Development of Unified Topology and Dimension Synthesis Methodology for Linkage Mechanisms and its Application to Conceptual Design of Automobile Suspensions

Abstract

Topology optimization of rigid-link mechanisms, a methodology for obtaining linkages that satisfy a set of user defined kinematic requirements without any a priori baseline design, is a new paradigm that can be usefully employed in industries such as automotive or aerospace engineering. In previous research, however, the methodology has been limited to simple planar linkages. In this research, a new formulation for synthesizing the topology and dimension of linkages is proposed. To design topology of link mechanisms by using the optimization method, a formulation which represents the DOF (Degree-of-Freedom) in differentiable form has to be considered. Herein, the DOF is the minimum number of actuators that is required to decide the position of the all link components. In previous research, motion compliance and load stiffness have been employed to avoid deficient-DOF state and redundant-DOF state, respectively. To this end, the motion compliance is the system flexibility under displacement excitation such as motion drive, and the load stiffness is the system rigidity under force excitation such as external resistance forces. However, in aspect of the multi-objective optimization, implementation of the DOF control by using the two functions, the motion compliance and load stiffness, contradictive to each other is quite particular about heuristic weighting factor decision issue. Meanwhile, as the work transmittance efficiency function suggested in this thesis is exploited to control the system DOF, there is no issue related to the preference decision between two objective functions. That is, only a unified objective function is used to avoid the deficient- and the redundant-DOF states. Therefore, it is possible to design complicated systems, unlike the previous research which is hard to consider it due to difficulties of the DOF control. Our approach is validated through several case studies. In the planar design case, benchmark type four-bar linkages and automotive steering systems are considered. For spatial linkage synthesis problems, automotive suspension mechanisms are designed by the suggested method. To find a better solution in suspension design, we employed a simultaneous topology and shape optimization method. As a result, a new type suspension mechanism is obtained by the unified topology and dimension synthesis method, especially when a smaller design space compared with nominal one is provided. To analyze the behavior of the newly designed suspension system, the screw-axis theory is applied. From this investigation, it is found that a new special module is included in the new-concept suspension and it works as a conventional link component. In this research, according to this property of the newly proposed concept, it will be called a hidden link suspension. It is also shown that the suspension installation space can be reduced compared with nominal multi-link type suspensions by exploiting the hidden link module. The synthesized suspension mechanism is the first successful industrial result obtained by the unified topology and dimension synthesis method. Especially, the proposed method can provide new insight to engineers who want to enhance the product quality by making use of totally different conceptual designs as shown in this research. In the near future, it will be possible to apply the suggested linkage synthesis method to other practical problems, beyond the automotive industry problems, to find more advanced mechanisms.