BLDC Rotor Position Detection Accuracy via Back-EMF Ringing Reduction

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Solution Overview

Problem

BLDC motors experience reduced accuracy in rotor position detection due to ringing oscillations in the back EMF voltage caused by RLC components during PWM waveform application, affecting efficient motor operation.

Innovation Solution

A method is introduced where a reverse pulse of shorter duration is inserted before a scheduled PWM on pulse in the floating phase of a BLDC motor, allowing back EMF monitoring during the final portion of the PWM on pulse, which reduces ringing oscillations and improves rotor position detection accuracy.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If PWM waveform is applied to drive the motor phases, then efficient motor operation is achieved, but ringing oscillations occur in the back EMF voltage causing reduced rotor position detection accuracy

Engineering Contradiction:

Improvemotor operation efficiencyVSAvoid rotor position detection accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

A reverse polarity pulse is applied to the floating phase before the PWM on pulse to pre-condition the RLC circuit, counteracting the ringing oscillations that would otherwise occur during the PWM switching. This preliminary counter-action reduces the amplitude and duration of the ringing, allowing for more accurate back EMF monitoring and rotor position detection while maintaining PWM-driven motor efficiency.

Inventive Principle:

Principle #9Preliminary anti-action

2Measurement precision

If back EMF monitoring is performed during PWM on pulse, then rotor position can be detected, but ringing oscillations during the pulse reduce detection accuracy

Engineering Contradiction:

Improve rotor position detection accuracyVSAvoidringing oscillations in back EMF

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The reverse polarity pulse is applied in advance of the PWM on pulse to prepare the circuit state, reducing the ringing oscillations before back EMF monitoring begins. This preliminary action modifies the initial conditions of the RLC circuit, resulting in diminished oscillations during the monitoring window and improved position detection accuracy.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The reverse polarity pulse, which initially appears as an additional control complexity, actually benefits the system by converting the harmful ringing oscillations into reduced-amplitude oscillations. This transformation allows the back EMF monitoring to proceed with significantly improved signal quality and rotor position detection accuracy.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Object-affected harmful factors

If reverse pulse is inserted before PWM on pulse, then ringing oscillations are reduced, but the PWM duty cycle timing is affected

Engineering Contradiction:

Improveringing oscillationsVSAvoidPWM duty cycle timing

Core Design Contradiction:

Object-affected harmful factorsVSLoss of time

Solution Approach 1:

The duration of the PWM on pulse is extended to compensate for the time consumed by the reverse polarity pulse. By adjusting the on pulse width parameter, the effective duty cycle is maintained despite the additional reverse pulse, thereby preserving both the ringing reduction benefit and the required PWM timing characteristics for motor control.

Inventive Principle:

Principle #35Parameter changes

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

This method effectively reduces the impact of ringing oscillations, enhancing the accuracy of rotor position detection and maintaining efficient motor operation by minimizing the effects of non-zero current derivatives and reducing the need for advanced EMC filtering.

Implementation Method 1

inserting a reverse pulse prior to a scheduled PWM on pulse... The reverse pulse may be a pulse of reverse polarity to the on pulse applied to the same stator phase... to reduce the effect of ringing oscillations

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Implementation Method 2

one phase is left floating (undriven) so that the back EMF generated by rotor rotation can be monitored... rotor position can be estimated... using Back EMF monitoring

Methodology Applied

Scientific EffectBack EMF generation: Electromagnetic Induction

Data Source

PatentUS8674639B2Accuracy of rotor position detection relating to the control of brushless DC motors

Publication Date: 2014.03.18 MELEXIS TECH NV

AI summary

During operation of a 3 phase BLDC motor it is driven by use of a PWM waveform applied to one of the driven phase (curve a). The other driven phase is connected thereto but no driving signal is applied (curve b). The third phase is left floating (curve c). This allows the back EMF in the third phase to be monitored for the purpose of determining rotor position by detection of zero crossing points. The rapid switching of the PWM pulses causes ringing in the back EMF signal indicated for one pulse by the ringed portions 1 of curve c. The ringing in the back EMF signal introduces inaccuracy into position calculations derived from back EMF signal measurement. In order to reduce this ringing, in the present invention, a reverse pulse is applied to the other driving coil shown (curve b) prior to a PWM on pulse. The reverse pulse has a polarity such that it drives the phase current through the linked coils in a direction opposite to that caused by the PWM on pulse. This reverse pulse reduces the magnitude of the ringing in the back EMF signal.