Dynamic Degauss Time Detection for BLDC Motor Control
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Solution Overview
Problem
Conventional sensorless trapezoidal commutation control techniques in BLDC motors face errors in detecting back-electromotive force (BEMF) zero-crossing due to fixed degauss time periods, leading to incorrect commutation points and potential motor stalling, especially during sudden changes in load or speed.
Innovation Solution
The method involves sampling electrical quantities at various times to identify the degauss time period and subsequent settling time, allowing for accurate detection of BEMF zero-crossing points, even during degauss periods, by comparing sampled values with references and determining when the floating phase voltage settles, thus enabling peak torque performance across a wide range of operational conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a fixed masking period is used to approximate the degauss time period, then the device complexity is reduced and ease of operation is improved, but measurement precision of BEMF zero-crossing deteriorates and reliability worsens under varying load and speed conditions
Solution Approach 1:
The masking period is made dynamic by determining it based on the actual electrical time constant of the motor phase winding. The control system calculates the masking period as a function of the measured electrical time constant, allowing the masking window to adapt automatically to different operating conditions, load variations, and speed changes, thereby maintaining measurement precision without sacrificing ease of operation
Solution Approach 2:
The masking period parameter is changed from a fixed value to a variable parameter that depends on the electrical time constant. By expressing the masking period as a function of the electrical time constant (e.g., masking period = k × electrical time constant), the system automatically adjusts the masking duration to match the actual degauss time period under different operating conditions, resolving the contradiction between ease of operation and measurement precision
2Device complexity
If a fixed masking period is used, then device complexity is reduced, but reliability deteriorates when load or speed changes suddenly
Solution Approach 1:
The system implements feedback by continuously measuring the electrical time constant and using this information to adjust the masking period. The control system monitors the phase current decay and calculates the actual electrical time constant, then feeds this information back to determine the appropriate masking period, ensuring reliable BEMF zero-crossing detection even during sudden load or speed changes
Solution Approach 2:
The masking period is made dynamic to respond to changing operating conditions. By calculating the masking period based on the measured electrical time constant rather than using a fixed value, the system automatically adapts to sudden changes in load or speed, maintaining reliability without significantly increasing device complexity
3Reliability
If the masking period is extended to cover all possible degauss scenarios, then reliability improves, but loss of time increases and productivity decreases
Solution Approach 1:
The masking period parameter is optimized by expressing it as a function of the electrical time constant rather than using a conservative fixed value. This allows the masking period to be precisely matched to the actual degauss time period for each operating condition, ensuring reliable detection while minimizing the time lost to masking, thereby improving productivity without sacrificing reliability
4Productivity
If BEMF detection is performed during the degauss time period, then productivity is improved by reducing masking time, but measurement precision deteriorates due to clamped floating phase voltage
Solution Approach 1:
The masking period is made dynamic and precisely matched to the actual degauss time period by calculating it based on the measured electrical time constant. This allows the system to extend the masking period just enough to cover the actual degauss duration without unnecessarily clamping the floating phase voltage beyond what is required, thereby improving productivity while maintaining measurement precision through optimized timing
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 eliminates the need for tuning parameters and provides accurate detection of BEMF zero-crossing points, ensuring peak torque performance and rapid start-up capabilities in BLDC motors, even under varying load and speed conditions.
Implementation Method 1
the floating phase voltage is usually clamped to either Vcc or ground. This clamped period is usually blanked out or masked
Implementation Method 2
Conventional sensorless trapezoidal commutation control techniques use back-electromotive force (BEMF) information of the floating phase voltage to detect rotor position
Data Source
AI summary
Example systems and processes compare sampled values of a floating phase voltage and/or outgoing phase current of an electric motor with a corresponding reference to identify a degauss time period. Post degauss time period identification, sampled values are compared with a threshold to identify a settling time period following the degauss time period. The threshold used to identify the settling time period depends on a slope of a floating phase voltage after the degauss time period, a modulation scheme being used, and a pulse width modulation ON/OFF state of the electric motor. When the threshold comparison test is not met, it is determined whether the slope of the floating phase voltage has inverted. Based on such processing, a back-electromotive force (BEMF) zero-crossing (ZC) is detected or estimated with respect to a floating phase voltage of the electric motor.