Patent Publication Number: US-2002011792-A1

Title: Method and apparatus for a control system of an automatic transmission

Description:
INCORPORATION BY REFERENCE  
       [0001] The disclosure of Japanese Patent Application No. 12-151884 filed on May 23, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of Invention  
       [0003] The present invention relates to a method and apparatus for a control system of an automatic transmission which is controlled based on an accelerator angle and a vehicle speed. The present invention also relates to a control method of the control apparatus.  
       [0004] 2. Description of Related Art  
       [0005] Up to the present, an automatic transmission in which a speed ratio is automatically controlled based on an accelerator angle and a vehicle speed has been known. According to this automatic transmission, it is not necessary for a person who drives a vehicle equipped with the automatic transmission to select a speed change shift by operating a shift lever or the like.  
       [0006] It is also known that a continuously variable transmission (hereinafter, referred to as “CVT” in an abbreviated form) belongs to the automatic transmission, and a speed ratio of the CVT can be changed continuously. By using the CVT, a driver can always select the most suitable speed ratio automatically.  
       [0007] Here, a relationship between a rotation speed of a power source which is connected to an input side of the automatic transmission and a rotation speed of an output side of the automatic transmission is uniquely determined. Here, the above-mentioned rotation speed indicates revolutions per minute. Accordingly, if an unsuitable speed ratio is set in the automatic transmission such as the CVT or the like, the rotation speed of the power source responding to the vehicle speed is determined to be unexpectedly excessive.  
       [0008] For example, the following details are described in Japanese Laid-Open Patent Application No. 6-42627. In this application, it is shown that a down-shift of a CVT is prohibited, when a rotation speed of an input shaft of the CVT is higher than an allowable maximum rotation speed of an engine caused by the down-shift due to the manual operation at high speed running of a vehicle. Here, the CVT has a manual mode by which a driver can manually set a speed change shift. By this invention, the rotation speed of the engine which is connected to the input side of the CVT can be prevented from overrunning, even when the driver operates the speed change shift by mistake.  
       [0009] In the above-mentioned example, however, only prohibiting the overrun of the engine caused by the manual operation of the driver is targeted, and a countermeasure responding to variable conditions and circumstances are not considered.  
       SUMMARY OF THE INVENTION  
       [0010] It is thus one object of the present invention to solve the aforementioned problems. An object of the invention is to provide a control system for an automatic transmission, and a control method of the control system. The control system can execute a suitable countermeasure to the automatic transmission so that the engine does not overrun, even when various conditions and circumstances change.  
       [0011] A control apparatus for an automatic transmission is installed in a vehicle. The automatic transmission is controlled on the basis of an accelerator angle and a speed of the vehicle. When an input rotation speed of the automatic transmission is equal to or greater than a predetermined value due to operating characteristics of a device or a plurality of devices installed in the vehicle, a down-shift of the automatic transmission is prohibited. As mentioned above, by prohibiting the down-shift of the automatic transmission when the input rotation speed of the automatic transmission is equal to or greater than a predetermined value, the input rotation speed of the automatic transmission can be prevented from further increasing, and the engine can be effectively prevented from overunning. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The above mentioned embodiment and other embodiments, objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of the preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:  
     [0013]FIG. 1 is a schematic view showing a power train structure of a vehicle to which a control apparatus of this invention is applied;  
     [0014]FIG. 2 shows a schematic view of an entire structure of a transmission including a CVT;  
     [0015]FIG. 3 shows a structure of a fluid pressure control circuit;  
     [0016]FIG. 4 is a flow chart showing a first embodiment of the control action of a control device;  
     [0017]FIG. 5 is a graph showing a control range and an upper limit input rotation speed;  
     [0018]FIG. 6 is a flow chart showing a second embodiment control action of the control device partially modified from the control action shown in FIG. 4; and  
     [0019]FIG. 7 is a block diagram showing a control of the control device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0020] In the following description and the accompanying drawings, the present invention will be described in more detail in terms of preferred embodiments. First, a transmission  3  including a CVT  14  will be explained. FIG. 1 is a schematic view showing a power train structure to which the embodiment of the present invention is applied. As a power source, here, an internal combustion engine  2  hereinafter, referred to simply as “engine”) is defined. An output power of the engine  2  transmitted to wheels  4  by way of the transmission  3  drives a vehicle equipped with a power train  1  including the transmission  3 . A control device  5  for controlling the power train  1  calculates control parameters for the engine  2  and the transmission  3  from parameters which indicate a driving condition of the vehicle such as an operating condition of the engine  2 , an operating condition of the transmission  3 , and the like. The control parameters are, for example, a throttle valve angle, a fuel injection amount of the engine  2 , a speed ratio of the transmission  3 , and the like. By controlling these parameters, the engine  2  and the transmission  3  are controlled in a predetermined condition.  
     [0021]FIG. 2 is a schematic structure view of the transmission  3  including the CVT  14 . (Automobile engineers sometimes refer to “continuously variable transmission (CVT)” as an entire transmission, when the transmission includes the CVT.) The output power of the engine  2  (not shown in FIG. 2), from the right side in FIG. 2, is transmitted to a torque converter  10  (which transmits torque by fluid). The power is then transmitted to a drive shaft  20  by way of a forward-reverse change mechanism  12 , the CVT  14 , a reduction mechanism  16 , and a differential  18 . The power of the drive shaft  20  transmitted to the wheels  4  (as shown in FIG. 1) drives the vehicle.  
     [0022] A front cover  22  included in the torque converter  10  is rotated by the power of the engine  2 . The rotation power of the front cover  22  is transmitted to a pump impeller  24  and an oil pump  26 . The oil pump  26  supplies pressure fluid to each pressure fluid control device of the transmission  3 . Furthermore, the pressure fluid also functions as lubrication oil. The pump impeller  24  pushes out pressure fluid contained in the torque converter  10  to a turbine runner  28 , so that the pressure fluid rotates the turbine runner  28 . The turbine runner  28  is coupled to an output shaft  30  of the torque converter  10 , so that the turbine runner  28  rotates together with the output shaft  30  as one body. The rotation power of the turbine runner  28  is consequently an output rotation power of the torque converter  10 . The pressure fluid flowing through the turbine runner  28  passes through a stator  32  and is sent to the pump impeller  24 . The stator  32  is supported and fixed by a case of the transmission  3  by way of a one-way clutch  34 . When a speed ratio (that is, a ratio of an input rotation speed against an output rotation speed) of the torque converter  10  is in a low range (that is, equal to or less than a clutch point), the one-way clutch  34  is engaged, and the stator  32  is fixed. In this case, the stator  32  changes a flowing direction of the pressure fluid pushed out from the turbine runner  28 . Furthermore, the stator  32  pushes out the pressure fluid to the pump impeller  24  from the rear of the pump impeller  24 , from the viewpoint of the rotating direction of the pump impeller  24 . By this action, torque of the turbine impeller  28  is amplified against the torque of the pump impeller  24 . On the other hand, when the speed ratio of the torque converter  10  is over the clutch point, the pressure fluid pushed out from the turbine runner  28  flows and strikes against back members of the stator  32 . The one-way clutch  34  is, then, released, and the stator  32  rotates idly. In this case, torque of the torque converter  10  is not amplified, and the torque converter  10  functions as a fluid coupling.  
     [0023] Furthermore, the torque converter  10  functions as a direct clutch. A direct clutch plate  36  is disposed such that the direct clutch plate  36  fronts to the front cover  22 . The direct clutch plate  36  is supported by the output shaft  30  of the torque converter  10  so that the direct clutch plate  36  rotates together with the output shaft  30  as one body and can slide in the direction of the axis of the output shaft  30 . A torsional damper  38  for absorbing a torsional shock or vibration is disposed between an outer circumference and a center portion of the direct clutch plate  36 . The outer circumference contacts the front cover  22 , and the center portion is supported by the output shaft  30 . When the direct clutch plate  36  is engaged, the pressure fluid from a fluid pressure control circuit  40  controlled by the control device  5  is supplied to a backside chamber  42  of the direct clutch plate  36 . By the pressure of the pressure fluid, the direct clutch plate  36  is slid to the right in FIG. 2 and is engaged to the front cover  22 . The power is, thus, transmitted not hydraulically but mechanically. In order to release the direct clutch condition, the pressure fluid is supplied to a front chamber  44  of the direct clutch plate  36 . By the pressure of the fluid, the direct clutch plate  36  is slid to the left in FIG. 2. Accordingly, the direct clutch plate  36  is detached from the front cover  22 .  
     [0024] The forward-reverse change mechanism  12  is a double-pinion type planetary gear mechanism including sets of double-pinions. A sun gear  46  is coupled to the output shaft  30  of the torque converter  10 . Double pinions  48  are supported by a carrier  50  so that the double pinions  48  rotate while moving along the outer circumference of the sun gear  46 . The carrier  50  is connected to the output shaft  30  of the torque converter  10  via a forward clutch  52 . Furthermore, the carrier  50  is coupled to an input shaft  54  of the CVT  14  (hereinafter, referred to as “CVT input shaft  54 ”). A ring gear  56  is fixed to a case of the transmission  3  by engaging a reverse brake  58 .  
     [0025] When the vehicle runs forward, the forward clutch  52  is engaged by supplying the pressure fluid from the fluid pressure control circuit  40 , and the CVT input shaft  54  is directly connected to the output shaft  30  of the torque converter  10 . When the vehicle runs back, on the one hand the forward clutch  52  is released, and on the other hand the reverse brake  58  is engaged by supplying the pressure fluid from the fluid pressure control circuit  40 . The ring gear  56  then stops, and the carrier  50  rotates in the opposite direction against the output shaft  30  of the torque converter  10 . That is, the both rotation directions of the front and rear sides of the forward-reverse change mechanism  12  are opposite. Incidentally, the transmission  3  is neutral when the forward clutch  52  and reverse brake  58  are released.  
     [0026] The CVT  14  comprises a primary pulley  60  rotating together with the CVT input shaft  54 , a secondary pulley  62 , and a belt  64  partially wrapping the primary pulley  60  and the secondary pulley  62 . The secondary pulley  62  rotates a CVT output shaft  66  and sends the rotation power to the reduction  16 .  
     [0027] The primary pulley  60  has a fixed sheave  68  and a movable sheave  70 . These sheaves  68  and  70  are disposed in parallel in the axis direction of the CVT input shaft  54 , and each face of both sheaves fronting to each other is shaped as a side face of a substantial cone or a substantially truncated cone. On the one side, the movable sheave  70  rotates together with the CVT input shaft  54  as one body. On the other side, the movable sheave  70  itself functions as a hydraulic actuator and moves in the axis direction of the movable sheave  70  by controlling a volume of the pressure fluid from the fluid pressure control circuit  40 . The movement of the movable sheave  70  changes a distance between the two confronting faces of both sheaves  68  and  70 .  
     [0028] In the same way, the secondary pulley  62  includes a fixed sheave  72  and a movable sheave  74 , each having a shape of a side face of a substantial cone or a substantially truncated cone. The movable sheave  74  moves in the axis direction thereof by controlling a volume of the supplied pressure fluid. A distance between the both sheaves  72  and  74  is thus changed.  
     [0029] A section of the belt  64  is shaped like a substantial trapezoid. In the sectional view as shown in FIG. 2, each side of the belt  64  contacts each confronting face of both sheaves  68  and  70 . In the same manner, each side of the belt  64  contacts each confronting face of the both sheaves  72  and  74 . The belt  64  is put between the fixed sheave  68  and the movable sheave  70 , and the belt  64  is also put between the fixed sheave  72  and the movable sheave  74  in the other portion of the belt  64 . In accordance with a change of the distance between the fixed sheave  68  and the movable sheave  70 , a radius is changed. Here, the radius is a distance between the common axis of the both sheaves and a center point of a line which the belt  64  and the fixed sheave  68  contact or the belt  64  and the movable sheave  70  contact. In the same way, the above-mentioned characteristic is applied to the second pulley  62 , that is, to the fixed sheave  72  and the movable sheave  74 . Moreover, since the abovementioned radius is changed in each of the input side (that is, the primary pulley  60 ) and the output side (that is, the secondary pulley  62 ), a speed ratio of the CVT  14  is changed. The speed ratio, here, indicates a ratio of the rotation speed (that is, revolutions per minute) of the CVT output shaft  66  against the rotation speed (that is, revolutions per minute) of the CVT input shaft  54 . Since the positions of the movable sheaves  70  and  74  can be continuously set at any position, the speed ratio of the CVT  14  can be continuously determined in a predetermined range.  
     [0030] The fluid pressure control circuit  40  supplies the pressure fluid from the oil pump  26  to suitable portions in accordance with the driving condition of the vehicle. The driving condition of the vehicle is detected by a speed sensor  76 , a NE sensor  78 , a shift sensor  80 , a pedal sensor  82 , an input revolution sensor  84 , and the like. The speed sensor  76  detects a vehicle speed SPD. The NE sensor  78  detects a rotation speed of the engine  2 . The shift sensor  80  detects a selected shift position of a shift lever. The pedal sensor  82  detects an operated amount of an accelerator pedal or an accelerator angle PA. Here, a throttle angle may be used instead of the accelerator angle PA. The input revolution sensor  84  detects a rotation speed of the input shaft  54  of the primary pulley  60  (that is, CVT input shaft  54 ). Incidentally, a return circuit of the pressure fluid is not shown in FIG. 2.  
     [0031] Next, the structure of the fluid pressure control circuit  40  will be explained according to FIG. 3. An up-shift flow control valve  92  comprises four ports ( 92   a ,  92   b ,  92   c  and  92   d ), a spool  92   s , a spring  92   f , a spring chamber  92   g , and a control pressure chamber  92   h . The spool  92   s  moves up and down in FIG. 3. The spring  92   f  pushes the spool  92   s  downward in the figure. The spring  92   f  is disposed in the spring chamber  92   g . Control pressure is introduced into the control pressure chamber  92   h . An up-shift electromagnetic valve  96  has three ports, that is,  96   a ,  96   b , and  96   c . When the up-shift electromagnetic valve  96  is on (shown in the right side of the valve  96  in FIG. 3), the ports  96   a  and  96   b  are connected to together. Furthermore, the up-shift electromagnetic valve  96  repeats to switch on and off at a constant cycle by a control signal from the control device  5 , when the up-shift electromagnetic valve  96  is on. Here, a pulse width of the control signal is controlled. By controlling a duty ratio of the control signal, the up-shift electromagnetic valve  96  controls the constant fluid pressure regulated by a regulator valve to a predetermined pressure between the atmospheric pressure and the constant pressure. This controlled pressure is set as the above-mentioned control pressure, that is an up-shift signal. The control pressure is supplied to the control pressure chamber  92   h  from the port  92   a  of the up-shift flow control valve  92 .  
     [0032] When the control pressure from the up-shift electromagnetic valve  96  is introduced to the control pressure chamber  92   h , the control pressure pushes the spool  92   s  upward against the spring  92   f  in the spring chamber  92   g . Subsequently, line pressure introduced from the port  92   c  by way of a fluid passage R 4  is supplied to the primary pulley  60 , from the port  92   d  via a fluid passage R 5 . In the above-mentioned way, a predetermined pressure is applied to the movable sheave  70 .  
     [0033] When the up-shift electromagnetic valve  96  is off (shown in the left side of the valve  96  in FIG. 3), the port  96   b  is connected to the port  96   c , and the pressure fluid in the control pressure chamber  92   h  drains off from the port  96   c . Consequently, the fluid pressure of the control pressure chamber  92   h  is decreased to the atmospheric pressure. The spool  92   s  of the up-shift flow control valve  92  is, then, moved downward by the spring force of the spring  92   f . Finally, the port  92   d  is closed.  
     [0034] A down-shift flow control valve  94  has four ports ( 94   a ,  94   b ,  94   c  and  94   d ), a spool  94   s , a spring  94   f , a spring chamber  94   g , and a control pressure chamber  94   h . The spool  94   s  moves up and down in FIG. 3. The spring  94   f  pushes the spool  94   s  downward in the figure. The spring  94   f  is disposed in the spring chamber  94   g . Control pressure is introduced into the control pressure chamber  94   h . A down-shift electromagnetic valve  98  has three ports, that is,  98   a ,  98   b , and  98   c . When the downshift electromagnetic valve  98  is on (shown in the right side of the valve  98  in FIG. 3), the ports  98   a  and  98   b  are connected to together. Furthermore, the down-shift electromagnetic valve  98  repeats to switch on and off at a constant cycle by a control signal from the control device  5 , when the down-shift electromagnetic valve  98  is on. Here, a pulse width of the control signal is controlled. By controlling a duty ratio of the control signal, the down-shift electromagnetic valve  98  controls the constant fluid pressure regulated by a regulator valve to a predetermined pressure between the atmospheric pressure and the constant pressure. This controlled pressure is set as the above-mentioned control pressure, that is a down-shift signal. The control pressure is supplied to the control pressure chamber  94   h  from the port  94   a  of the down-shift flow control valve  94 .  
     [0035] When the control pressure from the down-shift electromagnetic valve  98  is introduced to the control pressure chamber  94   h , the control pressure pushes the spool  94   s  upward against the spring  94   f  in the spring chamber  94   g . Subsequently, the line pressure introduced from the port  94   c  by way of a fluid passage R 6  is drained from the port  94   d . Consequently, a predetermined amount of the fluid is drained from the movable sheave  70  of the primary pulley  60  by way of the fluid passage R 6 .  
     [0036] When the down-shift electromagnetic valve  98  is off (shown in the left side of the valve  98  in the FIG. 3), the port  98   b  is connected to the port  98   c , and the pressure fluid in the control pressure chamber  94   h  drains off from the port  98   c . Consequently, the fluid pressure of the control pressure chamber  94   h  is reduced to the atmospheric pressure. The spool  94   s  of the down-shift flow control valve  94  is, then, moved downward by the spring force of the spring  94   f . Finally, the port  94   d  is closed.  
     [0037] In the above-mentioned fluid pressure control circuit  40 , when the up-shift signal is sent from the control device  5 , the up-shift electromagnetic valve  96  is turn on at a predetermined duty ratio. The control pressure in accordance with this duty ratio is introduced into the control pressure chamber  92   h  from the port  92   a  of the up-shift flow control valve  92 . As the result, the spool  92   s  is pushed upward against the spring  92   f  in the figure, the ports  92   c  and  92   d  are connected to together, and the pressure fluid is supplied to the movable sheave  70  of the primary pulley  60 . In this case, “off” is ordered to the down-shift electromagnetic valve  98 . Accordingly, the port  94   d  of the down-shift flow control valve  94  is closed, and the fluid pressure to the primary pulley  60  is maintained. Then, the aforementioned radius of the primary pulley  60  becomes longer. On the contrary, the radius of the secondary pulley  62  becomes shorter in accordance with the increased degree of the radius of the primary pulley  60 . The CVT  14  is up-shifted by the aforementioned actions.  
     [0038] On the other hand, when the down-shift signal is sent from the control device  5 , the down-shift electromagnetic valve  98  is turn on at a predetermined duty ratio. The control pressure is introduced into the control pressure chamber  94   h  from the port  94   a  of the down-shift flow control valve  94 . As the result, the spool  94   s  is pushed upward against the spring  94   f  in the figure, the ports  94   c  and  94   d  are connected together, and the pressure fluid is drained from the port  94   d  via the fluid passage R 6 . The fluid pressure of the movable sheave  70  of the primary pulley  60  is, then, decreased. In this case, “off” is ordered to the up-shift electromagnetic valve  96 . Accordingly, the port  92   d  of the up-shift flow control valve  92  is closed, and the fluid pressure to the primary pulley  60  decreases. The radius of the primary pulley  60 , then, becomes shorter. On the contrary, the radius of the secondary pulley  62  becomes longer in accordance with the decreased degree of the radius of the primary pulley  60 . The CVT  14  is down-shifted by the abovementioned actions.  
     [0039] The control pressure from the down-shift electromagnetic valve  98  is applied to the spring chamber  92   g  by way of a fluid passage R 17  and the port  92   b . When the down-shift electromagnetic valve  98  is turn “on,” the port  92   d  of the up-shift flow control valve  92  is closed. Consequently, the up-shift of the CVT  14  is prohibited by turning the down-shift electromagnetic valve  98  “on,” even when the up-shift electromagnetic valve  96  gets out of order and is kept “on.” 
     [0040] The control pressure from the up-shift electromagnetic valve  96  is applied to the spring chamber  94   g  by way of a fluid passage R 16  and the port  94   b . When the up-shift electromagnetic valve  96  is turn “on,” the port  94   d  of the downshift flow control valve  94  is closed. Consequently, the down-shift of the CVT is prohibited by turning the up-shift electromagnetic valve  96  “on,” even when the down-shift electromagnetic valve  98  gets out of order and is kept “on.” 
     [0041] Next, actions in high rotation speed that are controlled by the control device  5  will be explained. It may happen that a suitable speed ratio is not selected in the CVT  14  due to operating characteristics in a device or a plurality of devices installed in the vehicle. The devices which are affected by the rotation speed comprise: a speed sensor  76 , a NE sensor  78 , a shift sensor  80 , a pedal sensor  82 , an input revolution sensor  84 , an accelerator pedal, a speed shifter, a throttle valve, a transmission  3  and components thereof, an engine  2 , a power source, a power train, fuel injectors, a CVT  14 , a controller  5 , a hydraulic pressure control circuit  40  and devices that sense the operating parameters of the power train. In this case, the input rotation speed of the CVT  14  may be equal to or greater than a predetermined rotation speed and the engine  2  may overrun. It is necessary to avoid this problem.  
     [0042] The countermeasures will be explained with reference to FIG. 4. First, in step  11  (hereinafter, referred to simply as S 11 , and other steps will be described in the same way) the control device  5  reads the vehicle speed SPD from the speed sensor  76 , the accelerator angle PA from the pedal sensor  82 , the input rotation speed NIN from the input revolution sensor  84 , and the shift position from the shift sensor  80 . Next, in S 12  (that is, query step) whether or not the CVT  14  is neutral is determined. When it is “yes,” the routine proceeds to “END,” because the CVT  14  does not transmit power and the present control is not necessary.  
     [0043] When it is determined that the CVT  14  is not neutral, the routine proceeds to S 13 . It is determined in S 13  whether or not the present input rotation speed NIN is equal to or over a predetermined input rotation speed (or called upper limit input rotation speed) NINFAIL. Here, the predetermined input rotation speed NINFAL is set as shown in FIG. 5.  
     [0044] Target input rotation speed NINT is determined in the CVT  14  based on the detected vehicle speed SPD and the accelerator angle PA. The speed ratio of the CVT  14  is controlled according to a difference between the detected input rotation speed NIN and the target input rotation speed NINT, and the input rotation speed NIN is controlled to coincide with the target input rotation speed NINT. The target input rotation speed NINT is restricted within a control range shown in FIG. 5. The input rotation speed NIN is controlled so that the input rotation speed NIN is in the aforementioned control range. Here, the horizontal axis shows the vehicle speed SPD, and the vertical axis indicates the input rotation speed NIN.  
     [0045] The upper limit input rotation speed (that is, the predetermined input rotation speed) NINFAIL is set a little higher than the control range in FIG. 5. This indicates that the input rotation speed NIN could not reach such high speed as the upper limit input rotation speed NINFAIL when the control is executed normally.  
     [0046] When “yes” is determined in S 13 , the input rotation speed NIN should be decreased. The down-shift is prohibited in S 14 , because the input rotation speed NIN increases if the down-shift is done. This execution results in that the input rotation speed NIN is prevented from further increasing caused by changing the speed ratio of the CVT  14 . By the way, if the down-shift is prohibited, the input rotation speed NIN at this time is considered to be greater than the target input rotation speed NINT within the control range. The CVT  14  should be up-shifted, and subsequently the input rotation speed NIN should be decreased.  
     [0047] Next, the routine transitions to S 15 , and the output torque of the engine  2  is reduced. The CVT input shaft  54  is connected to the engine  2  via the forward-reverse change mechanism  12  and the torque converter  10 . Accordingly, by cutting fuel to the engine  2  by fully closing the throttle angle or other methods, the engine  2  functions as a brake. The input rotation speed NIN can be, thus, decreased. Incidentally, it is not always necessary to cut the fuel completely, and a control for decreasing the output torque of the engine  2  by stopping combustion in one or more cylinders in the engine  2  or by other methods is also available.  
     [0048] After the execution for reducing the input rotation speed NIN is done in S 14  and S 15 , the routine ends.  
     [0049] When it is “no” in S 13 , whether or not the down-shift is kept prohibited is determined in S 16 . When “no” is determined, the control is under the normal condition. The routine ends, because it is not necessary to cancel the prohibition. On the other hand, when the control is under the prohibition of the down-shift, the routine proceeds to S 17 . In S 17 , whether or not the input rotation speed NIN is within the control range is determined. If the control is normal as mentioned above, the CVT should be up-shifted because the present input rotation speed NIN is excessively high, and the input rotation speed should be decreased. Whether or not the control returns back to the normal condition can be determined by the above-mentioned input rotation speed NIN. When the input rotation speed NIN is not within the control range, the prohibition of the down-shift should be maintained, and the routine ends.  
     [0050] When the input rotation speed NIN is within the control range in S 17 , the routine proceeds to S 18 , and whether or not the accelerator angle PA is equal to or greater than 50% is determined. When “no” is determined in S 18 , the routine goes to “end.” On the other hand, when “yes” is determined in S 18 , the prohibition of the down-shift is canceled in S 19 , because the input rotation speed NIN not only returns within the control range but it is also surely permissible to increase the input rotation speed NIN by confirming the driver&#39;s intention. The decrease of the output torque in S 15  is canceled by the fact that the accelerator pedal is depressed, because the driver intends that the engine power should increase and the control should comply with the driver&#39;s intention.  
     [0051] In the above-mentioned embodiment, when the input rotation speed NIN is equal to or greater than the predetermined abnormal value, the down-shift is prohibited. Accordingly, by reducing the speed ratio of the CVT  14 , the input rotation speed NIN can be decreased. Furthermore, the input rotation speed NIN is decreased more effectively by reducing the output torque of the engine  2 .  
     [0052] By canceling the prohibition of the down-shift when the input rotation speed NIN returns within the control range, the control can be returned to the normal control. Especially, the prohibition of the down-shift can be canceled responding to the driving condition by confirming that the driver depresses the accelerator pedal more than a predetermined level and intends to accelerate the vehicle.  
     [0053] Next, a second embodiment which is partially modified is shown in FIG. 6. In the routine of FIG. 6, the prohibition of the down-shift in S 14  shown in FIG. 4 is replaced by an up-shift in S 20  in FIG. 6. As mentioned above, the up-shift is executed in order to return the control to be under the normal condition, when the input rotation speed NIN is very high, even when the down-shift is prohibited. In this second embodiment, the up-shift is executed as quickly as possible. That is, the maximum amount of the fluid is introduced from the fluid control valves to the movable sheave  70  of the primary pulley  60  in FIG. 2. Consequently, the up-shift is performed at the maximum speed, and the input rotation speed NIN can be instantly and smoothly decreased. By the way, the up-shift may be executed at a speed different from the maximum, and such a speed of the up-shift may be properly selected.  
     [0054] Next, the control device  5  will be explained according to FIG. 7.  
     [0055] Each input signal from the sensors shown in FIG. 2 enters an input signal processing device  502  in the control device  5 . The input signal processing device  502  repeatedly reads these input signals at a predetermined cycle time and sends the input signals to a NINT calculating device  504 . The NINT calculating device  504  calculates the target input rotation speed NINT based on data from the sensors. The target input rotation speed NINT is stored in the control device  5  as a datum in a map. Incidentally, it is preferable that the target input rotation speed NINT is changed corresponding to a temperature of the fluid.  
     [0056] The input revolution sensor  84  is attached on the CVT input shaft  54 . The rotation speed detected by the sensor  84  is sent to a difference computing device  508  via a NIN calculating device  506 . This difference computing device  508  calculates a difference between the target input rotation speed NINT from the NINT calculating device  504  and the actual input rotation speed NIN. The calculated difference is inputted into a computing device of feedback operational data  510 . The computing device  510  computes feed back operational data for actually driving the primary pulley  60 . Specifically, the computing device  510  calculates a control quantity QSC for controlling an opening degree of the up-shift flow control valve  92  or the downshift flow control valve  94 . When the up-shift control is performed, the control quantity QSC is sent to a computing device of up-shift duty ratio  512 . The computing device  512  calculates a duty ratio for the case where the up-shift electromagnetic valve  96  is “on.” When the down-shift is performed, the control quantity QSC is sent to a computing device of down-shift duty ratio  514 , and the computing device  514  calculates a duty ratio for the case where the down-shift electromagnetic valve  98  is “on.” 
     [0057] Output control pressure from the up-shift electromagnetic valve  96  or the down-shift electromagnetic valve  98  is controlled in accordance with the duty ratio from the computing device of up-shift duty ratio  512  or the computing device of down-shift duty ratio  514 . Subsequently, the opening degree of the up-shift flow control valve  92  or the down-shift flow control valve  94  is controlled by the abovementioned output control pressure. The position of the movable sheave  70  of the primary pulley  60  is thus controlled, and the speed ratio of the CVT  14  is determined.  
     [0058] A measuring device of pulse interval of input rotation speed  520  receives pulses responding to the input rotation speed NIN from the input revolution sensor  84 . The measuring device  520  measures the interval of pulses responding to the input rotation speed NIN. On the other hand, on the basis of a map stored in advance, a setting device of pulse interval of the upper limit input rotation speed (NINFAIL)  522  sets the interval of pulses corresponding to the upper limit input rotation speed NINFAIL which responds to the current vehicle speed. The interval of pulses of the actual input rotation speed NIN from the measuring device  520  and the interval of pulses of the upper limit input rotation speed NINFAIL from the setting device  522  are inputted to a computing device of overrun operational data  524 . The computing device of overrun operational data  524  compares both intervals of pulses and determines whether or not the interval of pulses of the present input rotation speed NIN is less than the interval of the upper limit input rotation speed NINFAIL. Whether or not the down-shift is prohibited depends on the above-mentioned determination.  
     [0059] Although an explanation concerning the secondary pulley  62  is omitted, a fluid pressure control for controlling the secondary pulley  62  is substantially the same as the fluid pressure control for controlling the primary pulley  60 . One difference between the control for the primary pulley  60  and the control for the secondary pulley  62  is that the movable sheave  74  moves in the opposite direction of the movable sheave  70 . The aforementioned fluid is usual oil and a pressure control system using the oil is adopted to the fluid pressure control system for this embodiment.  
     [0060] As mentioned above, when the input rotation speed of the automatic transmission is equal to or greater than the predetermined rotation speed, the downshift is prohibited. By this operation, the input rotation speed can be prevented from further increasing. Consequently, the engine overrun or the like can be effectively avoided. Especially, when the input rotation speed is equal to or greater than the predetermined value because of operating characteristics of a device or a plurality of devices installed in the vehicle, the control system can be returned to the normal condition by the above-mentioned operation.  
     [0061] The input rotation speed can also be quickly decreased by up-shifting in place of the prohibition of the down-shift.  
     [0062] Furthermore, the input rotation speed can be decreased by restricting the output torque of the power source.  
     [0063] As explained, the automatic transmission is the CVT in which the speed ratio can be continuously changed. Even though the automatic transmission is an ordinary automatic transmission which comprises a torque converter and a planetary gear, this invention is also applicable to the ordinary automatic transmission. Since the ordinary automatic transmission has usually a one-way clutch, the ordinary automatic transmission is under neutral condition when an accelerator pedal is released. On the contrary, the CVT does not have the one-way clutch. Accordingly, when the accelerator pedal is released, the engine is directly connected to the input shaft of the transmission, and engine braking occurs. Consequently, the overrun of the engine, or the strong engine braking, occurs easily. If the invention is applied to the CVT, the above-mentioned overrun can be effectively prevented.  
     [0064] In the illustrated embodiment, the controller  5  is implemented as a programmed general purpose computer. It will be appreciated by those skilled in the art that the controller can be implemented using a single special purpose integrated circuit (e.g., ASIC) having a main or central processor section for overall, system-level control, and separate sections dedicated to performing various different specific computations, functions and other processes under control of the central processor section. The controller can be a plurality of separate dedicated or programmable integrated or other electronic circuits or devices (e.g., hardwired electronic or logic circuits such as discrete element circuits, or programmable logic devices such as PLDs, PLAs, PALs or the like). The controller can be implemented using a suitably programmed general purpose computer, e.g., a microprocessor, microcontroller or other processor device (CPU or MPU), either alone or in conjunction with one or more peripheral (e.g., integrated circuit) data and signal processing devices. In general, any device or assembly of devices on which a finite state machine capable of implementing the procedures described herein can be used as the controller. A distributed processing architecture can be used for maximum data/signal processing capability and speed.  
     [0065] While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the preferred embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.