Full toroidal IVT for the off-highway market - Hydro-mechanical control system development

Torotrak is the world leader in full toroidal traction drive technology and in its application to transmission systems. The full toroidal variator is typically configured to form part of a mechanical shunt, and the resulting Infinitely Variable Transmission (IVT) provides a continuous ratio range from forwards through 'geared-neutral' to reverse. This system offers a quiet, efficient and highly flexible power transmission solution. For agricultural tractors, Torotrak has developed two generic types of transmission; the electronically controlled twin-regime IVT and the two-range hydro-mechanically controlled IVT. Following an overview of tractor transmission requirements, this paper focuses on the hydro-mechanical ratio control system. Detailed system dynamic models are used to show the inherent stability of the system and its ability to maintain constant ratio regardless of wheel torque disturbances. The validity of these theoretical models and the quality of transmission control achieved is verified using tractor test data.