Sensorless technique for BLDC motors

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Sensorless Technique for BLDC Motors

by

Daniele Gambetta, B.Sc (Hons)

Dissertation

Submitted in Fulfillment of the Requirements

for the Degree of

Master of Philosophy

at the

The University of Southern Queensland

Faculty of Engineering & Surveying

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Copyright by

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Abstract

Commutation is a fundamental feature of all DC machines. In conventional DC machines the commutation function is performed by the commutator and brushes. These act as both position sensors and switches. The mechanical commutator has obvious disadvantages. Overcoming those disadvantages has been a major reason behind the development of brushless DC (BLDC) machines. In brushless DC machines commutation is performed by power electronic devices forming part of an inverter bridge. However, switching of the power electronic devices has to be synchronised with rotor position. Position sensing is therefore an essential requirement. This can be done by using sensors such as Hall Effect devices or a sensorless approach may be adopted. Advantages of sensorless techniques include reduced cost and wiring. The most common sensorless method is based on detection of the zero crossing of back EMF signals. But this technique works only above a certain speed since back EMF is directly proportional to speed. As a result BLDC systems which rely solely on back EMF signals for commutation suffer from

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Certification of Thesis

I certify that the ideas, experimental work, results, analyses, software and conclusions reported in this dissertation are entirely my effort, except where otherwise acknowledged. I also certify that the work is original and has not been previously submitted for any other award, expect where otherwise acknowledged.

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Signature of Candidate Date

ENDORSEMENT

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Signature of Supervisors Date

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Acknowledgments

I would sincerely like to thank and acknowledge the following people for their assistance, guidance and support throughout the duration of this project.

First of all I would like to thank my supervisor Dr Tony Ahfock. This study would not have been possible without his continued guidance, support and advice. Throughout the duration of this project he has been extremely patient, inspiring and has been a pleasure to work with.

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I wish to thank the Faculty of Engineering and Surveying and the Office of Research and Higher Degrees for their support and assistance. My thanks also extends to all those at the University who made this such a pleasant experience.

Finally, I wish to thank my family for their continued support and encouragement.

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List of Tables

TABLE 1.1:BLDC MOTOR CONTROL SYSTEMS: PERFORMANCE CHARACTERISTICS... 8

TABLE 1.2:ROTOR POSITION INDICATION... 12

TABLE 3.1:MAGNETIC PROPERTIES OF THE CM120 MOTOR MAGNETS (SOURCE:BOMATEC AG) ... 27

TABLE 3.2:PHYSICAL AND MECHANICAL PROPERTIES OF THE CM120 MOTOR MAGNETS (SOURCE:BOMATEC LTD)... 27

TABLE 3.3:TEST MOTOR NAMEPLATE DATA... 28

TABLE 3.4:MEASURED STAR POINT VOLTAGE WAVEFORMS... 37

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TABLE 5.1:COMPARISON OF MOTOR CURRENT (DC) AND SPEED FOR GIVEN PWM DUTY RATIOS

... 86

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List of Figures

FIGURE 1.1:BLDC ROTORS WITH SURFACE MAGNETS (SOURCE:MILLER 1989)... 2

FIGURE 1.2:BLDC ROTORS WITH INTERNAL MAGNETS (SOURCE:MILLER 1989) ... 2

FIGURE 1.3:INVERTER FED BLDC STATOR WINDINGS... 3

FIGURE 1.4:IDEALISED CURRENTS SUPPLIED BY INVERTER... 4

FIGURE 1.5:BLOCK DIAGRAM OF TYPICAL BLDC MOTOR DRIVE INCORPORATING CLOSED LOOP SPEED CONTROL... 5

FIGURE 1.6:COMMUTATION STATES AND ROTOR POSITION (Θ) ... 9

FIGURE 1.7:COMMUTATION STATES AND ROTOR START-UP POSITION... 10

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FIGURE 2.3:INDUCTANCE VARIATION DUE TO SATURATION (SOURCE:CHIRICOZZI ET AL.,1998)

... 20

FIGURE 2.4:CURRENT DIFFERENCES (SOURCE:CHIRICOZZI ET AL.,1998) ... 20

FIGURE 3.1:ROTOR OF THE CM120 MACHINE (SOURCE:ELTRONIC LTD) ... 28

FIGURE 3.2:THREE PHASE BRIDGE INVERTER CONNECTED TO BLDC... 29

FIGURE 3.3:UNIPOLAR SWITCHING STRATEGY (CHALUPA,2001) ... 31

FIGURE 3.4:BIPOLAR SWITCHING STRATEGY (CHALUPA,2001) ... 32

FIGURE 3.5: D- AND Q-AXIS STEP RESPONSE (IN ALIGNMENT WITH PHASE A) ... 34

FIGURE 3.6:STAR-POINT VOLTAGE WAVEFORM... 36

FIGURE 4.1:MEASURED SELF-INDUCTANCES... 41

FIGURE 4.2:MEASURED MUTUAL-INDUCTANCES... 42

FIGURE 4.3:IDEALIZED BACK EMF WAVEFORMS... 43

FIGURE 4.5:COMMUTATION ALGORITHM... 47

FIGURE 4.6:INVERTER BRIDGE SUPPLYING A BRUSHLESS DCMOTOR... 47

FIGURE 4.7:THE TWELVE POSSIBLE INVERTER STATES... 49

FIGURE 4.8:EQUIVALENT CIRCUITS FOR INVERTER STATES A+B- AND B+A-... 50

FIGURE 4.9:ALGORITHM FOR ROTOR INITIAL POSITION DETECTION-... 58

FIGURE 4.10:TRANSFER TO OR FROM THE BACK EMF METHOD-... 61

FIGURE 4.11:CHANGE OF STATE OF EL FLAG WITH SPEED... 62

FIGURE 5.1:OVERALL SYSTEM MODEL... 64

FIGURE 5.2:HARDWARE BLOCK DIAGRAM... 65

FIGURE 5.3:THE CONTROL MODULE... 66

FIGURE 5.4:MOSFET AND SHUNT CONNECTIONS... 68

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FIGURE 5.5:BOTTOM-LAYER OF THE CONTROL MODULE... 69

FIGURE 5.6:THE DRIVE (CONTROL MODULE AND POWER SEMICONDUCTORS)... 69

FIGURE 5.7:STATE TRANSITION DIAGRAM... 70

FIGURE 5.8:MAIN PROGRAM... 71

FIGURE 5.9:PWM RELOAD ISR ... 73

FIGURE 5.10:SYNCHRONISATION OF PWM RELOAD ISR ... 73

FIGURE 5.11:ADC COMPLETE ISR ... 75

FIGURE 5.12:FLOW CHART FOR THE ADC COMPLETE ISR(PART1) ... 76

FIGURE 5.13:FLOW CHART FOR THE ADC COMPLETE ISR(PART 2)... 77

FIGURE 5.14:DEFINITION OF PARAMETERS USED IN ADC COMPLETE ISR ... 78

FIGURE 5.15:COMMUTATION TIMER (QTC0)OCISR ... 79

FIGURE 5.16:FLOW CHART FOR THE COMMUTATION TIMER (QTC0)OCISR ... 80

FIGURE 5.17:TIMING DIAGRAM SHOWING OPERATION OF THE COMMUTATION TIMER... 81

FIGURE 5.18:SPEED TIMER ISR... 83

FIGURE 5.19:START/STOP REQUEST ISR ... 83

FIGURE 5.20:START-UP TIMER ISR ... 85

FIGURE 5.21A :COMPARISON OF THE COMMUTATION INSTANT (DETAIL)... 87

FIGURE 5.21B :COMPARISON OF THE COMMUTATION INSTANTS (AT NO LOAD,100MA)... 88

Sensorless technique for BLDC motors