Patent Publication Number: US-6213236-B1

Title: Sensor for bicycle with assist engine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of our application entitled “BICYCLE WITH ASSIST ENGINE”, Ser. No. 08/692,876, filed Aug. 2, 1996 now U.S. Pat. No. 5,937,962 and assigned to the assignee hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a manually powered vehicle and more particularly to a manually powered vehicle such as a bicycle with an assist engine for assisting in the operation of the vehicle. 
     A wide variety of vehicles have been proposed which are designed to be primarily powered by manual power input of the rider. In order to expand the use of these types of vehicles, it has also been proposed to provide various forms of power assist. These power-assist devices incorporate a prime mover, such as an electric motor or an internal combustion engine, which can be operated so as to assist in the manual propulsion of the vehicle. One very successful power-assisted vehicle of this type employs a sensor that senses the actual manual force input by the operator, and then operates the prime mover so as to provide an assist force that is proportional to the manual force. 
     The advantage of this type of system is that the rider still maintains a “feel” in operating the vehicle. That is, the operator&#39;s manual input of force is necessary in order to have the vehicle propelled, and thus the exercise value of the manual propulsion system is maintained. By utilizing the power assist, this type of vehicle can be utilized by persons of varying physical capabilities. Also there is less need to govern the speed of the vehicle since the operator must always input a force to have any power assist. 
     With these types of power-assist mechanisms, the device generally utilizes a form of force sensor which senses the actual manual force input by the operator. This manual input force is then utilized to provide a control signal for controlling the appropriate amount of assist by the prime mover. 
     However, many of these types of vehicles are operated by mechanisms wherein the operator input is not constant. For example, many vehicles of this type utilize a pedal-operated crank mechanism for their manual input. Because of the angular relationship of the crank mechanism, the actual force varies cyclically during a single revolution. Thus, the power assist will also vary cyclically in the same manner. 
     This presents a particular problem when the prime mover is an internal combustion engine. If the engine output is varied cyclically along with the manual force, then because of the inherent uneven characteristics and quick response of internal combustion engines, a jerky operation will result. 
     It is, therefore, a principal object of this invention to provide an improved power-assisted vehicle that employs a powering internal combustion engine. 
     It is a further object of this invention to provide an improved internal combustion engine power-assisted vehicle wherein the power assist is proportional to the manual force input, but a smoothing of the applied power assist is accomplished to avoid jerky operation. 
     In addition to the problem of the cyclically varying manual input, these types of vehicles also employ brake systems for retarding the forward motion of the vehicle. There may be times when the brake is applied, and also some force may nevertheless be applied to the pedals. For example, the operator may be holding the vehicle stationary, but nevertheless may apply some force to the pedals. Since the force-sensing mechanism will sense an output, then the power assist may energize the engine and undesirable results may occur. 
     It is, therefore, a still further object of this invention to provide a power-assisted vehicle wherein the application of power assist can be totally disabled when the brake is applied. 
     It is a further object of this invention to provide an internal combustion engine-assisted manually operated vehicle wherein the relationship between the power assist and the operator force can be altered to suit certain conditions. 
     SUMMARY OF THE INVENTION 
     This invention is adapted to be embodied in an engine assisted, manually powered vehicle. The vehicle comprises a body assembly that is adapted to accommodate at least one rider. A propulsion device is provided for propelling the vehicle. A manual operator receives manual force inputted from the rider for driving the propulsion device. The manual operator is configured such that the force applied by the rider varies cyclically during a force application mode by the operator from a minimum value to a maximum value and back to a minimum value. An internal combustion engine is also provided for the vehicle. The internal combustion engine is coupled to the propulsion device for driving the propulsion device. The engine has a control for varying the output of the engine. A force sensor senses the force applied by the rider to the manual operator and outputs a control signal. A drive coupling couples the output from the force sensor to the engine control for providing an engine assist to the manual operator of the propulsion device in response to the degree of manual force applied. Means are provided for selectively modifying the connection between the force sensor output and the engine control for effecting changes in the amount of engine assist applied to the propulsion device. 
     In accordance with one feature of the invention, the modification of the connection between the force sensor output and the engine control reduces the change in rate of engine control in response to a rapid change in the force output from the sensor. 
     In accordance with another feature of the invention, the vehicle is also provided with a manually operated brake. The coupling between the force sensor output and the engine control is disconnected when the brake is actuated for precluding engine assist when the brake is applied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a bicycle constructed in accordance with an embodiment of the invention. 
     FIG. 2 is an enlarged cross-sectional view taken generally along the line  2 — 2  of FIG.  1  and shows the connection between the manual drive and the engine power assist for driving the rear wheel. 
     FIG. 3 is an enlarged partially schematic side elevational view looking generally in the same direction as FIG. 1, with portions broken away so as to show the construction of the engine and the cooperation of the force sensor with the engine control. 
     FIG. 4 is an enlarged cross-sectional view showing the portion of the control connection for reducing variations in engine speed during sudden changes in manual power input. 
     FIG. 5 is a graphical view showing the manual pedaling force, engine power assist, and resulting power, both with and without a feature of the invention. 
     FIG. 6 is a graphical view showing the pedaling force and throttle opening during changes in manual-force application to show how the system operates to minimize fluctuations in power assist upon sudden changes in manual input. 
     FIG. 7 is a series of cross-sectional views showing how the brake disconnect system operates in controlling the interrelationship between the force sensor and the engine throttle control. 
     FIGS. 8 and 9 are electrical diagrams showing how the brake disabling interconnection between the force sensor and the engine throttle control operates. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring now in detail to the drawings, and initially to FIG. 1, a manually powered vehicle embodying the invention is identified generally by the reference numeral  11 . As will become apparent to those skilled in the art, the invention may be utilized with a wide variety of types of vehicles which are primarily manually powered. Bicycles are a typical vehicle of this type, and therefore the vehicle  11  depicted is a bicycle. Although the invention is described in conjunction with a land vehicle of this type, it will be readily apparent to those skilled in the art how the invention can be applied to a wide variety of other types of vehicles, including watercraft. 
     The bicycle  11  is comprised of a body assembly, indicated generally by the reference numeral  12 , and which is primarily a frame assembly. This body assembly or frame is comprised of a head pipe  13 , main frame member  14 , and down tube  15 , which are all connected to each other in an appropriate manner. A seat  16  is adjustably supported by a seat post on a seat pillar  17  that is connected in an appropriate manner to the rear portion of the frame tubes  14  and  15 . This seat  16  is adapted to accommodate a rider in a well-known manner. 
     The bicycle  11  includes a front wheel  18  that is rotatably journaled at the lower end of a front fork  19 . The front fork  19  is dirigibly supported by the head tube  12  and is steered by a handlebar  21  that is connected through a handlebar post  22  to the upper end of the front fork  19  in a manner which is also well known in this art. 
     A rear wheel  23  is rotatably journaled at the end of a trailing arm  24 , which is, in turn, connected to the frame assembly  12  at the lower end of the seat pillar  17  in a known manner. A back stay  25  is interconnected between the trailing arm  24  and the seat pillar  17  for providing rigid support for the rear wheel  23 . 
     The rear wheel  23  has associated with it a sprocket that is driven by a chain  26  in a manner which will be described. The rear wheel  23  is primarily driven by a manually operated pedal device, indicated generally by the reference numeral  27 , which includes a pedal crank shaft  28  and which is coupled to the chain  26  in a manner which will be described for manual powering of the bicycle  11 . 
     In addition, a power assist in the form of a prime mover, preferably an internal combustion engine, indicated generally by the reference numeral  29 , is also provided for assist powered operation of the bicycle  11  in a manner which also will be described. As will become apparent, the pedal mechanism  27  has associated with it a force or torque detector. This force detector detects the force exerted by the rider on the pedals  31  mounted at the end of the pedal device  27  and controls the motor  29  to provide an assist which generally is related to the manual input power exerted by the rider on the seat  16 . There is, however, a control interconnection between the manual force sensor and the control for the engine assist provided by the engine  29  which incorporates the invention and which will be described in more detail later. 
     Referring now in detail primarily to FIG. 2, the manually powered crank assembly  27  is associated with an outer housing assemblage, indicated generally by the reference numeral  32 , and which is connected to the frame assembly  12  as thus far described. This housing assembly  32  is positioned at the lower end of the down tube  15  and the seat pillar  17 . The crankshaft  28  is journaled therein for rotation by a pair of transversely spaced bearing assemblies  33  and  34 . Pedal arms  35  are connected rigidly to the opposite sides of the crankshaft  28  and pivotally carry the pedals  31  at their outer ends. 
     The crankshaft  28  is connected in a manner to be described to drive a drive sprocket  36 , which is journaled on the housing member  32  via a drive sleeve  37 . The drive sprocket  36  is engaged with the aforenoted chain  26  for driving the rear wheel  23 . 
     The drive sleeve  37  is affixed to an driving assembly, indicated generally by the reference numeral  38 , which forms a portion of a planetary transmission, indicated generally by the reference numeral  39 , and which is contained within a gear casing cavity  41  defined by the outer housing  32 . The driving assembly  38  is connected to a ring gear  42  of the planetary transmission  39 . This ring gear  42  is engaged with a plurality of planet gears  43  that are journaled on stub shafts  44  of a carrier element  45 . 
     The carrier element  45  is connected by means of a one-way clutch  46  to the crankshaft  28  so as to provide a step-up transmission between the rotation of the crankshaft  28  and the rotation of the driving sprocket  36 . The one-way clutch  46  only permits drive from the crankshaft  28  about its axis A to the sprocket  36  when the crankshaft  28  is pedaled in the direction indicated at B in FIG.  2 . In other words, the drive sprocket  36  can overrun the rotation of the crankshaft  28 . 
     The planet gears  43  are further engaged with a sun gear  47  which is formed integrally with a portion of a force or torque sensing mechanism, indicated generally by the reference numeral  48 , and which has a construction as best seen in FIG.  3 . This torque sensing mechanism  48  is utilized so as to measure the torque or manual force exerted by the rider on the pedals  31 , or more specifically on the crank operating mechanism  27  so as to control the engine  29  so as to provide power assist in the manner which will be described shortly. 
     The engine  29  is, in the illustrated embodiment, of a single-cylinder, two-cycle, crankcase compression, spark-ignited type. Although this type of engine is depicted, it will be readily apparent to those skilled in the art that a wide variety of other types of internal combustion engines may be utilized in conjunction with the invention. However, because of their simplicity, the invention has particular utility in conjunction with such two-cycle engines. 
     The engine  29  includes a cylinder block  49  having a cylinder bore  51  in which a piston  52  reciprocates. The piston  52  is connected by means of a connecting rod  53  to a crankshaft  54  that is rotatably journaled within a crankcase chamber  55  of the engine. The engine  29  is mounted so that the crankshaft  31  rotates about an axis that generally intersects the axis A of the crankshaft  28  of the pedal-operated mechanism  27 . 
     An intake charge, which comprises a fuel-air mixture, is introduced to the crankcase chamber  55  by an induction system that includes a charge former in the form of a carburetor  56 . The carburetor  56  draws air from the atmosphere through an air inlet silencer device and filter  57  (shown primarily in FIG. 1) which is mounted to the rear of the engine  29  and beneath the seat pillar  17 . This air silencer and inlet device  57  supplies filtered and silenced air to an intake passage  58  of the carburetor  56 . 
     A sliding piston-type throttle valve  59  is slidably supported within the body of the carburetor  56  and is operated in a manner which will be described. This sliding piston  59  also is coupled to a metering rod for controlling the supply of fuel mixed with the incoming air and flowing in the direction indicated by the arrow  61  in FIG. 3 to the crankcase chamber  55 . Fuel is supplied to the carburetor  56  from a fuel tank  62  (FIG. 1) that is mounted behind the seat pillar  17  and adjacent the rear wheel  23  in an otherwise unused space of the bicycle  11 . 
     An intake passage  63  is formed in the cylinder block  49 , and the flow into this passage is controlled by a reed-type check valve  64 , as is well known in this art. The fuel-air charge is then transferred from the crankcase chamber  55  where it is compressed for further compression between the head of the piston  52  and a cylinder head  65  through one or more scavenge passages (not shown). This charge is fired by a spark plug (not shown) and then is discharged to the atmosphere through an exhaust system which includes a muffler  66  (FIG.  1 ). 
     The engine crankshaft  54  is connected to drive the sprocket  36  in assistance to the manual operation through a transmission, indicated generally by the reference numeral  67  and which is shown best in FIGS. 2 and 3, with reference first to the latter figure. This transmission includes a centrifugal clutch  68  which is driven by the crankshaft  54  and which drives the input of a reducing planetary transmission  69  when the speed of the crankshaft  54  exceeds a predetermined speed. 
     The output shaft  71  of this planetary transmission  69  is coupled to a pinion shaft  72  through a one-way clutch  73 . The one-way clutch  73  permits the planetary transmission output shaft  71  to drive the pinion shaft  72 . However, this one-way clutch  73  will not permit pedaling by the operator to crank the engine  29  in the event the engine  29  is stalled or stopped. 
     Referring now primarily to FIG. 2, the pinion shaft  72  is formed with an integral pinion gear  74 . This pinion gear  74  is enmeshed with a ring gear  75  which is affixed to the ring gear  42  of the planetary transmission  39 . Hence, the engine  29  can also drive the planetary transmission along with the operation of the pedal-operated crankshaft  28 , although the engine  29  cannot drive the crankshaft  28  because of the interpositioning of the one-way clutch  46 . 
     It has been noted that the torque or force sensor  48  is operative so as to control the amount of assist provided by the engine  29  to the manual input of the operator through the manually operated crank mechanism  27 . This sensor  48  will now be described by more detail through reference to FIGS. 2-4, and the theory of operation will be described by reference to FIGS. 5 and 6. 
     It has been noted that the torque sensor  48  employs the sun gear  47 . To this end, the sun gear  47  is connected to or formed integrally with a lever arm or disk-shaped member  76  that has an arm portion  77  which extends rearwardly and downwardly. This arm portion  77  has a flattened upper surface  78  that is engaged by the toe portion  79  of a lever  81 . The lever  81  is journaled on the housing  32  by means of a pivot pin  82 . The lever  81  has a further arm portion  83  that is connected by means of a connector  84  to one end of a wire actuator  85 . 
     The wire actuator  85  forms a part of an interconnecting mechanism for connecting the torque sensor  48  with the throttle valve  59  of the carburetor  56 . This interconnecting mechanism further includes a disconnect/reconnect device, indicated generally by the reference numeral  86 , that cooperates with a further wire actuator  87  for actuating the throttle piston  59  in opposition to the action of a return spring  88 . The return spring  88  normally urges the throttle piston  59  to its closed or idle position. 
     Referring again to FIG. 3, the control lever  81 , and specifically its toe portion  79 , is urged by a coil compression spring  89  toward engagement with the surface  78 . This spring  89  is contained within an integral post  89  of the housing  32 . The preload of this spring  89  is controlled by an adjustable stop  92 . Hence, the preloading of the spring  89  will determine the force necessary to effect pivotal movement of the actuating lever  81  from its idle position, as shown in solid lines, toward its full thrust position, as shown in phantom lines, as the pedaling torque increases between its low level or zero torque position and the maximum torque position. Thus, as the pedaling torque increases, the wire actuators  85  and  87  operating through the disconnect/reconnect mechanism  86  will move the throttle piston  59  from its idle position to its full throttle position so as to vary the amount of power assist generated by the motor  29 . This power assist is, therefore, generally directly related to the operator input torque. 
     As may be seen in FIG. 5, the actual force applied to the pedals, or torque exerted on the crankshaft  28 , varies cyclically generally in accordance with a sinusoidal wave as the respective pedals are pressed and released. The actual torque will be at a minimum zero position at top and bottom dead centers of each crank arm, and will be at their maximum at the 90-degree position. Thus, without any other change, it will be seen that the engine power assist will generally follow the same curve as shown in the dot-dash lines of FIG. 5, and thus there will be rather severe variations in the actual power applied to the rear wheel  23  during a single crank revolution. 
     This can give an unacceptable feel to the ride of the bicycle and to the feeling of the operator. Therefore, a damping mechanism, indicated generally by the reference numeral  93  and shown in most detail in FIGS. 3 and 4, is employed so as to reduce the drop off in power assist when the pedal is on its return or up stroke and no significant force is being exerted by the operator. Basically, this damping mechanism  93  operates as a shock absorber and/or damper to preclude reverse movement in a direction opposite to the direction indicated at B in FIGS. 3 and 4 when the pedal force falls off to zero or is reduced suddenly. 
     This mechanism is mounted in the outer housing  32  and includes a first cylindrical member  94  that defines a fluid chamber  95  by means of a bore  96  formed therein. A closure plug  97  closes the end of the chamber  95  and defines a further fluid chamber  98 . The chambers  95  and  98  are separated by a piston  99  which is slidably supported in the bore  96  and which is engaged at the bottom end of its stroke with a shoulder  101  formed by an extending portion of the closure plug  98 . A coil compression spring  102  is also contained in the chamber  98  so as to urge the piston  99  and a piston rod  103  integrally connected therewith toward engagement with a flattened shoulder surface  104  of the torque sensor member  77 . This piston rod  103  extends through a seal  105  so as to provide a fluid-tight seal for the chamber  95 . 
     The piston  99  is formed with a first restricted flow passage  106  that has an orifice diameter which is chosen to provide the desired damping effect. In addition, a larger passage  107  also extends through the piston  99  in parallel relationship to the damping passage  106 . A one-way check valve  108  of the reed type is contained within the chamber  98  and controls the flow through the passage  107  so that there can be relatively free flow from the chamber  95  to the chamber  98 . However, the valve  108  will close and preclude any reverse flow from the chamber  98  to the chamber  95  through the passage  107 . 
     The solid-line views in FIGS. 3 and 4 show, as previously noted, the position of the torque-sensing member  77  when no force is applied to the pedal mechanism  27 . The spring  89  and carburetor return spring  88  will act on the torque member  77  to hold it in this position. 
     As the operator exerts a force on the pedal mechanism  27 , the torque arm  77  will rotate in the direction indicated by the arrow B, and the lever  81  will actuate the throttle valve  59  to open it, as previously described. When this occurs, the flat  104  tends to move away from the plunger extension  103 . However, the spring  102  will urge the piston  99  upwardly and force fluid from the chamber  95  to the chamber  98  through the passage  107  upon opening of the check valve  108 . This movement can continue until the full-boost position, as shown in the phantom-line views, is reached. 
     When the pedal force exerted by the operator decreases, the device  93  will, however, preclude a rapid release of the throttle opening. Under this condition the springs  88  and  89  will act to pivot the torque-sensing lever  77  in a direction opposite to that of the arrow B. This exerts a force through the piston rod  103  on the piston  99 . Pressure will increase in the chamber  98 , and the check valve  108  will close. Hence, downward movement of the piston  99  will be restricted by the flow through the restricted orifice  106 . Thus, as seen in FIG. 5, the engine power will not fall off abruptly, but will gradually reduce until the next application of power. As a result, the power assist will vary more gradually than the actual pedal force, giving a better feel to the operator. 
     FIG. 6 also shows the type of hysteresis curve that results in that the opening movement occurs linearly, but the closing movement will fall off gradually due to the operation of the damping mechanism  93 . 
     In addition to tailoring the power assist from the motor  29  to the driving of the rear wheel  23  when the pedal pressure falls off due to the cyclic operation, there is also another situation when the power assist may not be necessary or desirable. This occurs when the operator of the bicycle is applying the brakes. The brakes of the bicycle and the interrelationship to the motor assist will now be described first by reference to FIG. 1, wherein the brake system is illustrated. 
     The handlebar assembly  21  carries a pair of brake levers  109 , each of which comprises a respective pivotally supported lever  111 . One of these levers  109  operates a front brake  112  through a respective wire actuator  113  so as to brake the rotation of the front wheel  18 . The other lever is connected through a wire actuator  114  to a rear wheel brake  115 . The operator may operate one or both of the brakes  112  and  115 , depending upon whether he wishes to slow the bicycle to make a turn or whether he wishes to hold the bicycle stationary. 
     In either instance, the operator may apply force to the pedals  31 , and the torque sensor  48  will thus normally call for operation of the motor  29  to assist in this manual force. The disconnect/reconnect device  86  is intended and serves the function of precluding engine assist when either or both of the brakes  112  and  115  are actuated. 
     This mechanism is, in the illustrated embodiment, electrically operated and includes a sensor  116  that senses when either or both of the levers  111  are actuated. This sensor switch  116  cooperates with an electric battery, which is shown schematically in FIGS. 8 and 9 and which is indicated generally by the reference numeral  117 . This battery operates the disconnect/reconnect mechanism  86  in a manner which will now be described by particular reference to the three views of FIG.  7  and also later by reference to FIGS. 8 and 9. 
     The disconnect/reconnect mechanism  86  includes an outer housing  118  that is fixed suitably to the body or frame of the bicycle  11 , and this connection may be directly to the housing  32 . The first wire actuator  85  has a protective sheath  119  that is fixed to the housing  118 , with a wire actuator  121  extending into this housing. This wire actuator  121  is connected by means of a connector  122  to a sliding piston  123  that is slidably supported within a bore  124  of the housing  118 . This connection permits the wire actuator  121  to pull the piston  123  to the right, but will not permit any movement of the piston  123  in the opposite direction if the wire actuator  121  moves in the opposite direction. 
     The piston  123  has mounted within it a reversible electric motor  125  that is energized through a circuit, as shown in FIGS. 8 and 9, in response to conditions which will be noted. This motor  125  drives a feed screw  126  in either a forward F direction of rotation or a reverse R direction of rotation, as indicated in FIG.  7 . The feed screw  126  has a threaded connection to a follower element  128  which has a headed portion  129  that is received in the member  123  and which is held against rotation relative the member  124 , but is free to move axially therealong. 
     The member  127  has an opening that receives a fastener  131  for connection with the wire actuating element  132  of the wire actuator assembly  87  which is connected at its other end to the throttle piston  59 . A protective sheath  133  of this wire actuator  87  is affixed to the housing  118  at one end and to the body of the carburetor  56  at the other end. The connection is such that, when the member  123  is moved to the right, as shown in the Figures, from the closed throttle position, as shown at the top view, to the full throttle position, as shown in the bottom view, the member  127  will follow it and actuate the wire  132  and open the throttle piston  59 . 
     In the condition shown in the top and center views, when the throttle pressure is released, the spring  87  will urge the wire actuator  132  to the left which will draw the housing member  123  and wire actuator  121  in the same direction to return to the figure shown in the upper view. Hence, in this condition the wire actuators  85  and  87  are interconnected for simultaneous operation in both the opening and closing directions as noted. 
     In order to uncouple this mechanism upon the actuation of either of the brakes  112  or  115 , the motor  125  is rotated in the direction F so as to advance the piston  129  and housing member  127  to move from the position shown in the top view to the position shown in the bottom view of this figure. When this rotation occurs, the spring  88  of the carburetor  56  can cause the throttle piston  59  to move to its fully closed position and regardless so the movement of the wire actuator  121 , even to its fully opened position under high torque as shown in the bottom view of FIG. 7, no opening of the throttle piston  59  will occur. Thus, the torque sensing device  48  is, in effect, uncoupled from the throttle position  59  under this condition and will not in any way control the position of the throttle piston  59 . 
     The way in which this is accomplished will now be described by primary reference to FIGS. 8 and 9. Before referring to these figures, however, a further discussion of the disconnect/reconnect mechanism  86  shown in FIG. 7 is necessary. 
     The outer housing  118  is provided with a receptive slot  134  in which two limit switches  135  and  136  are mounted. The limit switches  135  and  136  cooperate with the piston portion  129  of the member  127  so as to provide an indication of the relative position of the member  127  along the feed screw  126 . It should be noted that these switches  135  and  136  are carried by the piston  123  and, thus, move along with it. Hence, the switches  135  and  136  indicate the position of the piston  129  relative to the piston  123  and not relative to the outer housing  118 . 
     Referring now to FIGS. 8 and 9, the switches  135  and  136 , as well as the brake actuating detecting switch  116 , are all connected to a solenoid operated switching mechanism, indicated generally by the reference number  137 . This switching mechanism has a solenoid winding or coil  138  which is connected between the battery  117  positive terminal and the ground through the brake switch  116 . The solenoid  138  has an armature  139  that is connected to a first switch  141  and a second switch  142 . The switches  141  and  142  are connected to the leads for the electric motor  125 . 
     When the switches  141  and  142  are open, as shown in the solid lines of FIG. 9, the electric motor  125  can be energized in a direction so that the contact of the switch  141  is positive and the contact of the switch  142  is negative or grounded so that the motor rotates in the direction F so as to advance follower  127  to the left right and the position shown in the upper views of FIG.  7 . On the other hand, when the switches  141  and  142  are open due to movement of the armature  139  upwardly to the position shown in solid lines in FIG. 8, then the motor connection is reversed so that the motor  125  can be rotated in the reverse R direction and return the elements to the position shown in the lower view of FIG.  7 . This is, of course, also dependent upon the condition of the switches  135  and  136 . 
     Considering first the condition when the piston  129  is a position to open the switch  135  and the switch  136  is closed, if the brakes applied by closing the switch  116 , then the solenoid winding  138  is energized and the current flow operates through the motor  125  to drive it in the reverse direction R so as to cause the piston  129  to advance to the left as shown in FIG. 7 from the position shown in the top and center views to the position shown in the lower view. This movement continues until the switch  136  opens and at which time the switch  135  is then closed. 
     The switches then move to the position shown in FIG. 9 and, under this condition, the connection between the torque sensor  48  and the carburetor  56  is discontinued and no engine power assist can occur. When the brake is released, however, then the switch  116  will open from the position shown in FIG. 9 from the solid line view to the phantom line view. When this occurs, the spring acting on the armature  139  will cause the switches  141  and  142  to move from the position shown in solid line views to the position shown in phantom line views, and the motor  125  will be energized to rotate in the direction F so as to return the screw follower  127  to the position shown in the top and center line positions at which time the switch  135  will again be open, and the switch  136  closed to return to the position shown in FIG.  8 . Thus, actuation of the brake will cause deactivation of the connection between the torque sensor  48  and carburetor  56  so as to effectively disable any engine power assist under the condition when the brakes are applied. 
     Thus, from the foregoing description, it should be readily apparent that the described power assist system provides good and responsive engine power assist for the manual force and in an amount that is proportional to the manual force, but which is controlled in such a way as to provide smooth power application and smooth running without abrupt changes. In addition, the power assist can be cut off automatically when the brakes are applied so as to avoid unnecessary and unwanted power application. Of course, the foregoing description is that of preferred embodiments of the invention, and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.