Variable PWM Frequency Motor Driver for Sensorless BLDC Torque Ripple
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Solution Overview
Problem
In sensorless brushless DC motor systems, increasing the PWM frequency to enhance current polarity detection resolution leads to a decrease in the resolution of the drive control signal, resulting in increased torque ripple.
Innovation Solution
A motor driver device with a prediction processor that sets a detection prediction interval based on previously detected polarity inversion timings, allowing for frequency variable control to increase the target frequency within the detection interval, thereby improving current polarity detection resolution while maintaining drive control signal resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the PWM frequency is increased to enhance current polarity detection resolution, then the sampling frequency increases and detection resolution improves, but the resolution of the drive control signal decreases
Solution Approach 1:
The patent applies dynamics by making the PWM frequency variable rather than fixed. The frequency is dynamically adjusted based on the operational phase: a first (higher) frequency is used during detection prediction intervals to improve current polarity detection resolution, while a second (lower) frequency is used outside these intervals to maintain drive control signal resolution. This dynamic frequency adjustment resolves the contradiction by allowing the system to optimize for detection precision when needed and for control precision at other times.
Solution Approach 2:
The patent changes the parameter of PWM frequency based on operational conditions. By setting different frequency values (first frequency during detection intervals, second frequency outside detection intervals), the system adapts the detection resolution and control signal resolution to the current operational requirements, thereby resolving the trade-off between the two precision requirements.
2Measurement precision
If the PWM frequency is increased to improve sampling frequency, then the detection resolution of polarity inversion timing improves, but torque ripple increases due to decreased drive control signal resolution
Solution Approach 1:
The system dynamically adjusts PWM frequency to balance detection precision and motor performance. During detection prediction intervals, the higher first frequency improves polarity inversion timing detection resolution. Outside these intervals, the lower second frequency maintains adequate drive control signal resolution, preventing excessive torque ripple. This dynamic adjustment eliminates the need to continuously operate at high frequency, thus avoiding continuous torque ripple generation.
Data Source
AI summary
A motor driver device includes: a detector that detects a polarity inversion timing of a current flowing through a coil of a predetermined phase of a motor; a drive control signal generator that generates a pulse-width-modulated or pulse-density-modulated drive control signal for each phase based on the detection result; and a drive voltage supply that supplies a drive voltage corresponding to the drive control signal to a corresponding coil, wherein the drive control signal generator includes a prediction processor configured to set a detection prediction interval based on two or more previously detected polarity inversion timings, and executes a frequency variable control to set a variable target frequency to be higher in the detection prediction interval than outside the detection prediction interval, the variable target frequency being a frequency of the pulse-width-modulated drive control signal or a reciprocal of a minimum pulse width of the pulse-density-modulated drive control signal.