Modelling friction at the mechanical interface between the human and the exoskeleton

In virtual assessments of exoskeletons, often, friction is not modelled even though the actual interface consists of straps or moulded surfaces, where friction could play a significant role. In this work, the human-exoskeleton interaction during the use of a passive lower limb exoskeleton is modelled in three test cases through two different interface models. In particular, a model introducing friction at the human-exoskeleton interface is compared with a more conventional model that uses a kinematic joint to simulate the interface forces. Both the models show a good match between the empirical and predicted distribution of body weight between the subject and the exoskeleton. However, the results also show different trends of the moment required at the assisted joint by the different interface models, highlighting the importance of a realistic interface model to investigate the effectiveness of the exoskeleton in virtual assessments

An observational method for Postural Ergonomic Risk Assessment (PERA)

Monotonous, repetitive work characterizes production lines. Repetitive movements and awkward postures are the most prominent physical risk factors in the workplace. Various legislations have been enacted along with technical standards for ergonomic risk evaluation to ensure the safety of the operators. There are numerous methods to assess the ergonomic risk at work. However, most methods are not meant to be used for assessing cyclic work. This paper proposes a method, Postural Ergonomic Risk Assessment (PERA), which is suitable to evaluate the postural ergonomic risk of short cyclic assembly work. Its key features are simplicity and compliance with standards. The added value of the method is that it provides an analysis of every work task in the work cycle, which facilitates the identification of sources of high risk to the operator. The method has been verified on nine work cycles, constituted by 88 work tasks, and it demonstrates accordance with the European Assembly Worksheet (EAWS), which has been developed to comply with the relevant standards and is one of the most comprehensive tools for ergonomic risk assessment. Industrial relevance: The simplicity and the compliance with standards of the proposed method would allow for a quick check of every work task of the work cycle and identification of problem areas. With further work, it would be possible to integrate the method along with work design tools used in the industry

A Simple Multibody 2d-model for Early Postural Checks in Workplace Design

Posture prediction is one of the most important aspects of virtual modeling tools used for the workplace design: once the work point to reach is defined, the posture prediction module allows simulating, through inverse kinematics, the posture the operator is likely to assume. The paper presents a simple multibody 2D-model created for early postural checks in the design phase. The tool is a spreadsheet created in Microsoft Excel environment, with the support of Visual Basic. The principal output of the model in terms of angles of trunk bending and upper arm elevation, set in compliance with the technical standards, are compared to the results of an established software tool for ergonomic analyses, the 3D Static Strength Prediction Program (3DSSPP). Finally, possible differences in terms of moments on the L5/S1 and the shoulder joints introduced by the simplified kinematics of the 2D manikin are discussed

Investigation into the applicability of a passive upper-limb exoskeleton in automotive industry

The fourth industrial revolution faces the technological challenge of human-robot cooperation in manufacturing process. Aim of this study was to investigate the effectiveness and user’s acceptance of a passive exoskeleton for upper limbs. Three different tests, involving static and dynamic tasks, were performed by 29 automotive operators without and with the exoskeleton. Main aspects and results of the testing campaign are presented in the paper. Potential issues associated to the introduction of these auxiliary devices in the automotive industry are briefly addressed, together with the open questions on how to assess the biomechanical workload risk, especially in the design phase

Ergonomic analysis of motor vehicles

Design of motor vehicles had not initially focused around humans. Vehicles were merely designed to perform basic mechanical tasks. Only later designers took into account the human element, even though, in the beginning, ergonomic principles were often introduced as an interventional option at the end of the design process. The aim of ergonomics was mainly introducing further qualities, which were often perceived as accessories or part of the brand image. Only in the past decades, vehicle occupant packaging became a necessary design phase. The primary focus in occupant packaging is the driver‟s workstation, that is the location and adjustment ranges of the steering wheel and seat with respect to the pedals, the physical location of controls and displays with which the driver interacts, the analysis of interior and exterior driver visual areas, both direct and through mirrors. The objective of packaging is usually stated in terms of percentage accommodation on particular measures. Accommodation is quantified Design of motor vehicles had not initially focused around humans [2]. Vehicles were merely designed to perform basic mechanical tasks. Only later designers took into account the human element, even though, in the beginning, ergonomic principles were often introduced as an interventional option at the end of the design process. The aim of ergonomics was mainly introducing further qualities, which were often perceived as accessories or part of the brand image. Only in the past decades, vehicle occupant packaging became a necessary design phase. An increasing common approach to occupant packaging employs manikins to represent driver requirements. Use of three-dimensional computer graphic models has followed the development of low cost computers. Early human modelling software programs such as Sammie have been followed by Ramis, Jack and Safework among others. These digital human models (DHM) are now widely used for vehicle interior design and have often replaced SAE packaging tools. Manikins are fundamentally population models, in that they describe percentiles of a population, not the behaviour of any individual within the population. A panel of manikins would be needed to attain good estimates of population characteristics. In the attempt to reduce the number of computer analyses that must be performed, designers select the extremes that span a large percentage of the range of body dimensions in the target population. The five elements to be considered in the ergonomics of motor vehicles are:habitability, accessibility, reachability, internal and external visibility, and seating comfor

Bending stiffness and strength degradation in glass-carbon/epoxy hybrid laminates: cross-ply vs. angle-ply specimens

A hybrid glass-carbon non-woven fabric reinforced epoxy matrix composite, constituted by layers of biaxial carbon fabrics, biaxial glass fabrics and hybrid carbon-glass fabrics, was considered for its bending fatigue behaviour. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (ratio of 0.10) were conducted on two sets of standard specimens, having fibre orientation parallel to the loading direction (cross-ply specimens) and at 45° to the loading direction (angle-ply specimens). Specimens were subjected to different fatigue loading, with the initial maximum load level up to 85% of the laminate ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 106 cycles the fatigue test was stopped and residual properties were measured on tested specimens. Stiffness-based stress-number of cycle (SN) curves were drawn for the two sets of specimens. The amount of stiffness loss for cross-ply and angle-ply specimens was observed to depend on the fatigue load level. In particular, cross-ply specimens were observed to damage more significantly than angle-ply specimens only at high fatigue loading. This was attributed to different damage mechanisms for the two sets of specimens. Reduction in material strength and elastic modulus as measured after 106 cycles was also found to depend on the level of fatigue loading and to follow different trends for the two sets of specimens

Ruolo e dinamica di formazione di strati protettivi superficiali nei fenomeni di usura

This paper concentrates on the dynamics of formation of surface layers for an aluminum-silicon alloy sliding against a carburized steel disk under dry conditions. The effects of the apparent contact pressure, sliding velocity and time on the thickness and morphology of the layers are investigated, together with the influence of the accumulation of wear debris in the contact region favored by one of the pin-on-disk geometry used in the study. Emphasis is given to the aspect of subsurface deformation, since deterioration of the layer is known to be caused primarily by delaminatio