Measurement of two-dimensional distribution of pure axial force in active polishing

We present a new technique of real-time measurement for the pure axial force distribution exerted by a full size active polishing tool on large aspheric surfaces. The technique uses an array of custom developed flat line load cells incorporating semiconductor strain gauges that can withstand the large shear forces in polishing. Design, manufacture and characterization of the force sensor array and its instrumentation to the active polisher are described. In particular, thermal characterization of the sensors proved that the temperature change by a few degrees required for a polishing tool has negligible effects on sensitivity and zero-point offset. Effects of non-axial force components were compensated with four independent half-bridge signals inside a sensor unit. This was further removed with a gap condition between the sensor array and the structural membrane, of which the latter functions as a point of pushing and pulling the polishing lap. The sensor array shows a potential usage in detecting and controlling the non-axial force components in operation. The real-time graphical map of axial polishing force generated by the sensor array improved the polishing efficiency by at least 30% compared to the traditional polishing technique.