Improved track following in magnetic disk drives using adisturbance observer

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     

Head-positioning control using resonant modes in hard disk drives

The best way to enhance the input/output (I/O) performance of a hard disk drive is by increasing the spindle speed. Therefore, the effect of windage vibrations caused by the airflow increases as the spindle speed increases. The servo bandwidth is limited by the primary resonant frequency of the mechanical system. However, the frequencies of the windage vibrations are higher than the primary resonant frequency. Accordingly, these frequencies are also above the servo bandwidth and are too high to be controlled by a conventional control system. In response to this problem, we have developed two methods for designing a servo control system that can suppress the windage vibrations. One method uses a stable mechanical resonant mode, and the other uses a stable resonant mode created by a digital filter. By using these methods, the head-positioning system can control the vibrations above the frequency of the primary resonant mode and the servo bandwidth. Application of these methods to actual hard disk drives showed that they can greatly decrease the windage vibrations, in which the peak frequency is about six times the open-loop gain 0-dB crossover frequency.     

Microfabricated electrostatic actuators for hard disk drives

A dual-stage servo system which utilizes a high-bandwidth secondary actuator has been proposed for magnetic hard disk drives. Microfabricated actuators are promising candidates for this secondary actuator, since they offer the benefits of extremely small size and weight and may be batch fabricated for reduced production cost. This paper presents the design of an electrostatic microactuator which produces sufficient output force to move a 1.6-mg picoslider from a conventional disk drive at frequencies above 2 kHz. A linear second-order actuation model is developed and fit to experimental data. The prototype device is shown to have an open-loop resonance frequency of 532 Hz when loaded with the picoslider, and a DC-voltage-to-position gain of 0.12 μm/V. A closed-loop bandwidth of 2.5 kHz is achieved using a simple PD controller     

Wise drives [hard disk drive]

Considers how smarter disk drives could herald speedier data mining, tighten computer security, and improve data storage reliability, but only if drive and computer makers and software designers can agree on new interface standards.     

Novel reference signal generation for two-degree-of-freedomcontrollers for hard disk drives

This paper considers the determination of reference trajectories for use with a two-degree of-freedom (2-DOF) controller for the head positioning of hard disk drives (HDD). The reference trajectories are designed in two stages-off-line simulation and online reference generation. The HDD model used for the design includes the back electromotive force of the voice coil motor. In the second stage, the idea of structural vibration minimized acceleration trajectory is utilized to adjust the errors at the end of the trajectories. 2-DOF control with the proposed reference trajectory is compared to conventional mode-switching control, and its effectiveness is verified     

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.     

MEMS-based integrated head/actuator/slider for hard disk drives

We propose a new integrated head/actuator/slider concept for hard disk drives. This slider is batch fabricated on the sacrificial layer of a wafer together with an electrostatic microactuator via micromachining technology. We present the basic integrated head/actuator/slider design, the fabrication and flying-height tests of a micromachined slider body, and the design, fabrication, and testing of a prototype electrostatic microactuator. Slider warpage was analyzed and successfully suppressed to within 0.1 μm. The slider was flown over a glass disk at heights of 26-81 nm and at linear velocities of 5-13 m/s. The test results of the electrostatic microactuator showed a stroke of 0.55 μm, a very high mechanical resonant frequency of 34 kHz due to its low moving mass of 0.85 μg, and a large force, estimated to be 21.3 μN, generated by the actuator     

A simulation and control design environment for single-stage anddual-stage hard disk drives

We describe an integrated tool for the design and evaluation of the performance of different control algorithms and strategies, applied to hard disk drives (HDDs) with single-stage actuators and dual-stage actuators. The tool is composed of a set of procedures for the computer-aided design of servocontrollers and a HDD simulator. In order to perform realistic simulations, each part of the HDD is described with high level of detail. As for the mechanics, the usual inertia plus resonance model of the voice coil motor (VCM) has been enriched with nonlinear friction modeling and, in case of DSAs, with the dynamic coupling between primary and secondary actuator. The nonlinear friction model has been tuned on experimental data, using an experimental system presented in the paper. As for the electronics, the VCM driver model is included and quantizations in the position error signal measurement and computation are explicitly considered. Also, in order to evaluate the tracking performance of different servocontrollers, repetitive run outs and nonrepetitive run outs are included. Such disturbances have been obtained from commercial HDDs, with a procedure that is detailed in the paper. The simulator is then validated by comparing simulation and experimental results, both in open-loop and closed-loop conditions, thus, confirming the effectiveness of the developed tool     

Settling control and performance of a dual-actuator system for hard disk drives

This paper presents a settling control of a dual-actuator system for hard disk drives. The dual-actuator system consists of a voice coil motor (VCM) as a first stage actuator and a push-pull-type piezo-electric transducer (PZT) as a second-stage actuator. The settling controller is designed in three steps. In the first step, the VCM controller is designed so that the VCM feedback loop has basic performance and appropriate stability. In the second step, the PZT controller and a decoupling filter are designed in order to achieve superior performance of the dual-actuator system. The decoupling filter, which is placed between the PZT controller output and the VCM controller input, is a PZT output estimator so that the PZT actuator output is canceled at the VCM controller input. In the third step, the reference trajectory is designed for fast and smooth settling. In this study, the closed-loop sensitivity function is used as a performance index, and the gain and phase margins of the open-loop characteristic are used as stability measures. Experimental results show that the dual actuator system with the proposed settling controller achieves better performance than a single actuator system with the same VCM and a conventional settling controller.     

Short track seeking of hard disk drives under multirate control-computationally efficient approach based on initial value compensation

This paper proposes a design method for short track-seeking control based on one degree of freedom (ODOF) control and initial value compensation (IVC). IVC uses nonzero initial values of the feedback controller to improve the step reference response of the ODOF tracking control system. This makes feedforward control unnecessary to shape the transient response of short track seeking. As a result, the amount of computation during short track seeking may be minimal. The proposed design method minimizes tracking errors in multirate control framework for a step reference input taking into account the inter-sampling behavior. The effectiveness of the proposed method is shown by simulation and experiment.     

Disturbance suppression beyond Nyquist frequency in hard disk drives

In conventional hard disk drives, a control system compensates for vibration in which the frequency is higher than the Nyquist frequency by using a multi-rate filter that decreases the gain above the Nyquist frequency. However, such a control system can only avoid instability and cannot suppress disturbances above the Nyquist frequency. To overcome this problem, a control system design method that suppresses disturbances beyond the Nyquist frequency is proposed. This method uses frequency responses of a controlled object and a digital controller to calculate the gain of the sensitivity function in a sampled-data system without requiring complex calculations involving matrices, and realizes a stable resonant filter that decreases the gain of the sensitivity function above the Nyquist frequency. Significant suppression of the vibrations caused by the disturbances beyond the Nyquist frequency is demonstrated by implementing this method in the head-positioning system of a hard disk drive.     

A design for high noise rejection in a pseudodifferentialpreamplifier for hard disk drives

A study of supply and system noise rejection for a pseudodifferential amplifier is presented in this paper. This pseudodifferential amplifier is aimed at high data-rate disk drive signal sensing and preamplification applications. This high rejection was achieved by improving the rejection of the pseudodifferential amplifier and also by carefully designing the interconnect flex circuit where the preamplifier is mounted. The measured rejection to power supply, ground and system noise is above 50 dB over a 300 MHz bandwidth. This is significant for a pseudodifferential amplification system. The gain of the preamplifier is 47 dB and write mode to read mode switching time is 210 ns. This preamplifier currently supports disk drive data rates over 270 Mb/s     

Robust friction state observer and recurrent fuzzy neural network design for dynamic friction compensation with backstepping control

In this article, we investigate a robust friction compensation scheme for the purpose of accomplishing high-precision positioning performance in a servo mechanical system with nonlinear dynamic friction. To estimate the friction state and tackle the robustness problem for uncertainty, a recurrent fuzzy neural network (RFNN) and reconstructed error compensator as well as a robust friction state observer are developed. The asymptotic stability of the series of friction compensation methodologies are verified from the Lyapunov’s stability theory. Some simulations and experiments on a frictional servo mechanical system were carried out to evaluate the effectiveness of the proposed control scheme.     

Estimation method for unobservable settling vibration of head-positioning control in hard disk drives

To improve reliability in hard disk drives, we proposed an estimation method for unobservable settling vibration in head-positioning control systems. Our proposed method uses both observability and excitability of mechanical vibrations in settling time of a track-seeking control. To see the observability, we employed the estimation method of unobservable amplitude of oscillations caused by mechanical resonances. To see the excitability, we also employed the Shock Response Spectrum (SRS) analysis that can handle the transient characteristics of mechanical vibrations. Simulations and experiments results on the track-seeking control showed that the results of our proposed method were good approximate solutions of the unobservable settling vibrations. This means that we can estimate the risk of the unobservable settling vibrations which may lead to destruction of user data in hard disk drives by using the proposed method. As a result, we can chose the best suited solution to avoid unobservable vibrations in the head-positioning control systems.     

An analog DFE for disk drives using a mixed-signal integrator

An analog adaptive decision-feedback equalizer (DFE) is described. The DFE cancels intersymbol interference using four feedback taps, and a fifth tap cancels dc offset. The coefficient for each tap is adapted using a small mixed-signal integrator. The DFE dissipates 220 mW at a data rate of 150 Mb/s. The active area is 1.8 mm² in a 1-μm CMOS process     

On-line dead-time compensation method using disturbance observer

A new on-line dead-time compensation method for a permanent magnet (PM) synchronous motor drive is proposed. Using a simple disturbance observer without any additional circuit and off-line experimental measurement, disturbance voltages in the rotor reference dq frame caused by the dead time and nonideal switching characteristics of power devices are estimated in an on-line manner and fed to voltage references in order to compensate the dead-time effects. The proposed method is applied to a PM synchronous motor drive system and implemented by using software of a digital signal processor (DSP) TMS320C31. Simulations and experiments are carried out for this system and the results well demonstrate the effectiveness of the proposed method.     

Minimization of unobservable oscillation in repeatable run-out for magnetic-head positioning system of hard disk drives

A positioning control system for a magnetic head in hard disk drives (HDDs) is a sampled-data control system that has a sampler and a hold. As a result, the control system has unobservable oscillations in a repeatable run-out (RROs) whose frequencies are above the Nyquist frequency. These unobservable oscillations may lead to destruction of user data in HDDs. To solve this problem, this paper presents a minimization method for the unobservable oscillation in the RROs. This minimization method focuses on a relationship between RRO frequencies and a sampling frequency of the sampled-data control system. To estimate amounts of the unobservable RROs beyond the Nyquist frequency, this study employs two types of unobservable magnitudes: a maximum value based magnitude, and a root-mean-square value based magnitude. By using these unobservable magnitudes, we can choose the best suited sampling frequency to minimize the unobservable oscillations in RROs of HDDs.     

Nonlinear compensator design for active sliders to suppress head-disk spacing modulation in hard disk drives

As the slider's flying height (FH) continues to reduce in hard disk drives, the flying height modulation (FHM) due to disk morphology and interface instability caused by highly nonlinear attractive forces becomes significant. Based on the concept that the FH of a portion of the slider that carries the read/write element can be adjusted by a piezoelectric actuator located between the slider and suspension and that the FH can be measured by the use of the magnetic readback signal, a new 3-DOF analytic model and an observer-based nonlinear compensator are proposed to achieve ultralow FH with minimum modulation under short-range attractive forces. Numerical simulations show that the FHM due to disk waviness is effectively controlled and reduced.     

Adaptive friction compensation using neural network approximations

We present a new compensation technique for a friction model, which captures problematic friction effects such as Stribeck effects, hysteresis, stick-slip limit cycling, pre-sliding displacement and rising static friction. The proposed control utilizes a PD control structure and an adaptive estimate of the friction force. Specifically, a radial basis function (RBF) is used to compensate the effects of the unknown nonlinearly occurring Stribeck parameter in the friction model. The main analytical result is a stability theorem for the proposed compensator which can achieve regional stability of the closed-loop system. Furthermore, we show that the transient performance of the resulting adaptive system is analytically quantified. To support the theoretical concepts, we present dynamic simulations for the proposed control scheme