Shift dynamics modeling for optimizing variator slip control in a continuously variable transmission

V-belt type Continuously Variable Transmissions (CVT) are applied in an increasing number of vehicles as a result of their unparalleled shift comfort. Large ratio coverage allows for reduced engine speed, improved highway driving comfort and reduced fuel consumption. With the advent of the competing automatic transmissions with 6 or even 7 steps, it becomes increasingly important to further improve the performance in terms of efficiency, robustness and torque capacity of the CVT. Improvements on the efficiency of the push-belt CVT by reducing variator clamping forces to minimum values are well established. By reducing clamping such that the variator operates in its most efficient point, the mechanical load on this variator is minimized and hydraulic actuation losses are reduced. The control technique allows for the best possible transmission efficiency, combined with improved robustness for a slip damage. In earlier studies, the relationship between the variator slip and functional transmission properties has been described. Conditions for the optimum performance regarding efficiency and robustness were identified and validated for steady state conditions. However, the stability robustness during ratio changes proved to be insufficient during tests in an experimental vehicle. To deal with CVT slip dynamics during transient behaviour, a theoretical model is necessary. In this paper, a novel CVT shifting model, recently proposed by Carbone, is tested experimentally and compared with the model by Ide. The relationships among the clamping forces acting on the moving pulley sheave, the rate of change of speed ratio, the loading conditions, and the belt velocity, are investigated both from a theoretical and an experimental point of view.