The thesis introduces a new actuator design and appropriate control concepts for a hydraulic joint integrateable rotary servo actuator, based on displacement control. The use of a vane type swivel motor as final actor it is possible to integrate the actuator directly into the end-effector joints of mobile working machinery and manipulators, where the joint axes are directly driven by the motor. This concept is referred to as joint integrated rotary actuator (JIRA). Basic advantages are wide swivel angles (up to 270° with vane type motor), compact mounting space especially for multiple DOF-joints and significantly reduced primary energy consumption by using displacement control instead of valve control (allows also recovery of aiding energy).
Non-linear models were developed for the servo pump and the vane type motor for analysis of actuator dynamics and simulation. Both models were linearized for control synthesis. For mobile applications especially the variation of the effective inertia has shown to be the most crucial parameter for the controller design, where the effective parameter spread can reach ratios up to 1:10. Other effects like varying temperature, motor chamber volume, leakage and friction also have significant influence on the dominant actuator characteristics. For this reason the Ackermann parameter space method was used for robust controller design. For experimental validation of the control concepts a test rig was set up, where its technical design realizes load conditions very similar to those of mobile working machinery (variable load torque and inertia up to 30000 Nm and 3650 kgm², use of sliding bearings, swivel angles up to 220°, angular sensor precision of 0.01°).
The proposed control strategy is based upon a cascaded control concept for controlling the servo pump swash plate angle, the actuator velocity and the actuator position (from inside to outside). Further the problem of insufficient damping of the actuator was addressed, where the effective damping ratio dH of the actuator can move very close to zero, depending on the operating conditions. A damping ratio of at least dH = 0.7 had to be achieved in order to guarantee sufficient smooth velocity and positional transition in automated or semi-automated process control. To solve this problem, three different strategies were investigated: dynamic forward regulation of 2nd order, feedback of dynamic actuator pressure difference and feedback of actuator acceleration. All three concepts had shown to achieve sufficient damping. While the forward regulation scheme achieved sufficient damping at moderate dynamics, the acceleration feedback scheme had shown to achieve significantly improved damping and bandwidth compared to forward regulation by using only one additional sensor.
Design of complex systems involving for example fluid power components and subsystems is today carried out in a collaborative, distributed, and at the same time competitive fashion. It is important that simulation models and computational design methods can be managed efficiently in such design projects, where the models can be exchanged and integrated, design data managed, and proprietary information protected.
In this thesis, a framework is presented which facilitates management of simulation models, design data, and computational processes. The framework emphasizes a model-centric view, which means that models are represented on a high level of abstraction, without tool-specific information. By defining models using the eXtensible Markup Language, XML, it is a straightforward matter to apply generic operators to the model and transform the model to several tool-specific implementations.
The framework also enables integration of distributed computational resources. By implementing standards for so-called web service technology, simulation modules are published as remote model services, enabling partners in a project to deploy and invoke simulation models as black boxes. By publishing an interface to the model in a standardized format, and keeping the model at its original location, the model can be accessed and integrated without revealing proprietary information about the model. This approach also facilitates integration of existing so-called legacy tools and models.
A data management approach is also presented where design data and the computational processes are handled. Variability measures for the design data are included, enabling probabilistic methodologies and design optimization to be applied in a straight-forward way. The computational design process is formalized by representing the process as an executable process description enabling direct implementation, modification, and exchange of processes.
In the last part of the thesis, a number of evaluation examples are presented where some of the tools and concepts of this thesis are illustrated. These examples are mainly from the area of aircraft system design and actuation technology.
The design of complex hydro-mechanical systems involves modeling and simulation as means for early estimation of system properties. The models considered in the thesis are mathematical and are computed numerically in the simulation activity. For systems spanning several engineering domains such as mechanics, hydraulics and electronics, modeling and simulation are collaborative processes where engineers and analysts from the different domains take part, making use of a variety of modeling formalisms and software. In the thesis, tools are presented that improve interoperability between models and simulation environments as well as among the environments. This facilitates straightforward collaborative simulation-based design in which appropriate modeling formalisms and software can be applied in a given situation.
To fully exploit the advantages of simulation-based design, the models have to be both accurate and easy to create. For these reasons, the system is often decomposed and modeled in several environments specialized in each domain in terms of user interface etc. In order to simulate the system as a whole, the models are made coupled, either directly by merging them into one system model, or by connecting the simulation environments in which they reside. Due to a number of circumstances, which are explored in the thesis, often the only possible way of performing system simulation is to engage the solvers part of each simulation environment in coupled simulation, so called co-simulation. Unfortunately, due to the problem de-coupling, the error in the simulation results might grow unboundedly through time. Today, methods exist for making the error remain constant or decay, but few simulation environments contain the functionality demanded by these methods, such as redoing former computations. In the thesis, a few methods are presented and evaluated analytically and in practice, which can be applied to most co-simulations in order to have reliable results. One of the methods is successfully applied to the case of a wheel loader, simulated in its entirety in an industrial-scale project.
One of the practicalities that necessitate co-simulation is the strong dependence between models and the environments in which they are created. If one and the same model can be simulated in any environment, it has a longer life and a larger application area than implementation-dependent models. This is of importance since the modeling work needs to be brought down to a minimum; redundant work is not an option in industry. In the thesis it is shown how equation-based models can be made to fit into most simulation environments and other software. A minimum of information is used about the model and the environment, in which the model is to function properly. A framework for open-ended model compilers is also presented that enables a model compiler to adapt to a destination environment given the minimum information mentioned. For evaluation purposes, a model compiler that can adapt models to fit within two simulation environments with fundamental differences in terms of solvers etc is implemented for a subset of the Modelica equation-based language.
This thesis covers various aspects of hydraulic power assisted steering, HPAS, systems in road vehicles. Power steering is viewed as a dynamic system and is investigated with linear and non-linear modeling techniques.
With the help of the linear model, relevant transfer functions and the underlying control structure of the conventional power steering system have been derived and analyzed. The non-linear model has been used in concept validation of a new feature that can be added to traditional power steering units in order to increase functionality. This model also has the capability of co-simulation with a vehicle model in order to create realistic tie-rod loads to the power steering model.
This thesis treats energy aspects of power steering systems. In order to point out solutions to energy problems, different methods of reducing energy consumption are discussed.
The interest for including more functions in power steering systems in road vehicles has increased with the development of new active safety features such as Lane Keep Assist, LKA. The traditional HPAS system cannot meet these new demands, due to the control units pure hydro-mechanical solution. The Active Pinion concept is presented in this thesis, which is a novel concept for controlling the steering wheel torque for future active safety applications. The concept introduces an additional degree of freedom in the control unit. Active safety features are going to play an increasingly important roll in future safety strategies; therefore, it is essential that sub systems, such as power steering systems, in road vehicles are adjusted to meet new demands.
A load sensing (LS) hydraulic system is one in which the pump flow is adjusted to keep pressure across an orifice constant and independent of any variation in the load pressure. This ensures that the pressure losses across the orifice are kept to a minimum, which increases efficiency substantially. However, stability can become a problem. The Ph.D. research project was to theoretically and experimentally investigate stability of a typical LS system.
This research concentrated on identifying the relationship between system parameters and instability in one particular type of LS system. Due to the high degree of nonlinearity in LS systems, the instabilities were dependent on the steady state operating point. The study therefore concentrated first on identifying all of the steady state operating points and then classifying them into three steady state operating regions. A dynamic model for each operating region was developed to predict the presence of instabilities. Each model was then validated experimentally. This procedure, used in the study of the LS system, was also applied to a pressure compensated (PC) valve.
A system that combined a LS pump and a PC valve (for the controlling orifice) is called a load sensing pressure compensated (LSPC) system. This research, then, examined the dynamic performance of the LSPC system using the operating points and steady state operating regions identified in the first part of the research.
The original contributions of this research included: (a) establishment of three steady state operating conditions defined as “Conditions I, II & III”, which were based on the solution of steady state non-linear equations; (b) the provision of an empirical model of the orifice discharge coefficient suitable for laminar and turbulent flow, and the transition region between them; (c) and the development of an analytical expression for orifice flow which made it possible to accurately model and simulate a hydraulic system with pilot stage valve or pump/motor compensator. These contributions resulted in a practical and reliable method to determine the stability of a LS or LSPC system at any operating point and to optimize the design of the LS or LSPC system.
by H. J. Matthies and K. T. Renius
Publisher: Teubner
ISBN: 3-519-36318-6
Language: German
Edited by J. S. Stecki
Publisher: Fluid Power Net Publications
ISBN: 0-9578574-1-1
Publisher: ASME
ISBN: 0791837173
Edited by D. Will, H. Ströhl and N. Gebhardt
Publisher: Springer, Berlin
ISBN: 3-540-20116-5