For non-linearsystems, linear in the control, a relationship between the singularity of the timeoptimalcontrol problemand the differential controllability of the linearized dynamics along a time optimal solution that satisfies the minimum principle is established. Known results concerning the solution of the time optimal control problem for a two link planar articulated arm robot without friction are extended to a general rigid articulated arm robot with friction. Finally, these resultsare used to prove differential controllability of the linearized dynamics (called first order controllability) along any trajectory of rigid articulated arm robots with friction. © 1990 Taylor & Francis Group, LLC.