Automatic transmissions for motor vehicles typically include a plurality of torque transmitting devices, commonly referred to as clutches. The line pressure of the transmission is controlled according to a pressure schedule to selectively engage these clutches so as to establish different gear ratios between the input shaft and the output shaft of the transmission. A line pressure which prevents clutch slippage is utilized, since clutch slippage reduces the torque transmitted and generates heat. Although clutch slippage is prevented, typically the line pressure utilized imparts excess torque capacity. This results in higher than necessary line pressure, increasing pumping losses and reducing overall transmission efficiency. It would therefore be desirable to control the transmission line pressure to the minimum required, or optimal, value, thereby maximizing efficiency.
Various attempts have been made at automatic transmission line pressure control. For example, U.S. Pat. No. 4,805,750, issued to Nitz, discloses a steady-state slip detection/correction for a motor vehicle transmission. When significant clutch slippage is detected, line pressure is adjusted upward to increase the torque capacity of a clutch. If significant slippage is not detected over a period of specified driving cycles, the pressure is adjusted downward to decrease the torque capacity of the clutch, resulting in a low frequency clutch pressure limit cycle, enhancing the transmission efficiency while maintaining clutch slippage within acceptable limits. The presence of significant clutch slippage is detected by a routine which generates an accumulative indication of the slippage between input and output shafts of the transmission. If excessive slippage continues after the pressure has been increased, a clutch or gear failure is indicated and the transmission shift points are adjusted to reflect the loss of that gear.
Following the teachings of the Nitz reference, slip correction does not occur until after significant, unacceptable clutch slippage occurs. This methodology has inherent disadvantages since significant clutch slippage reduces transmission efficiency and degrades the durability of the clutch element.
Other U.S. Patents relating generally to controlling transmission line pressure include U.S. Pat. No. 4,958,538, issued to Yamashita et al., No. 4,972,738, issued to Narita, No. 5,082,097, issued to Goeckner et al., No. 5,086,669, issued to Fujiwara et al. and No. 5,111,718, issued to Iizuka.