Abstract:
An automatic transmission for a vehicle includes an input shaft, an output shaft, a first and second plurality of gears, and a series of engagement elements. The series of engagement elements includes at least one overrunning engagement element in mechanical communication with the second plurality of gears. A controller is operable to disengage a first engagement element associated with a first one of the first plurality of gears and apply a second engagement element associated with a second one of the first plurality of gears to achieve a speed change of the first plurality of gears. The controller is adapted to disengage one of the overrunning engagement elements from engagement with one of the second plurality of gears within a range of 20 to 120 milliseconds after the speed change of the first plurality of gears to accomplish a gear shift of the transmission.

Description:
FIELD OF THE INVENTION 
     The present invention relates to transmissions and more particularly to a six-speed transmission incorporating a double-swap shift control scheme. 
     BACKGROUND OF THE INVENTION 
     Generally, conventional automatic transmissions include a torque converter to transfer engine torque from an engine to an input of the transmission, planetary gearsets that provide various gear ratios of torque and thus various drive speeds, and fluid pressure-operated, multi-plate drive or brake clutches and/or brake bands that are connected to the individual elements of the planetary gearsets in order to allow shifts between the various gear ratios 
     In addition, some conventional automatic transmissions include one-way clutches (i.e., overrunning clutches) that cooperate with the multi-plate clutches to optimize power shift control and include a transmission controller for selectively applying and releasing elements to shift the gears. For example, the controller chooses the proper gear depending on system conditions such as the shift-program selected by the driver (i.e., Drive, Reverse, Neutral, etc.), the accelerator position, the engine condition, and the vehicle speed. 
     As an accelerator is further depressed, and the vehicle increases speed, the controller disengages appropriate clutches to sequentially shift up through each of the gears until the highest gear is engaged. Specifically, the controller initiates a “single swap” event that releases an engaged clutch and applies an idle clutch such that a shift from a lower gear to a higher gear is accomplished. As can be appreciated, the application and release are preferably controlled and timed such that a driver does not notice or feel the gear shift. 
     Once the highest gear is engaged, further depression of the accelerator will cause the controller to operate another single swap event such that a lower gear is chosen, and a requisite torque is supplied by the transmission. In this manner, the controller will downshift through the gears, each time applying and releasing a single pair of clutches to perform the requisite gear shift. 
     Thus, conventional transmissions only use a single applying clutch and a single releasing clutch for each individual shift event. Conventional transmissions do not use a “double swap” event involving more than two clutches to achieve a desired gear ratio. Therefore, while conventional transmissions adequately accomplish gear shifts that meet driving conditions through use of “single swap” events, some conventional transmissions, depending on the gear set arrangements, suffer from the disadvantage of not being able to use desirable and available gear ratios, as the exchange of clutches required to achieve the desired ratio involves more than two clutches. The transmission controls, thus, do not use all available gear ratios and thereby limit the driveability, performance and fuel economy of the transmission. 
     Therefore, a transmission capable of performing a double swap, to provide a desired gear ratio, is desirable in the industry. Furthermore, a transmission that reduces the requisite number of clutches and gears through use of double swap operations is also desirable. 
     SUMMARY OF THE INVENTION 
     An automatic transmission for a vehicle, including an input shaft, an output shaft, a first plurality of gears, a second plurality of gears, and a series of engagement elements movable between an engaged position and a disengaged position is provided. The series of engagement elements includes at least one overrunning engagement element in mechanical communication with the second plurality of gears. A controller selectively applies and releases the series of engagement elements between the engaged position and the disengaged position to selectively drive through the first plurality of gears and the second plurality of gears to achieve a desired speed ratio between the input shaft and the output shaft. 
     During a double swap sequence, the controller performs a single-swap upshift in the first plurality of gears, thereby releasing a first engagement element and applying a second engagement element associated with the first plurality of gears to achieve a speed ratio change of the first plurality of gears. When the single-swap shift has progressed to the point that the speed change has started, the controller releases one of the engagement elements associated with the second plurality of gears to achieve a downshift in the second plurality of gears. The release of one of the engagement elements associated with the second plurality of gears is performed so that the speed change in the second plurality of gears begins within a range of 20 to 120 milliseconds after the start of speed change in the first plurality of gears. This sequence provides acceptable shift quality by ensuring that the output torque loss associated with the downshift of the second plurality of gears does not occur until after the output torque loss of the upshift of the first plurality of gears is finished. The output torque from the upshift begins to rise at the beginning of the speed change. Hence, this timing minimizes the total torque disturbance, because the rising torque of the upshift cancels some of the torque loss associated with the downshift. 
     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 schematic representation of a transmission in accordance with the principals of the present invention; 
         FIG. 2  is a table showing gear ratio combinations and shift sequences for the transmission of  FIG. 1 ; 
         FIG. 3  is a graphical representation of the pressure curves for a double-swap shift sequence; and 
         FIG. 4  is a graphical representation of an output torque and speed curves for the double-swap shift sequence of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     With reference to the figures, a transmission  10  is shown having a main gear set  12  disposed in a main box  13  of the transmission  10 , a compounder gear set  14  disposed in an underdrive assembly  15  of the transmission  10 , and a series of engagement elements  16 . The engagement elements  16  selectively engage respective gears of the main gear set  12  and compounder gear set  14  to provide the transmission  10  with an optimum gear ratio, as will be discussed further below. 
     With particular reference to  FIG. 1 , the transmission  10  is shown operably connected to a torque converter  18  and a differential  20 . The torque converter  18  is a fluid coupling between a power plant  22 , such as, but not limited to, a combustion engine and the transmission  10  and serves to transmit a rotational force from the power plant  22  to the transmission  10 . The rotational force received from the power plant  22  via torque converter  18  is then used to drive a combination of the main gears  12  and/or compounder gears  14  to provide a desired output of the transmission  10 . The output of the transmission  10  is received by the differential  20  for use in driving one or more wheels of a vehicle (not shown) at a desired acceleration and speed. 
     The transmission  10  further includes an input sensor  26 , an output sensor  28 , and a transfer sensor  30  that monitor operating conditions of the transmission  10 . The input sensor  26  monitors a rotational speed of an input shaft  32 , which is generally indicative of the rotational speed of an output of the torque converter  18 , while the output sensor  28  serves to monitor a rotational speed of an output shaft  34  of the transmission  10 . The transfer sensor  30  monitors a speed of rotation of an output of the main gears  12  for use in determining which of the compounder gears  14  to engage to optimize transmission output, as will be discussed further below. 
     Each of the sensors  26 ,  28 ,  30  are connected to a transmission controller  36  and provide the controller  36  with operating conditions of the transmission  10 . The transmission controller  36  uses the operating data in conjunction with vehicle data received from vehicle sensors  38  in an effort to determine an optimum gear ratio for the transmission  10 . Vehicle sensors  38  monitor vehicle speed and operator inputs, such as braking and accelerator pedal position. Selection of the optimum gear ratio provides the differential  20  with an appropriate input via output shaft  34 , and thus, enhances the performance of a vehicle to which the transmission  10  may be tied. While the vehicle sensors  38  are described as monitoring vehicle speed, braking, and accelerator pedal position, it should be understood that such parameters are exemplary in nature and are therefore not limited as such. Other vehicle operating parameters having bearing on transmission gear selection, such as braking, vehicle speed, and accelerator pedal position, are considered within the scope of the present teachings. 
     The controller  36  adjusts the engagement elements  16  to selectively apply different gears from the main and compounder sets  12 ,  14  to provide the transmission  10  with an optimum output. As will be described further below, the controller  36  compares current operating conditions of the transmission (i.e., data received from sensors  26 ,  28 ,  30 ) with current vehicle operating conditions (i.e., data received from vehicles sensors  38 ) to determine an optimum gear ratio, and thus, an optimum transmission output. 
     The main gear set  12  includes a first and second planetary gear sets  40 ,  42  while the compounder gear set  14  includes a third planetary gear set  44 , as best shown in  FIG. 1 . The planetary gears  40 ,  42 ,  44  provide the transmission  10  with seven different gears and a reverse gear. The engagement elements  16  include a series of individual clutches A–G and an additional “overrunning” clutch H disposed in the under drive assembly  15 , which are selectively engaged to provide the transmission  10  with a number of different gear ratios. Specifically, the controller  36 , based on current operating conditions of the vehicle and the transmission  10 , selectively applies respective clutches A–H to engage varying combinations of planetary gears  40 ,  42 ,  44  to provide a desired output gear ratio of the transmission  10 . 
     As best shown in  FIG. 2 , clutches F and H are seemingly applied at the same time to achieve respective gears. However, it should be understood that clutch H is a so-called “overrunning” clutch and, is therefore, only engaged (i.e., carries torque) when the transmission  10  experiences a positive torque. Conversely, when the transmission  10  experiences negative torque, clutch F is overrunning (i.e., disengaged). Therefore, for positive torque shifts, clutch F is applied/released and is not involved in the torque exchange during positive-torque shifts. 
     With reference to  FIGS. 2–4 , the operation of the transmission  10  will be described in detail. When the vehicle is at idle, the torque converter  18  freely spins without transmitting a rotational force to the transmission  10  from the power plant  22  (i.e., in a braked or neutral condition). However, once enabled and in a drive mode, a user depresses an accelerator (not shown), and the vehicle sensor  38  sends a signal indicative thereof to the transmission controller  36 . As shown in  FIG. 2 , the controller  36  engages clutch A, E, and F so that the lowest, or first gear combination  1 , is selected (each selection represented by an “X” in  FIG. 2 ). It should be noted that while clutch H is not engaged for gear combination  1 , that clutch H is always available to carry torque. The lowest gear  1  includes the highest gear ratio (i.e., 3.921), and thus, provides the vehicle with the most torque. As can be appreciated, a higher torque value is desirable in that it provides the vehicle with the greatest acceleration from a rest position. 
     Once a predetermined speed is achieved, the controller  36  will engage clutch G with clutch H releasing automatically as the torque applied by the gearset drops to zero and becomes negative (the controller  36  will also release clutch F, but it&#39;s torque is zero since clutch H is carrying all of the torque). After the speed change is complete, and clutch G is fully engaged, the transmission  10  has shifted sequentially from first gear  1  to second gear  2  and has shifted to a lower gear ratio (i.e., 3.921 to 2.699). The second gear  2  includes a lower gear ratio, and thus, provides less torque to the output shaft  34 . However, it should be noted that while maximum torque is sacrificed, the overall efficiency is improved, as engine speed is reduced. The reduction in engine speed provides an increase in efficiency by reducing pumping losses in the power plant  22 . 
     The shift from gear  1  to gear  2  is accomplished by a “single swap” shift such that the gear ratio of the transmission  10  is changed by swapping clutch H, associated with the compounder gear set  14 , for clutch G also associated with the compounder gear set  14 .  FIG. 2  clearly shows that clutches F and H are released or overrunning, and clutch G is engaged, thereby indicating a single swap. Therefore, the shift from gear  1  to gear two  2  is accomplished entirely within the under drive assembly  15  and is a single swap shift. 
     When the vehicle increases speed, the controller  36  initiates a shift from lower gear  2  to a higher gear  3 , thereby changing the gear ratio from 2.699 to 2.169, as best shown in  FIG. 3 . The shift between gears  2  and  3  is accomplished by a “double swap” shift, meaning that two clutches are released and two different clutches are applied. In this case, two single-swap shifts occur at the same time and make up the “double-swap” shift. First, a 1.8 ratio step upshift is being made in the main gear set  12  while a 1.45 ratio step downshift occurs in the compounder gear set  14 . The combination between the 1.8 ratio step upshift and the 1.45 ratio step downshift combine to provide a 1.24 ratio step  2 – 3  upshift and achieve the third gear ratio of 2.169. 
     In making the shift from gear  2  to gear  3 , the controller  36  disengages clutch E and applies clutch D in the main box  13  and subsequently disengages clutch G and allows the torque to be carried by clutch H in the under drive assembly  15  (clutch F is applied after the shift is complete). The double swap shift only yields an acceptable shift if the shift in the main box  13  is timed correctly with the shift in the underdrive assembly  15 , as will be discussed further below. 
     The main box shift is initiated by the controller  36  in response to vehicle conditions, as read by vehicle sensors  38  and transmission speed sensors  26 ,  28 , and  30 . Once the controller  36  indicates that an upshift is required (i.e., from gear  2  to gear  3 ), the fluid pressure applied to clutch D is increased while the fluid pressure applied to clutch E is decreased, as best shown in  FIG. 3 . In addition, the fluid pressure applied to clutch G is also reduced to thereby reduce the pressure to a predetermined pressure. The drop in applied pressure eventually disengages clutch E such that clutch E no longer couples gear set  42  to the input and output shafts  32 ,  34  of the transmission  10 . Conversely, the increased pressure applied to clutch D eventually fully applies clutch D such that gear set  40  is coupled to the input and output shafts  32 ,  34  of the transmission  10 . 
     The release of clutch E and the engagement of clutch D is timed such that the exchange between clutch E and clutch D is slightly overlapped. In general, the releasing element (i.e., clutch E) will maintain some excess capacity until the applying element (i.e., clutch D) has enough capacity to hold engine torque. Once the applying element has enough capacity to hold the engine torque, the releasing element (i.e., clutch E) is disengaged. 
       FIG. 3  is a graphical representation of the aforementioned power shift from clutch E to clutch D, indicating the respective fluid pressure applied to each clutch E, D. From the plot, it can be seen that clutch E maintains engagement with gear set  42  until a sufficient fill volume is experienced by clutch D. If the controller  36  determines that the swap between clutches E and D is not properly timed (i.e., where a sufficient fill volume is not accurate for clutch D) one of two scenarios is possible. 
     In a first scenario, clutch D does not have enough capacity when clutch E has lost its capacity. In this situation, the controller  36  slightly increases the pressure of the releasing clutch E to maintain engagement with gear set  42 , as indicated by a spike Z in  FIG. 3 . The slight increase in pressure (Z) is maintained by the controller  36  until clutch D experiences sufficient capacity to prevent slip and maintain engagement gear set  40 . The spike Z is released by the controller  36  once there is sufficient capacity exerted on clutch D. Clutch D is being engaged while clutch E is being released to ensure a proper torque swap of the main box  13 . In a second scenario, clutch D has capacity while E still has capacity, thereby resulting in an overlap condition. In this situation, the volume of clutch D is modified to match the torque transfer on a subsequent shift. 
     To complete the shift from gear  2  to gear  3 , the under drive assembly  15  must also apply and release a set of clutches. Specifically, clutches F or H must take up the torque and clutch G must be released, as indicated in  FIG. 2 . The timing of the release of clutch G must be within a predetermined time after the main box  13  slips to ensure a proper output torque transition for the transmission  10 , as will be described further below. 
     Once the main box  13  slips (point X in  FIG. 3 ), the controller  36  will release the pressure applied to clutch G such that clutch G begins to slip at point Y of  FIG. 3 . As previously discussed, the output torque begins to rise, because main box  13  is beginning the speed change of the upshift. The under drive assembly  15  must slip within a predetermined time after the main box  13  slips to ensure that the downshift torque loss occurs at the same time as the upshift torque rise, thereby minimizing the overall torque disturbance of the 2–3 upshift of transmission  10 . To ensure a proper output torque, the time to slip between the main box  13  and the under drive assembly  15  should be within 20 to 120 milliseconds, and preferably between 40 and 70 milliseconds. 
     The time interval between the slipping of the main box  13  and the slipping of the under drive assembly  15  is generally given as the distance between lines X and Y of  FIG. 3 . If the under drive assembly  15  slips outside of the 20 to 120 millisecond window, the rate of change of the output torque of the transmission  10  will increase and the shift quality between gears  2  and  3  will deteriorate. 
     The increase in the rate of change of the output torque is shown in  FIG. 4  between lines X and Y. In addition, speed plots for an acceptable speed change (i.e., one falling between points X and Y) are provided indicating torque converter  18 , power plant  22  (i.e., engine), under drive assembly  15 , and transmission output speed. 
     If the distance between points X and Y in  FIG. 3  is less than 20 milliseconds the output torque curve takes a shape similar to A. If that distance is greater than 120 milliseconds it takes a shape similar to B. While a window between 20 and 120 milliseconds generally results in an acceptable shift, the distance between lines X and Y is preferably between 40 and 70 milliseconds, In addition, the clutch pressures of applying clutch D and releasing clutch G must be controlled to minimize the overall output torque disturbance. 
     As best shown in  FIG. 3 , the pressure applied to clutch G is released until clutch G slips. When slip occurs, the controller  36  increases the pressure on clutch G, to minimize the downshift speed change, torque loss, and uses engine torque management to minimize the output torque rise as shown in  FIG. 3  at point W which naturally occur at the end of a power downshift. The controller  36  uses open loop control to complete the full release of clutch G within the time in which main box  13  completes its speed change. 
     The increase in pressure on clutch G when it slips is a function of flow which minimizes the change in clutch pressure and the loss in torque during the downshift. The solenoid duty cycle controlling clutch G is chosen to result in zero flow. Just before the under drive assembly  15  reaches target speed, the pressure applied to clutch G will be decreased using open loop control. If the target speed hasn&#39;t been achieved within a 100 milliseconds, the duty cycle control will also enter open loop control. 
     The hold pressure initial duty cycle applied to control clutch G) is a function of oil temperature and input torque and may therefore be tailored to fit the particular system. To optimize slip time of the under drive assembly  15  with respect to the main box  13 , the oil temperature and input torque are monitored to adaptively correct the initial hold pressure (duty cycle) so slip is achieved in the desired 40 to 70 millisecond window. The initial hold pressure (duty cycle) is predicted from an adaptive surface such that the duty cycle required for the hold pressure is a function of oil temperature and input torque. The initial solenoid duty cycle used for the hold pressure is updated at the end of the shift, depending on the X-Y window. If the window is more than desired, the solenoid duty cycle is lowered and vice versa. The duty cycle is generally defined as a percentage a hydraulic fluid valve is open over a given time. Therefore, if slip is falls outside of the 20 to 120 millisecond window, the duty cycle can be increased or decreased to bring the slip within the desired range. 
     For example, if the slip time overshoots the 120 millisecond threshold, the duty cycle can be increased so as to supply more fluid to the applying clutch. In this manner, the applying clutch will slip sooner as hydraulic fluid is applied in a greater volume over a shorter period of time. Conversely, if the time to slip falls short of the 20 millisecond threshold, the duty cycle can be reduced such that less fluid is applied to the applying clutch over a longer period of time. In this manner, the reduction in duty cycle causes the time to slip to be extended and fall within the desired 20 to 120 millisecond window. 
     In addition to monitoring oil temperature, the duty cycle (i.e., the rate at which fluid is applied to clutch H) may also be turned off for approximately the first 100 milliseconds following the instruction to shift gears by the controller  36 . Toggling the duty cycle off for the first 100 milliseconds of a shift sequence will cause the pressure in clutch H to drop in a direction Q, as best shown in  FIG. 3 . Such manipulation of the duty cycle is especially important in cases of low input torque. Under such conditions, the time required to achieve speed change with clutch D in the main box might not be enough to reach the required level. As previously discussed, the hold pressure must be at a point that will allow the under drive assembly  15  to slip within 40 to 70 milliseconds after the main box  13  slips. Therefore, if the pressure applied to clutch H is too high initially, the under drive assembly  15  will not slip within the requisite time frame, and therefore the output torque and shift quality will be adversely affected. In the exemplary embodiment of the present invention, turning off the duty cycle for the initial period of time happens when a swap shift is performed. 
     In addition to the foregoing, the torque input to the system via power plant  22 , torque converter  18 , and input shaft  32 , may also be adjusted using torque management to improve shift quality. As can be appreciated, a lower input torque during the X-Y window will minimize the rate of increase on output torque. Conversely, a higher input torque will require an increase in pressure applied to clutch G to minimize the output torque rate of increase. 
     Once the gear change is complete, clutches A and D of the main box  13  are engaged, over running clutch H is carrying the torque of the under drive assembly  15  and the gear ratio of the transmission  10  has moved from 2.699 to 2.169 as best shown in  FIG. 2 . 
     The controller  36  will sequentially move through each of the remaining gears  3 – 6  by selectively engaging and releasing clutches A–H until the sixth gear  6  is achieved, as best shown in  FIGS. 1–2 . The sixth gear  6  is achieved when clutches B, D, and G are engaged and provides the transmission  10  with the lowest torque and the lowest gear ratio (i.e., 0.655). Again, the sixth gear  6  is the highest gear and is engaged when the vehicle is moving at a relatively high speed. Therefore, even though the sixth gear  6  includes a low torque value, a high torque value is not required to propel the vehicle because the vehicle is already in motion, as previously discussed. In this manner, the reduced torque value improves efficiency by choosing the highest gear with the lowest ratio which provides the lowest engine speed and best fuel economy. 
     At this point, the controller  36  has selectively engaged clutches A–H to sequentially move through each of the first six gears  1 – 6  until the sixth gear  6  with a gear ratio of 0.655 is selected, as best shown in  FIG. 2 . At this point, if acceleration is required, the vehicle sensor  38  will send a signal to the controller  36  to downshift the transmission  10 . 
     During the downshift operation, the controller  36  compares vehicle operating conditions to current transmission operating conditions and selects an optimum lower gear to accommodate the requisite acceleration, and will once again repeat the sequential shift sequences, employing the double swap operation to shift from gear  2  to gear  3  and the single swap operation to shift between each of the other gears. 
     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.