The effect of the object weight on the assembly posture satisfying certain visual demand is investigated. Based on the ergonomic requirements of manual assembly, workload on hands must not exceed 50 N at the average position (Table 3.2) and hence the object weight can vary between 0.1 kg and 5 kg. Its effect on the optimum hip displacement is shown in Figure 4.27 where the object height H = 700 mm and the distance D = 500 mm. In this diagram, more hip displacement is required when the object weight increases in order to satisfy different visual demands. As stated in Section 4.3.2, a load on hands only affects energy expenditure rate and does not interfere with joint discomfort. Consequently, the trend of optimum hip displacement leads to a recommendation of a squat posture based on the reduced total joint moment as well as the energy
Figure 4.26: The effect of distance to the operator.
expenditure rate when the object weight increases.
4.6
Conclusions
Assembly posture modelling using multi-objective optimisation (MOO) method was achieved in this chapter in order to improve the actuality and accuracy of manual assembly simulation and analysis. A 10-DOF, 4 control-points human model associated with the assembly features was proposed and demonstrated to represent assembly postures and task constraints correctly and effectively. The proposition of 4 control points allowed a more convenient and active manipulation of human model in the virtual environment. Its advantages can be summarised as: 1) establishing new constraints readily, for example, the hand control point and eye control point are used as the equality constraints respectively for the MOO solution; 2) reducing the number of unknown variables greatly and enhancing the optimisation efficiency.
Subsequently, optimum assembly postures were predicted using the MOO method. Two main problems were considered in the posture prediction, i.e. how to model the performance measures and how to combine them together. In this chapter, the joint discomfort model based on previous work by Marler et al. evaluated joint discomfort in its rotation position associated with its comfortable neutral position and its respective rotation limits. By arranging the metabolic energy rate formula, the energy expenditure model was simplified and established as a sum of weighted joint torques and the basal metabolic rate (BMR). A weighted sum method developed by Yoon and Hwang was applied for multi-objective combination. It was capable of assigning weight to each objective function automatically without a priori knowledge about its relative importance. The procedure of assembly posture prediction using the MOO method was verified via experiments in the Virtual Engineering Centre. The results have shown a high consistence on predicted postures and those captured from real operators. After verification, a series of assembly posture analysis was conducted in terms of different assembly constraints and conditions.
Stoop and squat postures had been studied widely in manual lifting tasks in order to reduce work related injuries and increase work efficiency [143–147]. In these studies, Garg suggested that the squat posture was superior to the stoop posture when the load to be lifted was close to the operator [143]. Park showed that the stoop posture was more favourable than the squat posture for loads greater than 5 kg [144]. Compared with manual lifting, manual assembly has higher demands on object positions (i.e. visual demands) which are summarised
and formulated in this research. When constrained by certain visual requirements, there is a trade-off between the squat posture and the stoop posture. In order to minimise joint discomfort and metabolic energy expenditure, the MOO method was applied to predict optimum squat depths for varying operators from 5th percentile to 95th percentile and the results indicate a preference of adopting squat postures for taller operators.
Analyses in Chapter 3 revealed that the height of assembly object from the floor, the distance to the operator and the weight played significant roles to the assembly postures. Assembly posture strategies for varying object heights, distances and weights were investigated under certain visual constraints. The results show that in order to reduce joint discomfort and energy expenditure, a squat posture is recommended when assembling 1) objects in lower positions, 2) objects which are closer to the operator, and 3) heavier objects (less than 5 kg).
Ergonomic Evaluation of
Assembly Sequence
5.1
Introduction
Assembly sequencing plays a key role in the strategic and operational aspects of integrative product design and production planning. Any delays in or modifications to assembly sequence planning after the completion of product design could lead to costly changes for rectification.
It is notable that in the past two decades, research in computer-aided assembly sequencing and planning has increased significantly. Advances have been made in both theory and practice of assembly sequencing as demonstrated by the emergence of new assembly planning systems [80–82, 86, 87]. However, such systems were mainly developed for automatic assembly. Ergonomic requirements for manual assembly are normally neglected or even violated in the system design. For instance, the visibility of the product and its components in the assembly environment and the accessibility of operator’s hands when performing assembly tasks, which are crucial to the assembly efficiency, product quality and operator well-being, are not always fulfilled.
DELMIA presents the basic vision analysis and hand access analysis functionalities as described in Chapter 3. In the vision analysis, the manikin’s field of view is provided through a vision window when its position and posture in the virtual assembly environment are determined; in the hand access analysis, the clearance between the manikin’s hand and obstructed assembly objects is detected. However, they are both lack of necessary evaluations regarding the
analysis results and hence leave ambiguous decisions to the analyst.
In this chapter, high-level consideration of the ergonomic requirements in the manual assembly process will be integrated into the assembly sequence planning. In sections 5.2, 5.3 and 5.4, several types of assembly sequences, assembly planning descriptions and representations of assembly sequences are described. Thus a general procedure to create all feasible assembly sequences can be set up and represented in Section 5.6, which only obeys the constraint arising strictly from the geometry of the assembly product itself. In Section 5.7, new ergonomic constraints considering assembly workstation layout, operator characteristics and working posture are proposed for objective evaluation and selection of manual assembly sequences, which consist of visibility criterion, accessibility criterion and both. Finally, a system called Liverpool Assembly Sequence Planning (LASP) is developed to achieve the integration by utilising different evaluation criteria. With LASP, optimum assembly sequences with the maximum viability and/or accessibility score are obtainable during the design stage and an illustrative case study of an air conditioner assembly is also presented in Section 5.8.