Synthesis of linkages considering dynamic aspects

• Getriebesynthese unter Berücksichtigung dynamischer Aspekte

Villegas Ulloa, Claudio Nabil; Corves, Burkhard (Thesis advisor); Hernandez Frias, Alfonso (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Nowadays, there is a constant demand of mechanisms that operate at higher speeds and with good accuracy without compromising its reliability. This requires that the mechanism possess good dynamic behavior. The dynamics are traditionally considered at the end of the mechanism design process. If any problem is detected in this stage, it is necessary to go back in the design process for recalculation, leading to undesirable loops. This is particularly challenging for linkages that operate at high speeds because of the inherent nonlinearity of the system. The nonlinear motion of the system generates periodic forces with multiple harmonics that can excite one of the eigenfrequencies of the system. If the forces are low and the eigenfrequencies of the system are far enough from the harmonic components of high amplitude, the dynamic response of the system can be of low amplitude. However, these facts are not commonly analyzed in the early stages of the design process.Methods to include the dynamic considerations in the early stages of the mechanism design process are presented in this dissertation. They are applied to function-generator linkages, namely four-bar linkages and Watt II linkages. The input link of the mechanisms rotates at constant rate while the output link swings. The latter link moves a body of constant inertia.Different approaches are studied starting with a parametric study of the effects that a swinging body has on the reaction forces on the joints of a four-bar linkage. Then, four bar linkages are synthesized for the case of moving an external body of extremely high inertia, and then, for a more general case. After that, six-bar linkages are synthesized with two heuristic methods, the genetic algorithm and the non-dominated sorting genetic algorithm modified (NSGA II). These approaches consider the force on the joints as a first indicator of good dynamic behavior and constrain the first eigenfrequency, among other variables. The resulting mechanism are compared against an elastic multibody simulation. Comparative results show a good correlation between resulting mechanisms and simulations, with predictable forces, relatively low vibration of the links and low deviation of the flexible output link with respect to a rigid model.

Institutions

• Chair and Institute of Mechanism Theory, Machine Dynamics and Robotics [411910]