Voltage Driven Hard Disk Drive with Voice Coil Model-based Control

In this paper we describe a new voltage driven head servo-positioning system for hard disk drives (HDDs) called Voice Coil Model-based Control (VCMC). Previous works [1],[2] have shown that the usual current amplifier used in driving the voice coil motor can be replaced by a simpler system, consisting a power voltage amplifier driven by an adaptive, multi-rate, zero-pole pre-filter that cancels out the VCM electrical pole. This fully digital solution can be implemented by using a switching power stage in place of the standard linear high power operational amplifier, with savings in both silicon area and power dissipation. The latter is further reduced with the voltage driver, since there is no need for the shunt resistor, placed in series to the VCM as a current sensor. The major drawback exhibited by a voltage driven HDDs is that the dynamics of the system to be controlled depend on the variations of the VCM coil resistance. Moreover, previous realizations, based on a multi-rate zero-pole pre-filter, did not account for the saturation of the driving power amplifier, causing a performance worsening for seeks with average size span. The drawback related to the dependence on the variations of motor resistance has been partially solved in [2], where the pre-filter is tuned at start-up, using the estimated VCM coil resistance given by an Extended Kalman Filter (EKF). This solution, however, presents a high computational complexity and cannot account for resistance variations when the HDD is in idle mode or during track following operations. During these phases, in fact, signal-to-noise ratio results not to be high enough to guarantee EKF convergence. The solutions proposed in this paper address all these issues. The VCMC is based on a new multi-rate pre-filter, which replicates the behavior of a current loop by using a model of the voice coil motor and its driver, including the saturation of the power stage. The performance obtained, when it is fully tuned, is the same as a current driven HDD during all operational phases. As for the pre-filter tuning, during seek operations, an adaptive algorithm based on a simplified least-square identification procedure, maintains the same performance level of the EKF with a lower computational complexity. Finally, several methods for estimating the value of VCM coil resistance during idle mode and track following operations will be compared. They are all based on the analysis of the information given by position error signal and VCM input. Experimental results, including those related to resistance estimation, will be presented in the final paper, showing that HDD servo-positioning performance obtained with the VCMC matches that obtained with a standard current loop, in both seek and track following operations.