Abstract:
Transmission system, in which a single double-effect actuator, used to engage/release the gears, comprises a piston which is mobile with reciprocating motion, and has different areas which face first and second chambers of the actuator. The first and second chambers are supplied respectively, directly by a source of pressurised fluid, and by a valve of the proportional type, which is interposed between the source of pressurised fluid and the actuator.

Description:
[0001]    The present invention relates to a transmission system for a motor vehicle.  
         BACKGROUND OF THE INVENTION  
         [0002]    Transmission systems are known, in which a gearbox of the mechanical type is connected to a hydraulic control circuit, which implements engagement/release of the gears by means of first and second actuators, which receive pressurised fluid supplied by respective first and second valves, in particular first and second solenoid valves of the proportional type. This hydraulic circuit generally comprises a third actuator, which is connected to a respective third valve, and is used in order to implement opening/closure of the clutch.  
           [0003]    In particular, it is known to control engagement of the even gears (R, 2, 4, 6) with a first solenoid valve, and engagement of the odd gears (1, 3, 5) by means of a second solenoid valve.  
           [0004]    The hydraulic circuits of a known type are however quite complex, somewhat costly, and liable to failures.  
         SUMMARY OF THE INVENTION  
         [0005]    The object of the present invention is to provide a transmission system which is provided with a hydraulic circuit to engage/release the gears, which is extremely simply and has a low cost.  
           [0006]    More particularly, the object of the present invention is to provide a hydraulic circuit which can be used to engage/release the gears, which uses a single valve and a single actuator.  
           [0007]    The preceding object is achieved by the present invention, in that it relates to a transmission system of the type described in claim 1. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The invention will now be described with particular reference to the attached figures, which represent a preferred, non-limiting embodiment of it, in which:  
         [0009]    [0009]FIG. 1 illustrates schematically a transmission system produced according to the dictates of the present invention; and  
         [0010]    [0010]FIG. 2 illustrates a block diagram of operations carried out by the system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    In FIG. 1, 1 illustrates as a whole a hydraulic circuit to control the transmission in motor vehicles.  
         [0012]    In particular, the circuit  1  is applied to a transmission system in which an endothermic (petrol or diesel) engine  2 , with an output shaft  3  which is connected, by means of interposition of a clutch  4 , to an intake shaft  5  of a gearbox  6  of a mechanical type, can supply as output mechanical power to the wheels (not illustrated) of the vehicle (not illustrated).  
         [0013]    In particular, the gearbox  6  is provided with a mobile unit  10 , which can be actuated with reversible angular motion (or reversible translation) by an actuator  12  (which is described hereinafter), and is used in order to implement engagement and release of the gears. The gearbox  6  is also provided with a further mobile unit (not illustrated), which can be displaced under the thrust of a respective further actuator (not illustrated), and used for selection of the rank of the gears.  
         [0014]    The clutch  4  is connected to single-effect actuator  14 , which is mobile with reversible motion, and can implement the opening/closure of the clutch itself.  
         [0015]    In particular, the actuator  14  is provided with resilient means  14   a , in particular with a Belleville washer or a spring coupled with the actuator  14 , which is integrated with the clutch disc, and can maintain the actuator in a position of rest, in which the clutch  4  is closed; the actuator  14  has a chamber  14   b , which can receive a pressurised fluid, in order to give rise to the axial movement of an output unit  14   c  of the actuator  14 , thus opening the clutch  4 .  
         [0016]    The actuator  12  is of the double-effect type, and generally comprises a cylindrical tubular casing  16 , which defines an inner cylindrical cavity, along which there slides a piston  17 , from which there extends a shaft  18 , which is coaxial to the casing  16 , and is used to move the unit  10 . In particular, the piston  17  (in the example the piston has a cylindrical shape) is delimited by a first surface  17   a , which has an area A, and by a second surface  17   b , from which the shaft  18  extends; in the embodiment illustrated, the second surface  17   b  has an efficient area which is smaller than the first, and is equal to A/2. It is therefore apparent that the second surface  17   b  would have a different value of efficient area; advantageously, the efficient area of the second surface can be variable between 0.3 and 0.7 times the value of the area of the surface  17   a.    
         [0017]    Together with the casing  16 , the first surface  17   a  of the piston delimits a rear chamber  19  of the actuator  12 , whereas, together with the casing  16 , the second surface  17   b  of the piston delimits a front chamber  20  of the actuator  12 .  
         [0018]    The hydraulic circuit  1  comprises an operating fluid (oil) tank  22 , which has an output  22   a , which is connected, by means of a pipe  23 , to an intake of a pump  25 , which is actuated by an electric motor  26 . The pump  25  has an output which communicates with a pipe  28 , along which there is disposed a non-return valve  30 , which can prevent return of the operating fluid towards the pump  25 . A by-pass pipe  31  extends from the end of the pipe  28  (downstream from the non-return valve  30 ), at the output  22   a ; along this pipe  31  there is disposed a maximum pressure valve  31   a,  which can open when the pressure along the pipe  28  reaches a threshold value Plim.  
         [0019]    The pipe  28  also communicates at its output with a pipe  32 , which supplies pressurised oil to the front chamber  20  of the actuator  12 .  
         [0020]    The pipe  28  also communicates at its output with a pipe  34 , which communicates with an intake  36   a  of a solenoid value  36  of the proportional type, which has an output  36   u,  which communicates, via a pipe  40 , with the rear chamber  19  of the actuator  12 . When it is disposed in a regulation position, the solenoid valve  36  can regulate continuously the flow which flows through the solenoid valve itself, and thus the pressure of the fluid output; the solenoid valve  36  can also be disposed in a discharge position, in which it puts into communication the output  36 , and thus the pipe  40 , with a discharge pipe  42  which extends from the solenoid valve  36  to the tank  22 .  
         [0021]    The pipe  34  also communicates with an intake  46   a  of a solenoid valve  46  of the proportional type, which has an output  46   u,  which communicates via a pipe  50  with a supply intake of the actuator  14 . When it is disposed in a regulation position, the solenoid valve  46  can regulate continuously the flow which flows through the solenoid valve itself, and thus the flow rate of the fluid output; the solenoid valve  46  can also be disposed in a discharge position, in which it puts into communication the output  46   u,  and thus the pipe  50 , with a discharge pipe  52  which extends from the solenoid valve  46  to the tank  22 .  
         [0022]    A control logic unit  60  is also provided, which controls switching on/switching off of the electric motor  26 , and controls the solenoid valves  36  and  46  by means of the methods which will be described hereinafter.  
         [0023]    The following comments are made as far as functioning of the actuator  12  is concerned:  
         [0024]    When the electric motor  26  is switched off, no pressure exists in the pipes  28 ,  31 ,  32  and  34 , and consequently the actuator  12  will tend to remain at a standstill, without exerting any action on the mobile unit  10 .  
         [0025]    When the electric motor  26  is activated, the pressure in the pipe  28  will continue to increase until the maximum pressure valve  31   a  is opened, which supplies oil to the tank  22 ; the increase in pressure is thus limited, and the pressure in the hydraulic circuit Plinea stabilises around a regulated value equal to Plim (calibration value of the maximum pressure valve  31   a ).  
         [0026]    In these conditions, if the solenoid valve  36  is in the discharge position, pressurised oil is supplied to the front chamber  20 , and the piston  17  will tend to move towards the rear chamber, since there is applied to it a force F which is equal to:  
         [0027]    F=A/2*Plinea  
         [0028]    If the solenoid valve  36  is disposed in the control position, pressurised oil is also supplied to the rear chamber  19 , by applying an action of contrast to the surface  17   a  of the piston with the larger surface area. The solenoid valve  36  makes it possible to modulate the pressure Pmod of the oil output.  
         [0029]    The force available Fris output from the actuator  12  is thus provided by the difference between the forces applied to the two surfaces of the piston, i.e.:  
         [0030]    Fris.=F1−F2  
         [0031]    Fris.=(Pmod.×A)−(Plinea×A/2)  
         [0032]    in which:  
         [0033]    Plinea is the line pressure determined by the valve  31   a ; and  
         [0034]    Pmod. is the pressure modulated by the solenoid valve  36 , which is variable between 0 and Plinea.  
         [0035]    By this means, the force exerted by the actuator  12  can vary continuously between the values:  
         [0036]    Fris.=(0×A)−(Plinea×A/2)=−Plinea×A/2 (minimum value)  
         [0037]    Fris.=(Plinea×A)−(Plinea×A/2)=Plinea×A/2 (maximum value).  
         [0038]    It is thus possible to implement reversible motion of the shaft  18 , by engaging the gears in the two directions (towards even gears and towards odd gears).  
         [0039]    In addition, in the absence of commands, when the pump  25  is switched off, residual forces are not exerted on the gear, since the line pressure is quickly cancelled out.  
         [0040]    Fris.=(0×A)−(0×A/2)=0.  
         [0041]    With particular reference to FIG. 2, a description will now be provided of the operations of change of gear implemented by the hydraulic circuit  1 , under the control of the control logic unit  60 .  
         [0042]    Initially, a block  100  is reached, which implements an initial phase characterised by the absence of commands. In this phase, the pump  25  is switched off, and the hydraulic circuit  1  is in pressure conditions which are virtually zero. Consequently, the pressure in the pipe  50  is zero, the actuator  14  is not activated, and is maintained in the position of rest by the Belleville washer  14   a , and the clutch  4  is closed. Similarly, the pressure in the pipes  32  and  40  is zero, and forces are not exerted on the mobile unit  10 , which can be used for engagement/release of the gears.  
         [0043]    Block  100  is followed by a block  110 , in which the hydraulic circuit  1  is activated; in particular, on the basis of a command for a change of gear or engagement of the clutch, received by the unit  60 , the logic unit  60  responds by activating the electric motor  26 , in order to pressurise the hydraulic circuit  1 .  
         [0044]    Block  110  is followed by a block  120 , in which the operation of change of gear is initiated.  
         [0045]    In particular, in this phase, the logic unit  60  controls the solenoid valve  36 , such as to create a situation of equilibrium, in which the resulting force on the cylinder  17  is substantially zero:  
         [0046]    Fris.=(Pmod.×A)−(Plinea×A/2)=0.  
         [0047]    In other words, the solenoid valve  36  regulates the pressure Pmod, such that it is equal to a value of equilibrium:  
         [0048]    Pmod=Plinea/2  
         [0049]    In the meantime, the logic unit  60  disposes the solenoid valve  46  in the position of regulation, by supplying pressurised oil to the actuator  14 , which operates opening of the clutch  4 .  
         [0050]    Block  120  is followed by a block  130 , which operates release of the gear. In particular, when the clutch  4  has been opened by a required quantity, the gear engaged is released.  
         [0051]    The release is obtained by interrupting the above-described situation of equilibrium, i.e. by varying (for example by increasing) the pressure Pmod regulated by the solenoid valve  36 , in comparison with the equilibrium value; this therefore provides movement of the shaft  18 , and angular rotation of the mobile unit  10 , in a first angular direction.  
         [0052]    After the gear has been released, the control logic unit  60  restores the equilibrium of the pressure forces generated on the two surfaces of the piston  17 .  
         [0053]    Block  130  is followed by a block  140 , which operates the selection of a new rank (in a known manner, by means of the further actuator, which is not illustrated).  
         [0054]    Block  140  is followed by a block  150 , which, after the new rank required has been reached, commands the solenoid valve  36  (for example by decreasing the pressure Pmod), thus modifying once again the equilibrium of the pressure forces of the actuator, in order to obtain motion of the actuator  12  in the opposite direction, and engagement of a new gear; this therefore provides the movement of the shaft  18 , and angular rotation of the mobile unit  10  according to a second angular direction.  
         [0055]    After the gear has been engaged, the control logic unit  60  restores the equilibrium of the pressure forces generated on the two surfaces of the piston  17 .  
         [0056]    Block  150  is followed by a block  160 , which modifies the regulation position previously reached by the solenoid valve  46  regulating the flow rate of the oil supplied to the actuator  14 , which moves under the thrust of the spring  14   a , and closes the clutch  4 .  
         [0057]    Block  160  is followed by a block  170 , in which the logic unit  60  commands switching off of the electric motor  26 , and thus deactivation of the hydraulic circuit  1 . The above-described operations (blocks  100 - 170 ) are repeated after a new request for a change of gear received by the logic unit  60 .  
         [0058]    From the foregoing description, the advantages of the transmission system according to the present invention are apparent, since a single actuator and a single solenoid valve are used in order to command engagement/release of the gears.  
         [0059]    By this means, the system which is the subject of the present invention permits a substantial reduction in the costs, and simplification of the hydraulic circuit.  
         [0060]    It should be noted that in some operating conditions, the pressure value Plinea is not determined sufficiently accurately, owing to the effect of the design tolerances of the valve  31   a , and of the internal friction of the actuator  12 .  
         [0061]    In these conditions, the unit  60  can implement a manoeuvre of self-learning, by means of which the solenoid valve  36  is sent a command which can give rise to limited movement of the piston  17  in two opposite directions.  
         [0062]    As a result of the effect of the friction forces (Fatt.), which oppose the movement inside the actuator  12 , two pressure values Pmod. 1 , Pmod.  2  will be obtained, relative to the movement of the actuator in the two opposite directions, i.e.:  
         [0063]    Pmod. 1=Plinea/2−Fatt./A  
         [0064]    Pmod. 2=Plinea/2+Fatt./A  
         [0065]    The control logic unit  60  will detect the values of the control signal, which are necessary in order to produce the two pressure values Pmod.  1  and Pmod.  2 . These values can be detected in different operating conditions, and can optionally be updated during the life of the vehicle, and stored in the non-erasable memory of the control logic unit  60 .  
         [0066]    Similarly, it is possible to detect characteristic values of the valve  36  for control of engagement/release of the gears. This makes it possible to increase the accuracy of the force control of the actuator  12 .  
         [0067]    Finally, it is apparent that modifications and variations can be made to the transmission system described, without departing from the scope of protection of the present invention.  
         [0068]    For example, the oil tank  22  need not be present, and in this case the oil would be collected directly from the gearbox, using the oil which is normally present inside the gearbox itself.