Tae-Yong Doh

Technical Communique: A Robust Approach to Iterative Learning Control Design for Uncertain Systems

This note proposes a frequency-domain design method to achieve an iterative learning controller for uncertain feedback control systems. For a linear time-invariant plant with multiplicative perturbations and a feedback controller, we derive a sufficient condition for the iterative process to converge for all admissible plant uncertainties. Based on the derived sufficient condition, we show that the iterative learning control design problem can be reformulated as a general robust control problem setup. Consequently, the iterative learning controller under plant uncertainty can be systematically designed by wellestablished robust control design procedures.

Robust direct seek control for high-speed rotational optical disk drives

This paper presents a new direct seek control scheme (SCS) that provides fast data access capability and robust performance for high-speed rotational optical disk drives (ODD). When a disk is rotated at a high speed to obtain fast data transfer in ODD, the magnitude and frequency of disturbances caused by eccentric rotation of the disk increase in proportion to the rotational speed of the disk. Such disturbances make it almost impossible for the conventional SCS to achieve stable and satisfactory seek performance. We analyze the problems that may arise when the conventional SCS are applied to high-speed rotational ODD and propose a new direct SCS that will solve such problems. In the proposed scheme, a seek control system is designed such that its performance is guaranteed for a set of plants with parameter perturbations. The performance of the proposed SCS are shown by experiments using a 24/spl times/CD-ROM drive.

Compensator design for a dual-stage actuator in the track-following servo system of optical disk drives

In this paper, a compensator design method is presented for track-following control in an optical disk drive, a common mechanism for which is a dual-stage actuator composed of a fine and coarse actuator. Most of previous works, however, have focused on tracking precision enhancement using only the fine actuator without systematic design method for a coarse actuator compensator. As the dual-stage actuator is categorized as a dual-input single-output system, the overall stability is affected not only by the fine actuator but also by the coarse actuator. In addition, the coarse actuator compensator should satisfy a performance specification to prevent overall system failure. Based on some obvious differences in dynamic characteristics between the two actuators, two compensators can be designed independently. Then overall system stability is ensured by the designed compensators, and performance specifications are also satisfied. Validity of the design method is proved by some simulation and experiment.

Robust ILC and current feedback for uncertain linear systems

To control an uncertain plant with iterative learning control (ILC), robust convergence is an important issue. The feedback controller plays a significant role as the learning controller does in the ILC system since it robustly stabilizes the uncertain plant and determines the convergence. To deal with not only convergence but also stability in ILC, we take into consideration of an ILC scheme with current feedback in this chapter. First, a sufficient condition for robust convergence and robust stability free from model uncertainty is obtained via structured singular value (µ) and linear fractional transformations (LFTs). Secondly, a synthesis method is presented, which is based on the proposed condition and D-K iteration. In this method, a feedback controller and learning controllers can be designed at one time and a weighting function is introduced to increase the learning performance. Finally, through a computational experiment, we confirm the feasibility of the proposed method.

An iterative learning control for uncertain systems using structured singular value

To deal with an iterative learning control (ILC) system with plant uncertainty, a set of new terms related with robust convergence is first defined. This paper proposes a sufficient condition for not only robust convergence but also robust stability of ILC for uncertain linear systems, including plant uncertainty. Thus, to find a new condition unrelated to the uncertainty, we first separate it into a known part and uncertainty one using linear fractional transformations (LFTs). Then, robust convergence and robust stability of an ILC system is determined by structured singular value (μ) of only the known part. Based on the novel condition, a learning controller and a feedback controller are developed at the same time to ensure robust convergence and robust stability of the ILC system under plant uncertainty. Lastly, the feasibility of the proposed convergence condition and design method are confirmed through computer simulation on an one-link flexible arm.

New fine seek control for optical disk drives

An optical disk drive has an excellent advantage of random accessibility. However, increased rotational velocity of a disk and limitations of mechanical structure have hindered implementation of direct seek control despite its effectiveness in reducing access time. A two-stage seek operation is instead adopted in conventional optical disk drives. In this case, fine seek control is restricted in its applicable range by the risk of misalignment between objective lens and laser beam axis, which causes the access time to increase enormously. In the paper, a control algorithm for extending the applicable range of fine seek is proposed with an appropriate control structure. The conventional fine seek technique utilized only a fine actuator without any maneuvering of a coarse actuator. With assistance of the coarse actuator, however, the misalignment of objective lens is compensated and the range of fine seek is extended in result. The proposed algorithm is applied to an optical disk drive to show its feasibility and some results are presented.

Auto-adjustment of the Objective Lens Neutral Position in Optical Disc Drives

Optical axis misalignment, which represents deviated neutral position of the objective lens from the optical axis, is an inevitable assembly error in an optical pick-up. Since the laser power intensity varies with respect to the distance from the optical axis, the misalignment leads to variation of the laser spot power intensity, which is one of the critical factors increasing data bit-error-rate in optical disc drives. In this paper, an auto-adjustment scheme of the objective lens neutral position is proposed to eliminate the undesirable variation of the laser spot power intensity in optical disc drives. An envelope of the data signal is extracted and utilized to detect the optical axis misalignment. Then an adjustment input is added to the driving input of the fine actuator to align the objective lens to the optical axis. Finally, the feasibility is verified by a series of experiments.

Enhanced Track Jump Stability in Optical Disc Drives

Track jump control is a random access strategy for short distance movement. The most common track jump scheme is a bang-bang control of a kick and brake manner. In a conventional track jump scheme, a track-following compensator is turned off during kick and brake periods, and restarted at a target track for track pull-in. The inevitable controller switching with non-zero initial condition results in undesirable transient response, and excessive overshoot in the transient response causes track pull-in failure. In this paper, a new track jump scheme is proposed for enhancing track jump stability. Instead of control switching, internal states of a track-following controller are artificially manipulated for kick and brake actions in a digital control environment. Experimental results are provided in comparison with conventional track jumps.

A QFT design of disturbance observer for the track-following control system of an optical disk drive

For track-following control in an optical disk drive, a compensator should be designed to satisfy disturbance attenuation property, overall system stability, and so on. In order to ease the compensator design procedure by reducing trial and error, a disturbance observer is added on to a conventional single loop track-following control system, which may cause control input saturation due to excessive loop gain. Because the track-following control system is conditionally stable, control input saturation leads to actual reduction of loop gain whereby the system becomes unstable. To preserve the overall stability during transient response, where control input saturation possibly occurs, the circle criterion based on the pull-in condition is utilized to check the stability under control input saturation. The constraints from the circle criterion and robust stability condition are converted into frequency domain bounds, which are represented on a Nichols chart. Then a design method using quantitative feedback theory (QFT) is applied to satisfy the constraints shown on the Nichols chart. To show the feasibility, some experiments are conducted, and the results are presented.