Sensorless BLDC Motor Control via Back EMF Zero Crossing
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Solution Overview
Problem
Current methods for controlling sinusoidally driven BLDC motors are complex and costly due to the need for high calculation power and multiple sensors, making them inefficient and prone to errors during load or speed changes.
Innovation Solution
A method that directly measures back EMF voltage and current samples to align phase current with back EMF voltage, using a single shunt resistor and microcontroller, allowing for efficient control without position sensors and robust performance under changing conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sensorless field oriented control (FOC) method is used to monitor rotor position, then rotor position monitoring capability is improved, but device complexity and cost increase due to requiring at least 2 shunt resistors and very high calculation power DSP
Solution Approach 1:
The patent extracts the essential function of rotor position monitoring from complex FOC algorithms and multiple sensors, using only back EMF zero crossing detection. This eliminates the need for shunt resistors and complex DSP calculations while maintaining position monitoring capability through direct measurement of back EMF voltage zero crossing points.
Solution Approach 2:
The patent replaces expensive high-power DSP and multiple precision shunt resistors with inexpensive microcontroller and single shunt resistor configuration. The solution uses basic analog-to-digital conversion and simple computational algorithms that can be implemented on low-cost microcontrollers, significantly reducing system cost while maintaining functionality.
2Measurement precision
If sensorless field oriented control (FOC) method is used to monitor rotor position, then rotor position monitoring capability is improved, but device complexity and cost increase due to requiring very high calculation power DSP
Solution Approach 1:
The patent replaces complex mathematical transformation algorithms (Park transformation, inverse Park transformation, current vector calculation) with direct back EMF zero crossing detection. This substitution eliminates the need for high calculation power by using direct measurement and simple timing-based position determination instead of iterative mathematical computations.
Solution Approach 2:
Instead of calculating rotor position from current and voltage measurements through complex transformations, the patent inverts the approach by directly measuring back EMF voltage and using its zero crossing points to determine position. This reverse methodology simplifies the computational burden significantly.
3Device complexity
If current zero crossing measurement method is used to control sinusoidally driven BLDC motor, then control simplicity is improved, but reliability deteriorates during speed or load changes or during PWM mode operation
Solution Approach 1:
The patent introduces back EMF voltage measurement as an intermediary parameter to bridge the gap between simple control and reliable operation. By measuring back EMF zero crossing points, the system obtains accurate rotor position information that remains valid under varying speed and load conditions, enabling reliable sinusoidal current control without complex algorithms.
4Manufacturing precision
If target current values are calculated and processed through current error detector and current control filter, then current control precision is improved, but computational complexity at least doubles
Solution Approach 1:
The patent performs preliminary action by directly determining the optimal current waveform parameters (amplitude and phase) based on back EMF zero crossing detection and load requirements. This eliminates the need for iterative current error detection and control filtering, as the optimal current profile is calculated directly from the measured back EMF characteristics.
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 simplifies the control of sinusoidally driven BLDC motors, reducing computational complexity and cost, while maintaining efficiency and robustness under varying load and speed conditions, suitable for implementation on a microcontroller.
Implementation Method 1
the induced back electromotive force based on the distance... typically it is desired that the BEMF zero crossing is aligned with the current zero crossing for optimum motor efficiency
Data Source
Figure 1A~1C
Figure 2
Figure 3
AI summary
In response to the determination or estimation of a back EMF zero crossing event for the phase, a time T1 is calculated, T1 being representative of the desired absolute maximum value of the phase current. Current samples are taken by the current sampling unit 104 symmetrically centred around T1. The values of the samples CS[1] to CS[10] are then input into the error function to calculate an error function value. The calculated error function value is input to the lead angle control unit 110 which calculates a value for lead_angle. The value of lead_angle is calculated to be the adjustment in phase angle of the driving voltage profile that will minimise the absolute value of the error function. In generating and adjusting the driving voltage profile the driving voltage generation unit 103 takes into account both lead_angle and the output of the position and speed estimation unit 108. The estimated motor speed may be used additionally in an outer speed control loop involving speed controller 111 to adjust the amplitude (AMP) of the effective driving voltage.