Shin Horng Chong

Practical control of non-friction mechanism for precision positioning

This paper describes the practical control of non-friction mechanism for precision positioning. Non-friction mechanism is often used for precision positioning. Even though it has a simple structure, still, plant identification is compulsory needed during designing a conventional controller. This makes the controller non-user-friendly and non-practical-used in industry. For overcoming this problem, practical controller design procedure based on NCTF (nominal characteristic trajectory following) controller is proposed. NCTF controller consists of a nominal characteristic trajectory (NCT) and a PI compensator, which is free from exact modeling and parameter identification. The NCT is determined using an open-loop time responses of the mechanism. The PI compensator is used to make the mechanism motion to follow the NCT and it is tuned without given model parameters. Non-friction mechanism has non-damping a characteristic and often has a short-working range. A suitable current input to stop the non-damping mechanism within a short working range in open-loop condition and to be able to improve the damping characteristic of the mechanism is necessary. The positioning performances of two different current inputs are examined and discussed. The positioning performance of NCTF control system is evaluated based on simulation and experimental results.

Positioning control of a one mass rotary system using NCTF controller

In this paper a practical control scheme is discussed for the positioning and tracking control of a one mass rotary system. The practical controller designed must have high speed performance, high accuracy, robust to disturbance and parameter variation and have high positioning response which always welcome to industry. Hence, a Nominal Characteristic Trajectory Control (NCTF) controller has been proposed to yield high motion control performance and high robustness. This controller does not require exact model and parameter identification which make it easy to design. Basically, NCTF controller consists of a simple structure comprising Nominal Characteristic Trajectory (NCT) and Proportional Integral (PI) Compensator. Apart from a Conventional NCTF controller, the NCTF controller also improves to Continuous Motion NCTF (CM-NCTF) controller. CM-NCTF controller has same design procedure as Conventional NCTF controller and it is able to produce a slightly better performance than the conventional one by producing slightly faster response and smooth tracking performance. To evaluate the controller performance, the Conventional NCTF controller and CM-NCTF controller, was compared to PID control through experiment.

AR-CM NCTF Control for Precision Positioning Systems -- Concept and Results

This paper describes the concept and application of the Acceleration Reference Continuous Motion nominal characteristic trajectory following (AR-CM NCTF) control to a non-contact mechanism, a simple and typical positioning mechanism. The AR-CM NCTF control is an improved version of the Continuous Motion NCTF (CM NCTF) control, in order to enhance the following characteristic of the object motion on nominal characteristic trajectory and improve the positioning and tracking accuracies of a system. It provides the advantages such as the high overshoot reduction characteristics and the low sensitivity disturbance. The basic concept and practical design procedure of the controller are introduced. The effectiveness and advantages of the AR-CM NCTF control are shown in the experimental positioning and tracking control results, in comparison with CM NCTF control.

Practical and robust control for precision positioning systems

This paper presents a practical and improved practical controller for enhancing precision motion performance. For practical use, high motion control performance, ease of design and controller adjustment are desired. A nominal characteristic trajectory following (NCTF) control has been studied to realize high performance and ease of application. The NCTF controller consists of a nominal characteristic trajectory (NCT) which is the reference motion of a control system and determined from experimental open-loop time responses of the mechanism and a compensator which makes the motion of the controlled object to follow NCT. The controller parameters can be determined easily, without any given model parameters. In the present paper, the Acceleration-Reference - Continuous Motion (AR-CM) NCTF controller has been proposed and implemented. The AR-CM NCTF considers the acceleration reference for object motion as additional controller elements besides the velocity reference. The AR-CM NCTF controller provides the advantage of high overshoot reduction and disturbance reduction characteristics. This paper then highlights how robustness the controller in the precision positioning systems. The issues considered include (i) variation of mass object; (ii) variation of disturbance force such as coulomb and viscosity frictions. The design procedure of the AR-CM NCTF controller remains easy, practical and is independent of friction characteristics. Validation and implementation of the AR-CM NCTF controller with a mechanism with friction are carried out in order to confirm its effectiveness. The robustness of the AR-CM NCTF controller is examined by comparing its motion performance with the PDDO (PD + disturbance observer) controllers. The comparative experimental results illustrate the achievable performance of the AR-CM NCTF controller in positioning and tracking control and the higher robustness to plant parameter variations, as compared to the PDDO controllers.

Nominal characteristics trajectory following control as practical controller: A review

Nominal Characteristic Trajectory Following (NCTF) control has been introduced and employed in positioning system since last two decades. It is well known with its practical and easy design procedure, which is independent of friction characteristics. Thus, designer does not need to consider the friction characteristic of a mechanism and it is a valuable controller approach in industry. Besides the straightforward design procedure, the NCTF controls are capable to achieve promising results in positioning and continuous motions and high robust performance as the end objective. The NCTF controller is comprehensive, comprising of a Nominal Characteristic Trajectory (NCT) and a PI compensator, which is free from exact modeling and parameter identification. There are three generations of the growth of NCTF controls, where it started as conventional NCTF control that emphasizes the motion of point-to-point. Then the NCTF control was improved in continuous motion where it has been named as Continuous Motion (CM) NCTF control. In order to enhance its motion accuracy and overshoot reduction characteristics, the acceleration element was added in the control structure, and namely Acceleration-Reference Continuous Motion (AR-CM) NCTF control. Although the control structure becomes slightly complicated, however, the design procedures of the three NCTF controls remain practical and easy. The usefulness of the NCTF controls have been clarified using the mechanisms with and without friction which in translation and rotary motions. The NCTF controls successfully demonstrated promising motion performances than the classical PID, PI-D and PDDO controllers in point-to-point, tracking and contouring motions.