Abstract:
A multistage automatic transmission comprises first and second clutches and a throttle open degree sensor. The first and second clutches are arranged to establish odd and even speeds of the transmission when engaged. A control unit of the transmission is configured to carry out determining a desired speed of the transmission upon receiving information signals; causing at least one the first and second clutches to be engaged to establish the desired speed; and applying a limitation to an engaging force produced by at least one of the first and second clutches when the throttle open degree sensor senses that the open degree of a throttle valve is smaller than a predetermined degree.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates in general to multistage automatic transmissions for wheeled motor vehicle, and more particularly to the multistage automatic transmissions of a constant-mesh type. More specifically, the present invention is concerned with the multistage automatic transmission having a speed change control system incorporated therewith.  
           [0003]    2. Description of the Related Art  
           [0004]    In order to clarify the task of the present invention, one of the multistage automatic transmissions of the above-mentioned type will be briefly described, which is disclosed in Japanese Laid-open Patent Application (Tokkai) 2002-357267. In this transmission, there are arranged first and second input shafts, synchromesh gear pairs, a first clutch for establishing ON/OFF connection between an associated engine and the first input shaft and a second clutch for establishing ON/OFF connection between the engine and the second shaft. By switching the first and second clutches, a speed change is carried out in the transmission. In this transmission, there are further provided mechanisms by which the ON/OFF operation of each clutch and a gear selection operation are automatically carried out.  
         SUMMARY OF THE INVENTION  
         [0005]    In the above-mentioned known transmission, the first and second clutches are each constructed to produce a 100% engaging force when assuming an engaged condition. Thus, in a speed change, there is inevitably produced a discontinuous flow of the engaging force in each clutch, like the flow as 100% engaging force (before shift)→0% engaging force (during shift)→100% engaging force (after shift). This means that in a final stage of the speed change operation, the clutch shows the engaging force of 100% even when the torque from the engine is still small. This tends to induce generation of an undesirable shift shock of the transmission.  
           [0006]    It is therefore an object of the present invention to provide a multistage automatic transmission, which is free of the above-mentioned shift shock.  
           [0007]    In accordance with a first aspect of the present invention, there is provided a multistage automatic transmission for use with an engine of a motor vehicle, which comprises a first clutch that is able to establish an odd speed of the transmission when engaged, a second clutch that is able to establish an even speed of the transmission when engaged; a first clutch actuator that adjusts an engaging force produced by the first clutch; a second clutch actuator that adjusts an engaging force produced by the second clutch; a throttle open degree sensor that detects an open degree of a throttle valve of the engine; and a control unit that issues instruction signals to the first and second clutch actuators upon processing information signals applied thereto, the control unit being configured to carry out determining a desired speed of the transmission upon receiving the information signals; issuing an instruction signal to at least one of the first and second clutch actuators to engage the corresponding one of the first and second clutches for establishing the desired speed; and issuing an instruction signal to at least one of the first and second clutch actuators for applying a limitation to an engaging force produced by the corresponding one of the first and second clutches when the open degree of the throttle valve becomes smaller than a predetermined degree.  
           [0008]    In accordance with a second aspect of the present invention, there is provided a multistage automatic transmission for use with an engine of a motor vehicle, which comprises a first clutch that is able to establish an odd speed of the transmission when engaged, a second clutch that is able to establish an even speed of the transmission when engaged; a first clutch actuator that adjusts an engaging force produced by the first clutch; a second clutch actuator that adjusts an engaging force produced by the second clutch; a throttle open degree sensor that detects an open degree of a throttle valve of the engine; and a control unit that issues instruction signals to the first and second clutch actuators upon processing information signals applied thereto, the control unit comprising a first section that determines a desired speed of the transmission upon receiving the information signals; a second section that issues an instruction signal to at least one of the first and second clutch actuators to engage the corresponding one of the first and second clutches for establishing the desired speed; and a third section that issues an instruction signal to at least one of the first and second clutch actuators for applying a limitation to an engaging force produced by the corresponding one of the first and second clutches when the open degree of the throttle valve is smaller than a predetermined degree.  
           [0009]    In accordance with a third aspect of the present invention, there is provided a method for controlling a multistage automatic transmission for use with an engine of a motor vehicle, the transmission comprising a first clutch that is able to establish an odd speed of the transmission when engaged, a second clutch that is able to establish an even speed of the transmission when engaged; a first clutch actuator that adjusts an engaging force produced by the first clutch; a second clutch actuator that adjusts an engaging force produced by the second clutch; and a throttle open degree sensor that detects an open degree of a throttle valve of the engine. The method comprises determining a desired speed of the transmission upon receiving information signals; issuing an instruction signal to at least one of the first and second clutch actuators to engage the corresponding one of the first and second clutches for establishing the desired speed; and issuing an instruction signal to at least one of the first and second clutch actuators for applying a limitation to an engaging force produced by the corresponding one of the first and second clutches when the open degree of the throttle valve is smaller than a predetermined degree. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic diagram of a speed change control system that is employed by a multistage automatic transmission according to the present invention;  
         [0011]    [0011]FIG. 2 is a schematic view of the multistage automatic transmission to which the present invention is practically applied;  
         [0012]    [0012]FIG. 3 is a flowchart showing programmed operation steps executed by the speed change control system when carrying out a clutch-slip control; and  
         [0013]    [0013]FIG. 4 is a time chart pictorially describing the slip state of two clutches under the clutch-slip control. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring to FIG. 1 of the drawings, there is schematically shown a speed change control system  100  that is practically employed by a multistage automatic transmission according to the present invention.  
         [0015]    In the drawing, denoted by numeral  60  is an engine, such as an internal combustion engine. Engine  60  has at its intake section an electronically controlled throttle valve  61  that electronically controls a throttle opening of an air intake passage. Engine  60  has also fuel injectors  62  each being exposed to a cylinder of engine  60  for injecting fuel thereinto. Denoted by numeral  20  is a multistage automatic transmission that is connected to an output member of engine  60  through an electrically controlled hydraulic clutch  40 . Engine  60  is controlled by an engine control unit (ECU)  70 , and transmission  20  is controlled by a transmission control unit (ATCU)  80 . Each of the control units  70  and  80  has a micro-computer that includes CPU (central processing unit), RAM (random access memory), ROM (read only memory), and input and output interfaces. If desired, these two control units  70  and  80  may be united for using one micro-computer commonly.  
         [0016]    As shown, an accelerator depression degree sensor  1 , an engine speed sensor  2  and a throttle open degree sensor  3  feed engine control unit  70  with corresponding information signals. Transmission control unit  80  is fed with a vehicle speed signal and a range signal (viz., up-shift signal and down-shift signal) that is issued when a shift mechanism  10  is manipulated by a driver. The range signal represents the range that is just selected by the driver. Engine control unit  70  and transmission control unit  80  are electrically connected to exchange the information signals therebetween.  
         [0017]    Upon processing the information signals fed thereto, engine control unit  70  issues control signals for controlling the throttle opening and the amount of fuel injected to each cylinder. Like this, upon processing the information signals fed thereto, transmission control unit  80  issues control signals for controlling a speed change actuator “A” and two actuators “a” and “b” of hydraulic clutch  40 .  
         [0018]    Referring to FIG. 2, there is schematically shown the multistage automatic transmission  20 . As shown, the transmission  20  has hydraulic clutch  40  operatively connected thereto.  
         [0019]    The clutch  40  is of a twin type that comprises a first clutch  40   a  which carries out engagement/disengagement between a first input shaft  21  and an engine output shaft  6  with the aid of clutch actuator “a” (see FIG. 1) and a second clutch  40   b  which carries out engagement/disengagement between a second input shaft  22  and engine output shaft  6  with the aid of clutch actuator “b” (see FIG. 1).  
         [0020]    As is seen from FIG. 2, second input shaft  22  is of a tube type that coaxially receives therein a part of first input shaft  21 .  
         [0021]    Disposed about first input shaft  21  are first and third speed drive gears  31  and  33  and a first reverse gear R 1 . First speed drive gear  31  and first reverse gear R 1  are integral with first input shaft  21 , while, third speed drive gear  33  is rotatably disposed about first input shaft  21 .  
         [0022]    Rotatably disposed about first input shaft  21  is the second input shaft  22 . Disposed about second input shaft  22  are second, fourth and sixth speed drive gears  32 ,  34  and  36 . These second, fourth and sixth speed drive gears  32 ,  34  and  36  are integral with second input shaft  22  to rotate therewith.  
         [0023]    Arranged in parallel with first and second input shafts  21  and  22  are a countershaft  23  and an auxiliary shaft  24 . Disposed on countershaft  23  are first, second, third, fourth and sixth speed driven gears  41 ,  42 ,  43 ,  44  and  46  and a third reverse gear R 3 . As shown, first, second, fourth and sixth speed driven gears  41 ,  42 ,  44 ,  46  and third reverse gear R 3  are rotatably disposed about counter shaft  23 , and third speed driven gear  43  is integral with counter shaft  23  to rotate therewith. For the purpose which will become hereinafter, a counter gear  26  is integral with countershaft  23  to rotate therewith.  
         [0024]    Disposed about auxiliary shaft  24  is a second reverse gear R 2  which is integral with the auxiliary shaft  24  to rotate therewith.  
         [0025]    As shown, at a position opposite to clutch  40  with respect to first input shaft  21 , there is arranged an output shaft  25  that is coaxial with first input shaft  21 . A counter driven gear (or fifth speed drive gear)  35  is integral with output shaft  25  to rotate therewith and meshed with the above-mentioned counter gear  26  that is integral with countershaft  23 . Although not shown in the drawing, driving road wheels are connected to output shaft  25  through a differential gear DG.  
         [0026]    As shown, about first input shaft  21 , there is arranged a 3-5 shift clutch  210  which selectively connects input shaft  21  with third speed drive gear  33  to achieve a third speed and connects input shaft  21  with counter driven gear  35  (viz., output shaft  25 ) to achieve a fifth speed.  
         [0027]    About countershaft  23 , there is arranged a 2-4 shift clutch  220  which selectively connects countershaft  23  with second speed driven gear  42  to achieve a second speed and connects countershaft  23  with fourth speed driven bear  44  to achieve a fourth speed.  
         [0028]    Furthermore, about countershaft  23 , there is arranged a 6-shift clutch  230  which selectively connects countershaft  23  with sixth speed driven gear  46  to achieve a sixth speed.  
         [0029]    Furthermore, about countershaft  23 , there is further arranged a 1-R shift clutch  240  which selectively connects countershaft  23  with first speed driven gear  41  to achieve a first speed and connects the countershaft  23  with third reverse gear R 3  to achieve a reverse state.  
         [0030]    3-5 shift clutch  210 , 2-4 shift clutch  220 , 6-shift clutch  230  and 1-R shift clutch  240  are all actuated by the above-mentioned speed change actuator “A” (see FIG. 1). That is, upon receiving a speed change instruction, each shift clutch  210 ,  220 ,  230  or  240  effects the engagement or disengagement for providing a torque transmission path that is needed by the speed change.  
         [0031]    In the following, various torque transmission paths provided by multistage automatic transmission  20  will be described.  
         [0032]    Under cruising, that is, when the associated vehicle is moving without carrying out the speed change, both first and second clutches  40   a  and  40   b  assume the engaged condition and a torque transmission from engine output shaft  6  to output shaft  25  of transmission  20  is achieved through either one of first and second clutches  40   a  and  40   b.    
         [0033]    [Neutral Condition] 
         [0034]    In this condition, both first and second clutches  40   a  and  40   b  are in their disengaged condition. It is to be noted that the neutral condition is also achieved even when first and second clutches  40   a  and  40   b  assume their engaged condition so long as the shift clutches  210 ,  220 ,  230  and  240  are in their neutral positions.  
         [0035]    [First Speed] 
         [0036]    First clutch  40   a  is engaged and by the work of 1-R shift clutch  240 , first speed driven gear  41  is united with countershaft  23 . Under this condition, the torque from the engine output shaft  6  is transmitted to first clutch  40   a , first input shaft  21 , first speed drive gear  31 , first speed driven gear  41 , countershaft  23 , counter gear  26 , counter driven gear  35 , output shaft  25  and then to differential gear DG and to the driving road wheels (not shown).  
         [0037]    [Second Speed] 
         [0038]    Second clutch  40   b  is engaged, and by the work of 2-4 shift clutch  220 , second speed driven gear  42  is united with countershaft  23 . Under this condition, the torque from the engine output shaft  6  is transmitted to second clutch  40   b , second output shaft  22 , second speed drive gear  32 , second speed driven gear  42 , countershaft  23 , counter gear  26 , counter driven gear  35 , output shaft  25 , differential gear DG and to the driving road wheels.  
         [0039]    [Third Speed] 
         [0040]    First clutch  40   a  is engaged, and by the work of 3-5 shift clutch  210 , third speed drive gear  33  is united with first input shaft  21 . Under this condition, the torque from the engine output shaft  6  is transmitted to first clutch  40   a , first input shaft  21 , third speed drive gear  33 , third speed driven gear  43 , countershaft  23 , counter gear  26 , counter driven gear  35 , output shaft  25 , differential gear DG and to the driving road wheels.  
         [0041]    [Fourth Speed] 
         [0042]    Second clutch  40   b  is engaged, and by the work of 2-4 shift clutch  220 , fourth speed driven gear  44  is united with countershaft  23 . Under this condition, the engine torque is transmitted to second clutch  40   b , second input shaft  22 , fourth speed drive gear  34 , fourth speed driven gear  44 , countershaft  23 , counter gear  26 , counter driven gear  35 , output shaft  25 , differential gear DG and to the driving road wheels.  
         [0043]    [Fifth Speed] 
         [0044]    First clutch  40   a  is engaged, and by the work of 3-5 shift clutch  210 , counter driven gear (or fifth speed drive gear)  35  is united with first input shaft  21 . Under this condition, the engine torque is transmitted to first clutch  40   a , first input shaft  21 , counter driven gear (or fifth speed drive gear)  35 , output shaft  25 , differential gear DG and to the driving road wheels.  
         [0045]    [Sixth Speed] 
         [0046]    Second clutch  40   b  is engaged, and by the work of 6-shift clutch  230 , sixth speed driven gear  46  is united with countershaft  23 . Under this condition, the engine torque is transmitted to second clutch  40   b , second input shaft  22 , sixth speed drive gear  36 , sixth speed driven gear  46 , countershaft  23 , counter gear  26 , counter driven gear  35 , output shaft  25 , differential gear DG and to the driving road wheels.  
         [0047]    [Reverse Condition] 
         [0048]    First clutch  40   a  is engaged, and by the work of 1-R shift clutch  240 , third reverse gear R 3  is united with countershaft  23 . Under this condition, the engine torque from the engine output shaft  6  is transmitted to first clutch  40   a , first input shaft  21 , first reverse gear R 1 , second reverse gear R 2 , third reverse gear R 3 , countershaft  23 , counter gear  26 , counter driven gear  35 , output shaft  25 , differential gear DG and to the driving road wheels. Due to insertion of second reverse gear R 2  between first and third reverse gears R 1  and R 3 , the torque from the engine output shaft  6  a reversed rotation of output shaft  25  thereby to move the vehicle backward.  
         [0049]    In the following, 1-2 Up-shift and 2-1 Down-shift will be described as exemplified shifting operation of the multistage automatic transmission  20 .  
         [0050]    [1-2 Up-shift] 
         [0051]    As is described hereinabove, under first speed, first clutch  40   a  is engaged and by the work of 1-R shift clutch  240  (that is, by moving a coupling means of this clutch rightward in FIG. 2), first speed driven gear  41  is united with countershaft  23 . In this first speed, also second clutch  40   b  assumes its engaged condition. Of course, in this case, other shift clutches  210 ,  220  and  230  are in their neutral conditions.  
         [0052]    Upon starting of the up-shift, second clutch  40   b  is fully disengaged, and then, the coupling means of 2-4 shift clutch  220  is moved rightward to unite second speed driven gear  42  with countershaft  23 . Since second clutch  40   b  has been fully disengaged, the uniting of second speed driven gear  42  with countershaft  23  does not induce any torque transmission at this stage. This condition is called as “pre-shift condition”.  
         [0053]    Then, the engaged condition of first clutch  40   a  is gradually released, and when the engine speed and the speed of second input shaft  22  become synchronized, the coupling means of 1-R shift clutch  240  is moved back to a neutral position to disconnect first speed driven gear  41  from countershaft  23 , and then, second clutch  40   b  is gradually engaged while gradually increasing the engaging force produced by the same. With these steps, the 1-2 up-shift is completed.  
         [0054]    [2-1 Down-shift] 
         [0055]    As is described hereinabove, under second speed, second clutch  40   b  is engaged, and by the work of 2-4 shift clutch  220  (that is, by moving the coupling means of this clutch rightward in FIG. 2), second speed driven gear  42  is united with countershaft  23 . In this second speed, also first clutch  40   a  assumes its engaged condition. Of course, in this case, other shift clutches  210 ,  230  and  240  are in their neutral conditions.  
         [0056]    Upon staring of the down-shift, first clutch  40   a  is fully disengaged, and then, the coupling means of 1-R shift clutch  240  is moved rightward to unite first speed driven gear  41  with countershaft  23 . Since first clutch  40   a  has been fully disengaged, the uniting of first speed driven gear  41  with countershaft  23  does not induce any torque transmission at this stage. That is, under this, the pre-shift condition is kept.  
         [0057]    Then, the engaged condition of second clutch  40   b  is gradually released in accordance with an after-described “clutch-slip control”, and when the engine speed and the speed of first input shaft  21  become synchronized, the coupling means of 2-4 shift clutch  220  is moved back to a neutral position to disconnect second speed driven gear  42  from countershaft  23 , and then, first clutch  40   a  is gradually engaged while gradually increasing the engaging force produced by the same in accordance with the after-described “clutch-slip control”. With these steps, the 2-1 down-shift is completed.  
         [0058]    Other up and down-shifts are carried out similarly. Since the operation steps of these up and down-shifts are substantially the same as those of the above-mentioned steps, explanation of them will be omitted.  
         [0059]    Referring to FIG. 3, there is shown a flowchart showing programmed operation steps that are executed by transmission control unit (ATCU)  80  in cooperation with engine control unit  70  for carrying out a so-called clutch-slip control in the 2-1 down-shift.  
         [0060]    It is to be noted that the clutch-slip control is the control applied to a clutch which is in a slip state wherein the clutch partially connects drive and driven members while allowing relative rotation therebetween to a certain extent.  
         [0061]    As is described hereinabove, under second speed, second clutch  40   b  is engaged, and by the work of 2-4 shift clutch  220 , second speed driven gear  42  is united with countershaft  23 . In this second speed, also first clutch  40   a  assumes its engaged condition. Other shift clutches  210 ,  230  and  240  assume their neutral conditions.  
         [0062]    At step S 101 , judgment is carried out as to whether a clutch-slip control executing instruction signal has been issued or not. If NO, that, when such instruction signal has not been issued, the operation flow goes to END. While, if YES, that is, when such instruction signal has been issued, the operation flow goes to step S 102 . The system may be so made that the clutch-slip control executing instruction signal is produced when the accelerator depression degree is smaller than a predetermined degree, that is, for example, when a throttle open degree is smaller than ⅛ of full throttle producing a smaller engine torque after completion of a speed change.  
         [0063]    At step S 102 , the engaging force of second clutch  40   b  is reduced by “ΔX1”. Then, the operation goes to step S 103 .  
         [0064]    At step S 103 , judgment is carried out as to whether a slip degree “SLIP” of second clutch  40   b  is higher than a predetermined degree “Δt” or not. If YES, that is, when the slip degree “SLIP” is higher than the predetermined degree “Δt”, the operation flow goes to step S 104 . While, If NO, that is, when the slip degree is lower than the predetermined degree, the operation step goes back to step S 102 . With repeating of this back flow, the engaging force of second clutch  40   b  is gradually reduced.  
         [0065]    At step S 104 , the operation for reducing the engaging force of second clutch  40   b  is stopped.  
         [0066]    Then, at step S 105 , an engaging force “T1” of second clutch  40   b  produced at the time when the clutch  40   b  shows the slip degree “SLIP” higher than the predetermined degree “Δt” is increased by “ΔX2”.  
         [0067]    At step S 106 , the value (viz., T1+ΔX2) determined at step S 105  is set as a pre-shift clutch engaging force limit value “LIM-1”.  
         [0068]    At step S 107 , the pre-shift clutch engaging force limit valve “Lim-1” is set as a full engaging force for second clutch  40   b.    
         [0069]    At step S 108 , judgment is carried out as to whether any movement for achieving the 2-1 down-shift has taken place or not. The movement may include a shift action by a driver and/or a state wherein due to the speed change action, the vehicle speed is reduced to a threshold for such 2-1 down-shift. If YES, that is, when there is any movement for achieving such 2-1 down-shift, the operation flow goes to step  109 . While, if NO, that is, when there is no such movement, the operation flow goes to step S 113 .  
         [0070]    At step S 109 , second clutch  40   b  is fully disengaged and engaging operation of first clutch  40   a  is started.  
         [0071]    At step S 110 , judgment is carried out as to whether the engine speed and the rotation speed of first input shaft  21  are synchronized or not. If YES, that is, when the engine speed and the rotation speed of first input shaft  21  become synchronized, the operation flow goes to step S 111 . While, If NO, that is, when the synchronization is not achieved, the operation flow goes back to S 109 .  
         [0072]    At step S 111 , an engaging force “T2” of first clutch  40   a  produced at the time when the synchronization is achieved is increased by “ΔX3”.  
         [0073]    At step S 112 , the value (viz., T2+ΔX3) determined at step S 111  is set as a post-shift clutch engaging force limit value “LIM-2”. Then, the operation flow goes back to step S 107 .  
         [0074]    At step S 113 , judgment is carried out as to whether with the post-shift clutch engaging force limit value being “LIM-2”, a slip degree “SLIP” of first clutch  40   a  is higher than a predetermined degree “Δt” or not. If YES, that is, when the slip degree “SLIP” is higher than the predetermined degree “Δt”, the operation flow goes to step S 114 . While, if NO, that is, when the slip degree is lower than the predetermined degree, the operation flow goes to step S 116 .  
         [0075]    At step S 114 , engaging force (LIM-2) of first clutch  40   a  is increased by “ΔX4”.  
         [0076]    At step S 115 , the value (viz., LIM-2+ΔX4) determined at step S 114  is set as a final clutch engaging force limit value “LIM-3”.  
         [0077]    At step S 116 , judgment is carried out as to whether a clutch-slip control canceling instruction signal has been issued or not. If YES, that is, when such instruction signal has been issued, the operation flow goes to step S 117 . While, if NO, that is, when such instruction signal has not bee issued, the operation flow goes back to step S 113 . The system may be so made that the clutch-slip control canceling instruction signal is produced when the accelerator depression degree becomes larger than a predetermined degree.  
         [0078]    At step S 117 , an engine torque demand signal is outputted to engine control unit (ECU)  70  for suppressing rapid increase of the engine torque.  
         [0079]    At step S 118 , the limiting to the engaging force of first clutch  40   a  is cancelled.  
         [0080]    Referring to FIG. 4, there is shown a flowchart that pictorially shows the clutch-slip control that is carried out in the 2-1 down-shift.  
         [0081]    At time “t1” when the accelerator becomes OFF, that is, when the accelerator depression degree becomes smaller than the predetermined degree, the clutch-slip control is started. From this time, the engaging force of second clutch  40   b  is gradually reduced one (ΔX1) by one (ΔX1). At time “t2”, due to reduction of the engaging force of second clutch  40   b , the slip degree “SLIP” of second clutch  40   b  becomes higher than the predetermined degree “Δt”. At this time “t2”, the pre-shift clutch engaging force limit value “LIM-1” that is equal to (T1+ΔX2) is provided for second clutch  40   b.  With this engaging force limitation, second clutch  40   b  is controlled to produce an appropriate engaging force and thus energy loss caused by excessive slip of mutually engaging elements of the clutch  40   b  and heat generation and wearing of such mutually engaging elements can be minimized.  
         [0082]    At time “t3”, due to reduction in speed of the vehicle, the down-shift is started actually. After this, the engaging force of second clutch  40   b  is sharply reduced toward 0 (zero), and first clutch  40   a  starts to produce the engaging force.  
         [0083]    At time “t4”, the engine speed and the rotation speed of first input shaft  21  become synchronized.  
         [0084]    At time “t5”, the post-shift clutch engaging force limit value “LIM-2” that is equal to (T2+ΔX3), is provided for first clutch  40   a.    
         [0085]    At step “t6”, there is produced a slip of first clutch  40   a  that is higher than the predetermined degree “Δt”. Upon this, the final clutch engaging force limit value “LIM-3” that is equal to (LIM-2+ΔX4), is provided for first clutch  40   a.  As shown, if such slip is not produced, the post-shift value “LIM-2” is kept for first clutch  40   a.    
         [0086]    At time “t7”, the clutch-slip control canceling instruction signal is issued due to a certain depression of the accelerator pedal by the driver. Upon this, the engaging force produced by first clutch  40   a  is increased to 100% (at time “t8”) to effectively and smoothly receive the engine torque. Thus, excessive slip of first clutch  40   a  and undesired engine roaring are suppressed or at least minimized during the speed change of the transmission  20 .  
         [0087]    As is described hereinabove, in the present invention, upon sensing the throttle open degree being smaller than a predetermined degree, the clutch-slip control for applying a limitation to the engaging force of the first or second clutch  40   a  or  40   b  is carried out. With this, the engaging force of the clutch  40   a  or  40   b  can be controlled to a degree appropriate for the torque applied to the transmission from the engine. This reduces the undesired shift shock.  
         [0088]    As is understood from the time-chart of FIG. 4, in the above-mentioned 2-1 down-shift, the engaging force of second clutch  40   b  is kept lowered (viz., LIM-1) before effecting the clutch change (viz., just before time “t4”). Thus, the full disengagement of this clutch  40   b  is achieved with a shorter time. Furthermore, the engaging force of first clutch  40   a  is set to show a lower level (viz., LIM-2) upon having the clutch change. Thus, first clutch  40  needs only shorter time for reaching such engaging force from zero level. These induce a reduction in time needed for the speed change by transmission  20 . In other words, higher responsiveness is obtained in the speed change.  
         [0089]    The entire contents of Japanese Patent Application 2003-105400 (filed Apr. 9, 2003) are incorporated herein by reference.  
         [0090]    Although the invention has been described above with reference to the embodiment of the invention, the invention is not limited to such embodiment as described above. Various modifications and variations of such embodiment may be carried out by those skilled in the art, in light of the above description.