Improvement of the fidelity of surface measurement by active damping control

This paper reports on the modification of a stylus instrument for studying dynamic effects in surface finish measurements. A feedback system allows control of the force from freely suspended to 10 mN and of the damping ratio from 0.02 to larger than one. Specimens of mild steel and copper have been traced repeatedly with damping ratios of 0.02 and 0.59 over the traverse speeds of 0.05-1 mm s^-1. From these profiles it is observed that more consistent profile shapes and surface finish parameters are obtained within an optimal damping range. The performance of the system is investigated over a large range of damping ratios by freely suspending the system while applying a force proportional to a previously measured profile. The closest correspondence between the stylus output and the original profile, taken to represent optimal fidelity, occurs with damping ratios in the range of 0.5-0.7. Dynamic forces at the stylus are evaluated by direct measurement using a load cell and by indirect estimation through friction measurements. To compensate the additional mass due to the force actuator and damping sensor, it is demonstrated that the state feedback control can be used to vary the natural resonant frequency and the damping ratio independently.