Abstract:
A hydraulic control system for a continuously variable transmission (CVT) includes a ratio control valve for distributing fluid to components in the CVT. The ratio control valve receives a system controlled pressure and distributes both a high pressure control fluid and a low pressure control fluid. A variable bypass valve distributes excess fluid pressure back to the inlet of a control pump. The bypass valve is responsive to the pressure differential between the high pressure control fluid and the system pressure controlled fluid to establish the operating point at which the excess fluid is bypassed. The system pressure controlled fluid is the highest pressure in the system. During operation of the CVT, the high pressure control fluid operates on one variable sheave of the CVT and the low pressure control fluid operates on another variable sheave of the CVT. The ratio control valve is responsive to a feedback control that is effective to determine the required ratio for the CVT and therefore the desired levels of the high and low pressure control fluids.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to controls for continuously variable transmissions and, more particularly, to the control of the belt positioning and pressure within the continuously variable transmission control system.  
         BACKGROUND OF THE INVENTION  
         [0002]    Continuously variable transmissions (CVT), which utilize a belt drive between variable diameter sheaves, have a pressure control system, which establishes the diameter of at least one of the sheaves of the CVT.  
           [0003]    The ratio of the CVT is controlled (in a well-known manner) by increasing the operating diameter of one sheave while decreasing the diameter of the other. This is generally accomplished through a hydraulic control system in which a positive displacement pump provides the fluid source and therefore the pressure, which operates on fluid motors on each of the variable diameter sheaves, controls the position of the sheaves.  
           [0004]    In current CVT systems, the pressure within the control system is generally held at a level significantly higher than the pressure required to control the positioning of the variable diameter sheaves. Control systems using a fixed or high-pressure source have two disadvantages. The excess pressure of the control system at the pump reduces the overall efficiency of the transmission system. The excess flow from the pump, which is not utilized by the control system especially during fixed ratio conditions, is exhausted over the regulator valve, which causes heat increase within the transmission. This additional heat in the fluid must be cooled and therefore a larger cooling system is required.  
         SUMMARY OF THE INVENTION  
         [0005]    It is an object of the present invention to provide an improved hydraulic control system for a continuously variable transmission ratio control.  
           [0006]    In one aspect of the present invention, a positive displacement pump supplies fluid to control the positioning of sheaves within a CVT.  
           [0007]    In another aspect of the present invention, the pressure from the pump is distributed by a ratio control valve, which serves as both the high-pressure control system and the low-pressure control system.  
           [0008]    In yet another aspect of the present invention, the discharge pressure of the pump is controlled by a variable bypass valve, which distributes excess fluid back to the pump inlet.  
           [0009]    In yet still another aspect of the present invention, the bypass valve is operated or controlled by the output pressure of the pump and by the highest pressure within the CVT control system.  
           [0010]    In a further aspect of the present invention, the pump pressure and the CVT control pressure are opposite forces on the bypass valve such that the output pressure of the pump is limited in maximum value by the maximum pressure within the belt or CVT control system. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic and diagrammatic representation of a continuously variable transmission ratio control system incorporating the present invention and having a mechanical feedback system.  
         [0012]    [0012]FIG. 2 is a schematic and diagrammatic representation of a continuously variable transmission ratio control system incorporating the present invention and utilizing a hydraulic ratio control mechanism.  
         [0013]    [0013]FIG. 3 is a schematic and diagrammatic representation of a bypass valve.  
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0014]    Referring to the drawings, wherein like characters represent the same or corresponding parts throughout the several views, there is seen in FIG. 1 a continuously variable transmission (CVT) ratio control system  10 . The control system  10  includes a CVT  12  and a hydraulic control  14 . The CVT  12  includes a pair of variable diameter sheaves or pulleys  16  and  18 , which are interconnected by a flexible torque transmitter such as a belt  20 . The sheave  16  includes a control chamber and piston  22  and the sheave  18  includes a chamber and control piston  24 . Each of the sheaves  16  and  18  are only shown in halves to permit simplicity within the drawings. Those skilled in the art will be well aware that construction and assembly of variable diameter sheaves, such as  16  and  18 , as well as the control piston and chambers  22  and  24 , which are employed therewith.  
         [0015]    The piston and chamber controls  22  and  24  are hydraulically operated devices, which receive fluid pressure through passages  26  and  28 , respectively, from the hydraulic control  14 . The hydraulic control  14  includes a positive displacement pump  30 , which draws fluid from a reservoir  32  through a passage  34  and delivers pressurized fluid to a passage  36 . The pump  30  is a conventional hydraulic device well known to those skilled in the art.  
         [0016]    The passage  36  communicates with a ratio valve  38 , a bypass valve  40 , and a system relief valve  42 . The system relief valve  42  is a conventional pressure regulator valve, which limits the maximum output pressure of the pump  30  in the passage  36 . The bypass valve  40  is also a conventional regulator valve having a control port  44 , a pilot port  46 , and a bias spring  48 . The bypass valve  40  is effective to return fluid from the passage  36  to a passage  50 , which communicates with the inlet of the pump  30  in such a manner as to supercharge the pump inlet in a well-known manner.  
         [0017]    The ratio valve  38  includes a valve body  52  having a bore  54  in which is slidably disposed a valve spool  56 . The valve spool  56  has four lands  58 ,  60 ,  62 , and  64 , and the valve body  52  has an inlet port  66 , a control port  68 , a second control port  70 , and a pair of regulated ports  72  and  74 . The inlet port  66  is in continuous communication with the passage  36  and therefore the pump  30 . The control port  68  is in fluid communication with the piston and chamber control  22  and with a conventional ball shuttle valve  76 . The control port  70  is in fluid communication with the piston and chamber control  24  and also with the shuttle valve  76 . The ports  72  and  74  are in fluid communication with a pressure regulator valve  78 , which is operable to limit the pressure within the ports  72  and  74  by limiting the pressure within a passage  80 , which communicates therewith.  
         [0018]    The pressure regulator valve  78  is controlled by a force motor pilot control mechanism  82 , which is a conventional electro-hydraulic device capable of providing a pressure control signal to the pressure regulator valve  78 , which is variable in response to a force motor on the control  82 . The force motor receives electronic signals or electrical signals from a conventional electronic control unit (ECU), not shown, which includes a preprogrammed digital computer. The control valve  82  receives inlet flow from the passage  36  to provide controlled pressure signals to the pressure regulator valve  78 .  
         [0019]    The valve spool  56  is mechanically connected through a link  84  with a control arm  86 . The control arm  86  and the link  84  are part of a mechanical feedback system, which control the positioning of the valve spool  56  of the ratio valve  38 . The arm  86  has a first end  88  operatively connected with the one half of the variable sheave  16  and a second end  90  operatively connected with a ratio actuator  92 . The center of the arm  86  is connected with the link  84 .  
         [0020]    The ratio actuator  92  has a spring-loaded piston member  94 , which cooperates with a cylinder  96  to form a chamber  98  in which control pressure is provided. The control pressure is established by a force motor regulator valve  100 , which is a conventional control device similar to the control  82 . The force motor regulator valve  100  also receives fluid pressure from the passage  36 . The regulator valve  100  issues pressure signals to the chamber  98  to adjust the position of the piston  94  within the chamber  98 . As the piston moves within the chamber  98 , the end  90  of the link  86  is also moved resulting in movement of the valve spool  56 . As the valve spool  56  is moved, fluid pressure is directed from the passage  36  through the valve  38  to either port  68 , which is connected with the control chamber  22  or port  70 , which is connected with the control chamber  24 .  
         [0021]    The ratio valve  38  is effective to connect the other of the ports  68  and  70  through the regulated ports  72  and  74 . The ports  72  and  74  have a minimum pressure imposed thereon as established by the regulator valve  78 . The higher pressure in either passage  26  or  28  causes an adjustment in the operating diameter of the respective sheaves  16  and  18 , which results in movement of the first end  88  of the arm  86 . As the arm  86  is moved with the sheave  16 , the valve spool  56  will be returned to the position, which will establish the exact operating pressure required for the CVT  12 . This operating pressure is determined by the ECU, which receives signals from the vehicle operator and from the vehicle.  
         [0022]    The shuttle valve  76  is operated on by the pressure in both ports  68  and  70 . The higher of these two pressures is directed through the shuttle valve  76  to a passage  102 , which communicates with the port  46 . The bypass valve  40  is responsive to the pressure in passage  36 , the pressure in passage  102 , and the bias spring  48 .  
         [0023]    The bypass valve  40  is effective to limit the pressure in passage  36  to a level determined by the highest required pressure within the belt control pressures for the CVT  12 . The bypass valve  40  is closed between the passage  36  and  50  until the pressure at port  44  is equal to the pressure at port  46  plus the force of spring  48 . At that time, the pressure at port  44  will cause the bypass valve to begin opening so that some of the fluid in passage  36  is bypassed back to the inlet of the pump  30 . As is well known, the inlet of the positive displacement pump can be designed so that incoming flow will cause a supercharging effect at the inlet of the positive displacement pump.  
         [0024]    If the control regulator valve  100  calls for a change in the ratio of the CVT  12 , the mechanical feedback mechanism will be actuated to require a change at the ratio valve  38 . This, of course, will produce a change in the pressures in passage  26  and  28 , such that a change in the setting of the bypass valve  40  will also occur. If the higher pressure is required at either chamber  24  or  22 , the pressure in passage  36  will increase accordingly, or if a lower pressure is demanded, then the pressure in passage  36  will decrease accordingly.  
         [0025]    A control  210  shown in FIG. 2 is operably the same as the control  10  shown in FIG. 1. The significant difference between the controls  10  and  210  is the operation of the ratio valve  38  in FIG. 1 and  238  in FIG. 2. The ratio valve  38  requires a mechanical feedback input while the ratio valve  238  employs a hydraulic control. The hydraulic control is in the form of a force motor regulator valve  200 , which is similar to the regulator valve  100 . The force motor control valve  200  receives fluid pressure from passage  36  through a filter  202  and distributes the fluid pressure through a passage  204  to a port  206  on the ratio valve  238 . The pressure in the passage  204  is determined by the ECU in response to operator and vehicle input signals.  
         [0026]    The ratio valve  238  includes a valve spool  256  that is slidably disposed on a valve bore  254 . The valve spool  256  has four equal diameter lands  258 ,  260 ,  262 , and  264 . The valve bore  254  has an inlet port  266 , two belt position control ports  268  and  270 , and two regulated ports  272  and  274 . The port  206  is also in communication with the valve bore  254 . The port  266  is in fluid communication with the passage  36 , the ports  268  and  270  are in fluid communication with the passages  28  and  26 , respectively, and the ports  272  and  274  are in fluid communication with the pressure regulator valve  78  which is controlled by the force motor control valve  82 .  
         [0027]    The valve land  258  cooperates with an end  259  of the valve bore  254  to form a chamber  261 , which when pressurized will urge the valve spool  258  rightward. The valve land  264  cooperates with an end  263  of the valve bore  254  to form a chamber in which a spring  265  is disposed. The spring  265  urges the valve spool  256  leftward in the valve bore  254  in opposition to the pressure in the chamber  261 .  
         [0028]    The force motor regulator valve  200  issues pressure signals to the chamber  261  in response to the operating condition set by the operator as well as the vehicle operating parameters. The fluid in chamber  261  cooperates with the spring  265  to position the valve spool  256  in the valve bore  254 . Depending upon the position that is established by these operating conditions, the fluid pressure in passage  36  is delivered to one of the passages  26  and  28  at a higher pressure level than the other passage. The lower pressure passage of passages  26  and  28  is controlled by the pressure regulator valve  78 . The higher pressure in one of the passages is directed to the respective control chambers and pistons  22  and  24  for the sheaves  16  and  18  so that the ratio of the CVT  12  is properly adjusted.  
         [0029]    As with the control system of FIG. 1, the bypass valve  40  will establish a position in which the pressure in passage  36  is equal to the pressure in passage  102  plus the force of the spring  48 . Thus, the maximum pressure in the control system is maintained at a level slightly greater than the maximum control system pressure required by the CVT  12 . Any excess fluid delivered by the pump  30  is bypassed by the valve  40  back to the pump inlet, which improves the operating condition and efficiency of the pump  30 .  
         [0030]    A diagrammatic example of the bypass valve  40  is shown in FIG. 3. As seen in FIG. 3, the bypass valve  40  includes a valve bore  300  formed in a valve body  302 . The valve body  302  is closed at end  304  and has a cover  306  closing an end  308 . A valve spool  310  is slidably disposed in the valve bore  300 . The valve spool  310  has two equal diameter lands  312  and  314 . The valve land  314  operates with the end  304  of the valve bore  300  to form a pressure chamber  315 , which is in fluid communication with the port  44  and therefore the passage  36 . The valve land  312  cooperates with the end  308  to form a chamber  316  in which a spring  318  is disposed to urge the valve spool  310  rightward in the valve bore  300 . The chamber  316  is also in fluid communication with the passage  102  through the port  46 .  
         [0031]    The valve land  312  also controls fluid communication between the passages  36  and  50 . In the position shown (the full rightward position), the passage  36  is closed to the passage  50 . The valve spool  310  is held in this position until the pressure in passage  36  at port  44  is sufficient to urge the valve spool  310  rightward to overcome the opposing forces in chamber  316  and spring  318 . When this occurs, the valve spool  310  is moved leftward until the passage  36  is controllably communicated to the passage  50  such that the pressure in passage  36  will not rise further unless there is a further increase in the pressure in passage  102 . When the pressure in passage  102  decreases, the pressure in chamber  315  will cause the valve spool  310  to move sufficiently to permit further increased fluid communication between passages  36  and  50  such that the pressure in passage  36  will decrease. The valve spool  310  will assume a new control position when the pressures in chambers  315  and  316  and the force in the spring  48  are again in balance.  
         [0032]    Obviously, there are a number of different designs for bypass valves, which will satisfy the present invention. Also in the present invention, as set forth above, the areas of valve lands  314  and  312  are equal, therefore the pressure in the chambers  315  and  316  will have an equal effect in changing the position of the valve spool  310 . The pressure difference between the chambers  315  and  316  is determined by the force in spring  318 . This differential can, of course, be determined by dividing the numerical value of the force in spring  318  by the area of the valve land  314 .  
         [0033]    In one example of the control system, this bias is set at 15 psi. Thus, the pressure in passage  36  will be 15 psi higher than the pressure in passage  102 . When the bypass valve  40  is regulated, the pressure in passages  102  and  36  will change accordingly. Any change in the pressure in passage  102  is reflected by an equal change in the pressure in passage  36 . The pressure in passage  102  will always be equal to the higher pressure required in the piston and chambers  22  and  24 . Thus, the output pressure of the pump  30  represented by the pressure in passage  36  will always be slightly higher than the maximum control pressure required by the CVT  12 . Those skilled in the art will recognize that the areas of the chambers  315  and  316  do not have to be equal. If they are unequal, a ratio of other than one-to-one will be present between the pressure in the passages  36  and  102 .