Hard Disk VCM Retract Control for Accurate Back-EMF Sensing
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Solution Overview
Problem
Conventional hard disk drives face issues with inaccurate speed control and generation of acoustic noise during head parking due to residual voltage effects in discontinuous driving modes, particularly at critical points like the end-of-travel point, which affects bandwidth and speed measurement accuracy.
Innovation Solution
A method is employed to modulate the operating frequency and duration of off-times in discontinuous driving based on the current amplitude, allowing for adaptive control of the speed control loop bandwidth and accurate detection of the end-of-travel point by varying the off-time duration as a function of the driving current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a constant off-time duration TOFF is used in discontinuous driving mode, then the bandwidth of the speed control loop is limited, but this ensures accurate BEMF measurement under worst-case current conditions
Solution Approach 1:
The patent applies dynamics by transitioning from a static, fixed off-time duration to a dynamic, variable off-time duration that adapts based on current amplitude. The control circuit adjusts TOFF in response to detected current levels, allowing the system to optimize between bandwidth and measurement accuracy at different operating conditions rather than being constrained by a single fixed value.
Solution Approach 2:
The patent implements parameter changes by modifying the off-time duration parameter TOFF as a function of current amplitude. When current amplitude is high, the control circuit reduces TOFF to increase bandwidth; when current amplitude is low, the control circuit increases TOFF to ensure accurate BEMF measurement. This dynamic parameter adjustment resolves the contradiction between fixed measurement accuracy and variable bandwidth requirements.
2Productivity
If the off-time duration TOFF is reduced to increase bandwidth, then the speed control responsiveness improves, but the BEMF measurement accuracy deteriorates due to residual voltage effects
Solution Approach 1:
The control circuit dynamically adjusts the off-time duration parameter TOFF based on real-time current amplitude detection. When current amplitude is high (which causes significant residual voltage), the control circuit increases TOFF to allow sufficient time for residual voltage to decay, ensuring accurate BEMF measurement. When current amplitude is low (where residual voltage is minimal), the control circuit decreases TOFF to increase bandwidth and improve speed control responsiveness.
Solution Approach 2:
The system employs feedback by continuously monitoring current amplitude and using this information to adjust the off-time duration TOFF. The control circuit receives feedback about the current state and modifies TOFF accordingly, creating a closed-loop system that automatically optimizes the balance between bandwidth and measurement accuracy based on actual operating conditions.
3Device complexity
If a fixed off-time duration is used throughout the retract procedure, then the control system is simple, but it cannot adapt to different current conditions and leads to suboptimal performance
Solution Approach 1:
The patent transforms the static, fixed off-time control into a dynamic, adaptive control system. The control circuit continuously monitors current amplitude and adjusts TOFF in real-time, allowing the system to adapt to varying operating conditions during the retract procedure. This dynamic adjustment improves reliability and speed control performance across different current conditions while maintaining reasonable system complexity through automated control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances speed control accuracy and reduces noise by increasing bandwidth and reducing latency in critical phases, ensuring precise head positioning without introducing errors in speed estimation.
Implementation Method 1
during an off-phase the current is interrupted and, if the speed is not null, a back electro-motive force (BEMF) voltage is generated across the motor winding(s). The BEMF voltage is (directly) proportional to the VCM motor speed and can therefore be used as a feedback signal in a speed control loop.
Implementation Method 2
During an on-phase, a current is caused to flow in the motor winding(s) and generates a motor torque
Data Source
AI summary
The disclosure relates to a voice coil motor (VCM) in a hard disk drive during a retract phase, wherein read-and-write heads of the hard disk drive are retracted by the VCM from a disk surface towards an end-of-travel parking condition with the heads climbing over a ramp starting from a ramp edge adjacent the disk surface. During the retract phase, the VCM is operated in a discontinuous mode comprising an alternation of on-times of duration TON and off-times of duration TOFF wherein supply of a drive current to the VCM is facilitated and countered, respectively. The duration TOFF of the off-times is varied to have a lower bound value at the ramp edge and to increase towards an upper bound value as the heads climb over the ramp towards the end-of-travel parking condition.