THE FOCUS OF this thesis is to evaluate and analyse secondary controlled hydraulic systems with linear loads. The main objective has been the hydraulic transformer, with special respect to Innas Hydraulic Transformer, IHT¹. One advantage of secondary controlled systems is that energy consumption can be kept low as energy can be regained when potential or kinetic energy is reclaimed, resulting in energy-efficient systems. The main obstacle to wider usage of secondary controlled systems is the difficulty of implementing linear loads, i.e. hydraulic cylinders, in an efficient way. The hydraulic transformer is the most interesting solution.

The principle of the IHT design is studied in detail. As the displacement control of the IHT design differs from conventional variable axial piston machines, port commutation will occur at other locations than the piston dead centres. At these positions the pistons will have considerable velocities, introducing pressure peaks and cavitation. Further, the design will cause driving torque variations, which in combination with torque losses of a stick-slip nature have a negative influence on low-speed controllability.

The presented study shows promising results concerning both efficiency and controllability of the IHT concept. The mechanism behind the torque variations is analysed and methods like shuttle technique and different port configurations are studied in order to minimise pressure peaks and torque variations. Problems and difficulties concerning low-speed controllability are also pointed out and different control strategies are studied.