A New Cost Effective Sensorless CommutationMethod for Brushless DC Motors WithoutPhase Shift Circuit and Neutral Voltage

南台科大電機系

Adviser : Ying-Shieh Kung Student : Chien-Hung Chen

陳建宏 M9920109

Cheng-Hu Chen and Ming-Yang Cheng, Member, IEEE PTT製作 100%

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OUTLINE ABSTRACT I. INTRODUCTION II. MATHEMATICAL MODELS OF EACH COMMUTATION STATE III. PROPOSED ZCP DETECTION APPROACH BY AVERAGE LINE

TO LINE VOLTAGE IV. ANALYSIS OF THE COMMUTATION ERROR V. EXPERIMENTAL EVALUATION VI. CONCLUSION REFERENCES

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Abstract(1/2)

This paper presents the analysis, design, and implementation of a high performance and cost effective sensorless control scheme for the extensively used brushless dc motors.

In an effort to decrease cost and increase ease of implementation, the commutation signals are obtained without the motor neutral voltage,multistage analog filters, A/D converters, or the complex digital phase shift (delay) circuits which are indispensable in the conventional sensorless control algorithms.

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Abstract(2/2)

In the proposed method, instead of detecting the zero crossing point of the nonexcited motor back electromagnetic force (EMF) or the average motor terminal to neutral voltage, the commutation signals are extracted directly from the specific average line to line voltages with simple RC circuits and comparators.

Theoretical analysis and experiments are conducted over a wide operating speed range and different back EMF waveforms to justify the effectiveness of the proposed method.

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I. INTRODUCTION(1/2) DURING the last two decades, a lot of research on sensorless control

techniques for brushless dc motors (BLDCMs) have been conducted. This

research can be divided into four categories.

Detection of the zero crossing point (ZCP) of the motor terminal

to neutral voltage with a precise phase shift circuit.

Back electromagnetic force (EMF) integration method.

Sensing of the third harmonic of the back EMF.

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INTRODUCTION(2/2)

The neutral voltage is required for comparison with the non-conducted back EMF or the average terminal voltage, in which it will introduce a high common-mode noise.

Since the zero crossing points of the conventional back EMF method are inherently leading 30 electric degrees of the ideal commutation points, a precise velocity estimator and a phase shift circuit (algorithm) are needed to process the zero crossing signals so that accurate commutation points

can be determined.

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II. MATHEMATICAL MODELS OF EACH

COMMUTATION STATE

Fig. 1 shows the equivalent circuit of a BLDCM and the inverter topology.

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Fig. 2 illustrates the relationship among the back EMF waveform of an ideal BLDCM, the armature current,

the commutation signals (H1–H3),

and the switching signals (S1–S6) for

the inverter.

According to the polarity of the

armature current as illustrated in Fig. 2,

the terminal voltage of each phase can

be divided into three sub-sections, i.e.,

positive, negative, and nonconducted.

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If the switch of the upper leg is conducted (e.g., S3 is on), the neutral voltage can be expressed as

According to (6) and (7), the neutral voltage can be written as

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If the switch of the upper leg is not conducted (e.g., S3 is off), the neutral

voltage can be expressed as

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III. PROPOSED ZCP DETECTION APPROACH BY

AVERAGE LINE TO LINE VOLTAGE The major problem of the conventional back EMF sensing techniques is

that they require noisy motor neutral voltage and a fixed phase shift circuit.

Since the noisy motor neutral voltage will introduce the common mode noise into the sensorless circuit ,a low pass filter is indispensable.

On the other hand, the fixed phase shift function over a wide speed range is hard to implement with analog circuits.

In order to cope with the aforementioned problems, the proposed method extracts the commutation points directly from the motor terminal voltages with simple comparators and a single stage low pass filter.

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If the terminal voltages are expressed in the average form (i.e., duty ratio), the switching states in (3), (4), (17), and (18) can be eliminated. The terminal voltages are rewritten as follows.

States I and II: Armature Current is Positive:

States III and VI: Armature is Open (Nonconducted):

States III and VI: Armature is Open (Nonconducted):

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According to (19)–(21), the ideal average terminal voltages for all three phases with different duty ratios are illustrated in Fig. 5.

Fig. 5. Ideal average terminal voltages under different duty ratios.

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The measured instantaneous (upper trace) and average (lower trace) terminal voltages as the duty ratio is increased from 10%,to 50%, to 100% are shown in Fig. 6.

Fig. 6. Measured instantaneous (first trace) and average (second trace) terminal voltages under different duty ratios. (a) Duty ratio = 10%. (b) Duty ratio = 50%. (c) Duty ratio = 100%.

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Equation (23) reveals that the zero crossing points of the average line to line voltage will occur at 30 and 210 electric degrees.

According to (22) and (23), Fig. 7 shows the phase relationship among the ideal back EMF, the average terminal voltage, and the average line to line voltage of phase “a ” and phase “c .”

It is clear to see that the average line to line voltage Vac lags 30 electric degrees compared with the back EMFea , namely the zero crossing points of the line to line voltage are in phase with the ideal commutation signals.

Fig. 7. Phase relationship among the back EMF, the average terminal voltage,and the average line to line voltage.

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Fig. 8. Proposed and conventional sensorless commutation circuits. (a) Proposed cost effective sensorless commutation circuit. (b) Conventional sensorlesscommutation circuit.

• Fig. 8(a) illustrates the practical circuit for implementing the proposed approach to obtain the commutation signals (namely the virtual Hall effect signals H1~H3 ). • Consequently, the circuit needed in the proposed approach is much simplercompared with that needed in the conventional circuit shown in Fig. 8(b).

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IV. ANALYSIS OF THE COMMUTATION

ERROR

A. Phase Delay by the Low Pass Filter and the Armature Impedance

B. Voltage Spikes by the Residual Current

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A. Phase Delay by the Low Pass Filter and the Armature Impedance

The phase delay angles caused by the input low pass filter and the armature impedance shown in Fig. 9(a) and (b) can be expressed as

Since the 30 (or 90 ) phase shift circuit shown in Fig. 8(b) is not required in the proposed approach, the corner frequency fc of the input low pass filter can be easily determined by the maximum motor speed RPMmax and the switching frequency fs , in which the value of fc can be chosen as

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B. Voltage Spikes by the Residual Current The voltage spikes shown in Figs. 4 and 6 are created by the residual current

when the armature current is blocked by the power switches. The voltage spike is the main cause for the commutation error in the conventional back EMF integration method and the window-captured back EMF method (detecting back EMF during the silent period) .

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V. EXPERIMENTAL EVALUATION Fig. 10 shows the block diagram

of the proposed sensorless control method. The system can be divided into several subblocks, including a velocity command generator, an open loop starting process, a line to line voltage based virtual Hall effect signal circuit, an electric commutation table, and a PWM generator.

Fig. 10. Block diagram of the overall system.

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Fig. 11. Structure of the employed BLDCMs. (a) Type I (segmented magnet), trapezoidal back EMF. (b) Type II (ring magnet), sinusoidal back EMF.

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Fig. 12. Measured back EMF waveforms of employed BLDCMs. (a) Type Imotor. (b) Type II motor.

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Fig. 13. Measured commutation signals under different duty ratios and back EMF waveforms (from top to bottom: average terminal voltage V a, average terminal voltage Vc , average line to line voltage Vac , estimated commutation signal, signal from Hall effect sensor). (a) Duty ratio = 10%, type I motor. (b) Duty ratio= 50%, type I motor. (c) Duty ratio = 100%, type I motor. (d) Duty ratio = 10%, type II motor.

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Fig. 14. (a) Duty ratio = 10%.

It can be seen that the signal from the conventionalsolution strongly depends on the operating speed; the mismatch angle is leading 21.8 in 10% full-speed .

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Fig. 14. (a) Duty ratio = 50%.

Lagging 14.4 in 50%full-speed.

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Fig. 14. (a) Duty ratio = 100%.

lagging 22.8 in full speed.

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VI. CONCLUSION Unlike conventional back EMF based sensorless commutation methods

which focus on detection of the ZCP of the motor terminal to neutral voltage, a novel sensorless commutation method based on the average line to line voltage is proposed in this study. Both theoretical analysis and experimental results verify that satisfactory performance can be achieved with the proposed sensorless commutation method. Compared with the conventional solutions, the proposed method has several advantages, including the following.

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Elimination of the motor neutral voltage: The neutral voltage is not required in the proposed method, only the

three motor terminal voltages need to be detected. Elimination of the fixed phase shift circuit:

The proposed specific average line to line voltage inherently lags 30 electric degrees compared with the phase back EMF. Moreover, experimental results have revealed that the phase relationship is insensitive to operating speed and load conditions.

Low starting speed: Since the amplitude of the line to line voltage is significantly larger

than the phase voltage, even a small back EMF can be effectively detected. Namely, a lower open loop starting speed can be achieved.

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REFERENCES