Patent ID: 8125173

Claim:
A control system of a lifting device that controls the rotation of a servo motor so that an operator can move a load in a direction and at a speed that he or she desires by applying a force for controlling to the load, wherein the load is hoisted up or down or maintains its position by means of a rope that is wound up or down by the rotation of the servo motor in the forward or the reverse direction, the system comprising: a means for measuring a total force that measures a force that is applied at the lower part of the rope caused by a force for controlling of the operator, the mass of the load, and the acceleration of the load, a first controller means in which the arithmetic part computes the direction and the speed of the servo motor based on the force that is measured by the means for measuring, and outputs a signal to the servo motor to have it operate, a second controller means that comprises an arithmetic part, wherein the arithmetic part determines a stable condition using a criterion for nonlinear stability, under which condition, when the load touches the ground, the input and output signals of the servo motor, which motor rotates in the forward and the reverse direction, are stable, and the arithmetic part computes the direction and the speed of the servo motor, and outputs a signal to the servo motor to have it operate, and a switching means that replaces the first controller means with the second controller means, at the right time, namely, when the value that is measured by the means for measuring becomes less than the threshold.