This paper presents a general necessary condition for singular time op
timal control of robotic manipulation moving along specified paths. Ea
rly work [1], [7] ignored the issue of singular control, assuming bang
-bang acceleration along the path. Recent work [6] has shown that the
time optimal control can be singular if one of the equations of motion
reduces to a velocity constraint. This paper derives a more general n
ecessary condition for singular control. It is also proven that singul
ar control cannot exist if the set of admissible controls is strictly
convex, as is demonstrated for a two-link planar manipulator with elli
ptical actuator constraints.