensor  1014  and a cadence sensor  1015 . Of course control unit  1000  may receive signals from any number of other inputs, such as the rider&#39;s physical characteristics, terrain data, and so on. Upshift switch  1008  and downshift switch  1012  typically are mounted at some convenient location on handlebar  50 , and they may take many different forms such as buttons, toggle switches, levers, twist grips coupled to switching mechanisms, and so on. Speed sensor  1014  typically comprises a conventional sensor mounted to frame  18  for sensing the passage of a magnet mounted to front wheel  46  or rear wheel  54 , but of course it may comprise any structure (e.g., optical or electromagnetic) that accomplishes the same purpose. Similarly, cadence sensor  1015  typically comprises a conventional sensor mounted to frame  18  for sensing the passage of a magnet mounted to pedal assembly  58 , but of course it may comprise any structure (e.g., optical or electromagnetic) that accomplishes the same purpose. 
     Control unit  1000  includes a motor drive command unit  1016  for providing commands that drive motor  804  (directly, or indirectly through a motor interface). Upshift switch  1008  and downshift switch  1012  typically are used for manually requesting an upshift or a downshift operation, respectively, and control unit  1000  causes motor drive command unit  1016  to provide commands to operate motor  804  accordingly. In this embodiment, control unit  1000  also includes an automatic control unit  1020  which causes motor drive command unit  1016  to provide commands to operate motor  804  automatically according to any number of the inputs and according to any desired algorithm. Such commands may comprise analog or digital messages, direct drive signals, or any other signal suitable for the particular application. Control unit  1000 , motor drive command unit  1016  and automatic control unit  1020  may comprise a suitably programmed microprocessor disposed on circuit board  990 , or any other suitably configured hardware, firmware or software implementation disposed or distributed anywhere that is convenient for the application. 
     The operation of this embodiment is rather straightforward. Input transmission drive link  924  ordinarily is located in the neutral position as shown in FIG.  24 (B) and determined by input drive member position sensor  1002 . If a downshift command is generated either by the operation of downshift switch  1012  or automatic control unit  1020 , then motor drive command unit  1016  generates commands to cause motor  804  to rotate drive cam  865  and thereby move input transmission drive link  924  in the downshift direction (counterclockwise) until input drive member position sensor  1002  senses input transmission drive link  924  in the downshift position shown in FIG.  24 (A). At this time, in this embodiment, control unit  1000  immediately causes motor drive command unit  1016  to generate commands to cause motor  804  to move input transmission drive link  924  in the opposite direction until input transmission drive link  924  returns to the neutral position shown in FIG.  24 (B). 
     Similarly, if an upshift command is generated either by the operation of upshift switch  1008  or automatic control unit  1020 , then motor drive command unit  1016  generates commands to cause motor  804  to rotate drive cam  865  and thereby move input transmission drive link  924  in the upshift direction (clockwise) until input drive member position sensor  1002  senses input transmission drive link  924  in the upshift position shown in FIG.  24 (C). At this time control unit  1000  immediately causes motor drive command unit  1016  to generate commands to cause motor  804  to move input transmission drive link  924  in the opposite direction until input transmission drive link  924  returns to the neutral position shown in FIG.  24 (B). 
     The signals provided by input drive member position sensor  1002  and output transmission member position sensor  1004  may be combined with suitable programming of control unit  1000  to provide a mechanism for detecting possible malfunctions of assist mechanism  14 . FIG. 28 is a flow chart showing a possible operation of control unit  1000  for that purpose. Assume a shift request is made in a step  1100 , either by pressing upshift switch  1008  or downshift switch  1012 , or by operation of automatic control unit  1020 . It is then ascertained in a step  1104  whether a battery condition (e.g., voltage) is sufficient to drive motor  804  for the desired shift. If not, then a possible error is processed in a step  1108 . Such a process could include a warning to the rider such as a warning tone and/or a visual signal such as an error message. Additionally, a prohibition condition could be set within control unit  1000  to prevent any further attempt to operate assist mechanism  14  by control unit  1000  until the matter is resolved. 
     If battery condition is acceptable, it is then ascertained in a step  1112  whether an upshift command has been made when the front derailleur  70  is already engaged with the outermost sprocket  66 . If so, then the appropriate error processing is performed in step  1108 . Otherwise, it is then ascertained in a step  1116  whether a downshift command has been made when the front derailleur  70  is already engaged with the innermost sprocket  66 . If so, then the appropriate error processing is performed in step  1108 . Otherwise, the shifting operation is allowed to begin in a step  1120 . This step may include resetting of a timer used to control the shifting operation as well as setting any other variables (such as a retry counter discussed below) used in the process. 
     In this embodiment, it is assumed that motor  804  can complete its operation to cause input transmission drive link  924  to move from the neutral position, to the desired upshift or downshift position, and back to the neutral position in approximately one second. Accordingly, it is then ascertained in a step  1124  whether less than one second has elapsed since the beginning of the shifting operation in step  1120 . If so, then motor drive command unit  1016  in control unit  1000  issues the appropriate commands to drive motor  804  in a step  1128 . Step  1128  represents whatever movement of motor  804  is needed to cause input transmission drive link  924  to move from the neutral position, to the desired upshift or downshift position, and back to the neutral position. It is then ascertained in a step  1132  whether input transmission drive link  924  has returned back to the neutral position. If not, then processing returns to step  1124 . Otherwise, motor  804  is stopped in a step  1136 . Motor  804  also is stopped if it is ascertained in step  1124  that more than one second has elapsed since the beginning of the shifting operation in step  1120 . In any event, step  1136  also represents the start of the mechanical phase of the assist operation wherein one of drive members  290  contacts rotating member engaging member  394  to assist the shifting operation. In step  1136 , various control variables may be initialized as is appropriate for the application. 
     It is then ascertained in a step  1140  whether input transmission drive link  924  has returned back to the neutral position. This step is optionally performed as a double check on the position of input transmission drive link  924 , but this step also may be used to determine whether a malfunction occurred if it is ascertained in step  1124  that more than one second has elapsed since the beginning of the shifting operation in step  1120  without the neutral position being ascertained in step  1132 . If input transmission drive link  924  is not in the neutral position at this time, then the appropriate error processing is performed in step  1108 . Otherwise, it is ascertained in a step  1144  whether the current gear indicated by output transmission member position sensor  1004  is the same as the requested destination gear. If so, then shifting is considered complete in a step  1148 . 
     In this embodiment, it is assumed that shifting will complete in ten seconds as long as pedal assembly  58  is rotating. Since many conditions can affect the shifting characteristics of any derailleur (such as the type of chain and sprocket used, whether the chain and sprockets are designed with any shift facilitating structures, the forces exerted by the rider and the bicycle, and so on), it is also assumed that it may take longer to shift the chain under some circumstances. Accordingly, the present embodiment retries the shifting operation three times when a failure is detected. To that end, it is ascertained in a step  1152  whether cadence sensor  1015  indicates that the pedal assembly  58  is rotating. If not, processing returns to step  1144 . Otherwise, it is ascertained in a step  1156  whether more than ten seconds has elapsed since the assist operation was begun in step  1136 . If not, then processing returns to step  1144 . If more then ten seconds has elapsed, then a retry counter programmed in control unit  1000  is incremented by one in a step  1160 , and it is then ascertained in a step  1164  whether more than three retries have been attempted. If so, then the appropriate error processing is performed in step  1108 . Otherwise, processing reverts back to step  1120  to retry the operation. 
     Of course, the foregoing electronic control system and method could be adapted to any type of bicycle transmission, such as internal hub transmissions, combination hub/derailleur transmissions, continuously variable transmissions, and so on. The system also could be adapted to uses other than bicycle transmissions. In all cases, the size, shape, location or orientation of the various components may be changed as desired. Components that are shown directly connected or contacting each other may have intermediate structures disposed between them. The functions of one element may be performed by two, and vice versa. The structures and functions of one embodiment may be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the scope of the invention should not be limited by the specific structures disclosed or the apparent initial focus on a particular structure or feature.