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Massimo Martelli, Pietro Marani, Silvia Gessi, and Damiano Chiarabelli
Continuously Variable Transmissions (CVTs) are typically implemented in agricultural tractors by means of a power-split configuration between the diesel engine 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. 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.
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