There, the experiment is performed where force command is zero and external force is not applied. The results of this experiment is shown in Fig. 12. Fig. 12 (a) shows the effect of force sensor noise in force control system. This result reveals that the force sensor noise makes the force control system unstable. On the other hand, Fig. 12(b) shows that the effect of encoder noise can be ignored in force control system. Consequently, the advantage of reaction force estimation observer is observed by this experiments.