Based on analysing the basic feature of scanning process, this paper proposes a new architecture to regulate autonomous scanning speed with the nominal vector DN. In addition, explains and demonstrates how to introduce the nominal vector DN into the tracer scanning for successive and self-adjusting scanning speed and autonomy contact level between stylus of trace and model according to the curved surface. The technical activities and experimental results shown that the parameter of nominal vector DN play a proper role in the case of retaining tracer contact on curved surface especially when stylus scanning from horizontal plane to the vertical plane.