Jingliang Zhang

Brief paper: Neural network compensation control for mechanical systems with disturbances

Two novel compensation schemes based on accelerometer measurements to attenuate the effect of external vibrations on mechanical systems are proposed in this paper. The first compensation algorithm exploits the neural network as the feedback-feedforward compensator whereas the second is the neural network feedforward compensator. Each compensation strategy includes a feedback controller and a neural network compensator with the help of a sensor to detect external vibrations. The feedback controller is employed to guarantee the stability of the mechanical systems, while the neural network is used to provide the required compensation input for trajectory tracking. Dynamics knowledge of the plant, disturbances and the sensor is not required. The stability of the proposed schemes is analyzed by the Lyapunov criterion. Simulation results show that the proposed controllers perform well for a hard disk drive system and a two-link manipulator.

Radial error propagation issues in self servo track writing technology

Conventional servo track writing needs external devices to position heads in hard disk drives to write servo patterns. In self-servo track writing (SSTW), the position reference is regenerated from previous written tracks and a servo pattern is propagated to cover the disk in a progressive mode. Errors will propagate and compound in this process. In this paper, the radial error propagation is discussed and a containment scheme is established. Furthermore, experimental results of SSTW on a head-disk assembly with blank media and off-the-shelf electronics are analyzed to show the effectiveness of error containment in self-radial propagation.

Disturbance Rejection for a Data Storage System via Sensitivity Loop Shaping and Adaptive Nonlinear Compensation

This paper investigates the disturbance rejection problem for a data storage servo system that contains a microactuator for high-accuracy positioning. For disturbances with known frequencies, their rejection is accomplished via the generalized Kalman-Yakubovic-Popov lemma together with the Youla parameterization approach, and a linear controller is computed via solving a number of linear matrix inequalities. To further improve the rejection of low-frequency disturbances such as nonlinear disturbances arising from friction torque or bias or other unknown disturbances, an adaptive nonlinear compensation scheme is adopted to cancel their effects on the positioning accuracy. Simulation together with implementation results demonstrate that the proposed design offers a remarkably improved positioning accuracy.

Disturbance modeling and control design for self-servo track writing

The major disturbances in self-servo track writing are identified and modeled. Based on the disturbance models, an H/sub 2/ controller together with a feedforward compensator is designed via linear matrix inequality approach to minimize the propagation tracking error from one track to the next. Furthermore, the error propagation containment effectiveness of the optimal H/sub 2/ control is compared with that of proportional-integral-derivative (PID) and proportional-derivative (PD) feedback controls with the feedforward compensators. Our results show that the propagation tracking error is improved by 27% with the H/sub 2/ control compared with that by PID control.

Phase lead peak filter method to high TPI servo track writer with microactuators

A servo track writing (STW) platform with dual piezoelectric (PZT) microactuator is proposed for higher track density hard disk drives (HDDs). We mainly deal with the servo control issues to achieve high track density. Vibration and noise analysis based on the measured position error signal (PES) indicates that the PES is significantly disturbed by narrow-band vibrations. Hence, we propose a general second-order phase lead peak filter (PLPF) that is applicable to reject narrow-band disturbances at all frequency ranges. The filter zero is designed to minimally degrade the closed-loop system stability and obtain a smooth sensitivity curve around the disturbance frequency. A feedback controller is employed to stabilize the servo loop and then a PLPF is embedded to further suppress the specific vibrations. The readback position errors are evaluated and 6.4 nm 3sigma value of the true PES NRRO is achieved, corresponding to 395k track-per-inch (TPI) track density

A generalized KYP lemma based control design and application for 425 kTPI servo track writing

The recently developed generalized Kalman-Yakubovic-Popov (KYP) Lemma establishes the equivalence between a frequency domain property and a linear matrix inequality (LMI), and allows to solve a certain class of system design problems with multiple specifications on gain/phase properties over several frequency ranges. This paper applies the KYP lemma in a control design for microactuator to suppress the narrow-band disturbances through shaping sensitivity functions in data storage systems such as servo track writers for hard disk drives (HDDs). The KYP Lemma is applied together with the Youla parameterization approach to allow a convex optimization for achieving the desired specifications of sensitivity functions. Both LDV based experiment and implementation in a servo track writer demonstrate the effectiveness of the design method in rejecting specific frequency disturbances. In particular, the design method improves the track density of the servo track writer with 425k track-per-inch (TPI)

Compensation of VCM Actuator Pivot Friction Based on an Operator Modeling Method

The pivot friction of voice-coil-motor (VCM) actuator is measured for a 1.8-in small disk drive with disk rotating and slider flying. The measurement is carried out under the conditions that the actuator is controlled and the head movement amplitude is growing by changing the references. The hysteresis of friction versus head position is then obtained. An operator-based modeling approach is adopted for the hysteresis, and an optimal model is obtained by minimizing the energy gain between the head position and the modeling error. It is also found that the frequency response of the actuator model with the inclusion of the hyteresis model matches well with the measured frequency response of the actuator. A friction compensation method based on the nonlinear hysteresis model is thus proposed. The simulation and implementation results demonstrate a significant improvement in disturbance rejection at low frequencies.

Vibration analysis and control design comparison of HDDs using fluid bearing and ball bearing spindles

This paper identifies the system model and major disturbances in a hard disk drive (HDD) with fluid bearing spindle motor and applies linear matrix inequality (LMI) approach to design an H/sub 2/ controller for the HDD. Experimental measurement of the heads position signal in ontrack mode is used to determine the spectrum of the position. The modeling of disturbances is based primarily upon the spectral decomposition of the position signal and is appropriate when the measurement point of position error signal (PES) is not available on HDD. Upon determining the disturbances, an H/sub 2/ controller is designed via LMI approach to achieve a minimal track mis-registration (TMR) for the HDD. The simulation results show the effectiveness of the H/sub 2/ controller to minimize TMR. Also, the performance comparison of H/sub 2/ control in the fluid bearing drive and a ball bearing drive is made.

External vibration compensation via loop shaping and H ∞ feedforward control design for hard disk drives in mobile applications

This paper focuses on compensation for external vibration effect on the positioning accuracy of hard disk drives. A feedforward controller is designed to attenuate the effect with the aid of a sensor to detect external vibrations. Loop shaping in frequency domain and H∞ method in state space are used to design the feedforward controller. The application results in a 1.8-inch disk drive with an accelerometer to measure acceleration signal show that 70% and 97% reduction of the position error signal can be achieved with the feedforward control designed by the loop shaping and H∞ method respectively. The robustness of the two methods is also evaluated and compared for external vibrations with amplitude variations.

Modeling and Compensation of Pivot Nonlinearity in Hard Disk Drives

In this paper the authors propose a new, straightforward approach to derive model of the pivot nonlinearity in the hard disk drive. The proposed nonlinear model is not based on classical pivot nonlinearity models. It is derived using frequency domain measurements obtained for different amplitudes of the actuator excitation current. One of the advantages of the obtained model is that it is derived using actual experimental measurements conducted under the real operating conditions of the hard disk drive. Experimental results show good match of the derived model with both the frequency and time domain measurements. The derived model is subsequently used for feedforward compensation of the pivot nonlinearity and the experimental results are presented. Simulation studies are carried out for adaptive neural network compensation using the derived model.