Voice Coil Motor Control Using Analog Voltage-to-Time Conversion
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Solution Overview
Problem
Existing methods for controlling the speed of voice coil motors in disk drives during emergency retraction face challenges in balancing velocity regulation and power consumption, particularly when using pulse width modulation (PWM) with high-resolution analog-to-digital converters, which consume more power and are not programmable for different disk drive models.
Innovation Solution
Implementing analog voltage-to-time conversion using a constant-current-charging-capacitor configuration to derive pulse width for PWM speed control, eliminating the need for high-resolution ADCs and allowing for programmable pulse widths by storing BEMF voltage in a capacitor and charging it to a target voltage, with optional pulse multiplier circuits to adjust pulse duration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-resolution ADC is used to provide finer control of the pulse width, then the speed control precision is improved, but the power consumption increases
Solution Approach 1:
The patent replaces the digital ADC-based voltage-to-time conversion system with an analog circuit implementation. The analog circuit directly converts the BEMF voltage to a time duration using capacitive charging/discharging, eliminating the need for high-resolution ADCs and digital processing while achieving fine speed control precision.
Solution Approach 2:
The patent changes the operating parameters by using different capacitor values to achieve different pulse widths for various speed control regions. By varying the capacitance value in the analog circuit, the system can provide fine control across multiple speed regions without requiring high-resolution digital conversion.
2Adaptability or versatility
If the pulse width is made programmable to accommodate different disk drive models, then the adaptability is improved, but the device complexity increases
Solution Approach 1:
The patent enables programmability by allowing different capacitor values to be selected based on the disk drive model. The analog circuit's behavior can be adjusted by changing the capacitance value, which provides adaptability to different drive models without requiring complex programmable logic or multiple ADC configurations.
Solution Approach 2:
The patent creates a universal analog voltage-to-time conversion circuit that can serve multiple disk drive models and applications. By using a single analog circuit topology with adjustable capacitor values, the system achieves multi-functionality and broad adaptability without needing model-specific control circuits.
3Measurement precision
If the number of BEMF profile regions is increased for finer adjustment, then the speed control precision is improved, but the power consumption increases
Solution Approach 1:
The patent replaces the digital region-decoding approach with an analog continuous comparison system. The analog circuit continuously compares the BEMF voltage against reference levels and generates appropriate pulse widths, providing fine control across multiple regions without the discrete steps and higher power consumption associated with high-resolution ADCs.
4Measurement precision
If a high-resolution ADC is used, then the speed control precision is improved, but the die area increases
Solution Approach 1:
The patent substitutes the large-area high-resolution ADC with a compact analog voltage-to-time conversion circuit. The analog implementation using capacitors, resistors, and comparators occupies significantly less die area while maintaining or improving speed control precision compared to digital ADC-based solutions.
Solution Approach 2:
The patent uses capacitive storage to copy and hold the BEMF voltage value temporarily during the voltage-to-time conversion process. This analog copying mechanism eliminates the need for complex digital sampling and holding circuits, reducing the overall die area while preserving measurement precision.
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 method provides fine control of voice coil motor speed with reduced power consumption and adaptability to different disk drive models, maintaining efficient motor control while minimizing the size of the motor speed control circuit, particularly important in integrated circuits where die area is limited.
Implementation Method 1
The speed may be determined by detecting the back-electromotive force (back-EMF, or BEMF) generated when current passes through the motor.
Implementation Method 2
analog voltage-to-time conversion may be used. If an analog signal is provided, the need for a digital speed decoder to allow a digital control signal to control the motor speed also may be eliminated.
Data Source
AI summary
Analog control of the pulse width used to control the speed of a voice coil motor may be implemented using a “constant-current-charging-capacitor” configuration where the time needed to charge the capacitor is directly related to how far the actual motor speed is from the target speed. The BEMF voltage, indicative of motor speed, is sampled, and then stored in a storage capacitor, which is allowed to charge/discharge to a target voltage level. The time required to charge/discharge the capacitor to the target voltage is directly proportional to the difference between the BEMF voltage and the target voltage, and may be used directly as the pulse width (i.e., the charging time) in the PWM velocity control system. To avoid larger capacitors, a pulse multiplier circuit can be added, allowing charging/discharging the sampled voltage to the target voltage to be repeated by a number, N, of times.