Developments in the field of electro-hydraulic linear actuators controlled by variable-speed pump drives have been increasing over the past decade. The majority of these benefits from the speed control loops realized in the associated electric drives, hence these variable-speed pump drives have been operated as flow control units. For this reason, control methodologies commonly used in valve operated linear drives can be adopted. Such approaches may be characterized as primary control strategies, meaning that there is a flow coupling between the variable-speed pump drive and the linear actuator, with a pressure reaction. Alternatively, other technologies like hydraulic transformers benefit from secondary control abilities, meaning that there is a pressure/force coupling between the actuating device and the linear actuator, with a flow reaction. This paper presents a study considering the application of secondary control principles in electro-hydraulic linear actuators, where the usage of only the electric motor current control loops allow for this to be realized. It is found that by feeding back the hydraulic force measurement in addition to the current loops, allow for a high force control bandwidth by virtue of state feedback controls. The high bandwidth of the force control enables alternative approaches to motion control of electro-hydraulic linear actuators.

Selective laser melting (SLM) or laser powder bed fusion (LPBF), is one type of metal additive manufacturing (AM) technology which is efficient in producing lightweight hydraulic components. However, the fabricating quality is poor when flow channels are built horizontally on the substrate due to residual stress on the large overhang region. Large surface roughness and dimensional error occurs which greatly affect friction factor but not studied. In this work, fluid channels with various diameters were built using SLM. The surface roughness and profile were characterized. Friction factor was then measured using a customed test rig. Results indicate that SLM fabricated fluid passages have high and uneven roughness and dimensional accuracy is poor. A new friction factor model was developed which can be used to calculate pressure loss in a SLM fabricated fluid channels.

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.

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.

Environmental and economic factors are driving the development of more fuel efficient off-highway vehicles. The pathway to fuel savings of greater than 50% in an excavator application through utilisation of system architectures unlocked by Digital Displacement technology is presented. Pump flow distribution using digital valves instead of traditional proportional control valves is demonstrated experimentally. The “Workbus” power distribution scheme is demonstrated on a small scale backhoe arm on a laboratory test rig. These tests do not include hydraulic energy recovery. A backward-facing simulation of an 18 tonne excavator is described. The simulation uses input data collected from grading and lorry loading duty cycles. Applying the workbus system architecture to the excavator in simulation, fuel savings of 31% to 48% are realized. With the addition of energy recovery capability via Digital Displacement Pump-Motors, simulated fuel savings are 53% to 58% compared to the original excavator hydraulic system.

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.

In this work, the authors present a robust and integrated procedure for the design of multi-stage gear pumps to be used in dry sump system applications. Based on the target delivery flow rate, rotational speed and fluid properties, the developed iterative method enables to directly obtain the geometrical features and the working parameters of the pump components, such as gearpair specifics, shaft and journal bearing dimensions, clearance values. The methodology is then applied to a case study in order to highlight its features and detail the achievable outcomes. Quality of the results is assessed by means of a CFD analysis, demonstrating the capability to obtain the expected volumetric efficiency.

In this paper, a piezoelectrically driven single piston pump is designed for high delivery pressure and large displacement. It is composed of a piezoelectric stack, a hydrostatic amplifier and a plunger with two check valves from a radial plunger pump commercially available. Passive check valves instead of active membranes in the pump allow higher pressure delivery, and the hydrostatic amplifier is adopted for larger displacement of the pump. In order to study delivery characteristic of a piezoelectrically driven single piston pump, numerical simulation of unsteady flow inside the pump is conducted by computational fluid dynamics (CFD). Moving mesh technique is adopted to characterize the behavior of two check valves for oil distributing. Influences of typical design and operation parameters on delivery performance of the single piston piezo pump are analyzed. The results show that, driving frequency and spring stiffness of the valves and chamber configuration should be properly designed for stable oil delivery with low flow ripple as possible. Numerical simulation is valid to describe the behavior of the pump, on target for theoretical reference to its counterparts used in miniature EHA actuators.

The paper deals with experimental tests and numerical simulations (3D and 0D fluid-dynamic modelling) of a conical poppet pressure relief valve with flow force compensation. The aim of the study was to realize a dynamic model of the valve, able to consider the effect of the pressure force, which arises downstream the metering edge and is determined by both the flow forces and the pressure recovery on the flow deflector. Starting from a 3D-CFD model, it was possible to study the interaction between the poppet opening and the pressure field, in order to evaluate the total pressure force as a function of the poppet displacement. The contribution of the pressure recovered on the deflector was separated from the traditional flow force and then parametrized as a function of some geometric features. It was then possible to develop a 0D fluid-dynamic model that is suitable not only for the considered valve but also for valves with similar geometries. Lastly, the model was validated using experimental data acquired on test bench for three different valves. The comparison of the predicted flow-pressure characteristics with the experimental ones indeed showed a good result matching. This model can also lead towards design considerations to study the behaviour of a larger number of valve geometries.

Hydraulic pumps, which reach pressures up to 3000 bar, are often realized as plunger-piston type pumps. In the case of a common-rail pump for diesel injection systems, the plunger is driven by a cam-tappet construction and the contact during suction stroke is maintained by a helical spring. Many hydraulic piston-based high pressure pumps include gap seals, which are formed by small clearances between the two surfaces of the piston and the bushing. Usually the gap height is in the magnitude of several micrometers. Typical radial gaps are between 0.5 and 1 per mil of the nominal diameter. These gap seals are used to allow and maintain pressure build up in the piston chamber. When the gap is pressurized, a special flow regime is reached. For the description of this particular flow the Reynolds equation, which is a simplification of the Navier-Stokes equations, can be used as done in the state of the art. Furthermore, if the pressure in the gap is high enough — 500 bar and above — fluid-structure interactions must be taken into account. Pressure levels above 1500 or 2000 bar indicate the necessity for solving the energy equation of the fluid phase and the rigid bodies surrounding it. In any case, the fluid properties such as density and viscosity, have to be modelled in a pressure dependent manner. This means, a compressible flow is described in the sealing gap. Viscosity changes in magnitudes while density remains in the same magnitude, but nevertheless changes about 30 %. These facts must be taken into account when solving the Reynolds equation. In this paper the authors work out that the Reynolds equation is not suitable for every piston-bushing gap seal in hydraulic applications. It will be shown that remarkable errors are made, when the inertia terms in the Navier-Stokes equations are neglected, especially in high pressure applications. To work out the influence of the inertia terms in these flows, two simulation models are built up and calculated for the physical problem. One calculates the compressible Reynolds equation neglecting the fluid inertia. The other model, taking the fluid inertia into account, calculates the coupled Navier-Stokes equations on the same geometrical boundaries. Here, the so called SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm is used. The discretization is realized with the Finite Volume Method. Afterwards, the solutions of both models are compared to investigate the influence of the inertia terms on the flow in these specific high pressure applications.

The apparent volumetric displacement of digital displacement pumps and motors is reduced with increasing fluid pressure. So-called pump shrinkage has been documented in previous publications, where fluid compressibility effects were assumed to affect input and output power equally. In this paper, the authors derive the torque and flow rate of an ideal digital displacement pump. It is shown that the output power shrinks slightly more than the input. The difference between input and output shrinkage is counted as a power loss according to the accepted definition of total efficiency. New equations are presented for calculating mechanical and volumetric efficiencies which are up to 2% more accurate than the previous method (which assumes equal shrinkage) and up to 5% more accurate than conventional equations (which assume no shrinkage). Compressibility effects may be even more significant depending on pump design parameters, fluid properties, flow control algorithms and operating conditions. Calculations of partial pump efficiencies require a derived displacement volume to be known. The derived displacement volume of digital displacement pumps is considered for the first time in this paper. The contributions of this work are instructive for understanding the unique characteristics of digital displacement pumps as well as check-valve type pumps in general.

There is a long trend in off-highway vehicles toward higher energy efficiency and electro-hydraulic control. Digital pumps and motors are poised to become a significant enabling technology in this trend. This paper analyzes a 20-tonne tracked excavator that was equipped with digital displacement pumps and evaluated for efficiency and productivity. Previous research by Artemis Intelligent Power demonstrated improvements compared to a conventional, negative flow control excavator hydraulic system. With support from Artemis, the Danfoss Digital Displacement Excavator (DDE) is a step forward in terms of technology and commercial readiness. DDE is based on a CAT 320 next-generation excavator with completely electronic controls and advanced sensor and operator-assistance features. In before/after testing, DDE showed 15% higher productivity (in meters of trench per hour) and 13% higher efficiency (in meters per liter of fuel) for a trench digging cycle. Static pump efficiency is only part of the story; dynamic response and controllability contributed to the measured performance gains. Potential causes for variation in measurement were analyzed including operator variation, flow variation, and machine response.

The lumped parameter approach based on equations describing of the physical behavior of the system still represents one of the most convenient way to simulate hydraulic control systems. The key advantages of this approach are given by its intrinsic simulation swiftness as well as the ease of deriving state space formulations for controller design purposes. However, the common limitation of lumped parameter models is the high level of simplification of for certain physical aspects. For the case of hydraulic control valves with complex geometry, the flow forces are usually one of the most difficult aspects to describe accurately. The present paper presents a lumped parameter model for counterbalance valves, which includes an accurate analytical approach to model the effect of the flow forces based acting on the valve poppet and piston. The model is based on a classic control volume scheme for the description of the flow through the valve, and it is coupled with a dynamic model for the descriptions of the motion of the moving parts inside the valve. The novelty of the proposed approach consists on the analytical description of the flow forces, which is based on fluid momentum considerations. After describing the modeling approach, the paper details the authors’ efforts for experimentally validate the model on the basis of tests performed on actual components. The comparison between simulation results and experimental data confirms the validity of the proposed model and also highlights the importance of accounting for flow forces while describing the operation of counterbalance valves, particularly for cases of high flow rates.

In hydraulic systems, the presence of foreign material in the system oil accounts for the majority of system troubles due to mechanical wear of components, sticking of different parts etc. Therefore, it is essential to maintain an adequate cleanliness level of the fluid at all times through filtration. Mechanical filters are used for this purpose, to separate solid particles from the system oil. As a hydraulic filter gets accumulated with dirt throughout its service life, the pressure drop over the filter element increases. This pressure drop is typically used for determining the lifetime of a filter element: once a predetermined pressure drop at certain flow conditions has been reached, the filter has accumulated enough dirt to require servicing or replacement. In this paper, a correlation model has been developed to describe the effects of flow and fluid properties on the dirt holding capacity and the service life duration of a hydraulic filter. For this purpose, extensive laboratory tests have been carried out in order to measure the pressure drop development of a filter unit at different oil flow rates, viscosities and gravimetric contamination levels. The work in this paper has been done as part of the initial research for investigating the effects of different flow and fluid parameters on hydraulic filtration. The aim of the overall research project is to develop an IoT-enabled smart filter unit that could predict its remaining lifetime, and estimate the condition of the system oil as well.

ISO 4409 is the most important international standard for measuring the efficiency of hydraulic pumps and motors, the latest edition being 4409:2007. The standard describes methods for determining the steady-state performance in terms of overall efficiency. It also defines equations for calculating the volumetric efficiency of pumps and motors. The hydro-mechanical efficiency is only defined for motors, not for pumps. This paper analyses the efficiency and losses of pumps and motors in an alternative way. The preference is on loss analysis instead of efficiencies. Especially the effects of the bulk modulus are considered in a different and more inclusive manner. The new methodology results in a higher total loss for motor and a lower total loss for pumps than the current ISO 4409 standard. Furthermore, it results in significant changes of the hydro-mechanical and volumetric losses. The differences between the new methodology and ISO 4409 become larger for high load pressures. The new methodology demands knowledge about the minimum volume of the displacement chamber. The ratio between this volume and the full displacement of a single displacement chamber strongly influences the hydro-mechanical and volumetric losses of the pump or motor. The new methodology is valid for all positive displacement hydrostatic pumps and motors. The volumetric efficiency, as defined in ISO 4409, can still be used as a flow rate factor, but should not be regarded as an energy conversion efficiency. The importance of adopting the proposed methodology is further demonstrated by analyzing and comparing the measurement data about a fixed displacement pump and motor, showing the differences in the loss analysis by means of ISO 4409 and the new equations. The methodology, observations and validation results presented in this paper are significant and can pave the road for improving the current ISO 4409:2007 standard, which would ultimately benefit the industry.

Agricultural tractors make massive use of hydraulic control technology. Being fuel consumption a big concern for agricultural applications, tractors typically use the latest state-of-the-art technology to allow efficient fluid power actuation. Nevertheless, the quantification of the energy loss within the hydraulic system of such applications represents an important step to drive the development of the current technology with cost-effective solutions. In this paper, the load sensing (LS) circuit that typically equips of the hydraulic remotes is taken as reference. A simulation model has been developed within the Amesim software with the aim of accurately predict the operation of the system including the energy flow from the hydraulic supply to the hydraulic user. The paper particularly details the models of the LS pump and the hydraulic remote valves. Within the research, experimental tests on a reference tractor were designed and executed to allow the model validation. The comparison between the experimental results and the simulation data shows the validity of the model. Furthermore, the model allows highlighting the energy losses in the different components of the system as well as identifying the most favorable operating conditions of the system with respect to energy efficiency. The model can be used in support of future research aimed at formulating a more efficient solution for the hydraulic circuit of agricultural tractors.

In an aircraft turbofan engine, a fuel metering unit meters and supplies the required fuel to the engine according to the flight situation. When a centrifugal fuel pump (CFP) is used as the fuel pump, the ratio of hydraulic power per weight can be increased by raising the rated rotational speed, so the weight of the fuel pump can be decreased compared to when using a gear pump (GFP). There is an advantage that it can be reduced significantly. However, since the operating range of the fuel pump is wide, it is not effective to use CFP in an extremely low flow rate region because the fuel temperature rises due to its PQ characteristics and a large loss. Therefore, it is considered effective to combine CFP and GFP as pressure sources, and to use GFP in the low flow region and CFP in the high flow region. For that purpose, it is necessary to have a pump mode switching mechanism. The disadvantage in this case is that changing the pump mode causes a large pressure change of the fuel pressure source, which in turn causes fuel flow pulsations. There are three possible ways to solve this problem. The first method is to keep the differential pressure control valve (DPCV) unit response constant, which keeps the metering valve differential pressure constant in FMS. A second method is to remove high frequency components that the DPCV cannot follow pressure changes in the fuel control system. A third method is to keep the pressure difference between the two fuel sources small and to reduce the amplitude of the applied disturbance. In this paper, the first method, which makes DPCV response high response, is verified by modeling and simulation, and its effectiveness is confirmed.

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.