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Hydrostatic transmissions for wheel loaders - control aspects of a multiple motor concept

Sannelius, M. / Palmberg, J.O. / Rydberg, K.E.

High power capacity, wide velocity range and good overall efficiency are important demands of a modern hydrostatic transmission intenden to be used in a heavy, off-road vehicle. Alternative solutions that can meet the demands are hydrostatic transmissions extended with smart mechanical gearboxes, split torque transmissions and multiple motor transmissions. A promising multiple motor concept is one using two sequence-controlled motors. The motors are connected to each other via gear and can therefore be driven at different rotational speeds. To utilise the capacity of the motors, the motor with the higher speed is able to control to zero-displacement and will be disconnected at high velocities. The challenge with the concept is the reconnection procedure where the disconnected motor must be accelerated, synchronised to the main motor ouput speed and finally locked to the output shaft of the transmission, all in a fraction of a second. In this paper control laws regarding the connection procedure using mainly the fluid power system are tested in a real wheel loader. The results show that connection using active control of the zero-motor works best at high load conditions. To make a fast, smooth and stable connection possible also at low pressures it is necessary to add special compensations. At moderate speeds, flow compensation using the pump is a neat solution to improve softness and stability. Otherwise, filtering of the zero-motor speed reference secures the stability and limitation of the zero-motor displacement setting improves both smoothness and setting time. These controls can be adapted to the current operating conditions. This implifies that information about state variables such as pressure is needed. However, in the future such sensors will become more and more ordinary, for example to simplify power division between work and drive hydraulic.