New Design for Sensorless BLDC Motor Using Half-Bridge Driver Circuit

Ming Lu1,2, Yaohua Li1

1. Institute of Electrical Engineering Chinese Academy of Sciences 2. Graduate University of Chinese Academy of Sciences

Beijing, China {luming,yhli}@ mail.iee.ac.cn

AbstractMost of the current sensorless control schemes for BLDC (Brushless DC) motor are based on full-bridge driver. However, half-bridge driver draws a lot of attention due to its simpler structure, smaller occupied volume, lower power loss, easier control scheme and lower production cost, and it has been successfully utilized in several applications. Inspired by these applications, this paper presents novel designs for sensorless BLDC motor based on half-bridge driver circuit with buck modulator. For different applications, two specific back EMF detection methods: terminal voltage detection method and line voltage detection method, are proposed. The simulation results verified the validity of the proposed design for sensorless BLDC motor, and by comparisons we further pointed out different characteristics and possible applications of the two back EMF detection methods.

Keywords- sensorless BLDC motor; half-bridge driver circuit; back EMF detection

I. INTRODUCTION In the past decades, sensorless control schemes for BLDC

motor have been widely studied. Among those existing schemes, back EMF detection is the most popular technique. Many direct and indirect back EMF detection methods have been proposed [1-5], most of which are based on full-bridge driver. Compared with full-bridge driver, half-bridge driver has simpler structure, smaller occupied volume, lower power loss, easier control scheme and lower production cost. Thus in many applications which have high requirements in reliability, power loss, volume, and cost, e.g., some military and household products, half-bridge driver would have its apparent superiority to full-bridge driver. Encouraged by these characteristics of half-bridge driver, this paper presents novel designs for sensorless BLDC motor using half-bridge driver circuit with buck modulator.

Existing back EMF detection methods are principally designed based on the full-bridge driver circuit. Iizuka et al. originally proposed a direct terminal voltage detection method in [1], which extracts the back EMF ZCP (zero-crossing point) by detecting voltage difference between terminal voltage of the floating phase and neutral point voltage, and then shifts 30 electrical degrees for commutation. To circumvent the unstable problem of the neutral point during PWM (pulse width modulation) switching, Shao presented an improved terminal voltage sensing scheme in which back EMF signals can be

directly extracted without detecting the motor neutral point [2].However, in both [1] and [2], the operation of shifting 30 electrical degrees is relatively complex. To reduce the computation complexity, line voltage detection methods are proposed in [3] and [4], which detects the commutation points by line phase voltages directly. In addition, several improved back EMF detection schemes are presented in [5], such as methods utilizing the integration of back EMF or the third harmonic of back EMF. However, all of the above methods cannot be directly utilized in the half-bridge driver circuit.

In order to realize sensorless BLDC motor based on half-bridge driver, in this paper we design corresponding back EMF detection methods. For different application cases, two improved back EMF detection methods are proposed respectively: terminal voltage detection method and line voltage detection method. Terminal voltage detection method for half-bridge driver extracts the true back EMF ZCP by detecting voltage difference between the terminal voltage of the floating phase and the control voltage modulated by buck modulator instead of the neutral point voltage. Compared with the neutral point, the control voltage modulated by buck modulator is stable during PWM switching. Line voltage detection method for half-bridge driver gets the commutation point directly by detecting ZCP of line voltage between two floating phases, while in full-bridge driver only one phase is floating at any time and the detection is relatively complicate. The simulation results verified the validity of the proposed design for sensorless BLDC motor, and by comparisons we learned that terminal voltage detection method has higher detection precision than line voltage detection method, while the latter one is simpler for realization. According to their different characteristics, these two methods could be utilized in different application cases in the future.

II. DRIVER CIRCUIT AND COMMUTATION MANNERIn this section, we would introduce the structure and

commutation manner of the half-bridge driver circuit.

The structure of the half-bridge driver circuit utilized is presented in Fig.1 (a). Here, a buck modulator is introduced to improve the performance of half-bridge driver. Four power devices T0-T3 are included in the driver circuit in total. T0,which is controlled by PWM signal in the buck modulator, is used to regulate the phase current. T1-T3 are all utilized for commutation.

978-1-4244-7161-4/10/$26.00 2010 IEEE

Fig.1 (b) shows three-step commutation manner for half-bridge driver. In three-step commutation manner, only one phase conducts current at anytime, and the conducting interval for each phase is 120 electrical degrees. In another word, half-bridge driver sould commutate every 120 electrical degrees. The back EMF (ea, eb, and ec), current (ia, ib, and ic), ZCP (TQa,TQb, and TQc) and commuting points (TQ1-TQ3) in the three-step commutation manner are illustrated in Fig.1 (b).

Compared with the structure of the full-bridge driver commonly utilized for BLDC motors, half-bridge driver utilizes fewer power components and has the simpler structure. Thus it has the lower power loss, smaller occupied volume, and would also have higher reliability. For the full-bridge driver, the conducting interval for each phase is 120 electrical degrees, but it should commutate every 60 electrical degrees to make current be in phase with the back EMF. Instead of six-step commutation for full-bridge driver, only a three-step commutation is required in the half-bridge driver. It is easy to learn that the half-bridge driver is easier to control than the full-bridge driver.

III. BACK EMF DETECTION METHODIn this section we discuss back EMF detection methods for

half-bridge driver with buck modulator. From Fig.1 (a), we could deduce the following equations:

d( )d

d( )dd( )d

= + + +

= + + +

= + + +

ap ag a a

bbg b b

ccg c c

iu u ri L M et

iu ri L M etiu ri L M et

(1)

Here, up is control voltage modulated by buck modulator. uxg is terminal voltage for phase x. ix is current for phase x. ex is back EMF for phase x. x could be any one of the phase a, b,and c. r, L and M is the resistance, self-inductance and mutual-inductance of the three phases.

Based on (1), two kinds of voltage detection methods are proposed, each of which would have its own application cases.

We referred to the basic ideas of back EMF detection in the full-bridge driver circuit. However, since here a different driver circuit is adopted, different methods should be designed according to the new circuit.

A. Terminal Voltage Detection Method Terminal voltage detection method extracts the true back

EMF ZCP by detecting voltage difference between the terminal voltage uxg and the control voltage up, and then shifting it 30 electrical degrees for commutation.

Assuming phase a is conducting current, phase b and c are floating, we can compute back EMF eb and ec from (1),

eb = up ubg (2)

ec = up ucg (3)

From (2) and (3), the ZCP TQb of phase b and TQc of phase ccan be directly extracted. There are 60 electrical phase difference existing between TQb and TQc, so the commutation point TQ2 for phase b which is 30 electrical degrees lagging behind TQb can be computed from,

212Q Qb

Qb Qc

T T T

T T T

= +

=

. (4)

In the similar way, the commutation point TQ1 and TQ3 for phase a and c can be obtained.

PWM switching noise and commutation noise are interferences to the system. In the half-bridge driver circuit, PWM switching noises have been filtered by the LC filter of buck modulator, they would have no impact on terminal voltage uxg and control voltage up. But the negative influence of commutation noises still remains. Commutation could cause a spike on terminal voltage and lead to wrong commutation points. So in this paper, sampling holder is further introduced to eliminate the commutation noises. When the commutation happens, the holder is triggered to hold the sampling results of terminal voltage until the commutation ends to make sure the spike on terminal voltage will not be brought in. This holding operation would not distort the waveform of back EMF at its

t

t

t

(a) Half-bridge driver with buck modulator (b) Three-step commutation manner

Figure 1. Half-bridge driver circuit and its commutation manner

ZCPs. That is to say, sampling holder has no negative influence on ZCP detection.

B. Line Voltage Detection Method In terminal voltage detection method, commutation points

are extracted by shifting 30 electrical degrees from the ZCPs of back EMF. This extra shifting operation makes the terminal voltage detection method relatively complicated. Line back EMF is of 30 electrical phase difference with back EMF, whose ZCPs coincide with the commutation points. So line voltage detection method can obtain commutation points directly by detecting the ZCPs of the line back EMF without the extra shifting operation.

In the three-step commutation, at any time only one phase is conducting and the other two phases are floating. So the true line back EMF can be computed from the line voltage between the two floating phases directly. Assuming phase a is conducting current, and phase b and c are floating, the line back EMF ebc between phase b and c can be deduced from (1),

ebc = ucg ubg = ucb. (5)

By detecting the ZCP of ucb, the ZCP of line back EMF ebcwould be obtained as the commutation point for phase c.

Similarly, when phase b is conducting, eca can be got by

eca = uag ucg = uac, (6)

and when phase c is conducting, eab can be got by

eab = ubg uag = uba . (7) By detecting the ZCPs of uac and uba, we can obtain the

commutation point for phase a and b. As mentioned previously, commutation noises can distort the line voltage and lead to wrong commutation points. Low pass filter can be used to filter the communication noise, but time delay will be brought in. We can use the same strategy of the terminal voltage detection method by introducing a sampling holder, but the sampling holder will delay the ZCPs because commutation point and the ZCP of line voltage occur at the same time. Here we adopted an ingenious solution from [6]. The solution is illustrated in Fig.2 (a). Line voltage uac, uba and ucb are inputs of zero-

crossing comparators, and HALL_A, HALL_B and HALL_C are outputs of the comparators. Pulse signal pulse_a, pulse_b and pulse_c are respectively generated at the rising edge of HALL_A, HALL_B and HALL_C. We take phase a for example to explain how the adopted solution works. HALL_A is sent to a sampling holder controlled by pulse_b and pulse_c. If either of pulse_b and pulse_c is valid, HALL_A will be held, otherwise HALL_A will be sampled. As in Fig.2 (b), the wrong position information in HALL_A is eliminated by pulse_b and pulse_c, and the true position signal hall_a is extracted. In the same manner, position signal for phase b and c can be got.

IV. SIMULATIONA simulating model built in Matlab Simulink is shown in

Fig.3. The model includes BLDC motor module, half-bridge driver with buck modulator module, position detector module, decoder module, speed regulator module and open-loop start module. The position detector module uses terminal voltage detection method or line voltage detection method to extract the position signals; the decoder module receives the position signals and outputs the commutation points; speed regulator is made up of speed loop and current loop, it controls the motor running at the speed of 6000 rpm (revolutions per minute); the open-loop start module is used to start the motor from standstill until the back EMF can be detected. The BLDC motor parameters in the model are shown in table I.

Firstly, terminal voltage detection method is utilized in the position detection module, the position signal is computed according to formula (4) and sampling holder is used to eliminate the commutation noises. The detected back EMF and position signal for phase a are compared with the true back EMF and position signal at speed of 6000 rpm in Fig.4 (a).The true back EMF and position signal are drawn with real lines, the detected back EMF and position signal are drawn with dashed lines.

TABLE I. BLDC Motor Parameters

Phase Pole Back EMF Coefficient Torque Coefficient

3 8 0.047V/(rad/s) 0.05Nm/A

Figure 2. Eliminating commutation noises in the line voltage detection method

(b) Illustrations for the process of eliminating commutation noises

apulse _

bpulse _

cpulse _

AND

AND

AND

ahall _

bhall _

chall _

zero

-cro

ssin

g co

mpa

rato

rze

ro-c

ross

ing

com

para

tor

zero

-cro

ssin

g co

mpa

rato

r

Puls

e ge

nera

ter

Pulse

ge

nera

ter

Puls

e ge

nera

ter

Sam

ple/

Hol

dSa

mpl

e/H

old

Sam

ple/

Hol

d

CHALL _

BHALL _

AHALL _

bau

cbu

acu

(a) Solution for eliminating commutation noises

pulse_c

hall_a

pulse_b

HALL_A

Time

Secondly, line voltage detection method is used in the position detection module. The position signal can be extracted according to the method described in Fig.2 (a). The detected line back EMF and position signal for phase a are compared with the true line back EMF and position signal at speed of 6000rpm in Fig.4 (b). The true line back EMF and position signal are drawn with real lines, the detected line back EMF and position signal are drawn with dashed lines.

From Fig.4, we can see at the speed of 6000rpm, position signal detected by terminal voltage is nearly the same compared with the true position signal, but position signal detected by line voltage has approximately 1% error compared with the true position signal. The reason is that terminal voltage detection method can extract the ZCPs of back EMF directly without distortion, its precision is mainly decided by the operation of shifting 30 electrical degrees. When the motor runs at 6000 rpm steadily, the computation error can be ignored, so it has high accuracy. In line voltage detection method, though the wrong commutation points created by commutation noise can be eliminated, ZCP of the line voltage is distorted unavoidable, because the ZCP of line voltage and the commutation point occur at the same time. However, line voltage detection method does not require shifting 30 electrical degrees operation, so it is more easily to realize than terminal voltage detection method.

V. CONCLUSIONThis paper presents new designs for sensorless BLDC

motor based on the half-bridge driver with buck modulator. The corresponding back EMF detection methods are developed including terminal detection method and line voltage detection method. Terminal voltage detection method

extracts the ZCP of back EMF between the terminal voltage of the floating phase and the control voltage modulated by the buck modulator. Sampling holder is utilized to eliminate the commutation noise. Line voltage detection method extracts the commutation points by measuring the line voltage between two floating phases. An ingenious solution is presented to eliminate the commutation noises while at the same time to avoid the time delay. Various simulation results verified the validity of the proposed design for sensorless BLDC motor. By comparing the two methods presented in this paper, we also learned that terminal voltage detection method has higher detection precision than line voltage detection method, while the latter one is much simpler for realization. According to their different characteristics, these two methods could be utilized in different application cases in the future.

REFERENCES[1] K. Iizuka, H. Uzuhashi., M. Kano, T. Endo and K. Mohri,

Microcomputer control for sensorless brushless motor, IEEE Trans. Ind. Appl., vol. IA-21, no. 4, pp. 595601, 1985.

[2] J. Shao, An improved microcontroller-based sensorless brushless DC (BLDC) motor drive for automotive applications IEEE Trans. Ind. Appl., vol. 42, no.5, pp.1216-1221, 2006.

[3] P. Damodharan and K. Vasudevan, Indirect back-EMF zero crossing detection for sensorless BLDC motor operation, 6th IEEE International Conference on Power Electronics and Drive Systems (PEDS2005), pp.1107-1111, 2005.

[4] P. Damodharan and K. Vasudevan, Sensorless Brushless DC Motor Drive Based on the Zero-Crossing Detection of Back Electromotive Force (EMF) From the Line Voltage Difference, IEEE Trans on Energy Convers.,vol. 25, no. 2, pp. 1-8, 2010.

[5] C. Xia, The control System of Brushless DC motor. Beijing: Science Press,2009, pp.127-161.

[6] C. Fan, Y. Wang, X. Lin and Y. Chen, A Driving Circuit for Sensorless Brushless DC Motor, Micromotors,vol.34, no. 3, 2001.

Figure 3. Simulating model for sensorless BLDC

0 0.625 1.25 1.875 2.50

1

hall_a

Time (ms)

0 0.625 1.25 1.875 2.5-40

-200

2040

ea(V)

Time (ms)

Figure 4. Simulation results for two back EMF detection methods at the speed of 6000 rpm (a) Simulation results for terminal voltage detection method (b) Simulation results for line voltage detection method

0 0.625 1.25 1.875 2.50

1

hall_a

0 0.625 1.25 1.875 2.5-100

-500

50100

eca(V)