Improved efficiency, better controllability and low noise are the most demanding features form a displacement machine now-a-days. Most of the conventional displacement machines are basically a reciprocating pumping element controlled by valve plates or with the help of check valve [1]. This kind of hydraulic machines loose efficiency dramatically at partial displacement because all of the pistons remain at high pressure at the cycle time and due to pressure inside the piston leakage and shear losses increases. One approach to improve the efficiency of the displacement machine can be controlling each hydraulic piston by using programmable faster valves called digital valve. As the total displacement will be controlled digitally, the total system is called Digital Displacement Technology. In digital displacement machine it is possible to disconnect some of the pistons from the load and the piston will connect only with the low pressure side, minimizing losses due to leakage and shear. As the valve will control directly with digital controller it will eliminate the necessity of servo-hydraulic control required by conventional systems. Digital valves can open fully and close again with the input signal within one revaluation of the shaft, so it gives better control to the pumping element results reduction in hysteresis and increase the linearity of the pumping element. In Digital Displacement machines by controlling the valves pistons are connected with the machine when pressure is equal, but in the traditional machines piston connection was pre-determined with the shaft angle. By doing the piston control efficiency of the machine will improve and the sound generates for the decompression flow will be reduced [17]. Also energy storage and recovery can be possible by using accumulator.

Linear Incremental Hydraulic Actuators combine one or more short-stroke cylinders, and two or more engaging/disengaging mechanisms into one actuator with long, medium, or even unlimited stroke length. The motion of each single short-stroke actuator concatenated by the engaging/disengaging mechanisms forms the motion of the linear incremental hydraulic actuator. The patterns of how these motions are concatenated form the gaits of a specific linear incremental hydraulic actuator. Linear incremental hydraulic actuators may have more than one gait. In an application, the gaits may be combined to achieve optimal performance at various operating points. The distinguishing characteristic of linear incremental hydraulic actuators is the incremental motion. The term incremental actuator is seen as analogous to the incremental versus absolute position sensor. Incremental actuators realize naturally relative positioning. Incremental motion means also that the behavior does not depend on an absolute position but only on the relative position within a cycle or step. Incremental actuators may realize discrete incremental or continuous incremental motion. Discrete incremental actuators can only approach discrete positions, whereby stepper drives are one prominent example. In contrast, continuous incremental actuators may approach any position. Linear electric motors are one example of continuous incremental actuators. The actuator has no inherent limitation in stroke length, as every step or cycle adds only to the state at the beginning of the step or cycle and does not depend on the absolute position. This led to the alternative working title Hydraulic Infinite Linear Actuator. Linear incremental hydraulic actuator provides long stroke, high force, and linear motion and has the potential to decrease the necessary resource usage, minimize environmental impact, e.g. from potential oil spillage, extend the range of feasible products: longer, stiffer, better, etc. This thesis presents an analysis of the characteristics and properties of linear incremental hydraulic actuators as well as the gaits and possible realizations of some gaits. The gait for continuous, smooth motion with two cylinders is comprehensively studied and a control concept for the tracking problem is proposed. The control concept encapsulates the complexity of the linear incremental hydraulic actuator so that an application does not have to deal with it. One other gait, the ballistic gait, which realizes fast, energy-efficient motion, enabling energy recuperation is studied.

<p>Energy conservation is a growing topic of research within various fields.  Digital Hydraulics is a division of fluid power that focuses on using on/off technology to improve the performance and efficiency of fluid power systems. One significant benefit of Digital Hydraulics is that it has enabled additional control over fluid power systems, which helps achieve component and system level improvements. Conventional radial and inline piston pumps use positive sealing valves, which mitigate leakage losses, compared to port plates commonly seen in variable displacement pumps. By using digitally controlled positive sealing valves on radial and inline piston pumps, leakage losses can be mitigated to develop a more efficient variable displacement pump. This work focuses on the design, modeling, and simulation of a mechanically actuated valving system developed for a commercially available radial piston pump. The design uses a ball screw actuation method to phase the cam during operation, changing the displacement. Using a modeling and simulation software, GT-SUITE, a simulation model was created for the digital pump that shows close correlation to the manufacturer’s data at high pressure. The parameters simulated, 50 – 200 bar, showed that the system could achieve a peak efficiency drop of approximately 11.0% from 87.0% to 76.0% from 100 – 25% displacement simulated at 200 bar and 500rpm. Compared to a typical variable displacement axial piston pump unit, the digital pump showed increased efficiencies across the bandwidth of 35-83% displacement, with a lower overall drop in efficiency across most of its operating conditions. In the comparison used, the pump is outside of its operating range and has not been optimized; thus, the simulation model created in this thesis will be used in the future to optimize the system and evaluate the system's potential performance and feasibility for future prototyping and testing as a proof of concept.  </p>