Global workpiece positioning system (GWPS) Part 2: Development of an ultrasonics-based system

Abstract

Abstract Datum establishment uncertainty is a major source of error affecting the geometric accuracy of machined features. In part one of this two-part paper, a novel concept, the Global Workpiece Positioning System (GWPS), was presented for datum establishment in a multiple-setup machining operation, where a laser triangulation based prototype was implemented and its concept feasibility validated. However, the approach based on a laser triangulation probe has several limitations. These include sensitivity to the optical quality and orientation of the artifacts, a short stand-off, and the necessity to operate in a scanning mode while held in the spindle of the machine tool. Ideally, it would be preferred if the sensors were fixed and outside of the workspace of the machine tool. This would eliminate the time required for a probe scanning cycle and potentially minimize the effect of machine tool positioning uncertainty on the locating accuracy of the GWPS. A solution to this problem lies in the development of a GWPS that is analogous to the current satellite-based Global Positioning System (GPS). In such a system, a set of receiver and transmitter pairs would operate in a manner similar to the satellite transmitters and ground-based receivers of the GPS, and a universal system to locate the workpiece in the work zone of the machine tool would be accomplished. To explore the potential of this approach, an ultrasonics-based GWPS was examined where, as an initial step, the laser triangulation probe was replaced with an ultrasonic receiver and a series of ultrasonic transmitters was affixed to the workpiece. This approach was found to permit a wider and longer detectable range of the artifacts (ultrasonic transmitters). Thus, the workpiece and artifacts can be deployed more freely. A 2D machining experiment demonstrates that the workpiece can be positioned by the ultrasonic GWPS without the use of physical contacts. An average positioning error of 0.185 mm (0.007 in.) was obtained in five test runs. Although the positioning accuracy is insufficient for machining applications, the ultrasonic GWPS may be applicable to less demanding situations where greater artifact sensor flexibility is needed. Furthermore, the use of time-of-flight ranging best embodies the general concept of GWPS. If advancements in electromagnetic GPS technology continue to occur at the current pace, it is anticipated that an electromagnetic time-of-flight GWPS will achieve the accuracy needed for machining applications in the future.