Sensorless DC Motor Positioning via Ripple Frequency Correction
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Solution Overview
Problem
Conventional methods for determining the position and speed of commutated DC motors using ripple counting are inefficient and prone to errors due to distortions from rotor eccentricity, brush seating, magnetic saturation, and other mechanical and electrical effects, often requiring additional sensors or introducing inaccuracies.
Innovation Solution
A method that monitors motor current, calculates ripple frequency, determines ripple count, compares it to a threshold value, and uses a motor model to estimate and correct the ripple frequency, thereby calculating a corrected ripple count to accurately determine motor position and speed without additional sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If ripple counting is used to determine motor position and speed, then additional sensors are not required, but measurement precision deteriorates due to distortions from rotor eccentricity, brush seating, magnetic saturation, and other effects causing missed or extra ripple counts
Solution Approach 1:
The system continuously monitors ripple frequency and compares it against expected values, using the deviation as feedback to detect and correct counting errors. When the ripple frequency falls outside a predetermined range, the system adjusts the ripple count accordingly to maintain measurement accuracy.
Solution Approach 2:
The system dynamically changes the parameter being monitored from simple ripple count to ripple frequency, and uses the frequency information to correct the count. This parameter transformation allows the system to detect errors in ripple counting and compensate for them, resolving the precision issue while maintaining sensorless operation.
2Measurement precision
If conventional error correction approaches are used for ripple counting, then measurement precision may be improved, but device complexity increases due to additional calculation requirements or injected inaccuracies
Solution Approach 1:
The system introduces ripple frequency as an intermediary parameter that mediates between the raw ripple count and the final position/speed determination. By using frequency as an intermediate step, the system can detect counting errors without requiring complex correction algorithms, thus improving precision while maintaining relatively simple device complexity.
Data Source
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AI summary
A motor control system (10) and method (100) are provided for detecting current ripple in a commutatcd DC motor (14) and further determining position and speed of the motor (14) based on the detected ripple current. Ripples in the motor current (Ia) are detected and a ripple frequency is calculated based on the time between successive ripples. A ripple count between successive frequencies is determined and the ripple count is compared to a threshold value, and an estimated ripple frequency is determined from a motor model (28) when the ripple count exceeds the threshold value. A corrected ripple count is calculated from a ratio of the calculated ripple frequency and the estimated ripple frequency, and motor position and motor speed are determined based on the corrected ripple count.