Hydraulic elevators with conventional long-stroke hydraulic cylinders are limited for use in low-rise buildings, up to five floors, due to low hydraulic stiffness, low natural frequency, low hydraulic pressure and large oil volume. With a new hydraulic actuation technology jointly invented at Linköping University and SAAB named the Hydraulic Infinite Linear Hydraulic Actuator (HILA), these short-comings for hydraulic actuators can be reduced and hydraulic elevators can be offered for mid-rise buildings.

The HILA technology provides long strokes, high system pressure, compactness and small chamber volumes. The actuator has a higher stiffness and a higher natural frequency compared to conventional hydraulic cylinders. The higher system pressure allows for an even more compact system design, with lower flow levels and a smaller reservoir.

The HILA technology combines two short-stroke cylinders with two engaging and disengaging clamping mechanisms into one actuator with long stroke length. The motion of each single short-stroke piston linked together by the clamping mechanisms creates the motion of the piston rod. In this way the two pistons are moving along the rod in a kind of rope climbing motion.

The challenge is to implement a control system which can provide a smooth motion without unacceptable jerk at load shift as seen with ordinary directional valves. Earlier research work on HILA technology has shown that a control system with fast servo valves can fulfil these requirements. This study shows promising results from simulation analysis combined with optimization techniques, using slightly modified standard directional hydraulic valves.

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