Hydrostatic drives are being used in heavy, low-speed vehicles to an ever-increasing extent. The reason for this is that a number of advantages can be achieved such as stepless gear ratios, simple power distribution and high stiffness. Increased demands have led to an interest for new conceptual designs for hydrostatic transmissions. Modern concepts often consist of several gear steps, use more than one motor or divide the power between a fluid power line and a mechanical line. In this thesis, a number of development tools are used for designing and evaluating a two-motor concept.

In the concept highlighted in this thesis, two motors are connected to each other via a gear unit and controlled in sequence. One of them can be controlled to zero displacement and is disconnected from the transmission at high vehicle speeds. The challenge with the concept is the reconnection procedure, in which the motor must be accelerated, synchronised and finally locked to the output shaft of the transmission, all in a fraction of a second.

Reconnection methods discussed in the thesis are based on either transmitting the acceleration power through a disc type clutch or through the fluid power system. Investigations focus particularly on a method in which the speed of the reconnecting motoris controlled using secondary control. Low inertia and load give this control method great capability at high pressures. With addition of control compensations, fast and smooth reconnection is possible even under low-pressure conditions.

Computer-aided tools were used to test reconnection and control methods. Pure simulation was utilised both in the early design phase and as a complement to hardware tests. Results were verified using hardware-in-the-loop (HWIL) simulations, first by letting the reconnecting motor act as hardware and add influences from the rest of the system by means of simultaneous real-time simulation. Secondly, in a real vehicle, but with the reconnecting motor represented by a simulation. Finally, full-scale laboratory testing was carried out with a complete two-motor transmission.