Brushless Motor Rotor Position Detection via Phase Current Saturation
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Solution Overview
Problem
Brushless permanent-magnet motors face challenges in determining rotor position, especially at low speeds, due to susceptibility of Hall-effect sensors to electromagnetic noise and reliance on back EMF transitions, which are unreliable at low speeds.
Innovation Solution
A method that involves sequentially exciting and freewheeling a phase winding, measuring changes in phase current or time intervals, and comparing these against a saturation threshold to determine rotor position without relying on back EMF, using inductance changes to accurately commutate the phase winding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Hall-effect sensor is used to determine rotor position, then rotor position can be detected, but the sensor signal is susceptible to electromagnetic noise and integration complicates design and manufacture
Solution Approach 1:
The patent extracts the rotor position detection function from the Hall-effect sensor and implements it through sensorless control using back EMF transitions. By removing the physical sensor and using electrical signal analysis instead, the solution eliminates electromagnetic noise susceptibility while maintaining position detection capability
Solution Approach 2:
The patent replaces the mechanical/electrical sensor system with a field-based measurement approach. Instead of using a physical Hall-effect sensor that generates electrical signals, the system uses computational analysis of back EMF transitions to determine rotor position, substituting physical sensing with field analysis
2Device complexity
If back EMF transitions are used to determine rotor position, then sensorless control is achieved, but transitions cannot be reliably determined at low speeds
Solution Approach 1:
The patent applies preliminary action by implementing a two-stage control approach: first using back EMF transitions for position detection at higher speeds, then transitioning to inductance-based detection at lower speeds. This preparatory switching strategy ensures reliable position determination across the entire speed range
Solution Approach 2:
The patent implements dynamics by making the position detection method adaptive to rotor speed. The system dynamically switches between back EMF-based detection and inductance-based detection based on operating conditions, optimizing reliability across varying speeds rather than using a fixed detection method
3Measurement precision
If saturation threshold is set low to detect aligned position accurately, then position detection precision improves, but the controller may miss the aligned position due to detection failure
Solution Approach 1:
The patent applies preliminary action by using a higher saturation threshold that provides a warning indication before the actual aligned position is reached. This early warning allows the controller to prepare for commutation in advance, ensuring reliable timing without missing the aligned position
Solution Approach 2:
The patent implements feedback by using the saturation threshold as a predictive indicator that triggers commutation preparation. The higher threshold provides feedback about approaching alignment, allowing the control system to adjust timing accordingly and maintain reliable operation
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 allows for reliable rotor position determination and commutation, even at low speeds, by employing a higher saturation threshold and variable commutation period, reducing the likelihood of missing aligned positions and improving motor control accuracy.
Implementation Method 1
transitions in the polarity of the back EMF induced in a phase winding may be used to determine the rotor position
Implementation Method 2
the method makes use of the changes in the inductance of the phase winding that arise as the rotor rotates from one aligned position to the next aligned position
Data Source
AI summary
A method of controlling a brushless permanent-magnet motor that includes sequentially exciting and freewheeling a phase winding of the motor is provided. The phase winding is freewheeled when the phase current exceeds an upper threshold. The method further includes measuring a parameter that corresponds to either: (i) the magnitude of the phase current during or at the end of freewheeling when the phase winding is freewheeled for the fixed period of time, or (ii) the time interval during freewheeling or during excitation when the phase winding is freewheeled until the phase current falls below the lower threshold. The measured parameter is then compared against a saturation threshold, and the rotor is determined to be at a predetermined position. In response to determining that the rotor is at the predetermined position, the phase winding is commutated after a commutation period has elapsed.