Abstract:
The invention concerns a method for adjusting the smooth engagement of a gear ratio shift wherein a planetary train ( 7 ) is engaged in direct gear by the action of centrifugal counter-weights ( 29 ) locking a clutch ( 18 ). The clutch release can be derived either from a hydraulic actuator ( 18 ) or from axial reactive forces of the ring helical toothing, reaction which is transmitted by an axial stop (B 2 ). In order to prevent the clutch ( 18 ) from being suddenly locked by the action of the counter-weights ( 29 ) in particular when the torque to be transmitted quickly disappears, a control unit ( 453 ) detects the instantaneous ratio (V E /V S ), and generates measured back pressure in the actuator ( 45 ) while the counter-weights ( 29 ) are locking the clutch ( 18 ). The invention is useful for adjusting the smooth engagement of gear ratio shifts.

Description:
DESCRIPTION 
     This invention relates to a method for adjusting the shifting—or transmission ratio change—progressiveness in a transmission device, in particular in an automatic transmission device with multiple ratios. 
     This invention also relates to a transmission device implementing such a method. 
     From WO-A-9207206, there is already known an automatic transmission in which a clutch selectively connects two rotary members of a differential gearing such as an epicyclic train, according to whether one or the other of two antagonistic forces dominates. The first force, tending to disengage the clutch, is for example a tooth reaction, more particularly an axial thrust produced by helical gear teeth mounted in an axially movable manner. The second force, tending to engage the clutch, can be produced by springs and/or by tachometric centrifugal means. When the clutch is disengaged, it is necessary to prevent rotation of a third rotary member of the differential gearing, and this can be ensured by a free wheel preventing the third member from rotating in the reverse direction. 
     This type of transmission is very advantageous since its basic operation does not require an external power source, nor sensors, nor a control circuit. The transmission device itself does produce the forces which will control it and these forces are at the same time a measurement of the parameters necessary for the control. 
     For modern transmissions which have to provide a high level of comfort and optimisation of the operation, the previously mentioned forces are advantageously completed by additional forces, produced for example by hydraulic actuators. The additional forces can serve to modify at will the speed and torque conditions under which the transmission changes ratio, or can lock the transmission in a given ratio when that is desired (PCT/FR 94/00 176). 
     On the other hand, it has been observed, according to the invention, that the shifting process under the effect of forces such as a centrifugal force or a tooth reaction could exhibit certain defects. 
     WO-A-97/08 478 proposes solutions intended to remedy certain types of defects. 
     A particular defect to which the present invention is relevant, is the brutality which the action of an uncontrolled force generator, such as a spring or even more so a centrifugal force generator can have, particularly in the case of the above-mentioned transmissions, when the antagonistic force disappears and suddenly releases the force generator. For example, if the antagonistic force is proportional to the transmitted torque and the latter disappears because the driver releases the accelerator pedal, the uncontrolled force generator might suddenly actuate a clutch and cause a dangerous and uncomfortable shock. 
     The purpose of the present invention is to provide better control of a shifting process involving actuation of at least one selective coupling means. 
     According to the invention, the method for adjusting progressiveness of a change from an old transmission ratio to a new transmission ratio, in a transmission device comprising: 
     a device for selective coupling between two rotary members; 
     a force generator means for urging the selective coupling means towards a predetermined one of its slipping and gripping states, corresponding to the new ratio; 
     an actuating means capable of urging the selective coupling means towards the other of the said states, corresponding to the old ratio; 
     is characterised in that the method comprises, whilst the generator means is actuating the selective coupling means towards its state corresponding to the new ratio, a step of controlling the actuating means so that it produces a measured resistant force, slowing down the transition between the old ratio and the new ratio. 
     By means of the actuating means, the action of the force generator means is counter-balanced in a measured manner, systematically or only when necessary, in order to prevent the force generator means from provoking a too-sudden change in the selective coupling device. 
     The amount of the contrary force applied by the actuating means can either be a predetermined unique amount or an amount chosen from a series of predetermined amounts, the choice being made according to a selection criterion, for example the speed of rotation which determines the centrifugal effect if the force generator means is of the centrifugal type. Such a predetermined amount preferably consists of a progressively decreasing force, which therefore progressively allows the force generator means to urge the selective coupling means towards the new coupling state. 
     Preferably, a physical value is detected, this value being one which is influenced by the progressive change from the old transmission ratio to the new transmission ratio, and the actuating means is controlled as a function of the result of this detection. 
     In the case of a transmission in which the shiftings are carried out spontaneously, that is to say without the intervention of a processing unit and, for example, are carried out according to the direction and value of the resultant of various forces such as a centrifugal force and a tooth reaction force indicating the transmitted torque, the detection of the said physical value has the function of detecting that a change of ratio is in progress in the transmission. The counter-balancing action of the actuating means is initiated, at least in certain cases, according to this detection. 
     Furthermore, the detection of the physical value can be used to produce a servo-control of progressiveness. It is possible, for example, to calculate the time derivative of the transmission ratio and to adjust the counter-balancing action such that this derivative remains as close as possible to a reference value. It is possible to choose references other than the derivative. For example it is possible to fix a law of evolution of the ratio over time as a reference. 
     When the transmission offers a number of transmission ratios which is high in comparison with the number of gear trains used, there is in general at least one shifting process which requires activation of one coupling and the release of another coupling whilst perfectly synchronising these two operations. Any imperfection in this synchronisation renders the change of ratio uncomfortable for the occupants of the vehicle and introduces stresses and/or shocks, which cause wear, in the transmission. 
     The selective coupling means which initiates the shifting process can in such a case be the selective coupling means which is counter-balanced as just described and this selective coupling means can be the one whose actuation varies the input speed of the transmission device in the sense corresponding to the change of ratio to be effected. When the detected physical value reaches a certain predetermined level, the actuation of the other coupling means is initiated in its turn. 
     As physical value which is characteristic of the evolution of the ratio change process, it is indeed advantageous to choose the input speed, or the ratio between the input speed and the output speed of the transmission or, more generally, between two speeds the ratio between which is affected by the shifting process in question. 
     In such a case, a progressiveness control regulating the counter-balancing provided by the actuating means as a function of the evolution of the overall transmission ratio provided by the two transmission mechanisms will allow to compensate for all of the imperfections of the ratio change process in the two mechanisms. The overall result will therefore be very satisfactory despite the complexity of the shifting process which is involved. 
     Another advantage in choosing as a physical value one that indicates the state of the whole of the transmission device, and not just the state of a specified subassembly such as an epicyclic train in a transmission device which comprises several of them, and in particular in choosing as a physical value the ratio between the input speed and the output speed in the transmission, is that this value indicates, according to the range of values within which it is currently varying, which change of ratio is actually occurring. For example, in a device producing four transmission ratios with only two epicyclic mechanisms in series, there are three possible changes, i.e. from the first to the second ratio, from the third to the fourth ratio and from the third to the second ratio, which comprise the engagement of the same clutch. Because of the overall detection, it is possible to distinguish which of the changes is occurring and, if necessary, to modify the control criteria accordingly. Furthermore, the overall detection makes it possible to use a same detector assembly for all of the selective coupling means for which it is desired to apply the method according to the invention. 
     According to a second aspect of the invention, the transmission device for a vehicle, comprising: 
     at least one gear train; 
     a selective coupling means able, by changing from an old coupling state to a new coupling state, to cause the gear train to change from an old transmission ratio to a new transmission ratio; 
     a force generator means capable of causing the selective coupling means to change from the old coupling state to the new coupling state; 
     an actuating means capable of applying to the selective coupling means an action tending to force it towards the old coupling state; 
     control means for controlling the actuating means; 
     characterised in that the control means comprise progressiveness means for causing the actuating means to apply a measured amount of force slowing down the change of the selective coupling means from the old coupling state to the new coupling state under the action of the force generator means. 
     In the rest of this description, following convention, a transmission ratio will be called “slow” when it corresponds to a high input speed with respect to the output speed. In the opposite case, the ratio is called “fast”. 
     Other features and advantages of the invention will furthermore emerge from the following description relating to non-limitative examples. 
    
    
     IN THE ACCOMPANYING DRAWINGS: 
     FIG. 1 is a diagrammatic half-view in longitudinal cross-section of a transmission device with two ratios according to the invention, in the rest state; 
     FIGS. 2 and 3 are views similar to FIG. 1 but relating to operation as a reduction gear and as a direct drive respectively; 
     FIG. 4 is a time-based diagram showing a version of the method according to the invention; 
     FIG. 5 is as diagrammatic half-view of a transmission device with four ratios according to the invention; and 
     FIGS. 6 and 7 are time-based diagrams illustrating the operation of the embodiment shown in FIG.  5 . 
    
    
     The transmission device with two ratios shown in FIG. 1, intended in particular for a motor vehicle, comprises an input shaft  2   a  and an output shaft  2   b  aligned along the axis  12  of the device. The input shaft  2   a  is connected to the output shaft of the engine  5  of a motor vehicle with the interposition of an input clutch  86  and possibly other means of transmission which are not shown. The output shaft  2   b  is intended to drive, directly or indirectly, the drive wheels of a vehicle. Between the output shaft  2   b  and the wheels of the vehicle there can, for example, be interposed another transmission device with two or more ratios and/or a manually controlled forward drive/reverse drive inverter and/or a differential for distributing the motion between the drive wheels of the vehicle. 
     The input  2   a  and output  2   b  shafts are axially immobilised with respect to a casing  4  of the transmission device, which is only partially shown. 
     The transmission device comprises a differential gearing formed by an epicyclic train  7 . The train  7  comprises a crown with internal teeth and a sun wheel  9  with external teeth, both meshing with planets  11  supported, at equal angular intervals about the axis  12  of the transmission device, by off-centred trunnions  14  of a planet holder  13  rigidly connected to the output shaft  2   b . The sun wheel  9  can rotate freely about the axis  12  of the transmission device with respect to the output shaft  2   b  which it surrounds. However, a free-wheel device  16  prevents the sun wheel  9  from rotating in reverse, that is to say in the opposite direction to the normal direction of rotation of the input shaft  2   a  with respect to the casing  4  of the transmission. 
     The crown wheel  8  is connected for common rotation with, but axially slidable with respect to the input shaft  2   a , by the intermediary of splines  17 . 
     A multi-disk clutch  18  selectively couples the crown wheel  8  with the planet holder  13 . 
     The stack of disks  19  and  22  of the clutch  18  can be axially clamped between a retaining plate  26  integral with the planet holder  13  and a movable plate  27  which belongs to a cage  20 , which is bound for common rotation with the planet holder  13 , but slidable with respect to the latter. 
     The cage  20  supports centrifugal flyweights  29  disposed in a ring around the clutch  18 . The flyweights are therefore bound for common rotation with the output shaft  2   b  of the transmission device. 
     The rotation of the planet holder  13  tends to cause a body  31  of each flyweight  29  to pivot radially outwardly about its tangential pivoting axis  28  under the effect of centrifugal force, in order to cause the flyweights to move from a rest position defined by abutment of a stop  36  of the flyweights against the cage  20  (FIGS. 1 and 2) and a separated position which can be seen in FIG.  3 . 
     This results in a relative axial displacement between a nose  32  of each flyweight and the pivoting axis  28  of the flyweight. This displacement, which brings the nose  32  towards the movable plate  27 , can correspond to a compression of a Belleville spring  34  fitted between the nose  32  and the retaining plate  26  and/or a displacement of the movable plate  27  towards the stationary plate  26  in the sense of clamping the clutch  18 . 
     When the transmission device is in the rest state as shown in FIG. 1, the Belleville spring  34  transmits to the cage  20 , by the intermediary of the flyweights abutted in the rest position, a force which engages the clutch  18  such that the input  2   a  of the transmission device is coupled in rotation with the output  2   b  and the transmission device constitutes a direct drive capable of transmitting torque up to a certain maximum defined by the clamping force of the Belleville spring. 
     Furthermore, the teeth of the crown wheel  8 , of the planets  11  and of the sun wheel  9  are of the helical type. Thus, in each pair of gears meshing under load, axially opposed thrusts appear which are proportional to the transmitted circumferential force, and therefore to the torque on the input shaft  2   a  and to the torque on the output shaft  2   b . The direction of helical inclination of the gear teeth is chosen such that the axial thrust Pac (FIG. 2) arising in the crown wheel  8  when it transmits a drive torque is applied in the direction in which the crown wheel  8  pushes the movable plate  27 , by the intermediary of a thrust bearing B 2 , in the direction separating the plates  26  and  27 , and therefore disengaging the clutch  18 . The force Pac also tends to bring the nose  32  of the flyweights  29  and the retaining plate  26  towards one another, and therefore to maintain the flyweights  29  in their position of rest and to compress the Belleville spring  34 . The planets  11 , which mesh not only with the crown wheel  8  but also with the sun wheel  9  undergo two opposite axial reactions PS 1  and PS 2 , which balance out, and the sun wheel  9  undergoes, taking account of its meshing with the planets  11 , an axial thrust Pap which is equal in intensity and opposite to the axial thrust Pap of the crown wheel  8 . The thrust Pap of the sun wheel  9  is transmitted to the casing  4  by the intermediary of a thrust bearing B 3 . 
     This is the situation shown in FIG.  2 . Assuming that this position is achieved, the basic operation of the transmission device will now be described. As long as the torque transmitted by the input shaft  2   a  is such that the axial thrust Pac in the crown wheel  8  suffices to compress the Belleville spring  34  and to maintain the flyweights  29  in the rest position shown in FIG. 2, the separation between the retaining plate  26  and the movable plate  27  of the clutch is such that the disks  19  and  22  slip against each other without transmitting torque between them. In this case, the planet carrier  13  can rotate at a speed different from that of the input shaft  2   a , and it tends to be immobilised by the load which the output shaft  2   b  must drive. The result of this is that the planets  11  tend to behave as motion reversers, that is to say to cause the sun wheel  9  to rotate in the opposite direction of rotation from the crown wheel  8 . But this is prevented by the free wheel  16 . The sun wheel  9  is therefore immobilised by the free wheel  16  and the planet carrier  13  rotates at a speed which is intermediate between the zero speed of the sun wheel  9  and the speed of the crown wheel  8  and of the input shaft  2   a . The module therefore operates as a reduction gear. If the speed of rotation increases and the torque remains unchanged, a time arrives at which the centrifugal force of the flyweights  29  produces on the movable plate  27  with respect to the retaining plate  26  an axial clamping force which is greater than the axial thrust Pac, and the movable plate  27  is pushed towards the plate  26  in order to achieve direct drive (FIG.  3 ). 
     The clutch  18 , as it is being engaged during the change to direct drive, increasingly transmits power directly from the crown wheel  8 , bound to the input shaft  2   a , to the planet carrier  13 , bound to the output shaft  2   b . Consequently, the teeth of the epicyclic train  7  work decreasingly, that is to say they transmit progressively decreasing force. The axial thrust Pac decreases and finally disappears. Thus, the axial thrust due to the centrifugal force can be applied fully in order to clamp the plates  26  and  27  against one another. 
     It can then happen that the speed of rotation of the output shaft  2   b  reduces, and/or that the torque to be transmitted increases, to such a point that the flyweights  29  no longer provide in the clutch  18  a sufficient clamping force to transmit the torque. In this case, the clutch  18  begins to slip. The speed of the sun wheel  9  reduces until it becomes zero. The free wheel  16  immobilises the sun wheel and the tooth force Pac reappears in order to disengage the clutch, such that the transmission device then operates as a reduction gear. Thus, each time that a change of operation from a reduction gear to operation in direct drive or vice-versa occurs, the axial force Pac varies in the sense which stabilises the newly instituted transmission ratio. This is very advantageous on the one hand to prevent too-frequent changes of ratio about certain critical operating points and, on the other hand, in order that the situations in which the clutch  18  is slipping are only transient. 
     As shown in FIG. 1, complementary means are provided to selectively cause operation of the transmission device as a reduction gear in conditions other than those determined  12  by the axial forces of the Belleville spring  34 , the flyweights  29  and the teeth of the crown wheel  8 . 
     For this purpose, the transmission device comprises a brake  43  which allows to immobilise the sun wheel  9  with respect to the casing  4  independently from the free wheel  16 . In other words, the brake  43  is mounted operatively in parallel with the free wheel  16  between the sun wheel  9  and the casing  4 . The piston  44  of a hydraulic actuator  43  is mounted in a axially sliding manner for selectively applying and releasing the brake  43 . The brake  43  and the piston  44  have an annular shape having as their axis the axis  12  of the transmission device. The piston  44  is adjacent to a hydraulic chamber  46  which can be selectively fed with pressurised oil in order to drive the piston in the direction of applying the brake  43 . 
     Furthermore, the piston  44  is rigidly connected to a push rod  47  which can press against the cage  20  by means of an axial thrust bearing B 4 . The assembly is such that when the pressure existing inside the chamber  46  pushes the piston  44  into the position of applying the brake  43 , the cage  20 , before the brake  43  is applied, is pushed back sufficiently for the clutch  18  to be released. 
     Thus, when the piston  44  is in the position of applying the brake (FIG.  2 ), the sun wheel  9  is immobilised even if the planet holder  13  is tending to rotate faster than the crown wheel  8 , as is the case when operating in engine braking mode, and consequently the module operates as a reduction gear, as allowed by the release of the clutch  18 . 
     The assembly  43 ,  44 ,  46 ,  47  which has just been described constitutes an actuating means which can be made available to the driver of the vehicle to force the module to change to operation as a reduction gear or to retain the operation as a reduction gear when the driver wishes to increase the engine braking effect, for example when going downhill, or when he wishes to increase the engine torque on the output shaft  2   b . When the torque is a driving torque, the brake  43 , if applied, carries out a redundant action with that of the free wheel  16 , but this is not harmful. 
     The feeding and draining of the chamber  46  are determined by the state of an electro-valve  69 . When it is in the rest state, the electro-valve  69  (FIGS. 1 and 3) connects the chamber  46  with a drain path  151  which is hydraulically resistant. When the electro-valve  69  is electrically energised (FIG.  2 ), it isolates the chamber  46  from the drain path  151  and connects it with the output of a pump  57  driven by the engine  5 . Irrespective of the state of the electro-valve  69 , the pump  57  can also serve to feed a lubrication circuit (not shown) of the transmission device. 
     The electro-valve  69  is controlled by control means  452  comprising a control unit  152  connected to a detector  153  of the speed V S  of the output shaft  2   b , a detector  158  of the speed V E  of the input shaft  2   a , a “manual/automatic” selector  154  made available to the driver, a detector of the position of the accelerator pedal  156  and a “normal/sport” selector  157  allowing the driver to choose between two different automatic behaviours of the transmission device. 
     The control unit  152  monitors the ratio between the input speed V E  and the output speed V S . As long as the device is operating as a reduction gear (the situation shown in FIG.  2 ), this ratio is equal to about 1.4. If the input speed V E  reduces with respect to the output speed V S , it is because the flyweights  29  have begun to engage the clutch  18  and consequently the transmission device has spontaneously initiated a change to direct drive operation. In this case, in order to ensure the progressiveness of this process and, more particularly, to ensure a certain duration of slipping of the disks  19  and  22  of the clutch, the control means  452  which have detected the reduction in V E  with respect to V S  control the feeding of the chamber  46  such that the piston  44  pushes the cage  20  in the direction tending to release the clutch  18 , in order to slow down the engagement process resulting in the situation shown in FIG.  3 . In practice, it is desired that the control means  452  cause as soon as possible the start of the action of the piston  44 . Taking account of the detection delay and of the inevitable response times, the action begins when the ratio V E /V S  becomes lower than about 1.3. 
     In the example shown, the control means  452  comprise, in addition to the control unit  152 , a progressiveness stage  453  which also receives the signals V E  and V S , continuously calculates the transmission ratio, detects the variation in the ratio V E /V S , resulting from the start of engagement of the clutch  18  and selectively produces on its output  454  a control signal producing a modulated energising of the electro-valve  69  such that the actuator  45  is fed as has just been described. 
     The method according to the invention will now be described more precisely with reference to FIG.  4 . 
     In this figure, the uppermost graph shows the evolution of the transmission ratio R=V E /V S , (vertical axis) with respect to time T (horizontal axis). The lowermost graph shows, along the same time scale T, the energisation level (the double line on the drawing) of the winding of the electro-valve  69 , and the pressure level PV (single line) in the chamber  46 . 
     According to a preferred feature used in this example, the intensity of the counter-balance effect provided by the actuator  45  is controlled by the progressiveness unit  453  by varying the width PW of electrical pulses applied to the electro-valve  69 . To do this, the signal on the output  454  is applied to a pulse generator  456  whose output  457  supplies the pulses to the electro-valve  69 . The width PW of the pulses varies from 0% (total absence of pulses) to 100%, corresponding to continuous working. The lower graph in FIG. 4 shows the evolution of the width of the pulses with respect to time, expressed in %. Small detail views show that a high percentage corresponds to a large pulse width and a low percentage corresponds to a small pulse width. 
     The pulse repetition frequency is constant and can for example be 50 Hz. The amplitude of the pulses outside of the cut-off periods is also constant, 12 volts for example. 
     In the example shown, the transmission ratio is initially equal to 1.4. Until the time T 1 the operation as a reduction gear is imposed by the actuator  45  since the width of the pulses applied to the electro-valve  69  is 100%. In this case it is a continuous signal applied by the control unit  152 . The actuator  45  is designed such that it is capable of overcoming the centrifugal force produced by the flyweights  29  even in the absence of tooth reaction force PAC for all speeds where this can be useful in practice. For example, if it is considered that the maximum speed V S  for which it can in certain cases be necessary to impose operation as a reduction gear is 3,000 r.p.m., the force of the actuator  45  when the pulse width is continuously 100% is at least equal to the force produced by the centrifugal effect on the noses  32  of the flyweights when the cage  20  is rotating at 3,000 r.p.m. 
     Operation as a reduction gear continues for a certain time until the time T 2  at which the transmission ratio suddenly starts to decrease. The duration T 1 −T 2  can be very short if, as from the end of feeding the actuator  45 , the axial force produced by the flyweights  29  is greater than the reaction P AC . The duration T 1 −T 2  can be longer in the opposite case, and if it is necessary consequently to wait for the unbalance between the force produced by the flyweights  29  and the tooth reaction force P AC  to begin to change direction. Whatever the case may be, the progressiveness unit  453 , which continuously calculates the ratio R=V E /V S  detects slightly after the time T 2  that a change from operation as a reduction gear to operation as a direct drive has begun. The unit  453  thus causes, as from the time T 3  and up to the time T 5  a predetermined energising of the actuator  45  in order to slow down the process of change to direct drive, by counter-balancing in a measured manner the force produced by the flyweights  29 . 
     Since the actuator  45  is capable of maintaining the clutch  18  disengaged against the effect of the flyweights  29  even in the absence of any tooth reaction P AC , a durable energising of the actuator  45  at PW=100% would have the effect not of slowing down the change to direct drive, but in most case of preventing it totally and of causing a return to operation as a reduction gear. 
     In the example shown, the adjustment of the counter-balancing effect consists in applying to the electro-valve  69 , as shown at the bottom of FIG. 4, a pulse width which varies from 100% to 0% linearly between the time T 3  and the time T 5 . The time interval T 3 −T 5  is in agreement with the duration desired for good progressiveness of the ratio change. 
     More particularly, the effect of the pulses is to produce a rise in the pressure PV in the chamber  46  of the actuator  45  up to a level which is however distinctly below that produced by a pulse width durably fixed at 100% (see the graph at the bottom of FIG.  4 ). Under the effect of the pulses, the electro-valve  69  oscillates between the open state and the closed state. When it connects the chamber  46  with the pump  57 , a pressure wave is sent into the chamber  46 . When the electro-valve  69  connects the chamber  46  with the drain channel  151 , the hydraulically resistant nature of this channel prevents an immediate discharge of the chamber  46 . This results in a counter-balancing force on the piston  44 , this force being modulated substantially according to the profile of pulse widths PW over time T, but with a certain delay. Consequently, the resultant force applied to the clutch in the sense of engagement thereof increases from a very small value at the time T 2  to a value equal to the force produced by the flyweights when, a certain time after the end of the pulses at the time T 5 , the pressure in the chamber  46  is eliminated. Thus, the transmission ratio, instead of suddenly dropping along the dotted line  401  shown in FIG. 4, decreases progressively from the time T 4  (slightly after the time T 3 ), to the time T 6 , after the time T 5  of the end of the pulses. The profile of the decrease in ratio can vary greatly from one case to another, depending for example on whether the change of ratio is due to an increase, necessarily progressive, in the speed of rotation V S , or to a disappearance, which can be sudden, of the torque to be transmitted. Typically, as shown in FIG. 4, the decrease profile resembles the decrease profile of the pulse width PW. 
     Improved progressiveness can also be provided when the control means  452 , as a function of the signals they receive on their inputs, must control shifting from direct drive operation to reduction gear operation, by means of the actuator  45 . 
     To do this, instead of suddenly changing the width of the pulses PW to be applied to the electro-valve  69  from 0% to 100%, the electro-valve can be subjected to a progressive increase in the width PW of the pulses which are applied to it. It can however be advantageous to begin with a few pulses of large width in order to fill the chamber  46  rapidly and to rapidly take up the various plays and possible deformations of the system. After that, the pulses drop down to a smaller width and then increase again up to a durable level of 100%. 
     Finally, according to a variant shown in dotted line in the lowermost graph of FIG. 4, it is possible for the train of pulses applied to the electro-valve  69  in order to slow down the change to direct drive to start from a value of PW below 100%, the influence on the pressure in the chamber  46  being correspondingly reduced. 
     In the embodiment diagrammatically shown in FIG. 5, the transmission device comprises two planetary trains fitted in series,  107 ,  207 . The planetary train  107  is similar to the one described with reference to FIGS. 1 to  3 : its crown wheel  108  is connected to the input shaft  2   a , its sun wheel  109  is connected to the casing  104  by the intermediary of a free wheel  116 , and its planet holder  114 , supporting planets  111  meshing with the crown wheel  108  and the sun wheel  109 , is connected to the output shaft  2   ab  of the mechanism  107 , which is also the input shaft of the mechanism  207 . A clutch  118  makes it possible to couple selectively the crown wheel  108  with the planet holder  114 , in other words the input shaft  2   a  with the intermediate shaft  2   ab  in order to achieve direct drive in the planetary train  107 . When the clutch  118  is released, the planetary train  107  operates as a reduction gear, the sun wheel then being immobilised by the free wheel  116 . The reduction ratio provided by such a planetary train, that is to say a planetary train with an input on the crown wheel and an output on the planet holder, is commonly of the order of 1.4. 
     The second planetary train  207  is different in that its input shaft, constituted by the intermediate shaft  2   ab , is connected not to the crown wheel  208 , but to the sun wheel  209 , the crown wheel  208  being connected to the casing  104  by the intermediary of a free wheel  216  preventing the crown wheel  208  from rotating in the reverse direction. The output shaft  2   b  is connected to the planet holder  214  supporting planets  211  each meshing with the crown wheel  208  and the sun wheel  209 . A clutch  218  allows to firmly connect the intermediate shaft  2   ab  with the output shaft  2   b  in order to achieve a direct drive in the second differential mechanism  207 . 
     When the clutch  218  is disengaged, the mechanism  207  operates as a reduction gear with the crown wheel  208  immobilised by the free wheel  216 . Taking account of the fact that the input is applied through the sun wheel  209  and the output is taken from the planet holder  214 , the reduction ratio is therefore typically equal to 3. 
     The clutches  118  and  218  are selectively engaged by a spring R 1  and respectively by the flyweights  229  driven in rotation by the planet holder  213 , and disengaged against the action of the spring R 1  and respectively of the flyweights  229 , by actuators A 1  and A 2  respectively, each controlled by an electro-valve V 1  and V 2  respectively, which are themselves controlled by the control unit  452 . Furthermore, in the case of the clutch  218 , a thrust bearing B 2  transmits the axial tooth force P AC  from the crown wheel  208  to the cage  220  in the sense of disengaging the clutch  218 . 
     The unit  452  receives on its inputs the signals V E  and V S  supplied by the detectors  158  and  153  respectively as well as the signal from the detector  156  indicating the position of the vehicle&#39;s accelerator pedal, which corresponds to a load parameter C of the vehicle&#39;s engine, which can be expressed for example as a percentage of the maximum load. 
     The transmission device which has just been described is capable of providing four different ratios. The first ratio, or slowest ratio, is established when the two clutches  118 ,  218  are disengaged and consequently the two planetary trains  107 ,  207  are operating as reduction gears. The transmission therefore provides a reduction ratio equal to 1.4×3=4.2. 
     For operation in the second ratio, the clutch  118  is engaged and the clutch  218  is disengaged, such that the planetary train  107  operates as a direct drive and the planetary train  207  as a reduction gear, which gives a total reduction ratio of  3  in the transmission device. 
     For operation in the third ratio, the opposite case applies, the clutch  118  is disengaged and the clutch  218  is engaged, such that only the first planetary train  107  is operating as a reduction gear. This provides an overall reduction ratio equal to 1.4. 
     For operating in the fourth ratio, or the fastest ratio, the two trains  107 ,  207  operate as direct drives, the overall ratio being equal to 1. 
     In the simple example which is illustrated, the changes of ratio in the first train are only controlled by the unit  452  according to the functional parameters V S  (output speed) and C (load) but more sophisticated versions are conceivable, the first train for example then being similar to that of FIGS. 1 to  3 . 
     In this transmission device, the change from the second to the third ratio is difficult to control because the clutch  118  must be disengaged at the time the clutch  218  must be engaged. If the synchronisation between these two operations is imperfect, there is a risk of having, for a short time, either a simultaneous disengagement of the two clutches, that is to say a brief return to the first transmission ratio probably with a risk of excess speed of the engine, or a simultaneous engagement of the two clutches, that is to say a brief situation of direct drive in the whole of the transmission with a risk of inadequate speed of the engine. In both cases, the passengers suffer shocks and the mechanics suffer shocks and useless stresses. Furthermore, these functional irregularities, if they were allowed to occur, would react on the functional parameters sensed by the unit  452 , and this would disturb the shifting process even more. 
     It can be seen from FIG. 6 that the detection by the unit  452  of the overall ratio of the transmission device R=V E /V S  allows the control unit to know what transmission ratio is being produced at any time and, consequently, upon variation of that ratio, what ratio change is in progress. 
     Consequently, when starting from the second transmission ratio, corresponding to R=3, the flyweights  229  of the second epicylic train  207  begin to engage the clutch  218 , the unit  452  detects that it is a change from the second ratio to the third ratio, a change for which it will be necessary to synchronise the action of the two clutches. 
     FIG. 7 illustrates the process which is used to avoid the above-mentioned disadvantages, and more generally to produce a virtually perfect transition between the second and the third transmission ratios. 
     In the example of FIG. 7 there is again found the time T 1  starting from which the control unit  452  authorises the engagement of the second clutch  218 , the time T 2  starting from which this engagement begins to happen, and the time T 3  starting from which the control unit  452 , in this example integrating the progressiveness unit, excites the actuator  245  in a measured manner to prevent a too-rapid engagement of this clutch. The train of pulses is also applied to the actuator  145 , which application advantageously initiates disengagement of the clutch  118 . 
     In this example, more perfected than the one shown in FIG. 4, the control unit  152  continuously calculates the ratio R and adapts the excitation (width of pulses) of the actuator  245  such that R varies according to a law defined with respect to time, which has been previously loaded into a memory of the unit  452 . In FIG. 7, this predetermined law is illustrated by a curve shown in dotted and dashed line  402 . Several types of servo-control are possible. For example it is possible at each instant to calculate the error between the value of R and a command value at that instant. It is also possible at each instant to calculate the time derivative of R and to correct the excitation of the actuator  245  to attempt to bring this derivative back to a predetermined command value. 
     At an instant T 7 , the unit  452  detects that R has passed through a threshold R S , for example R S =2. At that instant, the unit  452  commands continuous excitation, at PW=100%, of the actuator  145  in order to disengage the clutch  118  of the first train  107 . The hydraulic pressure in the actuator  145  is illustrated by the diagram at the bottom of FIG.  7 . Consequently, the train  107  progressively changes from operation in direct drive to operation as a reduction gear, as illustrated by the diagram  403  at the top of FIG. 7, its transmission ratio therefore changes from 1.0 to 1.4. Even if the coming into action of the actuator  145  is relatively sudden, this does not produce any shock on the input shaft  2   a  or on the output shaft  2   b  since the regulation provided by the actuator  245  affects the overall ratio of the transmission. Consequently, as illustrated by the curve  404  at the top of FIG. 7, if the coming into action of the actuator  145  is sudden, the regulation carried out by the actuator  245  will cause a corresponding sudden decrease of the transmission ratio in the train  207 , such that the overall ratio continues to follow the ideal profile  402  quite closely. 
     Returning to FIG. 6, when the unit  452  detects a change from the first to the second ratio or from the third to the fourth ratio, for each of which there is an engagement of the clutch  118  without modification of the state of the clutch  218 , the actuator  145  can be controlled as described with reference to FIGS. 1 to  4  or in a more sophisticated manner such that the transmission ratio or its time derivative follows a predetermined law or command. The pulses, also applied to the clutch  218  have no effect on the latter since the resulting pressure in the actuator  245  is insufficient. 
     In the right hand section of FIG. 6 there has also been illustrated the situations in which the transmission device causes a change to a slower ratio. In this case, the actuators can be controlled as described in WO-97/08 478, whose content is integrated in the present application by way of reference. 
     With regard to the change from the third to the second ratio (the right hand section of FIG.  6 ), the latter is initiated spontaneously by a slipping of the clutch  218  or on the intervention of the unit  452  provoking this slipping by an appropriate excitation of the actuator  245 . Starting from a time T 8  corresponding to the passing of a threshold which can be the threshold R S  as illustrated or a slightly different threshold, the unit  452  begins draining the actuator  145 . In this case, the method according to the invention can also be used, by maintaining, by means of width-modulated pulses, a measured resistance in the actuator  145  against the action of the spring R 1 . 
     The invention is not of course limited to the examplary embodiments described and shown. 
     The use of the invention is not necessarily coupled with other control functions of a transmission. 
     The invention is compatible with transmissions other than those actuated by centrifugal force and/or tooth reaction forces. 
     In an embodiment where it is necessary to modify simultaneously the state of two clutches such as  118  and  218  in FIG. 5, the invention could be applied only to the clutch which is subjected to the action of the force generator, for example such as described with reference to FIGS. 1 to  4 , and the progressiveness of the change of state of the other clutch could be regulated in another way, by using, in particular, the disclosures of WO-A-96/23 144 and of WO-A-97/08 478.