Computer Aided Design of Eight-bar Linkages

This dissertation describes research into an automated methodology for designing eight-bar linkage systems for specialized tasks. The user specifies the desired motion as five discrete task positions, and a backbone chain robot that can achieve those task positions. The design algorithm then uses the graph theory approach to find all the different ways of constraining the robot using mechanical constraints, which are rigid binary links (RR constraints), to generate single degree of freedom linkages. Linkage solutions synthesized using this method are only guaranteed to reach the five positions. They can exhibit branch defects which results in discontinuities during their motion in between the task positions. An analysis algorithm is used to discard the defective solutions and ensure that the filtered solutions will move the end-effector smoothly through all the five desired task positions. Once the original task positions are explored for finding linkage solutions, the design system runs the algorithm iteratively over randomized task positions, within acceptable variations provided by the user. This results in the generation of large number of defect-free linkage solutions. These solutions are further checked for a minimum and maximum allowed link length criteria to further filter out linkages. The result is a list of successful linkage solutions that are presented to the user. The RR constraining strategy for the user specified robot has provided some useful insights in to linkage design. It has been observed that as we increase the complexity of the starting serial or closed chain robot, the number of linkage solutions that could be synthesized increases rapidly. This increases the probability of finding useful solutions and hence more design options can be presented to the designer for selection. The design system has been implemented in a computer aided linkage software system called MechGen for four-bar and six-bar linkages, and has demonstrated its effectiveness to design better devices for several applications