Hand-arm mechanical impedance in presence of unknown vibration direction

This work describes the results of an experimental campaign whose aim was the identification of the hand-arm driving point mechanical impedance (DPMI) in presence of a stimulus having an unknown direction. This situation is relevant in case of cylindrical handles, where the tool can be gripped in different ways independently from the vibration direction. In this case, xh and zh ISO 10068 curves can be conveniently substituted by a unique parameter, here referred to as " radial impedance" Radial impedance values have been determined with an experimental campaign based on the classical setup in which the vibration is generated by an electrodynamic shaker and the force component parallel to the vibration is measured by an instrumented handle. The vibration direction was always vertical, while the different postures entailed a different stimulus orientation with respect to the hand, forearm and shoulder. The hand-arm posture was monitored using a purposely designed measurement system. The push forces transmitted by the operator to the handle were measured using load cells, while the grip force was measured before the tests using a pressure matrix. A statistical analysis on the radial DPMI data was performed using the analysis of variance (ANOVA) techniques. Results showed that the uncertainty deriving from the adoption of the radial impedance in presence of cylindrical handles does not always lead to a larger uncertainty with respect to the ISO 10068 data. Coherently with the existing literature studies, also the radial DPMI depends on the forces transmitted to the handle and on the wrist, elbow and shoulder postures. The opportunity of adopting (in specific cases) the radial impedance instead of the xh and zh is discussed. Relevance to industry: Hand-transmitted vibration may cause a variety of vascular and musculoskeletal disorders that often result in loss of work capacity and compromised quality of life. The concept of radial impedance described in this paper can be used in the design of devices meant to reduce the vibration transmitted to the workers, thus reducing the above mentioned risks.