Abstract:
An arrangement for controlling a variator in a continuously variable transmission such as a vehicle transmission is described. The arrangement comprises a user operable ratio control part such as a lever and a device for operatively coupling the ratio control part to a movable torque transfer part of the variator such as its rollers. The coupling device is, in the particular embodiment described, a hydro-mechanical arrangement. By moving the user operable ratio control part, the user exercises control over the variator ratio. In accordance with the invention the arrangement further comprises a torque release device such as a valve which is operable by the user to decouple the ratio control part from the movable torque transfer part. By doing so, the torque release device offers functionality which is in some ways analogous to that of a clutch in a conventional stepped ratio vehicle transmission.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application is a U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/GB2007/050258, filed on May 11, 2007, entitled ARRANGEMENT FOR CONTROL OF A CONTINUOUSLY VARIABLE TRANSMISSION, which claims priority to United Kingdom patent application number 0611265.0 filed Jun. 8, 2006. 
     FIELD 
     The present invention is concerned with control of continuously, variable transmissions. 
     BACKGROUND 
     A conventional motor vehicle transmission with a manual, stepped ratio gearbox typically has a user operable clutch between the engine and the gearbox serving to couple/decouple them. When the vehicle is launched—that is, when it moves away from rest—there is an inevitable initial mismatch of speeds between the transmission input and the engine output, and by allowing the clutch to slip at this stage the driver causes torque to be applied to the transmission to accelerate the vehicle, without applying an unsustainable load to the engine that would cause it to stall. In the case of conventional automatic, stepped ratio transmissions the initial mismatch of speeds is accommodated by a torque converter. The clutch and torque converter are examples of what will be referred to herein as “launch devices”. 
     In some continuously variable transmissions (“CVTs”) no launch device is required. Instead, such CVTs are able to provide an infinite speed reduction. That is to say, by adjustment of ratio and without physically decoupling the engine from the transmission output, they can achieve a condition in which the transmission output is stationary while the engine is running. This condition is referred to as “geared neutral”. Launch can be achieved simply by moving the CVT ratio away from geared neutral, and it is not necessary to decouple the transmission from the engine to bring the vehicle to a halt. 
     The inventors have recognised, however, that it would be advantageous if such a transmission could be made to emulate in some respects the functionality of a manual transmission having a user operable clutch. The use of a clutch is very familiar to many drivers. It offers positive assurance that no creep torque will be applied to the vehicle wheels. It is also useful e.g. in certain low speed manoeuvres. 
     Note that the word “engine” is used herein for the sake of brevity to refer to any form of rotary driver, and must be understood to encompass not only internal combustion engines but also electric motors, external combustion engines etc. 
     The word “variator” will be used herein to refer to a device that has a rotary input and a rotary output and that transfers drive from one to the other at drive ratio that can be continuously varied. Such a device is to be found in any CVT. Most, if not all, variators have some movable torque transfer part which is involved in the transfer of drive and whose position corresponds to the variator ratio. In the well known case of a toroidal race, rolling traction type variator, rollers serve as the movable torque transfer parts. They transmit drive from one toroidally recessed race to another, and their motion involves a change in roller inclination which is associated with a change in variator drive ratio. A force is applied to the movable torque transfer part to influence its position, and so to influence variator drive ratio. In principle, this force could be provided through some direct linkage to a user operable control such as a lever or foot pedal. In practice, excepting the case of very lightweight vehicles, the force required—which is related to the torques suffered by the variator—proves to be too large to be comfortably provided by the user. Achieving geared neutral with such an arrangement could also be problematic, since any minor deviation of the position of the torque transfer part from the position required for geared neutral would cause the transmission to adopt a very low drive ratio, potentially resulting in a large “creep torque”, albeit at low wheel speed. 
     Modern CVTs are typically reliant upon sophisticated electronic controllers to regulate the engine and the transmission. However there is a commercial need for simple physical devices for controlling CVTs. These would be particularly attractive for use in basic model tractors, for example. 
     SUMMARY 
     In accordance with a first aspect of the present invention there is an arrangement for controlling a variator, the arrangement comprising a user operable ratio control part, a device for operatively coupling the ratio control part to a movable torque transfer part of the variator to enable the user to exercise control over the variator ratio, and a torque release device operable by the user to decouple the ratio control part from the movable torque transfer part. 
     The ratio control part may for example be a foot pedal or lever operable by the user. The operative coupling of the ratio control part to the torque transfer part is preferably such that the position of the torque transfer part is determined by the position of the ratio control part. 
     In accordance with a second aspect of the present invention there is an arrangement for controlling a variator, the arrangement comprising a user operable ratio control part, a device for operatively coupling the ratio control part to a movable torque transfer part of the variator to enable the user to exercise control over the variator ratio, and a device for applying to the ratio control part a force which varies in sympathy with force experienced by the movable torque transfer part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: — 
         FIG. 1  is a highly simplified and schematic representation of a variator suitable for use in implementing the present invention; 
         FIG. 2  is a schematic representation of a CVT suitable for use in implementing the present invention; and 
         FIG. 3  is a schematic representation of a control arrangement embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  represents a variator of the well known toroidal race, rolling traction type. The present invention has been developed in connection with a CVT using this type of variator, which is particularly well suited to the purpose, but in principle variators of other types could be used. 
     The variator  10  comprises co-axially mounted input and output races  12 ,  14 , adjacent faces  6 ,  8  of which are semi-toroidally recessed and together define a generally toroidal cavity  16  containing a movable torque transfer part in the form of a roller  18 . In fact a practical variator typically has two or three such rollers spaced about the cavity  16  at circumferential intervals. Each roller  18  runs upon the faces  6 ,  8  of the respective races  12 ,  14  and so serves to transmit drive from one to the other. The roller  18  is able to move back and forth along a circumferential direction about the common axis  20  of the races  12 ,  14 . It is also able to precess. That is, the roller&#39;s axis is able to turn, changing the inclination of the roller axis to the disc axis. In the illustrated example, these motions are provided for by rotatably mounting the roller  18  in a carrier  22  coupled by a stem  24  to a piston  26  of an actuator  28 . A line  19  from the centre of the piston  26  to the centre of the roller  18  constitutes a precession axis about which the whole assembly can turn. Precession of the roller results in changes of the radii of the paths traced upon the races  12 ,  14  by the roller, and hence in a change of variator drive ratio. 
     Note that in this example the precession axis  19  does not lie precisely in a plane perpendicular to the common axis  20 , but is instead inclined to this plane. The angle of inclination is labelled CA in the drawing, and is known as the “castor angle”. As the roller moves back and forth it follows a circular path centred upon the common axis  20 . Furthermore the action of the races  12 ,  14  upon the roller creates a steering moment which tends to maintain it at such an inclination that the roller axis intersects the common axis  20 . This intersection of the axes can be maintained, despite movement of the roller back and forth along its circular path, by virtue of the castor angle. As the roller moves along its path, it is also steered by the action of the races, causing it to precess such as to maintain the intersection of the axes. The result is that the position of the roller along its path corresponds to a certain roller inclination and hence to a certain variator drive ratio. 
     The actuator  28  receives opposed hydraulic fluid pressures through lines  30 ,  32 . The force thus created by the actuator  28  urges the roller along its circular path about the common axis  20 , and at equilibrium it is balanced by forces exerted upon the roller by the races  12 ,  14 . The force exerted by the races is proportional to the sum of the torques externally applied to the variator races. This sum—the variator input torque plus the variator output torque—is the net torque that must be reacted to the variator&#39;s mountings, and is referred to as the reaction torque. 
     Looking now at  FIG. 2 , an engine is represented by a box ENG, the variator by a circle V and an epicyclic shunt gear by a box E. The variator input is coupled to the engine through gearing R 1 , R 2 . Its output is coupled to a first input shaft S 1  of the epicyclic shunt E. A second input shaft S 2  of the epicyclic shunt E is coupled through fixed ratio gearing R 1 , R 3  to the engine. An output shaft S 3  of the epicyclic shunt E is coupled through gearing R 4  to the point of power usage, in this case wheels W of a motor vehicle. The operation and construction of epicyclic gear is very well known. The speed of the output shaft S 3  can be expressed as a function of the speeds of the input shafts S 1 , S 2 . At some variator drive ratio, the speeds of S 1  and S 2  cancel each other out and the output speed at S 3  is zero whatever the speed of the engine. This is the “geared neutral” condition referred to above. Variator drive ratios to one side of geared neutral produce S 3  output rotation in one direction and variator drive ratios to the other side of geared neutral produce S 3  output rotation in the opposite direction. 
     Thus by adjusting the variator drive ratio, it is possible to move from forward drive, through geared neutral to reverse. 
     A control arrangement embodying the present invention will now be described with reference to  FIG. 3 , in which the variator&#39;s control actuator and piston are once more labelled  28  and  26  respectively. The arrangement serves to control the hydraulic pressures applied to the actuator which in turn control variator ratio. 
     A user operative ratio control part is seen at  50  in the drawing. The ratio control part is operatively coupled to the variator rollers. The user moves this part to control the ratio adopted by the variator and hence by the transmission as a whole. The variator ratio is a function of the position of the ratio control part. The ratio control part is movable through a continuous range, indicated by arrows in the drawing, from a maximum forward ratio position through a geared neutral position to a maximum reverse ratio position. The range of ratios in forward and reverse will typically be different, making higher outputs speeds available in forward than in reverse. The ratio control part is in this embodiment formed by a hand lever. It could alternatively be a pedal. Pedal mechanisms are known in which the driver, using both the ball and heel of the foot, can rock the pedal to either side of a neutral position. These would be well suited in this context, but an alternative would be to give the driver two pedals—one for forward drive and one for reverse. 
     The device used to operatively couple the ratio control part to the variator rollers is seen in the drawing and is hydro-mechanical. To briefly summarise its main components, it uses a comparator arrangement  52  which receives and compares (a) the position of the ratio control part and (b) the position of the variator rollers, and in response modulates a force to move the rollers toward the position dictated by the user through the ratio control part. This force is provided through a hydraulic pressure control arrangement  54  supplying fluid pressure to the actuator  28 . The device also serves to apply a feedback force, indicative of variator torque, to the ratio control part, to provide the user with information about torque. This is done through a force feedback arrangement  56 . The user is provided with a torque release control  58  which, acting through a torque release device  60 , serves to operatively decouple the ratio control part  50  from the variator and so to zero variator reaction torque, thereby providing functionality which is in some ways similar to that provided by a clutch in a conventional manual transmission. 
     These aspects will now be described in more detail, beginning with the comparator arrangement  52 . 
     In the present embodiment the comparator uses a system of mechanical levers. The lever forming the ratio control part  50  is pivoted about a fixed fulcrum  62  and extends beyond the fulcrum to a pivotal link with a bridging part  64 , which in turn has a first pivotal comparator linkage  65  to a comparator bar  66 . Hence moving the ratio control part  50  moves the comparator bar&#39;s first comparator linkage  65 . 
     The piston  26  is in the present embodiment coupled to the comparator bar through a cable linkage  68  and a feedback torque device  70  (the function of the feedback torque device  70  will be described below). The feedback torque device is pivotally coupled to the comparator bar  66  through a second comparator linkage  72 . Hence the position of the second comparator linkage corresponds to the position of the variator roller, and so to the variator ratio. 
     Between the first and second comparator linkage  65 ,  72 , the comparator bar  66  has a reference linkage  74  to a valve control bar  76  leading in turn to a pressure control valve  78 . 
     The effect of the lever arrangement is to set the state of the pressure control valve  78  on the basis of a comparison of variator ratio against the position of the ratio control part  50 . 
     The pressure control valve  78  forms part of the pressure control arrangement  54 . It has a port which receives pressurised fluid through fluid line  80  from a pump  82 . The pump  82  draws from a sump  84  and is provided with a pressure relief valve  86 . The pressure control valve has ports communicating with two supply lines S 1 , S 2  arranged to supply fluid respectively to opposite sides of the variator piston  26 . Pressure in S 1  urges the piston  26  one way. Pressure in S 2  urges it the other way. The pressure control valve  78  is a proportional valve with three states. In one, it applies pressurised fluid from a pump to S 1 . In another it applies the pump fluid to S 2 . In the third, intermediate, state, it isolates S 1  and S 2  from the pump pressure. 
     Consider what happens when, the system having been in a state of equilibrium, the user moves the ratio control part  50 . This produces a mismatch between the control part&#39;s position and the variator ratio. The first comparator linkage  65  is moved. In this example, let us take it the movement is to the left as viewed. The reference linkage  74  is thus also moved leftward, causing the pressure control valve to adopt its second state, applying pump pressure to S 2  and venting S 1  to the sump. Resultant pressure on piston  26  urges it to the left, as viewed, moving the piston and changing variator ratio. This motion is transmitted through the cable linkage  68 , moving the second comparator linkage to the right. When this rightward motion of the second comparator linkage is sufficient to cancel out the leftward motion of the first comparator linkage, the pressure control valve  78  returns to its third position to maintain the piston pressure and position. 
     This is in effect a servo system for control of roller position using hydraulic actuation and mechanical position feedback. 
     The user is in principle able to demand any ratio at any moment, and this can create difficulties. A very rapid change in the demanded ratio could result for example in excessive wheel torque. The system has been developed in connection with agricultural vehicles having low transmission ratios and high engine torques, and in this context it is entirely possible through incautious user input to cause the vehicle to ride up on two wheels, with its steering wheels in the air! To provide the user with information about the torques being created in the transmission, the force feedback arrangement  56  applies to the user operative ratio control part a force which corresponds to torque. As  FIG. 3  shows, the force feedback arrangement comprises a double acting arrangement of a piston  88  and cylinder  90 . To opposite sides of the piston are applied pressures taken directly from respective supply lines S 1 , S 2 . The force thereby applied to the ratio control part is thus proportional to the force applied to the variator rollers by the variator piston  26 , although by suitable choice of piston areas it is ensured that the former is smaller than the latter. As explained above, the force on the variator rollers is proportional to the variator reaction torque so the user experiences a feedback force which is directly related to reaction torque. 
     Turning now to the torque release control  58 , this may for example be a hand lever or foot pedal. By use of the control  58 , the driver is able to set to zero the force applied to the variator rollers. In this way variator reaction torque is likewise set to zero, and the variator is rendered incapable of sustaining an output torque to drive the vehicle wheels. The effect is akin to declutching in a conventional manual transmission, in that it prevents the transmission from applying torque to the vehicle wheels, but is achieved without any physical decoupling of the engine from the wheels. Instead it relies upon operatively decoupling the variator rollers from the ratio control part  50 . The torque release control part  58  acts upon a torque release device  60  formed in this embodiment as a torque release valve leading from one fluid supply line S 1  to the other S 2 . When open, the valve provides a route for equalisation of pressures in the supply lines. With little or no pressure difference across the piston, no significant force is applied to the variator rollers and so no significant reaction torque can be sustained. Closing the torque release valve  60  restores reaction torque. The valve  60  is a proportional valve so that the user can adjust its degree of opening. 
     The torque release control can be used analogously to the type of launch device described above, by first setting the ratio control part  50  to demand forward or reverse drive and then progressively closing the torque release valve  60  to bring the ratio in a controlled manned to the demanded value, causing the vehicle to accelerate away from rest. The torque release control can be used to gently “inch” the vehicle toward a desired position, as when parking. In this case it serves to limit the wheel torque, again in a manner very much akin to the conventional clutch. The torque release control can also be used to release any creep torque, e.g. when the vehicle is parked with the engine running. Note however that the user can also control the transmission without use of this control. For example, he/she can “shuttle” from forward to reverse and vice versa using only the ratio control part  50 . 
     The feedback torque device  70  has yet to be described. It serves to modify variator ratio based upon reaction torque. It comprises a cylinder  91  which is movable by the cable linkage  68  and which contains a piston  92  connected through a bridging part  94  to the second comparator linkage  72 . Within the cylinder  91  are springs on either side of the piston  92 , biasing it toward a certain position. The forces applied to the piston in controlling the pressure control valve  78  are not large, and the springs are stiff enough that little if any movement of the piston  92  in its cylinder is created by these forces. However the piston is also subject, via a shutoff valve  96 , to the S 1 /S 2  pressures. The result is that a sufficient reaction torque—and correspondingly a sufficient difference in the S 1  and S 2  pressures—causes the piston to move against the springs, changing the effective length of the linkage from the cable  68  to the comparator bar  66 . The effect is to modify variator ratio in a manner that reduces reaction torque, giving the system some compliance and protection against excessive torque. The shutoff valve removes this facility, by isolating the feedback torque device from the S 1 /S 2  pressures, when it is not required. 
     Items  98  and  100  in  FIG. 3  are hydraulic dampers. Their function is to damp unwanted oscillatory behaviour of the variator by providing some resistance to flow into/out of the piston  28 . Item  102  is a crossover valve used to reverse pressures applied to the variator for overrun conditions. 
     The above described embodiment serves as an example only of a possible implementation of the present invention. Numerous other ways of putting the invention into practice are possible. As an example, the lever arrangement used to compare roller position and demanded ratio could be replaced by a known type of valve in which the spool and sleeve are movable by the rollers and the ratio control part.