Patent Publication Number: US-6336890-B1

Title: Method for effecting a ratio shift, and transmission device for implementing same

Description:
This is a divisional of U.S. patent application Ser. No. 09/029,122, filed Feb. 23, 1998 which is a 371 of PCT/FR96/01291 Aug. 16, 1996. 
    
    
     The present invention relates to a method of controlling a change of ratio in a transmission device, in particular an automatic multiple-ratio transmission device. 
     The present invention also relates to a transmission device implementing such a method. 
     From WO-A-9207206 there is 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 is for example a gearing reaction, more particularly an axial thrust produced by a pinion with helical teeth mounted in an axially mobile manner, tending to disengage the clutch. The second force, tending to engage the clutch, can be produced by springs and/or by a centrifugal tachometric means. When the clutch is disengaged, it is necessary to prevent the rotation of a third rotary member of the differential gearing, and this can be ensured by a free wheel preventing this third member from rotating in the reverse direction. 
     This type of transmission is very advantageous as its basic functioning requires neither an external power source, nor sensors, nor a control circuit. It is the transmission device itself which produces 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 having to provide a high level of comfort and of optimisation of the operation, the previously mentioned forces are advantageously completed by additional forces, for example produced by hydraulic actuators. The additional forces can be used to modify at will the speed and torque conditions under which the transmission changes ratio, or for locking the transmission in a given ratio when this is required (PCT/FR 94/00 176). 
     From another aspect, it has been observed, according to the invention, that the change of ratio under the action of forces such as a centrifugal force or a gearing reaction could exhibit certain defects, such as excessive slowness. 
     Furthermore, 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 ratio change process which necessitates releasing one coupling and activating another, whilst synchronising these two operations perfectly. Any imperfection in this synchronization makes the change of ratio uncomfortable for the occupants of the vehicle and introduces stresses and/or shocks, which generate wear, in the transmission. 
     According to US-A-4 713 984, the coupling which must be activated receives an engaging force which is at first small and then increases progressively up to the maximum value, whilst the engaging force of the other coupling is progressively released. The hydraulic means for implementing this solution are complex, expensive and difficult to perfect. 
     The DE-A-41 19 078 teaches to adjust the hydraulic pressure which is used for performing a ratio change in an automatic transmission, this adjustment being made as a function of the position of the gas throttle of the engine. The DE-A-41 19 078 furthermore teaches to modify the hydraulic pressure only after a certain delay in case of a variation of the position of the gas throttle, thereby to take into account the delay of variation of the engine torque as a result of the new position of the throttle. The tuning of such a system to every possible practical situation is extremely complicated, and abrupt shocks or on the contrary excessive slippings cannot be avoided with certainty. The tuning of the system depends upon the correct operation of the engine and regresses as the engine and the transmission device wear. 
     The purpose of the present invention is to provide better control of the ratio change process involving the actuating of at least one selective coupling means. 
     According to the invention, the method for controlling progressive change from an old transmission ratio to a new transmission ratio in a transmission device offering at least two transmission ratios and comprising an actuator actuating a selective coupling means of the transmission device, wherein after initiation of the ratio change process, there is detected at least one operation-related physical magnitude and the actuator is controlled according to the detected value of the physical magnitude, is characterised in that the physical magnitude which is detected is a physical magnitude which is influenced by the progressive transmission ratio change process. 
     Instead of taking into account parameters which influence the behaviour of the transmission, such as the load of the engine, and the influence of which has to be predicted for trying to compensate the effect thereof on the quality of the ratio change, as is made according to DE 41 19 078, the invention takes advantage from a physical magnitude which on the contrary is influenced by the ratio change which is in progress. The actuator is accordingly controlled on the basis of the actually detected effects, instead of on the basis of the presumable effects of a circumstantial parameter. 
     As explained in WO-A-92 07 206, the initiation of the ratio change process can be due to a spontaneous slipping of the coupling means when the torque to be transmitted exceeds the transmission capability of the coupling means subjected to a well-determined engagement force which can for example be constant or increasing with a speed of rotation. 
     The invention applies in particular to the case in which two selective coupling means must change state in a synchronized manner. The initiation of the ratio change process can therefore be performed by the second selective coupling means, that is to say the selective coupling means other than the one which is controlled as a function of the physical magnitude. It is then advantageous to choose as the second selective coupling means the one whose actuation causes the input speed of transmission device to vary in the sense corresponding to the ratio change to be effected. For example, if the ratio change in the process of being carried out is a change to a higher ratio, which will therefore result in a reduction of the input speed of rotation of the transmission, action is carried out such that the ratio change process begins by actuating that one of the two coupling means which causes a reduction in the input speed of the transmission. When the detected physical magnitude reaches a certain predetermined value, the actuation of the other coupling means is initiated in its turn. 
     As a physical magnitude characteristic of the evolution of the ratio change process, it is, more particularly in light of the foregoing considerations, advantageous to choose the input speed, or the ratio between the input speed and the output speed of the transmission, or the ratio between a speed measured upstream and another one measured downstream of the one or more coupling means whose state changes when the ratio change process is initiated. 
     Preferably, the coupling means are controlled on the basis of two different truth tables. When the physical magnitude passes a predetermined threshold, there is a change from the first of the truth tables, which does not provide for the actuation of the first coupling means for the vehicle&#39;s operating conditions which prevail at the time of the ratio change, to the second of the said tables, which provides for the actuation of the first coupling means for the said conditions. 
     According to a second aspect of the invention, there is proposed a transmission device for a vehicle comprising at least one gear train and able to change from an old transmission ratio to a new transmission ratio by actuating a selective coupling means, the device comprising 
     initiating means to initiate a change of transmission ratio as a function of at least one operating parameter of the vehicle; 
     means of detecting a physical magnitude of operation; and 
     control means for controlling the actuation of the coupling means according to the evolution of the value of the said physical magnitude, 
     characterized in that the detection means are designed for detecting a physical magnitude which is capable of being influenced by the progressive change of transmission ratio after its initiation. 
     In the rest of the description, a transmission ratio will be referred to as “low” when it corresponds to a low speed at the output with respect to the input speed. In the opposite case, the ratio is referred to as “high”. 
     Other features and advantages of the invention will furthermore emerge from the following description, given with reference to non-limitative examples. 
     In the accompanying drawings: 
     FIG. 1 is a diagrammatic half-view in longitudinal cross-section of a two-ratio transmission device according to the invention, in the rest state; 
     FIGS. 2 and 3 are views similar to FIG. 1 but relating to the operation as a reduction gear and as a direct drive respectively; 
     FIG. 4 is a diagrammatic half-view of a four-ratio transmission device according to the invention; 
     FIG. 5 shows a flowchart for the control of the transmission device of FIG. 4; and 
     FIGS. 6 and 7 show two truth tables used by the flowchart of FIG.  5 . 
    
    
     The two-ratio transmission device shown in FIG. 1, intended in particular for a motor car, comprises an input shaft  2   a  and an output shaft  2   b  in alignment with the axis  12  of the device. The input shaft  2   a  is connected to the shaft of an engine  5  of a motor vehicle with the interposition of a clutch  86  and possibly of other transmission means which are not shown. The output shaft  2   b  is intended to drive, directly or indirectly, the driving wheels of a vehicle. Between the output shaft  2   b  and wheels of the vehicle there can, for example, be interposed another transmission device having two or more ratios and/or a manually controlled forward drive/reverse drive reversing gear, and/or a differential for distributing movement between the driving wheels of the vehicle. 
     The input  2   a  and output  2   b  shafts are immobilized axially with respect to a casing  4  of the transmission device. 
     The transmission device comprises a differential gearing formed by an epicyclic train  7 . The train  7  comprises a crown wheel  8  with internal teeth and a sun wheel  9  with external teeth, both of them meshing with planet wheels  11  supported, at equal angular intervals around the axis  12  of the transmission device, by eccentric spindles  14  of a planet wheels support  13  which is 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 output shaft  2   b  which it surrounds. However, a free wheel device  16  prevents the sun wheel  9  from rotating in the reverse direction, that is to say in the direction opposite 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 rotationally locked but is free to slide axially, 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 wheels support  13 . 
     The stack of disks  19  and  22  of the clutch  18  can be axially clamped between a mobile plate  27  and a retaining plate  26  which is integral with the planet wheels support  13 . The mobile plate  27  is part of a cage  20 , which is rotationally locked with the planet wheels support  13 , but able to slide with respect to the latter. 
     The cage  20  supports centrifugal fly-weights  29  disposed to form a ring around the clutch  18 . The fly-weights are therefore rotationally locked to the output shaft  2   b  of the transmission device. 
     Each fly-weight has a solid body  31  located radially to the outside of the disks  19  and  22  and an actuating arm  32  pressed against an external face of the fixed plate  26  by the intermediary of a Belleville disk spring  34 . 
     The rotation of the planet wheels support  13  tends to cause the bodies  31  of the fly-weights  29  to pivot radially towards the outside about their tangential axes  28 , under the effect of their centrifugal force, in order to make them move from a position of rest, defined by the abutment of a stop piece  36  of the fly-weights against the cage  20  (FIGS.  1  and  2 ), to a separated position which can be seen in FIG.  3 . 
     This therefore results in a relative axial displacement between the arm  32  and the axis of articulation  28  of the fly-weight. This displacement, which brings the arm  32  towards the mobile plate  27 , can correspond to a compression of the Belleville disk spring  34  and/or to a displacement of the mobile plate  27  towards the fixed plate  26  in the direction of engaging the clutch  18 . 
     When the transmission device is in the rest state as shown in FIG. 1, the Belleville disk spring  34  transmits to the cage  20 , by the intermediary of the fly-weights  29  which are against their stops in the rest state, a force which engages the clutch  18  such that the input  2   a  of the transmission device is rotationally coupled 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 engaging force of the Belleville disk spring. 
     Furthermore, the teeth of the crown  8 , of the planet wheels  11  and of the sun wheel  9  are of the helical type. Therefore, in each pair of teeth meshed under load, opposing axial thrusts appear which are proportional to the circumferential transmitted 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 teeth is chosen such that the axial thrust Pac (FIG. 2) arising in the crown  8  when it transmits a driving torque is applied in the direction such that the crown  8  pushes the mobile 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 arm  32  of the fly-weights  29  and the retaining plate  26  towards one another and therefore to keep the fly-weights in their position of rest and to compress the Belleville disk spring  34 . The planet wheels  11 , which mesh not only with the crown  8  but also with the sun wheel  9 , are subjected to two opposing axial reactions PS 1  and PS 2  which balance eachother, and the sun wheel  9  is subjected to, because of its meshing with the planet wheels  11 , an axial thrust Pap which is equal in value and opposite to the axial thrust Pac of the crown  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 situation is produced, 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  8  suffices to compress the Belleville disk spring  34  and maintain the fly-weights  29  in the position of rest shown in FIG. 2, the separation between the retaining plate  26  and the mobile plate  27  of the clutch is such that the disks  19  and  22  slide against each other without transmitting torque from one to the other. In this case, the planet wheels support  13  can rotate at a speed different to that of the input shaft  2   a,  and it tends to be immobilized by the load which the output shaft  2   b  has to drive. The result of this is that the planet wheels  11  tend to behave as motion reversers, that is to say to rotate the sun wheel  9  in the direction opposite to the direction of rotation of the crown  8 . But this is prevented by the free wheel  16 . The sun wheel  9  is therefore immobilized by the free wheel  16  and the planet wheels support  13  rotates at a speed which is intermediate between the zero speed of the sun wheel  9  and the speed of the crown  8  and of the input shaft  2   a.  The unit therefore operates as a reduction unit. If the speed of rotation increases and the torque remains unchanged, a time arrives when the centrifugal force of the fly-weights  29  produces, on the mobile plate  27  with respect to the retaining plate  26 , an axial engaging force greater than the axial thrust Pac, and the mobile plate  27  is pushed towards the plate  26  in order to achieve direct drive. 
     The clutch  18 , as it becomes engaged during the change to direct drive, increasingly transmits power directly from the crown  8 , which is bound with the input shaft  2   a,  to the planet wheels support  13 , which is bound with the output shaft  2   b.  Consequently, the teeth of the epicyclic train  7  work less and less, that is to say they transmit less and less force. The axial thrust Pac decreases and is eventually eliminated. Thus the axial thrust due to the centrifugal force can be applied fully in order to engage the plates  26  and  27  against one another. 
     It can then occur that the speed of rotation of the output shaft  2   b  reduces and/or that the torque to be transmitted increases to the point at which the fly-weights  29  no longer provide, in the clutch  18 , an engaging force sufficient to transmit the torque. In this case the clutch  18  begins to slip. The speed of the sun wheel  9  reduces until it disappears. The free wheel  16  immobilizes the sun wheel and the gearing force Pac reappears in order to disengage the clutch, such that the transmission device then operates as a reduction unit. Thus, each time a change from operation as a reduction unit to operation in direct drive occurs, or vice-versa, the axial force Pac varies in the sense of stabilising the newly prevailing transmission ratio. This is very advantageous, on the one hand in order to avoid too frequent changes of ratio around certain critical operating points and, on the other hand, in order that situations in which the clutch  18  is slipping are only transient. 
     As shown in FIG. 1, additional means are provided for selectively causing the transmission device to function as a reduction unit under conditions different from those determined by the axial forces due to the Belleville disk spring  34 , the centrifugal fly-weights weights  29  and the helical teeth of the crown  8 . 
     For this purpose, the transmission device comprises a brake  43  which makes it possible to immobilize the sun wheel  9  with respect to the casing  4  independently from the free wheel  16 . In other words, the brake  43  is functionally fitted in parallel with the free wheel  16  between the sun wheel  9  and the casing  4 . A hydraulic piston  44  is fitted in an axially sliding manner in order to apply and release the brake  43  selectively. The brake  43  and the piston  44  are of annular shape and their axis is the axis  12  of the transmission device. The piston  44  is adjacent to a hydraulic chamber  46  which can be supplied with oil under pressure selectively in order to force the piston  44  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 in the chamber  46  is pushing the piston  44  towards the position of applying the brake  43 , the cage  20 , before the brake  43  is engaged, 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 immobilized even if the planet wheels support  13  is tending to rotate faster than the crown  8 , as is the case when the engine operates to retard the vehicle, and consequently the unit operates as a reduction unit, as allowed by the disengagement of the clutch  18 . 
     The assembly  43 ,  44 ,  46 ,  47  which has just been described therefore constitutes a means which can be made available to the driver of the vehicle to force the unit to operate as a reduction unit when the driver wishes to increase the engine braking effect, for example when descending, or when he wishes to increase the drive torque on the output shaft  2   b.  When the torque is a driving torque, the brake  43 , if it is engaged, applies a redundant action with that of the free wheel  16 , but this is not disadvantageous. 
     The supplying and draining of the chamber  46  are controlled by an electro-valve  69 . When it is in the rest state, the electro-valve  69  (FIGS. 1 and 3) connects the chamber  56  with a leakage path  151  which is hydraulically resistant. When the electro-valve  69  is electrically powered (FIG.  2 ), it isolates the chamber  46  from the leakage path  151  and connects it to the outlet of a pump  57  driven by the engine  5 . Independently of the state of the electro-valve  69 , the pump  57  can also be used to supply a lubrication circuit (not shown) of the transmission device. 
     The electro-valve  69  is controlled by a control unit  152  connected to a detector  153  detecting the speed Vs of the output shaft  2   b,  a detector of the position of 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  making it possible for the driver to choose between two different automatic behaviours of the transmission device. 
     According to the present invention, an additional detector  158  detects the input speed V E  on the shaft  2   a.  At least when the device is functioning in direct drive, and consequently the piston  44  is not actuated, the control unit  152  monitors the ratio between the input speed V E  and the output speed V S . As long as direct drive is occurring, this ratio is equal to 1. If the input speed V E  increases with respect to the output speed V S , this means that the clutch  18  starts to slip and consequently the transmission device has spontaneously initiated a change to functioning as a speed reduction unit. In this case, in order to accelerate this process and to limit the duration of slipping of the disks  19  and  22  of the clutch, the control unit  152  which has detected the increase in V E  with respect to V S  commands the supply of the chamber  46  such that the piston  44  pushes the cage  20  in the direction of disengaging the clutch  18  in order to end in the position shown in FIG.  2 . For example, the control unit  152  can cause the action of the piston  44  to begin when the ratio V E /V S  becomes greater than 1.1. 
     In order that this function of the control unit  152  may be compatible with its other functions taking account of other operating parameters of the vehicle, it is advantageous that the control unit  152  should have in its memory two truth tables saying if the piston  44  must be activated, as a function of the operating parameters provided by the detectors  153 ,  154 ,  156  and  157 . When the control unit  152  detects that the ratio V E /V S  exceeds the said threshold, for example equal to 1.1, the control unit changes from the first truth table to the second truth table. For the operating conditions in force, the first truth table provided for the non-activation of the piston  44  whilst the second one provides for the activation of the piston  44  for the same conditions. 
     In a slightly different version, it is possible for the control unit  152 , when it detects that the ratio change process is complete, to suppress the activation of the piston  44 . The detection of the completion of the ratio change process consists, for example, in detecting that the ratio V E /V S  reaches the value, for example 1.4, corresponding to operation as a speed reduction unit. Suppression of the activation of the piston  44  does not causes a return to operation in direct drive since the operation as a reduction unit has caused the reappearance of the gearing forces Pac capable of stabilising the functioning as a reduction unit without the assistance of the piston  44 . 
     In the embodiment shown in FIG. 4, the transmission device, represented diagrammatically, comprises two planet gear trains  107 ,  207  mounted in series. The planet gear train  107  is similar to the one described with reference to FIGS. 1 to  3  in the sense that its crown  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 wheels support  114 , supporting planet wheels  111  meshing with the crown  108  and with the sun wheel  109 , is connected to the output shaft  2   ab  of the train  107  which is also the input shaft of the train  207 . A clutch  118  makes possible the selective coupling of the crown  108  with the planet wheels support  113 , in other words the input shaft  2   a  with the intermediate shaft  2   ab  in order to produce direct drive in the planet gear train  107 . When the clutch  118  is disengaged, the planet gear train  107  operates as a reduction unit, the sun wheel  109  then being immobilized by the free wheel  116 . The reduction ratio provided by such a planet gear train, that is to say a planet gear train with the input connected to the crown and the output connected to the planet wheels support, is commonly of the order of 1.4. 
     The second planet gear train  207  is different in that its input shaft, constituted by the intermediate shaft  2   ab,  is not connected to the crown  208  but to the sun wheel  209 . The crown  208  is connected to the casing  104  by the intermediary of a free wheel  216  preventing the crown  208  from rotating in the reverse direction. The output shaft  2   b  is connected to the planet wheels support  213  supporting planet wheels  211  each meshing with the crown  208  and the sun wheel  209 . A clutch  218  makes it possible to firmly connect the intermediate shaft  2   ab  to the output shaft  2   b  in order to produce direct drive in the second differential mechanism  207 . 
     When the clutch  218  is disengaged, the mechanism  207  operates as a reduction unit with the crown  208  immobilized by the free wheel  216 . Taking account of the fact that the input is connected to the sun wheel  209  and that the output is connected to the planet wheels support  213 , the reduction ratio is then typically equal to 3. 
     The clutches  118  and  218  are selectively engaged by springs, R 1  and R 2  respectively, and disengaged against the action of these springs 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  152 . 
     The unit  152  receives on its inputs the signals V E  and V S  provided by the detectors  158  and  153  respectively and 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 the lowest ratio, is established when both of the clutches  118 ,  218  are disengaged and consequently the two planet gear trains  107 ,  207  are operating as reduction units. The transmission then provides a reduction ratio equal to 1.4 ×3 =4.2. 
     In order to operate with the second ratio, the clutch  118  is engaged and the clutch  218  is disengaged, such that the planet gear train  107  operates as a direct drive and the planet gear train  207  operates as a reduction gear, which gives a total reduction ratio of 3 in the transmission device. 
     In order to operate with the third ratio, the reverse applies, the clutch  118  is disengaged and the clutch  218  is engaged, such that only the first planet gear train  107  operates as a reduction gear. This provides an overall reduction ratio of about 1.4. 
     In order to operate with the fourth ratio, or the highest ratio, both of the trains  107 ,  207  function as direct drives, the overall ratio being equal to 1. 
     In the simple example with is illustrated, the ratio changes are controlled only by the unit  152  in accordance with the functional parameters V S  (output speed) and C (load) but more sophisticated versions are conceivable. 
     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 same time as the clutch  218  must be engaged. If the synchronisation between these two operations is imperfect, there is a risk of having, over short periods, either a simultaneous disengagement of both clutches corresponding to a return to the first transmission ratio probably with a risk of excess engine speed, or a simultaneous engagement of both clutches, that is to say a brief situation of direct drive through the whole transmission with a risk of the engine speed being too low. In both cases, the passengers in the vehicle suffer shocks and the mechanics are subjected to useless shocks and stresses. Furthermore, these functional irregularities, if they are allowed to occur, would have an effect on the functional parameters recorded by the control unit  152 , which would interfere even more with the ratio change process. 
     In order to avoid these disadvantages, the control unit  152  firstly causes the engagement of the clutch  218  without disengaging the clutch  118 . This will reduce the input speed V E  with respect to the output speed V S  since this corresponds to the progressive change of the transmission device from the second ratio directly to the fourth ratio. During this partial process, the input speed V E  therefore changes in the sense corresponding to the required ratio change, that is to say the change from the second ratio to the third ratio. Conversely, if the clutch  118  had been actuated initially in the sense of disengaging it, an operation would have been carried out corresponding to a return to the first transmission ratio and therefore to an undesired increase in the input speed V E . 
     The engagement of the clutch  218  occurs progressively, especially if the valve  69  comprises a hydraulically resistant leakage path  151  as shown in FIGS. 1 to  3  in order to prevent the sudden draining of the hydraulic chamber  46 . Returning to FIG. 4, when during the progressive engagement of the clutch  218  the control unit  152  detects that the ratio V E /V S  falls below a certain threshold K 1 , it commands the disengagement of the first clutch  118 . 
     FIG. 5 shows an example of a flowchart which can be used by the control unit  152 . 
     The first step  301  consists in selecting the truth table T 1  shown in FIG. 6 which indicates the ratio to be selected for different values of the load C and of he speed V S . The truth table T 1 , does not in any case provide for the selection of the third ratio; only the ratios 1, 2 and 4 can be selected. 
     Then, returning to FIG. 5, a test  303 , preceded by a step  302  of reading the parameters V S  and C, determines if, according to the table T 1 , the ratio R 1 , must be selected or retained. Depending on whether the answer is “yes” or “no”, a command  304  to supply the actuators A 1  and A 2  is issued or not issued respectively. Then, in both cases, the parameters V S  and C are again read (instruction  306 ) and a test  307  determines if all the conditions are present for establishing the second ratio. If the answer is yes, an instruction  308  commands the draining of the actuator A 1  and the supplying of the actuator A 2  and then, after a new reading of the parameters V S  and C (instruction  309 ), a test  310  determines if all of the conditions are present for establishing the fourth ratio. If the answer to test  307  on the necessity of establishing the second ratio is negative, the logic passes directly to instruction  309  and to test  310  without passing through the instruction  308 . If the answer to the test  310  is negative, the logic returns to test  303  in order to determine if it is necessary to establish or to retain the first ratio. 
     If the answer to the test  310  is positive, it is either the third or the fourth ratio which must be established, because the truth table T 1  does not distinguish between these two cases. But whatever the case may be, an instruction  311  commands the draining of both actuators in order to initiate the change to the fourth ratio. Then, a test  312  determines if the ratio V E /V S  is below the threshold K 1 . If the answer is negative, the logic returns to instruction  309  in order to check the speed and load parameters of the vehicle, V S  and C respectively. If these parameters have not varied too much, the test  310  returns to instruction  311 . This loop can take place several times until the progressive engagement of the clutch  218  has caused the speed V E  to drop sufficiently for the answer to the test  312  (“V E /V S &lt;K 1 ?”) to be positive. In this case, the instruction  313  commands the change to the truth table T 2 , shown in FIG. 7, which distinguishes the cases in which the third ratio must be selected from those where it is necessary to select the fourth ratio. 
     After an instruction  314  for reading V S  and C, a test  315  determines if the third ratio must be selected. If the answer is yes, the supply of the actuator A 1  is initiated but the draining of actuator A 2  is maintained (instruction  316 ), then the logic loops back to instruction  314 . 
     If at a certain stage during the functioning in the third ratio the answer to test  315  becomes negative, the logic returns to instruction  309  and to test  310  to determine if the fourth ratio must be selected. If the answer is yes, shifting into, and then maintaining of the fourth ratio is carried out by looping through the successive steps  309  to  315 . In fact, as the starting point is the situation in which the third ratio is already established, the answer to test  312  is immediately positive, as from the first execution of the said loop. Because, from what has been said above, it is during the transition from the second to the third ratio that the answer to test  312  changes from “NO” to “YES”. 
     If on starting from operation in the third ratio the answer to test  315  and then the answer to test  310  are both negative, then it is necessary to establish the first or the second ratio and the logic returns to instruction  301  which reinstates the table T 1  and then the process which has already been explained at the beginning of the description of FIG. 5 begins again. 
     For the change from the second ratio to the third ratio, the flowchart in FIG. 5 has therefore consisted in starting a process of changing to the fourth ratio, by instruction  311  for draining the two actuators. As the starting point is a situation of functioning in the second ratio for which the actuator A 1  is already drained, the only effect of instruction  311  is to start the draining of the second actuator A 2 , and therefore to cause a change of state in the second train  207  whose jump between the two ratios (1:1 in direct drive and 3:1 as a reduction gear) is the biggest. Then, when the ratio V E /V S  becomes less than K 1  (test  312 ), it is then the first train  107  which begins to change state through the supplying of actuator A 1  (instruction  316 ). 
     In the example shown, it was not considered useful to manage the reverse change electronically, that is to say the change from the third to the second ratio, because this transition has proved less difficult in practice. 
     But such electronic management would have been possible. For this purpose, it would have sufficed that the table T 2  made no distinction between the cases in which the first and those in which the second ratio must be established. The process of changing from the third to the second ratio would begin as if there was going to be a direct return to the first ratio, simply by releasing the clutch  218 . 
     Then, only when the ratio V E /V S  becomes greater the a threshold K 2 , would occur the return to table T 1  which distinguishes between the cases in which the first ratio must be selected from those in which the second ratio must be selected, and only at this stage would begin the engagement of the clutch  118  of the first train. 
     Consequently, it can be seen that according to the invention there is always an advantage, when it is necessary to reverse the high and low ratio states of the two gear trains mounted in series, to begin the actuation with the gear train having the biggest ratio jump between its high ratio and its low ratio. In the example, the starting point is always the actuation of the second epicyclic train  207 , whose reduction ratio is three times greater when operating as a reduction gear than when operating as a direct drive, and it is only afterwards that the first epicyclic train  107  is actuated, the reduction ratio of which is only 1.4 times greater than the direct drive ratio. 
     In the example shown in FIG. 4, it would have been possible to bring centrifugal forces or gearing forces into play as shown in FIGS. 1 to  3 . 
     In the truth tables such as T 1  and T 2 , it would have been possible to have pairs of values C, V S  for which no choice of ratio is fixed which would signify that, for these particular conditions, the control unit  152  allows the centrifugal and gearing forces alone to control the transmission device. Even in such conditions, it would be possible to make provision for the control unit  152  to control the actuating input of an actuator to favour the change of state of one or more clutches during the transition between two transmission ratios. 
     If the unit  152  shown in FIG. 4 must take account of parameters other than C and V S  in order to decide on the choice of a ratio, the tables T 1  and T 2  can each be replaced by matrices with more than two dimensions.