Measurement of Two-dimesional 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.