Mutual Inductance Voltage Offset Compensation for Sensorless BLDC Motors
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Solution Overview
Problem
Existing brushless DC (BLDC) sensorless motor control circuits face challenges in accurately detecting rotor position due to mutual inductance voltage offset, especially at low speeds, leading to unreliable zero-crossing detection.
Innovation Solution
A control circuit that includes a driver circuit generating drive signals with different current amplitudes and a differencing circuit to sense and determine the difference between mutual inductance voltages, producing a difference signal to compensate for the offset and ensure accurate zero-crossing detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the motor is stopped or running at very low speed, then the mutual inductance voltage becomes very low, but the voltage offset becomes significant relative to the signal, making zero-crossing detection unreliable
Solution Approach 1:
The patent applies parameter changes by varying the current amplitude through multiple levels (first current amplitude, second current amplitude, third current amplitude) to generate corresponding mutual inductance voltages. By comparing voltages at different current parameters, the system can distinguish the actual mutual inductance signal from the fixed voltage offset, enabling reliable zero-crossing detection even at low motor speeds where the signal amplitude is small.
2Device complexity
If a single current amplitude is used for drive signals, then the control circuit is simpler, but the voltage offset cannot be compensated, leading to detection errors
Solution Approach 1:
The patent implements periodic action by cyclically switching between multiple current amplitudes (first, second, and third current amplitudes) in the drive signals. This periodic variation allows the system to sample mutual inductance voltages at different current levels and use the differences to calculate and compensate for the voltage offset, thereby improving detection accuracy without requiring complex additional hardware.
Solution Approach 2:
The patent employs feedback by using the detected mutual inductance voltages at different current amplitudes to determine the voltage offset, which is then used to compensate future measurements. The logic circuit continuously monitors the relationships between voltages at different current levels and adjusts the interpretation of the signals to account for the offset, creating a closed-loop system that improves detection accuracy.
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
The solution effectively cancels mutual inductance voltage offset, ensuring reliable rotor position detection and minimizing errors in zero-crossing detection, even at low motor speeds.
Implementation Method 1
the variable current flowing through the driven winding generates variable magnetic flux. The variable magnetic flux is concatenated to the undriven winding (thanks to the mutual inductance) and generates a voltage across the undriven winding itself (this being referred to in the art as the 'mutual inductance' voltage)
Implementation Method 2
The comparator 22 functions as a zero-crossing (ZC) detection circuit to detect instances where the voltage on the first input (i.e., the voltage on the undriven motor terminal) crosses over the voltage of the reference node 26
Data Source
AI summary
A control circuit controls the operation of a brushless DC (BLDC) sensorless motor having a first terminal connected to a first winding, a second terminal connected to a second winding and a third terminal connected to a third winding. A driver circuit applies drive signals to the first and second terminals and places the third terminal in a high-impedance state. The drive signals include first drive signals at a first current amplitude and second drive signals at a second, different, current amplitude. A differencing circuit senses a first mutual inductance voltage at the third terminal in response to the first drive signals and senses a second mutual inductance voltage at the third terminal in response to the second drive signals. The differencing circuit further determines a difference between the first and second mutual inductance voltages and produces a difference signal that is used for zero-crossing detection and rotor position sensing.