Takanori Chihara

Perceived discomfort functions based on joint moment for various joint motion directions of the upper limb.

The aim of the present study was to formulate the relationship between the perceived discomfort and the joint moment ratio for twelve joint motion directions of the upper limb by considering the between-subject variability, and to investigate the effect of joint motion direction. Three approximation models (i.e., linear, exponential, and logistic function models) were compared in terms of the accuracy of predicting the perceived discomfort, and the logistic function was selected because its average error was lowest. The concept of L-R fuzzy number was used to consider the individual variability of perceived discomfort, and a simplified distribution of perceived discomfort was represented. Cluster analysis showed that the twelve discomfort functions formed two clusters: one for elbow flexion and a second for the remaining joint motions. The data show that elbow flexion is more sensitive than other joint motions to increases in the joint moment ratio.

Evaluation function of drinking ease from aluminum beverage bottles relative to optimum bottle opening diameter and beverage type.

In recent years, aluminum beverage bottles having screw tops with opening diameters of 28 and 38 mm have been launched in the Japanese market in keeping with the modern-day drinking habits of consumers. Although Japanese consumers are familiar with such bottles, a majority of them feel that the 28 mm opening is too small and the 38 mm opening is too large. Therefore, we felt the need to develop a method for evaluating consumer feelings when they drink a beverage directly from the bottle opening. For this purpose, we propose an evaluation function of drinking ease that calculates the optimum opening diameter of the bottle. From results of our previous study, we know that there exists an ideal volume of beverage flowing into the mouth, at which consumers feel most comfortable while drinking directly from bottles. Therefore, we define the evaluation function of drinking ease in terms of the difference between the actual volume of fluid in the mouth and the expected ideal volume. If this difference is small, consumers probably feel comfortable while drinking the beverage. We consider a design variable, i.e., the opening diameter, and two state variables, i.e., the volume of beverage remaining in the bottle and the height of consumers, and construct the response surface of the evaluation function by using radial basis function networks. In addition, for investigating the influence of beverage type on the evaluation function, we select green tea and a carbonated beverage (Coke) as test beverages. Results of optimization of the proposed function show that when the opening diameters are 35.4 mm and 34.4 mm in the case of green tea and Coke, respectively, the actual volume of fluid in the mouth is closest to the ideal volume and the participants feel most comfortable. These results are in agreement with results of our previous study that an opening diameter of 33 mm is optimum for young Japanese adults. Thus, we confirm that the proposed function is accurate; it can be used to design bottle openings to suit consumers of various age groups and types of beverages.

Evaluation of multiple muscle loads through multi-objective optimization with prediction of subjective satisfaction level: illustration by an application to handrail position for standing.

Proposed here is an evaluation of multiple muscle loads and a procedure for determining optimum solutions to ergonomic design problems. The simultaneous muscle load evaluation is formulated as a multi-objective optimization problem, and optimum solutions are obtained for each participant. In addition, one optimum solution for all participants, which is defined as the compromise solution, is also obtained. Moreover, the proposed method provides both objective and subjective information to support the decision making of designers. The proposed method was applied to the problem of designing the handrail position for the sit-to-stand movement. The height and distance of the handrails were the design variables, and surface electromyograms of four muscles were measured. The optimization results suggest that the proposed evaluation represents the impressions of participants more completely than an independent use of muscle loads. In addition, the compromise solution is determined, and the benefits of the proposed method are examined.

Optimal design method with biomechanical analysis for a work environment reducing physical workload: illustration by application to work table height design

ABSTRACT A work environment should be designed to minimise physical workload. We propose an optimal work environment design method to accomplish this, in which joint moment ratios were calculated by biomechanical analyses through digital human modelling (DHM) and were used to indicate physical workload. The work environment design problem was formulated as a multi-objective optimisation problem, minimising the average and maximum joint moment ratio values to determine the optimal work environment. Sequential approximate optimisation, which improves the accuracy of the response surface by sequentially adding new sampling points from a simulation, was applied to efficiently obtain a precise optimal solution. The proposed method was applied to designing a work table height for light assembly tasks. This method determined the optimal work table height considering the anthropometric diversity of workers from a relatively small number of subjects. Through this case study, the validity of the proposed method is discussed.

Evaluation of Mental Workload Based on Pursuit Eye Movement

Mental workload (MWL) has been an important concept in Human Factors. In a human-machine system, MWL can explain operator errors at times. MWL is commonly measured by electrocardiography and electroencephalography, but during real-world tasks problems such as noise intrusion make it difficult to estimate mental workload using physical signals. Therefore, the aim of this study was to propose an evaluation index for MWL based on pursuit eye movement. It was examined for participants to follow two kinds of gaze points presented on a rear projector. As a result, it was shown that we experimentally confirmed the effectiveness of the proposed pursuit saccadic intrusions (PSIs) evaluation index, and suggested that it can be used to estimate MWL from pursuit eye movement.

Selection of Approximation Model on Total Perceived Discomfort Function for the Upper Limb Based on Joint Moment

The aim of this study is to formulate the relationship between the total perceived discomfort of the upper limb and perceived discomforts of each degree of freedom (DOF). The perceived discomforts of each DOF were formulated as functions of the joint moment ratio based on the results of previous study, and then the function approximation model for the total perceived discomfort was investigated. The summary score of the rapid upper limb assessment (RULA), which is assumed as the total perceived discomfort, and the perceived discomforts of each DOF were taken as the objective and explanatory variables respectively. Three approximation models (i.e., the average, maximum, and radial basis function (RBF) network) were compared in terms of the accuracy of predicting the total perceived discomfort, and the RBF network was selected because its average and maximum error were lowest.