Pair Pole Asymmetry Compensation in Back EMF Zero Cross Detection
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Solution Overview
Problem
Existing hard disk drives face significant challenges in maintaining consistent motor speed due to manufacturing tolerances, leading to variations in zero crossing timing, which result in jitter and errors in commutation step placement, affecting data storage and retrieval accuracy.
Innovation Solution
A system that detects and compensates for pair pole asymmetry by analyzing zero crossing signals over multiple rotations to identify systematic errors, modifying these signals to provide more accurate speed determination and refined commutation control, thereby reducing jitter and improving motor control precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If standard zero cross detection is used for motor control, then the system is simple to implement, but manufacturing tolerances cause jitter and speed variations
Solution Approach 1:
The system performs preliminary characterization of pair pole asymmetry by collecting zero cross interval data over multiple rotations before actual operation. This pre-measured asymmetry information is stored and used to compensate for systematic errors during motor control, eliminating the need for complex real-time correction mechanisms while maintaining high reliability.
Solution Approach 2:
The system implements feedback by continuously monitoring zero cross intervals, comparing them against the characterized asymmetry profile, and adjusting commutation timing accordingly. This closed-loop approach compensates for manufacturing tolerances and maintains consistent motor speed without requiring overly complex hardware modifications.
2Reliability
If manufacturing tolerances are reduced to eliminate pair pole asymmetry, then motor speed consistency improves, but manufacturing cost and complexity increase
Solution Approach 1:
The system performs self-characterization by automatically measuring its own pair pole asymmetry through zero cross interval analysis during initial operation. This self-measured data is then used to compensate for manufacturing imperfections, eliminating the need for precision manufacturing while achieving consistent motor control. The system essentially characterizes and corrects its own manufacturing errors.
Solution Approach 2:
The system changes the operational parameters of commutation timing based on the characterized asymmetry profile. By adjusting when commutation occurs relative to the asymmetric zero cross points, the system compensates for manufacturing tolerances in the motor construction, achieving consistent speed without requiring tighter manufacturing tolerances.
3Measurement precision
If zero cross detection resolution is increased to reduce timing errors, then speed determination accuracy improves, but system complexity and processing requirements increase
Solution Approach 1:
The system extracts only the essential asymmetry information from multiple zero cross measurements by calculating average intervals and identifying systematic deviations. Rather than processing all raw timing data in real-time, the system extracts the key asymmetry parameters and uses these simplified values for compensation, maintaining high precision while reducing processing complexity.
Solution Approach 2:
The system performs preliminary statistical analysis of zero cross intervals to establish the asymmetry profile before actual speed control begins. This pre-processing step converts complex timing variations into a simplified asymmetry characterization that can be easily applied during operation, achieving high measurement precision without complex real-time processing.
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
The solution enables more aggressive and precise control of disk drive motors, reducing spindle vibrations and improving data storage accuracy by accounting for pair pole asymmetry, allowing for better handling of vibrations and increased data density in varying environments.
Implementation Method 1
a back electromotive force sense circuit that measures the back EMF generated on the un-energized phase
Implementation Method 2
The selective energizing of phases in the stator in a predetermined sequence is known as commutation of the motor, which simply involves providing a series of timed commutation steps wherein energy is imparted to the motor to cause it to rotate
Data Source
AI summary
Disk drive spindle jitter is comprised of electrical noise, error due to pair pole asymmetry, and random disk speed variances. Error caused by pair pole asymmetry can be identified and compensated for by detecting over a single rotation of a rotor a plurality of zero cross signals. These signals can be statistically analyzed over a period of a plurality of revolutions of the rotor so as to identify the systematic error caused by pair poles. Once identified, this pair pole error can be used to modify zero cross signals and/or modify commutation signal driving the disk so as to arrive at a more accurate determination of disk speed and to precisely control the speed of the disk.