In order to analyze the form of motion and characteristics of steel ball in the process of detectionbased on the operating principles of equipment AVKIO detecting the defects automatically and produced by Czechcompany，the physical observation platform of deployable structure for steel balls was designed and established.Taking advantage of research method of inverse dynamic analysis，the high-speed camera was used to observe themovement trajectory of marker on steel ball surface. Then the geometric features of two types of spiral was analyzedrespectively，combining with the form of movement of detection probe relative to steel ball. The results indicate thatunder the action of unfolding wheel，the steel ball rotates in a way of space helix with equal length of arc to ensurethe unfolding of whole surface of steel ball，meanwhile the detection probe scans the surface in a way of homogeneous meridians relative to steel ball

which can guarantee the whole-surface unfolding and no duplicate detection ofsphere and provide theoretical basis for structure design of unfolding wheel.