Magnetic Head Bias Control for DFH Stability
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Solution Overview
Problem
In magnetic disk devices, the dynamic flying height (DFH) control leads to varying output levels of the magnetic head, causing error rates in data and servo signals to deviate from the required levels, which hampers the improvement of surface density and stability of AGC control during read and write processes.
Innovation Solution
The control unit adjusts the bias current of the magnetic head in response to the ON/OFF setting of the DFH control, changing it from a higher value to a lower value when the DFH power is turned on, ensuring the output level remains within the input dynamic range of the AGC circuit, thereby stabilizing AGC control and reducing error rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If DFH control is implemented to adjust magnetic spacing, then surface density is improved, but output level of magnetic head varies causing error rates to increase
Solution Approach 1:
The patent changes the bias current parameter of the magnetic head in response to DFH control activation. When DFH power is turned on, the bias current is reduced from a first value to a second value, which compensates for the output level variation caused by thermal expansion and maintains stable signal quality and error rates.
Solution Approach 2:
The system implements feedback control by monitoring the DFH control state and automatically adjusting the bias current accordingly. The control unit detects when DFH power is activated and responds by modifying the bias current to maintain optimal output levels, creating a closed-loop control system that stabilizes performance.
2Manufacturing precision
If DFH control is activated to improve surface density, then magnetic spacing is adjusted, but AGC control stability deteriorates
Solution Approach 1:
The patent applies preliminary action by adjusting the bias current before the output level deviation becomes problematic. When DFH control is activated, the bias current is proactively reduced to preemptively compensate for the expected output level increase, preventing AGC instability before it occurs.
3Reliability
If bias current is changed in response to DFH control, then output level is stabilized, but device complexity increases
Solution Approach 1:
The control unit is designed to perform multiple functions: it manages DFH control activation and simultaneously controls the bias current adjustment. This multi-functionality allows the system to stabilize output levels without adding separate dedicated circuitry, thereby limiting the increase in device complexity.
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 adjustment maintains the output level of the magnetic head within the dynamic range of the AGC circuit, improving error rates and surface density by stabilizing AGC and seek controls, even during transitions between DFH states.
Implementation Method 1
by making current flow through a heater mounted on the magnetic head to make the magnetic head thermally expand
Data Source
AI summary
According to one embodiment, there is provided a magnetic disk device including a magnetic disk, a magnetic head, a heater, and a control unit. The magnetic head reads out information recorded on the magnetic disk. The heater is configured to adjust a magnetic spacing of the magnetic head from the magnetic disk. The control unit is configured to change a bias amount of the magnetic head according to setting of current flow through the heater.