BLDC Motor BEMF Detection Using Tri-State Phase Control
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Solution Overview
Problem
Conventional methods for monitoring back electromotive force (BEMF) in brushless direct current (BLDC) motors suffer from inefficiencies, torque distortion, and noise sensitivity, particularly in low-speed and low-current operations, and are ineffective for core-less motors due to reliance on accurate current sensing.
Innovation Solution
The proposed solution involves tri-stating the phase U input during PWM periods and comparing the resulting voltage to ground to directly detect the BEMF zero cross point, eliminating the need for current sensing and position sensors, and improving noise performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If window-opening method is used to detect BEMF, then BEMF zero cross point can be determined, but control efficiency is low and torque distortion occurs
Solution Approach 1:
The patent implements periodic tri-stating of the floating phase at specific intervals during PWM periods, allowing BEMF detection only during these predetermined time windows. This periodic action eliminates continuous detection overhead while capturing sufficient zero cross information, thereby improving control efficiency without sacrificing detection accuracy.
Solution Approach 2:
The patent extracts BEMF detection from the continuous operation mode by isolating specific detection windows during PWM cycles. By separating the detection function from continuous control operations and performing it only during tri-stated intervals, the system achieves accurate zero cross detection while minimizing impact on control efficiency and torque production.
2Measurement precision
If window-opening method is used to detect BEMF, then BEMF zero cross point can be determined, but noise performance is poor
Solution Approach 1:
The patent introduces tri-state control as an intermediary mechanism that isolates the BEMF detection process from noisy motor operation. By switching the floating phase to a high-impedance tri-stated condition during detection windows, the system creates an electrically quiet measurement environment that filters out switching noise and electromagnetic interference, enabling accurate zero cross detection despite harsh operating conditions.
3Measurement precision
If windowless method is used to estimate BEMF, then continuous BEMF information can be obtained, but accurate current sensing is required which limits application range
Solution Approach 1:
The patent extracts BEMF detection from current-dependent estimation methods by utilizing the floating phase voltage directly during tri-stated intervals. This extraction eliminates the need for complex current sensing and calculation algorithms, replacing them with a simpler voltage comparison approach that works reliably across diverse motor types including core-less motors with varying electrical characteristics.
4Measurement precision
If conventional BEMF detection methods are used, then rotor position can be determined, but torque distortion occurs during control
Solution Approach 1:
The patent applies periodic tri-stating during PWM periods when motor current is naturally low or zero, specifically during intervals that do not coincide with active torque production phases. This temporal separation ensures that BEMF detection occurs during electrically quiet periods, obtaining accurate rotor position information without interfering with torque generation and avoiding torque distortion.
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 enhances BEMF zero cross detection efficiency, stability, and noise resistance, particularly at low speeds and in core-less motors, without distorting control torque or relying on current sensing, thereby improving overall motor control.
Implementation Method 1
A comparator 45 compares the voltage on the floating phase to the motor ground, which serves as a reference voltage for the comparator.
Implementation Method 2
A change detector 48 detects changes in the output of the comparator, with each change corresponding to a BEMF zero cross.
Data Source
AI summary
A back electromotive force (BEMF) zero cross may be detected in a brushless direct current (BLDC) motor that is controlled by pulse width modulation (PWM). A phase input of the BLDC motor is tri-stated during PWM periods in which the phase input conducts motor drive current, and the tri-stating of the phase input is used to determine whether a BEMF zero cross has occurred in the BLDC motor.