This paper presents a method to determine contact conditions between a planar end-effector and the environment, i.e., face-to-face, face-to-line, or face-to-point contact. First of all, two kinds of contact motion of a planar end-effector, i.e., passive and active contact motions, are described. A compliance controller is implemented to achieve the passive contact motion, and the "groping motion"is generated as the active contact motion. Then, novel robot-friendly expressions of the environment are proposed based on the concept of environmental modes. Discrete Fourier transform matrices are utilized as matrices transforming environmental information into environmental modes. In the experiments, a planar end-effector contacts with the environment with the passive and active contact motions, and the environmental data obtained from sensors are transformed into environmental modes. The profiles of the extracted environmental modes are utilized to determine the contact conditions. The validity of the proposed method is confirmed by the experimental results.
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