| Number | Name | Date | Kind |
|---|---|---|---|
| 3583325 | Melin | Jun 1971 | |
| 3987734 | Horn | Oct 1976 | |
| 4071135 | Ishikawa et al. | Jan 1978 | |
| 4557373 | Paurat et al. | Dec 1985 | |
| 4565276 | Dengs et al. | Jan 1986 | |
| 4635783 | Hofling | Jan 1987 | |
| 5237931 | Riedl | Aug 1993 | |
| 5267173 | Tanizawa et al. | Nov 1993 | |
| 5289778 | Romine | Mar 1994 | |
| 5570637 | Wagner | Nov 1996 |
| Entry |
|---|
| Current PRT and Related Systems Development Efforts. |