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The control of hydraulic valve-cylinder drives is still an active subject of research, and various linear and particularly nonlinear approaches has been proposed, especially in the last two-three decades. In many cases the proposed controllers appear to produce excellent tracking ability due to robust- and/or adaptive functionalities, but also often rely on full state feedback and/or cumbersome and elaborate parameter designs. Also, such sophisticated control approaches often lack tuning methods which, together with the lack of proven reliability, is likely to be the reason why such approaches generally has failed to break through in industry. This paper discusses the dominant properties necessary to take into account when considering position tracking control of hydraulic valve-cylinder drives, and presents two generally applicable, generic control design approaches that combines non-linearity compensation, feed forward control and linear control. The generic properties of the proposed control structures arise with the fact that the designs are based on analytic considerations and rely on meaningful design rules. This also means that the proposed design approaches does not utilize conventional model based linear analysis tools such as bode plots, root loci etc. in order for the controller parameterizations to be realized. However, the analytic approach presented takes offset in such analyzes, but in a generalized fashion. The proposed control structures are focused on industrial applicability, and are therefore constrained to utilize only measurements of the piston position, the valve spool position, the transmission line pressures and the supply pressure, as feedbacks. The control structures are generally targeting a high bandwidth of the controlled cylinder drive and accurate tracking ability in the entire range of operation, rather than reducing stationary errors, and may be parameterized from the desired gain margin, as well as linear model parameters. The proposed control design approaches are evaluated in an experimentally validated, nonlinear simulation model of a hydraulic valve-cylinder drive, and the results demonstrate the excellent tracking effort realized with the proposed design approaches.

Many mobile hydraulic systems are fitted with over centre valves for safety measures. However, it is well known that over centre valves in combination with pressure compensated flow control valves may lead to oscillatory and even unstable system behaviour. The traditional solution to overcome this problem is to use an over centre valve with a sufficiently low pilot ratio and/or include various damping orifices in the system. Both of these solutions are energy consuming and may decrease the control performance. An alternative approach is to use (electronic) pressure feedback — also referred to as active damping — to stabilise the system and damp pressure pulsations. This is not a new method, but the effect and adjustment of the filters is often misunderstood leading to incorrectly adjusted filters and degraded system performance. The focus of the current paper is therefore to explain and derive a set of guidelines for how to properly adjust a standard pressure feedback in system with an over centre valve when also considering model uncertainties, un-modelled dynamics and parameter variations. The paper takes its basis in a standard cylinder drive with an inertia load, over centre valve and a pressure compensated proportional valve. Based on a linearized model of the system, the system is analysed and it is shown that there is an optimal range for both the filter frequency and gain, which are closely linked. It is furthermore shown that these both closely affect the obtainable damping and dynamic response of the system. From the analysis a set of guidelines are derived for how to properly adjust the filter coefficients and the effect of different filter adjustments are shown and commented for the example system.

For wave energy to become a major future contributor of renewable energy it is a requirement that the efficiency and reliability of the Power Take-Off (PTO) systems is significantly improved. However, the cost of installing and testing PTO-systems at sea is very high. The focus of the current paper is therefore on the design and commissioning of a full scale wave simulator for testing PTO-systems for point absorbers. The challenge is to be able to design a system, which mimics the behavior of a wave when interacting with a given PTO-system – especially when considering discrete type PTO-systems. The paper presents the designed system, including the major design considerations. A model of the complete system is presented and controllers for the system are developed. These enable the system to emulate the wave behavior and the wave–float interaction. Finally both simulation and experimental results are presented, showing that the system is able to emulate real waves up to three meters in height and with a resulting force of more than 800 kN.

To compare different electroencephalography (EEG)-based regressors and their ability to predict the simultaneously recorded blood oxygenation level dependent response during blocked visual stimulation, simultaneous EEG–functional magnetic resonance imaging in 10 healthy volunteers was performed. The performance of different single-trial EEG regressors was compared in terms of predicting the measured blood oxygenation level dependent response. The EEG-based regressors were the amplitude and latency of the primary positive (P1) and negative (N2) peaks of the visual evoked potential, the combined P1–N2 amplitude, and the alpha power. Apart from peak latencies, all regressors showed significant positive or negative correlation with the blood oxygenation level dependent response in visual cortex. In addition, several EEG-based regressors were found to predict blood oxygenation level dependent variations in different occipital and extraoccipital cortical areas not explained by the boxcar regressor. The results suggest that the P1–N2 regressor is the best EEG-based regressor to model the visual paradigm, but when looking for additional effects like habituation or attention modulation that cannot be modeled by the boxcar regressor, it is better to include regressors based on individual peaks or alpha power. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.

In the past three decades an increasing amount of research has been performed in the field of tribodynamics of fluid power pumps and motors. The main incentives for this research are optimization of reliability and efficiency through the study of loss and wear mechanisms. These mechanisms are very difficult to study experimentally, whereby modeling and simulation are necessary. The modeling of tribodynamics is a multiphysics problem involving multibody dynamics, fluid mechanics, thermodynamics and solid mechanics. Consequently, the simulation durations can easily become impractical for parametric analysis or optimization. The coupling between multibody dynamics and fluid mechanics depend on the formulation of the solid body motion equations, where two approaches have historically been used. One approach is where the external forces on any lubricated joint are balanced by the fluid forces, such that solid body inertia is neglected. The other approach includes the inertia terms in the calculation of microdynamics. The inclusion of inertia terms entails a need for smaller time steps in comparison to the force balance approach, wherefore it is of interest to analyze the influence of the inertia term. In this paper the influence of the inertia term on the lubrication gaps of a radial piston motor are studied by a parametric analysis of the piston and cylinder density in a multibody tribodynamic simulation model. The motor is modeled as a digital fluid power displacement machine and a series of full-stroke displacement simulations are used as basis for the parametric analysis. From the parametric analysis a change, in the minimum film thickness as function of piston and cylinder density, is shown for certain operating modes of the digital fluid power displacement motor. This indicate a need for careful assessment of the applicability, of the force balance condition, if it is used in multibody tribodynamic simulations of radial piston digital fluid power displacement motors.Copyright © 2015 by ASME

The key objectives of wind turbine manufactures and buyers are to reduce the Total Cost of Ownership and Total Cost of Energy. Among others, low downtime of a wind turbine is important to increase the amount of energy produced during its lifetime. Historical data indicate that pitch systems accounts for a substantial part of the downtime of wind turbines. With a focus on fluid power pitch systems; this paper presents an overview of methods relevant to assessing and increasing the reliability and availability of such systems. Four major areas are identified and covered; failure analysis, reliability and availability assessment, condition monitoring and fault-tolerant control. Within each area a brief description of the state of the art is given. On basis of the state of the art, a discussion is given on where further work is required in order to obtain a systematic framework for designing reliable fluid power.Copyright © 2015 by ASME

Efforts to overcome the inherent loss of energy due to throttling in valve driven hydraulic systems are many, and various approaches have been proposed by research communities as well as the industry. Recently, a so-called speed-variable differential pump was proposed for direct drive of hydraulic differential cylinders. The main idea was here to utilize an electric rotary drive, with the shaft interconnected to two antiparallel fixed displacement gear pumps, to actuate a differential cylinder. With the design carried out such that the area ratio of the cylinder matches the displacement ratio of the two gear pumps, the throttling losses are confined to cross port leakage in the cylinder and leakage of the pumps. However, it turns out that the volumetric pump losses and the pressure dynamics of the cylinder and connecting pipes may cause pressure increase- or decrease in the cylinder chambers, which may seriously influence the dynamics and hence the performance during operation. This paper presents an analysis of these properties, and a redesign of the hydraulic system concept is proposed. Here the area- and displacement ratios are deliberately mismatched, causing inherent pressure build-up or cavitation in the return chamber, depending on the direction of motion. In order to avoid cavitation, a third gear pump is introduced, which provides a flow in the relevant cylinder chamber in one direction of motion, while operating in idle mode in the opposite motion direction. Together with two 2/2 way proportional valves, this design allows to control the lower chamber pressure levels, throttling excess compression flow to tank. The resulting design introduces additional losses due to throttling of excess compression flow, but also improves the dynamic properties of the system significantly. The proposed features are verified by comparison with the original pump concept and a conventional valve concept. Furthermore, significant improvement in energy efficiency is demonstrated under certain load conditions.Copyright © 2015 by ASME

The pitch system is a central part of modern wind turbines and good pitch control is essential for proper operation of the wind turbine. However, often when considering the pitch control, the pitch system dynamics is approximated by a simplified low order model, which may be acceptable for electrical pitch systems and turbine control purposes, but which does not capture the potential damping possibilities that may arise by actively using the fluid power systems to reduce the loads on the wind turbine structure. The focus of the current paper is therefore on the load reduction possibilities arising from applying an active damping filter in the fluid power systems to damp structural loads on the wind turbine, while maintaining the power production. Utilising the 5MW NREL reference wind turbine the paper presents a model of the fluid power pitch system, which is incorporated into the aeroelastic code FAST. Based on the model, an active damping approach is applied in combination with the standard pitch control to reduce oscillations in the pitch actuator force and hence the fatigue loadings on the mechanical structure. With basis in the implemented algorithm, the wind turbine is simulated under standard (IEC) load conditions and the load reduction possibilities analysed for the critical areas of the wind turbine along with performance of the wind turbine (pitch angle and power output).

The task of designing a digital fluid power displacement unit entails a choice of topology. Consequently, this choice entails a need for knowledge of the various applicable topologies to choose from. In this paper a morphological topology generation approach is used to derive a series of radial piston topologies. The morphological approach is combined with the Chebychev- Grubler-Kutzbach criterion, a fully rotatable shaft condition and a straight line path generation condition in order to obtain one degree of freedom single closed chain mechanisms applicable for wind turbines and wave energy converters. In conclusion it follows, that six single closed chain planar, one degree of freedom, mechanisms are suitable for digital fluid power displacement units in wind and wave energy applications.

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.

When modeling flexible robots and structures for control purposes, most often the assumed modes (AMs) method is used to describe the deformation in combination with a floating reference frame formulation. This typically has the benefit of obtaining a low-order, but accurate model of the flexible structure, if the number of modes and AMs are properly chosen. The basis for using this method is, however, that the vibrations (deflections) are time and position independent, i.e., the expression is separable in space and time. This holds for the classic Euler–Bernoulli beam equation, but essentially does not hold for translational links. Hence, special care has to be taken when including flexible translational links. In the current paper, different methods for modeling a hydraulic loader crane with a telescopic arm are investigated and compared using both the finite segment (FS) and AMs method. The translational links are approximated by a single beam, respectively, multiple beam elements, with both one and two modes and using different mode shapes. The models are all validated against experimental data and the comparison is made for different operating scenarios. Based on the results, it is found that in most cases a single beam, low mode order approximation is sufficient to accurately model the mechanical structure and this yields similar results as the FS method.

Fluid stiction may significantly influence the dynamic behavior when attempting to quickly separate two plates in close contact. The liquid fluid film, filling the gap between the plates, experiences a pressure drop resulting from an increasing distance, and cavitation may appear if sufficient separation speed and low plate distance are present. In the case of small initial plate separation, fluid tension is known to develop and the stiction force may exceed the maximum stiction force calculated by assuming strictly positive pressures in the fluid film. In this paper, a model for simulating the time dependent fluid stiction phenomenon, including a fluid tensile strength and cavitation effects, is proposed. The model is based on Reynolds theory, and the pressure distribution in the liquid zone is solved analytically for each time step, leading to a computationally efficient model without the need for finite element/volume methods. The considered geometry is two long parallel plates submerged in liquid, as present in many valve applications. The model is compared to experimental measurements, and it is found that the model is able to predict the stiction effect with reasonable accuracy given that proper selections of liquid tensile strength and initial plate distance are made.

Switching valves experience opposing fluid forces due to movement of the moving member itself, as the surrounding fluid volume must move to accommodate the movement. This movement-induced fluid force may be divided into three main components; the added mass term, the viscous term and the so-called history term. For general valve geometries there are no simple solution to either of these terms. During development and design of such switching valves, it is therefore, common practice to use simple models to describe the opposing fluid forces, neglecting all but the viscous term which is determined based on shearing areas and venting channels. For fast acting valves the opposing fluid force may retard the valve performance significantly, if appropriate measures are not taken during the valve design. Unsteady Computational Fluid Dynamics (CFD) simulations are available to simulate the total fluid force, but these models are computationally expensive and are not suitable for evaluating large numbers of different operation conditions or even design optimization. In the present paper, an effort is done to describe these fluid forces and their origin. An example of the total opposing fluid force is given using an analytically solvable example, showing the explicit form of the force terms and highlighting the significance of the added mass and history term in certain fast switching valve applications. A general approximate model for arbitrary valve geometries is then proposed with offset in the analytic model terms. The coefficients in this general model are determined based on CFD analyses, which are evaluated throughout the movement range of the moving member on an example valve geometry. The proposed model is compared to complete unsteady CFD simulations and found to generally predict the opposing fluid force well and gives accurate predictions under certain conditions. The proposed model is suitable for valve designers who need a computationally inexpensive fluid force model suitable for optimization routines or efficient dynamic models.Copyright © 2015 by ASME

Generally most hydraulic systems are intrensically non-linear, why applying linear control techniques typically results in conservatively dimensioned controllers to obtain stable performance. Non-linear control techniques have the potential of overcoming these problems, and in this paper the focus is on developing and applying several different feedback linearisation (FL) controllers to the individual servo actuators in a hydraulically driven servo robot to evaluate and compare their possiblities and limitations. This is done based on both simulation and experimental results.

This paper describes the development and application of a signal-based fault detection method for identifying gas leakage in hydraulic accumulators used in wind turbines. The method uses Multiresolution Signal Decomposition (MSD) based on wavelets for feature extraction from a~single fluid pressure measurement located close to the accumulator. Gas leakage is shown to create increased variations in this pressure signal. The Root Mean Square (RMS) of the detail coefficient Level 9 from the MSD is found as the most sensitive and robust fault indicator of gas leakage. The method is verified on an experimental setup allowing for the replication of the conditions for accumulators in wind turbines. Robustness is tested in a multi-fault environment where gas and external fluid leakage occurs simultaneously. In total, 24 experiments are performed, which show that the method is sensitive to gas leakage in the desired range and can be isolated from external fluid leakage. Additionally, the robustness to other operating conditions, such as wind speeds between rated and cut-off, turbulence intensity and ambient temperature is evaluated via simulations of a pitch system in a wind turbine using the Fatigue, Aerodynamics, Structures and Turbulence program (FAST). Simulation shows that robustness is affected at low ambient temperatures, however, detection is permitted in the range of 22-60 degC.

Digital Displacement (DD) fluid power machines are upcoming technology, improving the efficiency compared to traditional variable displacement machines, especially at low displacements where currently available fluid power pumps/motors suffer from mediocre efficiency. This efficiency improvement is made possible using independent electronically controlled seat valves for each pressure chamber, which is controlled corresponding to the rotation of the crankshaft. By control of these pressure chamber seat valves, the total displacement are controlled in discrete steps, and the pressure chambers not contributing to the displacement are not pressurized, which has been shown to improve the efficiency. To make this type of displacement control possible and energy efficient, the seat valves must be fast switching (ms range) and exhibit a very low pressure loss during operation, setting strict requirements for the valve design process. In this paper simulation of such fast switching valve is presented and the transient actuator performance is experimentally validated against transient Finite Element Analysis (FEA). Models predict a switching time of approximately 1ms for the valve and a pressure loss of 0.5 bar at 600 l/min, leading the way for highly efficient switching valves in DD machines.

Traditional valve controlled hydraulic drives have an inherent power loss, due to the throttling over the valves, which limits the maximum system efficiency. Pump controlled direct drives do not have this inherent limitation, but are limited when it comes to controlling asymmetric cylinders, why most solutions that have tried to overcome this problem have incorporated some kind of accumulator. In the present paper a new concept is presented, modelled and analyzed, and it is shown that the concept overcomes the problem with asymmetric cylinders, without the use of an accumulator. The paper first presents an analysis of the general concept, showing that both cavitation and excessive pressure build up needs to be handled by the system, after which the system is presented and modelled. Finally both simulation and experimental results are presented showing the validity of the concept.

An analytical thermal field theory is derived by a perturbation series expansion solution to the energy conservation equation. The theory is valid for small values of the Brinkman number and the modified Peclet number. This condition is sufficiently satisfied for hydraulic oils, whereby the analytical approach provides an alternative to existing computationally expensive numerical methods. The paper presents the dimensional analysis, which provides the foundation for the derivation of the analytical approximation. Subsequently, the perturbation method is applied in order to find an asymptotic expansion of the thermal field. The series solution is truncated at first order in order to obtain a closed form approximation. Finally a numerical thermohydrodynamic simulation of a piston-cylinder interface is presented, and the results are used for a comparison with the analytical theory in order to validate the modelling approach.Copyright © 2014 by ASME

This paper discusses the application of second order mode controls to hydraulic valve-cylinder drives with a special focus on the limitations resulting from nonlinear dynamic effects in flow control valves. Second order sliding mode algorithms appear highly attractive in the successive implementation of sliding mode control, achieving continuous control inputs, while maintaining the main properties of sliding modes. Under certain model assumptions, some of these controllers may even be applied as output feedback controllers. However, intrinsic nonlinear dynamic effects of hydraulic valves such as slew rates and time delays arising in the amplification stages, limits the applicability of such methods, and may lead to partial losses of robustness and limit cycles. These properties are analyzed and experimentally verified, and compensation methods are proposed. The application of the second order sliding algorithm known as the super twisting controller is considered for output feedback control and compared with conventional first order sliding mode control. The controllers under consideration are applied for position tracking control of a hydraulic valve-cylinder drive exhibiting strong variations in inertia- and gravitational loads. Results demonstrate that the super twisting algorithm may be successfully applied for output feedback control of hydraulic valve-cylinder drives, with modifications guaranteeing robust control performance in a small vicinity of the control target.

Digital Displacement (DD) fluid power machines utilizes electronically controlled seat valves connected to pressure chambers to obtain variable displacement with high operational efficiency and high bandwidth. To achieve high efficiency, fast valve switching is essential and all aspects related to the dynamic behaviour of the seat valves must be considered to optimize the machine efficiency. A significant effect influencing the valves switching performance is the presence of oil stiction when separating the contact surfaces in valve opening movement. This oil stiction force is limited by cavitation for low pressure levels, e.g. valves connected to the low pressure manifold, however for valves operated at higher pressure levels, the oil stiction force is dominating when the separating surfaces are close to contact. This paper presents an analytic solution to the oil stiction force for annular seat valves suitable for DD applications based on the Reynolds equation and considers contact surface curvature and attack angle. A dynamic cavitation zone is included in the stiction model, and cavitation is found to be present even for seat valves surrounded by high pressure levels.

This paper discusses the utilization of the so-called twisting algorithm when applied in output feedback position control schemes for electro-hydraulic cylinder drives. The twisting controller was the first second order sliding controller ever introduced, and can structure-wise be considered a straight forward extension of the simplest first order sliding controller, that is, a relay controller. Such a controller may be implemented without the knowledge of system time constants etc., as opposed to the surface based first order sliding controllers which has been presented in numerous contributions in literature. This paper considers the twisting algorithm when applied directly for output feedback control, and with the design based on a reduced order model representation of an arbitrary valve driven hydraulic cylinder drive. The consequence of implementing such a controller with the well-known saturation-, or boundary layer method is discussed, and the control operation inside- and out-side the boundary layer region is considered. Furthermore, the global stability of such a controller is discussed, with emphasis on possible local instability modes. Results demonstrate that the proposed output feedback controller may be successfully applied to hydraulic valve driven cylinder drives, with performance being on the level with a conventional surface based first order sliding mode controller.

Discrete fluid power force systems have been proposed as the primary stage for Wave Energy Converters (WEC’s) when converting ocean waves into electricity, this to improve the overall efficiency of wave energy devices. This paper presents a model based feasibility study of using bidirectional check valves in wave energy converters to improve the system efficiency. A single float arm of the Wavestar wave energy converter is modelled including the power take-off (PTO) system. The primary stage of the utilised PTO-system is a discrete fluid power force system consisting of a multi-chamber cylinder and multiple common pressure lines. The valve manifold, employed in the discrete PTO system and conventionally equipped with On/Off valves, is instead considered fitted with bidirectional check valves. The energy output from the primary stage is compared for PTO systems occupying respectively On/Off and bidirectional check valves. Based on the analysis it is found that the energy production may be slightly improved by using bidirectional check valves as compared to on/off valves, due to a decrease in switching losses. Furthermore a reduction in high flow peaks are realised. The downside being increased structural loads in the WEC.

Fluid power systems are the leading technology for power take off systems in ocean wave energy converters. However, fluid power systems often suffer from poor efficiency, especially in part loads. This degrades the PTO system efficiency and therefore lowers the energy production. To overcome the issues with poor system efficiency, a discrete fluid power system is proposed as a main part of the PTO system. For the discrete system to be feasible large fluid power switching valves are needed. The current paper presents a two stage 1,000 L/[email protected] bar multi-poppet on/off valve with a switching time less than 10 ms. The pilot stage is directly actuated and utilises internal valve pressure as supply and an external tank connection as drain. The current paper presents the multi-disciplinary design process leading to the final valve design. This includes the geometric design of the main stage, the choice of pilot valve, structural mechanical issues and modelling and simulation of various valve configurations. Hence, a mechatronic design process is utilised to choose the best valve configuration.

Sliding modes impose strong robustness toward parametric plant uncertainties and disturbances and provide for accurate tracking performance in control systems. However, in physical systems the application of sliding modes may give rise to undesirable chattering of the control signal due to actuator dynamics, possible excitation of unmodelled dynamics and structural resonant modes of load systems, etc. This may be avoided by application of smoothing functions imposing boundary layers on the control constraint, or by carrying out the design in relation to the control derivative. However, such boundary layers introduce additional design parameters and actuator dynamics may not allow the desired control accuracy to be reached. In this paper continuous controllers are proposed, with the designs taking their offset in some well-known sliding controllers. The proposed controllers preserve the finite-time convergence properties known from sliding control while at the same time avoiding control chattering, however, on the cost of robustness. Experimental results confirm the announced properties when applied to a hydraulic valve-cylinder drive, and demonstrates superior performance over conventional linear controllers.

Digital displacement fluid power is an upcoming technology setting new standards for the achievable efficiency in variable displacement fluid power pumps and motors. In the present work, an annular seat valve suitable for use in digital displacement units is considered, and the valve geometry is optimised considering both the mechanical strength during pressure loading and fluid flow restriction in the open valve state. Material stresses are modelled using finite element (FE) analysis including non-linear material behaviour, contact elements and fluid pressure penetrating load, closely reflecting the actual load of the seat valve connected to a fluid pressure chamber. Valve pressure losses are modelled using computational fluid dynamics (CFD). On basis of an overall physical size requirement and material specification, optimum valve geometry and stroke length are given as function of a defined normalised flow coefficient directly related to the machine efficiency.

Digital Displacement® (DD) machines are upcoming technology where the displacement of each pressure chamber is controlled electronically by use of two fast switching seat valves. The effective displacement and operation type (pumping/motoring) may be controlled by manipulating the seat valves corresponding to the piston movement, which has been shown to facilitate superior part load efficiency combined with high bandwidth compared to traditional displacement machines. However, DD machines need fast switching on-off valves with low pressure loss for efficient operation, especially in fast rotating operation, where switching times must be performed within a few milliseconds. These valve requirements make a simulation based design approach essential, where mechanical strength, thermal dissipation, fluid dynamics and electro-magnetic dynamics must be taken into account. In this paper a complete design method for DD seat valves are presented, taking into account the significant aspects related to obtaining efficient DD valves with basis in a given DD machine specifications. The seat area is minimized and the stroke length is minimized to obtain fast switching times while considering the pressure loss of the valves. A coupled optimization is finally conducted to optimize the electro-magnetic actuator, leading to a valve design based on the chosen valve topology. The design method is applied to an example DD machine and the resulting valve design fulfilling the requirements is presented.Copyright © 2014 by ASME

A conventional simplification of the thermal problem in fluid film lubrication analysis is performed by assuming that the main direction of heat flow is conduction through the film thickness, and thereby neglecting convection. However, in a significant amount of applications, convection is not negligible, whereby the majority of design engineers exclusively use numerical solvers. This paper presents a perturbation series expansion of the temperature field for small values of the Brinkman number. The derived perturbation solution and the more conventional analytical solution, where convection is neglected, are compared to a numerical test case. The comparison shows a significant improvement in the region where the convection term and conduction term are of the same order.

A simple two-particle model of excitons in conjugated polymers is proposed as an alternative to usual highly computationally demanding quantum chemical methods. In the two-particle model, the exciton is described as an electron-hole pair interacting via Coulomb forces and confined to the polymer backbone by rigid walls. Furthermore, by integrating out the transverse part, the two-particle equation is reduced to one-dimensional form. It is demonstrated how essentially exact solutions are obtained in the cases of short and long conjugation length, respectively. From a linear combination of these cases an approximate solution for the general case is obtained. As an application of the model the influence of a static electric field on the electron-hole overlap integral and exciton energy is considered.

We develop the nonlinear theory of transversal parametric oscillation in photorefractive sillenite crystals. The theory is nonlinear in the sense that the nonlinear feedback from the parametric space-charge field waves, above threshold of their excitation, is taken into account. In this manner, an analytical solution for the stationary state of the parametric waves is obtained. We analyze the stationary states' stability both against small perturbations in amplitude and phase (internal stability) and against excitation of new secondary waves (external stability). It is shown that the stationary state of transversal parametric oscillation is stable within certain regions of external and internal parameters. This is opposed to the degenerate case (K/2 subharmonic generation), which is unstable.

The application of sliding modes for control of hydraulic drives appear promising due to strong robustness toward plant uncertainties and disturbances. Especially high order sliding modes may be successfully implemented avoiding the discontinuous control seen in first order sliding controls. However, the very feature of switching about the control target may be undesirable due to finite sampling time and actuator dynamics, and may cause oscillating flow line pressures. This paper discusses a second order sliding controller based on the so-called prescribed convergence algorithm, when used for chattering elimination in hydraulic drive control applications. For this usage the algorithm suffers from poor convergence properties unless a high control gain is chosen, which in turn increases pressure oscillations. To negotiate the combined challenge the controller is extended with a proportional term for improved convergence speed, and the gain of the discontinuous control is made variable according to the control target itself. It is shown that the control error and its derivative are globally convergent to a vicinity of the target via Lyapunov arguments, with accuracy dependent on control parameters, and finite time convergence properties are considered via homogeneity reasoning. Results demonstrate improved control operation compared to the basic algorithm when implemented for position tracking control of a hydraulic drive.

Being a second order sliding algorithm, the super twisting algorithm is highly attractive for application in control of hydraulic drives and mechanical systems in general, as it utilizes only the control error while driving the control error as well as its derivative to zero for properly chosen algorithm parameters. However a discontinuous term internally in the control structure may excite pressures of transmission lines in hydraulic drives as the control structure strives to maintain the control error and its derivative equal to zero. In this paper a modified version of a controller based on the super twisting algorithm is proposed, with the focus of eliminating the discontinuous term in order to achieve a more smooth control operation. The convergence properties of the proposed controller are analyzed via a conservative phase plane analysis. Furthermore, homogeneity considerations imply finite time convergence of states to the origin under certain model assumptions. Results demonstrate the smooth control operation compared to the direct super twisting control approach while maintaining robustness properties in relation to position tracking of a hydraulic drive, under parameter perturbations, uncertainties and un-modeled dynamics.Copyright © 2013 by ASME

The increasing interest in hydraulic transmissions in wind and wave energy applications has created an incentive for the development of high efficiency fluid power machinery. Modeling and analysis of fluid power machinery loss mechanisms are necessary in order to accommodate this demand. At present fully coupled thermo-elastic models has been used to simulate and study loss mechanisms in various tribological interfaces. Consequently, a reasonable focus of further development is to couple the interface models and the rigid body mechanics of fluid power machinery. The focus of the current paper is a multibody dynamics model of a radial piston fluid power motor, which connects the rigid bodies through models of the transient hydrodynamic lubrication pressure in the joint clearance. A finite volume approach is used to model the pressure dynamics of the fluid film lubrication. The model structure and model equations are explained in this paper and simulation results are shown in terms macroscopic and microscopic dynamics.Copyright © 2013 by ASME

In this paper different linear and non-linear controllers applied to a hydraulically driven servo robot are evaluated and validated. The task is to make the actuators of the manipulator track a position reference with minimum error. Hydraulic systems are intrinsically non-linear and using linear control techniques typically results in conservatively dimensioned controllers to obtain stable performance. Non-linear control techniques have the potential of overcoming these problems and in this paper the focus is on applying simple nonlinear robust and adaptive controllers feasible for implementation in industrial servo drives. The different controllers are compared and evaluated from simulation and experimental results.

Digital Displacement Fluid Power is an upcoming technology setting new standards for the achievable efficiency of fluid power pumps and motors. The core element of the Digital Displacement technology is high performance electronically controlled seat valves, which must exhibit very low flow pressure loss and switching times within a few milliseconds to enable high efficiency operation. These valves are mechatronic components and special attention to both the mechanical, electromagnetic, fluid dynamical and control system design must be paid to ensure the needed performance. In the present work an annular seat valve suitable for use in Digital Displacement units is considered, and the ring geometry is optimized using finite element analysis including non-linear material behaviour, contact elements and fluid pressure penetrating load, closely reflecting the actual load of the seat valve connected to a fluid pressure chamber. The search for optimal design points is conducted using a brute force strategy with subsequent selection of the dominating design points.

Digital displacement fluid power motors have been shown to enable high-efficiency operation in a wide operation range, including the part load range where conventional fluid power motors suffers from poor efficiencies. The use of these digital displacement motors set new requirements for the valve technology, as very fast switching and low pressure loss is needed to obtain high-efficient motor operation.Two central parameters describing the valve operation are the valve switching time and the flow-pressure coefficient. These requirements are important inputs for the mechatronic valve design process, where both the electro-magnetic actuation, the valve moving mass and the material strength is pushed to the limits.In this present paper a system model describing the digital motor efficiency as a function of valve performance is developed, including a valve opening/closing control method. The valve transition time and flow-pressure coefficient are normalized, leading to a presentation of the general efficiency map of the digital displacement motor. Finally the performance of existing commercial valves with respect to digital motors is commented.

Discrete fluid power force systems consisting of a multi-chamber cylinder, a switching manifold and common pressure lines have been proposed as a technology for increasing the efficiency of the power take off system in ocean wave energy converters. However the force shifting of these discrete systems introduces large switching losses, especially when large pressure difference is present across the valves in the manifold. The current paper therefore focus on designing a bidirectional check valve for use in the switching manifold of the discrete force systems. The use of the bidirectional check valve enables passive force switching under minimal pressure difference, hence minimal energy loss. The bidirectional check valve is designed with a rated flow in the range of 1000L/min@5bar. The flow direction of the bidirectional check valve is set by the setting the pilot pressure. This paper presents a functionality test of a 125 L/min@5bar bidirectional check, leading to the design and modelling of a bidirectional check valve for ocean wave energy. It shows that a feasible bidirectional check valve may be configured by employing a multi-poppet topology for the main stage and utilising a 3/2 switching valve as pilot stage. The bidirectional check valve may operate both passive an active switching.Copyright © 2013 by ASME

In the later years, there has been an increased focus on new valve types, which yield the possibility to do Separate Meter-In Separate Meter-Out (SMISMO) control. This includes both digital valves, but proportional valves with separate metering spools and build in pressure sensors are also emerging. The possibility to independently control the meter-in and meter-out side not only increase the functionality of the system, but also opens up for better performance and/or lowered energy consumption. The focus of the current paper is therefore on investigation and comparison of what may be obtained using multivariable control strategies for SMISMO control of a single axis hydraulic system with a differential cylinder, when not taking other measures to improve performance. The paper first presents an experimentally verified model of the system considered, from which a linear model is derived. Based on the model, the control strategies are discussed and several H∞ controllers are designed, for which both simulation and experimental results are presented. The controllers are evaluated with regard to performance and robustness and compared to a simple SISO control. Based on the findings, the possibilities and limitations of the approach and the different controllers are outlined and discussed.Copyright © 2013 by ASME

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.

A fast rotating 1500 rpm radial piston digital displacement motor connected to a 350 bar high pressure manifold is simulated by means of transient 3D CFD analysis of a single pressure chamber. The analysis includes dynamic piston and valve movement, influencing the boundaries of the fluid domain. Movement of the low and high pressure valves is coupled to fluid forces and valve actuation is included to control the valve movement according to the pressure cycle of the digital displacement motor. The fluid domain is meshed using a structured/unstructured non-conformal mesh, which is updated throughout the simulation using layering zones as required by the moving fluid boundaries. The effect of cavitation at low pressures is included by implementing a pressure dependent density, based on an effective bulk modulus model. In addition, pressure dependent oil viscosity is included in the analysis. As a result of the CFD analysis, the dynamic response of the pressure chamber and valve movement of a digital displacement motor is presented, along with the total efficiency of a multi-chamber motor at full and partial displacement. Simulation predicts that pressure over- and undershoot may be reduced to a few percent of the high pressure manifold level by precise timing and fast switching of the seat valves.Copyright © 2013 by ASME

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.

Digital displacement fluid power pumps/motors offers improved efficiency and performance compared to traditional variable displacement pump/motors. These improvements are made possible by using efficient electronically controlled seat valves and careful design of the flow geometry. To optimize the design and control of digital displacement machines, there is a need for simulation models, preferably models with low computational cost. Therefore, a low computational cost generic lumped parameter model of digital displacement machine is presented, including a method for determining the needed model parameters based on steady CFD results, in order to take detailed geometry information into account. The response of the lumped parameter model is compared to a computational expensive transient CFD model for an example geometry.

The yaw system is the subsystem on a wind turbine which ensures that the rotor plane of the turbine always is facing the wind direction.Studies from [1] show that a soft yaw system may be utilized to dampen the loads in the wind turbine structure. The soft yaw system operates much like a suspension system on a car, leading the loads away from the turbine structure. However, to realize a soft hydraulic yaw system a new design concept must be found.As a part of the development of the new concept a preliminary concept evaluation has been conducted, evaluating seven different hydraulic yaw concepts, ranging from a one-to-one copy of the electrical drive (electrical drives replaced by hydraulic dittos), to floating suspension systemsmounted on hydraulic cylinders. Rough calculations of size and consequences of the different systems are presented ending up with the final concept for further investigation.Loads and yaw demands are based on the IEC 61400-1 standard for wind turbine design, and the loads for this examination are extrapolated from the HAWC2 aeroelastic design code. The concepts are based on a 5 MW off-shore turbine.

In this paper a control strategy based on second order sliding modes, generally applicable for position tracking control of electro-hydraulic valve-cylinder drives (VCD), is proposed. The main target is to overcome problems with linear controllers deteriorating performance due to the strong nonlinearities characterizing VCD's. The proposed controller requires pressure-, valve- and piston position measurements, and is based on the so-called super twisting algorithm and compensation of controlgain nonlinearities. Simulation results demonstrate strong robustness when subjected to large perturbations in coulomb friction.

Introducing a hydraulic soft yaw concept for wind turbines leads to significant load reductions in the wind turbine structure. The soft yaw system operates as a shock absorption system on a car, hence absorbing the loading from turbulent wind conditions instead of leading them into the stiff wind turbine structure. Results presented shows fatigue reductions of up to 40% and ultimate load reduction of up to 19%. The ultimate load reduction increases even more when the over load protection system in the hydraulic soft yaw system is introduced and results show how the exact extreme load cut off operates.Further it is analyzed how the soft yaw system influence the power production of the turbine. It is shown that the influence is minimal, but at larger yaw errors the effect is possitive. Due to the implemeted functions in the hydraulic soft yaw system such as even load distribution on the pinions, fatigue and ultimate load reduction and backlash compensation it is possible to increase the number of yaw activations without compromising the lifetime of the components. By increasing the number of activations, the average yaw error may be lowered and hence the energy production raised below rated wind speed.

To comply with the increasing demands for life time and reliability of wind turbines as these grow in size, new measures needs to be taken in the design of wind turbines and components hereof. One critical point is the initial testing of the components and systems before they are implemented in an actual turbine. Full scale hardware testing is both extremely expensive and time consuming, and so the wind turbine industry moves more towards simulations when testing.In order to meet these demands it is necessary with valid models of systems in order to introduce new technologies to the wind turbine market. A hydraulic yaw system is such a new technology, and so a mathematical model of the full scale system and test rig system is derived and compared to measurements from the system. This is done in order to have a validated model, which wind turbine manufacturers may use for test in their simulation environment. The model and the test rig are tested up against different design load cases and the results are compared. The experiments show that the model is valid for comparing the overall dynamics of the hydraulic yaw system. Based on the results it is concluded that the model derived is suitable for testing of the dynamic behavior in wind turbine manufacturer’s full scale aero elastic code.

The focus of the current paper is on a hydraulic soft yaw system, designed to reduce the loading of the turbine structure, by absorbing wind guest via the hydraulic system, but which also enables the system to be used as a self-aligning yaw system. The system is analyzed with basis in the NREL 5-MW turbine, modeled in FAST, in which a new robust method for implementing Coulomb friction is utilized. Based on this model and a model of the hydraulic system, the influence of friction and wind speed is investigated in relation to the possibility to use the system as a self-aligning yaw system. Similarly the behavior of the hydraulic system is analyzed and it is concluded that the hydraulic yaw system allows self-yaw under normal operating conditions for the turbine. Self-yaw control is possible in wind speeds above 12 m/s when yaw friction is kept below 1 MNm.

This paper discusses the application of second order sliding modes for position tracking control of electro-hydraulic valve-cylinder drives (VCD’s). Thetarget is to introduce increased tracking- and transient performance compared to conventional linear approaches, without extending the number of tuning parameters. The proposed controller utilizes basic component knowledge commonly available from data sheets, as well as pressure-, valve position-, piston position- and velocity measurements. Results demonstrate improved position tracking- and transient performance, compared to a linear control approach, and that control chattering is eliminated without introducing a boundary layer, normally seen in first order sliding mode controlled systems.

The design and development of a hydraulic yaw system for multi MWturbines is presented and the concept explained. As part of the development of the new concept a full scale test rig for a 5 MW wind turbine has been designed and constructed. The test rig is presented along with its unique design features. The design process is outlined to give insight in the design criteria driving the design. Loads and yaw demands are based on the IEC 61400-1 standard for wind turbine design, and the loads for this examination are extrapolated from the FAST aero elastic design software. The concepts are based on a 5 MW offshore turbine. After the system presentation, measurement results are presented to verify the behavior of the system. The loads to the system are applied by torque controlled electrical servo drives, which can add a load of up to 3 MNm to the system. This gives an exact picture of the system dynamics under real conditions. The behavior of the system is analyzed with regard to 20 years of operation. This is for example done by applying loads from different design load cases, e.g. normal turbulence, extreme turbulence and different fault scenarios on the turbine.The paper first presents an introduction with the current state of the art and problem description, followed by a system description, where the system is designed and dimensioned. Based on the design, results from the test rig are presented and analyzed. Finally a conclusion summing up the design, model and test results is given.