Autonome Optimierung des Verhaltens von Fahrzeugsteuerungen auf der Basis von Verstärkungslernen

Fahrsysteme oder Fahrerassistenzsysteme sind bereits seit einigen Jahren Bestandteil von Forschungsprojekten. Dabei umfassen diese meist zwei wesentliche Bestandteile: die Erfassung der Umwelt sowie die Generierung von Steuerungsbefehlen. Die Lernfähigkeit solcher Projekte gewinnt zunehmend an Bedeutung und ist der Hauptfokus der vorliegenden Forschungsarbeit - wobei Lernfähigkeit als Optimierung von Fahrverhalten verstanden wird, d.h. die Auswahl vom optimalen Verhalten für eine jeweilige Situation. <br /> Die vorliegende Arbeit setzt für die Lernfähigkeit erstmals ein System basierend auf Verstärkungslernen (Reinforcement Learning) ein – dies im Gegensatz zu bisherigen Arbeiten im ähnlichen Umfeld basierend auf Modellierung oder Neuronalen Netzen. Im Kern der Arbeit werden Situationen klassifiziert und für jede Situation eine mögliche Anzahl von Verhalten ermittelt. Durch Verstärkungslernen werden diese Verhalten bewertet und die entsprechenden Situationsbewertungen konvergieren über die Zeit. Fazit: es wird autonom ermittelt, welche Verhalten in einer Situation angemessen sind und welche nicht. <br /> Neben einer mathematischen Abhandlung über das Konvergenzverhalten von Verstärkungslernen-Systemen wird mit unterschiedlichen Testreihen die Funktionsweise des o.g. Ansatzes im Rahmen einer konkreten Implementierung nachgewiesen und die Konvergenz der Situationsbewertungen untersucht.Driver and Driver Assistance Systems move more and more into the focus of research projects. Corresponding research areas consist of two main parts: the understanding of the environment as well as the generation of steering commands. Learning capabilities gain more and more importance and is the main aspect of this research. In detail: the term “learning capability” represents the optimization of driving behaviour, i.e. the optimized situation-specific selection of actions. <br /> The current research implements first-time a system based on Reinforcement Learning (RL) – in contrast to many other research work using modeling or neural nets. In the light of Reinforcement Learning, situations are being classified and possible actions are being identified for each situation. Rewards following such actions are used for cumulated ratings which in turn converge over time. In the end, the cumulated ratings indicate as to how much an action was appropriate for the determined situation. <br /> After a mathematical analysis of Reinforcement Learning methods in general, test-series are being analyzed within the framework of a real implementation and convergence behaviour as well as driving capabilities are being achieved

Process design of the patterning process of profile grinding wheels

In production environment, grinding is often the last step along the process chain. At this step, the main share of the value chain is already manufactured. Correspondingly, the process result of this step directly influences the product quality. Thus, the avoidance of process induced damages is a major challenge in grinding. The major limiting factor in grinding is the thermal load on the workpiece, which leads to grinding burn and tensile residual stresses. This thermal load can be reduced, as previous fundamental studies have shown, by means of using microstructured grinding wheels. In this paper, the patterning process of profile grinding wheels is investigated with regard to the resulting geometry and the resulting grinding wheel topography. In detail, an analytical model is established and evaluated that enables a design of the patterning process of profile grinding wheels. The presented formulas describe the local depth and width of a pattern over its length of engagement. The influence of the inclination angle of the patterning tool and the profile angle of the grinding wheel on the resulting width and length of one pattern is investigated. Further influencing parameters on the size of a pattern that are investigated are e.g. the radius of the grinding wheel, the radius of the patterning tool, the corner radius of the patterning edge and the speed ratio between the grinding wheel and the patterning tool. In addition, grinding experiments were conducted to validate the process design. The results show a high correlation between the calculated and the resulting patterns on the grinding wheel as well as that a decrease in cutting forces can be achieved by this approach. When maintaining the workpiece and grinding wheel load, the productivity of the profile grinding process can be increased in this way

A New Tool Concept for Milling Automotive Components

Due to the rising number of car variants, the production systems in automobile industry are driven by a strong demand for flexible production processes. In the production of thin-walled workpieces, forming and cutting processes stand in concurrence to each other. In many applications, cutting processes facilitate higher flexibility regarding possible workpiece geometries. However, the required productivity is demanding. In this paper, a multi-sectional milling tool is developed to reach the required cutting performance by minimizing secondary processing times. Tool geometry is optimized with statistical methods to enable a target oriented tool development and reduce iterative development steps in milling tool design processes.BMBF/02PN218

Increasing productivity in heavy machining using a simulation based optimization method for porcupine milling cutters with a modified geometry

Porcupine milling cutters offer a high potential for increasing the metal removal rate in heavy machining of steel and titanium. Here, the available machine power and the maximum radial force represent important process limits. According to the current state of the art, mainly rectangular indexable inserts are used. Investigations show that the use of round inserts can significantly reduce the resulting radial force and cutting torque similar to serrated endmills. However, the design of such tools is a major challenge due to the complicated shape of cross-section of the undeformed chip. Therefore, this paper presents a new method for optimizing the position of individual indexable inserts by means of geometric material removal simulations. With the new method, the radial force can be reduced by 14%

Process stability of a novel roughing-finishing end mill

In this paper, stability investigations of a novel roughing-finishing end mill are carried out. This tool possesses two sharp finishing teeth and two radially recessed, chamfered roughing teeth. By applying the same tool for roughing and finishing operations, tool changes and process time can be reduced. For the stability investigations, the semi-discretization method for calculating stability charts was extended and made applicable for the novel tool concept by taking into account the radial recession of the chamfered cutting teeth. This is necessary because the radial recession leads to varying time-delays during the tooth engagement. Stability charts were then calculated for roughing-finishing tools with different radial recession as well as for conventional finishing and roughing tools. Furthermore, experimental stability charts were created. The results show a good agreement between calculated and experimental stability charts for the finishing tool. However, the calculated stability limits of the roughing-finishing tool and the roughing tool do not met with the experimental stability limits, which is attributed to inaccuracies in the modelling of process damping. Nevertheless, calculated as well as experimental stability charts indicate a significant increase of the stability limit of the roughing-finishing tool compared to the finishing tool

High performance peel grinding of steel shafts using coarse electroplated CBN grinding wheels

Grinding is widely known for its low material removal rates and high surface quality. However, recent developments in production processes for cubic boron nitride (CBN) abrasive grains have led to commercially available grain sizes larger than 300 µm. These superabrasive CBN-grains allow higher material removal rates during grinding of hardened steel components. Currently, these components are pre-machined with turning processes before hardening and finishing the work piece by grinding. However, the turning process can be eliminated by grinding with coarse CBN-grains since higher depths of cut are achievable when machining hardened components. This paper explores the limits of grinding wheels using grains with a size of B602 during soft and hard machining in comparison to conventional B252 grains. It is shown that the use of coarser grains leads to lower process forces, higher (tensile) residual stress and higher surface roughness. Residual stress and surface roughness are of less importance as these grains are to be used mainly in roughing operations with ensuing finishing operations for the required surface properties. Over all investigations, especially in hard machining, neither grain nor tool wear was observed for the B602 grains, whereas the B252 tool was severely clogged during the experiments. Additionally, the grinding force ratio indicates that the coarse grain tools have not yet reached their productivity limit as it increases over all investigated feeds. This indicates improving tool performance with lower amounts of rubbing for increasing feed rate during hard grinding and shows the potential for the industrial use of higher feed rates with larger grains

Influence of metal working fluid on chip formation and mechanical loads in orthogonal cutting

Metal working fluids are used in machining processes of many hard-to-cut materials to increase tool life and productivity. Thereby, the metal working fluids act on the thermal and on the mechanical loads of the tool. The changing mechanical loads can mostly be attributed to the changing friction between rake face and chip and changes in the chip formation, e.g., the contact length between rake face and chip. However, analyzing those effects is challenging, since a detailed look at the chip formation process is prevented by the metal working fluid. In this paper, a novel planing test rig is presented, which enables high-speed recordings of the machining process and process force measurements while using metal working fluids. Experiments reveal that process forces are reduced with increasing pressure of the metal working fluid. However, the average friction coefficient only changes slightly, which indicates that the reduced process forces are mainly the result of reduced contact lengths between rake face and chip

Acoustic properties of porous microlattices from effective medium to scattering dominated regimes

Microlattices are architected materials that allow for an unprecedented control of mechanical properties (e.g., stiffness, density, and Poisson's coefficient). In contrast to their quasi-static mechanical properties, the acoustic properties of microlattices remain largely unexplored. This paper analyzes the acoustic response of periodic millimeter-sized microlattices immersed in water using experiments and numerical simulations. Microlattices are fabricated using high-precision stereolithographic three-dimensional printing in a large variety of porosities and lattice topologies. This paper shows that the acoustic propagation undergoes a frequency dependent transition from a classic poroelastic behaviour that can be described by Biot's theory to a regime that is dominated by scattering effects. Biot's acoustic parameters are derived from direct simulations of the microstructure using coupled fluid and solid finite elements. The wave speeds predicted with Biot's theory agree well with the experimental measures. Within the scattering regime, the signals show a strong attenuation and dispersion, which is characterized by a cut-off frequency. The strong dispersion results in a frequency dependent group velocity. A simplified model of an elastic cylindrical scatterer allows predicting the signal attenuation and dispersion observed experimentally. The results in this paper pave the way for the creation of microlattice materials for the control of ultrasonic waves across a wide range of frequencies

Using tool wear to increase process stability when milling Al7075 and AISI 4140+QT

One of the main limits of productivity during cutting processes is the occurrence of regenerative chatter. Due to these self-excited vibrations, the load capacity of the machine components, the tool as well as the machine performance cannot be fully utilized. There are several methods to stabilize the milling process. One is the use of increased process damping, which results from the contact of the tool’s flank face and the workpiece. The flank wear land naturally increases the contact between tool and workpiece. However, this effect has not been used to increase productivity in milling processes. This paper investigates with experiments and numerical simulations how tool wear affects process stability in milling of aluminum and steel. Therefore slot milling and side milling tests were carried out with tools of various states of flank wear. It could be shown that increasing flank wear allows to raise the depth of cut ap up to 300% in machining aluminum and perform the machining process with a higher productivity

Production of chip breakers on cemented carbide tools using laser ablation

Chip breakers are essential for achieving reproducible and safe turning processes. In case of cemented carbide tools with standard ISO geometries, they are manufactured by form pressing before sintering. In form turning applications with individual tool geometries, chip breakers are typically avoided, because they require an additional and resource intensive grinding process. An alternative preparation strategy for chip breakers is laser ablation. In this paper, the influence of pulse fluence, areal fluence and pulse duration (pico- and nanosecond regime) on ablation mechanisms of cemented carbide tools is investigated. Different ablation mechanisms between pico- and nanosecond-lasers could be detected. Furthermore, a suitable laser ablation strategy using ns-laser is applied to create a chip breaker. Turning investigations showed distinct shorter chips when machining with modified tools. © 2020 The Authors. Published by Elsevier B.V