Pivot friction compensation using an accelerometer and adisturbance observer for hard disk drives

This paper considers a pivot friction compensator using the combination of an accelerometer and a disturbance observer. Noting that the pivot torque bias due to uncertain disturbances and nonlinearities such as pivot friction is approximately the difference between the scaled voice-coil motor (VCM) current and the scaled carriage angular acceleration, a disturbance observer to estimate the bias can be constructed using the above two continuously measured signals. By feeding back the bias estimate into the VCM amplifier, the frequency response from the VCM current to the carriage angular acceleration is linearized to be a constant gain within the observer bandwidth. The proposed cancellation scheme provides wider bandwidth bias rejection and better settling performance than traditional bias compensation schemes. Experiments have confirmed that the cancellation scheme is effective in the frequency range 0-700 Hz     

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.     

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     

Switching control in blu-ray disk drives

To improve the nominal PID-controlled servo performances of a blu-ray disk drive, a switching observer is studied. Switching is done using a deadzone switching gain having the following properties: large-magnitude signals induce extra gain, small-amplitude signals do not. Additionally the observer part assures that large-magnitude signals induce extra low-pass filtering. As a result the design has low-frequency disturbance rejection properties resulting from high-gain feedback but with a favorable high-frequency noise response related to the properties of low-gain feedback. Robust stability is guaranteed using a Lyapunov characterization of input-to-state stability, whereas improved performances under vibrations and disk surface defects are demonstrated using two blu-ray disk drive setups.     

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.     

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.     

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.     

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.     

Improved adaptive observer with optimal convergence

A simple technique for simultaneous parameter identification and state estimation of linear discrete systems is presented. The gradient method is used for identification and the global stability of the scheme is guaranteed by the use of Lyapunov's method. The optimal adaptive gains are also considered for more rapid convergence.     

Mode switching control design with initial value compensation andits application to head positioning control on magnetic disk drives

To meet an increasing demand for high performance and large-capacity in magnetic hard disk drives, both fast response and precise positioning are strongly required for the head positioning control. A mode switching control (MSC) system, which includes a track seeking controller, a track following controller, and a switching function, is widely used to meet this requirement. The issue for MSC is to confirm a design method of servo mode switching. This paper proposes the initial value compensation method (IVC). The IVC inputs certain initial values in the controller state variables at mode switching in order to improve the transient response after switching. There are two approaches to determine the values: (1) minimizing a linear-quadratic function of the plant state variables; and (2) cancelling the unfavorable poles of the transfer function between initial values and head position by relocating zeros. Some experimental results are shown to be effective     

A hybrid speed estimator of flux observer for induction motor drives

This paper proposes a hybrid speed estimator that gives the synergetic effect between the model- and the saliency-based field orientations for induction motor drives. The model-based field orientation consists of a flux observer with an adaptive speed estimator that has unstable regions at zero frequency and zero speed. Saliency-based flux orientation utilizes magnetic saliencies caused by saturation and high-frequency injection that causes the torque ripples due to the chattering. The chattering is caused by the higher cutoff frequency of the flux-angle estimation to keep its high dynamics. The proposed method compensates both faults and realizes complete speed estimation from zero to high-speed condition including zero stator frequency.     

A MRAS-based adaptive pseudoreduced-order flux observer for sensorless induction motor drives

The performance of vector-controlled sensorless induction motor drives is generally poor at very low speeds, especially at zero speed due to offset and drift components in the acquired feedback signals, and the increased sensitivity of dynamic performance to model parameter mismatch resulting especially from stator resistance variations. The speed estimation is adversely affected by stator resistance variations due to temperature and frequency changes. This is particularly significant at very low speeds where the calculated flux deviates from its set values. Therefore, it is necessary to compensate for the parameter variation in sensorless induction motor drives, particularly at very low speeds. This paper presents a novel method of estimating both the shaft speed and stator resistance of an induction motor. In this novel scheme, an adaptive pseudoreduced-order flux observer (APFO) is developed. In comparison to the adaptive full-order flux observer (AFFO), the proposed method consumes less computational time, and provides a better stator resistance estimation dynamic performance. Both simulation and experimental results confirm the superiority of the proposed APFO scheme for a wide range of resistance variations from 0 to 100%.     

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     

Stabilization of regenerating-mode operation in sensorless induction motor drives by full-order flux observer design

This paper deals with the full-order flux observer design for speed-sensorless induction motor drives. An unstable region encountered in the regenerating mode at low speeds is well known. To remedy the problem, a modified speed-adaptation law is proposed. Instead of using only the current estimation error perpendicular to the estimated flux, the parallel component is also exploited in the regenerating mode. Using current estimation error loci in steady state, a linearized model, simulations, and experiments, it is shown that the observer using the proposed speed-adaptation law does not have the unstable region. It is also shown that the effect of erroneous parameter estimates on the accuracy of the observer is comparatively small.     

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     

Improved digital current control methods in switched reluctancemotor drives

This paper proposes a method to avoid current feedback filters in fast digital-based current loops in switched reluctance drives. Symmetrical pulsewidth modulation (PWM) and synchronized sampling of the phase current allow a noise-free current sampling with no antialiasing filter. This paper also proposes more efficient methods to chop the two transistors in the asymmetric inverter used with switched reluctance drives. A fast field-programmable gate array (FPGA)-based test system is used for validation of the new methods. Test results show a significant improvement in dynamic and steady-state current loop control compared with traditional methods. The new chopping method is found to reduce the switching losses and increase the drive efficiency     

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