Abstract:
A transmission and control method are disclosed which ensure proper stroke pressure and minimize torque transients during a shift event. The transmission includes a clutch having a torque capacity based on a fluid pressure, a torque sensor adapted to measure a torque value that varies in relationship to the torque capacity, and a controller. The method includes varying the fluid pressure around a predetermined value, measuring a resulting torque difference with the torque sensor, and adjusting a clutch control parameter if the resulting torque difference is less than a threshold value.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a system and method to control ratio changes in an automatic vehicle transmission. 
     BACKGROUND 
     Known automatic transmissions for automotive vehicles include step ratio controls for effecting speed ratio changes in response to changing driving conditions. The term “speed ratio”, for purposes of this description, is defined as transmission input shaft speed divided by transmission output shaft speed. 
     An upshift occurs when the driving conditions require a ratio change from a lower numbered ratio (high speed ratio) to a higher number ratio (low speed ratio) in the transmission gearing. Similarly, a downshift occurs when the driving conditions require a ratio change from a higher numbered ratio (low speed ratio) to a lower number ratio (high speed ratio). The gearing can include, for example, either a planetary type gear system or a lay shaft type gear system. An automatic gear ratio shift is achieved by friction torque establishing devices, such as multiple disk clutches and multiple disk brakes. The friction torque establishing devices include friction elements, such as multiple plate clutches and band brakes, which can be actuated hydraulically. 
     A step-ratio automatic transmission uses multiple friction elements for automatic gear ratio shifting. A ratio change occurs in a synchronous clutch-to-clutch shift as one friction element, which may be referred to as the oncoming clutch (OCC), is engaged and a second friction element, which may be referred to as the off-going clutch (OGC), is disengaged. Failure to properly coordinate the engagement of the OCC with the disengagement of the OGC can be perceived by the vehicle occupants as an unpleasant shift event. More particularly, early engagement of the OCC relative to the release of the OGC can result in a phenomenon called tie-up. On the other hand, if the OCC is engaged too late relative to the release of the OGC, an engine flare can occur. 
     SUMMARY 
     In one embodiment, a method for controlling a transmission is provided. The method ensures proper clutch stroke and minimizes torque transients. During a downshift, a clutch pressure is set for an oncoming clutch at a predetermined stroke pressure. Then the clutch pressure is varied from the predetermined stroke pressure. A resulting torque difference is measured along a torque transmitting element with a torque sensor while the clutch pressure is varied. A clutch control parameter is adjusted if the resulting torque difference is less than a threshold value. 
     In another embodiment, the torque transmitting element can be, for example, an input shaft or an output shaft. 
     In yet another embodiment, varying the clutch pressure can involve pulsing the clutch pressure above the predetermined stroke pressure, pulsing the clutch pressure below the predetermined pressure, gradually increasing the clutch pressure in a ramp profile, or other means. 
     In some embodiments, the method can include setting the clutch pressure at a boost pressure higher than the predetermined stroke pressure for a boost duration before setting the clutch pressure at the predetermined stroke pressure. 
     In still another embodiment, the clutch control parameter to be adjusted can be, for example, the predetermined stroke pressure, the boost pressure, or the boost duration. 
     In one other embodiment, a method for controlling a transmission is provided. The method includes varying a clutch pressure around a predetermined value in advance of a torque phase of a shift event. A torque change is measured in a transmission element as the clutch pressure is varied. A clutch control parameter is adjusted in response to the measured torque change. 
     In another embodiment, the value can be increased if the change in measured torque is below a first threshold. 
     In another embodiment, the value can be decreased if the change in measured torque is above a second threshold. 
     In another embodiment, the shift event can be a downshift and the clutch can be the oncoming clutch for the downshift. 
     In on other embodiment, a transmission is provided. The transmission includes a clutch having a torque capacity based on a fluid pressure and a torque sensor adapted to measure a torque value that varies in relationship to the torque capacity. A transmission controller is configured to vary the fluid pressure from a predetermined value in advance of a torque phase of a shift event and adjust the predetermined value in response to a change in the measured torque value. 
     The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a transmission; 
         FIG. 2  is a schematic diagram of a transmission clutch or brake; 
         FIG. 3  is a graph illustrating a downshift under idealized clutch pressure control; 
         FIG. 4  is a graph illustrating a downshift under open loop clutch pressure control in which the oncoming clutch pressure is set too high; 
         FIG. 5  is a graph illustrating a downshift under open loop clutch pressure control in which the oncoming clutch pressure is set too low; 
         FIG. 6  is a flow chart illustrating a first embodiments of a closed loop pressure control algorithm; 
         FIG. 7  is a graph illustrating a downshift under the closed loop clutch pressure control system of  FIG. 6  in which the initial oncoming clutch pressure is set too high; 
         FIG. 8  is a graph illustrating a downshift under the closed loop clutch pressure control system of  FIG. 6  in which the initial oncoming clutch pressure is set too low; 
         FIG. 9  is a flow chart illustrating a second embodiments of a closed loop pressure control algorithm; and 
         FIG. 10  is a graph illustrating a downshift under the closed loop clutch pressure control system of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely examples of the invention that can be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
       FIG. 1  illustrates a six speed planetary transmission  18  with three planetary gear sets  20 ,  22 , and  24 . Each planetary gear set includes a sun gear, a ring gear, a planet carrier, and a collection of planet gears supported for rotation about the planet carrier and meshing with both the sun gear and the ring gear. The carrier of gear set  20  is fixedly connected to the ring gear of gear set  22 , the carrier of gear set  22  is fixedly connected to the ring gear of gear set  24 , and the carrier of gear set  24  is fixedly connected to the ring gear of gear set  20 . Input shaft  26  is fixedly connected to the sun gear of gear set  22  and output shaft  28  is fixedly connected to the carrier of gear set  24 . 
     Various power flow paths between input shaft  26  and output shaft  28  are established by the selective engagement of clutches and brakes. Brakes  30 ,  32 , and  34  selectively hold the sun gear of gear set  20 , the carrier of gear set  20 , and the sun gear of gear set  24 , respectively, against rotation. Clutches  36  and  38  selectively connect the sun gear of gear set  20  and the carrier of gear set  20 , respectively, to input shaft  26 . Table 1 indicates which clutches and brakes are engaged in order to establish each of the six forward and one reverse transmission ratios. Torque sensor  40  senses the torque transmitted to the output shaft and electrically communicates that information to controller  42 . The controller  42  can, for example, be part of a vehicle system control module or transmission control module or can be a stand-alone controller. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Brake 30  
                 Brake 32 
                 Brake 34 
                 Clutch 36 
                 Clutch 38 
               
               
                   
               
             
             
               
                 Reverse 
                   
                 X 
                   
                   
                 X 
               
               
                 1st 
                   
                 X 
                 X 
                   
                   
               
               
                 2nd 
                 X 
                   
                 X 
                   
                   
               
               
                 3rd 
                   
                   
                 X 
                   
                 X 
               
               
                 4th 
                   
                   
                 X 
                 X 
                   
               
               
                 5th 
                   
                   
                   
                 X 
                 X 
               
               
                 6th 
                 X 
                   
                   
                 X 
               
               
                   
               
             
          
         
       
     
     While an automatic transmission according to an embodiment of the disclosure can be a planetary type as shown in  FIG. 1 , it is also contemplated that the transmission can be a lay shaft type transmission. Similarly, a speed ratio change can be achieved by the friction elements as described above, or the friction elements can be plate clutches or band brakes. 
       FIG. 2  illustrates a representative cross section of a clutch, such as clutches  36  and  38  and brakes  30 ,  32 , and  34  in  FIG. 1 . A set of friction plates  44  is splined to a clutch hub  46 . The friction plates  44  are interspersed with a set of separator plates  48  that is splined to a clutch cylinder  50 . In the disengaged state as shown here in  FIG. 2 , there is space between the friction plates  44  and the separator plates  48  such that the hub  46  and the cylinder  50  are free to rotate at different speeds with respect to each other. 
     To engage the clutch, pressurized fluid is forced into the cylinder  50 . The pressure is supplied by a pump  52 . The controller  42  regulates the hydraulic pressure indirectly by setting an electrical current in a solenoid  54  which controls the position of a valve  56 . The pressurized fluid travels through a hydraulic passageway  58  to the clutch cylinder  50 . The pressurized fluid forces the piston  60  to slide within the cylinder  50  and squeeze the friction plates  44  and separator plates  48  together. Friction between the friction plates  44  and the separator plates  48  resists relative rotation of hub  46  and cylinder  50 . When the fluid pressure is removed, a return spring  62  forces the piston  60  to slide in the opposite direction returning the clutch to the disengaged state. 
     The torque capacity of the clutch depends upon the fluid pressure but the relationship is complicated by several factors. First, there is a time delay between when fluid starts flowing to the cylinder  50  and when the piston  60  has moved far enough to start squeezing the friction plates  44  and separator plates  48  together. The torque capacity of the clutch is nearly zero during this period before the piston  60  is fully stroked. When the piston  60  has moved such that it can apply force to the plates  44 ,  48 , the piston and clutch are said to be stroked. Secondly, some amount of pressure, called the stroke pressure, is required to overcome the force of the return spring  62  even after the piston  60  is stroked. 
     Once the piston  60  is stroked, the clutch torque capacity is proportional to the fluid pressure minus the stroke pressure. However, a variety of unpredictable noise factors influence the relationship between the solenoid  54  current as commanded by the controller  42  and the torque capacity so that the commanded torque capacity may not be accurately achieved. For example, variations in the coefficient of friction, frictional forces between the piston  60  and the cylinder  50 , and pressure variations in the passageway  58 , may cause the actual torque capacity to be either higher or lower than commanded. 
     These noise factors can make it difficult to achieve a smooth shift behavior without torque transient conditions that may be perceptible to a driver. A downshift from one speed ratio to another requires the coordinated application of one clutch and release of another. For example, to shift from sixth gear to fifth gear, brake  30  (the OGC) is released while clutch  38  (the OCC) is applied, as described in Table 1. As discussed above, noise factors make it more difficult to achieve a smooth shift behavior using only open loop control strategies. The disturbances associated with pressure control inaccuracy are best understood in relation to the intended behavior which is illustrated in  FIG. 3 . As discussed below, actual control strategies do not repeatably achieve this behavior. 
       FIG. 3  illustrates how a downshift process would ideally be executed if there were no noise factors and the controller could command precisely the right amount of torque capacity. The holding pressure for the OGC would be set to the pressure at which the torque capacity of the OGC equals the torque carried by the OGC in the initial gear. To initiate the shift, the controller would reduces the pressure to the OGC to a level slightly below the holding pressure as shown at  102 , marking the beginning of the inertia phase. 
     During the inertia phase, the input speed would increase to the correct multiple of the output speed for the destination ratio, as shown at  104 . The output torque would drop slightly, as shown at  106 , because some of the input power would be consumed to overcome the inertia of elements connected to the input. During the inertia phase, the OCC would be stroked in preparation for the torque transfer phase. The commanded pressure to the OCC would be elevated to a high pressure, P boost , for a short interval, t boost , to rapidly fill the cylinder with fluid and move the piston to the stroke position, as shown at  108 . Then, the commanded pressure would be maintained at a pressure near the stroke pressure. In  FIG. 3 , the actual pressure is shown equal to the stroke pressure at  110 , which would keep the piston stroked but not apply any torque. 
     Once the input speed reaches the correct multiple of the output speed at  112 , the torque transfer phase begins. During the torque transfer phase, the commanded pressure to the OGC would be gradually reduced  114  while the commanded pressure to the OCC is gradually increased  116 . Ideally, the torque capacity of the two clutches would be coordinated such that the input speed remains constant  118  and the output torque gradually increases  120 . The torque transfer phase is complete when the OCC pressure is above its holding pressure  122  and the OGC pressure is below its stroke pressure  124 . The commanded pressure of the OCC would then be further increased to provide some margin over the holding pressure as shown at  126 . 
     While  FIG. 3  illustrates an ideal system without noise factors, the actual pressure will generally only approximate the stroke pressure. In the absence of a feedback signal, it is difficult to determine if the commanded pressure has being achieved. 
       FIGS. 4-5  illustrate the potential problems associated with the noise factors and subsequent pressure control errors in an open loop control strategy.  FIG. 4  illustrates an effect of accidentally commanding an OCC pressure above the stroke pressure during the inertia phase  128 . Once the OCC is stroked, the torque capacity increases to a positive value  130 . Since the speed ratio at this point is below the speed ratio of the destination gear, torque capacity of the OCC produces a drop in the output torque  132 . The vehicle occupants perceive this fluctuation in output torque as a rough and jerky shift event. 
       FIG. 5  illustrates an effect of accidentally commanding an OCC pressure below the stroke pressure  134 . In this circumstance, the OCC is not fully stroked by the beginning of the torque transfer phase. As the commanded pressure of the OCC is increased in the torque transfer phase, there is a delay before the OCC torque capacity begins to increase  136 . During this delay period, the input speed continues to increase above the speed ratio of the destination gear as shown at  138 . This is called an engine flare. Eventually, the OCC torque capacity increases enough to bring the input speed back to the desired level  140 . The output torque changes suddenly  142  when the input speed returns to the destination gear speed ratio which occupants perceive as a rough and jerky shift event. 
       FIG. 6  illustrates a flow chart of a control system for a transmission using closed loop control during a ratio shift. As those of ordinary skill in the art will understand, the functions represented by the flowchart blocks can be performed by software and/or hardware. Also, the functions can be performed in an order or sequence other than that illustrated in  FIG. 6 . Similarly, one or more of the steps or functions can be repeatedly performed although not explicitly illustrated. Likewise, one or more of the representative steps of functions illustrated can be omitted in some applications. In one embodiment, the functions illustrated are primarily implemented by software instructions, code, or control logic stored in a computer-readable storage medium and if executed by a microprocessor based computer or controller such as the controller  50 . 
       FIG. 6  is a flow chart for one embodiment of the present disclosure for using a torque sensor for detecting improper stroke and using closed loop control during a ratio shift. Initially when a ratio shift is requested, the controller raises the OCC pressure to a boost pressure P boost  for a boost time t boost  in order to quickly move the piston to a substantially stroked position, as represented by blocks  60  and  62 . The boost pressure P boost  is a clutch control parameter significantly above the stroke pressure P stroke . For example, the boost pressure can be the maximum available pressure based on limits of the solenoid. The boost time t boost  is a clutch control parameter calculated to be long enough to substantially stroke the clutch and short enough that the clutch does not prematurely transmit torque. 
     Then, the controller commands the OCC to an estimated stroke pressure P stroke     —     est  and waits for a period t test  calculated to be long enough for the piston to reach an equilibrium position as represented by blocks  64  and  66 . Both P stroke     —     est  and t test  are clutch control parameters. Initial values for all clutch control parameters can be established experimentally based on vehicle testing and can be adjusted adaptively during vehicle operation. In this illustrative example, P stroke     —     est  is adjusted adaptively. 
     At  68 , the controller records a reference reading τ ref  from a torque sensor  40 . The torque sensor can measure the torque on the output shaft as shown in  FIG. 1 , the input shaft, or any other element that transmits torque in the destination gear. At  70 , the controller commands a pressure variation P test  above or below the estimated stroke pressure P stroke     —     est . The incremental pressure P test  is calculated to be enough of a pressure variation to generate a change in the torque measured by the torque sensor  40  if the clutch is fully stroked. However, the pressure variation can be small enough that the change in torque would not be objectionable or even noticeable to the vehicle occupants. 
     At  72 , the controller records a second reading τ test  from the torque sensor  40 . At  74 , the controller compares the two torque readings, τ ref  and τ test , to determine if the difference between τ ref  and τ test  differ by more than a threshold amount τ threshold . The threshold amount τ threshold  is calculated to be large enough that short term variations due to noise factors are not erroneously attributed to the change in commanded pressure. If the two pressures, τ ref  and τ test , differ by less than the threshold amount τ threshold , this is indicative that the piston was not fully stroked. If the piston is not fully stroked, then the estimated stroke pressure is increased as represented by block  76 . On the other hand, if the two pressures, τ ref  and τ test , differ by more than the threshold amount τ threshold , this is indicative that the piston was fully stroked. If the piston is fully stroked, then the estimated stroke pressure is decreased, as represented by block  78 . At  80 , the controller commands the revised estimated stroke pressure. 
     Finally, if there is time remaining before the end of the inertia phase, another adjustment is performed. Otherwise, the process ends and the revised estimated stroke pressure is utilized in future shift events involving that OCC. 
       FIG. 7  illustrates the results of utilizing the control strategy of  FIG. 6  when the initial estimated stroke pressure is higher than a required stroke pressure as shown at  144 . When the estimated stroke pressure is too high, the clutch is fully stroked and has positive torque capacity  146 . When the clutch is fully stroked, the upward  148  and downward  150  perturbations in commanded pressure produce measurable changes in torque as shown at  152  and  154  which are detectable by the torque sensor. For example, the upward perturbation  148  results in the measured torque reading τ test1    156 . The torque perturbation is compared to a reference torque value τ ref1    158 . If the difference between the measured torque readings, τ test1  and τ ref1 , is greater than a threshold amount τ threshold , then the estimated stroke pressure is decreased. The controller commands this decreased stroke pressure  160 . 
     Prior to a second perturbation  150 , a revised reference torque value τ ref2    162  is measured. Following the perturbation, a second torque reading τ test2    164  is measured. Even though the new commanded pressure is below the required stroke pressure, the torque difference still exceeds the threshold, resulting in another downward adjustment. The commanded pressure is set to the new adjusted value as show at  166 . Please note, the perturbations in pressure and torque may be exaggerated for illustrative purposes. 
       FIG. 8  illustrates the results when the initial estimate of stroke pressure is below the actual stroke pressure as shown at  168 . When the estimated stroke pressure is too low, the clutch is not fully stroked and has zero torque capacity  170 . In this unstroked condition, perturbations in commanded pressure do not produce a measurable change. For example, as illustrated, upward pressure pulse  172  and downward pressure pulse  174  do not affect output torque  176 . Consequently, then the estimated stroke pressure can be increased after each perturbation. The controller commands this increased stroke pressure as shown at  178  and  180 . 
       FIG. 9  is a flow chart for another embodiment of the present disclosure where the initial estimate of the stroke pressure is intentionally set slightly below the required stroke pressure and gradually increased until a measurable change is detected. Blocks  60  through  68  are identical to the previously described embodiment except that the initial estimate is decreased from the previous value at block  86 . Blocks  88 ,  90 ,  92 , and  94  form a loop in which the estimated stroke pressure and the commanded pressure is gradually increased until the torque sensor indicates a change in measured torque. The increment added to P stroke     —     est  in each iteration can be small compared to the increment used at blocks  76  and  78  of  FIG. 6  or block  86  of  FIG. 9 . 
       FIG. 10  illustrates the results of utilizing the control strategy described in  FIG. 9 . After the boost phase, the clutch pressure is set to a value below the stroke pressure at  182 . Because the clutch is not fully stroked, the clutch torque capacity is zero  184 . The reference torque value τ ref    186  is measured. Then, the commanded pressure is gradually increased, as shown at  188 . Once the commanded pressure reaches the stroke pressure, the clutch torque capacity will begin to increase above zero, as shown at  190 , and the output torque will begin to decrease, as shown at  192 . In each iteration, a new test torque τ test    194  is measured until the difference between the measured torque readings, τ test  and τ ref , is greater than a threshold amount τ threshold    196 . 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.