Today the digital simulation of fluid power systems has become a standard tool in the development departments. However, tools and methods which may integrate the simulation efficiently into the entire project planning procedure from the specification up to the final start up are still under construction.
By analyzing a load-sensing-system and a hydrostatic gear with strongly varying thermal operation conditions, the present work shows how already today essential aspects of project planning can be covered by CAE-Tools. The software tool DSHplus, available at the market, was extended for this reason in the process of the research work. The emphasis of the research work is on integration of neural nets as means of not knowledge-based information processing. In this manner, measured or computed data can be used very conveniently without extensive data processing while project planning.
Through an intelligent combination of digital simulation and methods for controller and system synthesis such as identification, adaptation and numeric optimization and the enlargement of the system description by the calculation of the fluid temperature, it is now possible to examine the dynamics of fluid power systems over the entire field of operation and for any load cycle with respect to the system temperature level. Moreover it is possible to optimize the systems automatically up to the parameterization of the controller hardware. Investigations on real systems which are coupled to the CAE-Tool over the controller hardware confirm the capability of the digital simulation and of the presented project planning method.
The losses in hydrostatic machines are mainly developing from the gaps between parts with relative movement. One function of these gaps is the separation and bearing of moving parts, another function is sealing chambers with different pressure level against each others. Minimization of friction and leakage and there from resulting losses of the complete machine is absolutely mandatory for energy and cost saving installations.
This thesis explains the PC based simulation of machine losses. Based on general flow equations, a method is used for describing laminar flow in mixed friction area for calculation of pressure distribution, friction and leakage. Out of this a simulation program is developed, which evaluates the losses of a hydrostatic machine in different points of operations. With the usage of appropriate test rigs having sensors for pressure, displacement, force and temperature, the calculation results are verified. Finally several examples demonstrate the capabilities of the simulation tool.
In this study, we deal with a bilateral master-slave system for tele-robotics composed of hydraulic or pneumatic servo-systems. In a tele-operated master-slave system, the master has to play two roles, firstly as a reference input device to the slave and secondly as a force display device. In order to produce a force display composed of hydraulic or pneumatic servo-systems, we must solve a problem called back-drivability, in which the actuator in a hydraulic or pneumatic servo-system cannot be operated freely by manual means. As a practical solution to this problem, we propose a driving method for the actuator that uses a force sensor attached to the actuator. As applications of the proposed force displays, we construct bilateral master-slave systems composed of hydraulic servo-system as a slave. Experimental results of the manipulation by these systems were presented and, consequently, the validity of these force-displays was confirmed.
Furthermore, we developed a six-DOF parallel link force display that is actuated by six hydraulic cylinders. The manipulability of the display, however, was insufficient at first, because the dynamic performance of each cylinder was not necessarily the same as that of the others. To overcome this problem, in the present study we have applied disturbance compensation to improve manipulability. To demonstrate a practical application of this force display, we also have constituted a master-slave system in which the display is adopted as the master and the same type of hydraulic manipulator is adopted as the slave. An operator manipulated the system so that the slave touched a flexible object serving as a load. Our experiments confirmed that the system was controlled with relatively good dynamic performance and that the operator was able to feel the load force sensitively through the force display.
This dissertation deals with vibration compensators for the attenuation of fluid flow pulsations in hydraulic systems. After a problem definition and an overview of conventional devices, several novel designs of vibration compensators, such as multi degree-of-freedom mass-spring oscillators, devices based on plate/shell elements and compact ?/4 side-branch resonators are presented and discussed. Wherever possible, the theoretical description of these devices is supplemented by simulation models, application examples etc.
However, the main contribution of this thesis is a compact and adjustable mass-spring resonator featuring a hydraulic spring. The main advantages of this design are:
After a description of the function principle and a discussion of some phenomena occurring at high frequencies, experimental results will be presented which prove the usefulness of the concept.
A section devoted to further work will finish this contribution.
Electropneumatic positioners are used in conjunction with pneumatic actuators for the open-loop and closed-loop control of process valves in industrial installations. The consequences of failed or malfunctioning process valves in plants used for continuously running processes not only include high repair, follow-up and outage costs; combined with loss of efficiency or leakages, they may also include long-term economic losses, as well as an environmental impact. All plant operators are therefore concentrating their efforts on finding methods that enable them to identify progressive wear and tear, initial indications of damage and signs of pending failures in good time in order to be able to respond appropriately in accordance with the process profile.
Use of a microcontroller in the positioner offers the user greater flexibility, as well as a means of communicating with centralized instrumentation and control centres via standardised field bus systems. This means that the positioner can also be used for diagnostic purposes, as well as for servo-control functions.
This project showed how signal-aided recognition methods, estimation techniques and neuronal networks can be used to draw conclusions regarding significant changes in the pneumatic actuator on the basis of the essential time-variant state variables that are available, such as the setpoint and actual displacement values, the manipulated variable and the pressure in the drive chambers. Appropriate techniques were tested and verified within the framework of simulation runs and practical experiments. Special attention was given to leakage in the drive, the friction level and backlash in the feedback path.
Other faults, such as leakage at the fitting cannot be correlated with the above-mentioned state variables in any measurable way. Heuristic methods are being developed to identify these faults using fault-specific sensors and these are being tested in field trials.
This study deals with the development of a convenient method for simulating transient responses of tapered fluid lines with both rigid and viscoelastic pipe walls.
Firstly, we develop a convenient method for simulating transient responses of tapered fluid lines, on the basis of theoretical analyzes established up to the present. Such a method is expected to be useful for predicting transient phenomena occurring in such fields as fluid power systems, fluidics and blood flow. In order to simulate transient phenomena of tapered fluid lines with high accuracy, an exact mathematical model of a distributed parameter system must be introduced. The exact model, in general, is described by a transfer matrix equation and is based on a fluid-line model in which the frequency-dependent effects of viscosity are considered. However, it is considerably troublesome to develop a simulation program for this exact model, because the transfer elements involve Bessel and hyperbolic functions. To overcome such problems the exact model, described by irrational formulae, must be simplified into a model developed by rational polynomials. In this study, therefore, in preparation for developing the simulation method, we first derive the rational polynomial model by employing the modal approximation. In order to confirm the validity of the approximated models, their frequency responses have been calculated and compared with those of the exact models. As a result of these processes, we intend to get a convenient method of simulation.
In addition to the analysis as stated above for obtaining the state variables at both ends of a fluid line, we deal, in the same way as before, with transient responses at arbitrary pipe positions of a line. In the simulation calculus we apply a conventional method of numerical integration for ordinary differential equations, the Runge-Kutta-Gill method. As a result of these processes, we intend to get a convenient method of simulation. In order to confirm the validity of the simulated results, they are compared with the corresponding analytical solutions, which were analyzed theoretically by considering Brown's approximations. As a consequence, the validity and applicability of the proposed method were well confirmed.
Secondly, our study deals with the dynamic characteristics of a liquid-filled tapered pipeline with a viscoelastic pipe wall. The viscoelastic pipe wall is modelled in the same way as proposed by Nakano. This model corresponds to a slightly modified version of the Voigt mechanical model, and it is distributed along the pipeline. In the modified version the effect of fluid compressibility is taken into consideration. The accuracy of the transfer matrix was investigated by a method of error estimation, and it then confirmed that the matrix is of considerably high accuracy, at least in the region. As a result, it is verified that the transfer matrix is accurate enough for practical applications.
For the convenience of simulation, we employed the same method, as the previous one, for modelling irrational transfer matrix elements of the line by modal approximations. The resulting models were estimated as highly accurate by comparing them with the corresponding exact solutions in frequency domain. By applying these approximation models to the block diagram representation of the line, simulation of time domain responses were executed and thus obtained the necessary results for different line end conditions. The step response solutions are also developed analytically in this study by introducing Brown's approximation method into the matrix equation and then by making the inverse Laplace transforms of thus approximated relations. The responses to stepwise changes in pressure or flow rate are theoretically obtained under open-end and closed-end condition.
In order to verify the appropriateness of thus derived solutions, their responses curves are compared with the corresponding ones obtained by a simulation analysis. As a result, it is confirmed that the analytical solutions obtained are sufficiently accurate under the condition of about ε <0.05. With the consideration that the dynamic characteristics of fluid line are largely influenced even by a slightly tapered line, it can be said that the solutions derived in this study are useful and valuable in practical applications.
Edited by G. Cantore
only published as CD-ROM
Publisher: Fluid Power Net International
ISBN: 88-88679-00-6
by A. Esposito
672 pages
Publisher: Prentice Hall
ISBN: 0130608998
by H.-W. Grollius
224 pages
by J. Saleh
1000 pages
Grundlagen der Hydraulik
Publisher: Fachbuchverlag Leipzig im Carl Hanser Verlag
ISBN: 3446215212
Fluid Flow Handbook
Publisher: McGraw-Hill
ISBN: 0071363726
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