I. Introduction
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. Humanoid robot also should be able to operate in environments with slopes, stairs and obstacles for working as human substitutes. 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 sensing system for realizing the interaction between robot and environment information. Recently, there is an increasing demand on various types of sensing devices for robots to obtain the detailed object information. Although computer vision is often employed to recognize the object shape with the position and orientation, tactile sensing is an essential ability for a robot [1]–[7]. Especially, the tactile sensor attached on the robot foot can sense the ground surface while the robot vision cannot get the occluded surface image. A variety of tactile sensing systems have been proposed not only for robots, however it's also for human-machine interfaces [8]–[9], force-feedback, pattern recognition and invasive surgery. For tactile or force sensing, there are a variety of techniques and sensing devices. Previous works have used traditional strain gauges [10], electro-magnetic device sensors [11], force sensitive resistors [12], capacitive tactile array [13], optical device [14], piezoelectric resonance [15] and the shape memory alloy (SMA) device as micro-coil actuators used for 2-D and 3-D tactile display [16]. Although the use of 3D or 6D force sensors located in the body can perform the similar tasks [17]–[18], such devices have many sensing elements such as strain gauges in the sensor inside. Moreover, the number of sensing elements is often more than three. Hence, such systems often become more complex than the proposed system. The proposed sensor foot is one of the simple implementations structured with three sensing elements (FSRs). Our idea is to use such devices with a layout specialized for object surface sensing. We have proposed tactile sensor units implemented on the end effectors of robotic arm with force sensitive resistors. In the previous works, we employed our sensing device for the active measurement of the object shape, 3-D object edge tracing, robot hand poses control and a human-robot cooperative work [19]–[23].