As the number of mobile robot applications increases, research related to physical contact between humans and robots is becoming an important issues in the service robot domain. In this paper, a mobile robot platform based on spring loaded casters is proposed as solution to problems in the service robot domain. A simple dynamic equation in two dimensions was computed and the dynamic characteristics according to variation of the elastic coefficient were analyzed. In order to detect force from a human, a linear equation was formulated by regression of the driving data. With the estimated force, an impedance model is applied to the controller for compliance. Through this process, it is shown that the proposed mobile robot platform is capable of a compliant reaction to external force through of its posture, similar to unstable mobile platform, though the mobile platform has the static stability.