In a servo-system of a conventional numerical control (NC) device, a delay in response or deformation is always observed on an output side with respect to an inputted speed command or position command and an actual output response behaves differently from a command value. Such delay or deformation of the output response constitutes a significant problem particularly in case of controlling one object simultaneously by a plurality of servo-system, which results in a remarkable defect in a controlling performance. According to this invention, when a transfer function of a servo-system of the type comprising a control system and a control object is designated by W, a device or a signal conversion system with an inverse function 1/W of the transfer function W is located a a pre-compensation system in advance of the control system thereby to input a speed or position to be controlled into the pre-compensation system. Thus, according to this invention, the output response to the inputted speed or position command can fully follow up, and moreover, in a system provided with a plurality of servo-system, the respective servo-system can be controlled with substantial no delay in response, so that it is possible to precisely control one object. Particularly, in a case where the servo-system of this invention is utilized for an NC device for a machine tool, a working operation with no working error can be realized especially in an arc cutting operation.
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