Enhanced open-loop control of automatic gear shifting in hydromechanical CVT for agricultural tractors

Continuously Variable Transmissions (CVTs) are typically implemented in agricultural tractors by means of a power-split configuration between mechanical transmission and infinitely-variable hydrostatic unit (IVHU). The present work is based on an optimized output-coupled/compound architecture developed by the authors for a 200-kW reference tractor, originally equipped with a standard input-coupled CVT. A two-stage planetary gearset – serving as the power-split device – is complemented by a four-gear mechanical gearbox, to efficiently cover the full velocity range, up to 40 km/h, with power-shift capability, to supply an uninterrupted traction torque even during gear change. The main topic is the control of the transmission, and the backbone of the algorithm is based on an open loop ideal mapping. At any given constant engine speed and for each gear, an ideal reference condition is considered – in which the hydraulic units have ideal efficiencies – to analytically define a theoretical open-loop function, providing the requested IVHU transmission ratio as a function of vehicle velocity, and the theoretical shaft synchronization point, to shift to the previous/next gear. Real-world efficiencies affect the synchronization dynamics and, consequently, the traction force while shifting gears. An enhanced control has then been devised, keeping the simple open-loop foundation, with an additional smoothing contribution, triggered by the shaft speed feedback when speed matching is detected. The problem is studied via a lumped-parameter simulation model, focusing on the effect of the control on the longitudinal dynamics of the vehicle-wheel-terrain system. An exploratory investigation is carried out to tune the function, based on vehicle speed, drawbar pull and gear, for a smooth operation in a suitable range of operating conditions.