Most proportional directional control valves currently in practical use are piston spool valves. A disadvantage of this type of design is the occurring leakage between spool and sleeve or housing. A second disadvantage is that the service life is limited by wear on the control edges. The ceramic flat slide valve is a promising concept, which allows to reduce the mentioned leakage and to increase service life. In contrast to piston spool valves, the main stage of the flat slide valve consists of three plates. Two control plates and a movable slide plate, which is located in between. An external force is applied to the plates by the so-called pressure compensation, which presses the plates together and thus counteracts a gap expansion and thereby an effective reduction of occurring leakage is accomplished. The plate-by-plate design of the main stage allows the use of technical ceramics, which are more resistance to abrasive wear and can thereby extend the service life of valves. A challenge in the realization of the flat slide valve is the design of the pressure compensation. If the force applied by the pressure compensation is not sufficient to prevent a gap formation between the plates, an increased leakage will occur, which results in a reduction of efficiency. In case the applied force is too high due to the pressure compensation, high frictional forces between the plates occur and an adjustment of the slide plate position through an actuator is no longer possible. Therefore, the design of the pressure compensation is essential for the success of the concept. This contribution presents the results of testing a main stage, which consists out of two metallic control plates and a slide plate made of special brass. The main stage is designed in such a way that a 4/3-way directional control valve with a proportional characteristic can be realized, as it is also achieved by valves currently available on the market. Therefore, in addition to the pressure forces, further influences on the resulting force of the pressure compensation were determined and their influence was estimated based on test bench results. With a program written in MATLAB, the forces prevailing in the valve can be calculated time-efficiently for different geometries and are validated with test bench trials.

In recent years, many works focusing on improving motion tracking accuracy of hydraulic actuators have been done by developing nonlinear control algorithms. Model nonlinearity compensation, such as valve deadband, nonlinear friction force, and flow leakages, is one of the key points in the control design. However, the inherently nonlinear valve flow characteristic is usually treated as linear for simplicity, which might lead to undesirable flow rate deviation and reduce the tracking precision especially when the cylinder moves at a low speed. In this study, an online valve flow nonlinearity compensation method is developed to improve the motion tracking precision of hydraulic actuators. Specifically, polynomials are used to mimic the nonlinear valve flow characteristic between the valve control input, pressure difference, and flow. The coefficients in polynomials are synthesized based on the offline identification experiments while the lumped discrepancy is updated online in the proposed control design. The adaptive robust control approach is also used to develop the closed-loop motion controller, which can handle the parametric uncertainties and uncertain nonlinearities in the electro-hydraulic system. Comparative experiments are conducted on an independent metering system with a differential cylinder and the results show the improved tracking precision with nonlinear valve flow comparing to the linear one, verifying the effectiveness of the proposed approach.

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.

Due to an increased focus on improving the energy efficiency and compactness of hydraulic linear actuators, the electro-hydraulic compact drive (ECD) has received increased attention lately. In this study the ECD consists of variable-speed electric motors and fixed-displacement pumps, which are directly connected to the cylinder, thus controlling the linear motion in a throttleless manner. Furthermore, ECDs are self-contained systems, i.e. based on a fully enclosed oil circuit, in order to avoid external contamination and air to enter the system and to increase system compactness. Conventionally a low-pressure gas-loaded accumulator is used as an oil reservoir to compensate for the flow imbalance occurring whenever utilizing single-rod cylinders in closed systems. The accumulator pressure is to be kept relatively low to stay within the required limits governed by the permitted pump housing’s pressure. Generally, this pressure is not allowed to exceed 1–3 bar. To avoid violating this limitation, the gas volume must be significantly larger than the actual oil volume, which needs to be stored in the accumulator. This requirement decreases the obtainable compactness of the ECD, especially for systems with a large cylinder stroke. Furthermore, the accumulator represents a potential of gas leakage, which ultimately could result in the ECD being non-functional. This paper presents a gasless reservoir solution, improving the system compactness and avoiding the risk of gas leakage. The proposed solution is based on a bootstrap reservoir which is charged by the lowest cylinder chamber pressure. This strategy is feasible for the class of ECDs that is capable of controlling the lowest cylinder chamber pressure alongside the cylinder motion. An ECD consisting of two electric prime movers is considered as a case study. It is shown how the gasless reservoir may be integrated into the system, and an analysis of how this affects the operating range and the dynamic couplings of the system is presented. This leads to the derivation of a control strategy for the Multi-Input-Multi-Output (MIMO) system based on state decoupling, by defining virtual inputs to control virtual outputs. A numerical study suggests that the reservoir volume may be reduced by approximately 50% for the given system dimensions. The proposed control strategy shows good position tracking performance while also being able to control the reservoir pressure within the pre-defined limits of 1 to 3 bar.

In order to increase efficiency of diagnostics of electro-hydro-mechanical systems (EHMS) it is advisable to have simulation models of typical faults. Such approach makes it possible to estimate in advance, even at the stage of mathematical modeling, the impact of different faults on functioning of hydraulic systems. This work is aimed at creating a database containing complexes of diagnostic features, which allow distinguishing types of faults, their causes and stages of development. In the paper, typical faults of EHMS are presented on the basis of statistical information from literature sources and experimental research. They include internal and external leakages, spool and sleeve sticking, degradation of power fluid. The causes of faults and their impact on hydraulic systems functioning are considered. Simulation models of typical faults are implemented and studied in the SimulationX software package. The static and dynamic characteristics of the systems are investigated in order to identify diagnostic signs of various faults. The impact of typical faults on various system parameters is discussed. During the research, the tasks of selecting the rational location of sensors of different types (pressure, flow, displacement, or force sensors), their quantity for recognition of a typical fault are solved. The results of theoretical and experimental studies of serviceable and faulty systems for cases of control and disturbance actions are presented. Comparative analysis of transient processes of serviceable and faulty EHMS is presented with assessment of difference between theoretical and experimental data. The results of the work allow to more rationally designing the diagnostic complex for more accurate identification of the type of fault, stage of its development and prediction of residual service life of EHMS.

When hydraulic fluid is compressible, the usual formula of hydraulic power being the product of pressure (p) and volumetric flow Q, which accounts only for the flow work, is not sufficient. By deriving an explicit formula for the stored compressible energy in accordance with the possibly pressure-dependent bulk modulus, a hydraulic effort (with symbol Φ) is defined to be the sum of the fluid pressure and the compressible energy density at that pressure. It is shown that Φ is the conjugate variable to volumetric flow (Q) such that that the compressible hydraulic power flow is the product Φ(p)Q. With proper understanding of compressible hydraulic power flow, the ideal relationships of pumps and motors with compressible fluid are then derived. These differ from incompressible formulae by replacing p by Φ and identifying an appropriate displacement. From these, definitions for mechanical efficiency, volumetric efficiency, and power efficiency are obtained. Unlike the previous attempts, these definitions are mutually consistent in that the product of mechanical and volumetric efficiencies is indeed the power efficiency. Furthermore, they are exact and are suitable even if the bulk modulus is pressure dependent. To gain insights into these new efficiency definitions, various modes of non-ideal effects in a piston pump/motor with variable valve timing are modeled and their effects on the mechanical, volumetric and power efficiencies are obtained. These non-ideal effects include piston-bore friction, leakage, valve throttling, and non-ideal valve timing.

Condition monitoring of axial piston pumps has seen considerable research in recent years, due to the attractive economic benefits of predictive pump maintenance rather than unscheduled failures. Often the health of the pump is well correlated to leakage, but directly measuring flow can be expensive and unreliable. Instead, some researchers have proposed using dynamic pressure measurements to infer leakage parameters, with some success. One of the major impediments to widespread adoption of this method is that large volumes of data are required to generate a useful model relating the dynamic measurements to leakage parameters, typically with high sensitivity to noise and prone to overfitting. This paper applies data dimensionality reduction techniques to this problem and evaluates their usefulness using a simulation study.

This work describes an efficient means to adjust the power level of an axial piston hydraulic pump/motor. Conventionally, the displacement of a piston pump is varied by changing the stroke length of each piston. Since the losses do not decrease proportionally to the displacement, the efficiency is low at low displacements. Here, with partial-stroke piston pressurization (PSPP), displacement is varied by changing the portion of the piston stroke over which the piston is subjected to high pressure. Since leakage and friction losses drop as the displacement is decreased, higher efficiency is achieved at low displacements with PSPP. While other systems have implemented PSPP with electric or cam-actuated valves, the pump described in this paper is unique in implementing PSPP by way of a simple, robust hydro-mechanical valve system. Experimental testing of a prototype PSPP pump/motor shows that the full load efficiency is maintained even at low displacements.

Cycloidal-toothed gerotors are formed by the combination of epicycloid and a hypocyloid arcs that use the pitch circles as their base circles. They are a common profile type used in industry likely because they can be generated by simple parametric equations. One of the problems with the cycloidal-toothed profile type is that the radius of curvature of both the inner and outer gear are zero when the gears contact at the pitch point which can lead to high contact stress. A gear generation algorithm has been developed that modifies the hypocycloid tooth form to eliminate contact in regions with very low radii of curvature that is yet to be described in scientific literature. Seven performance metrics are developed to evaluate size, flow ripple, adhesive wear, contact stress, radial gap leakage, lateral gap leakage, and inlet throttling that are used as objective functions in a multi-objective optimization. The pump geometry is optimized by applying the NSGA-II algorithm with a population size of 1000 for 500 generations to produce a pareto front, and the results are compared to two cycloidal-toothed gerotors used in the automotive industry. Two designs were found that significantly reduce the contact stress in the profiles while giving the same performance in the other six objective functions in comparison to the profiles used in industry. Another two designs are found that can significantly reduce several objective functions if the size of the pump can be increased slightly.

A large variety of options for the internal valves of a hydraulic motor are available. Poppet valves, rotary valves, and port plates are common options, but each have drawbacks such as actuation force, complexity, leakage, or friction. An alternative type that is also common is spool valves. Spool valves require minimal actuation force and have low throttling losses if sized correctly. This paper compares two drive options for a mainstage spool valve: a hydraulic pilot from a rotary valve and mechanical actuation. Dynamic models of the two valve configurations are created to carry out design studies and evaluate the performance of the motor. Compressibility effects, viscous drag, and flow forces are included in the model. Simulations show that the rotary valve pilot operation causes an unacceptable lag in the motion of the spool that cannot be corrected without reducing performance when running in reverse. Furthermore, the rotary pilot stage drive results in the motor timing becoming retarded at higher operating speeds. For these reasons, mechanically actuated valves are preferred over pilot operated valves for hydraulic motors operated bi-directionally or across a wide speed range.

Failures in pitch systems may cause fatal damage to industrial wind turbines. One of the main reasons for failures in pitch systems is gas leakages of hydraulic accumulators. Due to the limited accessibility of offshore turbines, automated fault detection algorithms potentially increase turbine availability. The gas leakage is detected without downtime by using a model-based approach together with a bank and extended Kalman filters (EKF’s). The residual is analyzed using multi-model adaptive estimation (MMAE). The applied accumulator model relies on a thermal time constant describing the heat flux from the gas to the surroundings. The thermal time constant has been empirically derived from a prepressure of 50 to 172 bar. The fault detection algorithm is tested experimentally in a laboratory on a 25 liters piston accumulator using a load scenario obtained from real turbine data and a prepressure range of 50–140 bar. The Bank of EKF’s can classify the prepressure within a range and thereby detect if a gas leakage has occurred before it results in failure.

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 presents research aimed at understanding the effects of geometrical imperfections and tolerances upon the internal leakage occurring around null in the main stages of servovalves. Specifically, a two-dimensional (2D) computational fluid dynamic analysis was used to predict the direct leakage flow as a function of the overlap and clearance between the spool and bushing sleeve, as well as the roundness on the edges of the spool and bushing sleeve. Predictions of direct leakage flow against edge overlap, which have general validity, are provided in the paper for three selected values of the pressure drop. For different values of the pressure drop, analytical correlations can be applied using the data retrieved from these graphs. The analysis shows that the leakage flow is highly affected by the above-mentioned geometrical parameters. As expected, for given values of overlap and radial clearance, the greater the roundness of the edges caused by manufacturing processes or wear, the higher the leakage flow. For low leakage and hence low power loss requirement, the radii on the spool and bushing sleeve as well as the clearance must be maintained as low as possible. In addition, it is well-known that overlap between the spool and its bushing sleeve can help to reduce the leakage flow at null, and the effect of edge roundness on this reduction is now revealed.

With a flat slide valve concept, a reduction in leakage and an increase in service life could be achieved compared to conventional piston spool valves. In order to achieve a reduction in leakage flows and guarantee the adjustability of the valves at the same time, a correct design of the pressure compensation is essential. The level of force depends on the operating point of the valve and the position of the slider. Due to the design of the flat slide valve, it is possible to use ceramic semi-finished products for the main stage, which consists of control plates and a slide plate. The geometries are simple enough to be inexpensively manufactured with sufficient accuracy from a ceramic pre-product using laser cutting technology. High-tech ceramics are a promising material, which can contribute to an improvement of the performance of the valves. The higher resistance of ceramic materials to abrasive wear further increases the service life of the valves. Furthermore, ceramic materials show lower friction coefficients compared to metallic materials, which is of great importance for the concept of the flat slide valve due to the sliding of the plates on top of each other. However, at the same time the use of ceramics, which are rarely used in hydraulics at the moment, means that the design must be appropriate to the material and must be taken into account when designing the main valve stage. This article presents the concept of the flat slide valve for a proportional 4/3-way valve. Therefore, the design of the pressure compensation is shown and the current design of the main valve stage is presented. This includes the design of the flow channels, which has to be suitable for ceramic materials. The preliminary design of the flow channels were executed with CFD-simulations. With the shown design the function of a 4/3-way valve, known from piston spool valves, can be implemented with a linear behavior between slide plate deflection and opening cross section.

Traditional variable displacement piston machines achieve high efficiency when operating at high displacements, but struggle with poor efficiency at low displacements. The pistons are connected to high pressure and low pressure in conjunction with the output shaft position and the displacement is changed by changing the piston stroke, resulting in almost constant friction, leakage, and compressibility losses independent of displacement. In digital displacement machines, the rotary valve is replaced by two fast switching on/off valves connected to every cylinder. By controlling the fast switching on/off valves, the cylinders can be controlled individually and friction, leakage and compressibility losses can be minimized resulting in high efficiency even at low displacements. Previous studies have shown that high efficiency digital displacement machines require fast switching valves with high flow capacity and optimal valve timing strategy. When the digital displacement motor is to start, stop or be controlled at low speeds, the on/off valves must be able to open against high pressure difference. When opening the valves actively, the valve timing has to be conducted properly to minimize valve throttling losses and flow and pressure peaks. First, this paper shortly describes a previously developed method to estimate valve characteristics like transition time and flow capacity for a digital displacement machine. Then the paper presents a novel method of describing the required valve accuracy and repeatability to keep the valve throttling losses low and machine efficiency high.

This paper proposes a fault diagnosis method based on subspace identification for the leakage fault detection of valve-controlled hydraulic cylinders. Firstly, the state-space model for the system is established, in which the external force on the piston of the hydraulic cylinder is selected as input signal, and the pressure of the two chambers, displacement and velocity of piston rod are chosen as state variables. Then, the estimation value of specific elements of the system matrix can be obtained in terms of the subspace identification theory. On this basis, the existence, type and level of the leakage fault are determined. Finally, the numerical simulation is conducted through MATLAB-Simulink to verify the proposed method. The results demonstrate that the proposed method is effective and has high accuracy.

A computationally efficient gerotor gear generation algorithm has been developed that creates elliptical-toothed gerotor gear profiles, identifies conditions to guarantee a feasible geometry, evaluates several performance objectives, and is suitable to use for geometric optimization. Five objective functions are used in the optimization: minimize pump size, flow ripple, adhesive wear, subsurface fatigue (pitting), and tooth tip leakage. The gear generation algorithm is paired with the NSGA-II optimization algorithm to minimize each of the objective functions subject to the constraints to define a feasible geometry. The genetic algorithm is run with a population size of 1000 for a total of 500 generations, after which a clear Pareto front is established and displayed. A design has been selected from the Pareto front which is a good compromise between each of the design objectives and can be scaled to any desired displacement. The results of the optimization are also compared to two profile geometries found in literature. Two alternative geometries are proposed that offer much lower adhesive wear while respecting the size constraints of the published profiles and are thought to be an improvement in design.

Water as a working fluid in hydraulic systems: the benefits of this particular hydraulic fluid are both numerous and consequential, but its implementation remains nontrivial for certain key applications. One of these key applications is the axial piston machine of swashplate type, which counts among its selling points efficiency, the possibility of variable displacement, and the ability to function in high-pressure systems [1]. Water as a working fluid tends to mar that last point with its extremely low viscosity — and the high leakages and low load support that stand as effects of that fluid property in the context of tribological interfaces. However, water’s environmentally friendly, fire resistant nature is coupled with a high thermal conductivity and high heat capacity favorable for keeping hydraulic systems cool, as well as a high bulk modulus that cuts slack in the exact execution of machine motions [2]. That makes it worth implementing in hydraulic systems, even in the face of the aforementioned troubles. This paper investigates the effects of a surface shape that can be applied to the cylinder bores of axial piston machines with the goal of improving load support while keeping down leakage in the critical piston cylinder tribological interface of axial piston machines operating at high pressures with water as their hydraulic fluid.

The leakage past the tooth flanks of the gears in transition contacts in involute external toothed gear pump is analyzed in details using CFD in Fluent ® . Analytical results support the experimentally visualized flow patterns [1]. Extending the approach of an earlier investigation [2] more rigorous analyses are carried out considering actual gear data of a pump and a full cycle of contact. However, the relieve groove and cavitation are not considered. The basic purpose is to establish the flow pattern analyses using CFD in Fluent-Ansys ® environment. The beginning and the end of entrapment, squeezing of fluid, pressure build up, separation and generation of gap at the active contact and instantaneous flow through the gap are estimated meticulously.

Reciprocating rod seals play an important role in the aviation hydraulic system. As a new type of the aerospace reciprocating seal, VL seal is gradually used in crucial and complicated working conditions. When the seal is installed in the groove, there is large deformation in the circumferential direction, which would cause the deviation while the traditional axisymmetric 2D model is used for analyzing its performance. Therefore the new 3D finite element model is developed in this paper to obtain more accurate results of the contact pressure and the stress of the VL seal. Furthermore a mixed lubrication model combined with the 3D model is implemented to obtain the friction and leakage characteristics of the VL seal, which is composed of the fluid lubrication model, the micro contact model and the deformation model. The performance of the seal in the outstroke and instroke is studied respectively and the effects of different working velocities on the seal are also discussed. The analysis results show that the 3D model can demonstrates more comprehensive performance than the traditional axisymmetric model when there is a tangential strain in the VL seal.