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Item type:Publication, Access status: without files , Trajectory tracking with collision avoidance for a parallel robot with flexible links(2021-06); ; End-effector trajectory tracking of a highly flexible three-link parallel robot is combined with collision avoidance in real-time. Typical real-time collision avoidance algorithms issue commands on actuator level which can cause large oscillations within flexible robots. Thus, the needed evasive motion at the points which are about to collide is used to adapt the desired end-effector trajectory of the underactuated system, even for actuator limits and points other than the end-effector. In contrast to existing real-time collision avoidance concepts, which are essentially completely based on rigid models, a dynamic flexible model inversion is then used for tracking. Thereby, output redefinition leads to a minimum phase system. The presented computationally efficient concepts, being actuator limit and obstacle collision avoidance, use sigmoid functions providing smooth trajectories. The former calculates a braking factor for the end-effector when an actuator approaches its limit. The latter avoids external dynamic obstacles based on repulsive vectors. Experiments validate both concepts with a flexible model inversion which clearly outperforms the classical rigid body approach.Publicationtype: Journal Article82 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Modeling and Trajectory Tracking Control of a New Parallel Flexible Link Robot(2018-12-27); ; ; A completely new compliant lightweight robot is presented with a kinematic loop and a highly flexible link. It is explained how to model such parallel robots accurately but still computationally efficient. The elastic deformations are described with the floating frame of reference approach. For the flexible components this allows to use linear finite element models, which can represent arbitrary geometries. These models are further reduced by modal truncation and a Component Mode Synthesis minimizing the number of elastic degrees of freedom, which is necessary for real-time control purposes. The obtained model of the underactuated robot is non-minimum phase for the end-effector as output. Thus, for the applied trajectory tracking controller which is based on servo constraints, the concept of stable inversion is used. The performance is compared to a relocated minimum phase output. Corresponding simulations are validated by first experimental results showing the need for and high accuracy of the flexible model and the trajectory tracking control.Publicationtype: Conference Paper200 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Nonlinear state estimation for trajectory tracking of a flexible parallel manipulatorLightweight robots can be advantageous when considering the lower energy consumption, the possibility to use smaller motors and the lower material cost. Nevertheless, in such lightweight structures non-negligible flexibilities are inherent which can lead to significant oscillations making the control more difficult. Within this research end-effector trajectory tracking of a parallel manipulator with a highly flexible link is considered. As centerpiece, a new approach to estimate the state of the system of differential algebraic equations is discussed to obtain the end-effector position and velocity. For the estimator, the kinematic loop is treated by a projection tangential to the constraint manifold which is based on a QR decomposition. Subsequently, an Unscented Kalman Filter is applied and the dependent coordinates are obtained by satisfying the algebraic constraint equations with the Newton-Raphson method. The utilized signals for the estimator are on the one hand position measurements of the actuators which are realized as direct drives and on the other hand measurements of strain gauges attached to the long and highly flexible link. As application, a basic end-effector output controller based on an equivalent rigid model and the transfer function of the dominant first bending eigenmode is utilized to enhance the tracking results. The concepts are applied to an experimental rig for validation purposes and to show first results for end-effector estimation with feedback control.Publicationtype: Journal ArticleScopus© Citations 8 184 191 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Control of vibrations for a parallel manipulator with flexible links — concepts and experimental resultsA comprehensive control approach is presented to reduce the vibrations of a parallel manipulator with a kinematic loop and two flexible links whereof the longer one can show significant oscillations. The control objectives are end-effector trajectory tracking and active vibration control. The system is modeled as a flexible multibody system and exact feedforward control based on the full dynamic flexible multibody system is applied to improve the end-effector trajectory tracking performance. Furthermore, the effect of different position control concepts for the two linear drives, such as gain scheduling for the utilized cascade control and a model based friction compensation, on the movers themselves as well as on the end-effector are discussed, which can be conflicting. Experimental results are presented illustrating the achievable accuracy of the end-effector tracking for different trajectories while showing significant error reductions for a feedforward control based on an elastic model in contrast to a rigid one. Finally, a model based curvature controller is utilized which actively controls the occurring oscillations of the parallel manipulator. Here, a proportional controller as well as a linear-quadratic regulator are applied and the impact of an additional curvature control on the end-effector tracking performance is investigated.Publicationtype: Journal ArticleScopus© Citations 2 216 476