Application of phase-locked loop to mass measurement using damped dynamic vibration absorber

Abstract

Phase-locked loop (PLL) is applied to mass measurement using damped dynamic vibration absorber. The frequency response curves of a mass-spring system with a dynamic vibration absorber pass through a point that is independent of damping in the absorber. The measurement system utilizes this fixed point to estimate mass independently from damping. To carry out such measurement, the excitation frequency must be tuned to equal the frequency of the fixed point. For such tuning, PLL is applied. The PLL system consists of two detection circuits, a microcomputer and a direct digital synthesizer. It adjusts the excitation frequency for the phase between the excitation signal and the displacement of the absorber mass to be 180 degrees. In the experiments, the frequency responses are measured to confirm the existence of the fixed point. The automatic tuning is achieved by the developed system. Mass measurements are carried out in the developed apparatus. The results demonstrate the efficacy of the measurement using PLL and that the measurement utilizing the fixed point is insensitive to the damping in the absorber.