Multi-stage servo control synthesis for hard disk drive actuator saturation
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Solution Overview
Problem
Hard disk drives face challenges in achieving high-bandwidth servo control systems due to limitations in voice coil motor bandwidth and microactuator stroke, particularly under external disturbances or aggressive seek trajectories, which can lead to actuator saturation.
Innovation Solution
A multi-stage servo control system is synthesized by dividing controller synthesis into multiple dual-stage steps, optimizing single-input-dual-output controllers to prevent actuator saturation and increase bandwidth, using a combination of voice coil motors and microactuators, including both translational and rotational designs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If voice coil motor bandwidth is increased to achieve high-bandwidth servo control, then servo control bandwidth is improved, but actuator saturation occurs under external disturbances or aggressive seek trajectories
Solution Approach 1:
The patent segments the single actuator system into multiple actuators (voice coil motor and microactuators) with distinct functional roles. The VCM handles low-frequency, large-range positioning while microactuators handle high-frequency, fine-positioning tasks, allowing the system to achieve high bandwidth without saturating individual actuators
Solution Approach 2:
The patent implements dynamic controller synthesis that adapts control distribution between actuators based on operating conditions. The multi-stage controller dynamically adjusts which actuator is active and how control authority is shared, optimizing performance while preventing saturation across varying disturbance levels and seek trajectories
2Length of moving object
If microactuator stroke is increased to handle aggressive seek trajectories, then seek capability is improved, but actuator saturation occurs under external disturbances
Solution Approach 1:
The patent divides the positioning task between actuators with different stroke characteristics. The VCM provides large stroke for aggressive seeks while microactuators provide smaller, precise adjustments. This segmentation allows the system to handle both large excursions and fine positioning without requiring any single actuator to have excessive stroke that would lead to saturation
Solution Approach 2:
The patent uses the VCM as an intermediary that absorbs large disturbances and positioning demands, preventing these from directly saturating the microactuators. The VCM acts as a buffer that handles the bulk of aggressive seek trajectories and external disturbances, allowing microactuators to operate within their linear range
3Device complexity
If single actuator system is used to simplify control design, then device complexity is reduced, but bandwidth and precision are limited
Solution Approach 1:
The patent segments the control design into multiple stages, each handling a subset of actuators. This modular approach manages complexity by breaking down the multi-actuator control problem into smaller, more tractable sub-problems while still achieving high bandwidth performance through the combined system
Solution Approach 2:
The patent employs dynamic controller synthesis that adapts the control strategy based on which actuators are active and system operating conditions. This dynamic approach simplifies control design by focusing on relevant actuator subsets while maintaining high bandwidth through coordinated multi-actuator operation
Data Source
AI summary
Implementations disclosed herein include a method to synthesize the controller of a multi-stage servo control system with multiple actuators in a hard disk drive by dividing controller synthesis into multiple dual-stage steps. The method includes measuring a first output of a first plant corresponding to a first actuator, measuring a second output of a second plant corresponding to a second actuator, designing a first controller responsive to the first output, designing a second controller responsive to the second output, and combining the first output and the second output to obtain a first combined output in a first dual-stage. The method also includes designing a third controller responsive to the first combined output, measuring a third output from a third plant corresponding to a third actuator, designing a fourth controller responsive to the third output, and combining the first combined output and the third output to obtain a second combined output.