Walking impairments are a common sequela of neurological injury, severely affecting the quality of life of both adults and children. Gait therapy is the traditional approach to ameliorate the problem by re-training the nervous system and there have been some attempts to mechanize such approach. We have recently presented the MIT-Skywalker; a novel device to deliver gait therapy, which, in contrast to previous approaches, takes advantage of the concept of passive walkers and the natural dynamics of the lower extremity in order to deliver more "ecological" therapy. In this paper we present a control scheme for the MIT-Skywalker, which is based on an artificial potential field applied at the foot workspace. It is used to improve sensory feedback to the patient, as well as to increase to normal the range of motion of the paretic leg. Simulation results prove the efficiency of the proposed controller.