In this paper, two disturbance observer (DOB) based explicit robust force control systems, in which a force sensor and a reaction force observer (RFOB) are used to detect contact forces, are compared in terms of stability, robustness and performance. A force sensor detects contact forces implicitly by estimating the strains of a strain gauge. The compliancy of a force sensor degrades the stability and performance of a force control system. Besides, noise is another challenging issue in a force sensor based force control implementation. Against a force sensor, a RFOB detects contact forces explicitly by using the acceleration and force/torque signals of a servo system. Inasmuch as a RFOB detects contact forces explicitly, the stability and performance of an explicit force control system are improved intrinsically. However, a RFOB has a model based control structure; therefore, the stability and performance of the robust force control system may deteriorate by the imperfect identification of system uncertainties. New analysis and design tools are provided for the DOB based robust force control systems. Simulation and experimental results show the viability of the proposals.