Michael Flad

Steering driver assistance system: A systematic cooperative shared control design approach

Several publications have shown that it is beneficial to design a driver assistance system using a shared control structure. For the steering task this structure can be realized with a setup in which driver and automation can apply a torque on the steering wheel in parallel. Thereby both, driver and assistance system, interact with the vehicle and each other over the haptical channel. In the system the driver is given and cannot be changed. The question is how to design the assistance system controller as an ideal complement to the driver. In this paper a formal design concept is applied to this problem which utilizes the fact that adding a controller to the overall system has to lead to a Nash equilibrium. Remaining degrees of freedom are used to optimize the designed controller with respect to a global objective function that specifies overall system performance. We refer the concept as “cooperative shared control design”. For the concept driver and vehicle are modeled as a differential game. We show systematically that this concept can be used to determine the optimal assistance system if the driver characteristics are known. Simulations prove the applicability of this concept.

Thermophysical Properties of Lead-Bismuth Eutectic Alloy in Reactor Safety Analyses

A consistent set of thermophysical properties of a lead-bismuth eutectic (LBE) alloy was developed for use in safety analyses of lead-alloy-cooled fast reactor systems. The vapor and liquid thermodynamic states of LBE were modeled up to and above the critical point based on a van-der-Waals type of equation. We assumed that LBE vapor is composed of monatomic lead and bismuth and diatomic, bismuth components, and that liquid LBE is a non-ideal mixture of lead and bismuth. Recommended equations were also presented for the transport properties and surface tension of liquid LBE.

Analysis of Severe Accident Scenarios and Proposals for Safety Improvements for ADS Transmuters with Dedicated Fuel

Abstract So-called dedicated fuels will be utilized to obtain maximum transmutation and incineration rates of minor actinides (MAs) in accelerator-driven systems (ADSs). These fuels are characterized by a high-MA content and the lack of the classical fertile materials such as 238U or 232Th. Dedicated fuels still have to be developed; however, programs are under way for their fabrication, irradiation, and testing. In Europe, mainly the oxide route is investigated and developed. A dedicated core will contain multiple “critical” fuel masses, resulting in a certain recriticality potential under core degradation conditions. The use of dedicated fuels may also lead to strong deterioration of the safety parameters of the reactor core, such as, e.g., the void worth, Doppler or the kinetics quantities, neutron generation time, and βeff. Critical reactors with this kind of fuel might encounter safety problems, especially under severe accident conditions. For ADSs, it is assumed that because of the subcriticality of the system, the poor safety features of such fuels could be coped with. Analyses reveal some safety problems for ADSs with dedicated fuels. Additional inherent and passive safety measures are proposed to achieve the required safety level. A safety strategy along the lines of a defense approach is presented where these measures can be integrated. The ultimate goal of these measures is to eliminate any mechanistic severe accident scenario and the potential for energetics.

An identification method for individual driver steering behaviour modelled by switched affine systems

This paper addresses the issue of modelling and identification of individual driver steering behaviour from a new point of view, incorporating the idea of human motion being built up by an individual and limited repertoire of learned patterns. We introduce a switched affine model structure to explain a measurable motion alphabet in the driving context and show that this leads to a new identification problem that differs from general hybrid identification issues. To solve this problem, we derive a multi-step model output error criterion and propose an algorithm to simultaneously identify switching times and subsystem parameters out of measurable movement data. We show that this algorithm is capable of identifying the true parameters of known systems as well as fitting real movement trajectories even though no a priori information is given about the true system complexity.

Safety aspects of oxide fuels for transmutation and utilization in accelerator driven systems

Abstract General safety aspects of fuels under development for accelerator driven systems (ADS) are reviewed and discussed. These fuels should allow a maximization of transmutation and incineration rates, which excludes fertile UO 2 as a component or matrix. The accumulated knowledge on data, phenomena and scenarios of fast reactors with (U,Pu)O 2 oxide fuels and sodium cooling serves as background for this review. For future ADS both the reactor system itself, the fuel and the coolant are innovative compared to traditional critical fast reactors. For the fuel, these boundary conditions lead to many open questions, starting from basic thermal physical, thermal mechanical and irradiation data to the behavior under transient conditions. The choice of fuel naturally has a significant impact on whole core behavior and safety too, including the influence on related neutronics parameters, on failure propagation and disruption behavior under accident conditions. Key safety issues are discussed and a first assessment of phenomena and scenarios is given. Areas of research and technology in which further work is required to resolve important safety issues are highlighted.

Necessary and sufficient conditions for the design of cooperative shared control

In a shared control system humans and machines cooperatively interact. From the control theoretic point of view this can be seen as a system which is controlled by several controllers that are either formed by a human or by a machine. Since all controllers influence the system they affect each other. However, the human parts are given and cannot be changed. Therefore, the question is how to design the non-human controller systematically stable and without experiments. In this paper a design concept for these controllers is proposed which is based on game theoretic modeling. We show that adding a controller to the overall system has to lead to a Nash equilibrium. We further show that remaining degrees of freedom may then be used to optimize the designed controller with respect to a certain global objective function that specifies the demands of the system designers. Based on this idea necessary and sufficient conditions for cooperative shared design are stated. Practical approaches to solve the design problem are presented for real world problems. An example shows the applicability of the concept.

Online Identification of Individual Driver Steering Behaviour and Experimental Results

Using switched systems, we model individual driver steering behaviour from a new point of view. This approach allows to incorporate the idea of human motion being built up by an individual and limited repertoire of learned patterns. The identification of the generating subsystem parameters of such individual motion primitives solely on measured output data requires a new identification method. We propose an algorithm using a multi-step model output error criterion and discuss different implementations in detail. We show that this method is capable of tracking real measurement data of driver steering motion trajectories with low model orders and number of switches respectively. The presented method is online-capable. Experimental driving results proof the concept.

Individual Driver Modeling via Optimal Selection of Steering Primitives

Abstract A shared lane keeping assistance system supports the driver in the steering task. In contrast to an autonomous system, the shared system works in parallel with the driver by applying an additional torque on the steering wheel. This means the system and the driver perform the steering task in cooperation. As the driver is still part of the control loop a driver model is required to predict the steering behavior of the actual driver. In this paper we introduce a new lateral steering model which is suited to characterize individual drivers. This model describes, in contrast to other models, the neuromuscular system, limbs and its control for a specific driver by using a set of dynamic primitives (so called movemes). These movemes build a gray-box model for the neuromuscular system. The steering wheel angle is the predicted output of the moveme model. In order to generate a steering angle trajectory suited for the desired maneuver, the steering model switches between these movemes. Therefore, the central component of the driver model is a framework which determines the optimal switching sequence of the movemes. For this task an optimal control strategy is introduced. The approach is validated using a simulation of an ISO-double lane change with movemes which were identified from a set of real driver trajectories. The results show that the steering trajectories of the driver model highly correspond with the recorded driver trajectories.

Optimization of Safety Parameters and Accident Mitigation Measures for Innovative Fast Reactor Concepts

Traditionally the analysis of the evolution of severe core disruptive accidents (CDA) is broken down into different phases. This is mainly done for a better focussing on the key phenomena of the accident phase and also allows the application of specific codes for the analysis. In the current paper we mainly deal with the initiating phase and the transition phase of an accident as the ULOF (unprotected loss of flow). The key phenomenon of the initiating phase is the start of boiling and the development of voiding; key phenomena of the transition phase are the progression of core melting and the occurence of recriticalities by fuel compaction. The first level of optimizing safety is oriented to the initiating phase by reducing the positive void worth in order to avoid that a ULOF accident would enter a severe development. If accident prevention is not achieved the transition phase, characterized by a progressive core degradation leading to the occurrence of recriticalities, can be mitigated by dedicated features that enhance and guarantee a sufficient and timely fuel discharge – e.g. by a controlled material relocation (CMR) - and influence and ‘brake’; the recriticality path. In the paper both phases are analyzed. The results presented are in agreement with the activities performed within the European Collaborative CP-ESFR project.

Cooperative Shared Control Driver Assistance Systems Based on Motion Primitives and Differential Games

In this paper, a cooperative shared control driver assistance system that supports the driver in the steering task is proposed. The aim behind this concept is a cooperation between the driver and the assistance system. Thereby, both, driver and assistance system, can apply a torque on the steering wheel. Mathematically, this structure is described as a differential game. As a primary condition to facilitate cooperation, it is essential to explicitly regard the aims and steering actions of the driver for the calculation of the optimal torque, which the assistance system should apply. This requires an appropriate model of the human driver. A model that describes the driver steering motion as a sequence of motion primitives, which can be identified, is proposed for this task. Next, a real-time capable implementation of this concept is proposed. The concept is validated in a driving study. The study indicates that the system improves the lane keeping performance of the participants and leads to a higher user rating compared with noncooperative driver assistance systems.