Investigation of Thermal Effects in Direct Driven Hydraulic System for Off-Road Machinery

Several types of off-road machinery, such as industrial trucks, forklifts, excavators, mobile cranes, and wheel loaders, are set to be operated in environments which can differ considerably from each other. This sets certain limits for both the drive transmissions and working hydraulics of these machines. The ambient temperature must be taken into account when selecting the hydraulic fluid since the viscosity and density of the fluid are changing at different operating temperatures. In addition to the temperature, energy efficiency can also be a problem in off-road machinery. In most off-road machines, diesel engines are employed to produce mechanical energy. However, there are energy losses during the working process, which causes inefficiency in produced energy. For better energy efficiency, hybridization in off-road machinery is an effective method to decrease fuel consumption and increase energy savings. One of the possible methods to save energy with hybrids is energy regeneration. However, it means that the basic hydraulic system inside off-road machinery needs to be modified. One solution for this is to utilize zonal or decentralized approach by means of direct driven hydraulic (DDH) system. This paper aims to investigate a DDH system for off-road machinery by means of modelling and analyzing the effect of the temperature. In the direct-driven hydraulic system, the actuator is controlled directly by the hydraulic pump which is operated by the electric motor. Specifically, it is a valveless closed-loop hydraulic system. Thus, there will be no energy losses caused by the valves, and the total efficiency is assumed to be significantly higher. In order to examine the DDH system, a thermo-hydraulic model was created. Additionally, a thermal camera was utilized in order to illustrate the temperature changes in the components of the DDH system. To reproduce the action of the system in different circumstances DDH system was run at different ambient temperatures, and the component temperatures in the system were measured and saved for the analysis. The thermo hydraulic model was proven capable to follow the general trend of heating up.

Advanced Virtual Prototyping of Axial Piston Machines

This paper explains how a combination of advanced multidomain numerical models can be employed to design an axial piston machine of swash plate type within a virtual prototyping environment. Examples for the design and optimization of the cylinder block/valve plate interface are presented.

A Numerical Study of High Pressure Flow Through a Hydraulic Pressure Relief Valve Considering Pressure and Temperature Dependent Viscosity, Bulk Modulus and Density

This paper investigates the flow through a hydraulic pressure relief valve at high levels of operating pressure up to 700 bar (10000 Psi). Following the flow path from the cold high pressure region before the metering edge to the warm low pressure region behind, the mean viscosity decreases by a factor of 16, the mean bulk modulus decreases by a factor of 2 and the mean density decreases by 6 %. Based on this preliminary considerations, a turbulent single phase flow considering pressure and temperature dependent viscosity, bulk modulus and density is modelled and steady state as well as transient calculations are performed. The results of this study show that a pressure and temperature dependent viscosity reduces the pressure drop and the spool force by 10 % compared to a simulation with constant fluid parameters. Moreover, it is shown that compressible flow modelling has negligible influence on pressure drop and spool force — nevertheless, it is required to describe the temperature correctly. Due to the effect of volumetric work an incompressible model approach predicts the mean temperature rise 20 % too high. Finally, it was found that the temperature on the spool exceeds 400 °C. Afterwards, this fact is experimentally validated obtaining tempering colors in high pressure tests.

Modelling of the Swash Plate Control Actuator in an Axial Piston Pump for a Hardware-in-the-Loop Simulation Test Rig

Hydraulic hybrid system solutions are promising in the quest for energy efficiency in heavy construction machines. Hardware-in-the-loop simulations, where hardware is included in software simulations in real time, may be used to facilitate the development process of these systems without the need to build expensive prototypes. In this paper, the displacement actuator of a prototype pump used in a hardware-in-the-loop simulation test rig is modelled and validated against hardware, in order to draw conclusions regarding its dynamic behaviour in a future control design. The results show that the dynamic response of the modelled displacement actuator is mainly determined by the system pressure as well as the response and geometry of the control valve.

Combined Kinetic and Potential Energy Recovery Solution Applied to a Reach Stacker

Reach stackers are heavy duty mobile machines mainly used for container handling on intermodal terminals or harbors. The increasingly restrictive legislation on pollutant emissions as well as the need to reduce fuel costs for operators motivate manufacturers to design more efficient machines. Previous studies highlighted three ways to improve the fuel consumption, namely: (i) improving the engine efficiency; (ii) recuperating the potential energy of elevated containers; (iii) recovering the kinetic energy of the vehicle during decelerations. The architecture proposed in this paper combines these three requirements while preserving most of the conventional components. It results in a moderately priced solution. Control strategies are also studied, especially focusing on the potential energy recovery system where an input-output linearization method is compared to a conventional linear controller. Simulation conducted on several duty cycles shows fuel savings of up to 18.4% and a good robustness to cycle variations.

Influence of the Lubricant Thermo-Piezo-Viscous Property on Hydrostatic Bearings in Oil Hydraulics

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

An Analytic Approach to Cascade Control Design for Hydraulic Valve-Cylinder Drives

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.

Design and Research on a Hydraulic Cylinder With Plastic Components

In mechanical engineering, there is a trend to use new materials which are an alternative to metals. This also applies to construction components and hydraulic systems, where more and more attempts are made to use plastics as construction material. This solution brings design, technological and economic benefits. The researchers from the Fluid Power Research Group of the Department of the Fundamentals of Machine Design and Tribology from Wroclaw University of Technology (www.fprg.pwr.wroc.pl), are working in this area, with an objective to create a complete hydraulic system whose basic elements such as the pump, valve and actuator are, entirely or in their substantial part, made of plastics. The paper presents the course and outcome of the design process and the research, the aim of which was to prepare a demonstration model of the hydraulic cylinder made of plastics. The work on the model of the actuator started from an analysis of traditional methods of designing hydraulic cylinders made of metal. The authors analyzed the course of the design process, paying particular attention to aspects of the strength of the actuators’ structure. It highlights the main elements and the important nodes occurring in the hydraulic cylinders, namely the sleeve, the bottom, the head, the piston, the piston rod, the fasteners, the hydraulic fluid ducts, the sealing, and the bolts. An algorithm for the procedure in a form of a block diagram was presented, and the necessary calculations were made. Taking the characteristics of the actuator and its respective nodes into consideration, it was found that a number of metal parts may be replaced by plastics. The result of the operations performed is the proposal of a model of the actuator elements made of plastics. For this solution, a 3D computer model was prepared and studied by means of the FEM. The obtained results allowed the identification of the place, the nature and the value of deformation. Based on the results of the theoretical research, it was found that the structure of the actuator will not be effected in the course of its work in the assumed conditions. A demonstration model of the actuator was created according to initial assumptions. The next step was to prepare and conduct preliminary studies on the actual model. The first tests were carried out with no load being applied. The tests were made with different speeds of the piston rod’s movement and the operation of the actuator was observed. Next, tests of the loaded actuator model were conducted. For that purpose, it was put on a special stand with a metal actuator in such a way that a linear displacement of the two rods along a common axis was provided. In that system, the conventional actuator enabled the loading of the model’s piston rod. Tests were carried out at different values of pressure and speed within the full motion range. Based on the prototype’s volumetric efficiency measurement results, the operation of the tested actuator featuring the elements made of plastic was proved correct. The theoretical and experimental research on the hydraulic actuator confirm the possibility of applying plastics as a construction material in devices of that type. The use of the actuators’ design algorithm showed that it can provide a theoretical basis for the design method of the actuators made of plastics. The algorithm, however, requires modifications taking into account the special properties of plastics due to their anisotropic nature. The development of a definitive method is planned in the context of further research. Additionally, the future development of a design solution for a cylinder of smaller dimensions, which could provide an alternative to traditional low-pressure actuators or pneumatic actuators. The future research direction is the analysis of the processes taking place in the individual parts of the plastic cylinders. A challenge of some kind may be to select sealing’s that will ensure long and trouble-free operation of the actuators.

Designing of the Gerotor Pump Body Made of Plastics

The article presents the design process of the gerotor pump body made of plastic. As the shape of the projected body a simple prism with a square base was adopted. That body shape is called the basic. It consists of three parts: front, middle and back body, which are connected by a screw joint. This shape was subject to strength analysis, using the FEM method. As a result of these analysis, the place and nature of the stress and deformation of the basic body shape was determined. It was established that the body is deformed in both axial ie. along the pump shaft axis, and radial directions, ie. perpendicular to the axis of the shaft. There have been made modifications to the basic shape of the body in order to reduce the stress and deformation. As a result of these modifications, the modified body shape was obtained, which was then subjected to strength analysis by FEM method. It was found that the modifications resulted in reduction of the stress and deformation. Reducing the stress values, enables loading of pump body with higher operating pressures. Reducing the deformation of the body leads to a reduction in axial and radial clearances in the pump and enables achieving higher efficiency.

Lyapunov Based Adaptive Control Method for Hydraulic Servo Drive

Adaptive control systems are one of the most significant research directions of modern control theory. It is well known that every mechanical appliance’s behavior noticeably depends on environmental changes, functioning-mode parameter changes and changes in technical characteristics of internal functional devices. An adaptive controller involved in control process allows reducing an influence of such changes. In spite of this such type of control methods is applied seldom due to specifics of a controller designing. The work presented in this paper shows the design process of the adaptive controller built by Lyapunov’s function method for a hydraulic servo system. The modeling of the hydraulic servo system were conducting with MATLAB® software including Simulink® and Symbolic Math Toolbox™. In this study, the Jacobi matrix linearization of the object’s mathematical model and derivation of the suitable reference models based on Newton’s characteristic polynomial were applied. In addition, an intelligent adaptive control algorithm and system model including its nonlinearities was developed to solve Lyapunov’s equation. Developed algorithm works properly and considered plant is met requirement of functioning with. The results shows that the developed adaptive control algorithm increases system performance in use devices significantly and might be used for correction of system’s behavior and dynamics.