Choong Woo Lee

Active high-frequency vibration rejection in hard disk drives

This paper presents an active vibration rejection control technique to reject actuator arm resonance modes using a PZT sensor attached to the actuator arm. A multi-rate digital implementation is proposed for high frequency active vibration rejection over the sector induced sampling frequency. The vibration control signal can be synchronized with the slow state feedback control input such that the two control signals are summed and applied to the voice coil motor. After obtaining a transfer function from head position to PZT sensor output, a method is proposed on how to design the vibration rejection controller. An application example demonstrates the utility of the proposed active vibration rejection scheme.

Design of a New Multi-Loop Disturbance Observer for Optical Disk Drive Systems

The disturbance observer (DOB) is known to be effective in enhancing the performance of dynamic systems in the presence of disturbances. In general, the disturbance rejection performance and robustness of DOBs depend on the order of the Q-filter, which is a low pass filter, and its time constant. In this paper, a new multi-loop DOB, improving the performance of DOB, is presented and its robust stability is proven using linear fractional transformation. It is possible to improve the sensitivity function with additional external-loop DOBs which do not significantly affect the complementary sensitivity function. The proposed new multi-loop DOB is applied to a commercial optical disk drive tracking servo system. Experimental results show that the proposed multi-loop DOB is an effective method in rejecting disturbance as well as improving the tracking performance.

Stability Analysis of an Electric Parking Brake (EPB) System with a Nonlinear Proportional Controller

In this paper, an Electric Parking Brake (EPB) system is modelled as a state-dependent switched system. The model involves screw friction which varies depending on the operation region. A new nonlinear proportional (P) controller is proposed and its stability is analyzed via Lyapunov and LaSalles theory. It is shown that the equilibrium point is locally uniform and ultimately bounded.

Design of Seeking Control Based on Two-Degree-of-Freedom Controller Using Frequency Shaped Final-State Control

Abstract In this paper, we introduce a new seeking control method based on the frequency shaped final-state control (FFSC). The seeking control method is a two-degree-of-freedom control, which is the plant-based feedforward control. The feedforward control input is designed through zero-order-hold using FFSC which imposes the constraints on control input magnitude and its frequency components to minimize residual vibrations. The reference generation is made through a feedforward path controller which is in the form of the zero-phase error tracking controller (ZPETC) of the nominal plant. The reference prefilter is designed to compensate the delay in the control system via the ZPETC. Mode switching control (MSC) is employed to enhance tracking performance after settling. An add-on type disturbance observer that is in the form of finite impulse response is used with the feedback controller. From the simulation results, the proposed method shows the improvement to the settling and disturbance rejection.

Design of the Tracking Controller for Holographic Digital Data Storage

In this paper we present a new tracking control method for holographic digital data storage systems. Holographic data storage systems that use disk media need a different tracking method from those of optical and hard disk drives. Here we present the design and implementation of the controller. Due to the intermittent characteristics of the tracking error signal (TES), the tracking controller is designed to regulate the tracking error only while valid TES is generated. Since the valid TES appears intermittently, we use a feedforward controller while the TES is not valid. Furthermore, the feedback control and feedforward control are mode switched depending on the radio frequency sum signal level. We also show the stability of the closed loop system employing the mode switching controller. Experimental results show the effectiveness of the proposed tracking control algorithm. For a plusmn100 mum repeatable runout at 1 Hz, we were able to reduce the TES to 3.13 mum (1sigma) with feedforward mode switching control.

An approach to discrete-time sliding mode control

This work presents a technique of discrete-time sliding mode controller design for assigning eigenvalues of sliding mode and determining a convergence rate to sliding surface. Firstly the sliding mode coefficient is designed via Ackermanns formula. Then a linear controller is designed to enforce sliding mode such that the resulting closed-loop yields the desired eigenvalues. As we use a linear control instead of nonlinear control for reaching mode, control chattering does not happen. Simulation and experimental results are included to show the effectiveness of proposed method.

Nonlinear Controller With the Dead-Zone and Saturation for Optical Disk Drive Systems in the Presence of External Shocks

In this paper, a nonlinear controller with dead-zone and saturation is proposed in order to improve the antishock performance. Since using dead-zone and saturation improves the stability margin, we can use a higher gain for the dead-zone than in the previous methods. In addition, we find that the overall controller with a linear lead-lag controller has an improved stability margin over controllers with the PID linear control in this application. The experimental results confirmed that the proposed method provides improved protection against external shock.

Minimum Residual Vibration Sliding Mode Servo Control for Computer Disk Drives

Abstract This paper presents a discrete time design of sliding mode servo control with minimum residual vibration that significantly affects settling of the disk drive head to a target track. By optimizing the acceleration rate, the convergence rate to the sliding hyperplane for seek is determined for minimum residual vibration at the end of track seek. The scheme retains the intrinsic nature of the sliding mode servo control enforcing the actuator to slide along the optimally designed reference profile such that no substantial seek performance degradation occurs at all. Simulation and experiment results are also presented to demonstrate utility of the proposed minimum residual vibration sliding mode controller.

Implementation of a Position Error Signal for Scanning Probe Microscopy-Based Data Storage Systems

In this paper, we propose a method of position error signal (PES) generation for read-only-memory (ROM) type scanning probe microscopy (SPM)-based data storage (SDS) systems. Conventionally, PES is generated by using servo bursts similar to those of hard disk drives. Recently, it was reported that stripe patterned media enables the generation of PES without using servo bursts as hard disk drives. In this paper, we report the fabrication of patterned media and its experimental results. Servo patterns are nano-imprinted on polymer medium film. We succeed in reading data and the servo patterns with a 1times 3 cantilever. We also demonstrate track follow control with the PES of the ROM type SDS through simulations.

Active high frequency vibration rejection in hard disk drives

This paper presents an active vibration rejection (AVR) control technique to reject actuator arm resonance modes using a piezoelectric sensor attached to the actuator arm to detect high-frequency off-track actuator vibration. The voice coil motor (VCM) is used as an actuator, while the piezoelectric sensor is used as a sensor only. A multirate digital implementation is proposed for high-frequency AVR over the sector-induced sampling frequency. The vibration control signal can be synchronized with the slow-state feedback control input such that the two control signals are summed and applied to the VCM. Throughout an application example the proposed AVR scheme, using the VCM as an actuator while the piezoelectric sensor as a sensor, is shown to be very prospective for high-frequency vibration rejection, provided a good vibration sensing scheme is used.