Standing-up characteristic of contact force during self-posture changing motions

The condition leading to self-posture changing motion (SPCM) of a link system is discussed. It is concluded that there exists a singular behavior in an SPCM, namely, contact force sharply increases when the link posture comes to a particular point depending on frictional conditions at the point of contact. That contact force keeps constant and small irrespective of the frictional coefficient, while the link posture is not so far away from the straight-line. Thus, an SPCM is highly robust for frictional coefficient when it is executed with the straight-lined link posture. It is shown that by introducing a non-dimensional angle, there exists a similarity law for the behaviors of contact forces for various frictional coefficients, and that a sufficient condition obtained in the authors´ earlier works has no space to be relaxed