Comparative Evaluation of Positioning Performance for a Direct Drive System: CM NCTF versus PTOS

This paper proposes a realization of continuous motion nominal characteristic trajectory following control (CM NCTF) as a practical controller for a single axis direct drive linear motor. Direct drive linear motors has the advantages of providing high performance motions with reduced mechanical components, but its major drawback is the load variation and parameter changes will degrade the positioning accuracy and compromise stability of the system. This will cause large deterioration in the motion profile. Although numbers of studies on control system have achieved nanometric accuracy with a direct drive linear motor, this research differs significantly in the ease of design and controller structure. The CM NCTF control has a simple structure and its design procedure does not require exact parameter identification, which makes it easy to design, understand, and adjust. It is comprised of two important elements which are nominal characteristic trajectory (NCT) and proportional integral (PI) control. The NCT is constructed based on the measured actual responses of the system, and the PI is designed in the stable region of the system. Extensive experimental validation was done to evaluate the positioning performance of the proposed controller using a direct drive linear motor. A Proximate Time Optimal Servomechanism (PTOS) controller was designed and comparative studies were done on the both controllers experimentally. Overall, the CM NCTF control has demonstrated superior tracking accuracy than the PTOS control and good robust performance in the presence of load variations, too.