Design and Analysis of a Foot Contact Sensor for Posture Control of a Biped Robot

Development of a planar biped robot is currently underway at Yeditepe University. The robot consists of lower extremities with a torso that are designed at anthropomorphic dimensions. This study describes the design and testing of a foot contact sensor for the biped robot. Dynamic stability of a biped robot is commonly measured by the zero moment point (ZMP) method. Experimentally, ZMP is measured by multi-component force/torque sensors. Due to their low cost and ease of use, force sensitive resistors (FSR) are used to build a foot contact sensor for the biped robot. Four FSRs are mounted at the corners of the robot’s foot to measure the ground reaction force and its moment. Hence, by utilizing the data from the foot contact sensors, a real-time ZMP computation scheme can be implemented. The performance of the designed foot contact sensor is presented by numerical simulations of a planar biped robot’s postural stability control. Results indicate that reaction force computation by the FSR based force sensors is a viable method to monitor postural stability of biped robots. Force sensors and their electronics are currently being built to be used for the actual tests. INTRODUCTION Natural adaptation in human postural balance control involves a number of feedback mechanisms, primarily, the visual, vestibular and proprioceptive systems [1]. Walking robots have borrowed some of the biological adaptation methods from humans. It is possible to derive joint trajectories for a biped robot based on anthropomorphic joint trajectories of human locomotion [2]. Another commonly referred biomimetic approach is to use central pattern generators (CPGs) for the generation of the joint motion [3]. The concept of Zero-Moment Point (ZMP) was introduced by Vukobratovic [4], and has been widely used as a measure of stability and active control of bipedal walking robots [2], [5], [6]. In this study, a ZMP based postural adaptation approach has been utilized. For a stable walk, most of the bipedal robots require measurement of the contact force between its feet and the ground [6], [7], [8]. Different technologies exist for the measurement of ground contact force such as strain gauge, piezoelectric, optical and force sensitive resistors (FSR). Mechanical design of the foot has a vital importance on robot’s stability. Li et al. [7] proposed a generic mechanical design of a biped robot foot by using six degree of freedom force/torque sensors and an impact absorption mechanism. FSR sensors differ from the others by their low cost, thickness, weight, and ease of mounting. Lebosse et al. [9] investigated the nonlinearities and dynamic behavior of these sensors. They provide an experimental setup by using a strain gauge sensor as a control group and a DC motor, in which an eccentric wheel is mounted on its shaft. This eccentric wheel actuates a cam and follower system, which converts rotational motion to a linear one. By the help of the results of the experimental setup, they identified the nonlinear properties of the sensor and proposed a compensation model. Che et al. [10] used FSR sensors and absolute encoders to build a wearable exoskeleton system for human gait analysis. Braun [8] proposed a planar biped robot foot design and a control approach. They mounted four FSR sensors to four corners of the foot and used them as an on/off switch type sensor to identify the states of the foot motion. Kong et al. [11] proposed a fuzzy stabilization algorithm using ground reaction forces that are measured by FSR sensors in ISHURO-II humanoid robot. Kim et al. [12] introduced a novel foot mechanism, which uses four FSRs in HanSaRam-VI humanoid robot project. They also proposed a new method to compensate for the landing impact force. Yang et al. [13] also Proce dings of the ASME 2010 10th i i l f r i ri t i l i July 12-14, 2010, Istanbul, Turkey