Blog over AS5147 Encoders Enhance BLDC Motor Performance Via Precise Rotor Alignment

BLDC motors, particularly PMSMs, rely on accurate rotor magnetic field angles for optimal performance. This ensures the stator's magnetic field maintains an ideal angle (typically 90 electrical degrees) with the rotor's field, maximizing torque. However, magnetic encoders like the AS5147 measure the absolute position of the rotor's magnet, not the electrical angle required by drive algorithms. This discrepancy introduces a fixed "offset" between the encoder's physical reading and the rotor's magnetic angle.

Uncorrected, this offset acts as a "translation error," causing the stator's magnetic field to misalign with the intended angle. The result? Suboptimal motor performance, especially in open-loop systems without current feedback. While some may dismiss this offset as negligible, it is critical for high-performance drives.

Manually adjusting the offset often feels like groping in the dark. The offset can vary with each motor or even with changes in power cable sequencing, sometimes becoming unstable. Traditional methods, such as locking the motor to align fields, prove inadequate due to mechanical complexities. Below, we outline a structured solution to eliminate guesswork.

Before addressing the offset, verify the motor responds correctly to commands. Incorrect phase sequencing or reversed connections can cause erratic behavior. Follow these steps:

1. Connection and No-Load Test: Connect the motor to the inverter (phase order irrelevant) and ensure the rotor spins freely.
2. Low-Speed Rotation Test: Run a program generating a rotating stator field (e.g., via SVM with a steadily increasing angle).
3. Direction Check: Observe the rotor's rotation. If it spins opposite to the expected direction, a phase error exists.
4. Correction: Swap PWM duty cycles for any two phases (e.g., A and B) to reverse the stator field's rotation.
5. Save Configuration: Store the correct phase sequence in non-volatile memory (e.g., Flash) for future use.

With the phase sequence corrected, measure the offset using one of these methods:

1. Apply Zero-Angle Field: Output a zero-angle SVM voltage vector (aligned with the a-axis) at a moderate amplitude—enough to resist manual rotation but avoid motor damage.
2. Read Encoder: The AS5147 reading at this stage approximates the offset between the rotor field and a-axis.
3. Error Note: Mechanical friction may introduce minor inaccuracies, but this provides a reliable initial estimate.

This approach averages measurements from opposite scans to cancel friction-induced errors.

1. Forward Scan: Run an SVM angle ramp in one direction. At each zero crossing, record and accumulate the encoder reading.
2. Forward Average: After several cycles, compute the mean (≈ offset + friction effect).
3. Reverse Scan: Repeat the process in the opposite direction.
4. Reverse Average: Compute the mean (≈ offset – friction effect).
5. Final Offset: Average the two means to eliminate friction bias.

With the offset known, refine your drive algorithm:

1. True Rotor Angle: Subtract the offset from the AS5147 reading to obtain the rotor's magnetic angle in the (a,b,c) frame.
2. SVM Input: For open-loop control, input the true angle ±90° (depending on desired rotation) to the SVM for precise torque output.
3. Dynamic Adjustment (Optional): Fine-tune the offset by comparing speeds under opposing voltage vectors, ensuring balanced performance.

By following these steps, you can systematically resolve encoder offset issues, achieving smoother and more efficient BLDC motor operation. Say goodbye to trial-and-error debugging and embrace the era of precision control.