M.Sc Thesis

M.Sc StudentReiner Ori
SubjectStability of Planar Spider Robots under Induced Linear
DepartmentDepartment of Mechanical Engineering
Supervisor PROF. Elon Rimon
Full Thesis textFull thesis text - English Version


Mobile robots are generally required to pass through non-flat terrains which may contain holes and obstacles. Limbed robots are adequate for performing this task. The goal of this research is to characterize stable equilibrium stances of a multi-limbed robot in a two-dimensional gravitational field. Such characterization serves as a basic tool for selecting the robot's stances. Furthermore, a sequence of stable equilibrium stances enables the robot to be navigated in a quasistatical gait pattern.   

A stiffness control method is used for inducing a desired stiffness at each contact point of the robot with the terrain. According to the induced stiffness, the mechanism is modeled as a compliant body having the same contact points and a variable center of mass. An analytic expression for the location of the body's equilibrium points is obtained. The stability of each equilibrium point is then analyzed. In addition, the constraints on the contact forces are computed. These constraints ensure that each contact point would neither break nor slip. The main contribution of this work is the method for computing the stable region where the center of mass must be located such that stable equilibrium and contact forces constraints are satisfied. A series of examples which demonstrate this region is provided. The curves which delimit the stable region are expressed analytically and can be applied to a spider-like robot with an arbitrary number of contacts. Simulation and experimental results are presented to demonstrate the stability of the robot's equilibrium stance. Finally, we discuss the effects of inaccuracies in the induced stiffness. Such inaccuracies may cause the appearance of an additional stable equilibrium point which does not satisfy contact forces constraints. We suggest a tool for determining the robust region for the location of the center of mass. Locating the center of mass within this region ensures that an additional stable equilibrium point does not exist.