Voice Coil Motor Velocity Control via BEMF Sensing
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Solution Overview
Problem
Conventional methods for controlling voice coil motors in hard disk drives lack precise velocity control, especially during load/unload operations, due to difficulties in accurately extracting back electromagnetic field (BEMF) voltage and motor resistance, which vary with temperature and torque, leading to potential crashes against the media or ramp.
Innovation Solution
A velocity control loop circuit that senses the voltage difference between the voice coil motor and a sense resistor, using a BEMF resistive network with resistors R1 and R2 to drive the motor and achieve bidirectional control, enabling precise velocity control by adjusting the R1/R2 ratio for temperature variations and current limiting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional discontinuous mode PWM control is used, then the VCM motor can be driven with sufficient off-time to sense BEMF, but the control precision and velocity accuracy deteriorate due to audible frequency limitations and insufficient control resolution
Solution Approach 1:
The patent employs periodic pulse signals with variable duty cycles to control the VCM motor velocity. By using periodic action with different pulse widths, the system achieves precise velocity control without requiring complex continuous modulation schemes, thus improving measurement precision while managing device complexity
Solution Approach 2:
The system implements feedback control by sensing the back electromagnetic field (BEMF) voltage during the off-state of PWM and using this information to determine the power required for the next cycle. This feedback mechanism enables precise velocity control by continuously adjusting the control signal based on actual motor performance
2Speed
If conventional continuous mode constant voltage control is used, then the control system is simple, but the maximum velocity is limited and velocity control precision deteriorates
Solution Approach 1:
The patent transitions from static constant voltage control to dynamic PWM control where the voltage applied to the VCM motor varies continuously based on the desired velocity profile. This dynamic control approach enables both higher maximum velocities and precise velocity control by adjusting the duty cycle of the PWM signal according to real-time requirements
Solution Approach 2:
The system changes the electrical parameters (voltage, current, duty cycle) dynamically to control the VCM motor velocity. By varying these parameters based on feedback from BEMF sensing and position information, the system achieves both high velocity capability and precise control, overcoming the limitations of constant voltage control
3Reliability
If motor resistance is not compensated for temperature and torque variations, then the control system is simple, but the velocity control accuracy deteriorates leading to potential crashes
Solution Approach 1:
The system performs preliminary characterization of the motor resistance as a function of temperature and torque. This pre-established resistance map is stored in memory and used during operation to compensate for resistance variations without requiring complex real-time measurement circuits, thus improving reliability while managing device complexity
Solution Approach 2:
The control system uses the BEMF sensing capability that already exists in the motor to indirectly measure and compensate for resistance changes. By utilizing the motor's own back EMF signal, the system achieves self-compensation for temperature and torque variations without adding separate sensing circuits, improving reliability while minimizing additional complexity
4Productivity
If the head is moved away from the disk at high speeds during unload, then the operation is fast, but the risk of crashing against the ramp or media increases
Solution Approach 1:
The system uses periodic velocity control updates during the unload operation to continuously monitor and adjust the actuator velocity. This periodic control ensures that even at high speeds, the system can detect and respond to potential crash conditions, maintaining both productivity and reliability
Solution Approach 2:
The velocity control loop provides continuous feedback during the unload operation, allowing the system to adjust the velocity profile in real-time. This feedback mechanism enables fast unload operations while preventing crashes by detecting approaching ramp or media and reducing velocity appropriately
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
Enables precise and continuous velocity control of the voice coil motor, preventing crashes during high-speed operations by accurately sensing BEMF and motor resistance, allowing for versatile velocity profiles and emergency braking routines.
Implementation Method 1
the back electromagnetic field (BEMF) voltage appearing across the VCM motor is sensed
Data Source
AI summary
A method of controlling the velocity of a voice coil motor (VCM), including sensing a voltage difference between the VCM and a sense resistor and driving a velocity control loop (VCL) based on the voltage difference. There is also a control loop circuit, including a current output connected to drive a voice coil motor, the voice coil motor producing a back electromagnetic field (BEMF) voltage. The circuit also includes a sense resistor connected to the BEMF output, and a BEMF resistive network comprising a first resistor and a second resistor. The circuit also includes a velocity control loop (VCL) connected to control the voltage output according to a voltage difference between (i) a junction of the sense resistor and the current output and (ii) a junction of the first resistor and the second resistor.