Chapter 1 Comparison of Feedforward Control Schemes for Real-Time Hybrid Substructuring (RTHS) Christina Insam, Mert Göldeli, Tobias Klotz, and Daniel J. Rixen Abstract In order to meet the high demands in testing, actuators must be able to follow their desired displacement with high precision. Feedforward control enables high tracking performance of actuators. In combination with feedback controllers, an actuator can follow a prescribed trajectory quickly, stably and robustly under varying conditions. In RealTime Hybrid Substructuring (RTHS), a method where parts can be tested under realistic boundary conditions, high tracking performance of the actuator is vital. It not only increases fidelity of the RTHS test outcome—meaning that the test replicates the environment and boundary conditions of the test specimen well—but it also prevents the RTHS loop from becoming unstable. Hence, research is carried out in the field of control schemes being applied to RTHS systems. In this work, the existing cascaded feedback control of the position controlled Stewart Platform is expanded by three different feedforward control schemes: model-based dynamic feedforward, modeling-free iterative learning control and velocity feedforward. The tracking performances are compared and discussed using a commanded sine trajectory. Results reveal that modeling-free iterative learning control and velocity feedforward outperform model-based dynamic feedforward and follow the desired trajectory with high amplitude and phase accuracy. Velocity feedforward is simple and requires almost no implementation effort. Thus it is recommended for applications with stiff actuators. In contrast, modeling-free iterative learning control is recommended for tasks where the actuator is not stiff compared to the test specimen. As all these feedforward control schemes improve the tracking performance compared to feedback control, the fidelity of the RTHS test will improve using them. Keywords Feedforward control for RTHS · Parallel manipulators · Model-based dynamic feedforward · Modeling-free iterative learning control · Velocity feedforward 1.1 Introduction In Real-Time Hybrid Substructuring (RTHS), mechanical components can be tested with realistic boundary conditions. By realistic boundary conditions we mean that the mechanical component is excited by the same forces and displacements that it will be subject to in future applications. Using RTHS, we investigate whether the test specimen will withstand the loads in operation, dynamically influence the movement of the whole structure as intended and functions correctly. This is achieved by running a co-simulation of the structure surrounding the mechanical component (test specimen) in the future application while testing the test specimen on a test bench. The test specimen is referred to as the experimental part (EXP) and the co-simulated surrounding structure is referred to as the numerical part (NUM). 1 The co-simulation and the test specimen are coupled at their interface points in real-time by a so-called transfer system (TS). It consists of an actuator, a force-torque sensor and a digital signal processor [1]. Current applications of RTHS can be found in civil and mechanical engineering. Applications in civil engineering include testing of buildings under earthquake loads, testing of tall buildings under wind loads and testing of road/rail bridges under wind and wave loads. In mechanical engineering, the method has been applied to common problems in the aerospace and 1Note, that instead of naming it parts, one can also find the naming components or substructures in literature. C. Insam( ) · M. Göldeli · T. Klotz · D. J. Rixen Chair of Applied Mechanics, Faculty of Mechanical Engineering, Technical University of Munich, Garching, Germany e-mail: christina.insam@tum.de; mert.goeldeli@tum.de; tobias.klotz@tum.de; rixen@tum.de © The Society for Experimental Mechanics, Inc. 2021 A. Linderholt et al. (eds.), Dynamic Substructures, Volume 4, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-47630-4_1 1
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