Patent Publication Number: US-6341680-B1

Title: Electric-power-assist transmission and its control method

Description:
This application is a continuation-in-part of application Ser. No. 09/151,660 filed on Sep. 11, 1998 now abandoned, the entire contents of which are hereby incorporated by reference. 
     This application is related to commonly assigned application Ser. No. 09/151,293, filed Sep. 11, 1998, the entire contents of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     In general, the present invention relates to a shift control method for an electric-power-assist transmission. In particular, the present invention relates to a shift control method wherein a gear shift as well as the operations to put the main clutch in an engaged or disengaged state are carried out electrically. More specifically, the present invention relates to a shift control method whereby the main clutch is put back in an engaged state after being released from an engaged state by rotation of the shift spindle from a first rotational position to a second rotational position, with a rotational speed of the shift spindle changed from a first speed to a second speed lower than the first speed with a predetermined timing. 
     2. Description of the Background Art 
     In the conventional transmission, a gear shift is carried out by operating both a clutch pedal (or a clutch lever) and a shift-change lever. On the other hand, in an electric-power-assist transmission disclosed in Japanese Patent Laid-open No. Hei 5-39865, a gear shift is carried out electrically by a motor. In the conventional technologies described above, a shift drum is intermittently rotated in both directions by a driving motor so as to actuate a desired shift fork in a gearshift-change operation. On the other hand, it is possible to put the main clutch in an engaged or disengaged state also by using a motor as well. 
     In such a case, when thinking of the conventional manual transmission, only by repeating the shift-change operation can the shift change be eventually completed even if the gear is not shifted smoothly. In addition, whether or not the clutch can be put in an engaged state smoothly after the shift change much depends on the operation of the clutch carried out by the driver. 
     As described above, in the conventional manual transmission, most of poor operability as evidenced by whether or not a shift change can be completed without repeating the shift-change operation or whether or not the clutch can be put in an engaged state smoothly-much depends on how the operation is carried out by the driver. In other words, the driver&#39;s learning effects allow good operability to be obtained. 
     By driving both the clutch and the shift-change lever by means a motor, on the other hand, elements dependent on the operation carried out by the driver do not exist any more. Thus, in a state where a gear shift is impossible, if the clutch is not put in an engaged state smoothly or not in accordance with the driver&#39;s intention, it is quite within the bounds of possibility that the driver feels a sense of incompatibility. 
     When putting back the clutch in an engaged state after being released from an engaged state, for example, it is desirable to put the clutch in an engaged state slowly in order to reduce the magnitude of a generated shift shock. On the other hand, the speed of a shift change is dependent on the engagement speed of the clutch. It is thus necessary to put the clutch in an engaged state quickly in order to implement a fast shift change. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to solve the problems described above by providing a shift control method to be adopted in an electric-power-assist transmission offering good operability wherein the time it takes to put the main clutch in an engaged state can be shortened and the magnitude of a generated shift shock can also be decreased as well. 
     In order to achieve the object described above, the present invention provides a shift control method to be main adopted in an electric-power-assist transmission wherein a clutch is put in an engaged or disengaged state in a manner interlocked with the rotation of a shift spindle rotated by a driving motor. The shift control method is characterized in that the clutch is put back in an engaged state after being released from an engaged state by rotation of the shift spindle with a rotational speed thereof changed from a high speed to a low speed with predetermined timing. 
     According to the configuration described above, in an operating zone having nothing to do with a shift shock, the clutch is driven at a high speed but, in an operating zone where the clutch is about to be put in an engaged state, the clutch is driven at a low speed. As a result, the time it takes to put the main clutch in an engaged state can be shortened and, at the same time, the magnitude of a generated shift shock can also be decreased as well. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: 
     FIG. 1 is a plan diagram showing an operation unit of a vehicle on which the electric-power-assist transmission provided by the present invention is mounted; 
     FIG. 2 is a diagram showing a partial cross section of the configuration of major components employed in a driving system of the electric-power-assist transmission provided by an embodiment of the present invention; 
     FIG. 3 is a conceptual diagram showing a state in which the sleeve and the gear are engaged with each other; 
     FIG. 4 is a diagram showing a perspective view of the sleeve provided by the present invention; 
     FIG. 5 is a diagram showing a perspective view of the gear provided by the present invention; 
     FIG. 6 is a diagram showing an enlarged portion of a outwardly-directed cog of the sleeve; 
     FIG. 7 is a diagram showing an enlarged portion of a inwardly-directed cog of the gear; 
     FIG. 8 is a diagram showing a state in which the outwardly-directed cog of the sleeve and the inwardly-directed cog of the gear are engaged with each other; 
     FIG. 9 is a diagram showing a perspective view of the conventional sleeve; 
     FIG. 10 is a diagram showing a perspective view of the conventional gear; 
     FIG. 11 is a functional block diagram showing a shift disabling system; 
     FIG. 12 is a diagram showing a model of engagement timing of the conventional sleeve and the conventional gear; 
     FIG. 13 is a diagram showing a model of engagement timing of the sleeve and the gear provided by the present invention; 
     FIG. 14 is a block diagram showing the configuration of major components employed in a control system of the electric-power-assist transmission provided by the embodiment of the present invention; 
     FIG. 15 is a block diagram showing a typical configuration of an ECU employed in the control system shown in FIG. 14; 
     FIG. 16 is a diagram showing Part I of a flowchart provided by the embodiment of the present invention; 
     FIG. 17 is a diagram showing Part II of a flowchart provided by the embodiment of the present invention; 
     FIG. 18 is a diagram showing Part III of a flowchart provided by the embodiment of the present invention; 
     FIG. 19 is a diagram showing Part IV of a flowchart provided by the embodiment of the present invention; 
     FIG. 20 is a diagram showing Part V of a flowchart provided by the embodiment of the present invention; 
     FIG. 21 is a diagram showing Part VI of a flowchart provided by the embodiment of the present invention; 
     FIG. 22 is a diagram showing operational timing charts of a shift spindle provided by the present invention; 
     FIG. 23 is a diagram showing operational timing charts of the rotational angle of a shift spindle and the rotational speed of the engine provided by the present invention in a shift-up operation; 
     FIG. 24 is a diagram showing operational timing charts of the rotational angle of a shift spindle and the rotational speed of the engine provided by the present invention in a shift-down operation; and 
     FIG. 25 is a diagram showing a relation between a PID (Proportional, Integral and Differential) sum value and a duty ratio. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will become more apparent from a careful study of the following detailed description of a preferred embodiment with reference to accompanying diagrams showing the embodiment. FIG. 1 is a plan diagram showing an operation unit of a vehicle on which the electric-power-assist transmission provided by the present invention is mounted. 
     As shown in the figure, the operation unit comprises a shift-up switch  51  for the electric-power-assist transmission and a shift-down switch  52  also for the electric-power-assist transmission, a dimmer switch  53  for changing the direction of a front light, a lighting switch  54  for turning on and off the front light, a start switch  55  for starting the engine and a stop switch  56  for stopping the engine. In the present embodiment, pressing the shift-up switch  51  once will raise the shift position by one stage. On the other hand, pressing the shift-down switch  52  once will lower the shift position by one stage. 
     FIG. 2 is a diagram showing a partial cross section of the configuration of major components employed in a driving system of the electric-power-assist transmission provided by an embodiment of the present invention. 
     In the configuration shown in the figure, a driving motor  1  which serves as an electric actuator rotates a shift spindle  3  in a normal or reversed direction through a reduction gear mechanism  2 . The rotational position (or the angle) of the shift spindle  3  is sensed by an angle sensor  28  which is installed at one end of the shift spindle  3 . A clutch arm  6  extends perpendicularly to the shift spindle  3 . At one end of the clutch arm  6 , there is provided a gear mechanism  8  for converting the rotational movement of the shift spindle  3  into a rectilinear movement. When the shift spindle  3  is rotated away from a neutral position by the driving motor  1 , the gear mechanism  8  releases the engaged state of a main clutch  5  without regard to the direction of the rotation in the course of the rotation. Elements  6  and  8  are indicated generally as a transmission mechanism  12 , which serves to put the main clutch  5  in an engaged or disengaged state in a manner which is interlocked with the rotation of shift spindle  3 . When the shift spindle  3  is rotated back to reach the neutral position in the opposite direction, on the other hand, the engaged state of the main clutch  5  is restored in the course of the rotation in the reversed direction. The clutch arm  6  and the gear mechanism  8  are configured so that the engaged state of the main clutch  5  is released at a point of time the shift spindle  3  is rotated outside of a predetermined angular range, typically +/−6 degrees, for example. The clutch arm  6  and the gear mechanism  8  engage one another by a roller mounted on the end of gear mechanism  8 . 
     One end of a master arm  7  fixed on the shift spindle  3  is engaged with a shift clutch mechanism  9  which is installed on a shift-drum mechanism  15 . When the shift spindle  3  is rotated by the driving motor  1 , a shift drum  10  is rotated in a direction determined by the rotational direction of the shift spindle  3 . The master arm  7  and the shift clutch mechanism  9  form such a clutch mechanism that, when the shift spindle  3  is rotated away from the neutral position in either direction, the master arm  7  and the shift clutch mechanism  9  get engaged with the shift spindle  3 , rotating the shift drum  10  and, when the shift spindle  3  is rotated back to the neutral position, the engaged state of the master arm  7  and the shift clutch mechanism  9  with the shift spindle  3  is released, leaving the shift drum  10  at a position where the engaged state is released. The master arm  7 , shift clutch mechanism  9 , shift drum  10 , and shift-drum mechanism  15  are indicated generally as gear shifting mechanism  13 , which acts to switch gears in a manner interlocked with rotation of the shift spindle  3 . 
     The edge of each shift fork  11  is engaged with an external circumference groove  31  of one of sleeves  30  to be described later by referring to FIG.  4 . When the shift drum  10  is rotated, the shift forks  11  are moved by the rotation of the shift drum  10  in a direction parallel to the axial direction of the rotation, moving one of the sleeves  30  determined by the rotational direction and the rotational angle of the shift drum  10  in a direction parallel to a main shaft  4 . 
     FIG. 4 is a diagram showing a perspective view of the sleeve  30  inserted in a state slidable in the axial direction of the main shaft which is not shown in the figure. On the circumference side surface of the sleeve  30 , a groove  31  is provided in the circumferential direction. The edge of a shift fork  11  cited earlier is engaged with the groove  31 . A plurality of outwardly-directed cogs  32  are provided on a ring-shaped flange  33  to form a single body on the circumference of the shaft hole of the sleeve  30 . The outwardly-directed cogs  32  are engaged with inwardly-directed cogs  42  of a gear  40  to be described by referring to FIG.  5 . 
     FIG. 5 is a diagram showing a perspective view of the gear  40  supported rotatably at a predetermined position on the main shaft which is not shown in the figure. A plurality of the inwardly-directed cogs  42  are provided on a ring-shaped flange  43  to form a single body on the circumference of the shaft hole of the gear  40 . As described above, the inwardly-directed cogs  42  are engaged with the outwardly-directed cogs  32  of the sleeve  30 . FIG. 3 is a conceptual diagram showing a state in which the outwardly-directed cogs  32  of the sleeve  30  and the inwardly-directed cogs  42  of the gear  40  are engaged with each other. 
     On the other hand, FIG. 9 is a diagram showing a perspective view of the conventional sleeve  38 , and FIG. 10 is a diagram showing a perspective view of the conventional gear  48 . As shown in FIG. 9, a plurality of stand-alone outwardly-directed protrusions  39  are provided on the sleeve  38  concentrically with respect to the shaft hole of the gear  48 . In order to assure the strength of each of the stand-alone outwardly-directed protrusions  39 , however, the area of the bottom surface of each of the stand-alone outwardly-directed protrusions  39  must be made relatively large. As a result, with the conventional technology, the ratio of the width of each of the outwardly-directed protrusions  39  to the length of the circumference on which the outwardly-directed protrusions  39  are provided increases, allowing only four outwardly-directed protrusions  39  to be created thereon as shown in FIG.  9 . This holds true of slits  49  bored on the gear  48  shown in FIG.  10 . 
     FIG. 12 is a diagram showing a model of relative positions of an outwardly-directed protrusion  39  on the conventional sleeve  38  and a slit  49  on the conventional gear  48 . As shown in the figure, the width D 2  of the slit  49  in the rotational direction is about twice the width D 1  of the outwardly-directed protrusion  39 . As a result, a period Ta during which the outwardly-directed protrusion  39  cannot be engaged with the slit  49  is long in comparison with a period Tb allowing the outwardly-directed protrusion  39  to be put in an engaged state with the slit  49 . The state of engagement of the outwardly-directed protrusion  39  with the slit  38  is referred to hereafter as an engagement state. 
     In the case of the present embodiment, on the other hand, the outwardly-directed cogs  32  are provided on a ring-shaped flange  33  to form a single body. Thus, as shown in FIG. 13, the width D 3  of the outwardly-directed cog  32  and the width D 4  of the inwardly-directed cog  42  in the rotational direction can be made sufficiently small, yet with adequate strength maintained. FIG. 13 shows a model of engagement timing of the relative positions of an outwardly-directed cog  32  on the sleeve  30  provided by the present embodiment and an inwardly-directed cog  42  on the gear  40  provided by the present invention. As a result, the period Ta during which the engagement state is impossible is short in comparison with the period Tb making an engagement state possible, increasing the probability of the engagement state. In this case, the engagement state is a state of engagement of the outwardly-directed cog  32  with a slit  46  on the gear  40 . 
     In addition, in the present embodiment, the difference between the width D 5  in the rotational direction of the slit  46  and the width D 3  in the rotational direction of the outwardly-directed cog  32  can be made small, allowing the play after the engagement of the outwardly-directed cog  32  with the slit  46  to be reduced. As a result, the magnitude of a shift shock and the amount of noise generated in the engagement can also be decreased. 
     In addition, in the present embodiment, the taper of the outwardly-directed cog  32  is bent to form a convex shape as shown in FIG. 6, while the taper of the inwardly-directed cog  42  has a straight-line shape as shown in FIG.  7 . Thus, the cogs  32  and  42  can be brought into line contact with each other in the axial direction as shown in FIG. 8, allowing concentration of stress to be avoided. As a result, the cog strength can be increased substantially and, at the same time, the durability and the resistance against abrasion can also be improved as well. 
     In the configuration described above, the sleeves  30  are moved in parallel by the shift forks  11  to a predetermined position, causing the outwardly-directed cogs  32  on one of the sleeves  30  to be put in an engaged state with the slits  46  of the gear  40 . In this engagement state, the gear  40  which has been supported in an idle state so far with respect to the main shaft is engaged with the main shaft by the sleeve  30 , being rotated synchronously with the main shaft as is generally known. As a result, a rotating force transferred from a clutch shaft to a countershaft is transferred to the main shaft by way of the gear. It should be noted that both the clutch shaft and countershaft are not shown in the figure. 
     It is worth noting that, while not shown explicitly in the figure, the engine of the vehicle employing the electric-power-assist transmission adopting the shift control method provided by the present invention is a four cycle engine. In a power transmission system for transferring power from the crankshaft to the main shaft, a power output by the engine is transferred through a centrifugal clutch on the crankshaft and a clutch on the main shaft. Thus, for an engine rotational speed lower than a predetermined value, the centrifugal clutch on the crankshaft stops the transfer of power to the clutch on the main shaft. As a result, the gear can be shifted to any speed if the vehicle is in a halted state. 
     FIG. 14 is a block diagram showing the configuration of major components employed in a control system of the electric-power-assist transmission provided by the embodiment of the present invention and FIG. 15 is a block diagram showing a typical configuration of an ECU  100  employed in the control system shown in FIG.  14 . 
     As shown in FIG. 14, the driving motor  1  described earlier is connected between motor+ and motor− pins of the Electronic Control Unit (ECU)  100 . Sensor-signal pins S 1 , S 2  and S 3  are connected respectively to a vehicle-speed sensor  26  for sensing the speed of the vehicle, an Ne sensor  27  for sensing the rotational speed Ne of the engine, and the angle sensor  28  described earlier for sensing the rotational angle of the shift spindle  3 . Shift-instruction pins G 1  and G 2  are connected to the shift-up and shift-down switches  51  and  52  described earlier. 
     A battery  21  is connected to a main pin of the ECU  100  through a main fuse  22 , a main switch  23  and a fuse box  24 . The battery  21  is also connected to a VB pin through a fail-safe (FIS) relay  25  and the fuse box  24 . An excitation coil  25   a  of the fail-safe relay  25  is connected to a relay pin. 
     As shown in FIG. 15, the main and relay pins of the ECU  100  are connected internally to a power-supply circuit  106  which is connected to a CPU  101 . The sensor-signal pins S 1 , S 2  and S 3  are connected to input pins of the CPU  101  through an interface circuit  102 . The shift-instruction pins G 1  and G 2  are connected to input pins of the CPU  101  through an interface circuit  103 . 
     A switching circuit  105  comprises a FET (1) and a FET (2) connected in series and a FET (3) and a FET (4) also connected in series. The series circuit of the FET (1) and the FET (2) and the series circuit of the FET (3) and the FET (4) are connected to each other to form a parallel circuit. One terminal of the parallel circuit is connected to the VB pin while the other terminal is connected to a GND pin. The junction point between the FET (1) and the FET (2) is connected to the motor− pin while the junction point between the FET (3) and the FET (4) is connected to the motor+ pin. The FETs (1) to (4) are selectively controlled by pulse width modulation (PWM) by the CPU  101  through a predriver  104 . The control of the FETs (1) to (4) carried out by the CPU  101  is based on a control algorithm stored in a memory unit  107 . 
     Next, the shift control method implemented by the electric-motor-assist transmission provided by the present invention is explained by referring to flowcharts a shown in FIGS. 16 to  21  and operational timing charts shown in FIG.  22 . 
     The flowchart shown in FIG. 16 begins with a step S 10  to form a judgment as to whether or not either the shift-up or shift-down switch  51  or  52  has been operated. If one of the switches is found turned on, the flow of control goes on to a step S 11  to form a judgment as to whether it is the shift-up switch  51  or the shift-down switch  52  that has been turned on. If it is the shift-up switch  51  that has been turned on, the flow of control proceeds to a step S 13 . If it is the shift-down switch  52  that has been turned on, on the other hand, the flow of control proceeds to a step S 12  at which the rotational speed Ne of the engine is stored in a variable Ne1. The flow of control then continues to the step S 13 . 
     At the step S 13 , the FETs employed in the switching circuit  105  of the ECU  100  are selectively controlled by PWM in dependence on whether it is the shift-up switch  51  or the shift-down switch  52  that has been turned on starting from a point of time T1 of the time chart shown in FIG.  22 . To be more specific, if it is the shift-up switch  51  that has been turned on, the FETs (2) and (4) are controlled by PWM at a duty ratio of 100% with the FETs (1) and (3) turned off. As a result, the driving motor  1  starts to rotate in a shift-up direction, driving the shift spindle  3  also to rotate in the shift-up direction as well in a manner interlocked with the driving motor  1 . 
     If it is the shift-down switch  52  that has been turned on, on the other hand, the FETs (1) and (3) are controlled by PWM at a duty ratio of 100% with the FETs (2) and (4) turned off. As a result, the driving motor  1  starts to rotate in a shift-down direction, a direction opposite to the shift-up direction, driving the shift spindle  3  also to rotate in the shift-down direction as well in a manner interlocked with the driving motor  1 . 
     By setting the duty ratio at 100% in this way, the speed of the shift can be increased, allowing the duration of the shift to be shortened. As a result, the clutch can be put in a disengaged state in a short period of time. It should be noted that the present embodiment is designed so that, by rotating the shift spindle by merely five to six degrees, the clutch can be put in a disengaged state. 
     The flow of control then goes on to a step S 14  at which a first timer not shown in the figure is started to measure time. Then, the flow of control proceeds to a step S 15  at which a rotational angle θ 0  of the shift spindle  3  detected by means of the angle sensor  28  is read in. Subsequently, the flow of control goes on to a step S 16  to compare the detected rotational angle θ 0  with a first reference angle θ ref  which is set at +/−14 degrees in the case of the present embodiment. To be more specific, the flow of control proceeds to the step S 16  to form a judgment as to whether or not the rotational angle θ 0  exceeds the reference angle θ ref . More specifically, the judgment formed at the step S 16  is a judgment as to whether or not the rotational angle θ 0  is equal to or greater than 14 degrees, or the rotational angle θ 0  is equal to or smaller than −14 degrees. It should be noted that, in the following description, the phrase stating “a quantity goes beyond a +/−value” is used to imply that either the quantity is equal to or greater than the + value, or the quantity is equal to or smaller than the value − for the sake of expression simplicity. 
     An outcome of the judgment formed at the step S 16  indicating that the rotational angle θ 0  goes beyond 14 degrees means that it is quite within the bounds of possibility that the sleeves moved in parallel by the shift forks  11  have arrived at a normal engaged (engagement) position. In this case, the flow of control goes on to a step S 17 . On the other hand, an outcome of the judgment formed at the step S 16  indicating that the rotational angle θ 0  does not go beyond +/−14 degrees means that it is quite within the bounds of possibility that the sleeves moved in parallel by the shift forks  11  have not arrived at the normal engaged (engagement) position. In this case, the flow of control goes on to a step S 30  to be described later. 
     When the fact that the sleeves moved in parallel by the shift forks  11  have arrived at the normal engaged (engagement) position is detected at a point of time t2 as a result of the comparison of the rotational angle θ 0  with the reference rotational angle θ ref , the flow of control proceeds to the step S 17  at which the first timer is reset. The flow of control then continues to a step S 18  at which the FETs employed in the switching circuit  105  of the ECU  100  are selectively controlled by PWM in dependence on whether it is the shift-up switch  51  or the shift-down switch  52  that has been turned on. To be more specific, if it is the shift-up switch  51  that has been turned on, the FETs (1) and (4) are controlled by PWM at a duty ratio of 100% with the FETs (2) and (3) turned off. If it is the shift-down switch  52  that has been turned on, on the other hand, the FETs (2) and (4) are controlled by PWM at a duty ratio of 100% with the FETs (1) and (3) turned off. As a result, the input pins of the driving motor  1  are short-circuited, providing a rotational load to the driving motor  1 . In this state, a braking effect is applied to the driving force working in the shift-up or shift-down direction of the shift spindle  3 , reducing the magnitude of an impact of the shift spindle  3  on a stopper. Such an impact is generated when the shift spindle  3  is brought into contact with the stopper. It should be noted that the rotational angle of the shift spindle  3  at which the shift spindle  3  is brought into contact with the stopper is 18 degrees. 
     The flow of control then goes on to a step S 19  shown in FIG. 17 at which a second timer not shown in the figure is started to measure time. Then, the flow of control proceeds to a step S 20  to form a judgment as to whether or not the time measured by the second timer has exceeded 15 ms. If the time measured by the second timer has not exceeded 15 ms, the flow of control continues to a step S 21  to execute control of the rotational speed Ne of the engine to be described later. The processing at the steps S 20  and S 21  are repeated until the time measured by the second timer exceeds 15 ms. As the time measured by the second timer exceeds 15 ms at a point of time t3, the flow of control goes on to a step S 22  at which the second timer is reset. 
     Subsequently, the flow of control proceeds to a step S 23  at which the FETs employed in the switching circuit  105  of the ECU  100  are selectively controlled by PWM in dependence on whether it is the shift-up switch  51  or the shift-down switch  52  that has been turned on. To be more specific, if it is the shift-up switch  51  that has been turned on, the FETs (2) and (4) are controlled by PWM at a duty ratio of 70% with the FETs (1) and (3) turned off. If it is the shift-down switch  52  that has been turned on, on the other hand, the FETs (1) and (3) are controlled by PWM at a duty ratio of 70% with the FETs (2) and (4) turned off. As a result, since the sleeves are pushed against the gear by a relatively weak torque, the load borne by each cog is reduced until the engaged (engagement) state is reached, allowing the engagement state to be sustained with a high degree of reliability. 
     The flow of control then goes on to a step S 24  at which a third timer not shown in the figure is started to measure time. Then, the flow of control proceeds to a step S 25  to form a judgment as to whether or not the time measured by the third timer has exceeded 70 ms. If the time measured by the third timer has not exceeded 70 ms, the flow of control continues to a step S 26  at which the control of the rotational speed Ne of the engine is executed. The pieces of processing at the steps S 25  and S 26  are repeated until the time measured by the third timer exceeds 70 ms. As the time measured by the third timer exceeds 70 ms at a point of time t4, the flow of control goes on to a step S 27  at which the third timer is reset. The flow of control then proceeds to a step S 28  to start clutch-on control to be described later. 
     It should be noted that the time-up time of the third timer adopted in the present embodiment is determined from the period Ta during which an engaged state cannot be established as described earlier by referring to FIG.  13 . To put it in detail, the time-up time of 70 ms is set so that the control to push the sleeves against the gear is executed at least until the period Ta is over. In the meantime, the outwardly-directed cogs are brought into contact with the inwardly-directed cogs. Since the duty ratio has been reduced to 70%, however, the load borne by each cog is light, being favorable to the strength of the cog. 
     In addition, the time-up time of the third timer does not have to be set at a fixed value. The time-up time can be set at a variable value determined as a function of gear setting. For example, the time-up time is set at 70 ms and 90 ms for the gear set at the range first to third speeds and the range fourth to fifth speeds respectively. 
     If the outcome of the judgment formed at the step S 16  shown in FIG. 16 indicates that the rotational angle θ 0  has not exceeded the first reference angle θ ref , on the other hand, the flow of control goes on to the step S 30  shown in FIG. 18 to form a judgment as to whether or not the time measured by the first timer has exceeded 200 ms. Since the outcome of the judgment formed for the first time indicates that the time measured by the first timer has not exceeded 200 ms, the flow of control goes on to a step S 31  at which the Ne control is executed before returning to the step S 16  shown in FIG.  16 . 
     As time goes by, the outcome of the judgment formed at the step S 30  indicates that the time measured by the first timer has exceeded 200 ms, implying that the shift change attempted this time ends in a failure. In this case, the flow of control goes on to a step S 32  at which the first timer is reset. The flow of control then proceeds to a step S 33  at which the value of a re-inrush flag to be described later is referenced. A reset state of the re-inrush flag, that is, a value thereof of zero, indicates that re-inrush control to be described later has not been executed. In this case, the flow of control continues to a step S 34  at which the re-inrush control is executed for the first time. The in-rush control is executed because, in some cases, the driver feels a sense of incompatibility if it takes a long time to accomplish a shift change. 
     On the other hand, a set state of the re-inrush flag, that is, a value thereof of one, indicates that the shift change was not successful in spite of the fact that the re-inrush control was executed. In this case, the flow of control continues to a step S 35  at which the clutch is put in an engaged state without making a shift change. At the same time, the re-inrush flag is reset. The flow of control then goes on to a step S 36  at which the clutch-on control to be described later is executed. 
     Next, a method adopted for the re-inrush control is explained by referring to the flowchart shown in FIG.  19 . Carried out when the sleeves driven by the shift forks into a parallel movement in the axial direction did not arrive at the normal engagement position, the re-inrush control is processing of making a re-movement (re-inrush) attempt to once reduce the movement torque before applying a predetermined torque again to the shift forks. 
     As shown in the figure, the flowchart begins with a step S 40  at which the duty ratio of the FETs under the PWM control is reduced to 20%. To be more specific, the duty ratio of the FETs (2) and (4) or that of the FETs (1) and (3) is reduced in a shift-up operation or in a shift-down operation respectively. As a result, the driving torque applied to the shift forks  11  is weakened. 
     The flow of control then goes on to a step S 41  at which a fourth timer not shown in the figure is started to measure time. Then, the flow proceeds to a step S 42  to form a judgment as to whether or not the time measured by the fourth timer has exceeded 20 ms. If the time measured by the fourth timer has not exceeded 20 ms, the flow of control continues to a step S 43  at which the Ne control is executed. If the time measured by the fourth timer has exceeded 20 ms, on the other hand, the flow of control goes on to a step S 44  at which the fourth timer is reset. The flow of control then goes on to a step S 45  at which the re-inrush flag is set. Then, the flow of control returns to the step S 13  shown in FIG. 16 at which the driving motor  1  is again controlled by PWM at a duty ratio of 100%, applying a large torque to the shift forks as usual. 
     As described above, in the present embodiment, if a shift change is not made normally, the torque applied to the shift forks is once weakened before being strengthened again to push forth the sleeves. As a result, the operation to re-inrush the sleeves can be carried out with ease. 
     Next, essentials and general operations of the Ne control and the clutch-on control cited above are explained by referring to FIGS. 23 and 24 respectively prior to a detailed description of the operations thereof. 
     As described by referring to FIG. 22, in the present embodiment, when the rotation of the shift spindle is started at the point of time T1, the engagement of the clutch is released at a point of time t2 and the rotation of the shift spindle is completed at the point of time t3. Later on, at the point of time t4, the control to push the sleeves is executed before a transition to the clutch-on control, control to put the clutch in an engaged state. 
     In the clutch-on control, the clutch is put in an engaged state slowly in order to reduce the magnitude of a generated shift shock. In other words, it is necessary to lower the rotational speed of the shift spindle  3 . On the other hand, the speed of a shift change is dependent on the rotational speed of the shift spindle  3 . It is thus necessary to increase the rotational speed of the shift spindle  3  in order to implement a fast shift change. 
     In order to satisfy the two requirements described above at the same time, according to the present invention, in a period from the point of time t4 to the point of time t5, the shift spindle  3  is rotated at a high rotational speed until a zone in close proximity to an angular range to put the clutch in an engaged state is reached whereas, after the point of time t5, that is, in the angular range to put the clutch in an engaged state, the shift spindle  3  is rotated at a low rotational speed as shown in the time chart of FIG.  22 . By executing such two-stage return control in the present embodiment, the magnitude of the generated shift shock and the time it takes to make a shift change can be both reduced simultaneously. 
     In addition, in the present embodiment, the timing to put the clutch in an engaged state is controlled to timing optimum for the operation of the accelerator pedal carried out by the driver. FIG. 23 is a diagram showing operational timing charts representing changes of the rotational angle θ 0  of the shift spindle in the clutch-on control and the rotational speed of the engine in the Ne control in a shift-up operation. On the other hand, FIG. 24 is a diagram showing operational timing charts representing changes of the rotational angle θ 0  of the shift spindle in the clutch-on control and the rotational speed of the engine in the Ne control in a shift-down operation. 
     As shown in FIG. 23, as a general practice in a shift-up operation, the control method comprises the steps of restoring the accelerator pedal, turning on the shift-up switch  51 , letting a shift change take place, putting the clutch back in an engaged state and opening the accelerator. In the mean time, the rotational speed Ne of the engine changes as shown by a solid line a. At that time, the shift spindle is controlled as shown by solid lines A and B. 
     It is also quite within the bounds of possibility, however, that the driver turns on the shift-up switch  51  without restoring the accelerator pedal or opens the accelerator before the clutch is put back in an engaged state. In such a case, it is desirable to put the clutch in an engaged state quickly since the driver usually desires a fast shift change. 
     In the present embodiment, changes in engine rotational speed Ne represented by a solid line b indicate that the driver has turned on the shift-up switch  51  without restoring the accelerator pedal. In this case, quick return control of the rotational angle θ 0  of the shift spindle to put the clutch in an engaged state immediately is executed as shown by a solid line C. On the other hand, changes in engine rotational speed Ne represented by a solid line c indicate that the driver has opened the accelerator with timing preceding timing to put the clutch in a re-engaged state. In this case, quick return control of the rotational angle θ 0  of the shift spindle to put the clutch in an engaged state immediately is executed as shown by a solid line D. 
     As a general practice in a shift-down operation, on the other hand, as shown in FIG. 24, the control method comprises the steps of restoring the accelerator pedal, turning on the shift-down switch  52 , letting a shift change take place, putting the clutch back in an engaged state and opening the accelerator. In the mean time, the rotational speed Ne of the engine changes as shown by a solid line a. At that time, the shift spindle is subject to two-stage control as shown by solid lines A and B. 
     In a shift-down operation, however, the engine may be revved. In such a case, it is desirable to put the clutch in an engaged state quickly since quick engagement of the clutch in such a state will generate a shift shock having a small magnitude. 
     In the present embodiment, changes in engine rotational speed Ne represented by a solid line b or c indicate that the engine has been revved. In this case, quick return control of the rotational angle θ 0  of the shift spindle to put the clutch in an engaged state immediately is executed as shown by a solid line C or D respectively. 
     Next, operations of the Ne control and the clutch-on control for implementing the two-stage control and the quick return control are explained in detail. FIG. 20 is a diagram showing a flowchart representing the method of the Ne control carried out at the steps S 21 , S 26 , S 31  and S 43 . 
     As shown in the figure, the flowchart begins with a step S 50  at which the rotational speed Ne of the engine is measured. The flow of control then goes on to a step S 51  at which a peak-hold value Nep or a bottom-hold value Neb of the rotational speed Ne of the engine measured so far is updated in dependence on the value of the rotational speed Ne of the engine measured at the step S 50 . Then, the flow of control proceeds to a step S 52  to form a judgment as to whether the shift change is a shift up or a shift down. If the shift change is a shift up, the flow of control continues to a step S 56 . If the shift change is a shift down, on the other hand, the flow of control continues to a step S 53 . 
     At the step S 56 , the rotational speed Ne of the engine measured at the step S 50  is compared with the bottom-hold value Neb updated at the step S 51  in order to form a judgment as to whether or not the difference between the two (Ne−Neb) is equal to or greater than 50 rpm. 
     This judgment is a judgment as to whether or not the accelerator is closed in a shift-up operation. A difference (Ne−Neb) equal to or greater than 50 rpm indicates that the driver has turned on the shift-up switch  51  without restoring the accelerator pedal or has opened the accelerator with timing preceding timing to put the clutch in a re-engaged state. In this case, the flow of control goes on to a step S 55  to set a quick-return flag F to suggest that the clutch be immediately put in an engaged state before finishing the processing. On the other hand, a difference (Ne−Neb) smaller than 50 rpm indicates that the normal control should be continued. In this case, the control of the rotational speed of the engine is completed without setting the quick-return flag F. 
     As described above, if the outcome of the judgment formed at the step S 52  indicates that the shift change is a shift down, on the other hand, the flow of control continues to the step S 53 . At the step S 53 , the rotational speed Ne of the engine measured at the step S 50  is compared with the rotational speed Ne1 of the engine stored at the step S 12  in order to form a judgment as to whether or not the difference between the two (Ne−Ne1) is equal to or greater than 300 rpm. If the difference between the two (Ne−Ne1) is equal to or greater than 300 rpm, the flow of control continues to a step S 54  at which the rotational speed Ne of the engine measured at the step S 50  is compared with the peak-hold value Nep updated at the step S 51  in order to form a judgment as to whether or not the difference between the two (Nep−Ne) is equal to or greater than 50 rpm. 
     This judgment is a judgment as to whether or not the driver has revved the engine in the shift-down operation. If the outcomes of the judgments formed at both the steps S 53  and S 54  are an acknowledgment (YES), the flow of control goes on to the step S 55  to set a quick-return flag F to suggest that the clutch be immediately put in an engaged state before finishing the processing. 
     FIG. 21 is a diagram showing a flowchart representing the method of the clutch-on control carried out at the steps S 28  and S 36 . 
     As shown in the figure, the flowchart begins with a step S 70  to form a judgment as to whether or not the speed of the vehicle is about zero. In the present embodiment, speeds of a vehicle up to 3 km/h are regarded as a vehicle speed of about zero. If the speed of the vehicle is about zero, the flow of control goes on to a step S 72  at which a target angle θ T  of the shift spindle  3  is set at a neutral position. The flow of control then proceeds to a step S 73 . This flow of control is implemented to make a shift at the time the vehicle is in an all but halted state. In such a case, it is desirable to make a shift change quickly since no shift shock will be generated anyway. 
     If the outcome of the judgment formed at the step S 70  indicates that the speed of the vehicle is equal to or greater than 3 km/h, on the other hand, the flow of control goes on to a step S 71  at which the target angle θ T  of the shift spindle is set at a second reference angle, an angle differing from an angle, at which the rotation of the shift spindle  3  is halted by the stopper, by 6 degrees. Since the angle, at which the rotation of the shift spindle  3  is halted by the stopper, is +/−18 degrees in the present embodiment, the second reference angle is +/−12 degrees. The flow of control then continues to a step S 73  at which the current rotational angle θ T  of the shift spindle  3  detected by the angle sensor  28  is input. Then, the flow of control goes on to a step S 74  at which the Ne control is executed. 
     Subsequently, the flow of control proceeds to a step S 75  at which a PID (Proportional, Integral and Differential) sum value for PID control is found. To put it in detail, a proportional (P) term, the integral (I) term and the differential (D) term are found and then added up. The P term is the difference (θ 0 −θ T ) between the current rotational angle θ 0  detected at the step S 73  and the target rotational angle θ T . The I and D terms are the integrated and differentiated values of the P term respectively. The flow of control then goes on to a step S 76  at which the PID sum value is used for computing the duty ratio of the PWM control. Then, the flow of control proceeds to a step S 77  at which the PWM control is executed. 
     FIG. 25 is a diagram showing a relation between a PID sum value and a duty ratio. As shown in the figure, a positive PID sum value gives a positive duty ratio while a negative PID sum value provides a negative duty ratio. The polarity of a duty ratio indicates a combination of FETs to be controlled by PWM. For example, a duty ratio of +50% means that the FETs (2) and (4) should be controlled by PWM at a duty ratio of 50%. On the other hand, a duty ratio of −50% means that the FETs (1) and (3) should be controlled by PWM at a duty ratio of 50%. 
     Subsequently, the flow of control goes on to a step S 78  to form a judgment as to whether or not the time measured by a sixth timer has exceeded 100 ms. Since the sixth timer has not been started yet to measure time initially, the time should have not exceeded 100 ms, causing the flow of control to proceed to a step S 79  at which a fifth timer is started to measure time. The flow of control then proceeds to a step S 80  to form a judgment as to whether or not the time measured by a fifth timer has exceeded 10 ms. Initially, the time measured by the fifth timer should have not exceeded 10 ms, causing the flow of control to return to the step S 73  to repeat the pieces of processing carried out at the steps S 73  to S 80 . 
     As time goes by, the time measured by the fifth timer exceeds 10 ms at a point of time t5 of the time chart shown in FIG.  22 . At that time, the flow of control goes on to a step S 81  at which the fifth timer is reset. The flow of control then proceeds to a step S 82  to form a judgment as to whether the quick-return flag F is in a set or reset state. If the quick-return flag F is in a set state, the flow of control continues to a step S 83  to catalog a new target angle set at a value smaller than the present target angle by two to four degrees for use in the execution of quick-return control. If the quick-return flag F is in a reset state, on the other hand, the flow of control continues to a step S 84  to catalog a new target angle set at a value smaller than the present target angle by 0.2 degrees. 
     The flow of control goes on from either the step S 83  or S 84  to a step S 85  to form a judgment as to whether or not the target angle is close to a neutral angle. If the target angle is not close to the neutral angle, the flow of control returns to the step S 73 . The pieces of processing carried out at the steps S 73  to S 85  are repeated until the target angle becomes sufficiently close to the neutral angle. Later on, as the target angle is found sufficiently close to the neutral angle at the step S 85 , the flow of processing proceeds to a step S 86  at which the neutral angle is cataloged as a target angle. The flow of control then continues to a step S 87  at which the sixth timer starts to measure time. 
     If the outcome of the judgment formed at the step S 78  indicates that the time measured by the sixth timer has exceeded 100 ms, on the other hand, the flow of control goes on to a step S 90  at which the sixth timer is reset. The flow of control then proceeds to a step S 91  at which the quick-return flag F is reset. Then, the flow of control continues to a step S 92  at which the PWM control of the switching circuit  105  is terminated. 
     It should be noted that, if the gear is shifted from a neutral state at a high engine rotational speed in the course of a high-speed cruise, a relatively large engine brake works, imposing an excessively large load on the engine. In order to solve this problem, in the present embodiment, there is provided a shift disabling system for preventing the control shown in FIG. 16 from being executed at a vehicle speed equal to or higher than 10 km/h or an engine rotational speed equal to or higher than 3,000 rpm even if the shift-up switch S 1  has been turned on. 
     FIG. 11 is a functional block diagram showing the shift disabling system. As shown in the figure, the shift disabling system employs a neutral-position detecting unit  81  for outputting an “H”-signal to indicate that the gear is placed at a neutral position. A vehicle-speed judging unit  82  generates an “H”-level signal for a speed of the vehicle equal to or higher than 10 km/h. On the other hand, an engine-rotational-speed judging means  83  generates an “H”-level signal for a rotational speed of the engine equal to or higher than 3,000 rpm. 
     An OR circuit  84  generates an “H”-level signal when the vehicle-speed judging unit  82  generates an “H”-level signal or the engine-rotational-speed judging means  83  generates an “H”-level signal. On the other hand, an AND circuit  85  generates an “H”-level signal when the neutral-position detecting unit  81  generates an “H”-level signal and the OR circuit  84  generates an “H”-level signal. With the AND circuit  85  outputting the “H”-level signal, the shift disabling system prevents the control shown in FIG. 16 from being executed even if the shift-up switch  51  has been turned on. 
     If a shift change is made to a neutral state by mistake at a vehicle speed equal to or higher than 10 km/h or an engine rotational speed equal to or higher than 3,000 rpm in the course of acceleration from the first speed, however, it takes time to accomplish re-acceleration. Thus, a system for disabling a shift to a neutral state in the course of a vehicle cruise, for example, at a vehicle speed equal to or higher than 3 km/h can be further added besides the shift disabling system described above. 
     According to the present invention, when putting back a clutch in an engaged state after being released from an engaged state, a shift spindle is rotated with a rotational speed thereof changed from a high speed to a low speed with predetermined timing. As a result, the time it takes to put the clutch in an engaged state can be shortened and, at the same time, the magnitude of a generated shift shock can also be decreased as well. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.