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### AN--PIC16F690--BLDC.pdf 关键知识点解析 #### 标题：AN--PIC16F690--BLDC.pdf - **应用领域**：本技术文档主要关注的是如何利用C8051F020微控制器（MCU）实现直流电机（DC）及无刷直流电机（BLDC）的转速控制。 - **核心内容**：介绍了一种基于C8051F020 MCU的应用方法，用于控制直流电机与无刷直流电机的转速。 #### 描述 - **关键词**：MCU、直流电机与无刷直流电机、转速控制 - **主要内容**： - 应用C8051F020进行直流电机和无刷直流电机转速控制的方法； - 软硬件配置实现； - 提供了关于硬件配置、软件编程等方面的具体指导。 #### 知识点详解 ##### 无刷直流电机（BLDC）优势 - **无需机械换向器**：使得电机可以达到更高的转速，因为没有机械换向器的限制。 - **瞬态响应改善**：与传统有刷直流电机相比，BLDC电机不受电刷性能的影响，因此其瞬态响应更好。 - **减少能量损耗**：由于不存在电刷接触，减少了电压降造成的能量损耗。 - **提高速度响应**：电枢电阻限制了反应电压，从而限制了最大转速，而在BLDC电机中这些限制得以减轻。 - **更好的散热性能**：BLDC电机的发热源在定子上，而有刷电机的发热源在转子上，因此BLDC电机更容易散热。 - **降低噪音**：无论是听觉上的还是电磁噪声都得到了显著降低。 ##### BLDC电机类型 - **相数**：定子中的相数不同，常见的有两相和三相电机。 - **极数**：转子中的磁极数量也有所不同。 - **结构布局**：转子相对于定子的位置不同，有的转子位于定子内部旋转，有的则位于外部旋转。 #### 背景 - 为了全面了解三相无刷直流电机，推荐阅读应用笔记“Brushless DC Motor Control Made Easy”（DS00857）。该文档详细介绍了如何使用传感器反馈来驱动无刷直流电机，并提供了先进的比较模式和软件技术，展示了一种改进的无传感器换向策略，具有更高的性能。 #### 电机控制 - **控制过程**： - 第一部分是高效地换向电机。 - 第二部分是在定义的参数内调节电机的转速。 - **目的**：本文档旨在说明一种可以在低成本微控制器上实现的优雅的无传感器技术。所有的演示软件都在开环模式下运行，不包含速度调节功能。 #### 硬件 - **组件**： - 电机功率驱动器：负责提供电机所需的电流和电压。 - 转子位置检测：通过检测反电动势（back EMF）来确定转子的位置，这对于无传感器控制至关重要。 #### 结论本文档不仅为读者提供了对C8051F020 MCU在无刷直流电机控制应用中的深入理解，还涵盖了硬件配置和软件编程的关键要素。通过学习这些内容，工程师们能够更好地掌握如何优化电机的性能并提高系统的整体效率。此外，通过对比不同类型的电机以及它们的特点，读者可以更灵活地选择适合自己应用场景的电机类型。

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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

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