VCM Back-EMF Measurement with Residual Voltage Compensation
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Solution Overview
Problem
In hard disk drives (HDDs), the residual voltage during discontinuous driving of voice coil motors (VCMs) limits the accuracy of measuring motor speed, especially when short TOFF times are desired.
Innovation Solution
A method for linearization and compensation of residual voltage across a VCM motor winding during discontinuous driving, achieved by measuring the voltage at least twice at different times and generating a signal from the mathematical average of these measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current is interrupted for a fixed time TOFF at regular intervals to measure BEMF, then motor speed measurement becomes possible, but residual voltage limits the accuracy of measurement especially when short TOFF times are desired
Solution Approach 1:
The patent segments the TOFF period into multiple measurement points, taking at least two voltage measurements at different time instants during the TOFF period. This segmentation allows the system to capture the decaying residual voltage characteristics and compensate for it through averaging, thereby maintaining measurement accuracy even with shorter TOFF times.
Solution Approach 2:
The patent performs preliminary voltage measurements at multiple time points during the TOFF period before the residual voltage completely decays. By capturing the voltage decay trajectory in advance and computing an average, the system prepares compensated BEMF values that are immune to residual voltage interference, enabling accurate speed measurement with reduced TOFF durations.
2Productivity
If TOFF times are reduced to improve response time, then productivity increases, but residual voltage compromises the accuracy of motor speed measurement
Solution Approach 1:
The patent divides the shortened TOFF period into multiple measurement segments, taking voltage samples at different time instants. This segmentation strategy enables the system to extract useful BEMF information even from brief interruption windows, maintaining productivity while preserving measurement accuracy through temporal diversification of measurements.
Solution Approach 2:
The patent changes the measurement parameter from a single voltage reading at a fixed time to multiple voltage readings at different time instants during TOFF. This parameter transformation allows the system to adapt to shorter TOFF durations by capturing the voltage decay profile and compensating for residual voltage effects through mathematical averaging.
3Measurement precision
If multiple voltage measurements are taken at different time instants and averaged, then residual voltage is linearized and measurement accuracy improves, but measurement complexity increases
Solution Approach 1:
The patent implements a feedback mechanism where voltage measurements taken at multiple time instants during TOFF are fed into an averaging computation. This feedback loop compensates for residual voltage effects by using the decay trajectory information, thereby improving BEMF measurement accuracy while keeping the additional complexity manageable through a systematic computational approach.
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 allows for accurate measurement of VCM motor speed even with reduced TOFF times, improving speed ripple and maximum driving torque performance.
Implementation Method 1
the current in the load, a VCM, is interrupted (discontinued) for a fixed time (TOFF times) at regular intervals to facilitate measuring the Back ElectroMotive Force, BEMF directly across the load
Data Source
AI summary
A voice coil motor (VCM) in a hard disk drive is operated in a discontinuous mode with an alternation of on and off times. A drive current to the VCM is facilitated and countered with a variable voltage across the during the on-times and off-times. The intensity of the drive current is controlled as a function of a Back ElectroMotive Force (BEMF) of the VCM. A method includes sampling during the alternation of on-times and off-times first and second values of the voltage across the VCM. The first value is sampled at a first time in response to the end of the off-time. The second value is sampled at a second time in response to the drive current of the VCM zeroing following the supply of drive current to the VCM being countered during the off-time. The BEMF is calculated as a function of first and second values.