Digital hydraulics uses simple and cheap on/off valves in order to replace expensive proportional valves. Furthermore, with fast switching hydraulic converters the energy efficiency can be raised compared to proportional valve control. The hydraulic buck converter (HBC) represents an energy efficient and cost-effective switched inertance system, because its inductance is realized by a simple pipe. In this paper, a prototype for a hydraulic linear cylinder drive controlled by an HBC is presented. Characteristic for this drive axis is that the HBC is directly mounted on the cylinder, which allows a reduction of the oil transport loss between the axis and the hydraulic power supply unit. Furthermore, piston accumulators are used for decoupling and pressure attenuation. Due to their robustness regarding the prepressure to operating pressure, the load pressure can be controlled arbitrary in the piston-sided chamber. The energy performance and the tracking behavior of the axis with a flatness-based control (FBC) are investigated by steady-state measurements and dynamic trajectories, respectively. The results are discussed and an outlook about further improvements of the concept is provided.

In digital hydraulic systems, switching valves have opening and closing times in the range of a few milliseconds. Due to this fast switching, high bandwidth pressure pulsation is excited, which is the stimulus for airborne noise up to some kilohertz. Since the human ear is very sensitive to audible noise in this frequency range, an analysis of the influence of the valve’s opening curve on the pressure surge in the pipe system is intended. The study is based on simulations employing dynamic pipe models for linear wave propagation and laminar fluid flow. In particular, a simple pipe system with a valve at one end and a pressure boundary at the other end of the pipe is investigated. It is shown, how the valve opening characteristics of spool and seat type switching valves influences the pipe responses. Also the role of parasitic inductances due to the valve block bores is discussed and it is shown how the switching characteristics influences the dynamical effects on the pressure pulsations in the pipe system.

This paper deals with the efficient computation of hydraulic switching systems with a check valve by a mixed time–frequency domain method; frequency-domain modelling is performed on the wave propagation in a pipe and time-domain modelling is applied to the switching valve and the check valve. The dual property of the check valve makes the complete problem have variational inequality properties. A solution method is presented which replaces the pressure and the flowrate of the check valve as a function of one new variable. The resulting system of non-linear algebraic equations is solved using a Newton–Raphson method in combination with a smoothing of the non-smooth properties of the check valve. The method is applied to a parameter study of a hydraulic buck converter.

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This paper is concerned with the application of fluid power in autonomous robotics where high power density and energy efficiency are key requirements. A hydraulic drive for a bioinspired quadruped robot leg is studied. The performance of a classical valve-controlled (“resistive-type”) and of an energy saving (“switching-control mode”) hydraulic actuation system are compared. After describing the bio-inspired leg design and prototyping, models for both drives are developed and energy efficiency assessments are carried out. It is shown through simulation that the switching-control mode hydraulic actuation can meet the challenge of legged robotic locomotion in terms of energy efficiency with respect to improving robot power-autonomy. An energy saving of about 75% is achieved. Limitations of the current system are identified and suggestions for improvements are outlined.