Identification and High Bandwidth Control of Hard Disk Drive Servo Systems Based on Sampled Data Measurements

One of the key factors that determines the track density of a hard disk drive (HDD) is the performance characteristics of the HDD servo system, the control system that positions the read/write magnetic head close to the center of the desired track of the HDD. For most HDD servo systems, the deviation of the position of the magnetic head from the center of the desired track is measured whenever the head passes over sectors, referred to as servo burst sectors that carry prewritten position data. The position error signal (PES) is, therefore, generated at discrete points in time, the rate of which is limited by the number of the prewritten servo burst sectors and the spinning speed of the disk. This limited natural sampling rate puts limitations on how high the bandwidth of the controller can be as well as making it difficult to identify the continuous time modes which occur at frequencies close to or higher than the sampling frequency. In this paper, we first introduce a method for identifying the open-loop continuous-time dynamics of such HDD servo systems by processing the available sampled PES data. The method is based on exciting the system with two orthogonal sinusoids and constructing a linear complex equation that relates the sampled PES points with the unknown values of the transfer function. Then, we design a high bandwidth HDD servo controller with improved performance characteristics. In our control design we employ the “ACORN estimator” that generates estimates of the head position by processing the servo burst and data sector measurements, to provide PES estimates at higher sampling rates during read. Our control design is tested on an actual disk drive. The effect of the higher sampling rate, generated by the ACORN estimator, on the seek time and steady-state performance is evaluated. While our results show that the use of a higher sampling rate provides some improvement, the presence of varying high frequency resonance modes limits further increase in bandwidth that is now possible due to the higher sampling rate. Consequently, the proposed control scheme is expected to have additional benefits in future generations of disk drives employing dual stage actuators.