CLUTCH UPSHIFT CONTROL

Abstract: A control method is provided for the torque-establishing devices (28 to 32) of a motor vehicle multiple speed-ratio automatic transmission (l4) that is connected to the engine (l2) of the vehicle by way of a fluid coupling (24), particularly in so-called step-out situations, in which an upshift is commanded in response to a rapid decrease in the engine throttle position. When step-out situations likely to produce a sharp reversal in the polarity of torque carried by the transmission input shaft (42) are identified, an activated torque-establishing device is de-activated at a time determined in relation to the speed ratio across the fluid coupling, thereby avoiding transmission gear lash-related output torque disturbances due to the torque reversal.

Description: This invention relates to clutch-to-clutch speed ratio control in a motor vehicle automatic transmission, and more particularly to the control of upshifts which are made in response to at least partial release of the engine throttle by the operator of the vehicle. Generally, a motor vehicle automatic transmission includes a number of gear elements for selectively coupling together the input and output shafts of the transmission, and a number of related torque-establishing devices such as clutches and brakes which are selectively engageable to activate certain gear elements for establishing desired speed ratios between the input and output shafts. The brakes can be of the band type or the disc type; engineering personnel in the automotive art tend to refer to disc-type brakes in transmissions as "clutches" or "reaction clutches". The transmission input shaft is connected to the vehicle engine by way of 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 position and vehicle speed, and generally involves releasing (disengaging) the off-going clutch or brake associated with the current speed ratio and applying (engaging) the on-coming clutch or brake associated with the desired speed ratio. Shifts performed in the above manner, using transmission clutches and/or reaction clutches, are termed clutch-to-clutch shifts, and require precise timing in order to achieve high-quality shifting, especially in situations in which the transmission is upshifted in response to at least partial release of the engine throttle by the operator of the vehicle. Shifts of this type are termed step-out or lift-foot upshifts, and may be accompanied by a sharp torque reversal at the input shaft of the transmission. Under such circumstances, the sharp torque reversal is carried through to the transmission output shaft prior to the shift via the active torque-establishing device, and the driveline experiences a significant torque disturbance due to lash in the transmission gear elements. A second disturbance is often experienced when the output torque is reversed to a positive sense by the application of the on-coming torque-establishing device during the impending upshift. For this and other reasons, automatic transmissions commonly include one-way devices connected in series with a reaction clutch, such one-way devices involving torque-transmitting free-wheel elements that are capable of transmitting torque in one direction only. As soon as the applied torque reverses, the driving element over-runs the driven element, such that no torque is transmitted through the one-way device. When the on-coming torque-establishing device is applied, the output torque increases from near zero, with little or no torque disturbance. However, due to the cost of one-way devices, it is highly desirable to limit their use in transmissions. The present invention is concerned with achieving high-quality clutch-to-clutch upshifts in a motor vehicle electronic transmission control system in response to partial or full release of the engine throttle by the operator of the vehicle, without the use of free-wheel (one-way torque-transmitting) elements. To this end, objectionable torque reversals across the transmission gear elements are avoided by identifying engine operating conditions which characteristically result in a sharp torque reversal, and introducing a neutral interval before the torque reversal is transmitted through the transmission. When the on-coming torque-establishing device is then applied to effect the upshift, the output torque increases from near zero, so that lash-related torque disturbance during the shift is substantially eliminated. If an upshift is not possible for some reason, a no-neutral interval is effected. The engine throttle position and the rate of change of engine throttle position are monitored to identify impending torque reversals, the severity of which is judged by comparing the monitored parameters with empirically determined reference values. In situations in which the impending torque reversal, if any, is expected to be relatively minor, the active torque-establishing device is maintained in an active state, but the parameters for the impending upshift are modified to avoid a sharp torque reversal to the positive sense when the on-coming torque-establishing device is applied. In more severe situations, the active (off-going) torque-establishing device is released concurrently with or just prior to the actual occurrence of the initial torque reversal. In such cases, to schedule the release the speed ratio across the torque converter is detected as a measure of the torque transmitted therethrough. If the release does not occur within a predetermined time, or if the operator re-applies the throttle before release occurs, the release is cancelled. If the transmission includes a torque converter clutch, and such clutch is applied, the speed ratio thereacross is unity, and the active clutching device is released as soon as operating conditions indicative of a severe torque reversal are detected. As in the less severe situations, the parameters for the impending upshift are modified to avoid a sharp torque reversal to the positive sense when the on-coming torque-establishing device is applied. In the drawings: Figures la and lb together schematically depict a computer-based vehicular electronic transmission control system for carrying out a control method in accordance with the present invention; Figures 2 and 3 graphically depict operation in conformity with the invention in response to step-out conditions of different severity, with Figure 2 depicting an upshift made in response to a relatively minor step-out condition, whereas Figure 3 depicts an upshift made in response to a severe step-out condition; and Figures 4 to 7 are flow diagrams representative of program instructions executed by the computer-based control system of Figure l for carrying out control functions in conformity with the invention. With reference now to the drawings, and more particularly to Figures la and lb, reference numeral l0 generally designates a motor vehicle drive train including an engine l2 and a parallel-shaft transmission l4 having a reverse-speed ratio and four forward-speed ratios. The engine l2 includes a throttle mechanism l6 mechanically connected to an operator-manipulated device such as an accelerator pedal (not shown) for regulating engine output torque, such torque being applied to the transmission l4 by way of the engine output shaft l8. The transmission l4 transmits engine output torque to a pair of drive axles 20 and 22 by way of a hydrodynamic torque converter 24 and one or more of a plurality of fluid-operated clutching devices 26 to 34, such clutching devices being applied or released according to a predetermined schedule for establishing the desired transmission speed ratios. With reference now more particularly to the transmission l4, the impeller (input member) 36 of the torque converter 24 is connected to be rotatably driven by the output shaft l8 of the engine l2 by way of an input shell 38. The turbine (output member) 40 of the torque converter 24 is rotatably driven by the impeller 36 by fluid transfer therebetween, and is connected to rotatably drive a transmission shaft 42. A stator member (reactor) 44 redirects the flow of fluid which hydraulically couples the impeller 36 to the turbine 40, the stator member being connected by way of a one-way device 46 to the housing of the transmission l4. The torque converter 24 also includes a clutching device 26 comprising a clutch plate 50 secured to the transmission shaft 42. The clutch plate 50 has a friction surface 52 formed thereon that is selectively engageable with the inner surface of the input shell 38 to form a direct mechanical drive between the engine output shaft l8 and the transmission shaft 42. The clutch plate 50 divides the space between the input shell 38 and the turbine 40 into two fluid chambers, namely an apply chamber 54 and a release chamber 56. When the fluid pressure in the apply chamber 54 exceeds that in the release chamber 56, the friction surface 52 of the clutch plate 50 is moved into engagement with the input shell 38, as shown in Figure l, thereby engaging the clutching device 26 to provide a mechanical drive connection in parallel with the torque converter 24. In this condition, there is no fluid-induced slippage between the impeller 36 and the turbine 40. When the fluid pressure in the release chamber 56 exceeds that in the apply chamber 54, the friction surface 52 of the clutch plate 50 is moved out of engagement with the input shell 38, thereby uncoupling such mechanical drive connection and permitting fluid-induced slippage between the impeller 36 and the turbine 40. The circled numeral 5 represents a fluid connection to the apply chamber 54, and the circled numeral 6 represents a fluid connection to the release chamber 56. A positive-displacement hydraulic pump 60 is mechanically driven by the engine output shaft l8 by way of the input shell 38 and the impeller 36, as indicated by the broken line 62. This pump 60 receives hydraulic fluid at low pressure from a fluid reservoir 64 and supplies pressurised fluid to the transmission control elements via an output line 66. A pressure regulator valve (PRV) 68 is connected to the pump output line 66, and serves to regulate the fluid pressure (hereinafter referred to as line pressure) in the line 66 by returning a controlled portion of the fluid therein to the reservoir 64 via a return line 70. In addition, the pressure regulator valve 68 supplies fluid pressure for the torque converter 24 via a supply line 74. The precise designs of the pump and pressure regulator valve are not critical to the present invention; a representative pump is disclosed in US-A-4 342 545, and a representative pressure regulator valve is disclosed in US-A-4 283 970. The transmission shaft 42 and a further transmission shaft 90 each have a plurality of rotary gear elements supported thereon. Specifically, gear elements 80 to 88 are supported on the shaft 42, and gear elements 92 to l02 are supported on the shaft 90. The gear element 88 is rigidly connected to the shaft 42, and the gear elements 98 and l02 are rigidly connected to the shaft 90. The gear element 92 is connected to the shaft 90 by way of a free-wheel (one-way) device 93. The gear elements 80, 84, 86 and 88 are maintained in meshing engagement with the gear elements 92, 96, 98 and l00 respectively, and the gear element 82 is arranged to be coupled to the gear element 94 by way of a reverse idler gear l03. The shaft 90, in turn, is coupled to the drive axles 20 and 22 by way of gear elements l02 and l04 and a conventional differential gear set (DG) l06. A dog clutch l08 is splined on the shaft 90 so as to be axially slidable thereon, and serves to rigidly connect the shaft 90 to either the gear element 96 (as shown) or the gear element 94. A forward-speed relation is established between the gear element 84 and the shaft 90 when the dog clutch l08 connects the shaft 90 to the gear element 96, and conversely a reverse-speed relation is established between the gear element 82 and the shaft 90 when the dog clutch l08 connects the shaft 90 to the gear element 94. The clutching devices 28 to 34 each comprise an input member rigidly connected to the respective transmission shaft 42 or 90, and an output member rigidly connected to one or more gear elements such that engagement of a clutching device couples together the respective gear element and shaft to effect a driving connection between the transmission shafts 42 and 90. The clutching device 28 couples the shaft 42 to the gear element 80; the clutching device 30 couples the shaft 42 to the gear elements 82 and 84; the clutching device 32 couples the shaft 90 to the gear element l00; and the clutching device 34 couples the shaft 42 to the gear element 86. Each of the clutching devices 28 to 34 is biased towards a disengaged condition by means of a return spring (not shown). Engagement of the respective clutching device is effected by supplying fluid pressure to an apply chamber thereof. The resulting torque capacity of the clutching device is a function of the applied pressure less the return spring pressure, hereinafter referred to as the working pressure ΔP. The circled numeral l represents a fluid passage for supplying pressurised fluid to the apply chamber of the clutching device 28; the circled numeral 2 and letter R represent a fluid passage for supplying pressurised fluid to the apply chamber of the clutching device 30; the circled numeral 3 represents a fluid passage for supplying pressurised fluid to the apply chamber of the clutching device 32; and the circled numeral 4 represents a fluid passage for directing pressurised fluid to the apply chamber of the clutching device 34. The various gear elements 80 to 88 and 92 to l00 are relatively sized such that the engagement of first, second, third and fourth forward speed ratios is effected by engaging the clutching devices 28, 30, 32 and 34 respectively, it being necessary for the dog clutch l08 to be in the position depicted in Figure l for a forward speed ratio to be obtained. A neutral speed ratio, corresponding to effective disconnection of the drive axles 20 and 22 from the engine output shaft l8, is effected by maintaining all the clutching devices 28 to 34 in a released condition. The speed ratios defined by the various pairs of gear elements are generally characterised by the ratio of the turbine speed N t to output speed N o . Representative N t /N o ratios for the transmission l4 are as follows: As is indicated above, shifting from a current forward speed ratio to a desired forward speed ratio requires that the clutching device associated with the current speed ratio (off-going device) be disengaged and the clutching device associated with the desired speed ratio (on-coming device) be engaged. For example, a shift from the first forward speed ratio to the second forward speed ratio involves disengagement of the clutching device 28 and engagement of the clutching device 30. The fluid control elements of the transmission l4 include a manual valve l40, a directional servo l60 and a plurality of electrically operated fluid valves l80 to l90. The manual valve l40 operates in response to operator demand, and serves, in conjunction with the directional servo l60, to direct regulated line pressure to the appropriate one of the fluid valves l82 to l88. The fluid valves l82 to l88, in turn, are individually controlled to direct fluid pressure to the clutching devices 28 to 34. The fluid valve l80 is controlled to direct fluid pressure from the pump output line 66 to the pressure regulator valve 68, and the fluid valve l90 is controlled to direct fluid pressure from the line 74 to the clutching device 26 of the torque converter 24. The directional servo l60 operates in response to the setting of the manual valve l40, and serves to properly position the dog clutch l08. The manual valve l40 includes a shaft l42 for receiving axially directed mechanical input from the operator of the motor vehicle in relation to the speed range the operator desires. The shaft l42 is also connected to an indicator mechanism l44 by way of a suitable mechanical linkage, as indicated generally by the broken line l46. Fluid pressure from the pump output line 66 is applied as an input to the manual valve l40 via an input line l48, and the valve outputs include a forward (F) output line l50 for supplying fluid pressure for engaging forward-speed ratios, and a reverse (R) output line l52 for supplying fluid pressure for engaging the reverse-speed ratio. Thereby, when the shaft l42 of the manual valve l40 is moved to the D4, D3 or D2 positions shown on the indicator mechanism l44, line pressure from the line l48 is directed to the forward (F) output line l50. When the shaft l42 is in the R position shown on the indicator mechanism l44, line pressure from the line l48 is directed to the reverse (R) output line l52. When the shaft l42 of manual valve l40 is in the N (neutral) or P (park) positions, the input line l48 is isolated, and the forward and reverse output lines l50 and l52 are connected to an exhaust line l54 that is adapted to return any fluid therein to the fluid reservoir 64. The directional servo l60 is a fluid-operated device, and includes an output shaft l62 connected to a shift fork l64 for axially shifting the dog clutch l08 on the shaft 90 to selectively enable either a forward or a reverse speed ratio. The output shaft l62 is connected to a piston l66 that is axially movable within the servo housing l68. The axial position of the piston l66 within the housing l68 is determined by the fluid pressures supplied to the chambers l70 and l72. The forward output line l50 of the manual valve l40 is connected by way of a line l74 to the chamber l70, and the reverse output line l52 of the manual valve l40 is connected by way of a line l76 to the chamber l72. When the shaft l42 of the manual valve l40 is in a forward-range position, the fluid pressure in the chamber l70 urges the piston l66 rightwardly as viewed in Figure l, to engage the dog clutch l08 with the gear element 96 for enabling engagement of a forward-speed ratio. Conversely, when the shaft l42 of the manual valve l40 is moved to the R position, the fluid pressure in the chamber l72 urges the piston l66 leftwardly as viewed in Figure l, to engage the dog clutch l08 with the gear element 94 for enabling engagement of the reverse-speed ratio. In each case, the actual engagement of the forward or reverse-speed ratio is not effected until engagement of the clutching device 30 has occurred. The directional servo l60 also operates as a fluid valve for enabling the reverse-speed ratio. To this end, the directional servo l60 includes an output line l78 connected to an electrically operated fluid valve l86. When the operator selects a forward-speed ratio and the piston l66 of the directional servo l60 is in the position depicted in Figure l, the passage between the lines l76 and l78 is cut off; conversely, when the operator selects the reverse-speed ratio, the passage between the lines l76 and l78 is open. The electrically operated fluid valves l80 to l90 each receive fluid under pressure at an input passage thereof from the pump 60, and are individually controlled to direct fluid pressure to the pressure regulator valve 68 or to respective ones of the clutching devices 26 to 34. The fluid valve l80 receives line pressure directly from the pump output line 66, and is controlled to direct a variable amount of such pressure to the pressure regulator valve 68, as indicated by the circled letter V. The fluid valves l82, l86 and l88 receive fluid pressure from the forward output line l50 of the manual valve l40, and are controlled to direct variable amounts of such pressure to the clutching devices 34, 32 and 28, as indicated by the circled numerals 4, 3 and l respectively. The fluid valve l86 receives fluid pressure from the forward output line l50 and the directional servo output line l78, and is controlled to direct a variable amount of such pressure to the clutching device 30, as indicated by the circled numeral 2 and the circled letter R. The fluid valve l90 receives fluid pressure from the line 74 of the pressure regulator valve 68, and is controlled to direct a variable amount of such pressure to the release chamber 56 of the clutching device 26, as indicated by the circled numeral 6. The apply chamber 54 of the clutching device 26 is supplied with fluid pressure from the output line 74 via the orifice l92, as indicated by the circled numeral 5. Each of the fluid valves l80 to l90 includes a respective spool element 2l0 to 220, axially movable within the respective valve body for directing fluid flow between input and output passages. When a respective spool element 2l0 to 220 is in its rightmost position as viewed in Figure l, the input and output passages are interconnected. Each of the fluid valves l80 to l90 includes an exhaust passage, as indicated by the circled letters EX, such passage serving to drain fluid from the respective clutching device when the spool element is shifted to the leftmost position as viewed in Figure l. In Figure l, the spool elements 2l0 and 2l2 of the fluid valves l80 and l82 are shown in their rightmost position, interconnecting the respective input and output lines, whereas the spool elements 2l4, 2l6, 2l8 and 220 of the fluid valves l84, l86, l88 and l90 are shown in their leftmost position, interconnecting the respective output and exhaust lines. Each of the fluid valves l80 to l90 includes a respective solenoid 222 to 232 for controlling the position of its spool element 2l0 to 220. Each of the solenoids 222 to 232 comprises a respective plunger 234 to 244 connected to the respective spool element 2l0 to 220, and a solenoid coil 246 to 256 surrounding the respective plunger. One terminal of each such solenoid coil 246 to 256 is connected to ground potential as shown, and the other terminal is connected to a respective output line 258 to 268 of a control unit 270 which governs solenoid coil energisation. As will be described, the control unit 270 pulse-width-modulates the solenoid coils 246 to 256 according to a predetermined control algorithm, to regulate the fluid pressure supplied to the pressure regulator 68 and the clutching devices 26 to 34, the duty cycle of such modulation being determined in relation to the desired magnitude of the supplied pressures. Although the fluid valves l80 to l90 are illustrated as spool valves, other types of valve could alternatively be used. By way of example, valves of the ball and seat type could be used. In general terms, the fluid valves l80 to l90 may comprise any three-port pulse-width-modulated valving arrangement. Input signals for the control unit 270 are provided on the input lines 272 to 284. A position sensor (S) 286 responsive to movement of the manual valve shaft l42 provides an input signal to the control unit 270 via the line 272. Speed transducers 288, 290 and 292 sense the rotational velocity of various rotary members within the transmission l4, and supply speed signals in accordance therewith to the control unit 270 via lines 274, 276 and 278 respectively. The speed transducer 288 senses the velocity of the transmission shaft 42 and therefore the turbine or transmission input speed N t ; the speed transducer 290 senses the velocity of the drive axle 22 and therefore the transmission output speed N o ; and the speed transducer 292 senses the velocity of the engine output shaft l8 and therefore the engine speed N e . The position transducer 294 is responsive to the position of the engine throttle l6, and provides an electrical signal in accordance therewith to the control unit 270 via the line 280. A pressure transducer 296 senses the manifold absolute pressure (MAP) of the engine l2, and provides an electrical signal to the control unit 270 in accordance therewith via the line 282. A temperature sensor 298 senses the temperature of the oil in the transmission fluid reservoir 64, and provides an electrical signal in accordance therewith to the control unit 270 via the line 284. The control unit 270 responds to the input signals on the input lines 272 to 284 according to a predetermined control algorithm as set forth herein, for controlling energisation of the fluid solenoid valve coils 246 to 256 via the output lines 258 to 268. As such, the control unit 270 includes an input/output (I/O) device 300 for receiving the input signals and outputting the various pulse-width-modulation signals, and a microcomputer 302 which communicates with the I/O device 300 via an address-and-control bus 304 and a bidirectional data bus 306. Flow diagrams representing suitable program instructions for developing the pulse-width-modulation outputs in conformity with the present invention are depicted in Figures 4 to 7. Graphs A to E of Figure 2 share a common time base and depict a 2-3 upshift made in response to a step-out condition where the accompanying torque reversal, if any, is expected to be relatively minor. Graph A depicts the engine throttle position; Graph B depicts the speed ratio N t /N e across the torque converter 24; Graph C depicts the pressure command P(OFG) for the off-going (active) clutching device 30; Graph D depicts the pressure command P(ONC) for the on-coming clutching device 32; and Graph E depicts the output or axle torque T o . At time t₀, the engine throttle is partially released from the relatively high torque position P h to a moderate torque position P m , and the engine speed N e drops somewhat. Accordingly, the ratio N t /N e increases at a faster rate, as seen in Graph B. Based on the absolute throttle position and its rate of change, however, the control unit 270 detects a step-out condition, but determines that this condition is relatively minor. That is, an initial torque reversal may occur due to the step-out, but it will be relatively minor (not sharp), and no significant driveline torque disruption is expected to result therefrom. In the situation depicted in Figure 2, no torque reversal occurs. In all such cases, the active clutching device 30 is maintained in the applied condition until the upshift occurs. At time t₁, a 2-3 upshift is commanded, with the fill phase of the shift taking place between times t₁ and t₂. Under normal, non-step-out conditions, the duration of the fill phase is calibrated relative to the fluid supply pressure so that its termination coincides with the point at which the on-coming clutching device is prepared to transmit torque. However, since the illustrated upshift is being made in response to a throttle release, the fill time t fill is shortened to soften the application of the on-coming clutching device 32. In addition, the apply and release rates for the on-coming and off-going torque-establishing devices 32 and 30 following the fill phase are lower than the schedules used in a non-step-out upshift. In a non-step-out upshift, the pressure command for the on-coming clutching device at the end of the fill phase steps immediately to a predetermined initial value P i , and then progressively increases over a predetermined time interval t f to a final value P f , as indicated by the broken trace in Graph D of Figure 2, the predetermined values being determined as a function of an input torque-related variable. Details relating to the development of the non-step-out pressure commands are set forth in EP-A-0230101 When the upshift is made in response to a detected step-out condition, however, the pressure command for the on-coming clutching device at the end of the fill phase drops immediately to near zero, and then progressively increases over the predetermined time interval t f to the final value P f , as shown by the solid trace in Graph D of Figure 2. The pressure command for the off-going clutching device is reduced in step with the pressure command for the on-coming clutching device so that the release of the off-going clutching device occurs simultaneously with the apply of the on-coming clutching device. In this way, the speed ratio transition is softened, and the occurrence of a sharp torque reversal across the transmission gear elements due to the upshift is avoided. At time t₄, the off-going clutching device 30 is fully released and the on-coming clutching device 32 is fully applied, so completing the shift. A relatively severe step-out condition is depicted in Figure 3, where the Graphs A to E depict the same parameters as are depicted in Graphs A to E of Figure 2. Also, as in Figure 2, the throttle release triggers a 2-3 upshift. Prior to time t₀, the clutching device 30 is applied to engage the second-speed ratio. The engine throttle is set at a relatively high torque position P h , and the torque converter clutching device 26 is released. Accordingly, the engine attains a relatively high speed, and the speed ratio N t /N e across the torque converter 24 increases with vehicle speed. At time t₀, the operator of the vehicle fully releases the engine throttle, and the engine speed drops sharply. Since the turbine speed N t does not change instantaneously, the ratio N t /N e also rises sharply. Due to the low absolute throttle position, and the high rate of change of throttle position, the control unit 270 determines that a relatively severe torque reversal is imminent. If an upshift is possible under such conditions, the active clutching device is released prior to the torque reversal and shift, to avert the gear lash-related driveline torque disturbance. The proper moment for releasing the active clutching device 30 is identified by monitoring the speed ratio N t /N e across the torque converter 24. When the ratio N t /N e is less than l:l, the engine is driving the vehicle, and positive torque is being transmitted through the torque converter 24. When the ratio N t /N e is greater than l:l, the vehicle is driving the engine, and negative torque is being transmitted through the torque converter 24. When the ratio N t /N e is exactly l:l, no torque is being transmitted through the torque converter 24. This characteristic relationship between the ratio N t /N e and torque is used to control the release of the active clutching device by defining a window of ratios within which the release of the active clutching device will prevent the torque reversal from being transmitted to the driveline. The upper boundary of the window is defined by the l:l ratio, and the lower boundary of the window is defined by the reference ratio N ref . When the actual ratio N t /N e enters the window as indicated at time t₁ in Graph B, the output torque T o is dropping rapidly, and the clutching device 30 is quickly released. Once the clutching device 30 is fully released at time t₂, the transmission l4 is effectively in neutral, and the output torque T o remains at zero. At such time the turbine speed N t falls substantially to the engine speed N e , and the ratio N t /N e floats up to a value of unity (that is, l:l). At time t₃, a 2-3 upshift is commanded, and a shortened fill phase takes place between times t₃ and t₄, as described in relation to Figure 2. Beginning at time t₄, the pressure commands for the on-coming and off-going torque-establishing devices are scheduled as described in relation to Figure 2, to progressively effect the upshift. As the on-coming clutching device 32 develops torque capacity, the output torque T o increases to a dynamic level as seen in Graph E, and then decreases at time t₅ to its lower, post-shift level. At time t₆, the off-going clutching device 30 is fully released, and the on-coming clutching device 32 is fully applied, so completing the shift. The timely release of the active clutching device (as in Figure 3) prevents step-out input torque reversals from being applied across the transmission gear elements. The characteristic initial driveline torque disturbance due to gear element lash prior to the impending upshift is thereby avoided. The modification of the impending upshift parameters (as in Figures 2 and 3) softens the speed ratio transition effected by the impending upshift, and avoids a lash-related driveline torque disturbance due to the upshift. Taken together, the two measures operate to provide a high-quality step-out upshift without the use of one-way torque-transmitting elements. In Figures 2 and 3, it is assumed that the torque converter clutching device is released at the time of the step-out. This is a reasonable assumption, especially in severe cases, because torque converter clutches are typically released under heavy throttle conditions, to permit higher engine speeds and multiplication of torque through the torque converter. Nevertheless, the torque converter clutching device may be applied at the time of the step-out, and the control system of this invention includes a provision for releasing it as soon as a step-out is detected, even if the step-out is not severe enough to warrant a release of the active clutching device. If the torque converter clutching device is applied when a step-out is detected, the drivetrain is significantly stiffer, and the active clutching device is released immediately to avoid the transmission of a torque reversal, and minimise the torque required to complete the impending upshift. The flow diagrams depicted in Figures 4 to 7 represent program instructions to be executed by the microcomputer 302 of the control unit 270 in implementing ratio-shifting control functions in conformity with the present invention. The flow diagram of Figure 4 represents a main or executive program which calls various subroutines for executing particular control functions as necessary. The flow diagrams of Figures 5 to 7 represent the functions performed by those subroutines which are most pertinent to the present invention. With reference now more particularly to Figure 4, reference numeral 320 designates a set of program instructions executed at the initiation of each period of vehicle operation for initialising the various registers, timers etc used in carrying out the control functions in conformity with the present invention. Following such initialisation, the instruction blocks 322 to 332 are repeatedly executed in sequence, as designated by the flow diagram lines connecting such instruction blocks and the return line 334. The instruction block 322 reads and conditions the various input signals applied to the I/O device 300 via the lines 272 to 284, and updates (increments) the various control unit timers. The instruction block 324 calculates various terms used in the control algorithms, including the transmission input torque T i , the speed ratio N t /N e , and the rate of change of throttle position ΔT. The instruction block 326 performs the step-out release logic to determine whether a release of the active clutch and/or a modification of the shift parameters is required. An expanded description of the instruction block 326 is given below in relation to the flow diagram of Figure 5. The instruction block 328 determines the desired speed ratio, R des , in accordance with a number of inputs including throttle position, vehicle speed and manual valve position. In transmission control, this function is generally referred to as shift pattern generation. The instruction block 330 determines the clutching device pressure commands for effecting a ratio shift, if required. The pressure commands for the pressure regulator valve PRV and non-shifting clutching devices are also determined. An expanded description of the instruction block 330 is given below in relation to the flow diagrams of Figures 6 and 7. The instruction block 332 converts the clutching device and PRV pressure commands to a PWM duty cycle based on the operating characteristics of the various actuators (empirically determined), and energises the actuator coils accordingly. As indicated above, the flow diagram of Figure 5 represents the step-out release logic generally referred to at the main loop instruction block 326 of Figure 4. A number of flags are used to indicate the status of the engine l2 and transmission l4. The "RELEASE ENABLE" flag is used to indicate whether a step-out of sufficient magnitude to warrant release of the active clutching device has been detected. The "STEP-OUT" flag is used to indicate whether a step-out of sufficient magnitude to warrant modification of the shift parameters has been detected. The "RELEASE" flag indicates that the active clutching device has already been commanded to the released condition. Initially, each of the above flags is reset by the initialisation instructions designated by the reference numeral 320 in Figure 4. The step-out release logic is initiated by determining at the decision block 336 whether the "RELEASE" flag is set. If so, the active clutching device has already been released, and the remainder of the routine is skipped, as indicated by the flow diagram line 338. If not, the decision block 340 is executed, to determine whether the "RELEASE ENABLE" flag is set. If it is determined at the decision block 340 that the "RELEASE ENABLE" flag is not set, it has not yet been determined that a release of the active clutching device is warranted, and the decision blocks 342 to 346 are executed to test the relevant input conditions. The decision block 342 determines whether the change in throttle position ΔTP is greater than a relatively high reference change ΔREF hi , such as 20%/msec. The decision block 344 determines whether the absolute throttle position TP is less than a relatively low reference position REF₁₀, such as l0%. The decision block 346 determines whether, given the present throttle position and vehicle speed, an upshift is possible. If any of the decision blocks 342 to 346 are answered in the negative, the throttle release (if any) is not severe enough to warrant a release of the active clutching device, and the flow diagram branch designated generally by the reference numeral 348 is executed, to determine whether a less severe step-out has occurred. If all the decision blocks 342 to 346 are answered in the affirmative, the instruction block 350 is executed, to set the "RELEASE ENABLE" flag and to start a RELEASE TIMER. Once the release enable conditions have been met and the release flag set, the decision block 352 is executed, to determine whether the ratio N t /N e across the torque converter 24 is within a ratio window defined in relation to a speed ratio of l:l. Such window is graphically depicted in Graph B of Figure 2. Algebraically, this condition is determined by comparing the expression (N t /N e - l.0) with a reference, N ref . Such expression is viewed as a measure of the torque being transmitted through the torque converter 24. If the ratio N t /N e is within the window, the instruction block 354 is executed, to set the "RELEASE" flag, to command the active clutching device to a released state, and to reset the "RELEASE ENABLE" flag. If the ratio N t /N e is not within the window, the execution of the instruction block 354 is skipped. A provision is also made for cancelling the release-enable mode after the release conditions have already been detected if the throttle position is no longer lower than the reference REF₁₀, or if the RELEASE TIMER exceeds a preset time reference, t ref . Thus if either of the decision blocks 356 or 358 is answered in the negative, the instruction block 360 is executed, to reset the "RELEASE ENABLE" flag. As indicated above, the flow diagram branch 348 detects the occurrence of a less severe step-out operation. The conditions which are tested to detect such condition are the same as described above in relation to severe step-out detection, except for the reference values. Once it is determined at the decision block 362 that the "STEP-OUT" flag is not set, the decision blocks 364 to 368 are executed. The decision block 364 determines whether the change in throttle position Δtp is greater than a moderate reference change ΔREF mod , such as 5%/msec. The decision block 366 determines whether the absolute throttle position TP is less than a moderate reference position REF mod , such as 30%. In practice, it has been found that the term REF mod should be scheduled as a function of engine speed N e so that at relatively high engine speeds a step-out condition will be detected even though the engine throttle position is not reduced to less than l0%. In other words, the reference REF mod increases with increasing engine speed N e . Preferably the increase in REF mod for a given engine speed is scheduled in relation to the running friction of the engine at that speed. The decision block 368 determines whether, given the present throttle position and vehicle speed, an upshift is possible. If any of the decision blocks 364 to 368 are answered in the negative, the step-out (if any) is not severe enough to warrant an adjustment of the shift parameters, and the instruction block 370 is executed to reset the "STEP-OUT" flag. If the decision blocks 364 to 368 are all answered in the affirmative, a significant step-out condition is indicated, and the instruction block 372 is executed, to set the "STEP-OUT" flag, and to command the torque converter clutching device 26 to a released state in anticipation of the impending upshift. In view of the above, it will be evident that in step-out situations for which the attendant torque reversal, if any, is expected to be relatively minor, the "STEP-OUT" flag is set. In more severe step-out situations for which the attendant torque reversal is expected to be sharp and result in an undesirable driveline torque disturbance, both the "STEP-OUT" flag and the "RELEASE" flag are set. However, the flow diagram of Figure 5 also provides that neither the "STEP-OUT" flag nor the "RELEASE" flag is set -- that is, no action is taken -- if it is determined that an upshift is not possible. Strictly speaking, this condition is not essential to the invention, and driveline torque disturbances can also be avoided when an upshift is not possible by releasing the active torque-establishing device as described above. The condition is thus optional; it is included in the preferred embodiment set forth herein because it has proved that releasing and subsequently applying the same torque-establishing device is not necessarily desirable in terms of driveability. The flow diagrams of Figures 6 and 7 set forth the clutch and PRV pressure determination algorithm generally referred to at the main loop instruction block 330 of Figure 4. As will be explained below, the flow diagram of Figure 7 is actually a subset of the flow diagram of Figure 6. On entry into the flow diagram of Figure 6, the blocks designated generally by the reference numeral 388 are executed, to set up initial conditions if a shift is in order. If a shift is in order, the blocks designated generally by the reference numeral 390 are executed, to develop pressure commands for the clutching devices involved in the shift. Thereafter, the instruction blocks 392 and 394 are executed, to develop pressure commands for the non-shifting clutches and the pressure regulator valve PRV, so completing the routine. As is indicated at instruction block 394, the pressure command for the regulator valve PRV is set to be equal to the highest of the pressure commands for the various clutching devices. The blocks designated by the reference numeral 388 include: the decision block 396 for determining whether a shift is in progress as indicated by the "SHIFT IN PROGRESS" flag; the decision block 398 for determining whether the actual speed ratio R act (that is, N o /N t ) is equal to the desired speed ratio R des determined at the instruction block 328 of Figure 4; and the blocks 400 to 404 for setting up the initial conditions for a ratio shift. The blocks 400 to 404 are executed only when the decision blocks 396 and 398 are both answered in the negative. In such case, the instruction block 400 serves to set the old ratio variable, R old , equal to R act , to set the "SHIFT IN PROGRESS" flag, to clear the shift timers, and to calculate the fill time t fill for the on-coming clutching device. The decision block 402 is then executed, to determine whether the "STEP-OUT" flag is set. If so, the instruction block 404 is executed, to decrease the shift parameter relating to the fill time, t fill , by a predetermined amount, t mod . If the "STEP-OUT" flag is not set, the fill time t fill determined at instruction block 400 remains intact. If a shift is in progress, the execution of the blocks 398 to 404 is skipped, as indicated by the flow diagram line 406. If no shift is in progress, and the decision block 398 is answered in the affirmative, the execution of the blocks 400 to 404 and the blocks designated by the reference numeral 390 is skipped, as indicated by the flow diagram line 408. The blocks designated by the reference numeral 390 include: the decision block 4l0 for determining whether the shift is an upshift or a downshift; the instruction block 4l2 for developing pressure commands for the on-coming and off-going clutching devices if the shift is an upshift; and the instruction block 4l4 for developing the pressure commands for the on-coming and off-going clutching devices if the shift is a downshift. The flow diagram of Figure 7 expands on the instruction block 4l2 -- upshift logic and control. On entry into the flow diagram of Figure 7, the decision block 420 is first executed, to determine whether the fill phase of the shift is complete, as indicated by the "FILL COMP" flag. If not, the flow diagram branch generally designated by the reference numeral 422 is executed; if so, the flow diagram branch generally designated by the reference numeral 424 is executed. The flow diagram branch 422 includes a fill initialising routine comprising the blocks 426 and 428, and a fill completion routine comprising the blocks 430 and 432. At the beginning of each shift, the "FILL COMP" flag is not set, and the decision block 426 of the fill initialising routine is executed to determine whether the fill phase has started, as indicated by the "FILL START" flag. Initially, the "FILL START" flag is not set, and the instruction block 428 is executed, to set the energisation duty cycle of the on-coming clutching device, DC(ONC), equal to l00%, to set the "FILL START" flag, and to start the FILL TIMER. Thereafter, the decision block 426 is answered in the affirmative, and execution of the instruction block 428 is skipped, as indicated by the flow diagram line 434. The decision block 430 of the fill completion routine determines whether the count in FILL TIMER is greater than or equal to the fill time t fill determined at the instruction block 400 of Figure 6. If so, the instruction block 432 is executed, to set DC(ONC) equal to 0% and to set the "FILL COMP" flag. If the decision block 430 is answered in the negative, the fill phase is incomplete, and execution of the instruction block 432 is skipped, as indicated by the flow diagram line 436. The flow diagram branch 424 includes a shift initialising routine comprising the blocks 438 to 446, and a shift completion routine comprising the blocks 450 to 458. The decision block 438 of the initialising routine determines whether the "FILL COMP" flag has just been set, as indicated by the status of the "FIRST FILL" flag. If so, the blocks 440 to 446 are executed, to set up the torque and inertia phases of the shift. The decision block 440 determines whether the "STEP-OUT" flag is set. If not, the instruction block 442 is executed, to determine the pressure parameters P i , P f and t f for the on-coming (ONC) and off-going (OFG) clutching devices according to a predetermined and torque-dependent pressure schedule. If the "STEP-OUT" flag is set, the instruction block 444 is executed, to determine a modified pressure schedule as described in relation to graphs D of Figures 2 and 3 and to reset the "STEP-OUT" flag. Then the instruction block 446 starts the inertia phase timer, IP TIMER, and resets the "FIRST FILL" flag. Thereafter, the decision block 438 is answered in the negative, and further execution of the blocks 438 to 446 is skipped, as indicated by the flow diagram line 448. In the inertia phase completion routine, the decision block 450 is executed, to determine whether the count in IP TIMER is at a maximum value, MAX. If so, the shift is complete, and the instruction block 452 is executed, to reset the "SHIFT IN PROGRESS" flag, to set the on-coming duty cycle DC(ONC) equal to l00%, to set the off-going duty cycle, DC(OFG), equal to 0%, and to reset the "RELEASE" and "RELEASE ENABLE" flags. If the decision block 450 is answered in the negative, the decision block 454 is executed, to determine whether the "RELEASE" flag is set. If not, the instruction block 456 is executed, to determine the on-coming and off-going pressure commands, P(ONC) and P(OFG), in the normal manner as a function of the P i , P f , t f and IP TIMER values. If the "RELEASE" flag is set, the instruction block 458 is executed, to set the off-going duty cycle DC(OFG) equal to zero, and to determine the on-coming pressure command P(ONC) as a function of the P f , t f and IP TIMER values to obtain the result described above in relation to graphs D of Figures 2 and 3. In the manner described above, the activation and de-activation of the torque-establishing devices serves to minimise or prevent driveline torque disturbances in step-out situations. The characteristic driveline torque disturbance due to gear lash prior to the impending upshift is avoided in severe step-out situations by de-activating the active (off-going) torque-establishing device, and the characteristic driveline torque disturbance associated with the normal activation of the on-coming torque-establishing device is minimised by adjusting the predetermined shift parameters in both minor and severe step-out situations. As a result, the objective of achieving a high-quality step-out upshift without the use of one-way torque-transmitting elements is achieved.

Claims: A method of operating a motor vehicle which prevents driveline torque disturbances due to lash in speed ratio mechanisms when a reduction in operator demand causes the torque carried by a transmission input shaft to reverse polarity, in a motor vehicle having an engine (l2) for producing output torque in accordance with operator demand, an automatic transmission (l4) having upper and lower speed ratio mechanisms for effecting predefined upper and lower speed ratios between input (42) and output (22) shafts thereof and a fluid-operated torque-establishing device (28 to 32) associated with each of the upper and lower speed ratio mechanisms, each such torque-establishing device (28 to 32) being effective when activated to establish a bidirectional path of torque transmission between the transmission input (42) and output (22) shafts via the associated speed ratio mechanism, a fluid coupling (24) for transmitting the engine output torque to the transmission input shaft (42), and a shift controller (270) for selecting a desired speed ratio in accordance with the operator demand and selectively activating and de-activating the torque-establishing devices (28 to 32) so as to establish the desired speed ratio, the method comprising the steps of: monitoring the operator demand and detecting the occurrence of a specified operating condition which is likely to result in a relatively sharp reversal in the polarity of torque applied to the transmission input shaft (42) by way of the fluid coupling (24); determining a measure of the torque transmitted through the fluid coupling (24) in relation to the relative speeds of the engine (l2) and transmission input shaft (42) following the detection of the specified operating condition; and de-activating any activated torque-establishing device (28 to 32) when the measure of torque becomes less than a reference amount, so that when the reversal in applied torque occurs the torque-establishing devices (28 to 32) are ineffective to transmit such torque to the vehicle driveline. A method according to claim l, characterised in that the measure of torque transmitted through the fluid coupling (24) is determined in relation to the ratio of engine speed to transmission input shaft speed. A method according to claim l or 2, characterised in that the engine (l2) has a throttle (l6) adjustable in accordance with operator demand, and the operator demand is monitored by sensing the throttle position. A method according to claim 3, characterised in that the specified operating condition is detected when the sensed throttle position is below a reference throttle position and the rate of change in sensed throttle position is greater than a reference throttle position rate. A method according to claim 3 or 4, characterised in that the method includes the step of: inhibiting the de-activation of an activated torque-establishing device (28 to 32) if the sensed throttle position increases above the reference throttle position after the specified operating condition has been detected. A method according to any one of claims l to 5, characterised in that the method includes the step of: inhibiting the de-activation of an activated torque-establishing device (28 to 32) if the measure of torque transmitted through the fluid coupling (24) fails to become less than the reference amount within a predetermined time after the specified operating condition has been detected. A method according to any one of claims l to 6, characterised in that the method includes the step of: inhibiting the de-activation of an activated torque-establishing device (28 to 32) if the detected operating conditions will not cause the shift controller (270) to effect an upshift from the lower speed ratio to the upper speed ratio. A method according to any one of claims l to 7, characterised in that: the step of monitoring the operator demand is effected to identify the occurrence of a first set of operating conditions likely to result in a relatively minor reversal in the polarity of torque applied to the transmission input shaft (42) and a second set of operating conditions likely to result in a relatively sharp reversal of the polarity of torque applied to the transmission input shaft (42); in response to the identification of the first or second set of operating conditions the predetermined shift parameters of the shift controller (270) are adjusted so as to soften the activation of the torque-establishing device (28 to 32) associated with the upper speed ratio mechanism in the impending upshift; and the step of determining a measure of the torque transmitted through the fluid coupling (24) is effected in response to the identification of the second set of operating conditions, with the de-activation of any activated torque-establishing device (28 to 32) being effected when such measure becomes less than a reference amount corresponding to a relatively low input torque level. A method according to claim 8, characterised in that, in a motor vehicle in which the engine (l2) has a throttle (l6) adjustable in accordance with operator demand, the identification of the first set of operating conditions is effected by comparison of the actual throttle position and the actual rate of change in throttle position with a first reference position indicative of a moderate throttle setting and a first reference rate of change indicative of a moderate rate of change in throttle setting, and the identification of the second set of operating conditions is effected by comparison of the actual throttle position and the actual rate of change in throttle position with a second reference throttle position indicative of a relatively low throttle setting and a second reference rate of change indicative of a relatively fast rate of change in throttle position. A method according to claim 9, characterised in that the adjustment of the predetermined shift parameters is inhibited if the actual throttle position increases above the first reference position after the occurrence of the first set of operating conditions has been identified, and the de-activation of activated torque-establishing devices (28 to 32) is inhibited if the actual throttle position increases above the second reference position after the occurrence of the second set of operating conditions has been identified. A method according to any one of claims 8 to l0, characterised in that the first reference position is determined in direct relation to the engine speed so that for a given throttle movement the first set of operating conditions will be more likely to be detected at a relatively high engine speed than at a relatively low engine speed. A method according to any one of claims 8 to ll, characterised in that the predetermined shift parameters of the shift controller (270) include a predetermined fill time during which the torque-establishing device (28 to 32) associated with the upper speed ratio mechanism is filled in preparation for activation, and the adjustment of the predetermined shift parameters comprises the step of shortening the predetermined fill time for the impending upshift to the upper speed ratio.

Inventors: NITZ, LARRY THEODORE
Applicants: GENERAL MOTORS CORPORATION
Application year: 1987
Main classification: B60K41
IPC codes: F16H59, F16H61, B62D6