Abstract:
A method for controlling ratio changes in a swap-shift transmission having a first simple planetary gear unit and a second compound planetary gear unit, the two gear units being arranged in series disposition, pressure-operated friction elements for separately braking the reaction member at each of the gear units to establish a change in ratio. The change in ratio consists of a downshift by releasing a reaction friction element for the first gear unit and upshifting the second gear unit by applying a second friction element to anchor a reaction member of the second gear unit in a non-synchronous fashion to achieve a consistent shift quality without precise matching of friction element application and release whereby inertia torque disturbances in the torque output elements of the transmission are avoided.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to a control method for establishing smooth ratio changes in a multiple-ratio geared transmission for an automotive vehicle wherein the gearing elements of the transmission are characterized by a swap-shift. 
     2. Background Art 
     A conventional multiple-ratio automatic transmission for automotive vehicles includes multiple-ratio gearing having gear elements that are controlled by pressure-operated friction clutches and brakes, which are applied and released to effect ratio changes. The multiple-ratio gearing may comprise two planetary gear units, which define plural torque flow paths arranged in series disposition. One such transmission is disclosed in U.S. Pat. No. 5,835,875, which is assigned to the assignee of the present invention. 
     The transmission of the &#39;875 patent comprises two planetary gear units that can be upshifted or downshifted independently, one with respect to the other, to effect gear ratio changes. The upshifting and the downshifting of the planetary gear units occur simultaneously to accomplish an overall gear ratio upshift or downshift. This involves downshifting the first planetary gear unit by releasing an overdrive brake for a reaction element of the first planetary gear unit while simultaneously upshifting the second planetary gear unit, which has multiple ratios, by applying a brake for a reaction element of the multiple-ratio planetary gear unit. This is commonly referred to as a swap-shift. 
     In the control system for the &#39;875 patent, the release of the reaction brake for the first planetary gear unit and the application of the reaction brake for the second planetary gear unit must be accomplished synchronously. Even if a small error in the synchronization is present, the shift quality may be accompanied by a large shift shock due to inertia torque disturbances. 
     Swap-shift transmissions of the kind shown in the &#39;875 patent require precise matching of friction element application and release in order to maintain consistent shift quality. This objective is difficult to achieve, however, under all operating conditions over the life of the automotive vehicle powertrain. This is due in part to the typical changes in the braking characteristics of the friction elements as well as operating variables in the control system for the friction elements. Manufacturing tolerances for the powertrain hardware also contribute to inconsistent shift quality, as does limited data communication speed for electronic digital microcomputer components typically used for controlling the vehicle powertrain. 
     Swap-shift transmissions of the kind disclosed in the &#39;875 patent rely upon precise control of the friction elements to synchronize the behavior of two separate planetary gear units to obtain consistent shift quality. It requires an overdrive friction brake capacity reduction for the first gear unit and an intermediate ratio friction brake application for the second gear unit. The application and release of the brakes are initiated simultaneously at the beginning of the shift interval. The overdrive brake drum speed increases while the intermediate ratio brake drum speed decreases. When the intermediate brake completes its engagement, the overdrive brake capacity is synchronously brought to zero. A small output torque peak may occur as a result of the inertia torque developed during the torque transfer from the overdrive brake to an overrunning coupling in the overdrive gear unit. Shift quality becomes very sensitive to a small mismatch in timing of the overdrive brake release and the intermediate brake engagement. 
     In a conventional swap-shift transmission, an early release of the overdrive brake causes a sudden and large torque transfer from the overdrive brake to the overrunning coupling while the intermediate brake is still rotating. This leads to a sudden increase of input torque into the second planetary unit. As a result, intermediate brake torque capacity is substantially increased before the intermediate brake drum motion completely stops in order to complete its engagement in time. A combination of a torque transfer from the overdrive brake to the overrunning coupling and an increase in intermediate brake torque capacity results in a large output torque spike if brake application and release are not precisely timed. 
     A large output torque spike is perceived by the driver as a shift shock. Further, if the intermediate brake completes its engagement before the overdrive brake is fully released, a low torque capacity developed at the overdrive brake results in a sharp output torque decrease. This also is perceived by the driver as a shift shock. 
     SUMMARY OF INVENTION 
     The invention is a robust control method comprising a strategy to achieve consistent shift quality in a swap-shift automatic transmission of the kind disclosed in the &#39;875 patent. Unlike prior art control systems, the strategy of the present invention does not require precise matching of the friction element application and friction element release timing to avoid a perceptible shift shock. 
     The method of the invention comprises measuring the carrier speed of a first planetary gear unit, which includes an overrunning coupling for effecting a direct-drive state and a pressure-actuated reaction brake for effecting an overdrive state. The speed of the carrier of a second planetary gear unit and the speed of the sun gear for the second planetary gear unit also are measured. The overrunning speed of the overrunning coupling is controlled during a shift interval so that the reaction torque on the sun gear of the first gear unit becomes zero in a first time increment. 
     The speed of the sun gear of the second gear unit also is measured during a second time increment so that a second pressure-actuated reaction brake for the second gear unit becomes fully applied before the first reaction brake is fully released. 
     The overrunning coupling speed control provides partial torque transfer from the reaction brake for the first gear unit to the overrunning coupling before the reaction brake for the second gear unit reaches a low target speed approaching zero speed. 
     The control routine for the overrunning speed of the overrunning coupling occurs simultaneously with the control routine of the sun gear speed for the second gear unit. Each control routine is achieved in a closed loop fashion using actuating pressure as a variable in the control of the pressure-actuated reaction brake for the respective gear units. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic representation of a swap-shift transmission having a first simple planetary gear unit and a second compound planetary gear unit arranged for second speed ratio operation in a torque flow path extending from an engine to a torque output shaft; 
     FIG. 1 a  is a schematic representation of the transmission of FIG. 1 when it is adapted for third speed ratio operation; 
     FIG. 1 b  is a chart showing the condition of the friction elements and the overrunning couplings for each of the two ratio steps of FIGS. 1 and 1 a;    
     FIG. 1 c  is an overall system block diagram of a vehicle powertrain including an engine, a transmission and a digital microcomputer powertrain controller for effecting application and release of pressure actuators for friction elements of the transmission; 
     FIG. 2 is a plot of the brake and coupling capacity and gear element speeds during a synchronized shift for a conventional swap-shift transmission of the kind shown in the &#39;875 patent; 
     FIG. 3 is a plot corresponding to FIG. 2 which shows the brake capacity and gear element speeds during a ratio upshift of a conventional transmission of the kind shown in the &#39;875 patent when the overdrive brake is released early rather than synchronously as in the case of FIG. 2; 
     FIG. 4 is a plot corresponding to FIGS. 2 and 3 wherein the overdrive brake is released late relative to the synchronized release of the overdrive brake of FIG. 2; 
     FIG. 5 is a plot of the characteristics shown in FIGS. 2,  3  and  4 , but which demonstrate the timing of the brake application and release and the corresponding gear element speeds during a shift interval at low throttle for a transmission controlled in accordance with the teachings of the present invention; 
     FIG. 6 is a plot corresponding to FIG. 5 when the engine is at medium throttle; 
     FIG. 7 a  is a plot showing the same characteristics as the plot of FIG. 5 when the overdrive brake capacity is lower, as evidenced by the increased slope of the overdrive drum speed plot; 
     FIG. 7 b  is a plot similar to FIG. 5 when the overdrive brake capacity is increased; 
     FIG. 7 c  is a plot similar to FIG. 7 b  when the overdrive brake release timing relative to intermediate brake release is at 50 radians/sec.; 
     FIG. 7 d  is a plot similar to FIG. 7 b  when the final overdrive brake release interval is 100 ms; 
     FIG. 7 e  is a plot similar to FIG. 7 b  when the final overdrive brake release time is reduced to 25 ms; 
     FIG. 8 is a flowchart illustrating the control strategy employed in practicing the invention; and 
     FIGS. 9 a ,  9   b ,  9   c  and  9   d  are plots illustrating the timing of overdrive brake servo pressure, the overrunning coupling speed for the first gear unit, the intermediate brake servo pressure, and the reaction gear speed, respectively, for the swap-shift transmission controls of the present invention. 
    
    
     DETAILED DESCRIPTION 
     1. General Description of Powertrain 
     FIG. 1 shows a swap-shift transmission of the kind shown in the previously described &#39;875 patent when the gearing elements are conditioned for second speed ratio operation. FIG. 1 a  shows the transmission of FIG. 1 when it is adapted for third speed ratio operation. The transmission forms a part of the powertrain system illustrated in FIG. 1 c.    
     In FIG. 1 c , the transmission is schematically shown at  10 . The engine is shown at  12 . Engine crankshaft torque is distributed through the impeller of a torque converter located in converter housing  14 , which forms a part of the transmission case  16 . An integrated engine and transmission electronic powertrain control comprises digital microcomputer  18  having memory registers for transmission control strategy and engine control strategy. The engine supplies engine speed input signals at  20 , manifold pressure signals at  22 , and engine coolant temperature signals at  24 , which are received by input signal conditioning ports of the microcomputer  18 . Driver selected signals illustrated at control block  26  include a throttle position signal  28 , a brake on-off signal  30 , a park/neutral mode signal  32 , an overdrive cancel switch signal  34 , and a manual lever position selection signal  36 . Both sets of signals are distributed to input signal conditioning ports for the microcomputer  18 . 
     The microcomputer  18  executes control strategy that results in a pulse width modulated pressure signal  38  for circuit pressure regulator  46 . Various variable-force solenoid signals  40  as well as shift solenoid signals  44  may be transferred to hydraulic control unit  42  for controlling shift valves and to effect pressure modification to achieve shift timing. The characteristics of the hydraulic control unit in the control of the transmission  10  are described in the previously mentioned &#39;875 patent. The disclosure of that patent is incorporated herein by reference. 
     A high clutch drum speed signal is distributed to the microcomputer  18 , as shown at  48 . A turbine speed sensor signal is distributed from the transmission  10  through signal flow path  50 . A manual lever position signal indicating actual manual lever position selected by the driver is distributed to the microcomputer  18  through signal flow path  52 . A transmission oil temperature signal is distributed to the microcomputer  18  through signal flow path  54 . An output shaft speed signal is distributed from sensor VSS to the microcomputer through signal flow path  56 . A vehicle speed sensor signal is distributed to the microcomputer  18  through signal flow path  58 . 
     The input signals for the transmission  10  control transmission actuators, which are illustrated in block diagram form in FIG. 1 c  at  60 . 
     In FIG. 1, the transmission case  16  encloses the hydrokinetic torque converter  62  and two planetary gear units  64  and  66 . Gear unit  64  is a simple planetary overdrive gear unit, and gear unit  66  is a compound planetary gear unit commonly referred to as a Simpson gearset. 
     Impeller (or pump)  68  is connected at  72  to the crankshaft of engine  12 . A torque converter lockup clutch  74  selectively connects the impeller  68  to turbine  70 . Turbine torque is distributed to the carrier  78  of gear unit  64  through a turbine shaft  80 . Sun gear  82  of gear unit  64  is connected to overdrive brake drum  84 , which is surrounded by selectively engageable, pressure-actuated brake  86 . A coast clutch  88  connects the carrier to the brake drum  84  when coast braking is desired. 
     Ring gear  90  is connected through friction clutch  92  to ring gear  94  of planetary gear unit  66  during fourth and fifth ratio operation. For purposes of the present dynamic analysis, however, this is not relevant to a swap-shift. 
     An overrunning coupling  96  (DOWC) connects the carrier  78  when the gear unit  64  is in a direct-drive mode. When the brake  86  anchors brake drum  84  and the sun gear  82 , the overrunning coupling  96  freewheels as the gear unit  64  enters an overrunning mode. The carrier  98  for gear unit  66  is connected directly to torque output shaft  98 . 
     The sun gear  100  for gear unit  66  is common to each of two gear units of the planetary gear unit  66 . The ring gear for the first gearset, as previously described, is identified by reference numeral  94 . The ring gear for the second gearset is identified by reference numeral  102 . The carrier for the second gearset, shown at  104 , is connected to low-and-reverse brake drum  106 , which is surrounded by band brake  108 . Brake  108  is applied during reverse drive operation and during manual low operation. It is capable of accommodating torque in the reverse direction as well as coasting torque during coasting operation in the low ratio. 
     During forward-drive operation in the lowest ratio, brake drum  106  is anchored by overrunning coupling (clutch)  110 . 
     The transmission case and the engine  12  are mounted on the vehicle chassis, as shown at  112 , through an engine mount system  114 . 
     The torque output shaft  98  is connected to the torque input gear of a final drive gear assembly  116 , which in turn distributes torque to a differential-and-axle assembly, generally indicated at  118 . The left traction wheel and the right traction wheel receive torque from the differential side gears through axle half shafts as shown. 
     The common sun gear  100  is connected to intermediate brake drum  120 , which is surrounded by intermediate brake  122 . 
     The schematic diagram of FIG. 1 shows a torque flow path for the transmission when the transmission is in condition for second speed ratio operation. The elements that transmit torque are illustrated by heavy lines. The elements that do not transmit torque are illustrated by light lines. 
     FIG. 1 b  indicates the brakes and clutches that are engaged or released to establish the second forward-driving ratio. When the transmission is in condition for second speed ratio operation, the overdrive brake  86  is applied, the overrunning coupling  96  (DOWC) is overrunning, the intermediate brake  122  (IM BAND) is released, and the low overrunning coupling  110  (LOWC) is engaged. Turbine torque is distributed to the carrier  78  of the first planetary gear unit  64 . 
     During second ratio operation, the gear unit  64  is upshifted since brake  86  is applied. Torque of ring gear  90  is distributed through the forward-drive clutch  92  to the ring gear  94 . 
     The forward-drive clutch  92  is continuously engaged during operation in each of the forward-driving ratios. For purposes of the present dynamic analysis dealing with the swap-shift strategy of the present invention, it can be assumed that there is a direct connection between ring gear  94  and ring gear  90 . 
     During operation in the first ratio, reaction torque for the second gear unit  66 , a Simpson planetary gear unit is accommodated by the overrunning coupling  110 . When the transmission is in condition for second speed ratio operation, the brake  86  is applied and brake  122  continues to be released. This causes the gear unit  64  to shift to an overdrive state as the reaction torque for the second gear unit continues to be accommodated by the overrunning coupling  110 . 
     A shift to the third ratio is obtained by releasing brake  86  and applying brake  122 . The planetary gear unit  64  thus is downshifted to a direct-drive ratio as the second planetary gear unit is upshifted with the sun gear  100  acting as a reaction element since it is anchored by the brake  122 . 
     Fourth ratio operation is achieved by engaging clutch  93 , thereby causing the second planetary gear unit to assume a one-to-one drive ratio mode as torque is transmitted through the overrunning coupling  96  (DOWC) in the first gear unit. 
     Fifth ratio operation is achieved when clutch  93  is engaged, which causes the second gear unit to assume a one-to-one drive ratio as the first planetary gear unit assumes an overrunning state as the sun gear  82  is anchored by brake  86 . 
     A ratio change from a second ratio to the third ratio is a swap-shift. As indicated in FIG. 1 b , this is accomplished by releasing brake  86  causing the overrunning coupling  96  to be engaged. Further, the intermediate brake  122  becomes applied as the overrunning coupling  110  (LOWC) begins to overrun. 
     FIG. 1 a  illustrates the torque delivery elements of FIG. 1 when the transmission is conditioned for third ratio operation. As in the case of FIG. 1, the elements that deliver torque are illustrated in heavy lines. The elements that do not deliver torque are illustrated in light lines. The first planetary gear unit  64  is downshifted in the case of FIG. 1 a  by releasing overdrive brake  86  while the second planetary gear unit  66  is operating in its intermediate ratio by applying intermediate speed ratio brake  122 . The overrunning coupling  96  now delivers torque as the first gear unit  64  assumes a one-to-one driving ratio. Further, sun gear  66  becomes a reaction element. As turbine torque is delivered through the first planetary gear unit to the ring gear  94 , carrier  98  becomes a torque output element as the overrunning coupling  110  freewheels. 
     2. General Description of Conventional Swap-Shift Controls 
     For the purpose of clarifying the differences between applicants&#39; invention and a conventional swap-shift control system, the plots of FIGS. 2,  3  and  4  first will be described. The performance demonstrated by FIGS. 2,  3  and  4  then will be compared to the performance of a swap-shift control system embodying the strategy of the invention. 
     In the plot of FIG. 2, the characteristics of a conventional swap-shift from the second ratio to the third ratio at low engine throttle is indicated. FIG. 2 represents speed, torque and brake capacity variations during a shift interval, shift interval time being plotted along the abscissa. Engine speed, shown at  130 , remains relatively uniform throughout the shift interval. In the example plotted in FIG. 2, the engine speed is approximately 2500 rpm. At the beginning of a shift interval, the overdrive brake capacity begins to decrease at point  132  and the intermediate brake becomes applied at point  134 . The intermediate drum speed then falls, as shown at  136 , as the overdrive brake capacity falls to a lower value as shown at  138 , where the overdrive brake drum begins to slip. The release of the overdrive brake and the application of the intermediate brake occur simultaneously. In the example of FIG. 2, this occurrence is at 4.0 seconds, which is also point  134  for the intermediate brake. 
     When the overdrive brake capacity decreases to the value shown at  138 , the overdrive drum speed increases, as shown at  140 . This occurs simultaneously with a decrease in the intermediate brake drum speed, as shown at  136 . In the example shown in FIG. 2, the intermediate brake completes its engagement at 4.3 seconds, as shown at  142 . Simultaneously, the overdrive brake capacity drops to zero, as shown at  144 . 
     Although the zero points for the intermediate brake drum speed and the overdrive brake capacity nearly coincide, a small undesirable, but acceptable, output torque peak is developed at  146 , which is the result of inertia torque disturbance due to the transfer of torque from the overdrive brake to the coupling DOWC. This torque transfer is shown at  148 . 
     As the intermediate brake capacity rises at the beginning of the shift at 4.0 seconds, as shown at  150 , the torque on the overrunning coupling LOWC decreases as shown at  152 . At the completion of the shift at approximately 4.3 seconds, the intermediate brake capacity rises to its maximum level, as shown at  154 , as the torque transfer to the overrunning coupling DOWC is completed. 
     If there is a mismatch in the overdrive brake release and the intermediate brake engagement timings, the shift quality deteriorates substantially, as shown in FIGS. 3 and 4. In the case of FIG. 3, the overdrive brake is released early, and in the case of FIG. 4, the overdrive brake is released late. 
     FIG. 3 shows that the final release of the overdrive brake is commanded at  156  before the intermediate brake engagement is complete at  158 . As a result of this mismatch, an input torque to the intermediate brake drum increases, which changes the intermediate brake drum dynamics. The intermediate brake torque capacity is increased to a holding level as shown at  160  just before the intermediate brake drum motion completely stops at  162 . This combination of torque transfer from the overdrive brake to the overrunning coupling DOWC, as shown at  164 , and an increased intermediate brake torque capacity at  160  results in a large output shaft torque spike, as shown at  166 . This causes a perceptible shift shock. Unlike the torque peak at  146  in FIG. 2, the torque peak at  166  in FIG. 3 is not acceptable. 
     In FIG. 4, the intermediate brake completes its engagement at  168  before the overdrive brake is released at  170 . The torque transmitted through the overdrive brake between 4.34 seconds and 4.39 seconds results in an output torque hole, as shown at  172 . This torque hole or depression in the output torque plot is perceived by the vehicle operator as a shift shock. 
     It is apparent from FIGS. 3 and 4 that slight deviations in the overdrive brake release timing and the intermediate brake drum apply timing cause sharp variations in the output shaft torque because of the high sensitivity in the timing of the friction engagements and release. Consistent shift quality is difficult to achieve with a conventional strategy. A minor variation in the completion of the application of the intermediate brake drum and the release of the overdrive brake, approximately 30 ms in the case of FIGS. 3 and 4, will cause a substantial inertia effect in the output shaft torque. 
     3. Description of Strategy of Invention 
     FIGS. 5,  6  and  7   a - 7   d  illustrate an improvement in the shift performance compared to the performance indicated in FIGS. 2,  3  and  4 . The performance indicated in FIGS. 5,  6  and  7   a - 7   d  is the result of a robust control that achieves consistent shift quality. Unlike the performance indicated in FIGS. 2,  3  and  4 , the strategy of the present invention does not require precise matching of the friction element application and release to avoid a perceptible shift shock. 
     FIG. 5 illustrates the performance resulting from the strategy of the present invention when the engine is at low throttle setting. The overdrive brake capacity decreases at the beginning of the overdrive brake release, as shown at  174  in FIG.  5 . This follows a completion of the torque phase of the intermediate brake engagement, which is shown at  176 , where the overrunning coupling torque for LOWC decreases, as shown at  178 . The decrease in the overdrive brake capacity begins at  180  after the torque phase  176  is ended. Precise control of the overdrive brake release is not critical since it is the intermediate brake that has the primary effect on the output torque because of its proximity of the output shaft. 
     The strategy evaluates the overrunning speed of the overrunning coupling DOWC or the overdrive brake drum speed at  182  and adjusts the overdrive torque capacity shown at  184  so that the overrunning speed of the coupling DOWC approaches zero before the intermediate brake completes its engagement at  186 . The timing when the overdrive brake drum achieves its peak corresponds to zero overrunning speed of the clutch DOWC. A partial torque transfer will occur from the overdrive brake to the overrunning coupling DOWC, as shown at  188 , when the overrunning speed of coupling DOWC becomes zero. Then, when the intermediate brake drum speed reaches a small target value (for example,  30  radians/sec.) as shown at  190 , the strategy commands a final reduction of overdrive brake torque capacity over an extended period of about 150 ms, as shown at  192 . This period for the overdrive brake capacity reduction should be long enough to extend beyond the point  186 , where the intermediate brake engagement is complete. It is possible to monitor the intermediate drum speed to ensure that point  186  occurs prior to completion of the overdrive brake capacity reduction, as shown at  194 . 
     The output shaft torque remains relatively stable, as shown at  196 , in comparison to the output shaft torque indicated in FIGS. 3 and 4. This stability in the output shaft torque is achieved, as previously described, without precisely matching the overdrive brake release and the intermediate brake engagement timings. 
     FIG. 6 is a plot similar to the plot of FIG. 5, although the data indicated in FIG. 6 represents a medium engine throttle shift rather than a low engine throttle shift. The intermediate brake drum speed in FIG. 6 reaches zero, as shown at  210 , in advance of the zero point for the overdrive brake capacity, as shown at  212 . This is due to the extended overdrive brake capacity reduction timing, as shown at  214 . It is due also to the initiation of partial torque transfer from the overdrive brake to overrunning coupling DOWC, as the overrunning coupling DOWC ceases to overrun, as shown at  216 . 
     FIGS. 7 a - 7   e  show system shift simulations at low engine throttle. These figures demonstrate the robustness of the strategy of the present invention with respect to variations in overdrive brake torque capacity level, the final release timing and the release interval. In the case of FIG. 7 a , it is assumed, for purposes of illustration, that after the initial release of the overdrive brake at  198 , the torque capacity level at  200  unexpectedly drops lower than the corresponding brake capacity level shown in FIG.  5 . As a result, the overrunning speed of the overrunning coupling DOWC reaches zero and starts transmitting torque at an earlier time, as shown at  202 . When the intermediate brake drum speed reaches 30 radians/sec. at point  204 , a final release of the overdrive brake takes place over a period of 150 ms, as shown at  206 . The strategy will result in an absence of significant torque disturbance at the output shaft, as shown at  208 . 
     In the case of FIG. 7 b , after the initial release of the overdrive brake at  198 ′, its torque capacity level will be assumed to stay unexpectedly higher than the corresponding level shown in FIG.  5 . As a result, the overrunning speed of the overrunning coupling DOWC reaches zero and starts transmitting torque at a later time, as shown at  202 ′. When the intermediate brake drum speed reaches 30 radians/sec. at  204 ′, shortly after the overrunning coupling DOWC starts transmitting torque at  202 ′, a final release of the overdrive brake takes place over an extended period (150 ms, as shown  206 ′). The strategy of the present invention does not lead to any significant torque disturbance at the output shaft, as evidenced by the output torque shaft plot. 
     In FIG. 7 c , the shift simulation is similar to that shown in FIG. 7 b  except that the final phase of the overdrive brake release takes place when the intermediate drum speed reaches 50 radians per second, as shown at  204 ″. However, the output torque profile is still very similar to that shown in FIG. 7 b . Again, there are no significant output torque disturbances comparable to the torque peak  166  in FIG. 3, nor to the so-called torque valley shown at  172  in FIG.  4 . 
     The shift simulation shown in FIG. 7 d  is similar to that shown in FIG. 7 b  except that the final release of the overdrive brake takes place over 100 ms, as shown at  206 ′″ instead of 150 ms, as shown at  206 ′. Notwithstanding this difference in the overdrive brake capacity release time, the output torque profile is still similar to that shown in FIG. 7 b.    
     In FIG. 7 e , the final overdrive brake release interval is further reduced to 25 ms, as shown at  206 ″″. This release interval is close to that of the conventional strategy, as shown in FIG.  2 . As a result, the overrunning coupling DOWC torque rapidly increases, as shown at  202 ″″, which causes an undesirable torque bump  166 ″″ comparable to the disturbance  166  in FIG.  2 . 
     These FIGS. 7 a - 7   e  illustrate that it is critical to gradually reduce the overdrive brake torque capacity at the end of the shift. 
     FIG. 8 shows in flow diagram form the method steps used in carrying out the strategy of the present invention. Three speed measurements are needed in carrying out the strategy. These are the input speed NC 1 , output speed NC 2 , and intermediate drum speed NS 2 , as shown in action block  218 . These speeds, which are identified in FIGS. 1 and 1 a , are continuously monitored. The values are distributed to the input signal conditioning portion of the microcomputer  18 . Sample readings are taken in each background control loop of the microcomputer and stored in RAM portions of memory. The central processor unit (CPU) in the digital microcomputer  18  calculates the speed of the ring gear  90  (NR 1 ). Since the forward clutch  92  is engaged, NR 1 =NR 2 , which is the speed of the ring gear  94 . The overrunning speed of the overrunning coupling DOWC is calculated as the difference between NC 1  and NR 1 . These calculations occur at action block  220 . 
     The ring gear speed NR 2  is computed using the known planetary gear constants of the Simpson gearset. The values NC 2  and NS 2  are measured and the value NR 2  is calculated. Since ring gear  90  rotates with ring gear  94 , the slip, or overrunning speed, across the first planetary gearset coupling DOWC (SLIP_DOWC) can be determined. 
     After filling and stroking the intermediate brake pressure servo, two simultaneously executed control loops are activated, as shown at  222  and  224 . Control loop  222  drives down to zero the overrunning coupling speed SLIP_DOWC until it reaches zero at a fixed time T 1  seconds using the overdrive servo pressure as a variable. This is done in action block  226 . Simultaneously, control loop  224  begins by driving down the value NS 2  to zero in a time interval of T 1  plus DELT plus T 3  seconds. This is indicated in the plot of FIG. 9 d , where the sun gear speed NS 2  is plotted against time during the shift interval. 
     Each control loop may be a conventional proportional-integral-derivative (PID) control with speed error as a feedback and a linear ramp to zero for a set point. The ramp rates for the speed set points are calculated at the beginning of the shift. 
     When the first control loop  222  has brought the value for SLIP_DOWC to zero, it is checked at decision block  228 . The pressure on the overdrive servo is held at a value of PRS_OD, as shown at action block  230 . The value for PRS_OD is the last value until NS 2  becomes less than the value N_ODMIN. It is tested at  234 . At that time, PRS_OD is ramped down to zero, as shown at  236 . 
     The second control loop at  224  will continue the control of intermediate brake servo pressure until NS 2  becomes zero. The engagement of the intermediate brake is completed in time T 1 +DELT+D 3 , as shown in FIG. 9 d . This occurs at action block  238 . 
     When the value SLIP_DOWC in the first control loop  222  reaches zero, it will not allow the pressure on the intermediate brake servo to decrease. It is only possible to increase the intermediate brake servo pressure. This is achieved by clipping the intermediate brake servo pressure PRS_IM to the greater of the new control loop value or the value last sent to the variable force solenoid of FIG. 1 c , which controls servo pressure. 
     The decision block  240  in FIG. 8 determines when the overrunning coupling speed DOWC reaches zero, which is seen at  188  in FIG.  5 . The action block at which the intermediate servo pressure is clipped is indicated in FIG. 8 at  242 . This clipped value for PRS_IM is shown in FIG. 9 c.    
     When the second control loop  224  makes NS 2 =0, the capacity on the intermediate servo is increased to a maximum. This is represented in FIG. 8 by decision block  244  and action block  246 . The capacity on the intermediate brake servo then is increased to its maximum value. At this point, the shift is complete. When the first control loop finishes its ramp of the overdrive brake servo pressure to a zero value in T 2  seconds, as shown at action block  236 , the maximum intermediate brake capacity is shown in FIG. 5 at  248 . 
     FIGS. 9 a  through  9   d  show the definitions for the times T 1 , DELT, T 2  and T 3 , and the relationship between these times and the two control loops  222  and  224 . Each control loop is used to drive their respective speeds to zero. Thus, the ramp rates for the set points can be determined as the initial value of speed divided by the desired time. The ramp rates can be determined if a normalized percent shift complete indicator for each control loop is used. This is done by computing a value equal to 1 minus current speed divided by the speed at the start of the shift. In the alternative, the ramp can be achieved by implementing an open loop fixed value substitute for the intermediate servo pressure PRS_IM rather than having the second control loop act on sun gear speed NS 2 . In this alternative implementation, the value of PRS_IM can be adaptively determined by measuring T 3  and adjusting intermediate servo pressure so that T 3  is less than T 2 , seen in FIG. 9 b , and time value T 3  is greater than zero (see FIG. 9 d ). 
     FIG. 9 c  shows a variation in the intermediate servo pressure during the shift interval together with the clipped value for intermediate servo pressure. FIG. 9 a  shows the variations in overdrive servo pressure during the shift interval. 
     In an alternative implementation of the strategy, the intermediate servo pressure may be adaptively determined. The intermediate servo pressure can be adjusted, as explained previously, because the shift quality is determined using the strategy of the present invention. It is relatively insensitive to variations in the value T 3 . 
     4. Summary 
     The present invention avoids the inertia torque disturbance, as shown at  166  in FIG.  3  and at  172  in FIG.  4 . The disturbance at  146  in FIG. 2 is due to the fast rise in the overrunning clutch torque or coupling DOWC at  148 . 
     If the overdrive brake is released early, as shown at  156  in FIG. 3, that increases the size of the peak torque disturbance shown at  166  due to a combination of a rapid torque increase in DOWC and increased intermediate brake capacity. 
     In FIG. 4, if the intermediate brake drum speed goes to zero, while the overdrive brake capacity has not yet started to go to zero, as shown at  168  and  170  in FIG. 4, a torque disturbance in a negative direction is created at  172  in FIG.  4 . 
     In FIG. 5, it is demonstrated that applicants&#39; strategy does not require a match between the overdrive brake release with the intermediate brake drum capacity increase. The torque on the overrunning coupling LOWC, shown at  178 , goes to zero, and only after that happens does the overdrive brake capacity decrease, as shown at  180 . The overdrive brake capacity decreases to the level at  184  in FIG. 5, at which time the overdrive drum speed begins to slip, but the overdrive brake is not released. The overdrive drum speed increases to its peak value, as shown at  182 . 
     The value for the overdrive brake capacity at  184  in FIG. 5 is less than the overdrive brake capacity value at  138  in FIG. 2 so that the overdrive drum speed rises faster to its peak as shown at  182 . The overrunning coupling DOWC at that instant starts transmitting torque, as shown at  188 . The rise in the curve at  188  is less compared to the rise at  148  in the case of the prior art design of FIG.  2 . 
     At point  190  in FIG. 5, the overdrive brake capacity begins to fall, as shown at  192 , over a non-critical time until point  194  is reached. This is what causes the torque on coupling DOWC to rise at a lower slope in the case of the design indicated in FIG. 5 compared to the prior art designs of FIGS. 3 and 4. The intermediate brake drum speed slows down to zero, as shown at  186  in FIG. 5, before the overdrive brake capacity reaches zero at point  194 . 
     The peak for overdrive drum speed at  182  must be reached before the intermediate drum speed reaches zero at point  186  in FIG.  5 . The overdrive brake capacity at  184  must be small enough to allow that to happen. 
     When point  190  is reached, the gradual release of the overdrive brake is triggered, as shown at  192 . That must give enough time for the intermediate drum speed to reach zero before point  194  is reached. 
     In the case of the shift demonstrated in FIG. 7 b , as in the case of the shift demonstrated in FIG. 5, the torque on the overrunning coupling DOWC must start increasing before the intermediate drum speed reaches zero. Then, overdrive brake capacity must be brought to zero over an extended time in order to minimize torque disturbance caused by the overrunning coupling DOWC. Aside from these requirements, there is no need to match the overdrive brake capacity release and the intermediate brake engagement, as in the case of the strategy demonstrated in FIG.  2 . 
     The strategy of the present invention does not require precise synchronization of the friction element engagement and release times. It achieves minimum output shaft inertia torque disturbances and has reduced shift sensitivity, regardless of varying operating conditions or unit-to-unit hardware variations, or friction element or other hardware degradation over extended use. It also simplifies transmission calibration and calibration time. 
     Having described an embodiment of the invention, it will be apparent that modifications may be made by persons skilled in the art without departing from the scope of the invention. All such modifications and equivalents thereof are intended to be covered by the following claims.