BLDC Motor Back-EMF Threshold Calibration
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Solution Overview
Problem
Brushless DC (BLDC) motors face timing errors due to incorrect back-electromotive force (Back-EMF) comparison thresholds, leading to increased power consumption, audible noise, and speed ripple, especially when Back-EMF sensing is done during PWM on-time, which is constrained by the need for a minimum PWM off-time.
Innovation Solution
A method and device for automatically selecting a correct comparison threshold for each motor phase by generating digitalized Back-EMF signals and controlling the threshold to reduce errors caused by low-pass filtering, low precision resistors, and power supply variations, allowing accurate Back-EMF sensing during PWM on-time without additional external circuitry and with low computational load.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Back-EMF sensing is done during PWM on-time, then timing accuracy is improved, but the PWM duty cycle is constrained to less than 100% due to minimum off-time requirements
Solution Approach 1:
The patent uses feedback by measuring the actual times between Back-EMF edges and comparing them against expected values. The system continuously monitors the first time (between homologous edges) and second time (between opposed edges), and adjusts the comparison threshold based on the error between measured and expected values, enabling accurate sensing during PWM on-time without requiring minimum off-time
Solution Approach 2:
The patent dynamically changes the comparison threshold parameter based on measured Back-EMF characteristics. By adjusting the threshold adaptively rather than using a fixed value, the system achieves accurate zero-cross detection during PWM on-time while allowing the PWM duty cycle to reach 100% when needed
2Device complexity
If a fixed comparison threshold of VDC/2 is used, then the control logic is simple, but timing errors occur due to variable attenuation and component tolerances
Solution Approach 1:
The system performs self-calibration by automatically measuring its own Back-EMF characteristics and adjusting the comparison threshold accordingly. The controller measures the times between Back-EMF edges, calculates the error from expected values, and autonomously adjusts the threshold without external intervention, eliminating timing errors while keeping the control logic relatively simple
Solution Approach 2:
The patent performs preliminary measurement and calibration actions to determine the correct comparison threshold before normal motor operation. By measuring Back-EMF edges and calculating the appropriate threshold in advance, the system prepares the optimal sensing parameters that account for specific hardware characteristics, ensuring accurate timing detection
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 improves Back-EMF sensing accuracy during PWM on-time, reducing timing errors and associated issues like power consumption and noise, while maintaining efficient operation without the constraints of a minimum PWM off-time.
Implementation Method 1
Control of a BLDC motor may involve sensing the back electro-motive force (Back-EMF), which may occur both during PWM on-time and during PWM off-time
Data Source
AI summary
A driver device for driving a DC motor using PWM modulated drive signals includes comparator circuits for producing digitalized Back-EMF signals having first and second values as a function of the Back-EMF signals being above or below a respective threshold, and an inverter for driving the PWM modulated drive signals in a phased relationship with the digitalized Back-EMF signals. The driver device also includes controller circuits configured for controlling the respective threshold by minimizing the error between a time measured between two consecutive opposed edges of the digitalized Back-EMF signal and half a time measured between two consecutive homologous edges of the digitalized Back-EMF signal.