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AN1175

INTRODUCTION

There is a lot of interest in using Brushless DC (BLDC)

motors. Among the many advantages to a BLDC motor

over a brushed DC motor, we can enumerate the

following:

• The absence of the mechanical commutator

allows higher speeds

• Brush performance limits the transient response

in the DC motor

• With the DC motor you have to add the voltage

drop in the brushes among motor losses

• Brush restrictions on reactance voltage of the

armature constrains the length of core reducing

the speed response and increasing the inertia for

a specific torque

• The source of heating in the BLDC motor is in the

stator, while in the DC motor it is in the rotor,

therefore it is easier to dissipate heat in the BLDC

• Reduced audible and electromagnetic noise

There are many different types of brushless motors,

and the differences are:

- The number of phases in the stator

- The number of poles in the rotor

- The position of the rotor and stator relative to

each other (rotor spinning inside the stator

vs. rotor spinning outside the stator)

This application note will discuss the three-phase

motors. Two-phase motors are discussed in AN1178,

“Intelligent Fan Control” (DS01178) while one-phase

motors are a degenerated form of two-phase motors.

BACKGROUND

For a full description of three-phase brushless motors,

read the application note “Brushless DC Motor Control

Made Easy” (DS00857). AN857 is an excellent

description of brushless motors and how to drive them

with sensor feedback for commutation. With more

advanced comparator modes and some new software

techniques, this application note demonstrates an

improved sensorless commutation strategy that has a

much higher performance.

MOTOR CONTROL

BLDC motor control consists of two parts. Part 1 is

commutating the motor at the most efficient rate. Part 2

is regulating the speed of the motor within defined

parameters. The purpose of this application note is to

illustrate an elegant sensorless technique that can be

implemented on low-cost microcontrollers. All demon-

stration software will operate within an open loop with

no speed regulation.

HARDWARE

The hardware for a BLDC system can be decomposed

into the following sections:

- Motor Power Drivers,

- Rotor position detection using back EMF

sensing

- Current Monitoring

- Microcontroller

- Microcontroller Power Supply

- Speed Set-point Input

Motor Power Driver

All BLDC motors require three half-bridge driver

stages. Each stage controls one phase of the motor, as

illustrated in Table 1 below:

Author: Joseph Julicher

Dieter Peter

Microchip Technology Inc.

Sensorless Brushless DC Motor Control with PIC16

AN1175

In this sample schematic, there are three P-Channel

MOSFETS controlling the current flow from +V

CC into

each phase. There are also three N-Channel MOS-

FETS controlling the current flow from each phase into

ground. Between the N-Channel MOSFETS and

ground there is a small resistor (R7) that allows the cur-

rent through the motor to be sensed as a small voltage

proportional to the current. Three BJT transistors are

used to drive the P channel MOSFETs. The N channel

MOSFETs are driven from the PIC

MCU I/O pins. For

small MOSFETS and/or bipolar transistor output

stages, MOSFET drivers are not required.

Back EMF Sensing

In order to learn the current position of the rotor, it is

critical that some form of rotor position sensing is

included. In a sensored design, the rotor position sens-

ing is provided by a series of Hall effect sensors that

react to the permanent magnetics in the rotor. For sen-

sorless designs, the rotor position is provided through

knowledge of when a magnetic pole crosses the non-

driven phase. During each commutation cycle, one

phase is left undriven so it can sense the passing of a

magnet on the rotor. The following circuit is self-biased

and uses one comparator to perform the back EMF

position sensing.

FIGURE 2: BACK EMF SYSTEM

Notice that the back EMF system consists of four

elements with three of them repeating. The purpose of

these elements is to detect the zero-crossing event

even when the V

DD voltages are changing. There are

two easy ways to detect the middle of a sine wave. The

first method is to make an inverted copy and compare

them. The point where the two waves cross is the

midpoint. The second method is to make a reduced

amplitude copy and compare them. Again, the point

where the two waves cross is the midpoint. The

simplest method is the second, because it only requires

a single comparator and a few resistors. Because this

motor is a three-phase system, there are six zero-

crossings per electrical rotation, the rising edge

crossings and three falling edge crossings. When the

commutation takes place, one of the three phase inputs

is selected by writing to the CMxCON0 SFR in the

microcontroller. To save cost, there is not a hardware

filter on the comparator input, therefore, a noisy motor

can cause false zero-crossings. The solution is a

software-based majority detector. To simplify this

majority detector, the polarity bit in the CMxCON0

comparator output polarity with each commutation

event, makes all zero-crossings look like a falling edge

on the comparator output.

Current Monitoring

Current monitoring is a nice feature for any motor con-

trol, but can be especially nice for BLDC motors. The

benefits of current monitoring are:

• High current, No zero-crossings indicate a stuck rotor

• Over-current limiting

• Torque control

Adding current monitoring is a simple task of inserting

a small sense resistor in the ground return path of the

half-bridge switching elements. An op amp may be

necessary if the sense resistor is very small.

The simplest possible over-current monitor is to simply

reset the microcontroller and restart commutation. This

method is shown in Figure 1. The current sense

resistor is used to drive the base of Q7. This transistor

will cause a Reset of the microcontroller, if external

MCLR

is enabled. If external MCLR is not enabled,

then the software can be extended to poll this input and

take corrective action if an over current condition is

detected.

SOFTWARE

The software accomplishes the following tasks:

• Start the motor

• Detect zero-crossing

• Commutate the stator

• Adjust commutation rate to match motor speed

Starting the motor

Starting the motor is the trickiest part of sensorless

drives. The simplest method to start the motor is to

simply start commutating at a slow rate and low duty

cycle. The commutating should “catch” the rotor and, at

some point, the zero-crossing detector will begin to see

crossings. Once zero-crossings can be measured, the

rotor has begun rotating in sync with the commutation,

and normal operation can begin. This method is very

simple, but there are a few problems:

• The motor can spin erratically until sync is achieved.

• The motor can sync at a harmonic of the actual speed

• It can take a long time for the motor to start-up

V_U

V_V

V_W

V_STAR

AN1175

To resolve these drawbacks, there are other methods

that can be used to map the stalled position of the rotor

and immediately start commutating from that point.

For many motors, the simple method of a time out on

the zero-crossing forcing a commutation will result in

satisfactory performance; therefore, this is the method

for this application note.

Zero-Crossing Detector

The zero-crossing system consists of switching the

inputs to a comparator synchronously with the

commutation and monitoring the output of the

comparator. The comparator output is filtered with a

majority detector. This filter is table-driven and looks for

a transition from mostly 1’s to mostly 0’s. Once the

transition is detected, the commutation can take place.

Zero-Crossing Majority Detector

In a noiseless system, zero-crossing events can be

determined by observing when the output of a

comparator sensing the back EMF voltage transitions

from one to zero. Switching high currents at high

voltages introduces a tremendous amount of noise into

the system (see Figure 3). Determining when a zero-

crossing event occurs in such an environment requires

some sort of filtering to mitigate the noise. Filtering with

discrete components adds too much delay to be useful,

especially at high motor speeds. Discrete filters also

vary with temperature, which adds to the complexity of

delay management. A better filter is one that has a

predictable delay that does not vary with the

environment.

A majority filter is one that can be implemented in

software. Software filters have a predictable and fixed

delay that is not affected by the environment. The filter

uses a series of comparator output samples to detect a

zero-crossing event. Zero-crossing is said to have

occurred when most of the first half of the samples are

ones and most of the last half of the samples are zeros.

For a six-sample window, a zero-crossing event is

detected when two or three of the first three samples

are ones and two or three of the last three samples are

zeros. Table 1 illustrates all the possible combinations

that satisfy these criteria.

FIGURE 3: TYPICAL ZERO CROSSING

WITH PWM GENERATED

NOISE

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