A vibratory sensor capable of locating parts by measuring their inertial properties is described in this paper. The sensor is designed to be fitted to the wrist of a robot and used to acquire parts from a stack or a tray. The initial coordinates of the parts need only be approximately known (say to ±25 mm and ±45°). The robot is thus said to operate in a “semi-ordered” environment which can be realised inexpensively as accurate pallets, fixtures or other parts presentation equipment are not required. Furthermore, the absence of complete disorder, as would exist with a “bin-picking” approach where parts are allowed to lie at random in a bin, reduces the degree of sophistication demanded of the sensor system. Consequently, the cost of the latter can also be kept low.
The proposed sensor has a circular platform mounted on the shaft of a motor such that the shaft is normal to the platform. The part whose location is to be determined is held on the platform by magnetic or other means. The assembly consisting of the part, platform and motor is constrained to vibrate about an axis parallel to the platform. Two methods for computing the location of the part relative to the platform are described. Both methods require the natural frequencies of vibration of the part-platform-motor assembly for various angular positions of the platform. A simulation study to examine the effect of various design and operational parameters (inertias of the platform and part, stiffness of the spring restraining the movements of the vibrating assembly, location of part on platform, accuracy of frequency measurements) on the accuracy of the location computed is presented. The simulation results clearly demonstrate the feasibility of the sensor.