06-10-2010, 05:39 PM
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AUTOMATIC WASHING MACHINE CONTROL SYSTEM
INTRODUCTION
The manufacturing of washing machines has lately been an important issue for the appliance industry. Current environmental awareness demands the improvement of washer efficiency. To this end, the use of closed-loop control instead of traditional open-loop approaches is being adopted. In addition, although horizontal-axis washers have a higher manufacturing cost, they are becoming more popular because it has been estimated that they consume less energy, water and detergent compared to the vertical-axis ones.
The reduction of washer mass is of crucial importance not only for environmental, but also for financial reasons. Unfortunately, washing machines remain big and heavy, weighing usually over fifty kilograms. This is due to the unbalanced rotation of the laundry mass during spinning. The rotating clothes are not evenly dispersed in the drum, resulting in significant centrifugal imbalance forces, which tend to destabilize the washer. This problem, which has been traditionally solved by adding a large concrete mass to the system, can have three modes: Translational slip, rotational slip and tip.
Depending on the spinning speed, the mass of the laundry and system geometry, each of these modes can become the most important destabilizing agent. A simplified two-dimensional model has been used to prove the existence of a threshold speed for translational slip and tip. The same work also showed that vertical-axis washers are slightly more stable than horizontal-axis washers. However, the rotational slip problem has not been addressed. In terms of stabilization techniques, research is focused on the use of suspension systems. More specifically, a lot of work is carried out in suspension system analysis, and optimization. In addition, it has been determined that there is a relationship between walk performance and suspension design.
Another study showed that this relationship can be explicitly formulated for impending walk. Naturally, all these techniques improve the washer’s dynamic behavior, but are insufficient unless the machine’s mass is over fifty kilograms. However, such a machine cannot be portable. An interesting approach to stabilization has been proposed by Zuoxin, who introduced the idea of passive balancing by adding a ring containing liquid at the drum. The idea takes advantage of the centrifugal forces and of the washer’s suspension to counteract the imbalance. This solution compensates for vibration caused by imbalance but introduces problems associated with system resonances.
Finally, Lemaitre introduced an out-of-balance detection system, but did not propose any method of counteracting these forces. This paper analyzes the problem of rotational slip and shows how it is related to translational slip and the design of a washing machine. It is proved that the critical speed for impending translational slip is higher than that of rotational slip. A design-based method is suggested, which increase the vertical force and contributes to the machine stability is allowing the reduction of the washer’s mass. Finally, an active method is proposed, which employs sensors, a micro-controller and stepping motors to minimize vibrations. It is also shown that an improved estimation of the drum angular position and velocity results in greatly reduced residual vibrations.
