1. United States Patent 6,506,850, January 14, 2003.
2. United States Patent 6,093,777, July 25, 2000.
3. United States Patent 5,602,208, February 11, 1997.
4. United States Patent 4,882,107, November 21, 1989.
5. United States Patent 6,136,905, October 24, 2000.