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High power density, good controllability, and low cost are the most appealing characteristics that make hydraulic systems the best choice for many applications. Current state-of-the-art hydraulic variable displacement pumps show high efficiency at high displacement while they have poor efficiencies at low displacement. This paper proposes a novel alternating flow (AF) variable displacement hydraulic pump to: 1) eliminate metering losses by acting as a high-bandwidth pump for displacement control; 2) achieve high efficiency across a wide range of operating conditions and displacements; and 3) allow multiple units to be easily common-shaft mounted for a compact multi-actuator displacement control system from a single prime mover. A dynamic model using first principles describes the cylinder pressure, flows between pairs of cylinders, and net inlet and outlet flows as a function of the pumps phase-shift angle. The model captures hydraulic check valve dynamics, the effective bulk modulus, leakage flows, and viscous friction. Piston kinematics and dynamics are discussed and energy loss models are presented. A first prototype of the AF pump was constructed from two inline triplex pumps that were modified so that three piston pairs were created. Experiment tests were conducted and used to validate the model. After model validation, simulation results from the model can offer an initial evaluation of this novel pump concept and potential applications and can be used to guide the design for future prototype of the AF hydraulic pump. [ABSTRACT FROM AUTHOR]

A digital displacement hydraulic motor uses actively controlled on/off valves instead of a valve plate for porting fluid. Displacement can then be varied by varying the timing of the on/off valves. To minimize throttling energy loss, the valve timing must also consider the precompression and decompression of the compressible fluid. In this paper, a rotary on/off valve, whose rotation controls the valve timing, is used. The desired motion of the rotary valve that encodes the ideal valve timing is a function of, and is periodic with respect to, the motor crank shaft angle. As the desired valve trajectory is not periodic in time unless the motor speed is constant, the conventional internal model based repetitive control is not applicable. A recently developed angle-domain repetitive control is used instead to control the rotary valve motion to achieve the desired valve timing. Experiments show that the proposed controller achieves good tracking performance for a broad range of varying motor speeds. The maximum error is reduced by six to ten times with the new control scheme over baseline fixed period repetitive or proportional-integral controllers. This in turn leads to efficiency gains for the digital hydraulic motor up to 45%. [ABSTRACT FROM AUTHOR]

McKibben actuators, comprised of an elastomeric tube wrapped in inextensible fibers, were developed in the 1950s and have been used primarily in pneumatic applications. Mc-Kibben actuators provide superior force and power density at a lower cost compared to many conventional actuators due to their simple design. This paper explores driving McKibben actuators with high-pressure hydraulics and reflects on some challenges associated with modeling the actuators at hydraulic pressures. Twelve actuators were designed, fabricated, and tested experimentally over a range of contraction ratios. Four modeling approaches were evaluated against experimental data including a new model that eliminates some of the limitations of existing models by predicting output force as a function of initial actuator geometry only and capturing the nonlinear variation in elastomer wall thickness as the actuator is strained. The experimental results suggest that the existing McKibben force models presented in this paper are not as accurate as the new model for tracking total axial force generation. The absolute value of the error from experimental values of each of the four models ranged from 9.1% for the new model developed in this paper to 9.9%, 10.0%, and 10.5% for the three other existing approaches, but the new model has unique advantages in modeling hydraulic McKibben actuators that require thicker tube walls. This study lays the ground work for future research in modeling soft robots comprised of compliant actuators powered hydraulically at high pressures. [ABSTRACT FROM AUTHOR]