1. Y. Ding, US Patent Number 6,632,555 (2003).
2. E.R. Eaton, W.H. Boon, C.J. Smith, US Patent Number 6,818,146 (2004).
3. T.F. Fuller, D.J. Wheeler, US Patent Number 6,068,941 (2000).
4. T.F. Fuller, D.J. Wheeler, US Patent Number 6,103,410 (2000).
5. J. Roberts, M. van der Geest, J. St-Pierre, D.P. Wilkinson, A. Lee, S. Moroz, US Patent Number 6,329,089 (2001).