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Item type:Publication, Access status: with files , Model predictive pose optimization for energy efficient robotic machiningOne focus of aviation research in recent years has been the reduction of greenhouse gases through weight savings in aircraft construction, e.g. by using lightweight materials such as CFRP components. Lighter aircraft lead to less fuel consumption, which impacts the global carbon footprint and is an important step towards CO₂-neutral flying. In order to achieve national climate goals, the environmental impact should already be assessed at the production stage. Most recently, hybrid kinematics consisting of serial kinematics (industrial robots on linear rails) and parallel kinematics (hexapod) mounted to the robot’s end effector, have been studied at Fraunhofer IFAM for the purpose of machining distorted CFRP-components. This work analyzes to which extent the redundant degrees of freedom of such hybrid kinematics can be exploited to determine energetically advantageous poses during machining. Therefore, the inverse kinematics are solved within an Optimal Control Problem (OCP) including the forward kinematics as well as minimizing the system’s energy consumption while following a reference trajectory.Publicationtype: Journal Article48 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental validation for the combination of funnel control with a feedforward control strategyCurrent engineering design trends, such as lightweight machines and human–machine interaction, often lead to underactuated systems. Output trajectory tracking of such systems is a challenging control problem. Here, we use a two-design-degree of freedom control approach by combining funnel feedback control with feedforward control based on servo-constraints. We present experimental results to verify the approach and demonstrate that the addition of a feedforward controller mitigates drawbacks of the funnel controller. We also present new experimental results for the real-time implementation of a feedforward controller based on servo-constraints on a minimum phase system.Publicationtype: Journal Article76 19 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Hierarchical refinement in isogeometric analysis for flexible multibody impact simulationsDetailed impact simulations in flexible multibody systems can be simulated based on reduced isogeometric analysis (IGA) models. However, a precise simulation of an impact requires a high element resolution in the contact area. Usually in IGA, global refinement methods are used, which are easy to implement. However, in the literature, also the use of hierarchical local refinement is proposed. The local refinement generates fewer countable degrees of freedom compared to an equivalent global refinement. Numerous application areas can be found in the literature, such as contact simulations, where the computational effort is reduced by local refinement. In this work, we introduce the inclusion of hierarchically refined IGA models within the floating frame of reference formulation. Thereby, the hierarchically refined IGA model is reduced and applied in impact simulations. In two application examples, we simulate the impact of two- and three-dimensional spheres and compare with an analytical solution. The focus here is on the comparison of calculation times and accuracy of globally and locally refined reference models. The third application example consists of two flexible double pendulums and is devoted to systems in which the bodies undergo both arbitrary rigid body motions and small elastic deformations.Publicationtype: Journal ArticleScopus© Citations 1 84 214 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Trajectory-tracking control from a multibody system dynamics perspectiveThe development of modern mechatronic systems is often driven by the desire for more efficiency and accuracy. These requirements not only result in more complex system designs, but also in the simultaneous development of improved control strategies. Therefore, control of multibody systems is an active field of research. This contribution gives an overview of recent control-related research from the perspective of the multibody dynamics community. A literature review of the research activity in the journal Multibody System Dynamics is given. Afterwards, the framework of servo-constraints is reviewed, since it is a powerful tool for the computation of a feedforward controller and it is directly developed in the multibody system dynamics community. Thereby, solution strategies for all possible system types, such as differentially flat systems, minimum phase and non-minimum phase systems are discussed. Selected experimental and simulation results are shown to support the theoretical results.Publicationtype: Journal ArticleScopus© Citations 3 167 393 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , End-effector trajectory tracking of flexible link parallel robots using servo constraintsWe apply the concept of servo constraints to end-effector trajectory tracking control of parallel robots with structural link flexibilities. Such servo constraints deliver the inverse robot model where solution approaches via projections are proposed, which transform the resulting differential-algebraic equations to ordinary differential equations. The applicable solution process depends on the existence and stability of the internal dynamics. When using the exact end-effector of flexible link robots as output, this internal dynamics is usually unstable. Then a two-point boundary value problem is considered in the framework of stable inversion to obtain the noncausal solution offline. This solution is used as a feedforward control, which is initially combined only with actuator feedback control. To also account for errors within the link flexibility, the well-known linear–quadratic regulator is adapted to end-effector trajectory tracking based on differential-algebraic equations. Finally, we propose a systematic input–output feedback linearization approach, which uses servo constraints for flexible link parallel robots. Here a minimum phase system is obtained by tracking a redefined end-effector output, which is an approximation of the exact end-effector position. All control concepts are validated experimentally with a parallel robot having a highly flexible link. The results allow us to compare different control approaches and show the superior performance of controllers that rely on a flexible multibody model in contrast to classical rigid multibody modeling.Publicationtype: Journal ArticleScopus© Citations 5 83 127 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Flexible multibody impact simulations based on the isogeometric analysis approachUsually detailed impact simulations are based on isoparametric finite element models. For the inclusion in multibody dynamics simulation, e.g. in the framework of the floating frame of reference, a previous model reduction is necessary. A precise representation of the geometry is essential for modeling the dynamics of the impact. However, isoparametric finite elements involve the discretization of the geometry. This work tests isogeometric analysis (IGA) models as an alternative approach in the context of impact simulation in flexible multibody systems. Therefore, the adaption of the flexible multibody system procedure to include IGA models is detailed. The use of non-uniform rational basis splines (NURBS) allows the exact representation of the geometry. The degrees of freedom of the flexible body are afterwards reduced to save computation time in the multibody simulation. To capture precise deformations and stresses in the area of contact as well as elastodynamic effects, a large number of global shape functions is required. As test examples, the impact of an elastic sphere on a rigid surface and the impact of a long elastic rod are simulated and compared to reference solutions.Publicationtype: Journal ArticleScopus© Citations 6 211 234 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On design sensitivities in the structural analysis and optimization of flexible multibody systemsThe structural analysis and optimization of flexible multibody systems become more and more popular due to the ability to efficiently compute gradients using sophisticated approaches such as the adjoint variable method and the adoption of powerful methods from static structural optimization. To drive the improvement of the optimization process, this work addresses the computation of design sensitivities for multibody systems with arbitrarily parameterized rigid and flexible bodies that are modeled using the floating frame of reference formulation. It is shown that it is useful to augment the body describing standard input data files by their design derivatives. In this way, a clear separation can be achieved between the body modeling and parameterization and the system simulation and analysis.Publicationtype: Journal ArticleScopus© Citations 5 189 408 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Topology optimization of members of flexible multibody systems under dominant inertia loading(2018-04-01); ; The topology optimization of members of flexible multibody systems is considered for energy-efficient lightweight design, where the gradient calculation has an essential role. In topology optimization of flexible multibody systems, where the function evaluations are very time consuming, the gradient information is necessary to accelerate the optimization process. Different approaches have been introduced and tested for the gradient calculation in the fully coupled topology optimization of flexible multibody systems. However, the computation and implementation costs of these methods are high, which limits the optimization size and the possible number of design variables. In this work, we present a modified gradient calculation based on the equivalent static load (ESL) method, which combines the time efficiency of gradient calculation of the ESL method with the higher accuracy of gradient calculation in the fully coupled methods. This modified approach, which takes into account the linear dependencies of inertial loads on acceleration, is tested on the application example of a flexible slider-crank mechanism, and the results are compared with the weakly coupled ESL method and a fully coupled optimization where gradients are calculated using the adjoint variable method.Publicationtype: Journal Article191 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Real-time trajectory control of an overhead crane using servo-constraints(2018-01-01); In this paper, the system dynamics of an overhead crane are inverted by servo-constraints. The inversion provides a feedforward control for trajectory tracking of the system output. The overhead crane is inherently underactuated and modeled as a two-dimensional mechanical system with nonlinear system dynamics. Actuators are modeled as first-order systems to simplify implementation and account for velocity-controlled drives. The control based on servo-constraints is shown to be an effective method of trajectory control for overhead cranes. It will be demonstrated that the formulation is solvable in real-time using linear implicit Euler integration. The feedforward solution is made robust by an augmentation with LQR as well as a sliding mode controller. Experiments are conducted on a laboratory crane of 13 m motion range.Publicationtype: Journal Article173 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Structural sensitivity analysis of flexible multibody systems modeled with the floating frame of reference approach using the adjoint variable methodFor the efficient analysis and optimization of flexible multibody systems, gradient information is often required. Next to simple and easy-to-implement finite difference approaches, analytical methods, such as the adjoint variable method, have been developed and are now well established for the sensitivity analysis in multibody dynamics. They allow the computation of exact gradients and require normally less computational effort for large-scale problems. In the current work, we apply the adjoint variable method to flexible multibody systems with kinematic loops, which are modeled using the floating frame of reference formulation. Thereby, in order to solve ordinary differential equations only, the equations of motion are brought into minimal form using coordinate partitioning, and the constraint equations at position and velocity level are incorporated in the adjoint dynamics. For testing and illustrative purposes, the procedure is applied to compute the structural gradient for a flexible piston rod of a slider–crank mechanism.Publicationtype: Journal Article166