Downsizing the Electric Motors of Energy-Efficient Self-Contained Electro-Hydraulic Systems by Using Hybrid Technologies

The ongoing tendency toward the electrification of hydraulic systems, mainly in the form of self-contained solutions, poses design challenges in high-power applications. An electric motor drives positive-displacement machines used to control the motion of the hydraulic actuator (nonhybrid systems encompassing one or two pumps exist in the technical literature). All the power managed by the actuator passes through the electric motor, which leads to often oversized arrangements. These detrimental characteristics are especially pronounced when the power level increases approximately above 35–40 kW. Therefore, this research paper presents and studies a self-contained, electro-hydraulic, hybrid architecture intended to downsize the electric motor while maintaining the high-power output of the nonhybrid counterpart. After introducing the sizing process for the energy storage device and developing a suitable control strategy for the hybrid subsystem, the proposed concept is validated via high-fidelity dynamic models. The rated power of the electric prime mover can be cut by 70% in the considered application (a mid-size, knuckle-boom crane with an installed power of about 46 kW) without altering the performance in terms of motion control. The additional mass (about 310 kg) of the hybrid system is not expected to affect the load-carrying capacity significantly. As a result, the hybridization of self-sufficient systems is technically feasible for high-power applications. Drawbacks related to the system cost-effectiveness might, however, be experienced. An application-driven cost analysis should be conducted before implementing such a solution.

A Design Procedure for Multistage External Gear Pumps

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.

Fast Meshless Solution With Lumped Friction for Water Hammer

The water hammer in pipelines, with the absence of fluid friction, could be solved by a time-domain exact solution, using a simple recursive process. No computational grid was needed, but the calculation time cost was extremely high. Its improved method, named as the fast meshless solution (FMS), was developed to speed the computation by introducing the time-line interpolation. For the purpose of practical applications, the attempt to consider fluid friction in the FMS is presented here. As there is no mesh grid in the distance-time plane, the distributed friction model can not be employed upon the presented method directly. The fluid friction lumped at the pipe end is proposed, and both steady and unsteady friction are studied. A benchmark problem of the water hammer in a reservoir-pipe-valve (RPV) system is employed for the validation and comparison. The water hammer considering lumped friction can be calculated fast by the FMS, and the accuracy is acceptable. The method discussed here may be of interest in a quick assessment of the piping water hammer.

Detailed Dynamic Modelling of an Integrated Hydraulic Actuator

Hydraulic servos are characterised by their high-performance nature but due to their size and weight are not suitable for robotics where new legged applications require high power density and excellent dynamic behaviour in a small size. As an answer to this need a new class of integrated smart actuators is being developed. These systems consist of a servo valve, hydraulic cylinder, sensors and a controller all in a single device. This paper outlines the detailed modelling of the smart actuator for use in simulation and control design. The result is a model consisting of the dynamics of the novel ultra-low leakage servovalve, the valve flow characteristics considering the properties of each spool land, the single-ended cylinder with friction and the pressure losses in the supply and return lines to the actuator. The models are a combination of empirical and theoretical development, validated with experimental data. The smart actuator’s unique properties; compactness, weight and efficiency, combined with high-performance hydraulics make it well suited to mobile robot applications.

Methodology for the Evaluation of Gear Pump Performance

Currently, most performance curves of gear pumps present volumetric efficiency as a function of one or more operating conditions. However, the nature of gear pumps is that volumetric efficiency is dependent on pump speed, pump pressure rise, and fluid viscosity. This dependency on multiple parameters impedes direct comparisons between pumps tested at disparate operating conditions or on different testbeds. A new method has been developed that formulates the volumetric efficiency as a function of a single parameter that captures pump speed, pressure, and fluid viscosity. The characteristics of the pump is then captured by curve fitting two constants to empirical data. This method allows extrapolation of pump performance beyond empirical data and direct comparison of the volumetric efficiency curves of two pumps tested under disparate conditions within a single plot. This work describes the analytical derivation of the methodology and the empirical data used for validations. Additionally, several possible applications of this method are presented.

Implementation of Bubble Dynamic Effects by Coupling the Gilmore Equation With Fluid Dynamic Equations Using the Method of Characteristics

In fluid power systems, performance as well as system dynamics are strongly influenced by the presence of bubbles — especially for low system pressures. While the static effect of dissolved air (especially the volume fraction of dissolved air) on the bulk modulus has been extensively investigated in the past, in hydraulics, the dynamic effects due to bubble dynamics have been neglected entirely. Thereby, the dynamic characteristics of the bubbles influence the compressibility of the disperse fluid and, as a consequence, the speed of sound in the mixture and the hydraulic system as a whole. In order to account for the bubble behavior in hydraulic simulation models, the present paper investigates a method for coupling bubble dynamics equations, such as the Gilmore or the Rayleigh-Plesset equation, with the fluid dynamic equations and their subsequent solution using the method of characteristics. Regarding the modeling, special attention is put on the distributed bubble nuclei sizes, since bubbles of the exact same size are unnatural and cannot be observed in reality. Since a dilute mixture — i.e. a small void fraction — is assumed, bubble-bubble interaction is neglected in this study. To account for the polydispersity, a discretized lognormal distribution for equilibrium bubble sizes is considered. In order to evaluate the discretization interval needed, case studies of different numbers of bubble size classes are presented and their results evaluated. Thereby, the question about the least required numbers of homogeneous bubble clusters shall be answered, as to reduce the computational effort that is needed. Using the method described in this paper, the profound effect of the bubble dynamics and the bubble size distribution on the fluid system dynamics is elaborated.

Underwater Flexible Manipulator Double-Loop Feedback Control Based on Built-in Binocular Vision and Displacement Sensor

A real-time and effective double-loop feedback control system for underwater flexible manipulators is raised in this paper. The research object is a kind of underwater flexible manipulator driven by McKibben water hydraulic artificial muscle (WHAM) that can grasp, swallow, and disgorge target objects in its interior space. To make up for the lack of flexibility, an underwater flexible manipulator collaborative working strategy is proposed. A more flexible and smaller flexible manipulator is placed inside the flexible manipulator to assist it in performing difficult underwater works. The control system feeds back the position of internal objects through a built-in binocular camera and the working state of the manipulator through displacement sensors. The control system setups including underwater flexible manipulator subsystem, hydraulic drive subsystem, PLC control subsystem, displacement sensor subsystem, built-in binocular vision subsystem, and upper computer subsystem is built. PYTHON-based built-in binocular vision software and C++-based underwater flexible manipulator control software are also developed to facilitate observation and recording. The underwater flexible manipulator collaborative experiment is designed to verify the performance of the control system and the control algorithm.

Simulation Study of Permanent Magnetic Actuation for a Hydraulic Valve With Hysteresis Response Behavior

For the realization of compact and lightweight digital hydraulic cylinder drives for exoskeleton actuation the hydraulic binary counter concept was proposed. This counter principle is based on hydraulically piloted switching valves which feature a hysteretic response with respect to the pilot pressure. In first prototypes of that counter bistable mechanical buckling beams realized the hysteretic response. Their performance suffered from high friction in the hinges and high local stresses. Furthermore, they require tight manufacturing tolerances not only of themselves but also of their bearing structure. In this paper, the usage of a permanent magnet concept to realize the hysteresis function in an alternative way is studied. The valve spool is made of a ferromagnetic material and is attracted or repelled by a permanent magnet made of a Neodymium-Iron-Bor. The expected benefits are lower friction, lower demands on manufacturing tolerances, and an easier assembly of the valve. To find an advantageous embodiment of this functioning principle ring or disc shaped magnets of different sizes are analyzed. The magnetic forces exhibited by these different magnetic circuit designs are simulated with the Magnetic Finite Element code ‘FEMM’. The quasi-static magnetic forces at different spool positions are computed. Magnetic saturation and remanence are considered in this analysis. The aim is to achieve the required force on the piston and, thus, ensure the valve’s functionality. At the same time, however, the valve should be designed as compact and light as possible. The Finite element simulations are compared with an analytical model which provides a compact understanding of the influence of the design parameters on the functional and non functional performance criteria.

Hydraulic Effort and the Efficiencies of Pump and Motors With Compressible Fluid

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.

Hopsan: An Open-Source Tool for Rapid Modelling and Simulation of Fluid and Mechatronic Systems

Hopsan is an open-source simulation package developed as a collaboration project between industry and academia. The simulation methodology is based on transmission line modelling, which provides several benefits such as linear model scalability, numerical robustness and parallel simulation. All sub-models are pre-compiled, so that no compilation is required prior to starting a simulation. Default component libraries are available for hydraulic, mechanic, pneumatic, electric and signal domains. Custom components can be written in C++ or generated from Modelica and Mathematica. Support for simulation-based optimization is provided using population-based, evolutionary or direct-search algorithms. Recent research has largely focused on co-simulation with other simulation tools. This is achieved either by using the Functional Mock-up Interface standard, or by tool-to-tool communications. This paper provides a description of the program and its features, the current status of the project, and an overview of recent and ongoing use cases from industry and academia.