Back-EMF Sensor Circuit for Brushless DC Motor Control
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Solution Overview
Problem
Existing methods for controlling three-phase brushless DC motors are prone to voltage offset issues, especially at low voltages and in noisy environments, leading to inconsistent comparisons and inefficient motor operation, including incorrect starting direction and slow rotation.
Innovation Solution
A microprocessor-controlled sensor circuit that measures back EMF during the off-time of the duty cycle to determine the order of voltages across phases, allowing for accurate commutation sequencing and pulse polarity adjustment to accelerate and sustain motor shaft rotation, using a power stage with a duty cycle less than 100% and a back EMF sensor circuit to monitor phase voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage comparison is performed using traditional comparator circuits, then phase angle detection can be achieved, but voltage offset causes inconsistent or ambiguous comparisons especially at low voltages and in noisy environments
Solution Approach 1:
The patent replaces traditional voltage comparison methods with a back EMF sensing approach. Instead of comparing voltages using comparators that are sensitive to offset and noise, the system measures the back EMF generated by the motor windings during the off-time of PWM operation. This substitution of measurement methodology eliminates the voltage offset problem and provides more reliable phase angle detection, especially at low voltages and in noisy environments.
Solution Approach 2:
The patent introduces back EMF as an intermediary measurement parameter. Rather than directly comparing phase voltages to determine commutation timing, the system uses the back EMF signal generated during the off-time as an intermediate indicator of rotor position. This intermediary measurement provides a cleaner, more reliable signal that is less susceptible to voltage offset and electrical noise.
2Measurement precision
If rotor angle sensors or phase angle detectors are used to determine phase angle, then accurate commutation can be achieved, but the circuits become complex and expensive
Solution Approach 1:
The patent extracts the essential information needed for commutation control from the back EMF signal without requiring complex rotor angle sensors or phase angle detectors. By taking out only the necessary voltage order information during the off-time and using simple comparison logic in the microprocessor, the system achieves accurate phase angle detection while avoiding the complexity and expense of dedicated sensor circuits.
Solution Approach 2:
The patent creates a simplified model of rotor position by measuring the order of back EMF voltages on different phases. Instead of using complex sensors to directly detect rotor angle, the system copies the essential position information through the back EMF voltage relationships, which can be measured with simple circuitry and processed by software logic.
3Ease of operation
If the motor is aligned with the actuated phase at initial position, then power can be applied, but the motor may not start, start in the wrong direction, or rotate too slowly resulting in insufficient back EMF for comparisons
Solution Approach 1:
The patent applies preliminary action by measuring the back EMF voltage order during the off-time before applying power pulses for acceleration. This preliminary measurement establishes the correct commutation sequence and ensures the motor will start in the correct direction. The system determines the appropriate phase to pulse and the correct polarity based on the measured back EMF relationships, preventing wrong-direction starting and ensuring sufficient back EMF is generated from the beginning.
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 provides reliable, efficient, and cost-effective motor control, ensuring consistent rotation direction and speed, particularly effective at low RPMs, and adaptable for use in radio-controlled models.
Implementation Method 1
A back EMF sensor circuit measures, during the off-time of the duty cycle, an indication of order with respect to voltage of a first phase of the motor with respect to one or more other phases
Data Source
AI summary
Control of rotational speed of a direct current multi-phase brushless motor is provided using an apparatus and method that works at low speed but does not depend upon Hall effect sensors. An apparatus for accelerating rotation of the motor shaft has a power stage circuit coupled to a back Electromotive Force (EMF) sensor circuit and a microprocessor. The power stage pulses at a duty cycle less than 100% under control of the microprocessor. The back EMF sensor circuit measures an order with respect to voltage of at least one phase relative to one or more other phases during off-time. The microprocessor determines one or more phases to be pulsed, and the polarity of the pulses based on the measured order. A method for sustaining rotation pulses the phases, measures order with respect to voltage of at least one phase relative to one or more other phases, and updates commutation state based on the measured order. A method of accelerating rotation generates one or more power pulses, determines an order with respect to voltage of one phase relative to one or more other phases, and generates subsequent pulses on phases and with polarity determined by the measured order.