Analysis, Synthesis, and Experiments of Standing up Methods for a Tripedal Robot

This paper presents the work addressing the issue of standing up after falling down for a novel three-legged mobile robot STriDER (Self-excited Tripedal Dynamic Experimental Robot). The robot is inherently stable when all three feet are on the ground due to its tripod stance, but it can still fall down if it trips while taking a step or if unexpected external forces act on it. The unique structure of STriDER makes the simple task of standing up challenging for a number of reasons; the high height of the robot and long limbs require high torque at the actuators due to its large moment arms; the joint configuration and length of the limbs limit the workspace where the feet can be placed on the ground for support; the compact design of the joints allows limited joint actuation motor output torque; three limbs do not allow extra support and stability in the process of standing up. This paper examines four standing up strategies unique to STriDER: three feet, two feet and one foot pushup, and spiral pushup. For all of these standing up strategies, the robot places its feet or foot at desired positions and then pushes the feet against the ground thus, lifting the body upwards. The four pushup methods for standing up were analyzed and evaluated considering the constraints such as, static stability, friction at the feet, kinematic configuration and joint motor torque limits, thus determining the suggested design and operation parameters. The motor torque trends as the robot stands up using pushup methods were investigated and the results from the analysis were validated through experiments. INTRODUCTION AND MOTIVATION STriDER can often fall down if it trips while walking or if external forces act on it. Thus, it is important to investigate a variety of standing up strategies specific to STriDER in order for the robot to stand up and complete its tasks. This paper focuses on four types of feet puhshup methods: three feet, two feet and one foot pushup, and spiral pushup. Generally, in all four methods, the robot first places its feet or foot at desired positions, then lifts its body by pushing the feet against the ground [1, 2]. The feet contact points are assumed to be stationary through the whole process. The unique structure and operation of STriDER makes the simple task of standing up challenging for a number of reasons; the tall height and long limbs of the robot require high torque from the actuators due to large moment arms; the joint configuration and length of the limbs limit the workspace where the feet can be placed on the ground for support; the compact design of the joints allows for limited actuator torque; and the number of limbs (three) does not allow extra support and stability in the process of standing up. A detailed analysis of the feet pushup methods is presented in this paper considering constraints such as, static stability, friction at the feet, kinematic configuration, link length ratios, and actuator torque limits. The objective of this analysis is to determine optimal design and operation parameters that will minimize actuator torques as the robot stands up. By minimizing actuator torque less power is consumed and the robot can stand up more efficiently. In addition, due to the size 1 Copyright c © 2009 by ASME Proceedings of the ASME 2009 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE 2009 August 30 September 2, 2009, San Diego, California, USA