Abstract:
The control system including four actuating forks, each associated to one or two gears and having a first engagement member, and a first hydraulically-operated actuator for controlling engagement of all gears. The first hydraulically-operated actuator includes: a shaft provided with four second engagement members for engagement each with a respective first engagement member of the actuating forks and arranged to turn about and translate along its axis; a cylinder coaxial to the shaft and arranged only to translate in the direction of its axis; and a pin drivingly connected either to the shaft or the cylinder and engaging in an S-shape groove provided either in the cylinder or shaft, respectively, so as to link the rotational and translational movements of the shaft with the translational movement of the cylinder. A second hydraulically-operated actuator is arranged to lock or release selectively the translational movement of the cylinder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a Divisional Application of U.S. application Ser. No. 11/176,204 filed Jul. 8, 2005; the entire disclosure of the prior application is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a servo-assisted control system of the electro-hydraulic type operable to control the shift between the various gears of a double clutch gearbox of a motor vehicle, in particular a six- or seven-speed gearbox. 
   A gear control system is known from U.S. patent application No. 013417 in the name of the Applicant, which is able to control the displacement of the four engagement sleeves of a six-speed gearbox of a motor vehicle, whether it is of the double clutch type or of the single clutch type with robotised control. This known system fundamentally comprises:
         a first control device for controlling the displacement of the coupling sleeves associated with the gears controlled by the first input shaft of the gearbox, that is to say the first, third, fifth and sixth gear, as well as the reverse gear; and   a second control device for controlling the displacement of the coupling sleeve associated with the gears controlled by the second input shaft of the gearbox, that is to say the second and fourth gear.       

   The first control device is provided with a drum which is mounted rotatably about its own axis and on the cylindrical lateral surface of which are provided three control grooves each of which engages a respective pin to displace it in the direction of the axis of the drum upon rotation of this latter. The three pins are each connected to a respective fork which controls the displacement of a respective coupling sleeve. The second control device is provided with a slidable rod carrying a fork for control of the displacement of the coupling sleeve of the second and fourth gear. 
   According to a first embodiment this known control system is electrohydraulically operated. The two control devices are alternatively controlled by a first proportional solenoid valve which controls the up shifting, and by a second proportional solenoid valve which controls the down shifting. The two solenoid valves modulate the pressure of the working fluid supplied by a pump in a delivery line and alternatively connect a first and a second input line of a six-way distributor with the delivery line from the pump or with a discharge line. The six-way distributor is further connected to the hydraulic actuator of the first control device through third and fourth output lines and to the hydraulic actuator of the second control device through fifth and sixth output lines. 
   This known control system, in combination with the double clutch six-speed gearbox described in the above document, makes it possible to perform multiple gear changes in “power shift” mode during the following downshift manoeuvres: from sixth to fourth or to second; from fifth to second; and from fourth to first. The remaining multiple downshift manoeuvres can however be performed in a traditional manner, that is with interruption of torque transmission. 
   A further example of a servo-assisted control system for the gears of a six-speed double clutch gearbox for a motor vehicle is known from German Patent Application DE 101 34 115. This known control system includes a hydraulic circuit arranged to control four double-acting hydraulic cylinders for actuation of four coupling sleeves, that is to say a first sleeve which effects engagement of the first or third gear, a second sleeve which effects engagement of the fifth gear, a third sleeve which effects engagement of the second or fourth gear and a fourth sleeve which effects engagement of the sixth gear or the reverse gear. The hydraulic circuit is subdivided into a first portion intended to control the odd gears and a second portion intended to control the even gears and the reverse gear. Upstream of each circuit portion is disposed a pilot valve which controls the supply of oil under pressure to the respective circuit portion. Each circuit portion includes a pair of proportional solenoid valves which control the two double-acting hydraulic cylinders to actuate the coupling sleeves of the gears associated with this circuit portion. Between the four double-acting hydraulic cylinders and the four proportional solenoid valves associated therewith is interposed a distributor. 
   This known control system makes it possible to perform multiple gear changes in “power shift” mode directly (that is non-sequentially) between gears not associated with the same input shaft of the gearbox. It has, however, the disadvantage of requiring a large number of components and of therefore having a high cost. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide an electro-hydraulic control system for a six or more speed gearbox of a motor vehicle, whether of the double-clutch or of the single-clutch type with robotised control, which has a smaller number of components and therefore a lower cost than the prior art, which makes it possible to perform the greatest possible number of multiple gear changes in “power shift” mode directly (that is non-sequentially) available from the gearbox architecture, and which is easily adaptable to different gearbox versions in such a way as to allow a further reduction in the costs of production. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention will become clearly apparent from the following detailed description, given purely by way of non-limitative example with reference to the attached drawings, in which: 
       FIG. 1  is a schematic illustration of a first embodiment of a servo-assisted gearbox control system according to the present invention; 
       FIG. 2  is a view in axial section of a known seven-speed double clutch gearbox for which the control system of  FIG. 1  is intended; 
       FIG. 3  is a schematic view which shows the two actuators of the control system of  FIG. 1  together with the respective coupling members; 
       FIG. 4  is a view on the arrow F of  FIG. 3  which shows the three engagement fingers associated with the actuator for controlling the odd gears and the reverse gear of the control system of  FIG. 1 ; 
       FIG. 5  is a view on the arrow F of  FIG. 3  which shows the two engagement fingers, associated with the actuator for controlling the even gears of the control system of  FIG. 1 ; 
       FIGS. 6A to 6H  show the sequence of operations necessary to effect the engagement of seventh gear starting from the neutral condition between the first and third gear of the control system of  FIG. 1 ; 
       FIGS. 7 and 8  are schematic illustrations of a second and, respectively, a third embodiment of a servo-assisted gear shift control system for a gearbox according to the present invention, both intended for a six-speed double clutch gearbox derived from the seven-speed gearbox of  FIG. 2 ; 
       FIG. 9  is a schematic illustration of a fourth embodiment of a servo-assisted gear shift control system for a gearbox according to the present invention, intended for a robotised seven-speed single clutch gearbox derived from the seven-speed gearbox of  FIG. 2 ; and 
       FIG. 10  is a schematic illustration of a fifth embodiment of a servo-assisted gear shift control system for a gearbox, according to the present invention, intended for a robotised six-speed single clutch gearbox derived from the seven-speed gearbox of  FIG. 2 . 
   

   Parts and components associated with the various forward gears of the gearbox are indicated in the drawings with the Roman numerals I, II, III, IV, V, VI and VII, respectively, for the first, second, third, fourth, fifth, sixth and seventh gear, whilst parts and components associated with the reverse gear are indicated with the letter R. 
   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2  shows in axial section a seven-speed double clutch gearbox for a motor vehicle forming the subject of European Patent Application No. 04425283.1 in the name of the Applicant. 
   The gearbox of  FIG. 2  comprises a first coupling sleeve  141  selectively displaceable to left and right to engage the first and the third gear, respectively, a second coupling sleeve  142  selectively displaceable to left and right to engage the seventh and sixth gear, respectively, a third coupling sleeve  143  selectively displaceable to left and right to engage the reverse gear and the fifth gear, respectively, and a fourth coupling sleeve  144  selectively displaceable to left and right to engage the second and fourth gear, respectively. The four coupling sleeves  141 - 144  are of type known per se and therefore will not be described in detail. 
   The gearbox of  FIG. 2  makes it possible to perform all the sequential gear shifts in “power shift” mode, except that between the sixth and seventh gear, the engagement of these two gears being controlled by the same sleeve  142 . Moreover, this gearbox makes it possible to perform multiple gear changes in “power shift” mode during the following downshift manoeuvres, from seventh to fourth or second, from sixth to third or first, from fifth to second and from fourth to first. 
   The engagement of the different gears of the gearbox of  FIG. 2  can be effected, according to a first embodiment of the invention, by means of the control system schematically shown in its entirely in  FIG. 1 . 
   With reference to  FIG. 1 , the control system fundamentally comprises a first actuator device  11  intended to control the displacement of the coupling sleeves  141 ,  142  and  143  in order selectively to engage one of the gears controlled by a first input shaft  110  of the gearbox, that is to say first, third, fifth, seventh and reverse, and a second actuator device  12  intended to control the displacement of the coupling sleeves  142  and  144  in order selectively to engage one of the gears controlled by a second input shaft  112 , that is to say second, fourth and sixth. As schematically illustrated in  FIG. 2  and as will be explained in detail hereinafter, the second coupling sleeve  142  associated with the sixth and seventh gear is controlled by a single fork indicated  27 , on which both actuator devices  11  and  12  can act. Preferably, the first actuator device  11  is of the so-called S-cam type, whilst the second actuator device  12  is of the twin axis type. 
   Referring now also to  FIG. 3 , the actuator device  11  comprises, in a manner known per se, a shaft  14  with axis x 1  (perpendicular to the axes of the input shafts  110  and  112  of the gearbox) and a cylinder  16  disposed coaxially to the shaft  14  and having an S-shape groove  18 . The shaft  14  can turn about its axis x 1  (as indicated by the arrow R 1 ) and translate in the direction of its axis (as indicated by the arrow T 1 ). The cylinder  16  on the other hand can only translate in the direction of its axis x 1 , as indicated by the arrow Z 1 . A pin  20  drivingly connected to the shaft  14  engages in the groove  18  of the cylinder  16  so as to connect the rotation and translation movements of the shaft with the translation of the cylinder. The shaft  14  is further provided with two engagement fingers  21  and  22  and a control lever  23  drivingly connected thereto. The first engagement finger  21  is arranged to engage in an engagement window  24  of a first actuating fork  25  associated with the first sleeve  141  (first and third gear) or in an engagement window  26  of a second actuating fork  27  associated with the second sleeve  142  (sixth and seventh gear). The second engagement finger  22  is arranged to engage in an engagement window  28  of a third actuating fork  29  associated with the third sleeve  143  (fifth gear and reverse gear). The two engagement fingers  21  and  22  are disposed along the shaft  14  in such a manner that each time only one of these is aligned with one of the three engagement windows  24 ,  26  and  28 . The S-shape groove  18  of the cylinder  16  comprises a pair of straight sections  18   a  and  18   b , which extend transversely of the axis x 1  on opposite sides with respect to this latter and are spaced from one another in the direction of the axis x 1  by a distance equal to the rank of the gears, and an inclined section  18   c  which joins the two straight sections  18   a  and  18   b.    
   The rotation of the shaft  14  is controlled by a double-acting hydraulic actuator  30  via the control lever  23 . The cylinder  16  is axially lockable by means of a locking device  32  formed for example as a single-acting hydraulic actuator, which in the rest condition leaves the cylinder free to translate axially along its axis. The two chambers of the hydraulic actuator  30  are connected to a three-input and six-output distributor  34  via a first output line OL 1  and a second output line OL 2 . On the other hand, a third output line OL 3  from the distributor  34  is connected to the actuator  32 . The distributor  34  is connected to a supply of fluid under pressure (not illustrated) via a first input line IL 1 , in which is disposed a first proportional pressure solenoid valve  35 , via a second input line IL 2 , in which is disposed a second proportional pressure solenoid valve  36 , and via the third input line IL 3 , in which is disposed an ON/OFF solenoid valve  37 . The distributor  34  is normally in a first working position such that the input line IL 1  is connected to the output line OL 1 , the input line IL 2  is connected to the output line OL 2  and the input line IL 3  is connected to the output line OL 3 . In this way, the actuator  30  is controlled by means of the two proportional solenoid valves  35  and  36  in order to control the rotation of the shaft  14  of the first actuator device  11 , whilst the actuator  32  is controlled by means of the ON/OFF solenoid valve  37  in order to lock or release the axial movement of the cylinder  16 . 
   In  FIG. 1  the control system is shown in the neutral condition between the first and third gear, in which the first engagement finger  21  is axially aligned with the engagement window  24  of the actuating fork  25  associated with the sleeve  141  of first and third gear and is disposed with clearance within this window. In this condition the pin  20  of the shaft  14  is positioned half way along the inclined section  18   c  of the groove  18  of the cylinder  16 . 
   If, now, the shaft  14  is driven to rotate anticlockwise as viewed from F in  FIGS. 1 and 3 , by supplying fluid under pressure to the actuator  30  via lines IL 1  and OL 1  under the control of the solenoid valve  35  (in such a manner that the actuator  30  translates rightwards with respect to the observer of  FIG. 1 ), the pin  20 , which is drivingly connected to the shaft  14 , forces the cylinder  16 , which can translate axially as the locking device  32  is not active, to slide along the inclined section  18   c  of the groove  18 , thus causing upward displacement of this latter. Moreover, the engagement finger  21 , which rotates rigidly with the shaft  14 , causes the fork  25 , together the sleeve  141 , to move leftwards thus engaging the first gear. If, on the other hand, starting from the neutral condition the shaft  14  is driven to rotate in the clockwise sense, the third gear is engaged. As is clearly shown in  FIG. 1 , the engagement window  24  of the fork  25  associated with the first and third gear has a width significantly greater than the other two engagement windows  26  and  28 . The clearance between the engagement finger  21  and the engagement window  24  is therefore correspondingly greater than that between the same finger and the engagement window  26  (associated with the seventh gear) or between the engagement finger  22  and the engagement window  28  (associated with the fifth gear and the reverse gear). This makes is possible to give the shaft  24  a small rotation in one direction or the other, starting from the neutral position between first and third, without thereby causing the engagement of the first or third gear. These two engagement start positions of the first and third gear serve during the engagement phases of the fifth and seventh gear, respectively, as will be explained in detail hereinafter. 
   Referring now to  FIGS. 6A to 6H , the engagement operation of the seventh gear will now be described. Starting from the neutral position between the first and third gear ( FIG. 6A ) the shaft  14  is driven to rotate clockwise through only such an angle as to cancel the clearance between the engagement finger  21  and the corresponding engagement window  24  ( FIG. 6B ). Passing from the condition of  FIG. 6A  to the condition of  FIG. 6B , the pin  20  of the shaft  14  rotates until reaching the point at which the horizontal section  18   b  of the groove  18  starts, whilst due to the inclined section  18   c  of the groove  18  the cylinder  16  translates axially downwards. At this point the locking device  32  is actuated by means of the solenoid valve  37  so as axially to lock the cylinder  16 . Then the shaft  14  is driven to rotate again, but this time anticlockwise. Since the cylinder  16  is locked, the pin  20  slides along the whole inclined section  18   c  of the groove  18  from the upper horizontal section  18   b  to the lower horizontal section  18   a , thus causing the shaft  14  to translate downwards one rank in such a way as to align the engagement finger  21  axially with the window  26  of the fork  27  of seventh gear ( FIG. 6F ). At this point, by continuing with the anticlockwise rotation of the shaft  14 , the pin  20  moves along the horizontal section  18   a  of the groove  18 , whilst the engagement finger  21  causes the fork  27  to move leftwards together with the sleeve  142 , thus engaging the seventh gear ( FIG. 6H ). 
   The engagement of the fifth gear starting from the neutral condition of  FIG. 1  takes place in a symmetric manner to that of the seventh gear (it being necessary in this case to drive the shaft  14  to rotate anticlockwise rather than clockwise, by using the solenoid valve  35 ) and will therefore not be described in detail. 
   To engage the reverse gear starting from the neutral condition of  FIG. 1  the following operations are performed in sequence:
         anticlockwise rotation of the shaft  14  up to the position of start of the engagement of the first gear in such a way as to displace the cylinder  16  axially upwards;   locking of the cylinder  16 ;   clockwise rotation of the shaft  14  up to the position of start of the engagement of the fifth gear, in such a way as to displace the shaft  14  axially until the engagement finger  22  is brought into alignment with the engagement window  28  of the fork  29  associated with the fifth gear and the reverse gear;   release of the cylinder  16 ; and   anticlockwise rotation of the shaft  14  in such a way as to displace the fork  29 , together with the sleeve  143 , leftwards to engage the reverse gear.       

   Returning to  FIG. 1 , the second actuator device  12  comprises a shaft  40  mounted so as to rotate about its axis x 2  (as indicated by the arrow R 2 ) and translate in the direction of its axis (as indicated by the arrow T 2 ). The shaft  40  is provided with two engagement fingers  41  and  42  drivingly connected thereto, of which the first engagement finger  41  is arranged to engage in an engagement window  44  of a fourth actuating fork  45  associated with the fourth sleeve  144  (second and fourth gear), whilst the second engagement finger  42  is arranged to engage in a further engagement window  46  provided on the second actuating fork  27  associated with the second sleeve  142  (sixth and seventh gear). 
   The shaft  40  is normally held, for example by the resilient action of a spring  48 , in a position such that its first engagement finger  41  is axially aligned with the respective engagement window  44  (neutral condition between the second and fourth gear), whilst its second engagement finger  42  is positioned outside the respective engagement window  46 . A single-acting hydraulic actuator  50  is arranged to displace the shaft  40  axially against the action of the spring  48  in such a way as to bring the first engagement finger  41  out of the respective window  44  and to align the second engagement finger  42  with the respective window  46  to permit engagement of the sixth gear. The rotational movement of the shaft  40  about its axis x 2  (gear engagement movement) is controlled by a double-acting hydraulic actuator  52 , advantageously identical to the actuator  30  of the first control device  11 , through a control lever  53  on the shaft  40 , advantageously identical to the control lever  23  on the shaft  14  of the first device  11 . 
   The two chambers of the double-acting hydraulic actuator  52  are connected to the distributor  34  via a fourth output line OL 4  and a fifth output line OL 5 , whilst the single-acting hydraulic actuator  50  is connected to the distributor  34  via a sixth output line OL 6 . The distributor can be controlled by an ON/OFF pilot solenoid valve  54  to be displaced to a second working position in which the input line IL 1  is connected to the output line OL 4 , the input line IL 2  is connected to the output line OL 5  and the input line IL 3  is connected to the output line OL 6 . In this way, by means of the two pressure proportional solenoid valves  35  and  36  in the two input lines IL 1  and IL 2 , the actuator  52  is controlled to drive the shaft  40  to rotate, whilst by means of the ON/OFF solenoid valve  37  in the third input line IL 3  the actuator  50  is controlled to displace the shaft  40  axially towards the selection position for the sixth gear. 
   In  FIG. 1  the second control device  12  is shown in the neutral condition between the second and fourth gear. If the shaft  40  is now rotated anticlockwise (as viewed from F in  FIGS. 1 and 3 ), by supplying fluid under pressure to the actuator  52  via the lines IL 1  and OL 4  under the control of the solenoid valve  35 , the engagement finger  41 , which rotates rigidly with the shaft  40 , causes the fork  45  to displace leftwards together with the sleeve  144 , thus engaging the second gear. If, on the other hand, starting from the neutral condition the shaft  40  is rotated clockwise, engagement of the fourth gear is obtained. To engage the sixth gear it is necessary first to displace the shaft  40  axially into the sixth gear selection position (condition of alignment of the engagement finger  42  with the engagement window  46  of the fork  27 ), by supplying fluid under pressure to the actuator  50  through the lines IL 3  and OL 6  under the control of the solenoid valve  37 . At this point the shaft  40  is rotated clockwise by supplying fluid under pressure to the actuator  52  through the lines IL 2  and OL 5  under the control of the solenoid valve  36 , in such a way that the engagement finger  42  displaces the fork  27 , together with the sleeve  142 , rightwards. 
   Thanks to the fact that the fork  27  which actuates the sleeve  142  of sixth and seventh gear is alternatively controllable by both the control devices  11  and  12 , the control system makes it possible to perform the greatest possible number of multiple gear shifts in “power shift” mode, starting from the seventh or sixth gear. In fact, starting from the condition of engagement of the seventh gear (by means of the first control device  11 ), the second control device  12  can simultaneously engage the second or fourth gear in such a way as to permit the direct change from the seventh gear to the fourth or second gear in “power shift” mode. The same gear change possibilities are offered starting from the fifth gear. 
   On the other hand, starting from the condition of engagement of the sixth gear (by means of the second control device  12 ), the first control device  11  can simultaneously engage the third or first gear in such a way as to permit the direct change from the sixth gear to the third or first gear in “power shift” mode. The same gear change possibilities are offered starting from the fourth gear. 
   Moreover, thanks to the fact that an initial condition of the control system is provided in which the first control device  11  is in the neutral position between the first and third gear and the second control device  12  is in the neutral position between the second and fourth gear, it is possible to perform the first four gear changes (from the neutral position to the fourth gear) without the need to perform any selection movement (axial displacement of the shafts  14  and  40  of the two control devices). It is in fact sufficient to control the pilot solenoid valve  54  to select the shafts  14  or  40  to control and the two proportional solenoid valves  35  and  36  to drive the selected shaft to rotate in one direction or the other. 
   To avoid the risk of an erroneous engagement of a gear by one of the two control devices, in particular the simultaneous engagement on two gears of the same input shaft of the gearbox, a safety system or so-called “interlock” system is provided which will now be illustrated in detail. With reference to  FIG. 3 , the “interlock” system comprises a first safety device  61  mounted on the shaft  14  of the first control device  11  and a second safety device  62  mounted on the shaft  40  of the second control device  12 . Each safety device  61 ,  62  is displaceable along the axis x 1 , x 2  of the respective shaft  14 ,  40  as a result of the axial translation movement imparted to this latter, and is locked against rotation by means of a restraint (not shown) provided by a fixed part of the gearbox. 
   The first safety device  61  forms a first arm  63  carrying an axial projection  64  engageable in the engagement window  24  of the fork  25  of first and third gear, a second arm  65  carrying an axial projection  66  engageable in the engagement window  26  of the fork  27  of sixth and seventh gear and a third arm  67  carrying an axial projection  68  engageable in the engagement window  28  of the fork  29  of reverse gear and fifth gear. Similarly, the second safety device  62  forms a first arm  69  carrying an axial projection  70  engageable in the engagement window  44  of the fork  45  of second and fourth gear and a second arm  71  carrying an axial projection  72  engageable in the other engagement window  46  of the fork  27  of sixth and seventh gear. 
   The three axial projections  64 ,  66  and  68  of the first safety device  61  are formed in such a way that each time two of them engage in the corresponding engagement windows, thus preventing the actuation of the respective fork, whilst the third projection disengages from the corresponding engagement window, which can therefore be engaged by the corresponding engagement finger. For example, in the operating condition illustrated in  FIG. 1  the projection  66  occupies the engagement window  26  of the fork  27 , thus preventing unwanted displacement of this fork which would cause engagement of the seventh gear. The projection  68  occupies the engagement window  28  of the fork  29 , thus preventing unwanted displacement of this fork which would cause engagement of the reverse gear or fifth gear. The projection  64  is, on the other hand, disengaged from the engagement window  24  of the fork  25  in such a way as to allow the engagement finger  21  to engage the first or third gear. The same applies to the second safety device  62 . 
   The “interlock” system is further suitably configured to prevent simultaneous actuation of the fork  27  of sixth and seventh gear by the two control devices  11  and  12 . To this end, as shown in the diagram of  FIG. 1 , the projections  66  and  72  of the two safety devices  61  and  62  associated with the fork  27  are arranged to cooperate with respective abutment surfaces  27   a  and  27   b  formed on the fork  27  in such a manner that:
         when the two control devices  11  and  12  are one in the neutral position between the first and third gear and the other in the neutral position between the second and fourth gear the projection  66  faces the abutment surface  27   a  so as to prevent leftwards displacement of the fork  27  and therefore engagement of the seventh gear, and likewise the projection  72  faces the abutment surface  27   b  so as to prevent rightwards displacement of the fork  27  and therefore engagement of the sixth gear;   when the shaft  14  of the first control device  11  is displaced axially (downwards) into the seventh gear selection position, the projection  66  moves away from the abutment surface  27   a  leaving the fork  27  free to displace leftwards to engage the seventh gear, whilst the projection  72  continues to prevent rightwards displacement of the fork  27  and therefore erroneous engagement of the sixth gear; and   when the shaft  40  of the second control device  12  is displaced axially (downwards) into the sixth gear selection position, the projection  72  moves away from the abutment surface  27   b  leaving the fork  27  free to displace rightwards to engage the sixth gear, whilst the projection  66  continues to prevent leftwards displacement of the fork  27  and therefore erroneous engagement of the seventh gear.       

   The control device  11  is further provided with a first detent mechanism  80  shown in  FIG. 3 , which controls the axial positioning of the shaft  14  by defining a first intermediate selection position of the fork  25  of first and third gear, a second selection position of the fork  27  of seventh and sixth gear and a third selection position of the fork  29  of reverse and fifth gear. The detent mechanism  80  comprises, in a manner known per se, a slidable segment  81  fixed to the shaft  14  and having three engagement seats corresponding to the said three selection positions (ranks) of the shaft  14  and a ball  82  intended to snap-engage under the action of a spring (not illustrated) into one of these seats. The shaft  14  is further provided with a second detent mechanism  85 , shown in  FIGS. 3 and 4 , which controls the angular positioning of the shaft  14  by defining a central neutral position and two opposite engagement positions. The detent mechanism  85  comprises, in a manner known per se, a catch element  86  fixed to the shaft  14  and having a central engagement seat  87  corresponding to the neutral position and a pair of lateral engagement surfaces  88  corresponding to the engagement positions, and a ball  89  intended to snap-engage, under the action of a spring (not illustrated) into the seat  87  or against one of the surfaces  88 . 
   The second control device  12  is provided with a detent mechanism  90 , similar to the detent mechanism  85  of the first control device  11 , which controls the angular positioning of the shaft  40 . The mechanism  90  comprises a catch element  91 , fixed to the shaft  40  and having a central engagement seat  92  corresponding to the neutral position and a pair of lateral engagement surfaces  93  corresponding to the engagement positions, and a ball  94  intended to snap-engage under the action of a spring (not illustrated) into the seat  92  or against one of the surfaces  93 . 
   Position sensors (not illustrated) are also provided on the first and second control devices  11  and  12  for providing signals indicative of the axial position (to identify the rank) and the angular position (to identify the neutral position or the engaged gear) of the two shafts  14  and  40 . 
   A second embodiment of a gearbox control system according to the invention, intended to control a six-speed double clutch gearbox obtained from the gearbox of  FIG. 2  simply by eliminating the seventh gear driven wheel on the first output shaft, is schematically illustrated in  FIG. 7 , where parts and elements identical or corresponding to those of  FIG. 1  have been given the same reference numerals. The actuator devices  11 ,  12  and the hydraulic circuit which controls the supply of fluid under pressure to the two devices are substantially identical to those of the control system of  FIG. 1  and will therefore not be described in detail. The only difference with respect to the first embodiment is that the first actuator device  11  is arranged to control only the coupling sleeve of first and third gear and the coupling sleeve of fifth gear and reverse gear. 
   A third embodiment of the invention, also intended to control a six-speed double clutch gearbox obtained from the gearbox of  FIG. 2 , is schematically illustrated in  FIG. 8 , where parts and elements identical or corresponding to those of  FIG. 1  have been given the same reference numerals. As opposed to the second embodiment of  FIG. 7 , in this case the actuator devices  11  and  12  which control the engagement of the odd gears (as well as the reverse gear) and of the even gears, respectively, are both of the twin axis type. 
   The second actuator device  12 , as well as the hydraulic circuit which controls the supply of fluid under pressure to the two devices  11  and  12 , are substantially identical to those of the first embodiment of  FIG. 1 , and will therefore not be described in detail. 
   The first actuator device  11  comprises a shaft  14  which can rotate about its axis x 1  (as indicated by the arrow R 1 ) and translate in the direction of this axis (as indicated by the arrow T 1 ). The shaft  14  is provided with two engagement fingers  21  and  22  drivingly connected thereto, of which the first engagement finger  21  is arranged to engage in the engagement window  24  of the actuating fork  25  associated with the first and third gear, whilst the second engagement finger  22  is arranged to engage in the engagement window  28  of the actuating fork  29  associated with the fifth gear and the reverse gear. 
   The shaft  14  is normally held, for example by the resilient action of a spring  47 , in such a position that its first engagement finger  21  is axially aligned with the respective engagement window  24  (neutral condition between the first and third gear), whilst its second engagement finger  22  is positioned outside the respective engagement window  28 . A single-acting hydraulic actuator  49  is arranged to displace the shaft  14  axially against the action of the spring  47  in such a way as to bring the first engagement finger  21  out of the respective window  24  and to align the second engagement finger  22  with the respective window  28  for engagement of the reverse gear or the fifth gear. The rotational movement of the shaft  14  about its axis x 1  (gear engagement movement) is controlled by a double-acting hydraulic actuator  51 . The two chambers of the double-acting hydraulic actuator  51  are connected to the distributor  34  of the hydraulic control circuit via the output lines OL 1  and OL 2 , respectively, whilst the single-acting hydraulic actuator  49  is connected to the distributor  34  via the output line OL 3 . 
   Advantageously, the two actuator devices  11  and  12  are identical to one another in such a way as further to reduce the overall cost of the control system. 
   A fourth embodiment of a gearbox control system according to the invention will now be briefly described, the system being intended to control a seven-speed single clutch robotised gearbox derived from the gearbox of  FIG. 2 . This embodiment is schematically illustrated in  FIG. 9 , where parts and elements identical or corresponding to those of  FIG. 1  have been given the same reference numerals. 
   Since the simultaneous engagement of two gears is not required, a single control device  11  of the S-cam type is sufficient to engage all the gears. The control device  11  is structurally identical to that previously described with reference to the embodiment of  FIG. 1 . In this case, however, a first engagement finger  21  is arranged to engage alternatively in an engagement window  24  of a first actuating fork  25  of first and second gear or in an engagement window  26  of a second actuating fork  27  of sixth and seventh gear. A second engagement finger  22  is arranged to engage alternatively in an engagement window  44  of a third actuating fork  45  of third and fourth gear or in an engagement window  28  of a fourth actuating fork  29  of fifth gear and reverse gear. 
   The rotation of the shaft  14  (engagement movement) is controlled by a double-acting hydraulic actuator  30  which is connected to a supply of fluid under pressure via first and second lines IL 1  and IL 2  in each of which is disposed a respective proportional pressure solenoid valve  35  and  36 . To lock the axial movement of a cylinder  16  of the control device  11  there is provided a locking device  32  formed for example as a single-acting hydraulic actuator controlled by an ON/OFF solenoid valve  37  via a third line IL 3 . 
   Finally, a fifth embodiment of a gearbox control system according to the invention, intended to control a six-speed single clutch robotised gearbox derived from the gearbox of  FIG. 2 , is schematically illustrated in  FIG. 10 , where parts and elements identical or corresponding to those of  FIG. 7  have been given the same reference numerals. This fifth embodiment differs from the fourth substantially only in the arrangement of the engagement windows, and will not therefore be described in detail. 
   The servo-assisted control system according to the invention has therefore the advantage of requiring a smaller number of solenoid valves than the prior art with the same operable functions (the possibility of performing directly several multiple gear shifts in “power shift” mode and the possibility of directly or indirectly performing the remaining multiple gear shifts, even if not in “power shift” mode) The following four solenoid valves are in fact sufficient: 
   the ON/OFF solenoid valve  54  which controls the positioning of the distributor  34 ; 
   the ON/OFF solenoid valve  37  which controls the locking device  32  of the first actuator device  11  to control the selection movement for the gears associated with this device and which, in the case of a double clutch gearbox, also controls the axial positioning of the shaft  40  of the second actuator device  12  (selection movement); 
   the two proportional pressure solenoid valves  35  and  36  which control the engagement movements of the first actuator device  11  and, in the case of a double clutch gearbox, also of the second actuator device  12 . 
   In the case of a control system intended for a double clutch gearbox the two actuator devices, respectively of the S-cam and of the twin axis type, use the maximum possible number of components in common, which allows to reduce the overall cost of the system. Moreover, both the control system intended for a double clutch gearbox and that intended for a single clutch gearbox preferably use an actuator device of the S-cam type. The two control systems can therefore share a large number of components and thus be fabricated in the same production installation. 
   The gear control system according to the invention further has a high flexibility of configuration, as it can be adapted to control a six- or seven-speed double clutch gearbox, or a robotised six- or seven-speed single clutch gearbox. 
   The gear control system according to the invention can further be provided as an ADD-ON version, in such a manner as to increase the possibilities of application with minimum modification to the manual gearbox.