Hard Disk Drive Servo Control Research Articles

Add-On Feedforward Compensation for Vibration Rejection in HDD

The applications of the hard disk drive (HDD) in mobile and compact devices pose higher requirement on the vibration-rejection capability of HDD servo control. A novel vibration-compensation method is proposed in this paper. It is an add-on feedforward scheme, and the add-on feature will be turned on only in the specific vibration environment. The scheme comprises two parts: the vibration detection through the analysis of the vibration sensor output, and the vibration compensation through the add-on feature. One major advantage of this method over conventional methods applied in HDD is that the vibration rejection in some frequencies can be improved without the compromise of the performance in other frequencies. The scheme is not based on an accurate modeling, therefore, it is less sensitive to the variation of the plant parameters. The transient response in switching on or off the add-on feature is shortened by the reset control. The scheme is implemented in real HDD, and the experimental results prove the effectiveness of the scheme.

Evolutionary design and implementation of a hard disk drive servo control system

The ever increasing demand for higher storage capacity and smaller magnetic hard disk drives have driven the need of developing a high performance head positioning servo control system. To meet the challenge, this paper presents the design and real-time implementation of a robust two-degree-of-freedom servo system for physical 3.5-in. hard disk drive with single voice-coil-motor actuator using a multi-objective evolutionary algorithm toolbox. Besides the simplicity in controller structure, such an evolutionary servo control system is capable of meeting various performance specifications of hard disk drives in both the time and frequency domains. It is shown that the servo system optimally moves the magnetic head onto the desired track with minimal control effort, and keeps it on the track robustly against plant uncertainties or runout disturbances. Validation results of the evolutionary servo control system are compared with classical PID and RPT controllers, which show excellent closed-loop response and robustness in the face of practical perturbations in HDD.

Fractional Order Disturbance Observer for Robust Vibration Suppression

For the first time, the fractional order disturbance observer (FO-DOB) is proposed for vibration suppression applications such as hard disk drive servo control. It has been discovered in a recently published US patent application (US20010036026) that there is a tradeoff between phase margin loss and strength of the low frequency vibration suppression. Given the required cutoff frequency of the low pass filter, also known as the Q-filter, it turns out that the relative degree of the Q-filter is the major tuning knob for this tradeoff. The solution in US20010036026 was based on an integer order Q-filter with a variable relative degree. This actually motivated the use of a fractional order Q-filter. The fractional order disturbance observer is based on the fractional order Q-filter. The implementation issue is also discussed. The nice point of this paper is that the traditional DOB is extended to the fractional order DOB with the advantage that the FO-DOB design is now no longer conservative nor aggressive, i.e., given the cutoff frequency and the desired phase margin, we can uniquely determine the fractional order of the low pass filter.

A DSP based hard disk drive servo test stand

The paper reports the development of a hard disk drive (HDD) servo test stand developed to demonstrate the principles of a hard disk drive servo system for teaching and research. The test stand consists of a servo written head disk assembly, electronic circuits to process the HDD signal to obtain position information, a 16-bit digital signal processor (DSP) and a spindle motor controller. Hard disk drive servo control related internal signals such as the read signal, peak detector output, servo gate and servo bursts are made available for probing. A host PC is used for down loading commands and controller parameters to the DSP and up-loading the results of the servo system for display and post-processing. Implementation results of a PTOS servo controller and system identification are presented.

A mode-switching controller with initial value compensation for hard disk drive servo control

In various kinds of servo positioning systems, both fast movement and precise positioning are required. To meet these demands, a mode-switching control (MSC) method, which has several controllers for each requirement and switches from one to the other, has been proposed. The issue raised by the use of this technique is the transient response after switching. This paper proposes the use of the initial value compensation (IVC) method to handle this problem. Under the IVC, initial values are placed in the controller state variables, or an additional input is added at switching. From experiments using hard disk drives, it is proved that the IVC improves transient responses after switching.