Xinqun Liu

Head-disk spacing variation suppression via active flying height control

A novel active flying height control method is proposed for the suppression of head-disk spacing variation in hard disk drives under shock or vibration conditions. The thermal signal detection method is used for real-time derivation of the spacing variation signal for feedback control. Design details of the servo control system are given. Simulation results are provided to demonstrate the effectiveness of the method.

Polarization interferometer for measuring small displacement

A homodyne polarization laser interferometer is presented for high-speed measurement of small displacements. No modulation technique is used, so the opto-mechanical setup is relatively simple. The dual-beam arrangement enables the displacement to be measured while the use of polarization interferometry enables the determination of the directional nature of the displacement. Another feature of this interferometer lies in the fact that it is also suitable for the measurement of head-media spacing of a hard disk drive. Combined with the electronics used at present, sub-nanometer resolution is achievable. Its measurement bandwidth is limited only by the sampling rate of the A/D board being used.

Real-time method to measure head disk spacing variation under vibration conditions

Three readback signal detection methods are presented for real-time head disk spacing variation measurement under vibration conditions. The proposed PW/sub 50/ parameter estimation method is not restricted by the isolated readback pulse signal requirement and, thus, can be used for spacing variation measurement in normally operated hard disk drives, especially for low-frequency variations. The thermal signal detection method, on the other side, is more effective for high-frequency spacing variation measurement. Amplitude detection gives a rough but simple way to study the head disk spacing change. Theoretical analysis and experimental results are given to illustrate the effectiveness of the methods.

Normal incidence polarization interferometry flying height testing

In this paper, a dual-beam normal incidence polarization interferometer is presented to measure the flying height or head-disk spacing. It has the advantages of both the intensity interferometry method and the oblique incidence polarization interferometry method. With this polarization interferometer, not only can the flying height be measured down to contact without losing accuracy, but also the pitch and roll of the head-slider can be detected dynamically. The optical parameters of the head-slider can also be determined. Design details and experimental results are given.

Improved intensity interferometry method for measuring head-disk spacing down to contact

In this paper, we give a detailed analysis of the working principle, application limits, and potential problems of the intensity interferometry flying height testing, one of the most popularly-used flying height testing techniques. Then, phase-shift methods are proposed to improve the sensitivity of this technique when the head-disk spacing is below 10 nm or near contact. This is based on fabricating the glass testing disk to have a specified thickness, or having it coated with a specified thickness thin carbon film. Theoretical analysis, numerical evaluation, and experimental considerations are presented.

Ultra low head-disk spacing measurement using dual beam polarisation interferometry

Abstract In this paper, a dual-beam normal incidence polarisation interferometer is proposed to measure the flying height or head–disk spacing. Using this interferometer, the head–disk spacing can be measured both in magnetic real disk condition and in glass disk condition. It has the advantage of both the currently popular intensity interferometry method and the oblique incidence polarisation interferometry method. With this polarisation interferometer, not only the flying height can be measured down to contact without losing accuracy, but the pitch and roll of the head-slider can also be detected dynamically. The optical parameters of the head-slider can also be determined. Design details and experimental study are presented.

Interferometry method for measuring head-disk spacing down to contact

In this paper, we give a detailed analysis of the working principle, application limits, and potential problems of the intensity interferometry flying height testing, one of the most popularly used flying height testing techniques. Then, a phase-shift method is proposed to improve the sensitivity of this technique when the head-disk spacing is below 10 nm or near contact, based on manufacturing the glass testing disk to have a thickness within specified tolerances. Theoretical analysis and numerical evaluation are presented.

Characterisation of MEMS Devices Using A Polarisation Interferometer

A potential application for ferroelectric thin films is micro positioning and actuation, as in MEMS devices. The amount of actuation possible is determined by a number of factors: the piezoelectric coefficient d 31 , geometric factors and the compliance of both the actuator and cantilever and the electric field across the film. It is important for their realisation as devices in applications that these micro-actuators are characterised. Based upon an interferometer developed in our laboratory to measure the flying height or head-disk spacing in a hard disk drive, an optical system is proposed which utilises the same principles to enable MEMS devices to be characterised with high-resolution and at high-speed.

Combined polarizing interferometer and optical beam deflection system for MEMS characterization

A potential application for ferroelectric thin films is micro positioning and actuation, as in MEMS devices. The amount of actuation possible is determined by a number of factors: the piezoelectric coefficient d31, geometric factors and the compliance of both the actuator and cantilever and the electric field across the film. It is important for their realization as devices in applications that these micro-actuators are characterized. One such means is to use optical beam deflection (OBD). However, whilst extremely simple to implement, optical beam deflection does not provide an absolute measure of displacement. For absolute displacement measurement, with directional determination, a dual-beam normal incidence polarization interferometer is required. Based upon an interferometer developed in our laboratory to measure the flying height or head-disk spacing in a hard disk drive, an optical system is proposed which enables both an OBD and a polarization interferometer to be combined in one compact system. Details of both systems and are presented and the combined system described.

A microprocessor based variable structure control system for synchronous high speed film cameras

A microprocessor-based fully digital DC phase lock servo system has been designed for the JX-300 series synchronous high-speed film cameras. A discrete variable structure integral controller was proposed and used in the system, both in speed loop and phase-locking loop. The controller is practically a quasi-sliding mode discrete variable structure controller cascaded with an integral controller, so it possesses the optimum blend of properties of these two controller types.