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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.

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.

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.

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.

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.

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.

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.

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

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.

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.

This paper presents an observer-based sliding mode control concept with chattering reduction, generally applicable for position tracking control of electro-hydraulic valve-cylinder drives (VCD's). The proposed control concept requires only common data sheet information and no knowledge on load characteristics. Furthermore the proposed scheme only employ ***piston-and valve spool positions- and pressure feedback, commonly available in industry. The main target is to overcome problems with linear controllers deteriorating performance due to the inherent nonlinear nature of such systems, without requiring extensive knowledge on system parameters nor advanced control theory. In order to accomplish this task, an integral sliding mode controller designed for the control derivative employing state observation is proposed, based on a generalized reduced order model structure of a VCD with unmatched valve flow- and cylinder asymmetries. It is shown that limited attention can be given to bounds on parameter estimates, that chattering is reduced and the number of tuning parameters is reduced to the level seen in conventional PID schemes. Furthermore, simulation results demonstrate a high level of robustness when subjected to strong perturbations in supply pressure and coulomb friction force, and that tracking accuracy may be reduced to the level of noise. Furthermore, the proposed controller tolerates significant noise levels, while still remaining stable and accurate.

In this paper a controller utilizing second order sliding modes, generally applicable for position tracking control of electro-hydraulic valve-cylinder drives (VCD), is proposed. The proposed controller requires pressure measurements, and only the signs of the valve spool position and piston position- and velocity. The main objective is to introduce a control concept that provide for increased performance compared to linear controllers, in the presence of the inherent nonlinear nature characterizing such systems. To accomplish this task, a controller based on the twisting algorithm and knowledge of system gain variations is proposed. Results demonstrate strong robustness when subjected to parameter perturbations and that control chattering is eliminated.

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 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.

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 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.

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.

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.

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® (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

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

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.

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.

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 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 presents the development of a robust tracking control concept based on accurate feedforward compensation of hydraulic valve-cylinder drives. The proposed feedforward gain is obtained by use of a generalized description of the valve flow that takes into account any asymmetry of valves and/or cylinders, not assuming these asymmetries to be matched, and utilizes pressure measurements. Hence the proposed compensator is generally applicable to any such drive. Simulations show that the proposed compensator poses superior performance and robustness when subjected to strong perturbations in supply pressure and coulomb friction, compared with a conventional constant gain type feedforward compensator.

This paper presents the development of a sliding mode control scheme with chattering elimination, generally applicable for position tracking control of electro-hydraulic valve-cylinder drives. The proposed control scheme requires only common data sheet information, no knowledge on load characteristics, and employs piston- and valve spool positions- and load- and supply pressure feedback. The main target is to overcome problems with linear controllers deteriorating performance due to the inherent nonlinear nature of such systems. In order to accomplish this task, an integral sliding mode controller is developed for the control derivative based on a reduced order model. Simulation results demonstrate strong robustness when subjected to parameter perturbations and that chattering is eliminated.

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

As wind turbines increase in size and the demands for lifetime also increases, new methods of load reduction needs to be examined. One method is to make the yaw system of the turbine soft/flexible and hence dampen the loads to the system, which is the focus of the current paper. The paper first presents work previous done on this subject with focus on hydraulic yaw systems. By utilizing the HAWC2 aeroelastic code and an extended model of the NREL 5MW turbine combined with a simplified linear model of the turbine, the parameters of the soft yaw system are optimized. Results show that a significant reduction in fatigue and extreme loads to the yaw system and rotor shaft are possible, when utilizing the soft yaw drive concept compared to the original stiff yaw system. The physical demands of the hydraulic yaw system are furthermore examined for a life time of 20 years. The duty cycles, based on the extrapolated loads, show that it is possible to construct a hydraulic soft yaw system, which is able to reduce the loads on the wind turbine significantly.

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

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

In mobile hydraulic application the actuating fluid power system is most commonly controlled using a hydro-mechanical control scheme called Hydraulic Load Sensing (HLS). However, with the demands for increased efficiency and controllability the HLS solutions are reaching their limits. Motivated by availability of electronic controllable fluid power components and the potential of increased dynamic performance and efficiency, this paper investigates how HLS can be replaced with electronic control, i.e. Electronic Load Sensing (ELS). The investigation is performed by taking a specific application, a teletruck, and replace the HLS control with ELS. To aid the controller design for the ELS system, a complete model of the teletruck’s articulated arm and fluid power system is developed. To show the feasibility, a preliminary control structure for the ELS system is developed. The controller is tested on the machine, validating that features such as pump pressure control, flow sharing and over pressure protection can be implemented using ELS and with improved energy efficiency.