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Most electrically actuated hydraulic valves utilize solenoids as the actuating element due to their robustness and simplicity. This goes for both on-off and proportional-type valves. However, despite their robustness, solenoid coil malfunction is the largest single failure mode in solenoid actuated valves. An outspoken fault is here solenoid winding short-circuit, i.e., two windings short-circuiting, which may ultimately lead to solenoid failure if more windings short-circuit. Research has therefore also focused on detecting winding short-circuits. Common for the approaches are that they, directly or indirectly, depend on the coil winding temperature, as this directly influences the coil resistance. Alternatively, the approaches are based on injection of high-frequency signals, which is typically a costly solution, which is not a feasible approach for use in hydraulic valves, with the limitations imposed by the control electronics. Therefore, this article focuses on a temperature-independent algorithm to detect coil winding short-circuit, which is easy to implement and only relies on existing position and current sensors. The proposed algorithm is based on an extended Kalman filter, which estimates the coil resistance. As this resistance estimate is indirectly dependent on the coil temperature, a window-based cumulative sum fault detection method is included to detect transient changes in the coil resistance while compensating for the temperature variations. The algorithm is developed based on an experimentally validated model of the valve, and has been tested for several different situations through both simulations and experimentally. Based on the presented results, it is found that the algorithm can consistently detect resistance changes down to 0.11 $\Omega$ for constant input signals and down to 0.17 $\Omega$ for sinusoidal-varying input signals. This while stillbeing robust to parameter variations, such as increased valve friction, spring coefficients, and sensor signal deviations.

For axial piston units, product optimisation plays an essential role in the development phase, where an efficient and price-attractive product design and manufacturing process is the key to the success of a product. Moreover, the market needs are on lighter, more compact, efficient, and reliable products designed to the limit. Therefore, reliability is the focus of this work, where the research includes method analysis of the material stress calculation applied to the cylinder block implemented in an axial piston pump. A simplified calculation model is presented and evaluated concerning load while remaining accessible for users without deep technical knowledge or access to expensive Finite Element (FE) simulation tools. The analytical calculation method delivers a stress distribution intended for different purposes like design evaluation or as load spectrum for lifetime calculation. Additionally, the developed calculation method is generalised, enabling the methodology to be used on any standard axial piston pump. The methodology utilises the stress determination model, which includes the load as a sum of forces caused by the external and internal influencing factors. To show the method’s success, a comparison demonstrates strong positive agreement between the calculated and simulated stress results obtained by Finite Element Analysis.

This article describes the development and application of a model-based scheme for detecting the early signs of coil failure in solenoid valves. Contrary to other works, the proposed method simply isolates the fault-induced change in the coil resistance from thermal effects and provides a cost-effective solution with no comprehensive equipment demand. The model-based detection method employs a simple thermal model of the solenoid and an extended Kalman filter (EKF) for generating coil current residuals. The EKF utilizes measurements of coil voltage, coil current, ambient, and fluid temperature. The CUmulative SUM of the current residual is found to be sensitive to the early signs of coil failure. An experimental setup is utilized for evaluating the robustness of the detection method in the event of changing ambient temperature, convection, and fluid flow conditions. Results show that the method is able to detect the early signs of coil failure, and robust behavior and high detection probabilities are achieved for realistic fault sizes. The realistic fault sizes and the number of fault instances before failure are determined through extensive accelerated tests, where solenoids are operated at high temperature until failure.

When considering digital displacement machines, one of the key elements are the fast switching valves. However, the dynamic requirements and conflicting design objectives mean that designing these fast-switching valves is a complex process, which pushes the technology to the limit. Optimization approaches are therefore required just to find feasible and, secondly, optimal solutions. However, dynamic CFD simulation is required to accurately describe dynamic fluid effects as fluid stiction, -drag and -end damping. This limits the possibilities to use optimization approaches due to the computational burden imposed, the need for dynamic mesh generation, and the design parameterization. Hence, this paper investigates analytical approximations and the error size introduced by simplifying dynamic fluid friction effects into a lumped parameter form. Specifically, the article presents a comprehensive parameter study of selected design parameters’ influence on the fluid forces in fast switching annulus valves, based on both analytical expressions and results derived from dynamic CFD simulations. The focus is primarily on describing the effects and size of the flow forces resulting from changes in parametric design parameters that influence the flow geometry. The results reveal a fair correlation between the fluid force predicted by the rapidly executable lumped model and the CFD model.

In marine applications, a cyclic varying pitch (CVP) propeller is a propeller in which the propeller blade can be cyclic-pitched. This cyclic pitching of the propeller blades is used to adapt to the local flow conditions in the non-uniform wake field that the propeller operates in, behind the ship hull. This has the potential to improve the performance of the propulsion system relative to a propeller which has fixed pitch for each revolution. The potential performance improvements include increasing the propulsion efficiency and reducing the cavitation, pressure pulses, vibrations and noise problems. However, the CVP propeller is not on the market today, and several challenges have to be addressed before the CVP propeller may be realized. One of these challenges is how to design the individual cyclic pitch mechanism for the propeller. However, before the cyclic pitch mechanism can be designed, it is necessary to know the requirements for it, such as the required pitching power and torque. The focus of the current paper is therefore to present a model for the propeller, by which it is possible to determine the loads acting on the CVP propeller blades during the cyclic pitching, and hence the actuator force/torque and power requirements. To illustrate the usefulness of the model, an example is presented, in which the loads on a CVP propeller are determined, together with the requirements for the individual cyclic pitch mechanism. The efficiency results presented are, however, not representative of the efficiency improvement that may be obtained, as neither the propeller nor the pitch trajectory has been optimised. The results do, however, serve to show the benefit and validity of the model.

Wind turbines have become a significant part of the global power production and are still increasing in capacity. Pitch systems are an important part of modern wind turbines where they are used to apply aerodynamic braking for power regulation and emergency shutdowns. Studies have shown that the pitch system is responsible for up to 20% of the total down time of a wind turbine. Reducing the down time is an important factor for decreasing the total cost of energy of wind energy in order to make wind energy more competitive. Due to this, attention has come to condition monitoring and fault detection of such systems as an attempt to increase the reliability and availability, hereby the reducing the turbine downtime. Some methods for fault detection and condition monitoring of fluid power systems do exists, though not many are used in today’s pitch systems. This paper gives an overview of fault detection and condition monitoring methods of fluid power systems similar to fluid power pitch systems in wind turbines and discuss their applicability in relation to pitch systems. The purpose is to give an overview of which methods that exist and to find areas where new methods need to be developed or existing need to be modified. The paper goes through the most important components of a pitch system and discuss the existing methods related to each type of component. Furthermore, it is considered if existing methods can be used for fluid power pitch systems for wind turbine. Wind turbines have become a significant part of the global power production and are still increasing in capacity. Pitch systems are an important part of modern wind turbines where they are used to apply aerodynamic braking for power regulation and emergency shutdowns. Studies have shown that the pitch system is responsible for up to 20% of the total down time of a wind turbine. Reducing the down time is an important factor for decreasing the total cost of energy of wind energy in order to make wind energy more competitive. Due to this, attention has come to condition monitoring and fault detection of such systems as an attempt to increase the reliability and availability, hereby the reducing the turbine downtime. Some methods for fault detection and condition monitoring of fluid power systems do exists, though not many are used in today’s pitch systems. This paper gives an overview of fault detection and condition monitoring methods of fluid power systems similar to fluid power pitch systems in wind turbines and discuss their applicability in relation to pitch systems. The purpose is to give an overview of which methods that exist and to find areas where new methods need to be developed or existing need to be modified. The paper goes through the most important components of a pitch system and discuss the existing methods related to each type of component. Furthermore, it is considered if existing methods can be used for fluid power pitch systems for wind turbine.

The pitch system is a crucial part of today’s wind turbines and is used for power regulation when operating above rated wind speed. According to studies the pitch system is responsible for up to 20% of the total downtime of a wind turbine. As an attempt of increasing the reliability and availability attention has come to fault detection and condition monitoring of such systems. This paper presents a State Augmented Extended Kalman Filter, SAEKF, for detecting both internal and external rod leakage. Furthermore, an external load torque is included in the filter to improve the performance in the presence of external load. The SAEKF is tested experimentally for different levels of both internal and external rod leakage under working conditions similar to the ones experienced for a pitch system located in a wind turbine. The working conditions include unkown fast varying external loads. The experiments show that the SAEKF is capable of detecting internal and external leakage down to 0.10 l/min and 0.34 l/min, respectively. The internal leakage is detected with a estimation error of maximum 0.04 l/min while the external leakage estimation error is going up to 0.43 l/min for high levels of external leakage.

This article presents a numerical framework for investigating transient features of hydraulic valves, specifically poppet type designs suited for digital hydraulics. The objective is to determine the validity of the state-of-the-art lumped parameter models (LPM) of fluid dynamical phenomena during switching of such valves. Knowledge about analytically valid models for simple situations are used together with complex Computational Fluid Dynamics simulations, which is not a novel concept, but the procedure at which this is done is. The idea behind the research is to consider a numerical framework with dynamic capabilities as a sufficiently accurate representation of reality. Thereby, strategic simulation cases can be applied to understand the features of the design. The sought dependencies of the valve was revealed by conducting several ‘experiments’ through the simulation framework which allows analysis of practically difficult operating conditions. The results of the numerical framework reveals how and where the state-of-the-art LPM deviate (e.g. that fluid displaced by the plunger itself is not directly proportional to velocity, that fluid inertia is significant and that the fluid-induced force is non-linear with changes in velocity).

This paper presents a qualitative design tool for evaluation of the risk for fluid power pitch systems. The design tool is developed with special attention to industry standard failure analysis methods and is aimed at the early phase of system design. Firstly, the concept of Fault Tree Analysis is used for systematic description of fault propagation linking failure modes to system effects. The methodology is conducted solely on a circuit diagram and functional behavior. The Failure Mode and Effects Criticality Analysis is subsequently employed to determine the failure mode risk via the Risk Priority Number. The Failure Mode and Effect Criticality Analysis is based on past research concerning failure analysis of wind turbine drive trains. Guidelines are given to select the severity, occurrence and detection score that make up the risk priority number. The usability of the method is shown in a case study of a fluid power pitch system applied to wind turbines. The results show a good agreement to recent field failure data for offshore turbines where the dominating failure modes are related to valves, accumulators and leakage. The results are further used for making design improvements to lower the overall risk of the pitch system. Copyright © 2017 John Wiley & Sons, Ltd.

Pitch systems impose an important part of today’s wind turbines, where they are both used for power regulation and serve as part of a turbines safety system. Any failure on a pitch system is therefore equal to an increase in downtime of the turbine and should hence be avoided. By implementing a Fault Detection and Diagnosis (FDD) scheme faults may be detected and estimated before resulting in a failure, thus increasing the availability and aiding in the maintenance of the wind turbine. The focus of this paper is therefore on the development of a FDD algorithm to detect leakage and sensor faults in a fluid power pitch system. The FDD algorithm is based on a State Augmented Extended Kalman Filter (SAEKF) and a bank of observers, which is designed utilizing an experimentally validated model of a pitch system. The SAEKF is designed to detect and estimate both internal and external leakage faults, while also estimating the unknown external load on the system, and the bank of observers to detect sensor drop-outs. From simulation it is found that the SAEKF may detect both abrupt and evolving internal and external leakages, while being robust towards noise and variation in system parameters. Similar it is found that the scheme is able to detect sensor drop-outs, but is less robust towards this.

This paper covers a reliability analysis as a qualitative method, especially focused on axial piston units. The method is based on Fault Tree Analysis (FTA) and results in risk and reliability assessment at the components level. Especially, the development of the reliability assessment as a methodical tool is the core of the paper. Moreover, the FTA is combined with the industrial standard method known as Design Failure Mode Effects Analysis (DFMEA) which is typically used in the development phase of the design. The evaluation and the usability of the FTA methodology is analyzed in connection with field data. Thus, the deviation of the theoretical valuation from the field data was utilized as a success indicator of the method. The analysis of the fault spreading covers the assessment of component faults and links failure states with unit effects. The analysis of the axial piston unit as a system is made on idealized/theoretical design and functional behavior only. Hence, the failure rating and the effect is subsequently applied to determine the fault risk in form of the Risk Priority Number (RPN). The failure modes and effects are based on engineering experience of past decades, supported by existing DFMEAs of axial piston units. Thus, the assessment of the risk priority number is based on previous data, yielding the given severity, occurrence and detection quantification. This approach opens new opportunities of design assessment and the results show a good agreement to the damage accumulation seen in real field data. Furthermore, the connection between theoretical design assessment and field data do support the failure ranking improvement of the DFMEA.

This paper studies reduced-order-models for the fluid flow problem of a digital valve, and whether it may efficiently be formulated by a deep Artificial Neural Network (ANN) to model e.g. the valve flow, flow-induced force, stiction phenomena and steep local pressure gradients that arise before plunger impact, which may otherwise require CFD to be accurately modeled. Several methodologies are investigated to evaluate both the required computation time and the accuracy. The accuracy is benchmarked against CFD solutions of flows and forces. As basis for comparison an analytical model is proposed where some fitting parameters are allowed, and the equation is tested outside its fitting range. A similar model is built as a deep ANN which is trained with data from the analytical model to investigate the amount of data required for an ANN and its fitting capabilities. The results show that in higher dimensions the required training data can be maintained low if data is structured by a Latin Hypercube, otherwise the amount becomes infeasible. This makes an ANN surrogate feasible when compared to a look-up table, and may be expanded to higher dimension where dynamical effects are included. However, the required data and computational cost for this is too extensive for the valve design considered as basis for the analysis. Instead, for this specific problem, the derived analytical model is sufficient to describe the valve dynamics and reduces the computation time significantly.

In the pursue of lowering the cost of energy for ocean wave energy devices the energy conversion efficiency of the Power Take Off (PTO) system has attained increased focus. A discrete fluid power force system has been proposed as a possible solution to improve the conversion efficiency. Transferring from a continuous fluid power PTO-system to a discrete poses the question of configuration and control of the discrete fluid power system utilised in a wave energy converter (WEC). The current paper presents a method for determining the optimal configuration of a discrete fluid power force system for the PTO-system in a WEC. A model based optimisation is utilised to identify the system configuration leading to the highest energy output. It is shown how the time distribution of wave conditions affects the choice of system configuration. Based on the current paper the preferred PTO system configuration consists of a two chamber cylinder and three common pressure lines in the tested sea conditions.

Fast-switching seat valves suitable for digital hydraulic pumps and motors utilize direct electromagnetic actuators, which must exhibit superior transient performance to allow efficient operation of the fluid power pump/motor. A moving coil actuator resulting in a minimum valve switching time is designed for such valves using transient finite-element analysis of the electromagnetic circuit. The valve dynamics are coupled to the fluid restrictive forces, which significantly influence the effective actuator force. Fluid forces are modeled based on transient computational fluid dynamics models. The electromagnetic finite-element model is verified against experimental measurement, and used to design an optimum moving coil actuator for the application considering different voltage-current ratios of the power supply. Results show that the optimum design depends on the supply voltage-current ratio, however, the minimum switching time obtained is nearly independent on this voltage-current ratio. Selecting a suitable power supply based on thermal considerations yields a switching time just above one millisecond for a travel length of 3.5 mm while submerged in oil. The proposed valve has a pressure drop below 0.5 bar at 600 L/min flow rate, enabling efficient operation of digital hydraulic pumps and motors.

The consideration of the drivetrains behavior under real operating conditions is becoming increasingly important. While in the past decades it was good enough to have a rough knowledge about the operation conditions in field, a more accurate and detailed picture of the duty cycles is required today. Caused by specific and complex operation conditions the load deviation between different applications is significant. In many cases like e.g. lifetime calculation, a precise knowledge of field conditions leads to the more accurate calculated or simulated results. This paper covers the duty cycles investigation of a forwarder application in field including the measurement concept and implementation. Especially, the analysis of the load conditions, clustering algorithm and lifetime calculation is included. The load spectrum generation is consequently done based on comprehensive load cycle determination. The Aim of this work is to verify the calculation accuracy of the load spectrum by using the clustered and raw data. The outcome of the investigation gives an indicator for the load degree of the drivetrain. Here the developed method creates a new and fast possibility of data analysis and is applicable to most drivetrain applications. Furthermore, the collected data is used as reference for future investigation of the forwarder evolution.

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 explores the challenges regarding designing a heuristic control algorithm for a dynamic non-linear system with multiple inputs and outputs. The presented algorithm aims to shape the voltage input (both magnitude and timing) applied to fast switching valves in a Digital Displacement® unit. This consists of multiple sub-systems, where optimal decisions must be made, based on the system design and performance criteria. In this regard good performance are defined as: low electrical energy required for switching, accurate switching timing and low plunger velocity near the seat. The proposed algorithm examines the design-space in a user-defined manner combined with stochastic decision making. The randomness of the algorithm is based on the standard deviation between located elite designs. This reveals several feasible input sequences to achieve the goal, and the optimums are benchmarked with a differential evolution algorithm. The techniques are demonstrated by simulation and the results compared showing similar performance of the optimums.

This paper presents a framework to evaluate different power losses in regards to hydraulic valves used in Digital Displacement ® Units (DDU). The feasibility of DDU may be compromised if internal leakage in the hydraulic valves develops over time, introduction of a time-dimension is the objective of this work. The actual leakage that will develop over time due to mechanical wear is unknown, which is the main reason why previous work has not addressed this research topic. The underlying assumption of the developed framework is that internal leakage propagates as a consequence of the amount of valve switching cycles. This assumption is combined with physical knowledge about a DDU into one theoretical framework, which is defined both as non-linear differential equations and as a timeaveraged static model to visualize the difference in accuracy when put into context of a specific machine. Thereby reducing the required simulation time and enhancing design possibilities. The main contribution of this work is a revision of the conventional efficiency curve of a DDU, where it now includes a novel trade-off between averaged displacement and efficiency (without partial strokes). Over a period of 25 years an average displacement of 55%. is shown to be optimal from an energy perspective.

This paper proposes a design methodology for designing the mechanical topology in hydraulic Active Check Valves (ACVs). These valves may have optimised flow geometries to obtain fast switching and high flow rates. Furthermore, ACVs comprise large pressure differentials and high deceleration during plunger and seat impact, which introduces a concern about the severity of long term fatigue and wear. A modelling framework of ACVs with the desired capabilities is multidisciplinary involving analysis of mechanics, fluid dynamics, contact mechanics and material science. Therefore, a state-of-the-art focusing on the significant contributions of these disciplines has been conducted. The literature review is focusing on lifetime models and wear concerned with valve seat inserts (VSI) in different fatigue cycle regimes. No studies with direct application have been found, making this work an infant step towards lifetime prediction of ACVs. However, studies of similar systems have pinpointed the mechanisms that result in wear particles. It is the main objective to uncover the parameters with greatest significance when considering lifetime estimation of ACVs and a conceptual formulation of a lifetime framework is established. This paper proposes a design methodology for designing the mechanical topology in hydraulic active check valves (ACVs). These valves may have optimised flow geometries to obtain fast switching and high flow rates. Furthermore, ACVs comprise large pressure differentials and high deceleration during plunger and seat impact, which introduces a concern about the severity of long-term fatigue and wear. A modelling framework of ACVs with the desired capabilities is multidisciplinary involving analysis of mechanics, fluid dynamics, contact mechanics and material science. Therefore, a state-of-the-art focusing on the significant contributions of these disciplines has been conducted. The literature review is focusing on lifetime models and wear concerned with valve seat inserts in different fatigue cycle regimes. No studies with direct application have been found, making this work an infant step towards lifetime prediction of ACVs. However, studies of similar systems have pinpointed the mechanisms that result in wear particles. It is the main objective to uncover the parameters with greatest significance when considering lifetime estimation of ACVs and a conceptual formulation of a lifetime framework is established.

Fluid power systems are inherently nonlinear and typically suffer from very poor damping. Despite these characteristics, it is not uncommon that traditional linear type controllers are applied. This typically results in conservative adjustment of the controllers, or when more advanced controllers are applied, that they do not benefit from the possibilities for improving the damping by applying (internal) pressure feedback in the systems. In both cases, the result will be degraded performance compared with what may be obtained. Pressure feedback, also referred to as active damping, is not a new method. However, it is an often overlooked method, and when it is applied, it is not adjusted optimally to also account for model uncertainties, unmodeled system dynamics, and parameter variations. The focus of this paper is to explain various types of pressure feedback that may be applied; how these affect a given system, and how to adjust the parameters of the pressure feedback to obtain the best results. This is done for both a traditional symmetric cylinder servo system and a system with a differential cylinder using both pressure and nonpressure compensated proportional valves. Based on the presented models, the analysis yields results for how to properly adjust the parameters.

In this paper the robustness of a class of learning control algorithms to state disturbances, output noise, and errors in initial conditions is studied. We present a simple learning algorithm and exhibit, via a concise proof, bounds on the asymptotic trajectory errors for the learned input and the corresponding state and output trajectories. Furthermore, these bounds are continuous functions of the bounds on the initial condition errors, state disturbance, and output noise, and the bounds are zero in the absence of these disturbances.

A fundamental part of a digital fluid power (DFP) pump is the actively controlled valves, whereby successful application of these pumps entails a need for control methods. The focus of the current paper is on a flow control method for a DFP pump. The method separates the control task concerning timing of the valve activation and the task concerning the overall flow output control. This enables application of linear control theory in the design process of the DFP pump flow controller. The linearization method is presented in a general framework and an application with a DFP pump model exemplifies the use of the method. The implementation of a discrete time linear controller and comparisons between the nonlinear model and the discrete time linear approximation shows the applicability of the control method.

The dominant physical phenomena in hydraulic drives are generally well known, why the model equations describing the dominant dynamics may be established with a high level of certainty. To some extend, this is also the case for the model parameters when these are based on data sheet information. However, parameters such as the effective bulk modulus, leakage, external disturbances etc. may be difficult to evaluate, and may furthermore be varying. In regard to control design, linear methods may be difficult to apply and stability margins difficult to evaluate, unless dynamic models are established prior to the control design. This problem may be overcome using adaptive controllers, adjusting themselves to uncertainties/variations. This often shifts the problem from adjusting the controller parameters, to adjusting the parameters of the control parameter adaption mechanism, which in many cases involves a significant number of parameters. This paper considers a novel adaptive control algorithm, theoretically applicable to systems of arbitrary orders, and potentially only with three tuning parameters. The proposed algorithm is considered in relation to the position control of a hydraulic winch drive, and results imply that excellent motion tracking performance may be achieved utilizing only state feedback.Copyright © 2017 by ASME

Proper functioning of the pitch system is essential to both normal operation and safety critical shut down of modern multi megawatt wind turbines. Several studies on field failure rates for such turbines show that pitch systems are a major contributor to failures which entails an increased risk. Thus, more reliable and safe concepts are needed. A review of patents and patent applications covering fluid power pitch concepts, reveals that many propose closed-type hydraulic systems. This paper proposes a closed-type concept with a bootstrap reservoir. In contrary to a conventional system where a common supply delivers to cylinder drives mounted at each blade, the concept using bootstraps are fully contained in the rotating hub and act as standalone actuators. Clear advantages of such systems are a reduction of high pressure leakage paths, but in turn more components are used for each supply circuit. To allow for comparison, this paper deals with a risk-based analysis of these two types of pitch concepts. The risk-based analysis is conducted according to a qualitative failure analysis method which is verified against recent field failure data for hydraulic pitch systems. The method combines Fault Tree Analysis and Failure Mode and Effect Criticality Analysis in a systematic framework that lowers the bias issues normally encountered for qualitative studies. Under the assumption of similar components, the results indicate an equal risk of the two concepts. A decreased reliability is seen for the bootstrap concept due to additional components in the supply circuit compared to the conventional system. It is noted that careful selection of high reliable pumps and relief valves may significantly reduce risk and increase reliability of the bootstrap concept.

The application of valve driven hydraulic winch drives is related to substantial power losses, primarily due to throttle generated valve flows. More energy efficient solutions are also commonly applied in terms of conventional hydrostatic closed circuit drives as well as so-called secondary controls. Such solutions are typically constituted by many and rather expensive components, and are furthermore often suffering from low frequency dynamics. In this paper an alternative solution is proposed for winch drive operation, which is based on the so-called speed-variable switched differential pump, originally designed for direct drive of hydraulic differential cylinders. This concept utilizes three pumps, driven by a single electric servo drive. The concept is redesigned for usage in winch drives, driven by flow symmetric hydraulic motors and single directional loads as commonly seen in e.g. active heave compensation applications. A general drive configuration approach is presented, along with a proper control strategy and design. The resulting concept is evaluated when applied for active heave compensation. Results demonstrate control performance on level with conventional valve solutions in terms of motion tracking, however with improved efficiency, especially in the event that the electrical servo drive can realize four quadrant operation.

The application of sliding mode algorithms for control of hydraulic drives has gained increasing interest in recent years due to algorithm simplicity, low number of parameters and possible excellent control performance. Both application of first- and higher order sliding mode control algorithms in hydraulic drive controls have been presented in various literature, demonstrating the possible successful application of these. However, a major drawback is the presence of e.g. valve dynamics which often necessitates the usage of continuous approximations of discontinuities in order to avoid control chattering, which on the other hand compromises the robustness properties. This may also be the case when discontinuities are only present in the control derivative. This fact suggests that sliding mode algorithms may be more appropriate for assisting the control, i.e. for state observation, disturbance observer based control etc., and several examples of such approaches have been presented in literature. The latter case appear especially interesting as a sliding mode actually takes place, but only the low-pass filtered sliding mode algorithm output is used in the actual control input. However, the successful implementation relies heavily on the low-pass filter design where the drive dynamics, sample rate etc. play a significant role. In this paper the utilization of the super twisting algorithm for disturbance compensation is considered. The fact that the discontinuity here is nested in an integral causes less restrictions on the used low-pass filter, enabling the possibility for a wide range of usage when the sampling frequency is in the range of industrial grade control hardware. The proposed control structure is designed and considered in relation to a valve driven hydraulic winch drive, and results demonstrate that excellent and high precision control may be achieved in the presence of large and highly varying external loads.

The fluid power accumulator is a high-risk component in the safety shutdown system of most modern wind turbines. Recent studies of failures in wind turbines have shown a high failure rate of such accumulators. This paper proposes an estimation method for detecting one of the most common faults for accumulators, namely gas leakage. The method utilizes an Extended Kalman Filter for joint state and parameter estimation with special attention to limiting the use of sensors to those commonly used in wind turbines. The precision of the method is investigated on an experimental setup which allows for operation of the accumulator similar to the conditions in a turbine. The results show that gas leakage is indeed detectable during start-up of the turbine and robust behavior is achieved in a multi-fault environment where both gas and external fluid leakage occur simultaneously. The estimation precision is shown to be sensitive to initial conditions for the gas temperature and volume.

This paper considers the analysis and control strategy for a novel direct hydraulic cylinder drive, that is over-actuated in the sense that it has more inputs than sensible outputs. Efforts to overcome the inherent loss of energy due to th+rottling in valve driven hydraulic drives are many, and various approaches have been proposed by research communities as well as the industry. Recently, a so-called Speed-variable Switched Differential Pump was proposed for direct drive of hydraulic differential cylinders. The main idea with this drive is to utilize an electric rotary drive with the shaft connected to three oppositely oriented fixed displacement gear pumps to actuate a differential cylinder. To ensure a high stiffness of the drive, this is constructed such that the transmission line pressures will increase for pump output flows exceeding pump leakages, and proportional valves provides the ability to bleed off flow from the transmission lines to achieve reasonable pressure levels. This design renders the drive over-actuated as the line pressures and the cylinder piston motion cannot be controlled independently, due to the pressure difference being motion generating. In order to achieve satisfactory performance of this drive, a state coupling analysis is presented along with a control strategy based on state decoupling synthesized from input-output transformations. This includes control schemes for the transformed system. The proposed control strategy is experimentally verified on a drive prototype, and results demonstrate that satisfactory overall performance-, and in particular highly accurate position tracking is achieved.

Discrete fluid power systems featuring transmission lines inherently include pressure oscillations. Experimental verification of a discrete fluid power power take off system for wave energy converters has shown the cylinder pressure to oscillate as force shifts are performed. This article investigates how cylinder pressure oscillations may be reduced by shaping the valve opening trajectory without the need for closed loop pressure feedback. Furthermore the energy costs of reducing pressure oscillations are investigated.

Motion control design for hydraulic drives remains to be a complicated task, and has not evolved on a level with electrical drives. When considering specifically motion control of hydraulic drives, the industry still prefers conventional linear control structures, often combined with feed forward control and possibly linear active damping functionalities. However difficulties often arise due to the inherent and strong nonlinear nature of hydraulic drives, with the more dominant being nonlinear valve flow- and oil stiffness characteristics, and furthermore the volume expansion/retraction when considering cylinder drives. A widely used approach with electrical drives is state controlor cascade control, that may by successfully applied to manipulate the drive dynamics in order to achieve high bandwidths etc., due to the nearly constant parameter-nature of such drives. Such properties are however, unfortunately not present in valveoperated hydraulic drives. This paper considers a cascade control approach for hydraulic valve-cylinder drives motivated by the fact that this may be applied to successfully suppress nonlinearities. The drive is pre-compensated utilizing a pressure updated inverse valve flow relation, ideally eliminating the system gain variation, and the linear model equations for the pre-compensated system is used for the cascade control design. The cascade design does not utilize e.g. bode plots, root loci etc., and is based on an analytic approach, emphasizing the exact influence of each control parameter, resulting in an easily comprehensible control structure. Two versions of this cascade control approach is presented, with the first utilizing pressure-, piston velocity- and piston position feedback, and the second utilizing only pressure- and piston position feedback. The latter may be especially interesting in an industrial context, as this does not use the velocity feedback, which is rarely available here. Furthermore, the position control loop is designed analytically to guarantee a user defined gain margin. The proposed control design approach is verified through simulations, and results demonstrate the announced properties.

The last decade an increasing amount of work are performed with the aim of enhancing efficiency and reliability of fluid power components. Consequently, the loss mechanisms of lubricated tribological interfaces are of particular interest. This has led to development of sophisticated three-dimensional thermo-elasto-hydrodynamic lubrication models of fluid power components. The computational efforts involved in simulation with such models entail that design optimization are to some extend impractical. However, such models are also pursued in theoretical tribology with the aim to study loss and wear mechanisms, which is very difficult to study experimentally. In consequence, advanced numerical models are the state of the art approach in theoretical fluid power tribology research. However, a downside of modern numerical models is the inability to provide a practical tool for wide-scale parameter sweep investigations, due to computational effort, whereby analytical research in loss mechanisms still have certain advantages. In this paper, the thermo-viscous effect of a lubricant is included in an analytical study of the friction and energy dissipation of oil hydraulic thin-films. This analytical study is based on an asymptotic approximation of the laminar lubrication thermal field at low reduced Peclet and Brinkman number, where viscosity is included as a function of temperature. The asymptotic series is truncated at first order and used to derive an expression of the viscous friction on a sliding surface. This reveal an influence from the surface temperature gradient on the viscous friction, which is not revealed when applying classical isothermal analysis. The significance of the thermo-viscous effect on friction and energy dissipation is analyzed analytically in order to provide a qualitative insight to the relation between thermodynamic properties, film thickness, sliding velocity and viscous friction.

Discrete Fluid Power Force Systems is one of the topologies gaining focus in the pursuit of lowering energy losses in fluid power transmission systems. The cylinder based Fluid Power Force System considered in this article is constructed with a multi-chamber cylinder, a number of constant pressure lines and a valve manifold. The valve manifold is used to control the connections between the cylinder chambers and the pressure lines and hereby the resulting force form the cylinder. The valve manifold is equipped with fast on/off valves. However, shifting between pressure lines may yield pressure oscillations in the cylinder chamber, especially for systems with long connections between the cylinder and the valve manifold. Hose pressure oscillations will induce oscillations in the produced piston force. Hence, pressure oscillations may increase the fatigue loading on systems employing a discrete fluid power force system. The current paper investigates the correlation between pressure oscillations in the cylinder chambers and valve flow in the manifold. Furthermore, the correlation between the pressure shifting time and the pressure overshoot is investigated. The study therefore focus on how to shape the valve flow in the manifold to reduce the added fatigue loads. A simple transmission line model is developed for the analysis. Two inputs are given in the Laplace domain and the time domain solution of the cylinder pressure to the given inputs are derived through inverse Laplace transformation. Based on the time domain solutions the pressure overshoot for various pressure shifting times is investigated. With the two input functions defined by the pressure shifting time, T, the main results of the current paper show the correlation between the minimum shifting time and the pressure overshoot in a given cylinder chamber with a given line connection.

In fluid power machinery hydrostatic bearings are frequently used, and a first approximation approach to design is determination of a balance ratio by analytical calculation of the hydrostatic pressure force. Usually this is performed assuming that the thermo-piezo-viscous property can be neglected. However, in applications as piston machines, where pressure in many cases exceeds 200 Bar, such assumption leads to considerable error in the balance ratio prediction, due to the piezo-viscous property of the lubricant. Furthermore, the thermo-viscosity relation also has a significant influence, which adds to the discrepancy of such simple design approach. In this paper the hydrostatic pressure force calculation is reviewed in terms of thermohydrodynamic (THD) lubrication theory, and simple analytical approximations of the hydrostatic pressure force, incorporating the piezo-viscous and thermo-viscous property of the lubricant, are presented. In order to investigate validity of the approximations a numerical THD model is developed. A comparison study of the numerical and analytical predictions is performed in order to validate the simple design approach. In addition, the assumptions that form the basis of these analytical approximations are explored in order to clarify the limits of application. In conclusion, it is found that the spatial gradient of the thermal field on the bearing surface is the significant factor in the thermo-viscous effect on the hydrostatic pressure profile, which leads to the conclusion that design engineers need to understand the thermodynamics of hydrostatic bearings, when using the conventional simple analytical approach, neglecting thermo-piezo-viscosity, in hydrostatic pressure force calculations.

Over the past 20 years an increasing focus on efficiency and reliability in fluid power displacement machines has provided an incentive to study loss and wear mechanisms. One example is the hydrostatic fluid power transmission systems for wind and wave energy applications. The loss and wear mechanisms are mainly attributed to the tribological interfaces in fluid power machines. Consequently, optimization of efficiency and reliability of fluid power machines imply consideration of tribological interface design. The majority of the work done by researchers and engineers on the study of loss and wear mechanisms in the lubricating gaps in fluid power machines is confined to simulation models, as experimental treatments of these mechanisms are very difficult. This means, that a complete verification of the theoretical work is difficult. The aim of this paper is a state of the art review on the theoretical work for the design and optimization of fluid power displacement machines, and also the work done to validate the theoretical models. This review is not a complete historical account, but aim to describe current trends in fluid power displacement machine tribology. The review considers the rheological models used in the theoretical approaches, the modeling of elastohydrodynamic effects, the modeling of thermal effects, and finally the experimental validation of the theoretical models.

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.