Rotor Position Determination Using EMF and Inductance
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Solution Overview
Problem
Existing methods for determining the rotor position of electronically commutated multiphase DC motors, such as BLDC motors, face accuracy issues as rotational speed increases, particularly when relying on either back EMF voltage or inductance variations, as these methods either neglect or minimize the effects of the other parameter.
Innovation Solution
A method that generates test voltage pulses across different phase positions, measures current responses, and uses a periodic approximation function composed of a fundamental wave for EMF and a first harmonic for inductance to determine the rotor position, leveraging both EMF and inductance variations for accurate positioning across varying rotational speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If back EMF voltage analysis is used to determine rotor position, then the method is simple to implement, but the accuracy deteriorates as rotational speed increases
Solution Approach 1:
The patent combines two different measurement approaches (back EMF voltage analysis and inductance variation analysis) into a unified rotor position determination system. By merging these methods and selecting based on operating conditions, the system maintains high accuracy across all rotational speeds while keeping implementation complexity manageable.
Solution Approach 2:
The patent changes the measurement parameter based on rotational speed conditions. At low speeds where back EMF is weak, it switches to inductance variation measurement. At high speeds where back EMF is strong, it uses back EMF analysis. This dynamic parameter selection resolves the accuracy-degradation problem at different speeds.
2Measurement precision
If inductance variation method is used to determine rotor position, then accuracy is maintained at high speeds, but the device complexity increases due to additional measurements
Solution Approach 1:
The patent implements a dynamic measurement system that adapts its operation mode based on rotational speed. The control unit dynamically selects between back EMF-based measurement and inductance-based measurement, making the system flexible and condition-dependent rather than static. This reduces unnecessary complexity by only performing inductance measurements when needed.
Solution Approach 2:
The patent changes the measurement parameter based on rotational speed conditions. At low speeds where back EMF is weak, it switches to inductance variation measurement. At high speeds where back EMF is strong, it uses back EMF analysis. This dynamic parameter selection resolves the accuracy-degradation problem at different speeds.
3Ease of operation
If rotor position is determined using a single method, then the system is simple to operate, but reliability deteriorates across varying operating states
Solution Approach 1:
The patent implements a dynamic measurement system that adapts its operation mode based on rotational speed. The control unit dynamically selects between back EMF-based measurement and inductance-based measurement, making the system flexible and condition-dependent rather than static. This reduces unnecessary complexity by only performing inductance measurements when needed.
4Loss of information
If test voltage pulses are applied at all phase positions, then complete rotor position information is obtained, but the energy consumption increases
Solution Approach 1:
The patent applies test voltage pulses only at necessary phase positions rather than all possible positions. By using a reduced set of measurement points and interpolating or inferring intermediate positions, the system obtains sufficient rotor position information while significantly reducing the number of required pulses and associated energy consumption.
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 method enhances the reliability and accuracy of rotor position determination by considering both EMF and inductance effects, providing robustness across all operating states, including high rotational speeds and low EMF conditions, and resolves the 180° ambiguity through additional measurements and weighting.
Implementation Method 1
generating in the winding system by means of a commutation device (3) a plurality of test voltage pulses
Implementation Method 2
the rotor produces a magnetic asymmetry because the reluctance is greater in the direction of the magnetisation of the rotor (d-axis) than in the transverse direction (q-axis). This results in an inductance of the BLDC motor that is dependent on the rotor position
Implementation Method 3
the current rotor position is ascertained by analysing the zero crossovers of the induced back voltage (EMF) in the winding phases
Data Source
AI summary
The invention relates to a generic method for determining the rotor position of an electronically-commutated multi-phase direct current motor, characterized in: (a) generating of a plurality of test voltage pulses in the winding system with a specified switch-on duration ΔT by means of the commuting device in different phase distributed over 360°; (b) measurement of the current values of the current responses of the test voltage pulses on expiration of the switch-on period of the respective test voltage pulse; (c) approximation of the measured current values by means of a periodic approximation function from a superimposition of a preferably sinusoidal fundamental wave with an amplitude IEMK and the associated first harmonic with an amplitude Und as a factor of the phase of the test voltage pulse, wherein the sinusoidal fundamental wave follows the chronological progression of the counter EMF voltage of the stator and the first harmonic follows the chronological progression of the stator of the direct current motor; (d) determination of an amplitude ratio value IEMF/IInd of the fundamental wave and the first harmonic of the approximation function; (e) determination of the rotor position as the EMF angle from the argument of the first harmonic of the approximation function as a factor of the amplitude ratio IEMF/IInd, wherein the EMF angle indicates the phase offset of the sinusoidal progression of the counter EMF voltage of the stator relative to the approximation function and the inductivity angle indicates the phase offset of the sinusoidal progression of the inductivity of the stator relative to the approximation function.