Transcranial Magnetic Stimulation (TMS) is a non-invasive technique that produces excitatory depolarization in the neurons located in the cerebral cortex. In order to proficiently stimulate a specific cerebral area it is of main importance the correct positioning and maintaining of the magnetic coil, while avoiding the mismatch due to subject head-coil relative movement. Hot Spot Hound is a novel robot-assisted experimental platform for enhancing TMS stimulation performance. It integrates a commercial optoelectronic neuronavigation system (E.M.S. srl, Italy) for gathering data on the relative pose (position and orientation) of subject's scalp and TMS coil, and a service robotic arm designed for human interaction (LWR system by KUKA, Germany). Besides integrating and synchronizing different platform sub-systems, we implement a control strategy to center the stimulation point and compensate for involuntary subject movements. Specifically, the proposed control maintains constant over time the homogeneous transformation matrix between the pose of the coil and of the head. The value of position stiffness has been chosen in order to assure the better compromise between coil position and orientation error, and the safety of the experimental subject. In this paper we show that Hot Spot Hound can provide coil position and orientation errors well beyond the ones achieved by manual experimenter, while assuring safety in the physical interaction with the stimulated subject.