This paper presents new analysis and synthesis tools for the robust force controllers implemented by Disturbance Observer (DOb) and Reaction Torque Observer (RTOb) in state space. The proposed design tool allows to use different plant and disturbance models in the observer synthesis, thus providing great flexibility in the robust force controller implementations. For example, the conventional DOb-based robust force controller can be synthesised using constant disturbance and interaction torque models in the DOb and RTOb synthesis. Moreover, the proposed tool allows to easily derive the transfer functions between the estimated interaction torque and exogenous reference, noise and disturbance inputs. This provides significant benefits when tuning the design parameters, such as the bandwidths of the observers, and analysing the stability and performance of the robust force controller. The proposed analysis method shows that the stability and performance of the robust force controller are significantly affected by the design parameters of the RTOb. For example, as the identified inertia is increased in the RTOb synthesis, an open-loop zero moves towards the right-half plane, leading to poor stability and performance limitations in practice. To synthesise a minimum-phase robust force controller, the identified inertia should be set smaller than or equal to the exact inertia. The stability and performance of the robust force control system are also affected by the other design parameters such as the bandwidths of the observers and environmental dynamics. To mitigate the latter effect, the design parameters of the force controller should be tuned by considering the dynamics of the environment that the servo system interacts physically. The proposed analysis and synthesis tools are verified by simulations.