Most existing real-time control systems use ad hoc static priority scheduling methods, in spite of the fact that the rate monotonic scheduling algorithm was proved to be the optimal static priority scheduling algorithm over 20 years ago. In this paper, we discuss a task library we are using for the real-time control of autonomous robots. The task library comprises a preemptive rate-monotonic scheduler which provides guaranteed optimal scheduling when certain conditions of processor utilization are met. The task library has been implemented as a collection of lightweight threads, which operate entirely in user-space for maximum efficiency. We show the performance advantages resulting from the reduced overhead of this approach, compared with commercial operating systems. The task system is robust, extensible, and portable, and has been successfully used to control the autonomous mobile robot Yamabico-11 developed at the Naval Postgraduate School.