Slip-based shift control system

DETAILED DESCRIPTION Accordingly, an object of this invention is to provide powershift transmission control system which prevents abrupt shifts and which produces smooth shifts.
Another object of this invention is to sense the torque transmitted by an off-going clutch in order to distinguish between torque present at the transmission output shaft and torque present at an external application, such as a PTO application.
Another object of this invention is to improve transmission shifting by controlling the torque transmitted by an off-going clutch during a shift.
Another object of this invention is to control such an off-going clutch so that it transmits, during a slipping (dynamic friction) condition, the same torque that it transmitted during a locked up (static friction) condition.
This and other objects are achieved by the present invention wherein clutch slip is monitored during a shift.
Shaft speeds are measured on both sides of a clutch and calculations based on gear ratios are performed during a shift to measure slip speed between the clutch friction disks and separators.
The clutch control pressure is gradually reduced or ramped downward until relative rotation (slip) between the off-going clutch friction and reaction plates is detected.
The pressure at which slip occurs in the off-going clutch is then used to calculate the portion of the engine load passing through the transmission.
This updated engine load number is then used to set various other parameters which are essential to a smooth shift.
The clutch control pressure is then increased so that the same torque is transmitted through the slipping clutch without locking up and returning to a static friction condition.
Target clutch control pressures may be calculated or modeled for known or measured load conditions and known clutch friction behavior.
Clutch control pressure may be held constant, stepped higher or lowered to compensate for the difference in static and dynamic coefficients of friction in the two clutch operating modes