Dirk Zimmer

A new framework for the simulation of equation-based models with variable structure

Many modern models contain changes that affect the structure of their underlying equation system, e.g. the breaking of mechanical devices or the switching of ideal diodes. The modeling and simulation of such systems in current equation-based languages frequently poses serious difficulties. In order to improve the handling of variable-structure systems, a new modeling language has been designed for research purposes. It is called Sol and it caters to the special demands of variable-structure systems while still representing a general modeling language. This language is processed by a new translation scheme that handles the differential-algebraic equations in a highly dynamic fashion. In this way, almost arbitrary structural changes can be processed. In order to minimize the computational effort, each change is processed as locally as possible, preserving the existing computational structure as much as possible. Given this methodology, truly object-oriented modeling and simulation of variable-structure systems is made possible. The corresponding process of modeling and simulation is illustrated by two examples from different domains.

Real-time models for wheels and tyres in an object-oriented modelling framework

This article presents models for wheels and tyres in the application field of real-time multi-body systems. For this rather broad class of applications it is difficult to foresee the right level of model complexity that is affordable in a specific simulation. Therefore we developed a tyre model that is adjustable in its degree of complexity. It consists of a list of stepwise developed sub-models, each at a higher level of complexity. These models include semi-empirical equations. The stepwise development process is also reflected in the corresponding implementation with the modelling language Modelica. The final wheel model represents a supermodel and enables users to select the right level of complexity in an unambiguous way.

Model-Based Energy Management Functions for Aircraft Electrical Systems

Intelligent software functions for energy management form a crucial element for aircraft electrical and thermal systems. In the electrical system, these are currently electrical load or power management functions that can cut and reconnect loads based on fixed priorities. The main aim of these functions is to prevent overload in failure mode of electrical generators, for example if one generator fails and another one has to take over its loads. For more-electric or all-electric aircraft, these functions should also cut loads during normal operation, since the electrical systems will not be sized to simultaneously provide maximum power to all loads. Additionally, energy management functions shall deal with multiple, parallel sources and should split power off-take in a way to reach maximum system efficiency. This paper provides an object-oriented tool and a method that enable a more intuitive development of an energy management function using economic models. They can deal with different criteria like efficiency and priorities while keeping computational effort low. For each load and each generator, a cost-over-power function is provided. A set of rules limit the characteristics of these functions. This enables a reliable and easy setup of the management function. This approach enables dynamic time-shifting of loads like galleys or environmental control systems (ECS) without defining fixed timetables in advance.

Custom Annotations: Handling Meta-Information in Modelica

Annotations and attributes form an important part of the Modelica language. They are used to include various meta-information such as documentation, external C-code, compilation hints, etc. Given the increasingly wide field of potential applications the set of useful annotations becomes too large to be included in the language specification. Hence we present a proposal how a Modelica modeler may define his own annotations and how such custom annotations can be organized within Modelica libraries. In the long term, the goal is to move the definition of standardized annotation, as well as of attributes, from the Modelica specification to a standard library.

WRAPPING MULTI-BOND GRAPHS: A STRUCTURED APPROACH TO MODELING COMPLEX MULTI-BODY DYNAMICS

Bond graphs have established themselves as a reliable tool for modeling physical systems. Yet, they are highly abstract due to their domain independence. Wrapping techniques allow the modeler to preserve the better of two worlds: the flexibility and reliability of bond graphs on the one hand, and the intuitive appeal and familiarity offered by a domain-specific modeling methodology on the other. The talk introduces a new multi-bond graph library for Dymola that includes a partial re-implementation of Dymolas standard multi- body systems library using wrapped multi-bond graphs.

Object-oriented modelling of wind turbines and its application for control design based on nonlinear dynamic inversion

ABSTRACT This article presents the object-oriented modelling of wind turbine aero-elastics. The article details the undertaken modelling approach and demonstrates its validity by comparing the results to a domain-specific simulation tool. The resulting object-oriented models can be generically adapted to meet different tasks or combined with models from other domains. Also the combination object-oriented Modelica and functional mock-up interface technology enable a rapid design of nonlinear controllers. As one possible application example, the article presents the model-based design of a corresponding controller. Its goal is to extend the turbine lifetime by reducing the structural loads. To this end, a modern control scheme is proposed that takes full advantage of the underlying object-oriented modelling approach. The controller is based on nonlinear dynamic inversion control methods combined with pseudo control hedging. The controller uses wind speed measurement information to adjust to wind gust load. Simulation-based comparisons to conventional control designs show a large potential reduction of the gust load on the wind turbine using the proposed controller.

Towards a Model-Based Energy System Design Process

Advanced modeling and simulation techniques are becoming more important in todays industrial design processes and for aircraft energy systems in specific. They enable early and integrated design as well as validation of finalized system and component designs. This paper describes the main methods and tools that can be applied for different phases of the energy design process. For demonstration, the object-oriented modeling language Modelica was chosen, since it enables convenient modeling of multi-physical systems. Based on this standard, common modeling guidelines, a modeling library template, and common interfaces have been provided. A common modeling infrastructure is proposed with considerations on additional libraries needed for local tasks in the energy design process. The developed methods and tools have been tested by means of some predefined use cases, which are performed in cooperation with diverse aircraft industrial partners. Each use case represents a specific modeling, simulation, or design task. This use case approach covers a wide range of the overall energy system design process.

A Reference-Based Parameterization Scheme for Equation-Based Object-Oriented Modeling Languages

Abstract This paper presents a reference-based parameterization scheme for equation-based, object-oriented modeling languages such as Modelica. It is demonstrated, how simple language constructs can be designed that enable a general and powerful parameterization of models. Furthermore, the computational process required for this parameterization scheme is outlined. To validate our concepts, an experimental language has been developed and implemented. It is called Hornblower and it represents an attempt to embrace the core ideas of Modelica while reorganizing the higher-level modeling tasks that have evolved during time.

Representations of equation-based models are not created equal

For equation-based modelling languages, modelling experts have many degrees of freedom when building a model from scratch. One of the most basic choices the expert faces is the mode of representation. The same system can be represented for instance as a block-diagram, by writing down the physical equations, by writing an algorithm, or by graphically connecting ready-made subcomponents. To give some guidance in this aspect, an experiment was conducted to measure the effects of different representations on various tasks. Participants had to identify models and predict their transient response. Both the time to execute the task and the correctness of the answer were measured. Participants also had to rate their confidence regarding the models. Results showed that tasks were executed much faster for graphical representations than for block-digrams. Equation-based and algorithm-based models can be grouped in the middle. The same results hold for rated confidence. Interestingly, the amount of errors was similar for all representations. Apparently, modelling experts largely compensate for difficulty by taking their time.

Exploitation Strategies of Cabin and Galley Thermal Dynamics

The thermal inertia of aircraft cabins and galleys is significant for commercial aircraft. The aircraft cabin is controlled by the Environment Control System (ECS) to reach, among other targets, a prescribed temperature. By allowing a temperature band of ± 2 K instead of a fixed temperature, it is possible to use this thermal dynamic of the cabin as energy storage. This storage can then be used to reduce electrical peak power, increase efficiency of the ECS, reduce thermal cooling peak power, or reduce engine offtake if it is costly or not sufficiently available. In the same way, also the aircraft galleys can be exploited. Since ECS and galleys are among the largest consumers of electrical power or bleed air, there is a large potential on improving energy efficiency or reducing system mass to reduce fuel consumption of aircraft. This paper investigates different exploitation strategies of cabin and galley dynamics using modelling and simulation. Modelica models of the thermal and the electrical system are used to assess and compare these different strategies. Potential impacts on passenger comfort are discussed. Additionally, the gained performance is compared to more conventional storage elements like electrical batteries. Finally, the potential of fuel reduction will be quantified using a reference aircraft model and the optimal strategy is selected.