Sensorless DC Motor Control via BEMF Asymmetry Detection
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Solution Overview
Problem
Existing methods for controlling single-phase brushless DC motors without position sensors, such as Hall sensors or optical encoders, face challenges in determining the direction of rotation and commutation instants due to the lack of natural intervals for back electromotive force (BEMF) sensing, and require additional windings or complex estimation methods that are not suitable for most motors.
Innovation Solution
A control circuit with a feedback loop regulator that generates a control signal to manage current in the motor windings, allowing for the determination of rotational position and direction of rotation based on characteristics of the control signal, such as the rate of variation of the BEMF waveform, without direct measurement of the BEMF signal, using pulse width modulation to derive commutation moments and cinematic quantities like angular displacement and speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If back EMF sensing is used to determine commutation instants in single-phase brushless DC motors, then motor control can be achieved without position sensors, but single-phase motors lack natural intervals when one phase is open-circuited making BEMF sensing inapplicable
Solution Approach 1:
The patent uses an intermediary winding (quadrature axis winding) that is positioned to sense BEMF without being affected by the stator winding magnetic field. This intermediary structure enables BEMF detection in single-phase motors where direct sensing is not possible, resolving the contradiction between ease of control implementation and difficulty of BEMF detection.
Solution Approach 2:
The control system processes the BEMF signal from the intermediary winding to achieve multiple functions: determining commutation instants, detecting rotation direction, and controlling motor operation. This multi-functional approach allows a single sensing mechanism to address multiple control requirements simultaneously.
2Difficulty of detecting and measuring
If an additional quadrature axis winding is added to sense BEMF signal, then BEMF sensing becomes possible in single-phase motors, but the motor fabrication complexity increases and cannot be used with the vast majority of single phase brushless DC motors
Solution Approach 1:
The patent utilizes the motor's existing stator windings to generate an asymmetrical magnetic field that creates detectable asymmetry in the BEMF signal during normal operation. This self-service approach eliminates the need for additional sensing windings, allowing the motor to detect its own operational state using its inherent structural characteristics.
Solution Approach 2:
The patent detects changes in the BEMF signal parameters (amplitude asymmetry, zero-crossing timing) that occur naturally during motor operation due to the asymmetrical magnetic field. By monitoring these parameter changes, the system determines commutation instants and rotation direction without modifying the motor structure.
3Extent of automation
If BEMF sensing is used to determine commutation instants, then sensorless control is achieved, but the method does not allow determination of the sense of rotation of the motor
Solution Approach 1:
The patent exploits the asymmetry in the BEMF signal characteristics (amplitude, timing, waveform shape) that occurs when the motor rotates in different directions. By detecting which side of the commutation point the BEMF zero-crossing occurs and analyzing the asymmetrical signal patterns, the system determines rotation direction while maintaining sensorless control.
Solution Approach 2:
The control system continuously monitors the BEMF signal characteristics and uses this feedback information to adjust commutation timing and determine rotation direction. The feedback loop analyzes the relationship between applied voltage and resulting BEMF to extract directional information without requiring additional sensors.
4Device complexity
If stator windings are used for both driving and sensing BEMF signal through time-sharing, then component count is reduced, but access to BEMF signal is unavailable when stator windings are required for driving
Solution Approach 1:
The patent uses the intermediary quadrature axis winding as a dedicated BEMF sensing channel that operates independently from the main driving windings. This intermediary structure provides continuous BEMF signal access during motor operation without requiring time-sharing, eliminating the conflict between driving and sensing functions.
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
Enables effective control of single-phase brushless DC motors without dedicated sensing circuitry, addressing the 'dead point' issue and handling asymmetries in the magnetic field, thereby improving starting reliability and motor control accuracy.
Implementation Method 1
The feedback loop regulator can be a Pulse Width Modulation Regulator
Implementation Method 2
During operation of the motor, the windings see a time-varying magnetic field generated by the permanent magnet. That time varying magnetic field induces a back electromotive force (BEMF or back EMF) in the windings.
Implementation Method 3
circuitry to extract a characteristic (DC) of the control signal, said characteristic varying in function of time as the back electromotive force in the windings
Data Source
AI summary
A circuit for determining a direction of rotation of an electric motor, the motor having asymmetry and/or eccentricity in a profile of back electromotive force as a function of angular position of a rotor with respect to a stator, the circuit receiving a signal representing the BEMF, and use the corresponding asymmetry and/or eccentricity in the signal to derive the direction of rotation. The signal representing the back emf can be generated by a control circuit. The control circuit can have a feedback loop regulator to generate a control signal (TL or TR) to control a current drive circuit (11,12) to control an amplitude of current (iw) in the windings, the feedback loop regulator being arranged to compare the amplitude of the current (iw) in the windings with a reference value (iset), and use the control signal to provide the signal representing the back electromotive force.