Method of controlling gear change in an automatic transmission

Abstract: In an automatic transmission 16, upshifting is controlled by ramping down the pressure in an off-going clutch C1-C5 associated with a high speed ratio and increasing the pressure in an on-coming clutch C1-C5 associated with a lower speed ratio. The on-coming clutch C1-C5 has commanded pressure at a maximum value for an initial fill time, a lower increasing ramp value during the ramp of the off-going clutch until the on-coming clutch starts to engage, and then a closed-loop control until the clutch C1-C5 reaches a synchronous speed. The closed-loop control parameter is on-coming clutch slip speed which is controlled to a calculated slip speed profile. The profile has a first steep slope and a final less steep slope so that the actual slip can smoothly go to zero in a given time period.

Description: This invention relates to a method of controlling the change in an automatic transmission, and more particularly, for a clutch-to-clutch change. Generally, a motor vehicle automatic transmission includes a number of gear elements coupling its input and output shafts, and a related number of torque establishing devices, such as clutches and brakes which are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts. The brake can be of the band-type or disc-type. Automotive engineers commonly refer to disc-type brakes in transmissions as "clutches" or "reaction clutches". As used herein, the terms "clutches" and "torque transmitting devices" will be used to refer to brakes as well as clutches. The input shaft is connected to the vehicle engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle wheels. Shifting from one forward speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing or disengaging the clutch (off-going) associated with the current speed ratio and applying or engaging the clutch (on-coming) associated with the desired speed ratio. The speed ratio is defined as the transmission input speed or turbine speed divided by the output speed. Thus, a low gear range has a high speed ratio, and a higher gear range has a lower speed ratio. To perform an upshift, a shift is made from a high speed ratio to a low speed ratio. In the type of transmission described below, the upshift is accomplished by disengaging a clutch associated with the higher speed ratio and engaging a clutch associated with the lower speed ratio thereby to reconfigure the gear set to operate at the lower speed ratio. Shifts performed in the above manner are termed clutch-to-clutch shifts and require precise timing in order to achieve high quality shifting. US-A-4,527,678 discloses a method of controlling a clutch-to-clutch shift including the steps in the preamble of claim 1. An aspect of the present invention provides a method of controlling the change in an automatic transmission from an off-going fluid pressure operated torque transmitting device associated with a first speed ratio to an on-coming fluid pressure operated torque transmitting device associated with a second speed ratio characterised over US-A-4,527,678 by the features in the characterising portion of claim 1. The invention can provide a method of controlling a clutch-to-clutch shift by establishing a slip speed profile and controlling the slip speed of the on-coming clutch to the profile by adjusting the pressure on one of the clutches to minimize the slip speed error. Preferably, the method generates a slip speed profile which quickly and smoothly achieves on-coming clutch synchronization. In an embodiment, the method controls an upshift in an automatic transmission by disengaging one clutch and engaging another clutch in a controlled manner and using transmission speed feedback to manage the control. In particular, a motor vehicle automatic transmission with an electronic control system can achieve high quality shifts by monitoring input and output speed to determine the effects of the clutch operations on the input speed and calculating on-coming clutch slip for closed-loop clutch control. The present invention is also directed to apparatus comprising means for carrying out the steps of the described method. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1a is a system diagram of a fluid operated motor vehicle transmission, including several solenoid operated fluid pressure control valves and a control unit; Figure 1b is a table illustrating the clutch engagements required to establish the various speed ratios of the transmission of Figure 1a; Figures 2, 3a and 3b are flow diagrams of an embodiment of method of controlling gear change by means of the controller of Figure 1a; Figure 4, graphs A, B and C, are graphs illustrating turbine speed, off-going pressure command and on-coming pressure command, respectively, for clutch-to-clutch shifting; Figure 5 is a slip diagram for the closed-loop operation of the on-coming clutch; Figures 6 through 9 are flow diagrams illustrating an embodiment of upshift logic for the on-coming clutch; and Figures 10 and 11 are flow diagrams illustrating an embodiment of upshift logic for the off-going clutch. Referring now to Figure 1a, the reference numeral 10 generally designates a motor vehicle drive train which includes a throttled internal combustion engine 12, a fluidic torque converter 14, a six-speed fluid operated power transmission 16 and a differential gear set (DG) 18. The engine 12 is connected to the torque converter 14 via shaft 20, the torque converter 14 is connected to the transmission 16 via shaft 22, the transmission 16 is connected to the differential gear set 18 via shaft 24 and the differential gearset is connected to a pair of drive wheels (not shown) via the prop shafts 26 and 28. Gear shifts are accomplished by selectively engaging and disengaging brakes and clutches, herein called torque transmitting devices or clutches. These clutches are actuated by hydraulic pressure and upon engagement, require a fill time before torque is transmitted between a driving and a driven friction element. The speed and torque relationships between the engine 12 and the drive wheels of the vehicle are controlled by a fluid operated torque converter clutch, designated TCC, and five fluid operated transmission clutches, designated C1 - C5. The torque converter clutch TCC is selectively engaged by the solenoid operated control valve 30 to connect mechanically the impeller I and turbine T of torque converter 14. The clutches TCC, C1, C2, C3, C4, C5 are selectively engaged and disengaged by the solenoid operated control valves 30, 32, 34, 36, 38, 40 according to the diagram shown in Figure 1b, selectively to establish a desired transmission speed ratio. The illustrated transmission gear set provides one reverse ratio and six forward ratios, and is described in detail in U.S. Patent 4,070,927. An operator actuated accelerator pedal 41 positions the engine throttle for controlling the engine power output. The operation of the solenoid operated control valves 30 - 40 is controlled by a computer-based control unit 42, shown in dotted outline, via lines 44 - 54 in response to various input signals representative of system parameters. Such inputs include an engine throttle position signal %T on line 56, an engine output shaft speed signal Ne on line 58, a torque converter output shaft speed signal Nt on line 60, a transmission output shaft speed signal No on line 62, a system supply voltage signal Vb on line 64, a transmission fluid temperature signal Tsump on line 66 and an operator range selector position signal RS on line 68. The system voltage is supplied by the storage battery 70, and the input signals are obtained with conventional electrical transducers such as potentiometers, thermistors and magnetic speed pickups. Internally, the control unit 42 comprises a number of conventional devices including a microcomputer (uC) with internal clock and memory, an input/output device (I/O) and an array of pulse width modulated signal generators (PWM) and drivers (DR). As indicated below, a PWM generator and a driver (DR) are dedicated to each solenoid operated control valve 30 - 40. The PWM generator outputs are delivered to the respective drivers (DR) and are used to energize the respective solenoid operated control valves. The duty cycle of the PWM generator outputs determine the hydraulic pressure supplied by the solenoid generated control valves, with a low percent duty cycle yielding a low pressure and a high percent duty cycle yielding a high pressure for a normally closed valve. The hydraulic circuit of transmission 16 includes a positive displacement pump 82 for supplying pressurized hydraulic fluid from the sump or reservoir 84, to the clutches TCC and C1 - C5 through various hydraulic and electro-hydraulic valving mechanisms. After passing through a main circuit filter 86, the fluid output of pump 82 is directed to a main pressure regulator valve 88 which develops regulated fluid pressures in lines 90 and 92. The fluid in line 90, generally referred to as converter feed pressure, is directed through the torque converter 14, as schematically designated by the converter shell 97. After passing through a cooler 100 and cooler filter 102, the converter fluid is then regulated down to a lower pressure by the regulator valve 104 and directed to the transmission lube, or lubricant circuit 106. The fluid in line 92, generally referred to as main or line pressure, is supplied as an input to the solenoid operated control valves 30 - 40, and also to the control pressure regulator valve 96. The control pressure regulator valve 96 develops a somewhat lower pressure in line 98, referred to herein as the control pressure, such pressure being directed to the solenoid of each control valve 30 - 40. The fluid in line 94, referred to as the converter clutch pressure, is supplied directly by solenoid 30 to the torque converter clutch TCC to engage the same. This pressure is also supplied to the main pressure regulator valve 88 to provide a lower regulated line pressure in the converter lock-up mode. Figures 2, 3 and 6 - 11 are flow diagrams representative of computer program instructions executed by the computer-based control unit 42 of Figure 1 in carrying out the shift control technique described below. Figure 2 represents an executive or main loop program which directs the sequential execution of various subroutines. Block 130 designates a series of instructions executed at the initiation of each period of vehicle operation for setting the various timers, registers and variable values of control unit 42 to predetermined initial values. Thereafter, the blocks 132 - 140 are sequentially and repeatedly executed as indicated by the flow diagram lines. Block 132 reads the various input signal values and outputs the required control signals to the PWM generators and drivers for solenoid controlled valves 30 - 40. Blocks 134 - 138 contain diagnostic, shift scheduling, and adaptive flag logic. The clutch control logic block 140 analyzes the various system input signals described above in reference to Figure 1a, develops pressure command signals PCMD for application to the solenoid operated control valves at the next execution of block 132, and computes adaptive corrections based on the adaptive flags at shift completion. Block 140 also effects pulse-width-modulation of the solenoid drive voltage to carry out the pressure commands for specific shift operations. Block 140 is detailed in the flow chart of Figures 3a - 3b. The flow diagram of Figures 3a - 3b sets forth the program for making decisions as to the type of range shift in progress, if any, and determines the specific control for the on-coming and the off-going clutches. The program also checks whether a shift has performed within specifications, and if not, certain shift parameters are changed at shift completion according to predefined adaptive logic to correct the shift. First, lockup clutch control is executed 142 if a lockup shift is in progress 144. Then it is determined (from the shift schedule) whether a range shift is in progress 146. If not, the clutch control logic is exited. If a range shift is in progress 146, it is determined whether it is an upshift 150, a downshift 152, a neutral shift 154, or a garage shift 156. A garage shift is a shift from neutral to either drive or reverse, or a shift from drive to reverse or from reverse to drive. The control flows from either the upshift, downshift, neutral shift or the garage shift block to the end-of-shift test 160. Once the shift is completed 160, adaptive shift parameters are changed if required 162 and the duty cycle command is output 163. If the shift has not ended 160, the duty cycle command is output 163 before returning to the main loop of Figure 2. If an upshift is indicated 150, the upshift on-coming clutch control 164 and the upshift off-going clutch control 166 are activated. If a downshift is indicated 152, it is next decided whether it is a closed throttle downshift or a powered downshift 168. If it is closed throttle, a closed throttle in progress flag is set 169, the closed throttle on-coming clutch control is activated 170 and the closed throttle off-going clutch control is activated 172. If the downshift is not at closed throttle 168, the closed throttle flag is checked 173. If the flag is not set, the powered downshift on-coming clutch control 174 and the powered downshift off-going clutch control 176 are activated. If the closed throttle flag is set 173, the throttle opened during the course of the closed throttle downshift and a transition to powered downshift may be necessary; in such case, the appropriate transition logic is invoked 178. If the shift is a neutral shift 154, the neutral shift clutch control executes shifts from drive to neutral or from reverse to neutral 158. Each control phase operates by setting pressures, pressure increments, times or other values to predefined calibrated values which are herein generally called "set", "preset", "given" or "certain" values. Each such value is chosen from a table of calibrated values for each specific transmission condition, throttle range and shift type. Thus, different values are supplied for upshift, downshift, etc. as well as each range shift, e.g., 1-2, 2-1, 4-3, 5-4, etc. Converter and lockup modes may also require separate sets of calibration values. Figure 4, graphs A, B, and C, detail the controlled clutch pressure for an upshift as well as the turbine speed or input speed. Graph A is a graph of turbine speed versus time, graph B, shows the commanded pressure versus time for the off-going clutch, and graph C shows the commanded pressure versus time for the on-coming clutch. Graph A curve is typical for the case of increasing vehicle speed prior to shift initiation, and indicates the turbine speed during the first range, the speed decrease during shifting, and increase again at a lower level after shifting. The peak of the turbine speed is due to the slowing action of the on-coming clutch, causing slip of the off-going clutch, and is indicative of "turbine pulldown". Turbine pulldown is detected by sensing when the turbine speed falls a set amount below the output speed multiplied by the higher speed ratio. The speed after shifting is "synchronous speed", i.e., the turbine speed (Nt) equals the output speed (No) times the lower speed ratio (SR) or Nt = No * SR . Referring to graphs B and C, it may be observed that initially, at the time of the shift command, the off-going pressure is reduced to an intermediate value, Pint, for a brief time and is then reduced to an initial value, Pioff, and then ramps down until off-going clutch slip (or turbine pulldown) is detected and then drops to zero. The brief intermediate value, Pint, is effective to reduce clutch pressure undershoot caused by solenoid dynamics. For the on-coming clutch, graph C shows that maximum pressure is commanded for a fill time. The fill time allows nearly complete stroking of the clutch plates and obtains clutch torque capacity. Then the command pressure drops to an initial value, Pin, and ramps up slowly until it causes turbine pulldown. The combination of the on-coming upward ramp and the off-going downward ramp results in a torque transition from the off-going to the on-coming clutch. Then the on-coming clutch control enters a closed-loop control period, wherein the pressure is adjusted to maintain the on-coming clutch slip close to a calculated slip profile. When the turbine speed, Nt, reaches synchronous speed, the pressure command is increased to maximum value to engage the clutch fully and complete the shift. The upshift process of this embodiment has several features which contribute to smooth and efficient operation. The on-coming and off-going pressure ramp commands reduce clutch timing sensitivity to the initial pressure commands. Thus, variations in clutches due to temperature or other factors do not impose critical demands on the timing of torque transition because the two ramps can continue for a variable time, subject to a limit value, so that the actual assumption of torque by the on-coming clutch determines the time of transition. Also, the immediate release of the off-going clutch following pulldown detection reduces clutch tie-up which might result in a braking action. The closed-loop control of the on-coming clutch reduces shift variation and the end-of-shift torque disturbance. The closed-loop control is better explained with reference to Figure 5 which shows the on-coming slip speed profile in solid lines and actual slip speed in dashed lines. Slip speed is determined by comparing the turbine speed to the output speed. Specifically, slip speed is the absolute difference (times a conversion factor K) between turbine speed and the product of the output speed and the speed ratio of the lower range or SLIP = K*ABS (Nt- No*SR) . Thus, as soon as a shift command is issued there is a slip speed across the on-coming clutch. The initial slip speed, SLIPI, is the slip speed value at the detection of turbine pulldown. The slip speed profile begins at that point and decreases at a fixed rate, called the first slope. Then, at a determined point, the rate reduces to a second slope. The slopes are chosen so that, ideally, the actual slip speed can be made to smoothly go to zero within a given time period. The second slope is less steep than the first slope and reduces end of shift torque disturbance by more closely matching the acceleration rates on both sides of the on-coming clutch. By using on-coming clutch slip speed as the control target, both turbine and output speeds are taken into account when controlling the shift duration. To determine the slopes, three constants are defined. C1 is a fraction of the initial slip value SLIPI at which the second slope begins; i.e., if SLIP =< C1*SLIPI , the slope changes to slope 2. C2 is the desired time to utilize the first slope. C3 is the desired overall closed loop time. C2 and C3 are used only for the slope calculation and not for direct timing purposes. Then the first slope shall be SLOPE1 = [SLIPI-(C1*SLIPI)]/C2 . The second slope shall be SLOPE2 = C1*SLIPI/(C3-C2) . The arrival at synchronization speed is determined by making several measurements in consecutive control loops. This assures that true synchronization has been achieved and maintained. If synchronization is assured, full clutch pressure is immediately applied. In the event the turbine speed goes below the synchronization speed, as shown in dashed lines in Figure 4, it is smoothly pulled up to synchronization speed by ramping the pressure up over a fixed period. This feature prevents full application of the on-coming clutch (during closed throttle shifts) when the on-coming clutch is not completely stroked. The "below sync" condition can only result when turbine pulldown is achieved through a "neutral" condition caused by lack of on-coming clutch capacity. The ramp application of the on-coming clutch will significantly reduce end-of-shift torque disturbance. The flow charts of Figures 6 - 11 illustrate the control logic incorporated in the program for shift control. Figure 6 shows the logic flow for the upshift on-coming clutch. SHIFT PHASE is a pointer indicating the stage of shift progress. If the SHIFT PHASE is set to FILL PERIOD 200, the fill period routine is executed 202, as shown in Figure 7, otherwise, it is determined whether the SHIFT PHASE is set to INIT PRESS 204. If so, the initial pressure routine is executed 206; that routine sets the initial calibration pressure, Pint, and sets the SHIFT PHASE pointer to RAMP. If the SHIFT PHASE is RAMP 208, the ramp routine is executed 210, as shown in Figure 8. If the SHIFT PHASE is CLOSED LOOP 212, the closed-loop routine is executed 214, as shown in Figure 9. If SHIFT PHASE is set to BELOW SYNC 216, the below sync routine is executed 218. The below sync routine increments the pressure by a set amount in each control loop until maximum pressure is reached. If SHIFT PHASE is not BELOW SYNC the maximum pressure is commanded 220. The fill period routine is shown in Figure 7. If it is the first pass through this loop 222, the timers and closed-loop parameters are initialized 224 and fill time is computed 226; the fill time is expressed by a counter as a number of control loops and sets a maximum time for filling. If it is not the first pass, the fill time counter is decremented 228 and the maximum pressure is commanded 230. If the fill time counter is not expired 232 and turbine pulldown is not detected 234, the loop repeats. Otherwise, SHIFT PHASE is set to INIT PRESS 236 and the program control goes to that phase. The ramp routine, as shown in Figure 8, increments pressure by a set amount 240. If turbine pulldown 242 or timeout 244 occurs, the closed-loop profile is initialized 246 and SHIFT PHASE is set to CLOSED LOOP 248. Otherwise, the ramp routine repeats until one of the above requirements is met. The timeout is based on the previously calculated fill time plus a constant value to limit the total time elapsed between the shift initiation and the initiation of closed loop. The closed-loop routine as shown in Figure 9, comprises computing the closed-loop profile 250, computing a closed-loop slip speed error 252, computing the closed-loop pressure required to reduce the error 254, and test for achieving sync 256 and below sync condition 258. If neither condition applies, the loop repeats to control the clutch slip. If a set number "N" of consecutive sync speeds are detected 256, the SHIFT PHASE is set to END 260 which causes maximum pressure to be applied 220. If the below sync condition is detected 258, SHIFT PHASE is set to BELOW SYNC 262. The upshift off-going clutch control 166 is shown in Figure 10. If SHIFT PHASE is set to INITIAL 270, the initial period routine is executed 272 as shown in Figure 11. If the SHIFT PHASE is set to OFF-GOING RAMP 274, the off-going ramp routine 276 is executed. That routine decrements pressure by a set amount if the on-coming SHIFT PHASE is not in CLOSED LOOP or commands an exhaust state if it is in CLOSED LOOP. In the latter case, the minimum pressure command 278 is executed. In Figure 11, the initial period routine 272 initializes a timer 280 if it is the first pass 282, or decrements the timer 284 if it is not the first pass. Then, if the timer is not expired 286, the initial off-going pressure plus an offset is commanded 288 to establish the intermediate pressure, Pint. If the timer is expired, the initial off-going pressure value Pioff, is commanded 290 and the SHIFT PHASE is set to OFF-GOING RAMP 292. Reference is made to our co-pending applications EP-A-0,435,374, EP-A-0,436,977, EP-A-0,435,377, EP-A-0,435,372 and EP-A-0,436,978 filed and published the same day as this application.

Claims: A method of controlling the change in an automatic transmission for a vehicle from an off-going fluid pressure operated torque transmitting device (C1-C5) associated with a first speed ratio to an on-coming fluid pressure operated torque transmitting device (C1-C5) associated with a second speed ratio, wherein the off-going torque transmitting device is disengaged substantially concurrently with the engagement of the on-coming torque transmitting device, the automatic transmission including an input (22), an output (24), a turbine (14) connected to the input to couple driving torque thereto and turbine speed (Nt) and output speed (No) sensing means (60, 62); the method comprising the steps of applying a progressively decreasing off-going pressure on the off-going torque transmitting device (C1-C5); preparing the on-coming torque transmitting device (C1-C5) to provide torque by applying a substantially sufficient pressure thereto; initiating a closed-loop control period for controlling the on-coming torque transmitting device; during the closed-loop control period, establishing a slip speed profile representative of desired slip during the closed-loop period, measuring the slip speed of the on-coming torque transmitting device, and progressively changing the pressure on one of the off-going and on-coming torque transmitting devices to control the measured slip speed on the basis of the slip speed profile, and reducing slip speed towards zero; characterised in that the step of establishing a slip speed profile includes the steps of generating a ramp beginning at an initial slip speed and having a first slope and a second slope, the first slope being relatively steeper than the second slope so as to reduce slip speed at a first greater rate, and the second slope being adapted to effect synchronization of the on-coming torque transmitting device. A method according to claim 1, wherein the step of preparing the on-coming torque transmitting device to provide torque comprises the steps of applying sufficient pressure on the on-coming torque transmitting device to cause slip of the off-going torque transmitting device, detecting slip of the off-going torque transmitting device on the basis of the turbine speed and the output speed, and initiating the closed-loop control period on detection of slip of the off-going torque transmitting device. A method according to claim 1 or 2, wherein the step of establishing the slip speed profile includes the steps of obtaining a fraction of the initial slip speed at which the first slope is to terminate, a desired first slope time during which the ramp is to have the first slope, and a desired overall slip time; determining the gradient of the first slope on the basis of the initial slip speed, the fraction and the desired first slope time; determining the gradient of the second slope on the basis of the initial slip speed, the fraction, the desired first slope time, and the desired overall slip time; and changing the gradient of the ramp from the first slope to the second slope when the actual slip speed reaches the fraction of the initial slip speed. A method according to any preceding claim, wherein the change is from a higher speed ratio, with which the off-going torque transmitting device is associated, to a lower speed ratio, with which the on-coming torque transmitting device is associated. A method according to claim 4, wherein the step of applying a progressively decreasing off-going pressure on the off-going torque transmitting device is carried out during a pressure relief period; the method comprising the steps of, during the pressure relief period, applying on-coming pressure to the on-coming torque transmitting device for a set time up to an initial on-coming pressure, and increasing the on-coming pressure at a rate calibrated to achieve a predetermined decrease in turbine speed, and removing the off-going pressure from the off-going torque transmitting device; and determining synchronization of the on-coming torque transmitting device from a comparison of the turbine speed and the output speed and, when turbine speed decreases to a synchronization speed, increasing the on-coming pressure to a maximum value to complete the change. A method according to claim 5, wherein the step of applying a progressively decreasing off-going pressure on the off-going torque transmitting device comprises the step of applying an intermediate off-going pressure for a set period, applying an initial off-going ramp pressure, and then applying a decreasing off-going ramp pressure. A method according to claim 5 or 6, including the step of comparing turbine speed and the product of output speed and the higher speed ratio to detect turbine speed pulldown during the pressure relief period, wherein the steps of removing the pressure from the off-going torque transmitting device and initiating a closed-loop control period of the on-coming torque transmitting is carried out on detection of turbine speed pulldown. A method according to claim 5, 6 or 7, wherein synchronization is determined to have occurred when the turbine speed is substantially equal to the product of the output speed and the lower speed ratio. A method according to any one of claims 5 to 8, wherein the step of increasing the on-coming pressure comprises the step of gradually increasing the on-coming pressure to the maximum value to complete the change, if turbine speed decreases to a value below synchronization value.

Inventors: HIBNER JOHN ANTHONY, LENTZ CARL ALBERT, HIBNER, JOHN ANTHONY, LENTZ, CARL ALBERT
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1990
Main classification: F16H61
IPC codes: F16H61, F16H59