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Spiral Servo Writing in Hard Disk Drives Using Iterative Learning Based Tracking Control
Abstract and Figures
The servo control in a hard disk drive (HDD) system relies heavily on the position information obtained from the servo sectors. As the tracks-per-inch (TPI) gets higher and higher in HDDs, writing of servo sectors takes a longer and longer time. One way to solve this problem is to use servo track writers to write spiral servo patterns on the disks and then perform self servo writing (SSW) within each HDD unit. The quality of the spirals is the key to achieve high TPI in SSW. This paper discusses high precision tracking control algorithms for the writing of spiral servo patterns. An iterative learning scheme is introduced, which combines a disturbance observer with the zero phase error tracking algorithm, and develops a new way of designing asymptotically optimal feedforward inputs for a general two degree of freedom control systems. In learning control schemes, non-repetitive disturbances have an adverse influence on the tracking performance. The paper analyzes this situation, and develops a robust learning scheme to deal with the problem.
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... Furthermore, this combination can be linear or nonlinear, and it is a typical intelligent control method by learning from the previous system information. In recent years, ILC has been successfully applied in robot control, 25,26 hard disk drive, 27 linear motor, 28 chemical engineering, 29 and so on. Considering the rigid rotor characteristics of the PHC and the requirement of the periodic component extraction in the AMB rotor unbalance control, an open-loop differential ILC-2 with variable forgetting factor and variable learning gain is proposed and expressed as shown in equation (7) ...
As one of the key technologies of high-temperature gas-cooled reactor, primary helium circulator–equipped active magnetic bearing provides driving force for primary helium cooling system. However, repetitive periodic vibration produced by rotor imbalance may introduce risks to primary helium circulator (even for high-temperature gas-cooled reactors). First, this article analyzes a periodic component extraction algorithm which is widely used in active magnetic bearing rotor unbalance control methods and points out the problem that the periodic component extraction algorithm occupies numerous computing resources which cannot satisfy the real-time request of active magnetic bearing control system. Then, a novel iterative learning control algorithm based on the iteration before last iteration of system information (iterative learning control-2) and a plug-in parallel control mechanism based on the existing control system are put forward, meanwhile, an integrated independent distributed active magnetic bearing control system is designed to solve the problem. Finally, both the simulation and experiment are carried out, respectively. The corresponding results show that the control method and control system proposed in this article have significant suppression effect on the repetitive periodic vibration of the active magnetic bearing system without degrading the real-time requirement and can provide important technical support for the safe and stable operation of the primary helium circulator in high-temperature gas-cooled reactor.
This paper presents an improved generalized predictive control (GPC) scheme integrated with a disturbance compensation scheme that combines iterative learning control (ILC) and real-time feedback control (RFC). A least mean square error (LMSE) estimator has been used to estimate the output error caused by repeatable disturbances. The use of this estimated error information in the ILC component aims to reduce the effect of real-time disturbances in the learning process. On the other hand, the inclusion of the current cycle error information, handled by the RFC component, allows the controller to make more immediate corrections with respect to disturbances occurring during the on-going operation. The proposed GPC-ILC-RFC-LMSE method is simulated on a two-link planar robotic manipulator that is to track a circular trajectory repeatedly. A discrete-time model of the robotic manipulator is used to predict the system output over a prediction horizon such that optimal control inputs that minimize the angular position and velocity trajectory errors can be determined. The proposed GPC-ILC-RFC-LMSE scheme succeeds to reduce the trajectory tracking errors significantly where the average MSE values is merely 40% of that of the GPC-ILC counterpart. In addition, the proposed controller is more robust if compared to the existing GPC learning methods where smoother control input profiles has been achieved.
A digital feedforward control algorithm for tracking desired time varying signals is presented. The feedforward controller cancels all the closed-loop poles and cancellable closed-loop zeros. For uncancelled zeros, which include zeros outside the unit circle, the feedforward controller cancels the phase shift induced by them. The phase cancellation assures that the frequency response between the desired output and actual output exhibits zero phase shift for all the frequencies. The algorithm is particularly suited to the general motion control problems including robotic arms and positioning tables. A typical motion control problem is used to show the effectiveness of the proposed feedforward controller.
The Q-filter cut-off frequency in a Disturbance Observer (DOB) is restricted by the model mismatch between the plant and its nominal model. DOB Robustness is usually poor when the cut-off frequency of Q-filter increases. In this paper, robustness of the DOB is improved by shaping the true plant with a cascaded frequency shaping filter, while the nominal plant model remains fixed. Once the shaping filter is optimally designed based on DOB robust stability, the cut-off frequency of Q-filter can be greatly increased without loss of stability. The proposed algorithm is implemented on a typical tracking problem, i.e., the spiral writing in hard disk drives.
There are many control methods to guarantee the robustness of a system. Among them, the disturbance observer (DOB) has been widely used because it is easy to apply and the cost is low due to its simplicity. Generally, an output signal of the system is required to construct a DOB, but for some systems such as magnetic/optical disk drive systems, we cannot measure the position output signal, but only the position error signal (PES). In order to apply a DOB to such systems, we must use an error signal instead of an output signal. We call it the error-based disturbance observer (EDOB) system. We analyze the differences between a conventional DOB system and EDOB system, and show the effectiveness of the proposed EDOB through simulations and experiments. Also, this paper proposes criteria to enhance the robustness of an EDOB system, and reveals the disturbance rejection property of the EDOB system. Finally, we propose a new method of a double Q system to improve the track-following performance. This is also verified through experiments for a DVD 12× optical disk drive system.
The disturbance observer (DOB) has been widely utilized for high-precision and high-speed motion control applications. In this note, we suggest the robustness measure of the DOB as a criterion to design the robust DOB systems. Also, we suggest its design guidelines especially for second-order systems. Experimental results for an optical disk drive system show the validity of design guidelines.
The hard disk drive is one of the finest examples of the precision control of mechatronics, with tolerances less than one micrometer achieved while operating at high speed. Increasing demand for higher data density as well as disturbance-prone operating environments continue to test designers’ mettle. Explore the challenges presented by modern hard disk drives and learn how to overcome them with Hard Disk Drive: Mechatronics and Control. Beginning with an overview of hard disk drive history, components, operating principles, and industry trends, the authors thoroughly examine the design and manufacturing challenges. They start with the head positioning servomechanism followed by the design of the actuator servo controller, the critical aspects of spindle motor control, and finally, the servo track writer, a critical technology in hard disk drive manufacturing. By comparing various design approaches for both single- and dual-stage servomechanisms, the book shows the relative pros and cons of each approach. Numerous examples and figures clarify and illustrate the discussion. Exploring practical issues such as models for plants, noise reduction, disturbances, and common problems with spindle motors, Hard Disk Drive: Mechatronics and Control avoids heavy theory in favor of providing hands-on insight into real issues facing designers every day.
To achieve ultra high track density in hard disk drives, the track-following performance should be improved by using micro-actuators and other new technologies. For practical use of an ultra high-density drive, it is essential to develop a new servo track writer. This paper introduces the high-precision, high-efficiency, low-priced servo track writers we have developed that use a newly developed sheet scale.
Improving the position control of the disk drive read/write heads
is an important step in increasing the storage capacity of a drive,
especially in the presence of internal and external disturbances. To
address this problem, the typical feedback loop of a disk drive servo
system was augmented with a disturbance observer. The disturbance
observer uses the position error signal and a nominal model of the plant
to create an estimate of the disturbance. This estimate is then used to
compensate for the disturbance effects. No additional sensors are
required, which is particularly relevant in products such as disk drives
where cost is a major concern. The effectiveness of the disturbance
observer in rejecting shock and vibration disturbances is demonstrated
in simulation and shake table experiments. The vibration experiments
showed a decrease in the position error of 61%-96% at frequencies below
100 Hz. The maximum position error due to an experimental shock
disturbance was decreased by 59%. The effects of noise in the position
error signal are also discussed
In this paper, we address the problem of precision motion control of permanent-magnet linear motors (PMLMs) under the influence of significant disturbances. We establish a mathematical model of a PMLM driven by a sinusoidal pulsewidth-modulated (PWM) amplifier, obtaining it from a describing function analysis of the essentially nonlinear characteristics. The overall model (PWM+PMLM) inevitably inherits uncertainties in the face of load changes, system parameter perturbation, noise, and inherent system nonlinearities, etc., all of which constitute disturbances to the control system that will adversely affect the precision and accuracy. We propose a robust control scheme employing a disturbance observer to address the sensitivity of the control performance to the disturbances. Real-time experimental results are provided to verify and confirm the practical effectiveness of the proposed approach.
A new access control method called SMART (Structural Vibration
Minimized Acceleration Trajectory) has been developed for hard disk
drives. The access formula is derived from a minimum-jerk cost function.
The closed loop of access mode is based on velocity control and the
target velocity to keep minimum-jerk is easily calculated as a function
of position by a DSP controller. With SMART control the high harmonics
of the actuator acceleration are suppressed in comparison with
conventional control. Residual vibration after the access operation and
spacing fluctuation of the head slider are both greatly decreased
Method and apparatus for writing spiral servo pattern in a disk drive, US Patent 7710682 Method and apparatus for writing servo data in a disk drive using spiral servo patterns
- S Mizukoshi
- M Yatsu
- H Sado
- K Ueda
- T Matsunaga
- S Kouhara
- S Nakajima
Mizukoshi, S., Yatsu, M., Sado, H., Ueda, K., Matsunaga, T., Kouhara, S., Nakajima, S., et al. (2010). Method and apparatus for writing spiral servo pattern in a disk drive, US Patent 7710682. Sado, H., Yatsu, M., Ueda, K., Matsunaga, T., Mizukoshi, S., Nakajima, S., and Kouhara, S. (2008). Method and apparatus for writing servo data in a disk drive using spiral servo patterns, US Patent 7477472.