Brushless Motor Rotor Position Determination via Mutual Inductance
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Solution Overview
Problem
Existing methods for determining the rotor position in synchronous motors without sensors are limited by the reliance on motion-induced voltage and phase inductances, which are sensitive to noise and have limited accuracy.
Innovation Solution
The method involves measuring the voltage induced by mutual inductance at different instants, calculating the voltage difference to overcome offset errors, and using pulse width modulation (PWM) to apply voltage and current to motor phases, allowing for the determination of rotor position, speed, and torque without additional sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional sensorless technologies use motion induced voltage or phase inductances to determine rotor position, then the rotor position can be determined without sensors, but the measurement accuracy is limited due to noise sensitivity
Solution Approach 1:
The patent introduces mutual inductance as an intermediary parameter to indirectly determine rotor position. Instead of directly measuring position through motion-induced voltage or phase inductance, the method measures the mutual inductance between stator phases, which varies with rotor position but is less sensitive to noise. This intermediary measurement approach filters out harmful noise while preserving position information.
Solution Approach 2:
The patent replaces the traditional electromagnetic measurement approach (motion-induced voltage) with a mutual inductance measurement approach. By substituting the measurement mechanism from direct voltage sensing to mutual inductance calculation through voltage measurements during PWM off-periods, the system achieves better noise rejection while maintaining sensorless operation.
2Productivity
If mutual inductance measurement is performed during PWM on-period, then the measurement can be taken during normal operation, but offset errors and motion induced voltage contaminate the measurement
Solution Approach 1:
The patent employs periodic measurement during the PWM off-periods, utilizing the natural switching cycles of the motor drive. By measuring mutual inductance periodically during each PWM off-period rather than continuously during on-periods, the system captures position information at regular intervals while avoiding the contamination present during active voltage application.
Solution Approach 2:
The patent extracts the mutual inductance measurement from the contaminated on-period and relocates it to the clean off-period. By separating the measurement action from the voltage application period, the method extracts pure mutual inductance information free from offset errors and motion-induced voltage contamination, while still maintaining continuous operation through periodic measurements.
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 accurate rotor position and speed estimation by exploiting slight variations in mutual inductance, overcoming noise and offset errors, and enabling efficient control of motor operations.
Implementation Method 1
the mutual inductance between the coils of the stator varies according to the angular position of the rotor
Implementation Method 2
using pulse width modulation (PWM) to apply voltage and current to motor phases
Data Source
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AI summary
The invention relates to a method for determining the position of a rotor in a brushless motor or in a stepper motor, said method including the following steps: applying a voltage in at least one powered phase of the motor; measuring, in an unpowered phase of the motor, the voltage induced by mutual inductance between phases; and determining said position and/or the speed of the rotor from said induced voltage.