The appliance industry has tried to design washing machine with large capacity, high energy efficiency, low vibration and low noise levels. As the installation place of a washer becomes closer to the living space, vibration and noise problems become more important challenges.

The four legs of the washing machine should be attached to a floor. If not so, it may cause severe vibration or walking in the spin-dying process [1,2]. Unfortunately, the floor of an ordinary house is bumpy in general, and the consumers will not accept bolting washing machines to a foundation; moreover, sometimes they move the location of their washing machines to utility rooms or bath rooms or kitchens and do not exactly level the legs.

In this study, an auto-leg system is devised that prevents the occurrence of abnormal vibration and the walking of washing machines. To investigate the mechanism of a walking washing machine, we use a simplified two-dimensional model. For the more detailed analysis, the three-dimensional dynamic model is developed through LMS CADSI's simulation software, DADS [3].

Bushing elements of DADS represent elasticity and damping for the leg material, and contact elements are designed to reflect the contact normal forces. Dummy bodies between the legs and a floor using friction force elements were used because a model with only contact elements supplied in DADS is insufficient to exactly express the tangential friction force. Since the characteristics of rubber affect analysis results in the dynamic behaviour of washing machines, a compression test of rubber was performed to estimate the Young's modulus of the legs.

Some experiments were implemented to show the dynamic characteristics of the three-dimensional auto-legged washing machine model when located either on the even or the uneven ground. Experimental and numerical results show that the proposed dynamic model can be used appropriately for this type of contact problem. The proposed auto-leg system is simply composed of a spring and a friction damper. In this paper, for the optimization the sequential quadratic programming algorithm supported in the package, MATLAB optimization toolbox is used. We have made a process to evaluate function values from the software, DADS, automatically. DADS Model, DADS Graph, and MATLAB are integrated in the batch process for the optimal design.

A spring parameter is optimized to adjust the length of the auto-leg system automatically for up to 10mm irregularity, and the friction damper is designed to decrease a resonance induced by the spring of the auto-leg system. Some numerical results show that placing the proposed auto-leg system in a washing machine provides good performance with low vibration, as well as low noise, regardless of the unevenness of the floor.

1

D.C. Conrad and W. Soedel, "On the problem of oscillatory walk of automatic washing machines", Journal of Sound and Vibration, 188(3), 301-314, 1995. doi:10.1006/jsvi.1995.0595

2

E. Papadopoulos and I. Papadimitriou, "Modeling, design and control of a portable washing machine during the spinning cycle", Proceedings of the 2001 IEEE/ASME International Conference on Advanced Intelligent Mechatronics Systems, 899-904, 2001. doi:10.1109/AIM.2001.936786

3

DADS(Dynamic Analysis and Design System) User's Manual for Revision 9.6, CADSI(Computer Aided Design Software Inc.), 1997.

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