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Highly Computationally Efficient White Noise Dithering Of A PWM Carrier In Sensorless BLDC Motor Drives

IP.com Disclosure Number: IPCOM000035305D
Original Publication Date: 2005-Jan-20
Included in the Prior Art Database: 2005-Jan-20
Document File: 1 page(s) / 44K

Publishing Venue

IBM

Abstract

A method for randomizing the PWM carrier of a motor drive control signal is presented that relies on the pseudo random sampling of back-EMF of the motor in conjunction with said PWM signaling to mitigate pure tone emission at the PWM carrier frequency.

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Highly Computationally Efficient White Noise Dithering Of A PWM Carrier In Sensorless BLDC Motor Drives

This invention introduces a random frequency modulated element to the main PWM carrier used to control power to the electric motor.

     Traditional code to introduce this dithering is large as it requires random number generation.

     This invention circumvents that complex computation by taking advantage of intrinsic random noise in the system.

How this works:

          Back-EMF waveforms are trapezoids synthesized by a PWM carrier at 10Khz in our case. The back-EMF has a normalized value ranging anywhere from 0 to 1024 bits. the PWM carrier has a value of 2000 bits in our case.

The above code masks off the lower 7 bits of the current BEMF value sampled every 25uSec and applies it to the motor's carrier every 100uSEC, using an XOR function to enhance randomness.

     This needs to be speed adaptive, as this will interfere with precise commutation at high speeds. This is okay because this problem only leads to acoustic emission at low speeds where the air flow is relatively quiet.

     Additionally, this technique works by adding this dither and decreasing the number of bits of dither as speed increases gradually, as well as simply turning it off over a certain speed.

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