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Andrea Katharina Fuchs

Synchronous Rectificationfor a Permanent Magnet

Brushless DC Motor

Studies on Mechatronicsand

Bachelors Thesis

Systems and Control CentreEcole des Mines (ENSM) Paris

Measurement and Control LaboratorySwiss Federal Institute of Technology (ETH) Zurich

Supervision

Philippe Martin

Prof. Lino Guzzella

June 2008

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Contents

Abstract v

Acknowledgement vii

Symbols xi

I Studies on Mechatronics 1

1 Introduction 3

2 Brushless DC-Motor 5

2.1 Brushed DC-Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Brushless DC-Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.1 Permanent Magnet Brushless DC-Motor (PMBLDC-Motor) . 5

2.3 System/Anatomy of Permanent Magnet BL Motor . . . . . . . . . . 7

3 Commutation 9

3.1 Power Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Six Step Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Pulse Width Modulation (PWM) . . . . . . . . . . . . . . . . . . . . 10

3.4 Scheme A: PWM on High Side, 1 on Low Side . . . . . . . . . . . . 12

3.4.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.4.2 Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.5 Scheme B: PWM Simultaneous on High and Low Side . . . . . . . . 14

3.5.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.5.2 Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.6 Scheme C: PWM on High Side, Non-Simultaneous on Low Side . . . 15

3.6.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.6.2 Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.7 Scheme SR: Synchronous Rectification . . . . . . . . . . . . . . . . . 16

3.7.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.7.2 Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.8 Dead Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4 Position Detection 19

4.1 Hall Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.2 Sensorless: BEMF Detection . . . . . . . . . . . . . . . . . . . . . . 19

4.2.1 Conventional BEMF Detection with Virtual Neutral Point . . 20

4.2.2 Direct BEMF Detection during PWM Off Time . . . . . . . . 21

4.2.3 Direct BEMF Detection during PWM On Time . . . . . . . . 22

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II Bachelors Thesis 25

5 Simulation of Different Commutation Schemes 27

5.1 Simulation of Scheme A . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.1.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.1.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . 28

5.2 Simulation of Scheme B . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.2.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


5.3 Simulation of Scheme C . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.3.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30


5.4 Simulation of Synchronous Rectification . . . . . . . . . . . . . . . . 31

5.4.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31


5.5 Comparison of all four Schemes . . . . . . . . . . . . . . . . . . . . . 32

6 Hardware 33

6.1 Microcontroller AT90PWM3B . . . . . . . . . . . . . . . . . . . . . . 33

6.1.1 Generate PWM Signal . . . . . . . . . . . . . . . . . . . . . . 33

6.2 AVR STK500 Evaluation Board . . . . . . . . . . . . . . . . . . . . . 35

6.3 AVR STK520 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.4 ATAVRMC100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.5 Printed Circuit Board (PCB) . . . . . . . . . . . . . . . . . . . . . . 36

6.5.1 Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.5.2 MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.5.3 Voltage Divider / Low Pass Filter . . . . . . . . . . . . . . . 36

6.5.4 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

7 Synchronous Rectification 39

7.1 Precedent Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.2 Implementation of SR . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.2.1 Testprogram . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7.2.2 PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.2.3 Dead Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.2.4 Position Control . . . . . . . . . . . . . . . . . . . . . . . . . 41

7.2.5 Velocity Control . . . . . . . . . . . . . . . . . . . . . . . . . 417.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.3.1 Test 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7.3.2 Test 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

8 Sensorless Commutation 47

8.1 Direct BEMF Detection . . . . . . . . . . . . . . . . . . . . . . . . . 47

8.1.1 During On Time . . . . . . . . . . . . . . . . . . . . . . . . . 47

8.1.2 During Off Time . . . . . . . . . . . . . . . . . . . . . . . . . 48

8.1.3 Commutation Filter . . . . . . . . . . . . . . . . . . . . . . . 48

8.2 Final Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . 49

9 Conclusions and Outlook 51

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A Photos and Schematics 52A.1 Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

A.1.1 Permanent Magnet Brushless DC-Motor . . . . . . . . . . . . 52

A.1.2 PCB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53A.2 Schematic Diagram of Card . . . . . . . . . . . . . . . . . . . . . . . 54

B Codes 55B.1 With Hall Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

B.1.1 Scheme A, 2 Ramp Mode, 1 directly on Low . . . . . . . . . 55B.1.2 SR, 2 Ramp Mode, 15.6 kHz . . . . . . . . . . . . . . . . . . 66B.1.3 SR, 2 Ramp Mode, 7.8 kHz . . . . . . . . . . . . . . . . . . . 77B.1.4 SR, 2 Ramp Mode, 3.9 k Hz . . . . . . . . . . . . . . . . . . . 88

B.2 Sensorless . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99B.2.1 Scheme A, Centre Aligned Mode . . . . . . . . . . . . . . . . 99B.2.2 SR, Centre Aligned Mode . . . . . . . . . . . . . . . . . . . . 114

C AVR Butterfly 129C.1 C Programming for Microcontrollers . . . . . . . . . . . . . . . . . . 129C.2 AVR Butterfly kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129C.3 Personal Recommendation for Learning to Program Microcontrollers 129

Bibliography 131

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Abstract

In this thesis an advanced control scheme called synchronous rectification is ex-plained, analysed and implemented, and its application to motors driving quadro-tors is discussed. These motors require high durability and low energy losses. Toobtain the required durability, permanent magnet brushless DC motors are chosen,

and the control functions preferably in a sensorless mode. The energy efficiencyis improved with synchronous rectification by controlling the current flow throughtransistors instead of diodes. With the help of synchronous rectification, it is pos-sible to change the direction much faster, enabling the motor to be more reactiveand dynamic. In this project, synchronous rectification is effectively implementedin a motor system both ways with and without sensors. Also, the projects imple-mentation and necessary tools are described to support further research.

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Acknowledgement

I would like to express my gratitude to all those who gave me the chance to do thisthesis. I want to thank the staff of the MAVT department administration office,the student exchange office of the ETHZ and Pierre Baladi of the international af-fairs office at Ecole des Mines for their administrative organisation of my exchange

semester.Furthermore, I have to thank my supervisor Philippe Martin who guided me through-out my project and always supported me. In the same way, I have to thank Prof.Lino Guzzella for his confidence and everlasting support.I am deeply indebted to my colleague in Paris, Caroline Claasen. She was verypatient and helped me wherever she could. It was a pleasant collaboration and shemotivated me to continue with my work.Additionally I have to thank my friends Eva Frommelt, Candy Brakewoo, FdadyAssassa, Fabian Ehrich and Lara Montini, who looked closely at the final version ofthe thesis for English style and grammar, correcting both and offering suggestionsfor improvement.Lastly, I thank my parents for funding and encouraging me during my exchange.

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List of Figures

2.1 Equivalent Circuit Diagram of the Motor . . . . . . . . . . . . . . . 62.2 Equivalent Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Signals for Six Step Mode . . . . . . . . . . . . . . . . . . . . . . . . 103.3 Results of Six Step Mode . . . . . . . . . . . . . . . . . . . . . . . . 103.4 Pulse Width Modulation . . . . . . . . . . . . . . . . . . . . . . . . . 113.5 Pulse Width Modulation in Six Step Mode . . . . . . . . . . . . . . 113.6 High side and low side closed . . . . . . . . . . . . . . . . . . . . . . 123.7 High side open, low side closed . . . . . . . . . . . . . . . . . . . . . 123.8 Duty cycle of PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.9 High side and low side closed . . . . . . . . . . . . . . . . . . . . . . 143.10 High side and low side open . . . . . . . . . . . . . . . . . . . . . . . 143.11 D uty cycle of PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.12 Non simultaneous with four states . . . . . . . . . . . . . . . . . . . 153.13 High side and low side closed . . . . . . . . . . . . . . . . . . . . . . 153.14 High side and low side open . . . . . . . . . . . . . . . . . . . . . . . 153.15 Signals for Synchronous Rectification . . . . . . . . . . . . . . . . . . 163.16 H A and LC closed . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.17 HA open, LA and LC closed . . . . . . . . . . . . . . . . . . . . . . . 163.18 Dead Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.1 Sensorless Detection Techniques Overview . . . . . . . . . . . . . . . 204.2 BEMF in Phase with Current and Zero Crossing . . . . . . . . . . . 204.3 Model of Virtual Neutral Point . . . . . . . . . . . . . . . . . . . . . 204.4 Current during PWM off time . . . . . . . . . . . . . . . . . . . . . . 214.5 Current during PWM on time . . . . . . . . . . . . . . . . . . . . . . 23

5.1 Simulink Model for Simulation of PWM on High Side, 1 on Low Side 28

5.2 Current through Phase A . . . . . . . . . . . . . . . . . . . . . . . . 285.3 Zoomed Current Ripple . . . . . . . . . . . . . . . . . . . . . . . . . 285.4 Simulink Model for Simulation of PWM Simultaneous on High and

Low Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.5 Current through Phase A . . . . . . . . . . . . . . . . . . . . . . . . 295.6 Zoomed Current Ripple . . . . . . . . . . . . . . . . . . . . . . . . . 295.7 Simulink Model for Simulation of PWM on High Side, Non-Simultaneous

on Low Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.8 Current through Phase A . . . . . . . . . . . . . . . . . . . . . . . . 305.9 Zoomed Current Ripple . . . . . . . . . . . . . . . . . . . . . . . . . 305.10 Simulink Model for Simulation of Synchronous Rectification . . . . . 315.11 Current through Phase A . . . . . . . . . . . . . . . . . . . . . . . . 315.12 Zoomed Current Ripple . . . . . . . . . . . . . . . . . . . . . . . . . 315.13 Current of all four Schemes . . . . . . . . . . . . . . . . . . . . . . . 32

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5.14 Angular velocity of all four Schemes . . . . . . . . . . . . . . . . . . 32

6.1 Experimental Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.2 Sensorless Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336.3 2 Ramp Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346.4 Centre Aligned Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 356.5 ATMELs STK500 with STK520 . . . . . . . . . . . . . . . . . . . . . 356.6 Printed Circuit Board . . . . . . . . . . . . . . . . . . . . . . . . . . 366.7 Voltage Divider / Low Pass Filter . . . . . . . . . . . . . . . . . . . 376.8 AREF Voltage Divider / Low Pass Filter . . . . . . . . . . . . . . . 376.9 Testing Low and High Side Driver and MOSFET . . . . . . . . . . . 386.10 Oscilloscope Measurement . . . . . . . . . . . . . . . . . . . . . . . . 38

7.1 Voltage at Phase B . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397.2 Experimental Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . 407.3 Simple PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407.4 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407.5 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427.6 PWM signals with a Duty Cycle of 50/256 . . . . . . . . . . . . . . . 437.7 Phase A and Signal to HA . . . . . . . . . . . . . . . . . . . . . . . . 447.8 Phase A and Signal to HA . . . . . . . . . . . . . . . . . . . . . . . . 44

8.1 ZC Detection During On Time . . . . . . . . . . . . . . . . . . . . . 478.2 Negative Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488.3 ZC Detection During Off Time . . . . . . . . . . . . . . . . . . . . . 488.4 Experimental Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

A.1 Photo of the Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

A.2 Backside of the Board . . . . . . . . . . . . . . . . . . . . . . . . . . 53A.3 VD/LPF on Phases . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

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Symbols

Symbols

ua, ub, uc Phase voltages on stator

ia, ib, ic Phase currents

ea, eb, ec Phase back electromotive forces

LS Inductor of the stator windings

RS Resistor of the stator windings

Flux induced into the stator through the permanent magnets of the rotor

Rotor angle

J Combined inertia of rotor and load

p Number of pole pairs

Angular velocity of the rotor

Te Electromagnetic torque

TL Shaft mechanical torque

D Duty cyclef Cut-off frequency

Acronyme and Abbreviation

PM BLDC-Motor Permanent Magnet Brushless Direct Current Motor

C Microcontroller

PWM Pulse Width Modulation

HA, HB, HC High Side Driver and/or Transistor of Phase A, B, C

LA, LB, LC Low Side Driver and/or Transistor of Phase A, B, C

BEMF Back Electromotive ForceZC Zero Crossing

SR Synchronous Rectification

PSC Power Stage Controller

VD Voltage Divider

LPF Low Pass filter

PLL Phase-locked Loop

DAC Analogue to Digital Converter

ADC Digital to Analogue Converter

ISP In-System-Programming

RPM Revolutions Per Minute

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Part I

Studies on Mechatronics

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Chapter 1

Introduction

This Bachelors thesis explains the function and the implementation of an ad-vanced commutation scheme called synchronous rectification for a permanent mag-net brushless DC motor.The motor is used for driving a quadrotor. The quadrotor is a helicopter drivenby four fixed rotors on every end of the two axes. It moves by varying the inducedtorques resulting from the rotor speed. Therefore, each rotor needs to be controlledseparately by motor.The first quadrotors were built in the early 1920s and were designed to transportpassengers. With the advancement of swashplates for normal helicopters, quadro-tors fell into oblivion. In recent years, the control of unmanned quadrotors receivedmore and more interest. Due to improvements of electronics and sensor systems,the quadrotor can achieve fast pitching moments, and its construction becomes lesscomplicated.

Each rotor of the quadrotor is driven by a permanent brushless DC motor. Thismotor is driven by a microcontroller, and the control system is designed to obtainhigh energy efficiency.Synchronous rectification is a control technique with the main purpose to lower theenergy losses. Another benefit of the synchronous rectification is the possibility offast velocity changes and turnarounds.The motor control is achieved by a power bridge. This power bridge generates a3-phase voltage connected to the motor. To vary the supplied voltage on the motor,transitors on the power bridge are driven by pulse width modulation. By appliyingsynchronous rectification, the current flows through the transistors instead of theflyback diodes, which decreases the energy losses.The major topic of this thesis is the analysis of the feasibility of the synchronous

rectification, and its implementation for the permament magnet brushless DC mo-tor. The thesis consists of two parts, the studies on mechatronics and the Bachelorsthesis. In the first part, chapters 2 - 4, the theory behind the motor, the commu-tation modes and the sensorless control is explained. The second part simulatesand implements the synchronous rectification. In chapter 5, simulations of differentcommutation schemes in Matlab/Simulink are demonstrated and discussed. Thenecessary tools are illustrated in chapter 6. The implementation of synchronousrectification is explained in chapter 7 and the results are discussed. Chapter 8 ex-plains the implementation of synchronous rectification in a sensorless control of apermanent magnet BLDC motor. A sensorless control advantageously simplifiesthe construction of the quadrotor, decreases the maintenance and reduces costs.Different sensorless techniques exist, the most applicable solutions are explained inchapter 4.

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4 CHAPTER 1. INTRODUCTION

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Chapter 2

Brushless DC-Motor

An electric motor consists of a fixed stator and a moving rotor. Electrical energy istransformed into mechanical energy by the magnetic force that acts on the rotor. Asynchronous motor is characterised by a synchronous turning of the rotor and theoscillating electromagnetic field or, for a permanent magnet rotor, the oscillatingcurrent in the stator. In general, DC motors need direct current to run. However,there are many DC motors that run with alternating current without any problems.The most common constructions of DC motors are the brushless and the brushedversion. Both are discussed in this chapter. Additionally, the operating mode ofthe motor used in this project and its mathematical model is explained.

2.1 Brushed DC-Motor

The stator of a brushed DC-motor is made up of a constant magnetic field in whichthe rotor with one or several windings is placed. Due to the Lorentz force, a torqueacts on the rotor. By continuously changing the poles of the rotor windings, thecommutator keeps the current flowing through the rotor windings. This current isalways perpendicular to the static magnetic field, which induces a Lorentz force onthe rotor, and therefore it continues to rotate.

2.2 Brushless DC-Motor

The rotor and the stator of a brushless DC-motor function differently from thebrushed one. The rotor consists of a constant magnetic field, provided by either a

permanent or an electric magnet. The poles of the stator windings are electricallycommutated, and therefore no brushes are required. This simplifies the maintenanceand extends the life of the motor. One advantage of a brushless DC-motor is thatit does not have any slip unlike normal induction motors. Compared to a brushedDC-motor, a brushless motor has a higher efficiency.

2.2.1 Permanent Magnet Brushless DC-Motor (PMBLDC-Motor)

The rotor of the motor used in this project consists of a permanent magnet. Apermanent magnet BLDC-motor requires a trapezoidal current to obtain a uniformflux profile in the rotor. While the rotor is turning, a back electromotive force isinduced in the stator windings. This force also has a trapezoidal shape. Contraryto this, the permanent magnet synchronous motor (PMSM), which is constructed

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6 CHAPTER 2. BRUSHLESS DC-MOTOR

similarly, is designed to have a sinusoidal BEMF and must therefore be driven si-nusoidally.

The motor used in this project has three windings on the stator and hence it worksin 3-phase. These three phases can be modelled as three equivalent circuits in astarlike arrangement. Every phase consists of a winding, its resistance and theBEMF, as depicted in figure 2.1.

Vn

R

R

R

L

L L

-

-

-

+

++

e

ee

Figure 2.1: Equivalent Circuit Diagram of the Motor

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2.3. SYSTEM/ANATOMY OF PERMANENT MAGNET BL MOTOR 7

2.3 System/Anatomy of Permanent Magnet BLMotor

The models used in this chapter are based on the model of the Permanent MagnetBL DC-motor of SymPowersystem of Matlab/Simulink [8]. The equivalent circuitof the motor from figure 2.1 is arranged more clearly in 2.2. The voltage over eachindividual phase consists of the sum of the induced voltage plus the voltage changeover the resistance plus the BEMF. Equations (2.1), (2.2) and (2.3) describe thevoltages of Phase A, B and C.

ua= RS ia+ LS dia

dt + ea (2.1)

ub= RS ib+ LS dib

dt + eb (2.2)

uc = RS ic+ LS dic

dt + ec (2.3)

RS

LS

+ -ia

+ -ib

+ -ic

ea

eb

ec

ua

ub

uc

A

B

C

RS

RS

LS

LS

Figure 2.2: Equivalent Circuit

with

ua, ub, uc Phase voltages on stator

ia, ib, ic Phase currents

ea, eb, ec Phase back electromotive forces

LS Inductance of the stator windings

RS Resistance of the stator windings

The back electromotive force is proportional to the rotational speed of the rotor, andit is also a function of the rotor angle. For a permanent magnet BL DC Motor theBEMFs have a trapezoidal shape, and the phases are delayed 120 electrical degrees.Equations (2.4), (2.5) and (2.6) describe the BEMF. Function e() is 2-periodicand trapezoidal between -1 and 1, e() [1, 1].

ea= ea= e() (2.4)

eb= e

b= e( 2

3 ) (2.5)

ec = e

c = e( 4

3 ) (2.6)

with

Flux induced into the stator through the permanent magnets of the rotor

Rotor angle

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8 CHAPTER 2. BRUSHLESS DC-MOTOR

The angular acceleration of the rotor is a function of the electromagnetic torque Te,the mechanical shaft torque TL and the combined inertia of the rotor and load J.When friction is neglected, the equation (2.7) depicts this correlation as

d

dt =

1

J(Te TL). (2.7)

The electromagnetic torque is related to the current through the phase and is definedas

Te= p (e

a ia+ e

b ib+ e

c ic). (2.8)

By inserting the definition of the electromagnetic torque in equation (2.7), then themotor equation (2.9) results in

Jd

dt =

1

(ia ea+ ib eb+ ic ec) TL (2.9)

with

J Combined inertia of rotor and load

p Number of pole pairs

Angular velocity of the rotor

Te Electromagnetic torque

TL Shaft mechanical torque

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Chapter 3

Commutation

The main objective of commutation is to obtain a constant torque. Therefore,the current through the stator windings needs the correct shape and has to bevaried correctly. This chapter explains how to control the motor with differentcommutation modes and pulse width modulation to obtain a desired average voltage.An electrical rotation of the rotor is divided into six parts. When the rotor turnsto the next part, the current has to be commutated. With advanced commutationschemes, energy can be saved and higher efficiency achieved.

3.1 Power Bridge

To obtain the required trapezoidal shape of the current, a power stage of threehalf bridges is used. Each half bridge consists of two transistors and two flyback

diodes across them. Figure 3.1 shows the set-up of the power stage. Each ofthe three phases A, B and C of the motor is connected to a half bridge. Thetransistors connected to the voltage are called high side transistors (HA, HB, HC)and the others that are connected to the ground are called low side transistors(LA, LB, LC). The transistors are activated by digital signals. When receiving a 1,the transistor closes the connection and when receiving a 0, it opens. With theseswitches, the voltage connection on the motor can be changed. In each step, onephase is connected to the positive supply voltage by closing the high side transistorand one phase is connected to the negative by closing the low side transistor. Thethird phase is potential-free, while both transistors are open. Six commutation stepsresult from these voltage connections of the three phases. Although the motor actsin three phases, the power stage is fed with a DC voltage source and thus the motor

is called DC motor.

+

-

M

HA

LA

HB HC

LB LC

A

B

C

Figure 3.1: Power Stage

9

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10 CHAPTER 3. COMMUTATION

3.2 Six Step Mode

A complete electrical revolution of the rotor is divided into six parts, each of them

includes 60 electrical degrees. The number of pole pairs affects how many electricalrotations result in an angular rotation. For example, when the rotor has two polepairs, two electrical revolution result in one physical rotation of the motor. In sixstep mode, every phase is connected to the voltage ground during 120 electricaldegrees. During the next 120 electrical degrees, it remains floating, before connect-ing to the ground for the remaining 120 electrical degrees. Figure 3.2 shows thesignals for the transistors in six step mode. During the first step, phase A has tobe connected to the supply voltage, so the high side transistor HA has to be closedand receives a 1. Phase C has to be connected to the ground and consequently thelow side transistor LC has to be closed by receiving a 1. The third phase is floating,so both transistors are open. The resulting behaviour of the rotor speed, the torqueand the current through the stator windings are illustrated in figure 3.3.

Step 1

HA

LA

HB

LB

LC

HC

Step 2 Step 3 Step 4 Step 5 Step 6

Figure 3.2: Signals for Six Step Mode

Figure 3.3: Results of Six Step Mode

3.3 Pulse Width Modulation (PWM)

With pulse width modulation, a preset average value of the supply voltage can beobtained. Therefore, the connected voltage alternates between two values, usually,the supply voltage and zero. Compared to voltage dividers, a higher power efficiencycan be achieved when using PWM. As voltage dividers reduce the voltage through

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3.3. PULSE WIDTH MODULATION (PWM) 11

heat emission over resistors or capacitors, they cause high energy losses. In PWMmode, the transistors work only in full conduction or locking mode. Thus the energylosses are minimised as the power supply is not constantly connected to the motor.

Due to this variation from a constant frequency, the overall power consumption canbe described by an average value. This average value is a desired percentage of themaximum value. The duty cycle describes this percentage and affects the on andoff time of the signal. With a duty cycle of 0.2, the signal is switched on for 20%of the time and switched off for 80%. This results in a overall power consumptionof 20% of the maximum value. Three different duty cycles and their signals areillustrated in figure 3.4.

PWM periodT

T

T

2T

2T

2T0.2 T

0.5T

0.9T

20%

50%

90%

on time

off time

average value

Figure 3.4: Pulse Width Modulation

The average value of the connected voltage to the motor is proportional to the rotorspeed. When a higher voltage is connected, a stronger current flows through the

stator windings and induces a higher force on the rotor. With PWM, the desiredaverage voltage on the motor and the resulting rotor speed can be achieved. Whileone phase is connected to the supply voltage, the high side transistor is activatedwith a PWM signal, and therefore the motor is connected with the resulting aver-age voltage. In figure 3.5, the six step mode with PWM signals on the high sidetransistor is illustrated.

Step 1 Step 2 Step 3 Step 4 Step 5 Step 6

HA

LA

HB

LB

LC

HC

Figure 3.5: Pulse Width Modulation in Six Step Mode

The frequency of the PWM should be much higher than the motor frequency. Thusthe higher frequency can be filtered out, and the two frequencies do not influenceeach other. To obtain the desired PWM signal, a counter with the PWM frequencyis compared with the value depending on the duty cycle. If the counter value corre-sponds with the reference value, the PWM signal changes. The specific applicationis described in chapter 6.1.1.One of the disadvantages of PWM is the high switching noise, which is even higherif multiple transistors switch at same the time.

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3.4 Scheme A: PWM on High Side, 1 on Low Side

As this scheme is the most common and the simplest, it has been already described

in the section before and is illustrated in 3.4.

3.4.1 Design

The temporary active high side transistor is driven by a PWM signal while the lowside is permanently switched on during the active phases. When both transistorsare closed, the current flows through two phases as pictured in figure 3.6. Whenthe high side transistor is open, the current flow through the flyback diode and thelow side transistor is closed, as shown in figure 3.7.

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA LB LC

i

i

V

A

B

C

Figure 3.6: High side and low sideclosed

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA L B LC

i

i

V

A

B

C

Figure 3.7: High side open, low sideclosed

When both the high and low side transistors are closed, the following voltages areconnected to the phase A and C:

va = V =ua+ vN (3.1)

vc = 0 = uc+ vN (3.2)

Using equations (2.1) and (2.1), Voltage V results in

V =ua uc = Rs (ia ic) + Ls (dia

dt

dic

dt) + ea ec. (3.3)

The current is constant, so didt = 0. It flows through the transistors HA and LC

ia= i (3.4)

ib= 0 (3.5)

ic = i (3.6)

The BEMF is proportional to the angular speed multiplied by the induced flux ofthe magnet.

ea = e

a (3.7)

ec = e

c (3.8)

e is a trapezoidal function between 1 and -1. With equations (3.7), (3.8) and thecurrents of (3.4) and (3.5), the voltage in (3.3) can be written as

V = 2 Rs i + 2 (3.9)

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3.4. SCHEME A: PWM ON HIGH SIDE, 1 ON LOW SIDE 13

The current i can be calculated from the motor equation (2.9) by inserting thecurrents and BEMFs

Jd

dt = 0 =

1

(ia ea+ ib eb+ ic ec) TL

= 1

(i ( ) + 0 + (i) ( )) TL (3.10)

The resulting current is then:

i=TL

2 (3.11)

The applied duty cycle of the PWM signal can be calculated with the source voltageUDCand the average voltage V(3.9), which depends on the angular speed.

Figure 3.8: Duty cycle of PWM

V T =D T UDC D = V

UDC(3.12)

3.4.2 Effects

The motor acts in 2-quadrants, which means that it can drive forwards and back-wards. This simple commutation scheme has high switching losses, but good costefficiency and can be easily implemented.

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3.5 Scheme B: PWM Simultaneous on High andLow Side

3.5.1 Design

The low and high side transistors used for this design are controlled by the samePWM signal. Therefore, a new state results when all transistors are open and thecurrent flows through the flyback diodes. As shown in figure 3.10, the negativesupply voltage is then connected to the motor.

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA L B LC

i

i

V

A

B

C

Figure 3.9: High side and low sideclosed

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB

LA L B LC

i

i

HC

V

A

B

C

Figure 3.10: High side and low sideopen

The connected voltage varies between +UDCand UDC, which is pictured in figure3.11. Thus the duty cycle is defined as

V T =D T UDC+ (1 D) T (UDC) D =V + UDC

2 UDC. (3.13)

Figure 3.11: Duty cycle of PWM

3.5.2 Effects

Unfortunately, the switching losses are high, but the motor can function in 4-quadrant mode. Beside operating in both directions, the motor can also breakfrom forward and reverse. Because the high and low side transistor switch at thesame time, the switching noise is high.

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3.6. SCHEME C: PWM ON HIGH SIDE, NON-SIMULTANEOUS ON LOW SIDE15

3.6 Scheme C: PWM on High Side, Non-Simultaneouson Low Side

3.6.1 Design

The PWM signal on the low side transistor is delayed by half a PWM mode tothe signal on the high side. Four connection states result of this mode and areillustrated in 3.12.

HA

LC

I II III IV

Figure 3.12: Non simultaneous with four states

In state I, no voltage is connected to the motor what is pictured in figure 3.13.While discharging the inductances, the current flows through the closed high sidetransistor and the flyback diode on the high side of the other phase. State II is thesame as before in 3.9. As it can be seen in figure 3.14, the current in state III flowsthrough the closed transistor and the low side diode for unloading the inductances.In state IV, all transistors are open and the current flows just like in figure 3.10with the reverse supply voltage connected to the motor.

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB

LA LB LC

i

i

HC

V

A

B

C

Figure 3.13: High side and low side

closed

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB

LA LB LC

i

i

HC

V

A

B

C

Figure 3.14: High side and low sideopen

3.6.2 Effects

Due to the four states during one PWM cycle, the connection is changed twice morethan by the other commutation schemes. The switching noise is lower, because thetransistors do not switch at the same time.

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3.7 Scheme SR: Synchronous Rectification

3.7.1 Design

The low side transistor is activated in a manner opposite to the high side one.The low side transistor of the other active phase in this step is still closed. Thecommand signals are illustrated in 3.15. The current flows through the closed lowside transistor instead of the diode, which is pictured in figure 3.17. The duty cycleand the current can be similarly calculated as in section 3.4.

Step 1 Step 2 Step 3 Step 4 Step 5 Step 6

HA

LA

HB

LB

LC

HC

Figure 3.15: Signals for Synchronous Rectification

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA L B LC

i

i

V

A

B

C

Figure 3.16: HA and LC closed

+

-

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA L B LC

i

i

V

A

B

C

Figure 3.17: HA open, LA and LCclosed

3.7.2 Effects

The current flows through the transistors instead of the diodes. When the conduc-tion losses through the transistors are less than through the diode, energy can besaved with this mode.

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3.8. DEAD TIME 17

3.8 Dead Time

It is important to avoid a short circuit when driving the motor by a power stage.

The high and low side transistors should never be closed at the same time, becausethis will result in a short circuit. Therefore a so-called dead time is implemented,if both transistors are driven after each other or by PWM. The dead time has toaccount for both the normal conduction time of a transistor as well as switch onand turn off time. It is also important to note that transistors usually require moretime to turn off than to switch on. The figure 3.18 illustrates the synchronousrectification signal of the high and low side transistor by inserting the dead time.

dead time dead time

high side

low side

Figure 3.18: Dead Time

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Chapter 4

Position Detection

The rotor position is necessary to control the motor. The motor works in a sixstep mode, and therefore the sole information required is the sector in which therotor is located momentarily. This can be obtained by Hall sensors or by sensorlesstechniques.This chapter explains the different position detection techniques, especially BEMFdetection. The BEMF detection will focus on the method used by Jianwen Shao[9].

4.1 Hall Sensors

Three Hall sensors are placed in the stator on the non-rotating end. The distancebetween two sensors can be 60 or 120. When the N or the S pole of the rotor ispassing by a sensor, a high or a low signal is transmitted. The sensor responds tothe passing magnetic field of the rotor, which is rectangular to the current throughthe sensor. The three sensor signals detect the instantaneous rotor position.

4.2 Sensorless: BEMF Detection

There are two primary techniques of sensorless position control that are illustratedin the literature. The first method measures the back electromotive force induced in

the stator windings. The second estimates the position using the motor parameters.Figure 4.1 shows a survey of the different sensorless detection modes. In this thesis,only the BEMF detection is discussed, as it is the most common sensorless applica-tion. The BEMF detection with a virtual neutral point and the direct detection areanalysed. Some other BEMF detection techniques exist, (position detection basedon the third harmonic of the voltage, BEMF integration etc.), but they are notfurther explained here.

To obtain a high energy efficiency, the torque-to-current ratio should be optimal.Therefore current through the stator windings should be controlled such that it isin phase with the BEMF. The rotor position affects the commutation steps and theform of the BEMF. When the BEMF can be detected, the commutation can be setat the right moment. The BEMF detection consists of measuring the zero crossingof the BEMF, which occurs in the middle of a commutation step. In figure 4.2, thecurrent in phase with the BEMF and the moment of the zero crossing is illustrated.

19

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20 CHAPTER 4. POSITION DETECTION

Sensorless

BEMF Parameters

Virtual Neutral Point Direct BEMF others

PWM on time PWM off time

Figure 4.1: Sensorless Detection Techniques Overview

Current

BEMF

Step

Zero Crossing

Figure 4.2: BEMF in Phase with Current and Zero Crossing

4.2.1 Conventional BEMF Detection with Virtual NeutralPoint

The BEMF is applied to the undriven voltage phase, which can be measured if thevoltage at the neutral point of the motor is known. Due to the inaccessibility of theneutral point, a model is derived from three resistors to estimate the voltage at theneutral point, as presented in figure 4.3.

One clear disadvantage of this detection technique is the instability of the virtual

Vn

R

R

R

L

L L

-

-

-

+

++

e

ee

Figure 4.3: Model of Virtual Neutral Point

neutral point as a consequence of the PWM drive. To measure a steady value, lowpass filters and voltage dividers, which complicate the detection mode, are necessary.

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4.2. SENSORLESS: BEMF DETECTION 21

4.2.2 Direct BEMF Detection during PWM Off Time

The advantage of this approach for BEMF detection is that measurements during

the PWM off time avoid the instability of the virtual neutral point. When the highside transistor is driven by the PWM and the corresponding low side transistor isclosed, the voltage at the floating phase can be measured. For example, the firststep is examined when HA receives a PWM signal and LC is closed. During thePWM on time, the voltage is supplied at phase A and charges the inductors. DuringPWM off time, the transistor HA is open and the inductors can be discharged. Thatgenerates a current flowing through the phases A and C, as illustrated in figure 4.4.At the floating phase B, the phase voltage ub can be measured.

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA LB LC

i

i

Comparator

VRef

VDC

ua

ub

uc

vN

Figure 4.4: Current during PWM off time

During PWM off time, no current flows through phase B, and therefore the measuredvoltage ub consists of the BEMF of the phase B eb and the voltage at the neutralpoint vN:

ub= eb+ vN (4.1)

Because phase A and C are connected to the ground, the voltage vNcan be describedthrough

vN = 0 vd RS i LS di

dt ea (4.2)

and

vN = 0 + vt+ RS i + LS di

dt ec (4.3)

withvt as the voltage over the transistor and vd as the voltage over the diode.When adding (4.2) and (4.3) and dividing by two, this results in

vN = 1

2(ea+ ec) +

vt

2

vd

2 . (4.4)

vt is the voltage over the transistor during conduction, which is very low and canbe ignored. The voltage over the conducting diode vd is approximately 0.7 V. Thevoltage at the neutral point is

vN = 12

(ea+ ec) vd2 . (4.5)

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The three BEMFs build a 3-phase system and equation (4.6) describes the BEMFs.

ea+ eb+ ec= e3 (4.6)

For further processing, the third harmonics are always ignored. Equation (4.6) istherefore rewritten as

ea+ eb+ ec= 0. (4.7)

From equation (4.5) and (4.7), the voltage at the neutral point becomes

vN = eb

2

vd

2 . (4.8)

By applying this result to equation (4.1), the measured voltage at phase B is

ub= eb+ vN =3

2 eb

vd

2 . (4.9)

For a general case and for simplification, the voltage over the diode is neglected.The measured voltage at phase B can be written

ub= 3

2 eb. (4.10)

As it can be seen in equation (4.10), the measured voltage at the floating phase isdirectly proportional to the BEMF. If the voltage at the floating phase is detected,the BEMF can be calculated directly. As it has been shown, the knowledge of thevoltage at the neutral point is not required when measuring during PWM off time.

The zero crossing of the BEMF can be detected with a comparator that checksthe measured voltage with respect to the reference value. This occurs in the middleof a step and with the information of the zero crossing time, the commutation canbe set.The advantage of this type of BEMF detection is, as it has been shown before, thatthe voltage at the neutral point is not necessary. Because the BEMF is not inducedat the beginning of the motor rotation, an additional start up process has to beprogrammed. This supplementary work is one of the disadvantages of the directBEMF detection. Another drawback of this BEMF detection is that it runs onlyfor low PWM duty cycles. So if the PWM off time for high velocities is too short,measurements are impossible.

4.2.3 Direct BEMF Detection during PWM On Time

To avoid the problem of a PWM off time that is too short or nonexistent, theBEMF is measured during PWM on time. Therefore, the first commutation step isanalysed again. During PWM on time, phase A is connected to the supply voltageand phase C to ground, as illustrated in figure 4.5.The voltage at the neutral point is the sum of the voltages at the phases, over theinductors and the resistors and the BEMF, with vt as the voltages over the twotransistors.

vN =vDC vt RS i LS di

dt ea (4.11)

vN = 0 + vt+ RS i + LS didt ec (4.12)

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4.2. SENSORLESS: BEMF DETECTION 23

+ -

+ -

+ -

RS

RS

RS

LS

LS

LS

BEMF

BEMF

BEMF

HA HB HC

LA LB LC

i

i

Comparator

VRef

ua

ub

uc

vN

HA

LA

VDC

Figure 4.5: Current during PWM on time

By adding (4.11) and (4.12) and dividing by two, vN becomes

vN = vDC

2

1

2(ea+ ec) (4.13)

As in the previous section, the third harmonics are neglected and the BEMF are3-phase.

ea+ eb+ ec = 0 (4.14)

The voltage at the neutral point results from equations (4.13) and (4.14):

vN = vDC2 +

eb

2. (4.15)

Applying this equation to (4.1), the measured voltage at phase B can be written as

ub= eb+ vN=3

2 eb+

vDC

2 . (4.16)

The measured voltage at phase B consists of one and a half times the BEMF andhalf of the supply voltage. When comparing the voltage ub with half of the supplyvoltage, the zero crossing of the BEMF can be detected.This BEMF detection mode can be used for high velocities when the PWM off timeis small or nonexistent. At high velocities, the BEMF amplitude is high, so thecommon mode voltage does not influence the BEMF too heavily. For low velocities,

the BEMF detection during off time is preferred.The direct BEMF detection is easier to implement during on time, because thecomparison with half of the supply voltage is more secure than with zero.

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Part II

Bachelors Thesis

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Chapter 5

Simulation of DifferentCommutation Schemes

For demonstration of the various effects, the commutation schemes described insection 3.4, 3.5, 3.6 and 3.7 have been simulated in Simulink. The models of the ex-perimental setup are built with SimPowerSystems tools in Matlab/Simulink. Thischapter shows the set-up of the simulation and discusses the results.

The following values are used for all simulations:

RS= 2.875 Stator resistance

LS= 8.5 mH Stator inductance

= 0.175 Wb constant flux induced by magnets

TL= 1 Nm Mechanical Torque120 Back EMF flat area

J= 0.001 kg m2 Inertia

F = 0.01 Fms Friction factor

p= 1 Pairs of poles

UDC= 100V Supply Voltage

The permanent magnet synchronous machine of SimPowerSystems is used to simu-late the motor. The flux distribution is set to trapezoidal shape and the mechanicalinput is torque. The DC Voltage Source is connected to the universal bridge, whichgenerates the three phase voltage. The bridge receives the signal to open or closethe transistors. This signal as well as the desired voltage is created differently in

every mode. The frequency of the repeating sequence, which is used for generatingthe PWM signal, is 10kHz.

27

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28CHAPTER 5. SIMULATION OF DIFFERENT COMMUTATION SCHEMES

5.1 Simulation of Scheme A

5.1.1 Model

Figure 5.1: Simulink Model for Simulation of PWM on High Side, 1 on Low Side

For PWM on high side and 1 on low side mode, the model 5.1 is built in Simulink.The signalfunction block generates the signal by checking the instantaneous stepwith the measured rotor angle. This signal is a binary six digit and three of themare overlaid by the PWM signal. The desired duty cycle is calculated with equation(3.12) in the voltage block and sent to the PWM block. There, the signal is set to

1, when the value of the repeating sequence is higher than the duty cycle and to 0,if it is lower. With this PWM signal, the high side transistors are activated.

5.1.2 Results and Discussion

0 0.1 0.2 0.3 0.4 0.5 0.6 0.76

4

2

0

2

4

6PWM on High Side, 1 on Low Side

Time

Current

Figure 5.2: Current through Phase A

0.39 0.395 0.4 0.4053.85

3.9

3.95

4

4.05

4.1PWM on High Side, 1 on Low Side, Zoom

Time

Current

Figure 5.3: Zoomed Current Ripple

The measured current during the first half of the simulation through phase A ispictured in figure 5.2. The percentage of the current ripple can be roughly estimatedby measuring the value of the current and its ripple at any instant. In figure 5.3 theaverage value of the current ripple is measured and compared with the amplitude.The ripple is 4.03 % of the amplitude.

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5.2. SIMULATION OF SCHEME B 29

5.2 Simulation of Scheme B

5.2.1 Model

Figure 5.4: Simulink Model for Simulation of PWM Simultaneous on High and LowSide

For the simulation of the commutation mode with simultaneous PWM signal onhigh and low side, model 5.4 is built. Therefore, only the overlaid PWM signaland the calculated duty cycle differ from model 5.1. Both the high and low side

transistors are now activated by the same PWM signal. The duty cycle is evaluatedusing equation (3.13).


0 0.1 0.2 0.3 0.4 0.5 0.6 0.74

3

2

1

0

1

2

3

4

5

6PWM Simultaneous on High and Low Side

Time

Current


0.525 0.53 0.535 0.543.5

3.55

3.6

3.65

3.7

3.75

3.8

3.85PWM Simultaneous on High and Low Side, Zoom

Time

Current


The current ripple can be estimated out of the measurements in 5.5 and 5.6. Thepercentage current ripple for this commutation mode is 7.17 %. The current rippleis almost two times the current ripple of the commutation scheme from the previoussection. As said in section 3.5.2, the main advantage of this mode is that the motorcan drive in 4-quadrant mode. Unfortunately, the current ripple is high.

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5.3 Simulation of Scheme C

5.3.1 Model

Figure 5.7: Simulink Model for Simulation of PWM on High Side, Non-Simultaneouson Low Side

The model for PWM on high side and non-simultaneous on low side mode is illus-trated in 5.7. For generating the non-simultaneous PWM signal on the low sidetransistors, a second repeating sequence is built and overlaid on the low side signal.Because this repeating sequence is delayed by half a PWM cycle, it has to be in-

verted. The PWM block sets the normal PWM signal on the high side signal andthe delayed one on the low side. The duty cycle is calculated with equation (3.13).


0 0.1 0.2 0.3 0.4 0.5 0.6 0.74

3

2

1

0

1

2

3

4

5PWM on High Side, Nonsimultaneous on Low Side

Time

Current


0.525 0.53 0.535 0.543.55

3.6

3.65

3.7

PWM on High Side, Nonsimultaneous on Low Side, Zoom

Time

Current


The percentage current ripple can be again estimated from the current measurement5.8. Figure 5.9 shows the current ripple, which is only 2.17 % of the amplitude.As described in section 3.6.2, the voltage connection on the motor changes two timesmore during one PWM cycle than with scheme A. Therefore, the frequency of thecurrent ripple is twice the PWM frequency, which makes the current ripple smaller.

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5.4. SIMULATION OF SYNCHRONOUS RECTIFICATION 31

5.4 Simulation of Synchronous Rectification

5.4.1 Model

Last

voltage

omega

tmv fcn

velocity

60

powergui

Discrete ,

s = 5e-006 s

low side

uy fcn

inverse PWM

v

triangleg fcn

high side

uy fcn

complementary

uy fcn

Universal Bridge

g

A

B

C

+

-

Torque1

Three-Phase

V-I Measurement

Vabc

Iabc

A

B

C

a

b

c

Scope8

Scope7

Scope6

Scope5

Scope4_2

Scope3

Scope2

Scope1

Scope

Repeating

Sequence

Permanent Magnet

Synchronous Machine

Tm

mA

B

C

PWM

v

triangleg fcn

Logical

Operator2

OR

Logical

Operator1

AND

Logical

Operator

AND

Gain

1/100

DC Voltage Source

Figure 5.10: Simulink Model for Simulation of Synchronous Rectification

The command to the power bridge in 5.10 is made up of three signals. The normal

PWM signal is applied on the high side. At the same time, the low side is activatedby the complementary PWM signal. The third signal closes the low side transistorduring one step and therefore is set to 1. The duty cycle is evaluated using (3.12).


0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.46

4

2

0

2

4

6Synchronous Rectification

Time

Curr

ent

Figure 5.11: Current through PhaseA

0.39 0.395 0.4 0.4053.85

3.9

3.95

4

4.05

4.1Synchronous Rectification, Zoom

Time

Curr

ent


In figure 5.11 the measured current is illustrated, and figure 5.12 shows the currentripple. The percentage current ripple is 3.97 % of the amplitude. The current isalmost the same as in scheme A, but the motor can act in 4-quadrant mode. Addi-tionally, energy can be saved through the lower conduction losses of the transistors.

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0.4 0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.51

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

All four Commutation Modes

Time

Current

Scheme A

Scheme B

Scheme C

Scheme SR

Figure 5.13: Current of all four Schemes

5.5 Comparison of all four Schemes

In figure 5.13, the currents of the simulated commutation modes are pictured. Itcan be seen that modes A and SR have almost the same current profile. The dutycycle of the PWM are the same for both schemes and are calculated with equation(3.12). The other two modes, B and C, have similar current profiles. The soledifference between these two schemes is the lower current ripple in scheme C, whichis caused by the double frequency of the current.As can be seen in figure 5.13, the period of the current in modes A and SR is shorterthan the current in the other two modes. Therefore, the phases change faster, sothe angular velocity of the rotor is higher. Figure 5.14 shows the speed of all fourschemes.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 15

0

5

10

15

20

25

30

35Speed of all four Commutation Modes

Time

Speed

Scheme A

Scheme B

Scheme C

Scheme SR

Figure 5.14: Angular velocity of all four Schemes

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Chapter 6

Hardware

This chapter describes the different hardware tools that have been used for motorcommutation. Every experimental setting includes a microcontroller, a power stage,including drivers and MOSFETs, and of course the motor. In figure 6.1 the set-up with Hall sensors is illustrated. The computer programs the C over a serialconnection to the flash memory and interacts over the second serial connection.TheC controls the MOSFETs by sending signals to the drivers. The power bridgecontrols the applied voltage on the motor. The instantaneous position of the rotoris detected by Hall sensors and sent back to the microcontroller. When usingsensorless commutation, the C determines the position from the BEMF. As itcan be seen in figure 6.2, the sole difference to the senorless set-up is how the Cdetermines the rotor position. Different combinations of tools have been used forthe experimental set-up. In this chapter only the tools that are important anduseful for the commutation implementation are explained.

ComputerMicrocontroller

Motor

Rotor Position

Flash

TransitorCommands Voltage

Power BridgewithSupply Voltage

Figure 6.1: Experimental Setting

ComputerMicrocontroller Power Bridge

withSupply Voltage

Motor

BEMF

Flash

TransistorCommands Voltage

Figure 6.2: Sensorless Setting

6.1 Microcontroller AT90PWM3B

The serial of ATMELs microcontroller AT90PWM is specially designed for motorcontrol applications. The AT90PWM3B is used for all experiments in this project.It features 8 kilo bytes of flash memory and has ten channels with advanced PWM.As it is explained later in this chapter, it also includes three high speed power stagecontrollers, which are required for generating the desired PWM signal. Moreover,eleven ADC channels and one DAC are in the microcontroller.

6.1.1 Generate PWM Signal

There are several different modes for the power stage controller in AT90PWM3B togenerate a PWM signal. Here only the two modes that are used in the project arediscussed. The 2 ramp mode is used for the first implementation of synchronousrectification and the centre aligned mode for sensorless commutation. Each of the

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34 CHAPTER 6. HARDWARE

three power stage controllers has a counter, which can count up and down. Addi-tionally, four reference values can be stored. By comparing the reference value withthe counter value, the two output signals and the countering can be changed. Since

each of the three PSC has two outputs, it is perfectly made for driving three halfbridges and a DC motor.

2 Ramp Mode

In every power stage controller, four output compare register values can be set:OCRnSA, OCRnRA, OCRnSB, OCRnRB. The counter enumerates two times, firstto the value of the output compare register RA and then to RB. The values SAand SB have to be set by considering the dead time. With SA and RA the highside transistor is activated and with SB and RB the low side is activated. One PSCcycle lasts for counting up to RA and RB. The 2 ramp mode is illustrated in figure6.3.

OCRnRA

OCRnSA

OCRnSB

OCRnRB

PSC Cycle

PSCOUTn0

PSCOUTn1

dead time dead time

Figure 6.3: 2 Ramp Mode

First, the PSC cycle and the on time of the high side transistor are defined. Theduration of one counting step is set with the phase-lock loop (PLL) frequency.Depending on this frequency, the counting values are set. The compare registersare calculated as follows:

OCRnSA = dead timeOCRnSB = dead timeOCRnRA = dead time + on time of high sideOCRnRB = PSC cycle - (dead time + on time of high side)

One of the big benefits of the 2 ramp mode is the guarantee that the two outputsignals are not switched on at the same time. When driving a low and a hightransistor of a half bridge, this mode can be wisely used for new commutations andexperiments.

Centre Aligned Mode

In centre aligned mode, the PSC counts from the set value RB down and up again.With the compare register SA, the on time of the first output can be set and withSB, the on time of the second output can be set. This mode is shown in figure6.4. As the fourth compare register is not required, it can be used for adjusting theanalogue to digital conversion (ADC) synchronisation (see 8.2).

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6.2. AVR STK500 EVALUATION BOARD 35

PSCOUTn0

PSCOUTn1

OCRnSB

OCRnSA

OCRnRA

OCRnRB

On time 1

On time 2

Figure 6.4: Centre Aligned Mode

6.2 AVR STK500 Evaluation Board

The AVR STK500 starter kit of ATMEL is a development system for differentmicrocontrollers (figure 6.5). Six places for temporary microcontrollers are available.The STK500 can be practically used for tests and prototypes, as the microcontrollerdoes not need to be brazed on the board. The flash memory can be programmedfrom the computer over the RS-232 interface, and it communicates over the secondRS-232. The STK500 interacts over ISP with external devices on the board.

PC PC

STK 520

Commands to Power Bridge

AT90PWM3B

Figure 6.5: ATMELs STK500 with STK520

6.3 AVR STK520

The AVR STK520 is an additional module for the STK500 board. It can be placedon the STK500 as shown in 6.5. The STK520 board includes two places for micro-controllers and allows for the use of all advanced features of the AT90PWM series.The microcontroller is placed on one of the temporary station.

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6.4 ATAVRMC100

The ATAVRMC100 board is an evaluation kit especially arranged for BL DC-motor

control with hall sensors, or it is sensorless by detecting the BEMF. It includes aAT90PWM3 microcontroller, drivers and MOSFETs (3 half bridges). This deviceis used for testing the synchronous rectification program.

6.5 Printed Circuit Board (PCB)

A special PCB for driving the motor by itself is constructed. For more detailedinformation, its schematic diagram is illustrated in A.2 of the Appendix. It consistsof a power bridge of three MOSFET half bridges driven by 6 drivers, an in-systemprogramming (ISP) interface and a AT90PWM3B. In this project the board isused without the microcontroller. Instead of the microcontroller on the board,

the AT90PWM3B is installed on the STK520. One possibility for further researchwould be to use the board with the microcontroller on it.

Figure 6.6: Printed Circuit Board

6.5.1 Driver

The IR2101S drivers from International Rectifier have been chosen, because thelogic input voltage is in phase (non inverted), and the high and low side outputchannels work independently. When the input voltage is in phase, the driver closesthe MOSFET by receiving a 1.

6.5.2 MOSFET

The selected MOSFETs are the IR8910 from International Rectifier with followingcharacteristics:

Turn-On Delay Time 6.2 nsRise Time 2 10 nsTurn-Off Delay Time 3 9.7 nsFall Time 4.1 ns

6.5.3 Voltage Divider / Low Pass Filter

The voltage divider and low pass filter has a common design. The desired outputvoltage and the cut-off frequency determines the values of the components. The

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6.5. PRINTED CIRCUIT BOARD (PCB) 37

Uin

Uout

R1

R2 C

2C

1

Figure 6.7: Voltage Divider / Low Pass Filter

capacitors filter out high frequencies and the resistors divide the voltage. Theequivalent circuit is shown in figure 6.7.The cut-off frequency is given by the equation:

f= 1

2

R1+ R2R1 R2 (C1+ C2)

(6.1)

At low frequencies, the circuit acts only as a voltage divider with the gain:

Uout

Uin=

R2

R1+ R2(6.2)

In the application note AVR444 [6], the sum ofR1 and R2 are recommended asvalues from 10k to 100k.

Voltage Divider / Low Pass Filter on Motor Terminal VoltagesFor sensorless commutation and its BEMF detection, the voltage at each motorphase has to be measured. This voltage has to be scaled down, and therefore, aVD/LPF as described before is used. The following values are chosen: R1 = 10k,R2 = 5.6k, C1+ C2= 150pF

Voltage Divider / Low Pass Filter for ADC Reference Voltage

The scaled down motor supply voltage is used as the reference voltage AREF, whichis 5V. For lowering ripples ofVDC, a low pass filter is required. On that account,the previously described VD/LPF is assembled, as illustrated in figure 6.8. Theinterior ADC input resistance of the AT90PWM3B is 30k and the input capacitor10nF. So the values are set to R1 = 10k, R2 = 6.8k, C= 2.2nF.

Uin

Uout

= AREF

R1

R2 C 10nF30k

ADC inputresistance

ADC inputcapacitor

Figure 6.8: AREF Voltage Divider / Low Pass Filter

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6.5.4 Testing

After composing the board as designed, the drivers and the MOSFETs need to be

checked to guarantee that they are well connected and function as desired. EveryDiver/MOSFET has to be tested alone, so six test runs are required. Therefore aload is connected to the relevant phase and some simple PWM signals (0V...5V)are transmitted to the driver as shown in figure 6.9. When testing the low side,the high side transistor is deactivated and a load is lying between the phase and a12V Voltage. By sending signals to the lower transistor, the measured voltage ofthe phase should react inversely to the signal.When the high side transistor is tested, it receives signals while the load is connectedto ground and the low side transistor is disabled. The measured voltage at the phaseshould react the same way as the sent signal.Different PWM signals are sent to the drivers to check them (different duty cycleand frequency).

Driver

12 V

5 V

0 V

0 V

RV

Ground

12 V

0 V

Driver

12 V

Ground

5 V

0 V

0 VV

R

12 V

0 V

Figure 6.9: Testing Low and High Side Driver and MOSFET

As an example, the high side driver/MOSFET of phase B is tested. In figure 6.10,

the voltage measured with the oscilloscope is displayed. The PWM frequency is15.6kHz with a the duty cycle of 100/256. The supply voltage is set to 12.02V anda current of 0.04A flows through the device.

Figure 6.10: Oscilloscope Measurement

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Chapter 7

Synchronous Rectification

The Synchronous Rectification is an advanced commutation mode to obtain a morerefined control. The operating mode is described in section 3.7 and simulated in 5.4.The current flows through the MOSFETs instead of the diode when the high sidetransistor is open. As the MOSFETs generally have lower conduction losses than theflyback diode, the efficiency can be increased by using the synchronous rectification.Another advantage of SR is that it allows the current to change direction withina given PWM period. Therefore, the motor becomes more reactive and dynamic.This chapter explains the implementation of SR and its most important settings,such as PWM signals and dead time.

7.1 Precedent Problem

When driving the motor at low velocities and with a low PWM frequency, thefollowing problem emerges. During the PWM off time, the voltage at the floatingphase is not always zero. As pictured in figure 7.1, the voltage changes after a while,but it does not equal zero. To measure this, the PWM frequency is 3.9kHz and theduty cycle 50/256.

Figure 7.1: Voltage at Phase B

This problem occurs because the inductor is discharged and the diode voltage is con-nected to the phase. When using synchronous rectification, this problem disappearsbecause the phase is directly connected to the ground.

7.2 Implementation of SR

Some different commutations have been programmed and tested to study the influ-ences of synchronous rectification. The drivers of the AVRMC100 board are usedbecause they do not work inversely. So they close the MOSFET by receiving a

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40 CHAPTER 7. SYNCHRONOUS RECTIFICATION

one. The microcontroller AT90PWM3B of the STK 520 evaluation board is used togenerate the instruction signals for the drivers. The position is measured by threeHall sensors, which transmit the measurements to the STK500 evaluation board.

This experimental setting is illustrated in 7.2.

M

STK 500

STK 520

MC 100

AT90PWM3(not used)

AT90PWM3B

Drivers

A

B

C

3 ha sensors!C"#$D

Brid%e

#&

T&

Po'er sta%e#S232

#S232

Figure 7.2: Experimental Set-Up

7.2.1 Testprogram

First, a simple PWM commutation is programmed to test the experimental set-up.Therefore, the two ramp mode is chosen. However, only the output compare reg-isters OCRnSA and OCRnRA are used while OCRnSB and OCRnRB are set tozero. The construction of the signal is pictured in figure 7.3.

Figure 7.3: Simple PWM

Figure 7.4: Measurement

After programming and activating the motor, the voltage on phase B is measured,and it is illustrated on the oscilloscope in figure 7.4. The duty cycle is set to 50/256.The measured t is the on time of the transistor. A PWM cycle should be 64.1s,and the corresponding on time is 12.52s. The measurement is not precise, so themeasured 12.80s corresponds well to the theoretical value.

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7.2. IMPLEMENTATION OF SR 41

7.2.2 PWM

Three programs with different PWM frequencies have been tested. When setting

the counting value of PSC cycle to 2048, the clocks have been set as follows toachieve the different PWM frequencies:

PLL PSC input PSC Prescaler PWM Frequency

Test 1 32 MHz PLL / 1 = 32 MHz PSC / 1 = 32 MHz 32 MHz / 2048 = 15.625 kHz



The PWM signal is built with the 2 ramp mode, described in 6.1.1. One PWMCycle lasts 1 / PWM frequency:

Test Number PWM Cycle

1 1 / 15.625 kHz = 64 s2 1 / 7.8125 kHz = 128 s3 1 / 3.90625 kHz = 256 s

7.2.3 Dead Time

The MOSFETs on the ATAVRMC100 are SUD35N05-26L of Vishay with charac-terstics:

Turn-On Delay Time 8 nsRise Time 2 30 nsTurn-Off Delay Time 3 30 nsFall Time 150 ns

This results in a dead time of 218 ns. To safely avoid a short circuit, the cho-sen dead time is 250 ns, which corresponds to a counting value of 8 for test 1, 4 fortest 2, and 2 for test 3.

7.2.4 Position Control

An interrupt on the analog comparator output toggle is applied when the Hall sen-

sors detect a sector change. The program enters a subroutine that reads the analogcomparator output bits. During this subroutine, the bits in the PSC configurationregister are set so that the PSC cycle cannot be disturbed. There are seven differ-ent cases that are implemented, six steps and one default case that stops the motorimmediately. During one step the high and the low driver/MOSFET of one phaseare activated to receive some signals. The low side transistor of the second phaseis set to one, so it is closed. All the other transistors are deactivated.

7.2.5 Velocity Control

When a command on the RS-232 is received, the program enters a subroutine andtests eleven cases:

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Comand Reactionr run motors stop motor

0 set duty cycle to 0/2561 set duty cycle to 25/2562 set duty cycle to 50/2563 set duty cycle to 100/2564 set duty cycle to 200/2565 set duty cycle to 255/256+ set old duty cycle + 1/256- set old duty cycle - 1/256default stop motor

The highest velocity is set to 255/256 to avoid complications when generating thePWM signal. By setting the velocity to the preferred value, the on times of boththe high side and low side transistors are adjusted.

7.3 Results and Discussion

All three frequencies have been successfully tested, and the motor turned withdifferent velocities. The phase voltages and the signals sent to the drivers havebeen measured and illustrated with an oscilloscope. Here, tests 1 and 3 are furtheranalysed.

7.3.1 Test 1

As mentioned before, the PWM frequency is set to 15.6 kHz. Therefore, a PWMcycle lasts 64.1s. In figure 7.5 the measured voltage on phase A and B with the

oscilloscope is pictured when driving the motor with a duty cycle of 50/256.

Figure 7.5: Measurement

The velocity of the rotor can be calculated from the duration of the steps. Anelectrical rotation includes 6 steps and lasts for the chosen duty cycle 11.2 ms.The rotor of the used motor consists of four pole pairs. Therefore, four electricalrotations build a mechanical revolution of the rotor. The corresponding revolutionsper minute (RPM) is

n= 14 t

. (7.1)

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7.3. RESULTS AND DISCUSSION 43

For the example of a 50/256-duty cycle, the RPM becomes

n50/256 =

1

4 11.2ms= 1339.3 rpm. (7.2)

The RPM for the different duty cycles are

Command Duty Cycle Duration of 6 Steps RPM

1 25/256 22.6 ms 663.7 rpm2 50/256 11.2ms 1339.3 rpm3 100/256 5.52 ms 2717.4 rpm4 200/256 2.72 ms 5514.7 rpm5 255/256 2.13 ms 7042.25 rpm

A quadrotor generally works around 3000 rpm.

When zooming in the plot, the PWM signal can be viewed and analysed. Forthe example with the duty cycle of 50/256, the on time should last 6.26 s andis illustrated in figure 7.6. On the bottom, the signal sent to the driver of powerbridge is measured.

Figure 7.6: PWM signals with a Duty Cycle of 50/256

The following current values through the motor and on time of the PWM signalsare measured:

Command Duty Cycle Current Theoretical On Time Measured On Time1 25/256 0.08 A 6.26 s 6.4 s2 50/256 0.10 A 12.52 s 12.8 s3 100/256 0.14 A 20.04 s 25.6 s4 200/256 0.24 A 50 s 50.4 s5 255/256 0.29 A 64.4 s 63.85 s

They cannot be measured very precisely with the oscilloscope, but the measuredvalues correspond approximately to the theoretical values.

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7.3.2 Test 3

Without synchronous rectification, the problem described in 7.1 occurs when driving

the motor with low PWM frequencies and low velocities. The SR is tested by settingthe PWM frequency to 3.9 kHz. A PWM cycle lasts 256 s. The measured voltageon phase A during 6 steps is pictured in figure 7.7. On the bottom, the signal sentto the high side driver of phase A is measured.

Figure 7.7: Phase A and Signal to HA

By increasing the resolution of the oscilloscope plot, the PWM signals can be anal-ysed. As it can be seen in the measurement 7.8 with a duty cycle of 50/256, theproblem described in 7.1 is corrected. The signal stays at zero volts during thewhole off time.

Figure 7.8: Phase A and Signal to HA

The measured PWM cycle is 258 s. For the different velocities, the following cur-rent and off times are measured:

Command Duty Cycle Current Theoretical On Time Measured On Time

1 25/256 0.08 A 25 s 26 s2 50/256 0.11 A 50 s 52 s3 100/256 0.16 A 100 s 102 s4 200/256 0.25 A 200 s 202 s5 255/256 0.31 A 255 s 256 s

The corresponding RPM can be calculated with equation (7.1). The RPM forthe different duty cycles are

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7.3. RESULTS AND DISCUSSION 45

Command Duty Cycle Duration of 6 Steps RPM

1 25/256 21.7 ms 694.4 rpm2 50/256 11 ms 1363.6 rpm3 100/256 5.4 ms 2777.8 rpm4 200/256 2.62 ms 5725.2 rpm5 255/256 2.14 ms 7009.3 rpm

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Chapter 8

Sensorless Commutation

This chapter describes the implementation of synchronous rectification into sensor-less commutation. The direct BEMF detection during on time has been previouslyimplemented by Caroline Claasen [1]. In this thesis, the SR implementation isintegrated in this existing program.

8.1 Direct BEMF Detection

8.1.1 During On Time

The floating phase is analysed and used for BEMF detection. The PWM signal isgenerated with the centre aligned mode described in section 6.1.1. Two additionalcompare matches (OCR1B and OCR1A) are required for counting the commutation

moment. The fourth unused output compare register of the PSC starts the ADconversion. The converted signal is compared to half of the supply voltage. Whenthe voltage achieves this value, the rotor is in the middle of a commutation step.The time since the last commutation occurred is filtered and set to OCR1B. Thecounter begins to enumerate again from zero to OSRB1B without disturbance. Atthe beginning of each step, the signal is not yet stable. Therefore, during countingup to the compare register OCRnA, the ZC is disabled during the first two PWMcycles. In figure 8.1, the operating mode of this implementation is illustrated.

Next step

Zero crossing

OCR1B

hold off

OCR1A

Figure 8.1: ZC Detection During On Time

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48 CHAPTER 8. SENSORLESS COMMUTATION

8.1.2 During Off Time

The differences between the implementation of the direct BEMF detection during

off time and during on time are theoretically not strong. During off time, themeasured value has to be compared to the zero voltage. Unfortunately, voltagesaround zero are always difficult to measure. As marked in figure 8.2, the voltageduring off time is sometimes lower than zero. This measured voltage consists ofthe diode voltage between the ground and the measured phase. This can causeproblems by AD conversion of the voltage because of the negative voltage.

0V

< 0V

Figure 8.2: Negative Voltage

Figure 8.3 shows the ZC detection during off time. The sole difference between thedetection during on time is the value for comparison with the zero crossing.

Next step

Zero crossing

OCRnB

OCRnA

hold off

Figure 8.3: ZC Detection During Off Time

8.1.3 Commutation Filter

When measuring the time between commutation and zero crossing, the time canvary. To decrease the probability of an incorrect time measurement, a digital filteris implemented:

tk = t

k+ 3 tk14

(8.1)

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8.2. FINAL IMPLEMENTATION 49

tk is the measured time between commutation and ZC, tk1 is the old time.This filter prevents the possibility of an incorrect measurement of the zero crossing,which destroys commutation.

8.2 Final Implementation

The synchronous rectification is implemented into BEMF detection during on time.The implementation is done with the AT90PWM3B on the STK520 and the powerbridge of the board described in 6.5. The experimental set-up is illustrated in figure8.4.

M

STK 500

STK 520

PCB Card

AT90PWM3B Drivers

A

B

C

Bridge

RS-232

RX

TX

Power stage

RS-232

ISP

2!

"R#$%DB&M'

AR&'

Figure 8.4: Experimental Setting

The implementation is achieved with several sub-steps. The existing program isadjusted to the board with non-inversed drivers. Then, the synchronous rectificationis implemented. The low side transistors are activated by two separately generatedsignals. To generate synchronous rectification, the opposing PWM signals of thehigh side transistors are sent to the power bridge. During the conduction steps, thelow side transistors should close and receive a 1.

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50 CHAPTER 8. SENSORLESS COMMUTATION

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Chapter 9

Conclusions and Outlook

In this thesis the functionality of the synchronous rectification was presented, andits implementation proved to be successful. The implementation was done for bothmodes with and without sensors. The synchronous rectification is therefore appli-cable for the control of the permanent brushless DC motors. For future research inthe area of motor control, synchronous rectification can be implemented based onthe explanations in this thesis.The next step for future projects would be the implementation with the micro-controller placed on the board. Therefore, the connections could be tested beforeusing the C. The implementation should be achieved stepwise to avoid possiblemalfunctions.More measurements could be done to achieve a better understanding of the system.For example, the energy losses avoided due to synchronous rectification could bestudied precisely. Additionally, the current flow through the motor could be anal-

ysed more precisely to check the performance of the system during commutation.

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Appendix A

Photos and Schematics

A.1 PhotosA.1.1 Permanent Magnet Brushless DC-Motor

Figure A.1: Photo of the Motor

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A.1. PHOTOS 53

A.1.2 PCB

Figure A.2: Backside of the Board

Figure A.3: VD/LPF on Phases

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54 APPENDIX A. PHOTOS AND SCHEMATICS

A.2 Schematic Diagram of Card

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Appendix B

Codes

B.1 With Hall Sensors

B.1.1 Scheme A, 2 Ramp Mode, 1 directly on Low

1 / / A n d r e a K a t h a r i na F u c h s

2 //ENSMP_CAS

3 //16.04.08

4

5 //

6

7 / / = = = = > > > >

8

/ / p o u r A T 90 P WM 3 B 9

10 / / m o di f ie d v e rs i on o f B r u sh l e ss _ H al l _ B_ 1 5 _6

11 / / c h a n ge s : n o Z C d e t e c ti o n

12 //

13

14 / / 2 R am p M od e , w i th o ut S y nc h r on o u s R e ct i fi c at i on , P WM o n H s ,

1 o n L s

15 / / W e d on t n ee d t he O CR nS X a nd O CR nR X , s o w e h av e o nl y o ne

r am p m od e , m ad e b y O CR nR A

16

17 / / P W M f r eq u en c y

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