Abstract:
An actuator for actuating a gear shift mechanism in a vehicular transmission system, the actuator comprising: an actuator body ( 40 ) having an opening ( 43 ) for the transport of fluid into and out of the actuator body; an actuating member ( 42 ) within the actuator body, moveable by the fluid; and a control device ( 410, 416 ) operable to control independently the pressure of the fluid and the flow of the fluid.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a drive transmission, in particular, a drive transmission comprising shift rails which are hydraulically actuated.  
       BACKGROUND OF THE INVENTION  
       [0002]     One known form of drive transmission is a dual clutch transmission, such as the DSG transmission used in the Audi TT. A typical dual clutch transmission is shown schematically in  FIG. 1 . In a vehicle system, drive from an engine  12  is coupled to the wheels (not shown) via a dual clutch transmission designated generally  14 . The transmission  14  includes first and second clutches  18  and  20  and respective first and second input shafts  22  and  24 . The input shafts  22  and  24  carry respective first and second gear sets  26  and  28 . In the embodiment shown, the first gear set  26  carries gears  1 , 3 ,  5  and  7  (designated schematically) and the second gear set  28  carries gears reverse,  2 , 4  and  6  (designated schematically). A gear of either or both gear sets  26 , 28  is selectively engagable with a corresponding gear of an output gear set  30  on an output shaft  32  which is coupled to the vehicle wheels by an appropriate intermediate linkage. It will be appreciated that the arrangement shown is simplified and in practice the arrangements may be more complex although operating on the same principal. For example, the clutches  18 , 20  can be mounted co-axially.  
         [0003]     In operation, when the vehicle is, for example, in fifth gear, the first clutch  18  is fully engaged with the engine  12 , with the relevant gear of gear set  26  engaging the relevant gear of gear set  30  on the output shaft  32 . The second clutch  20  is fully disengaged from the engine  12  but can be engaged with the output shaft  32  by means of a gear in the gear set  28  and the second input shaft  24 . If a gear shift is required (for example to fourth gear) and the required gear on the second input shaft  24  is not already engaged with the output shaft  32 , then, with the second clutch  20  remaining disengaged from the engine, the appropriate gear of the second gear set  28  is synchronized with the relevant gear of the output gear set  30  and then engaged such that clutch  20  spins freely and not in synchronization with the engine speed. In the downshift from fifth to fourth gear, the second clutch  20  will typically be rotating faster than the engine speed and so, to synchronize the engine speed with the speed of the second clutch  20 , the first clutch  18  is disengaged until slipping starts, at which point the engine speed rises. As the engine speed approaches the second clutch speed, the second clutch  20  is engaged and the first clutch  18  disengaged.  
         [0004]     As explained with reference to  FIG. 1 , an input shaft  22 , 24  is provided for each gear set  26 , 28 . A shift rail  34  is provided to shift a selector fork  36  between a neutral position and an engaged position in which a synchronizer clutch  38  is moved into engagement with a gear on an input shaft, which is itself already in engagement with a gear of the output gear set  30 . To move a selector fork  36 , hydraulic fluid is passed through conduits to actuate an associated actuator  40  connected to the shift rail  34  and hence cause the associated selector fork  36  to move as required and the associated synchronizer clutch  38  to move into engagement with a gear on an input shaft.  
         [0005]     Another known type of transmission is an automated manual transmission (AMT), for example of the type sold by Alfa Romeo under the trade name “Selespeed™”. In an AMT, a single clutch and associated gear pack couples with an output gear pack. When a gear change is required, the clutch disengages, the new gears synchronize and the clutch re-engages. A problem with AMTs is that, during gear changes, there is a significant torque interrupt as well as a rapid change in engine speed when the gear re-engages. To overcome these problems, previous approaches have been to speed up the synchronization process, which can damage the related components, or to control engine speed separately, which can give rise to additional control requirements.  
         [0006]     Typically, hydraulic systems are used to actuate the displacement of shift rails and each hydraulic circuit is controlled by electrically actuated hydraulic spool valves.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention is defined in the accompanying claims. Thus, there is provided a hydraulic circuit which enables both the pressure and the flow rate into and out of a gear selection actuator to be controlled. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention will now be described further, by way of example only, with reference to the accompanying drawings in which:  
         [0009]      FIG. 1  is a schematic diagram of a known dual clutch transmission;  
         [0010]      FIG. 2  shows a schematic diagram of a hydraulic control system for controlling movement of a gear in accordance with the invention;  
         [0011]      FIG. 3  shows a more detailed schematic diagram of the hydraulic control system of  FIG. 2 ;  
         [0012]      FIGS. 4   a  and  4   b  show examples of displacement/time profiles for a gear; and  
         [0013]      FIGS. 5   a  and  5   b  show examples of pressure and flow profiles with respect to time used to generate the example profiles shown in  FIGS. 4   a  and  4   b , respectively. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     A vehicle typically has a plurality of gears including reverse and typically at least four forward gears: first, second, third and fourth. In the embodiment to be described, an eight speed gear box will be described which provides gears as follows: reverse, 1, 2, 3, 4, 5, 6, 7.  
         [0015]     In the transmission system shown, different sets of gears are locked and unlocked to an output shaft to achieve the various gear ratios required. Locking and unlocking of the gears is performed by actuating a selector fork  36 . To move the selector fork  36 , hydraulic fluid is passed through conduits to actuate an actuator  40  and hence cause the selector fork  36  to move as required. Each selector fork  36  is controlled by a double acting actuator  40  (shown in more detail as actuators  402 , 404 ,  406 , 408  in  FIG. 2 ) with pistons which are mechanically connected to the shift rails  34  within the transmission. The rails  34  support the gear selection forks  36  for the movement of splined selector rings  38  to lock and unlock the gears. The shift rails and actuators are arranged such that when the actuator pistons are energized in the neutral position, the actuator will be equidistant from both gears on the rail, with neither gear locked. The position of the selection forks is monitored via a linear displacement sensor, which forms the feedback loop to the control system.  
         [0016]      FIG. 2  is a schematic diagram of circuitry according to the invention used to control movement of a shift rail. The embodiment shown in  FIG. 2  has eight gears: reverse, first, second, third, fourth, fifth, sixth and seventh. These are provided in pairs. In the embodiment shown, the following gear pairs are provided: an actuator  402  for the first and third gear; an actuator  404  for the fourth and sixth; an actuator  406  for the reverse and second; and an actuator  408  for the fifth and seventh.  
         [0017]     Each actuator has a pair of flow control valves  410  associated with it. In  FIG. 2 , numeral  410   l  indicates the flow control valve associated with the “into gear” side of the first gear,  4102  indicates the flow control valve associated with the “into gear” side of the second gear, and so on. These valves typically take the form of electrically actuated hydraulic spool valves. The pressure and flow of the hydraulic fluid entering and exiting the valves  410  controls the force and speed, respectively, with which the associated actuator is moved. The direction of movement of the actuator is dependant upon the controlled direction of fluid flow through the relevant pair of flow control valves  410 .  
         [0018]     The valves  410  are supplied with hydraulic fluid by a pressure control valve  416  and controlled by a flow control input  414 . The pressure control valve  416  regulates the pressure of the hydraulic fluid supplied to the valves  410 . The control input  414  of each flow control valve  410  controls the rate of flow of hydraulic fluid into the chamber of the actuator. The flow control valves  410  and pressure control valve  416  are themselves controlled by a control device  420  such as a microprocessor. (For simplicity, some of the flow control inputs  414  of the flow control valves  410  are shown un-connected to control device  420  although in practice they will be connected.) The control device  420  may also be used to control other features of the transmission or vehicle as a whole.  
         [0019]     Gear change is controlled by the control device  420 . The control device  420  may decide that a gear change is needed in response to a number of factors, e.g. engine speed (rpm), engine torque, terrain etc.  
         [0020]      FIG. 3  shows the actuator arrangement in more detail. Only one actuator  402  is shown, however it will be appreciated that the other actuators  404 , 406 ,  408  are arranged in a similar manner. Each actuator comprises an actuator body  41  and an actuating member  42  in the form of a piston. The piston is connected to a shift rail  34 . The actuators shown are double-acting actuators, i.e., they have openings  43  at opposite ends of the actuator body  41  for fluid to enter and exit the actuator body, one on the left and one on the right as shown in  FIG. 3 . To enable the gear actuating member  42  to be moved, a proportional 3-way/2 position flow control solenoid spool valve  410  is connected, via a control port  44 , to each opening  43  of the actuator  402 , 404 ,  406 , 408 . Each supply pressure port  46  of the flow control valves  410  is connected to the control port of a proportional 3-way/2 position pressure control solenoid spool valve  416 . The supply pressure port of this valve  416  is connected to a hydraulic supply  418 , consisting of a pump and an accumulator. The electrical control input  414  of each flow control valve  410  is connected to the microprocessor  420 .  
         [0021]     The proportional flow control valves  410  regulate the flow rate of the fluid from the control port  44  to the drain port  47  (when on the “drain” side of the actuator), or the pressure port  46  to the control port  44  (when on the “into gear” side of the actuator) in relation to the current applied to the solenoid of the valve  410 . This affects the velocity and direction of movement of the actuating member  42 . The fluid is typically oil, although other liquids or gases may be used.  
         [0022]     The proportional pressure control valve  416  regulates the pressure of the fluid at its control port (and hence the pressure at the pressure port  46  of the flow control valves  410 ) in relation to the current applied to the solenoid of the pressure control valve  416 . This affects the force applied to the actuating member  42 .  
         [0023]     To enable movement of the gear selector actuator, each of the valves  410 , 416  are operated in a suitable manner to achieve the desired velocity and force. This is achieved by use of the flow control valves  410  to determine the direction of movement of the actuator while the pressure control valve  416  regulates the supply pressure and hence the force generated by the actuator piston. The velocity of the actuator can be regulated by use of the flow control valve on the drain side of the actuator controlling the flow between its control and tank ports.  
         [0024]     To enable swift selection (locking) of a synchronized gear in the transmission, the actuating member  42  (and hence the selection rail) is moved from the neutral position (i.e. both gears disengaged), towards the gear by placing the flow control valve  410  on the ‘into gear’ side of the actuator in a state where it commands maximum flow from its pressure to control ports, and the flow control valve  410  on the ‘drain’ side of the actuator in a state where it can regulate the flow out of the actuator by controlling flow between its control and tank ports.  
         [0025]     To move a synchronizer clutch  38  of the first or second gear set ( 26 , 28  respectively) into engagement with a gear of the gear set, the appropriate actuator is actuated. This causes the associated shift rail  34  to move and hence the selection fork  36  and the associated synchronizer clutch  38  to be moved. Clearly, the synchronizer clutch  38  needs to move into the engaged position as quickly as possible but also as smoothly as possible. In a preferred embodiment, sequential gears are provided on different shift rails. This means that the transmission does not have to move directly from one gear on an input shaft to another gear on the same input shaft. This aids a smooth transition. The ideal gear shift is one that is smooth but fast. In each gear change, the teeth of the engaging gears and the synchronizer clutch  38  have to be brought into engagement in the smoothest possible manner. Clearly, since the gears are rotating at the time, this may be a delicate and precise operation.  
         [0026]     An example of the position profile of an actuator is shown in  FIG. 4 .  FIG. 4   a  shows the position profile of a selector fork  36  when moving into engagement with gear A and  FIG. 4   b  shows the position profile when moving out of engagement with gear A.  FIG. 5  shows the corresponding pressure/flow profile for the gear.  FIG. 5   a  shows a pressure/flow profile when moving into engagement with gear A and  FIG. 5   b  shows the pressure/flow profile when moving out of engagement with gear A. The pressure characteristics are shown in the graph marked P and the flow demand characteristics are shown in the graph marked F.  FIG. 5   a  shows an example of a pressure and flow profile with respect to time. Alternatively, the pressure and flow profile with respect to the position of the selector fork may be stored. A separate pressure and flow profile may be stored for each gear.  
         [0027]     Considering  FIGS. 4   a  and  5   a , the pressure control valve  416  initially ( 1 ) is regulated to accelerate the actuator member  42 , and therefore the selection rail  34 , towards the gear to be selected according to the profile P. The ‘into gear’ flow control valve ( 410 ) is regulated to give maximum pressure to control port flow, as it is for the entire gear selection. The flow control valve on the ‘drain’ side of the actuator is regulated, also, to accelerate the actuator according to the profile F. As the actuator approaches a displacement (A) where the synchronizer clutch  38  for the gear will be engaged, the pressure is reduced ( 2 ) by the pressure control valve  416 , and the flow control valve  410  on the ‘drain’ side of the actuator restricts the exhausting flow such that the actuator can be decelerated quickly. In this way, the effect of the operation of the synchronizer clutch on the output shaft torque of the transmission is minimized. As the synchronizer clutch  38  engages ( 3 ) with the gear, the actuator is prevented from further movement by the operation of a sychronizer clutch blocker. At this point, the pressure control valve  416  has a slowly rising pressure ramp applied ( 3 ) such that the synchronization takes place. During this time, the ‘drain’ side flow control valve  410  is regulated to allow maximum control-to-tank-port flow ( 3 ). Upon synchronization (S), the synchronizer blocker is released and the actuator is again free to move. The actuator is accelerated by an increasing pressure command ( 4 ) from the pressure control valve  416 , and regulated on the ‘drain’ side of the actuator by the flow control valve  410 . As the actuator approaches a position (B) where the gear is fully engaged ( 5 ), the actuator is decelerated by a reduction ( 5 ) in the regulated pressure of the pressure control valve  416 , and the restriction ( 5 ) of the exhausting flow on the ‘drain’ side of the actuator by the flow control valve  410 .  
         [0028]     When the control device  420  decides that a gear change is required and which gear is to be engaged, the control device  420  retrieves the flow/pressure profile for the gear being brought into engagement. The control device sends a signal to the appropriate pressure control valve  416 . This, in turn, supplies a controlled pressure supply to the appropriate flow control valve  410  and hence the pressure of the fluid applied to the associated actuator. A control signal is also sent from the control device  420  to the control input  414  of the flow control device  410  to control the flow rate of the hydraulic fluid through the valve  410  and hence into, or out of, the associated actuator.  
         [0029]     This may be illustrated by considering the example of changing the gear from second to fourth. In second gear, the gear of the second gear set  28  is engaged with a gear of the output gear set  30 . When the control device  420  decides that a gear change from second to fourth is required, the control device  420  accesses the gear profile for the fourth gear and, in response to data read from the profile, sends a control signal to the pressure control valve  416  and the control input  414  of the actuator  404  for the fourth gear. In response, the pressure control valve  416  sets the pressure of the hydraulic fluid applied to the flow control valve  4104  for the fourth gear as set in the fourth gear profile and the control device  414  sets the flow of the hydraulic fluid of the actuator  4104  of the fourth gear. The control signals to the control devices  416  and  414  alter in line with the profile as the position of the gear for the fourth gear alters.  
         [0030]      FIGS. 4   b  and  5   b  show examples of actuator displacement and pressure/flow profiles respectively for disengagement of a gear. In the case of disengagement, a command is sent to the pressure control valve  416  according to profile P and a command is sent to both flow control valves associated with an actuator (e.g. flow valves  410 , and  4103  associated with actuator  402 ) to allow flow from the pressure port to the control port according to profile F, such that the actuator accelerates towards neutral.  
         [0031]     As the actuating member nears the neutral position, the actuator is then slowed ( 7 ) by reducing the regulated pressure of the pressure control valve  416  and reducing the flow to both flow control valves  410 . In this way, the severity of mechanical noise caused by the actuator pistons stopping suddenly in the neutral position is reduced.  
         [0032]     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.  
         [0033]     For instance, the invention may be applied to gearbox technologies other than that described with reference to the drawings (e.g. AMTs).