Back EMF Extrapolation for Sensorless BLDC Motor Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Brushless DC motors face challenges in determining rotor position due to the inability to measure back EMF voltage at zero crossing points, especially when demagnetization occurs or when driving voltages are pulsed or chopped, making direct measurement of back EMF signals impossible.
Innovation Solution
A controller system that samples back EMF voltages at predetermined points offset from the zero crossing within each commutation cycle segment, using a linear relationship between voltage and angular position to determine the center point of the segment, allowing synchronization of the commutation cycle with motor speed and position without direct zero crossing measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct zero crossing measurement of back EMF is used to determine rotor position, then measurement precision is improved, but reliability deteriorates when demagnetization occurs or voltages are pulsed/chopped
Solution Approach 1:
The system performs preliminary sampling of back EMF voltage at multiple predetermined points within the commutation cycle segment before the zero crossing occurs. By collecting voltage data at these advance points and using linear extrapolation, the system determines the zero crossing position without requiring direct measurement at the zero crossing instant itself, thus avoiding the reliability issues caused by demagnetization or pulsed voltages.
2Measurement precision
If back EMF voltage is sampled at zero crossing point, then measurement precision is improved, but ease of operation deteriorates due to inability to sample during demagnetization or pulsed periods
Solution Approach 1:
The commutation cycle segment is divided into multiple discrete sampling points at predetermined time intervals. Instead of attempting a single measurement at the zero crossing point, the system segments the measurement process into multiple samples taken at different moments, which are then processed through linear extrapolation to determine the center point position. This segmentation allows operation in various drive configurations including demagnetization and pulsed voltage modes.
3Reliability
If multiple back EMF voltage samples are taken at predetermined points, then reliability is improved, but device complexity increases due to additional sampling and calculation functions
Solution Approach 1:
The controller incorporates intermediary processing functions including a sampling function that collects voltage data at multiple predetermined points, a storage function that holds the sampled values, and a center point locating function that performs linear extrapolation calculations. These intermediary elements mediate between the raw back EMF signals and the final commutation control decisions, enabling reliable sensorless operation while maintaining a manageable controller structure through functional decomposition.
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
Enables reliable sensorless commutation of brushless DC motors by accurately determining commutation cycle center points, eliminating the need for separate position detectors like Hall effect sensors, even in configurations where direct zero crossing measurement is not feasible.
Implementation Method 1
the back EMF voltage changes from the voltage applied during the previous segment to the voltage to be applied in the following segment... the change in voltage is substantially linearly related to the change in angular dimension
Data Source
AI summary
A controller is provided for operating a DC motor through a commutation circuit and for synchronizing the commutation circuit to motor position and speed by measuring back EMF voltages along commutation cycle steps during which the measured phase is not being driven. In connection with each such step, the back EMF voltage is measured at two points offset from the center of the step. The controller uses the measured voltages and their corresponding locations to locate the center point of the step through extrapolation. The motor can be analyzed for determining suitable locations for the measuring points.