Patent ID: 7761187

Claim:
A method of controlling tub oscillation of a drum type washing machine utilizing a ball balancer, the method comprising: maintaining an rpm of a drum in a predetermined rpm level for a predetermined period of time before a tub is subjected to oscillation that is greater than a predetermined level during a dehydration operation to reduce a differential rotation speed between the drum and a ball; determining a time point of re-increasing the rpm of the drum based on a variation of a tub oscillation signal when the rpm is maintained in the predetermined rpm level for the predetermined period of time and re-increasing the rpm of the drum such that the tub is not subjected to the oscillation that is greater than the predetermined level, wherein the determining the re-increasing time point includes calculating an oscillation period in the predetermined rpm level using a difference between a first reaching time at which an oscillation frequency reaches a first peak point after the oscillation frequency starts to increase with a positive gradient after the rpm of the drum has reached the predetermined rpm level, and a second reaching time at which the oscillation frequency reaches a second peak point after the oscillation frequency starts to re-increase with a positive gradient.