Sensorless BLDC Control with Back-EMF Filtering Using a Majority Function

AN1160 Sensorless BLDC Control with Back-EMF Filtering Using a Majority Function SENSORED CONTROL VERSUS Author: Daniel Torres Microchip Technology Inc. SENSORLESS CONTROL The BLDC motor is used for both consumer and industrial applications owing to its compact size, INTRODUCTION controllability and high efficiency. Increasingly, it is also This application note describes a sensorless Brushless used in automotive applications as a part of strategy to Direct Current (BLDC) motor control algorithm that is eliminate belts and hydraulic systems, to provide ® Digital Signal Controllerimplemented using dsPIC additional functionality and to improve fuel economy. (DSC). The algorithm works utilizing a majority function The continuing reduction in cost of magnets and the for digitally filtering the Back-Electromotive Force electronics required for the control of BLDC motors has (BEMF). Each phase of the motor is filtered to contributed to its use in an increasing number of determine when to commutate the motor drive applications and at higher power levels. voltages. This control technique excludes the need for The BLDC motor is usually operated with one or more discrete, low-pass filtering hardware and off-chip rotor position sensors since the electrical excitation comparators. It should be pointed out that all the must be synchronous to the rotor position. For reasons discussions here, and the application software, of cost, reliability, mechanical packaging and especially assume a 3-phase motor has to be used. The motor if the rotor runs immersed in fluid, it is desirable to run control algorithm described here has six main parts: the motor without position sensors, which is commonly * Sampling trapezoidal BEMF signals using the known as sensorless operation. dsPIC Analog-to-Digital Converter (ADC) It is possible to determine when to commutate the * Reconstructing the Motor Virtual Neutral Point motor drive voltages by sensing the back-EMF voltage * Comparing the trapezoidal BEMF signals to the on an undriven motor terminal during one of the drive reconstructed motor virtual neutral point to detect phases. The obvious cost advantage of sensorless the zero crossing points control is the elimination of the Hall position sensors. * Filtering the signals coming from the comparisons Although there are some disadvantages to sensorless using a majority function filter control: * Commutate the motor driving voltages * The motor must be moving at a minimum rate to * Control loop generate sufficient back-EMF to be sensed * Abrupt changes to the motor load can cause the The purpose of this application note is to explain the BEMF drive loop to go out of lock different parts of the implementation of this new sensorless BLDC motor control technique in a basic * The BEMF voltage can be measured only when and simple form. It also shows that this new control the motor speed is within a limited range of the method is a single-chip dsPIC DSC device-based ideal commutation rate for the applied voltage solution, which does not requires external hardware * Commutation at rates faster than the ideal rate except for a couple of resistors used to conditioning the will