Recent Researches in Circuits, Systems and Signal Processing

Hardware Implementation of Field-Weakening BLDC Motor Control RÓBERT ISTVÁN LŐRINCZ, MIHAI EMANUEL BASCH, DAVID CRISTEA, IVAN BOGDANOV, VIRGIL TIPONUT Electronics and Telecommunications “Politehnica” University of Timişoara Bd. Vasile Pârvan, nr.2, Timişoara, Timiş ROMANIA [email protected], [email protected], [email protected], [email protected], [email protected] Abstract: - This paper presents a new approach of sensored BLDC (Brushless DC) motor driving in the field weakening (also called phase advanced drive) region, in order to extend the maximum speed of the BLDC motor. The basic concept is the usage of a special position sensor ASIC (Application Specific Integrated Circuit) for which the zero rotor position offset is programmable. Whit this technique the position sensor itself generates phase-advanced rotor position signals to the BLDC motor controller, Therefore the computational power needed to calculate the timing of the phase-advanced commutation points can be significantly reduced, to a simple position angle advance command to the position sensor. Key-Words: - Field-weakening, BLDC, hall sensor, ASIC, Phase Advance

1 Introduction

Rx

T1

Vdc/2

Today trend of industrial applications is replacing the conventional technologies which imply a DC motor with brushless DC (BLDC) motors, because of their higher efficiency, generated torque per size, longer lifetime, silent operation and low electromagnetic emissions. Among of these industries is the automotive industry, in which the tendency is to replace the conventional DC motor driven actuators, pumps and fans with BLDC motor driven technologies. This technology change also affects the control electronics and control algorithms of the automobiles in the same time increasing their robustness, lifetime and the automobile driving comfort. The most employed in the automotive applications is the three phased BLDC motor type. The diving of these motors requires a three phased inverter circuit which converts the DC voltage of the automobile battery into three phased synchronously alternating trapezoidal shaped phase voltages (Fig. 1). These phase voltages must be synchronous to rotor position in order to move the rotor in the desired direction and required torque. This is ensured by position sensing of the rotor, implemented using: hall-effect based sensors, optical sensors or inductive position sensors. There is also a possibility of sensor-less driving of the BLDC motor, estimating the rotor position from the back-EMF (back Electro Motive Force) signal. At low rotational speeds the rotor position is estimated using the inductance variation of the phase windings according to rotor

ISBN: 978-1-61804-017-6

idc

Vdc

T3

U Rx

T5

V

T2

W

T4

T6

GND (N) Shunt

RU

RV

RW

LU

LV

LW

~ eV

~ eW

~ eU

O

Fig. 1. Brushless dc motor and power electronics circuit

100 110

101

010

001

100 HALL1 HALL2 HALL3

011 Fig. 2. Six commutation steps

position. A comprehensive overview of these sensor-less driving methods is presented in [1]. However these sensor-less driving of BLDC motors are not suitable for all applications, as example like actuator applications

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where precise movement and position control of the rotor is a must. Therefore in case of these applications rotor position sensors are used. In the automotive industry mainly hall-effect based position sensors are used, due to their high reliability and low price. According to the BLDC motor driving strategy, different types and number of hall position sensors are used. For the classic six step commutation (120° block commutation) method, three hall position sensors are required [2] to encode the rotor position. Fig. 2 presents the encoding of the rotor positions by the three hall signal logic values and fig. 3 presents the three hall sensor signals H1, H2 and H3 respective to the phase voltages. In case of twelve step commutation (60° block commutation) control method, position information from six hall sensors is required. However it is possible to implement the twelve step commutation method using only three hall position sensors, every second commutation signal being estimated by software calculations. The torque output of a brushless motor is constant over a speed range limited by the power electronic converter ability to maintain the demanded phase currents at the required level. Fast and accurate control of the phase winding current is only possible if the supply voltage is larger than the back-EMF voltage amplitude, to be able to force current changes into the motor. The speed at which the back-EMF voltage effective amplitude is equal to the supply voltage is referred as the maximum normal operating speed or base speed. Fig. 4 presents the torque versus speed characteristics of a BLDC motor. The motor can run over its base speed in the field-weakening mode, in which a component of the phase winding current produces a magnetic field opposing the permanent magnet field and reducing the effective back-EMF voltage amplitude. Field weakening can be accomplished by increasing the phase angle by which the current leads the back-EMF voltage [3]. This method is also called phase-advanced drive. Fig. 5 presents the vector representation of the field weakening. The graph from Fig. 6 presents the normalized speed output of a BLDC motor in respect to its base speed versus the phase advance angle under no load conditions [4]. This phase advance allows fast current rise before the “occurrence” of the back-EMF. (assuming a PM span angle aPM < 150° - 160°) An approximate way to estimate the advance angle required αa, for 120° conduction, may be based on linear current rise to the value I:

αa 120  ωr Ls  I ; Vdc

ωr  2  p  n  π

ISBN: 978-1-61804-017-6

100

Commutation step

101

001

011

010

110

100

101

HALL1

HALL2

HALL3

U

V

W

Fig. 3. Six step commutation signals

Torque Peak Torque

Intermittent Torque Zone

Rated Torque

Field weakening region

Continuous Torque Zone Speed Rated Speed

Maximum Speed

Fig. 4. Torque VS Speed characteristic of a BLDC motor

Fsq

Fsq

Fs αa Stator

Stator

ωr

Rotor

Rotor

Frd

Frd

(a) Normal operation

(b) Field weakening

Fig. 5. Field weakening vector representation 2.6 2.4 Output Speed

2.2 2 1.8 1.6 1.4 1.2 1 0

(1)

10

20 30 40 50 Phase advance (electrical degrees)

60

Fig. 6. Normalized speed Output in respect to base speed

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calculate the phase advance in time which will give the desired phase angle advance [5]. The implementation of this phase advancing methods on microcontrollers requires significant computational power which is not available in most of automotive ECU’s (electronic control unit) which controls BLDC actuators. As an example a double clutch automatic transmission control unit uses up to four BLDC motor actuators to control the automatic gearbox. Beside the control of the BLDC motors the system microcontroller has several other tasks assigned to like the shifting algorithm, diagnosis of the system, read and interpret signals from several different sensors, communication with the rest of the automobile ECU’s etc. Therefore in most of the systems the computational power required for a high performance field weakening algorithm implementation is not available. This paper presents a new mode of implementation of the field weakening, using hardware implementation therefore reducing considerably the computational power required.

Hall - Cells

Rotor magnets

Fig. 7. Hall cells angular displacement for a four magnet poles rotor configuration

Rotor Stator

Housing

2 HW field weakening implementation method

BLDC motor

2.1 Classical hall sensor based rotor position sensing As mentioned in the last chapter for the six steps (120° block commutation) control method three hall position signals are required. The placement of the hall sensors for six step commutation (BLDC motor with four pole pairs) is presented in Fig. 7 for an inner rotor configured BLDC motor. The hall sensors must be placed with a certain electrical angle difference to each other according to the driving strategy. The actual mechanical angular distance between them is dependent on the motor construction. The following equation shows how to determine the actual mechanical angle (αHall) distance between two hall sensors:

Hall Cells

Fig. 8. Assembly drawing of a BLDC motor with three hall sensors

 Hall 

- where with n denoting the rotor speed in revolutions per second (rps), VDC the supply voltage, LS the phase inductance and p the number of magnetic pole pairs of the rotor. This phase advance usually is implemented by software. The motor control algorithm commands the power inverter to switch to the next commutation step with a defined time before the actual next commutation point, indicated by the hall signals. In order to achieve this, the control algorithm must estimate the time when the next hall signal commutation will accrue, according to the rotor speed, acceleration / deceleration and

ISBN: 978-1-61804-017-6

360 2 step _ number  polepairs (2)

In case of six step commutation and four pole pairs (as in Fig. 7) the result will be:

 Hall 

360 360  2 2   30o step _ number  polepairs 64 (3)

Fig. 8 presents an assembly drawing of a BLDC motor actuator showing the hall cells placement. Note that the hall cells can be mounted also with 30° + 90°

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(360° electrical degrees in case of rotor with four pole pairs as show in the picture). This ensures easier mounting of the hall cells. With this mechanical configuration the advancing of the hall signals by hardware is not possible due to the fix position of the hall cells.

Magnet

BLDC motor

2.1 Phase advance implementation using rotary encoder Rotor position sensing is also possible to implement with advanced rotary encoders developed specially for BLDC motor control. The rotor position is sensed by the rotary encoder circuit placed exactly beneath the rotor shaft Fig. 9 (a). On this shaft a small magnet is attached to, with the magnetic field poles configuration as shown in Fig. 9 (b). The rotary encoder is a special BLDC dive optimized ASIC. It provides three output hall position signals (U, V, W outputs from Fig. 10) as the three separate halls cells. Fig. 10 presents the internal block schematic of such a rotary encoder developed by Austria Micro Systems, the AS5134. The small magnet used for the position sense is a two pole magnet therefore the encoding period of the internal hall cells of the sensor is a complete mechanical 360° degree. The internal logic of the sensor divides this complete mechanical period to several complete electrical periods (1 to 6) according to the number of magnetic pole pairs of the employed BLDC motor. Before this encoder can be used as rotor position sensor and provide the correct hall signals the following steps must be followed: - Configure the number of rotor magnetic pole pairs; - Calibrate the “zero position” of the encoder; The initial “zero position” must be calibrated together with the BLDC motor in order to align the BLDC motor zero position and with the rotary encore zero position. Fig. 11 presents the zero position calibration procedure. First the rotor of the BLDC motor is aligned with the “zero position”, by applying a voltage vector which will move the rotor in the desired position. Then the rotary encoder angle indication is read out from the sensor and stored. Than the application software can set the rotary encoder zero position via SPI command. There is also a possibility to program this zero angle position in the chip OTP memory in case no further change is done by the application. The original intention of this programmable zero angle is the calibration of the sensor itself, to match the zero position angle of the BLDC motor rotor.

ISBN: 978-1-61804-017-6

(a) Motor assambly with rotary encoder

Rotary encoder

(b) Rotary encoder magnet configuration Fig. 9. Rotary position encoder assembly

Fig. 10. Magnetic rotary encoder ASIC block diagram [6]

START

Align rotor with zero position Read / store encoder position

Write encoder zero position

END Fig. 11. Flowchart of zero position calibration

This programmable zero positions of the rotary encoder gives the possibility of the implementation of a hardware phase advanced drive of the BLDC motor. In case the initial position is programmed with a certain

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angle offset the rotary encoder will generate the three hall signals with a pre-advanced angle. Therefore the software application task is only the set of the phase advanced zero angle position and the sensor itself will generate the hall signals with a constant advance in phase with the desired angle. With this method saving the computational power needed for the phase advance from the system microcontroller, now done very precisely by the rotary encoder regardless of rotor speed or acceleration, the phase advance is in every case equal with the predefined angle. As example if the zero position of the rotor is corresponding to α0 = 50° (mechanical) of the encoder, the encoder will always subtract from his measured position angle αe the 50°. This can be described as follows:

 r  e  0

System microcontroller S12XF384 PWM

DIR

USB Laptop

EN

BLDC motor

3 phase inverter

BLDC motor controller

U V W

ASIC

H1 H2 H3 SPI

Rotary encoder

Fig. 12. Experimental setup block diagram

Table 1. Parameters of the employed BLDC motor

(4)

- where αr represents the current rotor position. Based on this equation (4) and considering the number of pole pair of the rotor the phase advanced rotor position can be expressed as follows:

Number of stator poles

12

Number of rotor poles

8

Rated DC voltage

12V

Max phase current

50A

Back-EMF

kE=2V/krpm (Ellpk/krpm)

 advance 



 r   e   0 

 (5) polepairs 



- where αadvance represents the desired phase advance electrical angle. From this equation we can derive the zero position command α0 to the encoder which advances the hall signals with the desired angle:

 '0   0 

 advance polepairs

BLDC Motor

(6)

Brake

Fig. 13. Motor test bench setup

PWM duty cycle, motor direction, read and write the rotary encoder registers, etc). The parameters of the employed BLDC motor are presented in Table 1. To evaluate the motor performance change according to the applied phase advance, the motor was evaluated using a motor evaluation bench. The picture of the motor evaluation bench, Kistler 4503A2L00 type [7], is presented in Fig. 13. During the laboratory evaluation the motor was set with different zero angle programmed for the rotary encoder resulting in -40°, -20°, 0°, 20° and 40° electrical angle phase advance (±5° , ±10° and 0° mechanical angle advance). The battery voltage used for this evaluation was set to 14V, the applied duty cycle of the PWM driving signal was set to 100%. The speed versus torque characteristics of the motor was evaluated for each case, the results are presented in

3 Experimental Results To demonstrate and validate the concept a test setup was build which block diagram is presented in Fig.12. The used BLDC motor is equipped with a magnetic rotary encoder AS5134, the three hall output signals of this sensor are used as rotor position information for the three phased inverter controller ASIC. The BLDC motor controller ASIC has its input signal from the system microcontroller PWM, DIR (direction) and EN (enable); the rotary encoder hall signals. The six step commutation table is implemented inside the ASIC and provides the six gate signals for the inverter MOSFET’s. The system microcontroller is connected via USB interface to a PC application from which the system operational parameters are set (like activate motor, set

ISBN: 978-1-61804-017-6

Torque and speed sensor

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Fig. 14. It can be observed that the phase advancing causes increase in the maximum speed and torque for the same operating conditions like battery voltage and applied PWM duty cycle.

0.8

Torque [Nm]

+20°

4 Conclusions A hardware implementation of phase advanced method drive of BLDC motors was presented in this paper. In contrast to the classical software implementation of the phase advanced driving, this hardware implemented method requires significantly reduced computational power while maintaining the advanced angle accuracy even better than most of the existing software algorithms can perform. In addition the price difference of these rotary encoders for BLDC motor applications compared to the three separate hall sensor solution is insignificant or even cheaper. A disadvantage of the current encored technology is that there is no possible to change the phase advance during the motor operation, because the encoder has to be set into configuration mode which disables the position signal generation. Nevertheless the future version of this encoder type (already under development) will be able to change the offset of the zero angle position during the motor operation.

0.6

0°

0.5

-20° -40°

0.4 0.3 0.2 0.1 0 0

1000

2000

3000 4000 Speed [rpm]

5000

6000

7000

Fig. 14. Torque VS Speed for different phase advance angles

References: [1] P. P. Acarnley, J. F. Watson, "Review of Position Sensorless Operation of Brushless PermanentMagnet Machines", IEEE. Trans. on Industrial Electronics, vol. 53, no. 2, April 2006; [2] Microchip AN 885; ”Brushless DC (BLDC) Motor Fundamentals,” 2003; www.microchip.com [3] K. Safi, P. P. Acarnley, and A. G. Jack, “Analysis and simulation of the high-speed torque performance of brushless DC motor drives,” Proc. Inst. Elect. Eng.-Electr. Power Appl., vol. 142, no. 3, pp. 191– 200, Mar. 1995; [4] K.N. Leonard, C.M. Bingnarn, D.A. Stone, P.H. Mellor, “Implementing a Sensorless Brushless DC motor Phase Advance Actuator Based on the TMS320C50 DSP” Texas Instruments application note SPRA324, ESIEE, Paris, Sept 1966; [5] Han Kong, Jinglin Liu, Guangzhao Cui, "Study on Field-Weakening Theory of Brushless DC Motor Based on Phase Advance Method," Measuring Technology and Mechatronics Automation (ICMTMA), 2010 International Conference on, vol.3, no., pp.583-586, 13-14 March 2010 doi: 10.1109 ICMTMA.2010.112 [6] Austria Micro Systems, “AS5134 -360 Step Programmable High Speed Magnetic Rotary Encoder” Component datasheet 2010 www.austriamicrosystems.com; [7] Kistler Group “Dual-Range Sensor with Brushless Transmission 4503A type” 2008, www.kistler.com

Acknowledgement This work was partially supported by the strategic grant POSDRU/88/1.5/S/50783, Project ID50783 (2009), cofinanced by the European Social Fund – Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013. This work was partially supported by the strategic grant POSDRU 6/1.5/S/13, Project ID6998 (2008), cofinanced by the European Social Fund – Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013. This work has been partially supported by Continental Automotive Romania. This work was supported by the grant CNCSIS – UEFISCDI PNII – IDEI Grant No. 599/19.01.2009.

ISBN: 978-1-61804-017-6

+40°

0.7

213