Experimental Modal Analysis Study of a Standing Soldier and Rifle System

Abstract

The response of the human body to shock and vibration has been a subject of interest to many researchers in the aerospace and automotive industry. In a new study, an experimental modal analysis of a rifle-armed, standing soldier in a standard firing position was performed. The purpose of the study was to determine the modes of vibration of the soldier-weapon system in order to gain an understanding of its response during the firing event. The weapon firing accuracy, especially during closely-repeated semi-automatic or fully-automatic fire, as well as the energy transmitted to the body of the soldier depend not only on the weapon itself but also on the soldier body’s dynamic characteristics. Rapid weapon fire does not allow a soldier to consciously control muscles needed to bring back the weapon to its original position, so the weapon location after each fire is influenced more by the dynamic characteristics of the human-rifle system. The experimental modal analyses were performed using multi-averaged, impact-force and electrodynamic shaker force excitation (mainly sine sweeps) and roving triaxial acceleration response at various locations on the body. It was observed that testing of human subjects poses significant difficulties since an increasing number of measurement averages could lead to muscle fatigue and ensuing tremors that could negatively influence coherence. The soldier stance during the tests could also change due to the unconscious need to adjust to a more comfortable body position. The positioning of the accelerometers was difficult since attachment could be made to skin only. While there was in general large variability in soldier size, mass and body strength, the study allowed the identification of some lower modes of interest which appeared to have the same mode shape, albeit at different frequencies, for all the various individuals tested. The signals were acquired with a National Instruments hardware and processed using ModalView software. For an average size soldier the first mode occurred at approximately 2 Hz. The first mode shape exhibited combined bending (backward) and twisting characteristics which are generally seen in a “up and to the right” motion of a right-shoulder held weapon during firing. More modes in the 0–20 Hz range were identified.