One-Shunt Brushless Motor Control for Accurate Current Sampling
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Solution Overview
Problem
The one-shunt current detection method in three-phase brushless motors leads to inaccurate two-phase current acquisition due to ringing and fluctuating actual currents during vector control, especially when rotation speed varies, making it difficult to converge rotation speed and angle to target values.
Innovation Solution
The control method adjusts the two-phase current acquisition timing and control period to avoid periods of inaccurate current detection, ensuring accurate current measurement by setting the control period to be double the duration of current acquisition disabled periods and synchronizing current acquisition with PWM pulses to minimize differences between detected and actual currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the one-shunt current detection method is used to perform vector control on a three-phase brushless motor, then the current detection complexity is reduced, but the measurement precision of two-phase current deteriorates due to ringing and inaccurate acquisition
Solution Approach 1:
The patent applies preliminary action by predicting the two-phase current value before actual acquisition. The prediction unit calculates predicted two-phase current values based on previously acquired accurate two-phase current values and one-shunt detected current values, preparing advance estimates that can be used when accurate acquisition is not possible due to ringing periods.
Solution Approach 2:
The patent uses prediction values as an intermediary between the one-shunt current detection system and the vector control system. When accurate two-phase current cannot be acquired during PWM switching periods, the prediction unit provides intermediate predicted values that bridge the gap, allowing continuous vector control without direct reliance on potentially inaccurate shunt resistor measurements during critical periods.
2Speed
If the PWM switching frequency is increased to improve control response, then the control speed improves, but the duration of ringing periods increases relative to the control period, worsening current acquisition accuracy
Solution Approach 1:
The patent applies dynamics by making the control period adaptive rather than fixed. The control period is dynamically adjusted based on the PWM switching frequency and the duration of periods when accurate two-phase current cannot be acquired. This allows the system to maintain accurate current sampling windows even as PWM frequency changes, preventing the ringing period from occupying too large a portion of the control cycle.
Solution Approach 2:
The system performs preliminary calculation of the control period based on expected PWM switching characteristics. Before entering a control cycle, the system determines an appropriate control period length that accounts for upcoming PWM switching events and their associated ringing periods, ensuring accurate current acquisition windows are preserved.
3Productivity
If vector control is performed using detected current values that differ from actual currents, then the control system operates continuously, but the actual current fluctuates greatly, worsening system stability
Solution Approach 1:
The patent implements feedback by continuously comparing one-shunt detected current values with predicted two-phase current values and using this information to refine future predictions. The correction unit uses the difference between detected and predicted values to adjust subsequent prediction calculations, creating a feedback loop that reduces errors over time and stabilizes current control despite using one-shunt detection methodology.
Solution Approach 2:
The system performs preliminary correction of current values before they are used in vector control calculations. The correction unit adjusts the two-phase current values based on prediction errors from previous cycles, preparing corrected current data in advance that compensates for systematic errors in one-shunt detection, thereby preventing current fluctuations before they occur.
Data Source
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AI summary
A three-phase brushless motor control device uses one shunt resistor disposed on a bus of an inverter as a drive circuit driving a three-phase brushless motor and acquires a two-phase current of the three-phase brushless motor at a predetermined timing to vector-control the three-phase brushless motor. At this time, in accordance with a duration in which the two-phase current cannot be accurately acquired, the control device varies an acquisition timing of the two-phase current or varies a period during which the three-phase brushless motor is vector-controlled.