Hand vibration: non-contact measurement of local transmissibility

The transmission of vibration from tool handles to operator hands can be reduced using special anti-vibration gloves [1-3,9]. Hand transmissibility (TR) is usually measured as the ratio between accelerations measured on the hand palm and on the handle [3,5]. This approach has been widely discussed in literature [2,4,7-11]. Being the hand a complex mechanical structure, its acceleration may also vary across its surface [12,13]. Therefore, TR can be used to observe the input-output relationships across the hand-surface [6]. Objectives of the present paper are: To present an original non-contact measurement procedure for measurement of local hand TR; To report TR values and spectra measured on 6 different positions of the hand of 9 subjects. To report the correlation between TR and grip force, push force, hand volume and BMI. Tests have been conducted on 9 healthy male volunteers. Tests have been carried out using a cylindrical handle mounted on an shaker. Excitation signal was random vibration: 16 – 400 Hz with un-weighted rms acceleration of 6 m/s2. Laser Doppler vibrometer is used to measure the acceleration [6]. Push force is measured using a force platform while grip force is measured using a capacitive pressure sensor matrix wrapped around the handle [10]. Rms values of the acceleration data series, and handlej a , are calculated for each j-th test. k TR is calculated for each point k, as:   j handle Results demonstrate how TR values measured on distal points are higher respect to the proximal points. In particular for all the distal points the TR is > 0.91, while for proximal point TR is < 0.56. The highest value of TR is 1.06. A resonance peak is present for all the measured points in the band 55 – 80 Hz. ANOVA analysis show that TR is not significantly dependent on: BMI, hand volume and push force alone. While TR is significantly dependent on: grip force alone, measurement positions alone and grip and push force together. The use of LDV [6] to measure hand-arm vibration transmissibility along the main vibration axis is here discussed and results demonstrate how the TR is significantly different on the point where the acceleration is measured. In accordance with [4], we have observed no significant dependence of TR on BMI and hand volume. Feed force alone is also not significantly influencing TR. On the contrary, the measurement position as well as the grip force are significantly influencing the TR. ANOVA tests demonstrate that coupling forces together are significantly influencing TR. The design of ergonomic handles therefore