Disk drive actuator design and control for robust non-operational shock performance

Abstract

As disk drives become more of a commodity based product, quality and cost control have become top industry priorities. Disk drive manufacturers are now heavily focused on efforts to reduce costs while maintaining quality and improving performance. Shock damage of the head/disk interface is one area that causes many quality issues and cost constraints. The most damaging shocks, which can be both linear and rotational in nature, occur during the non-operational state. Several methods are used to combat shocks, many of which preserve servo control performance at the expense of cost increases and reliability issues. The research investigates the combination of improved mechanical designs and advanced control methods to improve servo controller performance while maintaining shock resistance, improving product quality, and providing cost reduction.

A voice coil motor actuator is designed to meet specific seek performance requirements. A low cost, magnetic bias feature on the actuator arm is independently designed to compensate for non-operational rotary shock. Because the resulting bias is nonlinear and uncertain, a model-based adaptive controller was developed to meet the seek performance requirements and simultaneously handle effects of the nonlinear bias. Extensive experiments were conducted to characterize the actuator physical parameters and verify the controller performance. A representative sample of the results is presented and discussed.

To address linear shock resistance, the research investigates the feasibility of a ramp load/unload (L/UL) controller using a conventional, non-L/UL disk drive actuator. Therefore, disk drives with lower cost, higher torque actuators can realize the linear shock resistance benefits of ramp loading. A unique disk drive actuator is designed and optimized for L/UL operation. While on the ramp, there exists a set in the state space where the actuator dynamics are uncontrollable. A sufficient condition is determined that guarantees actuator passage through the uncontrollable region. A state trajectory is generated that, when tracked, moves the actuator through the uncontrollable set for a successful load onto the disk at the desired load velocity. Exponentially stable state-feedback and output feedback controllers are designed for tracking. Experiments are performed to validate and verify the new design.