Development of a sub-shift control algorithm for an agricultural tractor with hydro-mechanical transmission

Abstract

A sub-shift control algorithm was proposed for an agricultural tractor with hydro-mechanical transmission. The hydro-mechanical transmission investigated in this study consists of a hydro-static unit and planetary gear sets which provide a continuously variable transmission function and four sub-shift gears. The sub-shift needs to be carried out at the hydro-static unit stroke where the speed of the off-going and on-coming clutches is synchronized. To reduce the hydro-static unit stroke error due to the nonlinear characteristic, the hysteresis characteristic was investigated in the test bench and a hysteresis compensator was developed. Using the compensator in the feedforward loop, a hydro-static unit stroke control algorithm was proposed and validated via experiment. To prevent the torque interruption during the sub-shift, a time delay of the off-going clutch was proposed. In addition, a hydro-static unit stroke duration which maintains the target stroke during the sub-shift was suggested to reduce the speed difference between the clutch plates. The sub-shift control algorithm including the hysteresis compensation, time delay of the off-going clutch, and hydro-static unit stroke duration was evaluated via experiment. It was found from the experimental results that the sub-shift was performed smoothly without the torque interruption, even in the acceleration state when large inertial torque change occurred.