In general, automatically shiftable transmissions include a number of gear elements and selectively engageable friction elements (referred to herein as torque transmitting mechanisms) that are controlled to establish one of several forward speed ratios between an input shaft and an output shaft of the transmission. The input shaft is coupled to an engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the drive wheels of the vehicle through a differential gearset.
Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a torque transmitting mechanism (off-going torque transmitting mechanism) associated with the current speed ratio and engaging a torque transmitting mechanism (on-coming torque transmitting mechanism) associated with the new speed ratio. Each such shift includes a preparation or fill phase during which an apply chamber of the on-coming torque transmitting mechanism is filled with pressurized fluid in preparation for torque transmission. Once filled to a predetermined amount, the fluid pressure supplied to the torque transmitting mechanism is lowered to allow smooth torque transmitting mechanism engagement.
Typically, the fill phase is carried out by commanding a maximum on-coming torque transmitting mechanism pressure for an empirically determined fill time. See, for example, the U.S. Pat. No. 4,707,789 to Downs et al., issued on Nov. 17, 1987, and assigned to the assignee of the present invention and hereby incorporated by reference in its entirety. In Downs et al., the fill time for any given torque transmitting mechanism is obtained from a look-up table as a function of the commanded fill pressure, less the torque transmitting mechanism return spring pressure, and then adjusted as a function of fluid temperature to compensate for variations in fluid viscosity. In operation, the stored fill times are then adaptively adjusted based on the observed inertia phase delay, in order to compensate for sources of error that influence the fill time. A similar approach is found in the U.S. Pat. No. 5,216,606 to Lentz et al., issued on Jun. 1, 1993, and assigned to the assignee of the present invention and hereby incorporated by reference in its entirety. In Lentz et al., the fill time is determined by table look-up based on the pump speed, as compensated for efficiency and fluid viscosity, and further modified based on the time since the last shift and a hydraulic restriction factor.
Traditional torque transmitting mechanism engagement methods provide a trim phase subsequent to the fill phase to allow smooth engagement and enhance shift quality. The trim phase provides a steady increase in torque transmitting mechanism apply pressure to the on-coming torque transmitting mechanism. Pressure variations or control system instability during the trim phase may cause poor shift quality.