I. Introduction
Recently, a variety of reports on human–robot interaction have been published. Although most robots have vision and auditory sensors [1], haptic sensor is another important equipment to interact with human and environment [2], [3]. This paper proposes a haptic sensing system implemented on the humanoid robots' feet. Humanoid robot is a bipedal architecture mechanism which is one of the most versatile setups for walking robot. This type of robot is highly suitable for working in human environments. It should be able to operate in various environments. For instance, a humanoid robot should be able to walk on the slopes, stairs [4], and obstacles [5] to work as human substitutes or to work together with humans. However, the complex dynamics involved in the walking mechanism make this type of robot control a challenging task. To complete this task, the robot requires the sensors and the control concept of a biped robot to realize the interaction between the robot and environment [6].!--note-ref7a--> Some researchers use accelerometers as a sensor for biped walking system [7]. Some researchers propose a flexible shoe system for biped robots to optimize energy consumption of the lateral plane motion [8].