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In this paper, two Discrete Displacement Cylinders (DDCs) are used to drive the boom of a knuckle boom crane. DDCs operate by connecting one of several available pressure levels to each chamber in order to produce different forces. A trade-off exists with such systems, between the accuracy of tracking and energy dissipation due to switching. A popular way to approach this problem is a Force Shifting Algorithm (FSA). However, in this paper, the trade-off is managed by use of a Model Predictive Control (MPC) algorithm. The tracking accuracy and energy efficiency of the MPC and FSA strategies for DDCs are compared to a PID strategy for standard cylinders. The comparison is obtained by use of a computer simulation of a knuckle boom crane performing a realistic load cycle. The load cycle consists of the crane extending to pick up a load and then retracting to place it at an appropriate location. The main results show that MPC can deliver smoother and more accurate motion than FSA, while using less energy. Compared with standard cylinders and PID control, MPC uses less energy, but due to the switching of chamber pressures, the motion is smoother with the standard strategy. Both FSA and MPC can have degraded performance when a large change in load is introduced.

The need for improvement of lifetime prediction of hydrostatic axial piston units is driven by partly outdated knowledge dating back to the 1970s and is also motivated by a significant evolution of hydrostatic drivetrains. However, the evolution of the load life is unknown and must be investigated. An additional aspect for the method improvement is the potential decrease of qualification effort caused by required evaluation background. The goal is an increase in the prediction accuracy of the lifetime by using of qualitative and quantitative calculation methods. This paper covers the analysis phase of the lifetime calculation improvement. Hence, the currently used calculation method is investigated and the improvement potential builds the research approach. The improvement potential covers calculation model of load-dependent treatment, where the characteristics of sub-components build a system assessment. The aim includes the representative load cycle acquisition for typical applications in the off-road market.

This paper discusses problems involved in the procedure for offshore installation of blades in wind turbines, due to wind loads. In general the high winds at sea provides for nearly optimal conditions for harvesting energy via wind turbines due to the often high wind speeds and low turbulence intensity. However, the very same features also call for great difficulties during installation of the wind turbine blades, making this process extremely difficult, expensive and time consuming. Often the blades are hoisted to the wind turbine hub via cranes and sought held in appropriate positions by so-called taglines/wires, but still significant motion oscillations of blade root ends are experienced, even at rather low wind speeds. The paper considers the possibility to dampen the oscillating motions via control of the tagline lengths. The main control strategy considered, is the possibility to control the orientation of the blade, i.e. the pitch, yaw and roll angles via a decentralized control approach. An overall dynamic model structure of such systems is presented, and a system analysis and control design is presented based on various considerations on the system. In order to evaluate the proposed control design, a scaled test system has been established for this purpose, and results demonstrate the proposed features of the controlled system.

The efficiency of fluid power motors and pumps is a subject to research, which has generated numerous publications during the last three decades. The main incentives for this research are optimization of reliability and efficiency through the study of loss and wear mechanisms, which are very difficult to study experimentally, whereby modeling and simulation are necessary. A common approach to theoretical investigation of the pressure generated in the lubricated joints is the use of Reynolds equation, in which the oil viscosity is modelled with dependency of both pressure and temperature. In this paper the derivation of Reynolds equation from the continuum assumption is reviewed and it is shown that the validity of Reynolds theory based pressure field solutions in oil hydraulic thermo-piezo-viscous lubrication models are subject to maximum bounds on the pressure and temperature field gradients. These bound must be evaluated a posteriori to validate that model results is complying with the principle of conservation of mass and momentum.

This paper deals with high-performance position control of hydraulics servo systems in general. The hydraulic servo system used is a two link robotic manipulator actuated by two hydraulic servo cylinders. A non-linear model of the hydraulic system and a Newton-Euler based model of the mechanical system were constructed and linearized. Controllers are implemented and tested on the manipulator. Pressure feedback was found to greatly improve system stability margins. Passive gain feedforward shows improved tracking performance for small changes in load pressure. For large changes in load pressure, active gain feedforward shows a slightly improved performance. Computed-Torque Control shows better performance, but requires a well described system for best performance. A novel Adaptive Inverse Dynamics Controller was tested and the performance was found to be similar to that of Computed-Torque Control, but without requiring any previous system knowledge.

This paper discusses a novel approach for state estimation in dynamical systems, with the special focus on hydraulic valve-cylinder drives. The proposed observer structure is based on the framework of the so-called twisting algorithm. This algorithm utilizes the sign of the state being the target for the observation, not directly implying its applicability for state observation. However it is shown that in the discrete case this approach may be applied to obtain a real time derivative using an intermediate integral process. Furthermore, different types of gains for the algorithm are suggested and analyzed via Lyapunov arguments, and a special version of the observer is proposed specifically for hydraulic drives utilizing online available plant information. The proposed observer with different observer gains are subjected to simulation studies in order to evaluate the exact properties, and results confirm that the proposed observer structure is easily implemented, produces very limited phase shift and is highly robust toward measurement noise.

Digital Displacement (DD) Fluid Power is an upcoming technology setting new standards for the achievable efficiency of fluid power pumps and motors. To enable high efficiency operation fast switching electronically controlled seat valves are needed, preferably exhibiting very low flow pressure loss and switching times within a few milliseconds. These valves are mechatronic components and attention to both the electromagnetic, fluid dynamical, mechanical and control system design must be paid to ensure the needed performance. In the present work, a quick response linear electro-magnetic actuator for DD seat valves is developed, and the resulting dynamicresponse of the seat valve is presented. Requirements for the valve actuator is established based on the DD application, and three feasible actuator topologies are analyzed by means of transient electro-magnetic FEA simulation. From this analysis a moving coil actuator topology is selected for the DD valve. A lumped parameter model including magnetic diffusion effects of this chosen actuator is developed, which is included in the dynamic valve simulation. Result show a switching time below 2 ms and a pressure loss below 0.5 bar at 500 litres/minute for the proposed DD seat valve.

As wind turbines increase in size and the demands for lifetime also increases, new methods of load reduction needs to be examined. One method is to make the yaw system of the turbine soft/flexible and hence dampen the loads to the system. This paper presents work previous done on this subject with foucus on hydraulic yaw systems. By utilizing the HAWC2 aeroelastic code and an extended model of the NREL 5MW turbine studies shows that a significant reduction in fatigue loads to the yaw system and rotor shaft is possible by the soft yaw drive concept.

Traditionally mechanical linked meter-in and meterout spool valves are used for velocity control of hydraulic differential cylinders. However with the demand for energy efficient systems the individual meter-in and meter-out valves draws massive attention. This paper propose an energy efficient actuator control and combines this with energy efficient system control. Finally energy neutral performance improvments are suggested for the subjacent actuator in an ELS system.

Currently mobile hydraulics is in a transition phase, where electronic sensors and digital signal processors are becoming standard on a high number of machines, hereby replacing hydraulic pilot lines and offering new possibilities with regard to both control and feasibility. As most open-circuit pumps are still hydraulically controlled, there is however still a need for being able to generate a hydraulic pilot pressure. The focus of the current paper is on the analysis and controller design of an electrohydraulic servo pressure regulator, which generates a hydraulic LS-pressure for a variable displacement pump based on an electrical reference. The paper first presents the considered system and an experimentally verified model of this. A linearized model and a stability analysis is then presented, based on which an H∞control strategy is selected. A nominal performance and a robustly stable controller are designed and tested in simulation and experimentally. Finally both controllers are compared to the reference system, finding that similar performance may be obtained, but the used FPGA limits the performance.