The importance of harmonic versus random excitation for a human finger

Abstract

In hand-held devices the vibration excitation is typically multi-harmonic (e.g., as a result of the eccentricity in rotating motors) on top of the random excitation. The excitation amplitude, excitation frequency and the push force typically vary with the application or the usage conditions. Typically, random excitation is used for finger or hand excitation. This paper investigates the importance of harmonic versus random excitation for a human index finger. Under different testing conditions the finger’s response was researched via the apparent mass. The harmonic excitation was studied using a sine sweep profile where the continuous wavelet transform was used for the extraction of the instantaneous apparent mass. The results show significant differences in the identified apparent mass for harmonic excitation when compared to random excitation (e.g., at frequencies close to 10 Hz the apparent mass was up to 3 times higher for a random excitation than for a harmonic excitation). As hand tools are frequently excited with a single or multiple harmonics, the identified differences should help in the process of designing safer hand tools.