Abstract:
A powertrain for a vehicle includes an engine having an output and a transmission selectively driven by the output of the engine. A torque converter is disposed between the engine and the transmission for selectively coupling the output of the engine to the transmission. A controller is in communication with the engine and the torque converter and controls a pressure: within the torque converter based on an input parameter supplied to the engine.

Description:
FIELD 
     The present invention relates to control systems and more particularly to a control system for a torque converter. 
     BACKGROUND 
     Vehicles incorporating an automatic transmission typically include a torque converter disposed between the automatic transmission and an engine of the vehicle. In a first mode, the torque converter transmits rotational energy from the engine to the transmission to allow the transmission to rotate wheels of the vehicle. In a second mode, the torque converter receives rotational energy from the engine but prevents such energy from rotating the transmission and, thus, the wheels of the vehicle. The torque converter essentially acts as a fluid coupling between the engine and the transmission that allows the engine to drive the wheels of the vehicle via the transmission in the first mode while allowing the engine to continue running without driving the wheels of the vehicle (i.e., when the vehicle is stopped, for example) in the second mode. 
     The input to the torque converter from the engine rotates generally at a higher speed than an output of the torque converter. For example, a conventional torque converter may include an impeller directly driven by the engine and a turbine coupled to an input of the transmission and rotatably driven by movement of fluid within the torque converter caused by rotation of the impeller. The impeller typically rotates at a higher speed than the turbine during operation. This difference in speed between impeller and turbine is referred to as “slippage,” which directly affects performance of the vehicle, as the slippage rate dictates how far an accelerator must be depressed prior to a vehicle being moved from rest, for example. The degree of slippage may be controlled by selectively applying a force to a converter clutch disposed within the torque converter, which, when applied, causes rotational speed of the impeller to more closely approximate that of the turbine. Generally speaking, a high degree of slippage indicates a high torque transfer and a high torque multiplication. Such high slippage also results in high energy losses due to the friction loss associated with directing fluid from the impeller towards the turbine when operating at high speeds. 
     Conventional control systems may be used in conjunction with a torque converter to apply a form of feedback control. For example, a feedback control system using an error signal that measures slip across the converter clutch may be used to control a pressure of fluid disposed within the torque converter and, thus, the degree to which the converter clutch is applied. While conventional control systems adequately control slip between the impeller and the turbine, conventional control systems mainly employ feedback control and therefore are typically slow to react to a change in driving conditions. 
     For example, when an accelerator is depressed, the error measured across the converter clutch (i.e., the difference in speed between the impeller and turbine) is great relative to the desired slip speed. As such, some time is required to allow oil pressure to sufficiently build up within the torque converter and exert a force on the converter clutch to allow the turbine speed to approximate that of the impeller to drive the transmission and, thus, the turbine, at a desired slip speed. This increased time results in a delay in acceleration of the vehicle and/or an oscillation in slip speed, and therefore reduces the performance and efficiency of the torque converter and vehicle. 
     SUMMARY 
     A powertrain for a vehicle includes an engine having an output and a transmission selectively driven by the output of the engine. A torque converter is disposed between the engine and the transmission for selectively coupling the output of the engine to the transmission. A controller is in communication with the engine and the torque converter and controls a pressure within the torque converter based on an input parameter supplied to the engine. 
     A method of controlling a vehicle includes detecting a torque demand on an engine of the vehicle, generating a signal based on the torque demand, and supplying the signal to a control valve. The method further includes operating the control valve based on the supplied signal, supplying fluid to a torque converter associated with the engine based on the opening of the valve, and driving a transmission based on an output of the torque converter to propel the vehicle. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a partial cross-sectional view of a torque converter associated with a transmission and a vehicle engine; 
         FIG. 2  is a partial cross-sectional view of the torque converter of  FIG. 1  in communication with a control system in accordance with the principles of the present teachings; and 
         FIG. 3  is a schematic representation of the control system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     A torque converter  10  is provided and includes a housing  12 , an impeller  14 , a turbine  16 , and a clutch  18 . The housing  12  is rotatably driven by an engine  20  and selectively transmits rotational energy received from the engine  20  to a transmission  22  via the impeller  14 , turbine  16 , and clutch  18 . In one mode of operation, the torque converter  10  receives rotational energy from the engine  20  and transfers the rotational energy to drive the transmission  22  via the impeller  14 , turbine  16 , and clutch  18  to propel a vehicle (not shown) at a desired speed. 
     As shown in  FIG. 1 , the housing  12  is fixed for rotation with the impeller  14  such that when the housing  12  is rotated by the engine  20 , the impeller  14  is concurrently rotated therewith. Conversely, the turbine  16  is fixed for rotation with an input to the transmission  22  and is therefore not directly coupled to the impeller  14 . Rotation of the turbine  16  and clutch  18  is accomplished when the housing  12  and impeller  14  are rotated and the rotational energy of the impeller  14  is transferred to the turbine  16  via a fluid medium, such as, but not limited to, oil or transmission fluid. Rotation of the turbine  16  and clutch  18  may also be accomplished when the clutch  18  is either fully engaged with the housing  12  (i.e., directly attached to the housing  12  for rotation therewith) or positioned in close proximity to the housing  12 . In either configuration, rotational energy from the impeller  14  is transferred to the turbine  16  at least partially by the clutch  18  acting on or near the housing  12 . 
     The impeller  14  includes a series of blades  24  that circulate fluid within the torque converter  10 . The turbine  16  similarly includes a series of blades  26  that receive fluid from the impeller  14  and cause rotation of the turbine  16  relative to the impeller  14  when the impeller  14  is rotatably driven by the engine  20 . 
     When the housing  12  is rotated by the engine  20 , the impeller  14  is concurrently rotated therewith such that the blades  24  of the impeller  14  impart a force on the fluid disposed within the torque converter  10 . The force applied to the fluid causes the fluid to move generally away from the impeller  14  and towards the turbine  16 . Sufficient movement of the fluid away from the impeller  14  and towards the turbine  16  causes the turbine  16  to rotate. Because the turbine  16  is rotated under fluid force received from the impeller  14 , the impeller  14  and the turbine  16  act as a “fluid coupling” between an output  21  of the engine  20  and an input  23  of the transmission  22 . 
     The clutch  18  is attached to the turbine  16  via a damper  28  that connects the clutch  18  to the input  23  of the transmission  22 . The clutch  18  selectively engages a clutch piston or bracket  30 , which is fixed for rotation with the damper  28 . When the clutch  18  is fixed for engagement with the bracket  30 , the clutch  18  is fixed for rotation with the housing  12  and impeller  14 . Because the clutch  18  is fixed for rotation with the turbine  16  (i.e., through the damper  28 ), the turbine  16  is similarly fixed for rotation with the housing  12  and impeller  14  when the clutch  18  is fixed for rotation with the bracket  30  and housing  12 . 
     Movement of the clutch  18  within the torque converter  10  is accomplished by regulating the pressure of fluid disposed within the torque converter  10 . For example, the pressure within the torque converter  10  may be regulated by increasing the volume of fluid within the torque converter  10  to selectively move the clutch  18  towards the housing  12 . When the clutch  18  is moved in close proximity to the bracket  30 , the turbine  16  rotates at a speed that approximates the speed of the impeller  14 . In other words, the closer the clutch  18  is to the housing  12 , the closer the rotational speed of the turbine  16  approximates that of the impeller  14 . When the clutch  18  is fully engaged with the housing  12 , such that the clutch  18  is fixed for rotation with the housing  12 , the speed of rotation of the turbine  16  is substantially identical to that of the housing  12  and impeller  14 . 
     As noted above, the pressure acting on the clutch  18  generally dictates the speed of the turbine  16  relative to the impeller  14  (i.e., the slip speed). For example, when the pressure within the torque converter  10  is high, the impeller  14  more closely approximates the speed of the turbine  16  due to the proximity of the clutch  18  to the housing  12  and the inertial forces acting on the transmission  22 . 
     The energy imparted on the impeller  14  by the engine  20  is transferred to the turbine  16  via a fluid medium (i.e., the fluid disposed within the torque converter  10 ), some of the energy imparted on the fluid by the impeller  14  is lost due to friction and heat associated with the rotating impeller  14  and moving fluid. Therefore, the turbine  16  typically rotates at a slower speed when compared to the rotational speed of the impeller  14 . This difference in rotational speed between the impeller  14  and the turbine  16  is referred to as “slip.” Applying a force to the clutch  18  such that the clutch  18  moves in close proximity to the housing  12  reduces the slip across the torque converter  10  and causes the impeller  14  to approximate the rotational speed of the turbine  16 . When the clutch  18  is fully engaged with the housing  12 , the turbine  16  is essentially fixed for rotation with the impeller  14  and therefore rotates at substantially the same speed as the impeller  14 . When the turbine  16  is fixed for rotation with the impeller  14 , the torque converter  10  is operating in a “zero-slip” state. 
     With continued reference to  FIG. 1 , operation of the torque converter  10  will be described in detail. When a vehicle (not shown) is initially started, the vehicle is at rest and the engine  20  is providing a rotational output. The rotational output is received by the housing  12  and causes the housing  12  and impeller  14  to rotate. Rotation of the housing  12  and impeller  14  applies a force on the fluid disposed within the torque converter  10  via the blades  24  of the impeller  14 . If the vehicle is at idle and brakes (not shown) of the vehicle are applied, the rotational energy supplied to the housing  12  and impeller  14  does not cause sufficient rotation of the turbine  16  to overcome the force applied to wheels (not shown) of the vehicle and the vehicle remains at rest. 
     When an accelerator  32  is depressed, a greater torque demand is required of the engine  20 . A throttle  34  of the engine  20  responds to the increased torque demand and causes the output of the engine  20  to be increased. Increasing the output of the engine  20  causes the housing  12  and impeller  14  to rotate at greater speeds. The increased rotational speed of the impeller  14  similarly causes the blades  24  to rotate at a higher speed and impart a greater force on the fluid disposed within the torque converter  10 . The increased force applied to the fluid causes the fluid to further rotate the turbine  16  and propel the vehicle. At this point, the vehicle will be driven forward unless a sufficient force is applied to the brakes to maintain the vehicle at rest. 
     Assuming the brakes are released and the depression of the accelerator  32  causes the vehicle to move forward, slippage between the impeller  14  and turbine  16  is experienced such that energy is lost in transferring fluid force from the impeller  14  to the turbine  16 . To mitigate these losses, more fluid may be introduced into the torque converter  10  to apply pressure on the clutch  18  and cause the clutch  18  to move into close proximity to the housing  12 . 
     As described above, movement of the clutch  18  into close proximity with the housing  12  causes the turbine  16  to more closely mimic the rotational speed of the impeller  14 . Allowing the turbine  16  to mimic the rotational speed of the impeller  14  allows the vehicle to be more responsive to engine speed and to operate more efficiently. 
     With particular reference to  FIG. 2 , a control system  36  is provided for use with the torque converter  10 . The control system  36  includes a feed-forward module  38  and a feedback module  40 . The feed-forward module  38  and feedback module  40  cooperate to provide an output signal for controlling a valve  42 . The valve  42  may be a solenoid valve such as, for example, a variable-force solenoid (VFS) or a pulse-width modulated (PWM) solenoid. Controlling the valve  42  directly controls the volume of fluid supplied to the torque converter  10 , and thus, controls the pressure within the torque converter  10  to control the proximity of the clutch  18  relative to the housing  12 . 
     The feed-forward module  38  attempts to mitigate a delay between depression of the accelerator  32  and the torque required to maintain the same amount of slippage when the engine speed increases (i.e., caused by depression of the accelerator  32 ) by estimating the required pressure of fluid needed within the torque converter  10  based on a position of the accelerator  32 . When the accelerator  32  is initially depressed, a volume of fluid enters the torque converter  10  and applies a force on the bracket  30 . Because the force applied to the turbine  16  from the impeller  14  and movement of the clutch  18  into close proximity to the housing  12  is largely dependent on the pressure of fluid disposed within the torque converter  10 , there may be a delay between depression of the accelerator  32  and the torque required to compete with the increasing engine torque. 
     The feed-forward module  38  attempts to mitigate this delay by anticipating the required pressure within the torque converter  10  based on the angle (i.e., the depression) of the accelerator  32 . While the angle of the accelerator  32  will be described hereinafter, the feed-forward module  38  may use other vehicle operating parameters that provide an indication of the torque demand on the engine  20 . For example, the feed-forward module  38  may receive information regarding the position of the throttle  34 , which indirectly supplies information as to the angular position of the accelerator  32 . 
     Once the feed-forward module  38  receives information as to the torque demand on the engine  20 , either from the position of the accelerator  32  and/or the position of the throttle  34 , the feed-forward module  38  may estimate the requisite pressure needed within the torque converter  10  to achieve a desired torque capacity of the torque converter  10  and maintain the slip across the impeller  14  and turbine  16 . 
     Anticipating the pressure required within the torque converter  10  quickly reduces slip between the turbine  16  and the impeller  14  by exerting a force on the clutch  18  via the added fluid. For example, if the vehicle is traveling at a relatively low speed and the accelerator  32  is depressed such that a large increase in speed and, thus, a large increase in torque demanded on the engine  20  are required, a great difference in rotational speed between the impeller  14  and turbine  16  is experienced. When the vehicle is operating at the lower speed, the slip between the impeller  14  and the turbine  16  is greater than when the vehicle is operating at a higher speed. The difference in slip between the impeller  14  and turbine  16  may be overcome by supplying the torque converter  10  with an increase in fluid generally within the torque converter  10 . 
     This increase in fluid applies a force to the clutch  18  and allows the clutch  18  to move into close proximity with the housing  12 , thereby allowing the turbine  16  to rotate at a speed that more closely approximates the impeller  14 . Anticipating the volume of fluid required within the torque converter  10  to sufficiently move the clutch  18  into proximity with the bracket  30  to achieve a desired slip between the impeller  14  and the turbine  16  allows the vehicle to operate more efficiently and directly respond to depression of the accelerator  32 . 
     The feedback module  40  works in conjunction with the feed-forward module  38  to “fine tune” the estimation performed by the feed-forward module  38 . The feedback module  40  may receive the slip speed between the impeller  14  and turbine  16  and output an error. The output error may be fed into the feed-forward module  38  to adjust the amount of fluid within the torque converter  10 . The signal output from the feedback module  40  may be a proportional, integral, derivative (PID) signal that continuously varies the duty cycle of the valve  42 . 
     The following summarizes operation of the feed-forward module  38  and feedback module  40  and provides algorithms for use by both modules  38 ,  40  in controlling the torque converter  10 . During operation of the torque converter  10 , the turbine  16  is engaged with the transmission  22 . Therefore, the inertia of the turbine  16  can be assumed as an infinite when compared with that of the engine  20 . When pressurized fluid is supplied to the torque converter  10  to engage and release the clutch  18 , controlling the slip speed indirectly controls the engine speed. The following equation demonstrates that any acceleration change of the engine  20  is equal to a difference between engine torque and the sum of the torque converter transmitted torque and clutch torque.
 
 T   e   −T   t   −T   cc =( I   e   +I   t )α e  
 
     Solving for the torque of the clutch  18 , yields the following relationship.
 
 T   cc   =T   e   −T   t −( I   e   +I   t )α e  
 
     The above relationship demonstrates that the torque in the clutch  18  is equal to the engine torque (T o ) minus the torque transmitted by the torque converter (T t ) and engine turbine inertia torque ( 1   e α e ). The clutch torque capacity at given clutch pressure is given by the following relationship, where μ f  is the friction coefficient of the friction material, R f  is the effective radius of the friction material, and A f  is the friction material area.
 
T cc =μ f R f A f P cc  
 
     During control of the torque converter  10 , a duty cycle of the valve  42  can be continuously modulated to adjust the pressure exerted on the clutch  18  to control the torque of the clutch  18 . The following relationship provides an expression that yields the clutch torque. 
     
       
         
           
             
               P 
               cc 
             
             = 
             
               
                 
                   
                     T 
                     e 
                   
                   - 
                   
                     T 
                     t 
                   
                 
                 
                   
                     μ 
                     f 
                   
                   ⁢ 
                   
                     R 
                     f 
                   
                   ⁢ 
                   
                     A 
                     f 
                   
                 
               
               + 
               
                 
                   
                     
                       I 
                       e 
                     
                     + 
                     
                       I 
                       t 
                     
                   
                   
                     
                       μ 
                       f 
                     
                     ⁢ 
                     
                       R 
                       f 
                     
                     ⁢ 
                     
                       A 
                       f 
                     
                   
                 
                 ⁢ 
                 
                   
                     e 
                     s 
                   
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     t 
                   
                 
               
             
           
         
       
     
     From above equations, we can see that the duty cycle control of the valve  42  for the clutch pressure is not only dependent on the slip speed error (i.e., the error between the impeller  14  and the turbine  16 ), but also must include the engine torque and torque converter torque. If engine throttle data is used to predict the engine and torque converter torques, the first term 
             (         T   e     -     T   t           μ   f     ⁢     R   f     ⁢     A   f         )         
in the above equation for clutch torque (P cc ) is a feed forward control term and the second term
 
             (           I   e     +     I   t           μ   f     ⁢     R   f     ⁢     A   f         ⁢       e   s       Δ   ⁢           ⁢   t         )         
is a feedback control term that accounts for an inertia (I t ) of the housing ( 12 ) and impeller ( 14 ), as well as a desired acceleration (e s /Δt). The first term may be used by the feed-forward module  38  while the second term may be used by the feedback module  40 .
 
     The feed forward module  38  is an anticipatory control to reduce system delay, as described above. The feedback module  40  determines a slip error based on an error between a current operating slip and a desired slip required to produce a desired clutch pressure on the clutch  18 . The feedback module  40  may generate a signal that is used to vary the slip speed and is fed back to the feed-forward module  38  to enhance stability of the duty cycle signal supplied to the valve  42 . 
     During steady state, the feedback module  40  may employ PID control to continuously adjust the slip error without considering the engine torque and torque converter torque. However, the system delay would inevitably cause slip speed swing and oscillating during periods of transient operations. To improve control quality, engine and torque converter torques should be taken into consideration. One algorithm for controlling the torque converter  10  is provided below. 
     The first step in controlling the torque converter  10  is to calculate the slip error using the following equation.
 
 N   s   =N   e   −N   t,   N   ds   =f ( THR,N   t ),  e   s   =N   s   −N   ds  
 
     Once the slip error is determined, the minimum fluid pressure for application on the clutch  18  can be determined using the following relationship when the clutch  18  is at or about the zero point (i.e., when the torque converter  10  is operated solely by fluid pressure and not by movement of the clutch  18  into engagement with or close proximity to the housing  12 ) where T em  is a temperature of the fluid disposed within the torque converter  10 .
 
 P   min   =P   ffad (0, T   em )+ P   offset ( N   t )
 
     The bypass clutch torque may be determined by the following expression, where T con  is the torque-converter torque.
 
 T′   cc   =T   eng   −T   con ( N   e   ,N   t )
 
     A correction for the clutch torque during periods of idle or steady-state part throttle is provided by the following equation.
 
 T   c   =T   eng   −T   con ( N   e   ,N   t )
 
     Finally, the corrected clutch torque is a follows.
 
 T   cc   =T′   cc   −T   c  
 
     The anticipated clutch torque may then be determined by the following relationship. 
     
       
         
           
             
               T 
               antcc 
             
             = 
             
               
                 T 
                 eng 
               
               + 
               
                 
                   K 
                   tt 
                 
                 ⁢ 
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     THR 
                   
                   THR 
                 
               
               - 
               
                 
                   T 
                   con 
                 
                 ⁡ 
                 
                   ( 
                   
                     
                       N 
                       ds 
                     
                     , 
                     
                       N 
                       t 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     The anticipated clutch torque may then be applied to the torque converter  10  to drive the torque converter  10  at a desired slip speed. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.