J. Sykulski, M. Rotaru, M. Sabene, and M. Santilli. The International Journal for Computation and Mathematics in Electrical and Electronics Engineering, (October 1998)
K. Sugawara, and T. Watanabe. Proceedings of the IEEE/RSJ International Conference on Intelligent Robots and Systems, page 2702--2707. Lausanne, Switzerland, (2002)