In this paper, a method of generating virtual desired signals in real-time for previously designed control systems is proposed. This virtual desired signal is introduced to improve performance in tracking the real desired signal. Two strong points of the proposed generation method are real-time signal generation and utilization of the previously designed control system informations. These strong points are realized by feedback of the state variables of the previously designed control system and by feedforward from the real desired signal. In addition to a basic method of generation of the virtual desired signal, a generation method which includes preview action is proposed. An optimal preview control system which make use of this real-time preview virtual desired signal is designed and applied to an air-slide linear brushless dc motor. Some experimental studies are carried out, confirming the effectiveness of the new methodologies.