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In order to stabilize the inherent unstable system like the inverted pendulum on a cart, severe judgment of situation is required. Accordingly, it can be expected that human operators exhibit complex behavior intermittently. This paper investigated the identification of the individual difference of human operator's behavior from time series data by using fuzzy inference and acquired individual skill of human operator. It also investigated the chaotic behavior of human operator and the formation of a complex system in the learning process of human operators with objects difficult to control. The operators in the experiment are skilled to some extent in stabilizing the inverted pendulum by training, and the data of ten trials per person were successively taken for an analysis, where the waveforms of pendulum angle and cart displacement were recorded. The maximum Lyapunov exponents were estimated from experimental time series data against embedding dimensions. It was found that the rules identified for a fuzzy controller from time series data of each operator showed well the human-generated decision-making characteristics with the chaos and the large amount of disorder and the individual difference of chaotic and complex human operation can be identified with fuzzy inference.

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The lightweight racket with handle-light configuration and large head size is recent tendency of high-tech tennis rackets, increasing power or post-impact ball velocity with an increasing racket swing speed. This paper investigated the performance of lightweight balanced racket with super-large head size with 120 square inches in terms of feel or comfort. It predicted the effect of the mass and mass distribution of super-large head sized rackets on the impact shock vibrations of the racket handle and the player's wrist joint when a player hits a flat forehand drive. The prediction is based on the identification of the racket characteristics, the damping of the racket-arm system, equivalent mass of the player's arm system and the approximate nonlinear impact analysis in tennis. A lightweight balanced racket (mass: 292 g, the center of gravity L G : 363 mm from the butt end) and a conventional weight and weight balanced racket (349 g, L G : 323 mm) are selected as representatives. These two super-large head sized rackets made of carbon graphite have the same head size and same geometry. The result showed that the shock vibration of the lightweight balanced racket with super-large sized head is much larger than that of the conventional weight balanced type racket. It also showed that the sweet area of the former in terms of the shock vibration shifts from the center to the topside on the racket face compared to the latter. This is because the location of the grip on the racket handle is further from the location of the node on the handle of the first mode of lightweight balanced racket than that of the conventional weight balanced racket.