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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 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

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

U V

T1

T2

T3

eU

T4

W

Vdc

T5

T6

idc

GND (N)

~ ~ ~eV eW

O

LU

Shunt

LV LW

RU RV RW

Vdc/2

Rx

Rx

Fig. 1. Brushless dc motor and power electronics circuit

101110

010 001

011

100

1 0 0

HALL1 HALL2 HALL3

Fig. 2. Six commutation steps

Recent Researches in Circuits, Systems and Signal Processing

ISBN: 978-1-61804-017-6 208

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:

πnp; ωV

ILωα r

dc

sra

2

120 (1)

U

V

W

HALL1

HALL2

HALL3

100 101 001 011 010 110 100 101Commutation

step

Fig. 3. Six step commutation signals

Continuous Torque Zone

Intermittent Torque Zone

Torque

Speed

Peak Torque

Rated Torque

Rated Speed

Maximum Speed

Field weakening region

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

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 10 20 30 40 50 60 70

Ou

tpu

t Sp

ee

d

Phase advance (electrical degrees)

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

Frd

Fsq

Rotor

Stator

Frd

Fsq

Rotor

Statorωr

αa

Fs

(a) Normal operation (b) Field weakening Fig. 5. Field weakening vector representation


ISBN: 978-1-61804-017-6 209

- 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

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.

2 HW field weakening implementation

method

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:

2_

360

polepairsnumberstepHall

(2)

In case of six step commutation and four pole pairs

(as in Fig. 7) the result will be:

o

Hallpolepairsnumberstep

3046

23602

_

360

(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°

Hall - Cells

Rotor

magnets

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

configuration

Hall

Cells

Housing

BLDC

motor

Stator

Rotor

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


ISBN: 978-1-61804-017-6 210

(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.

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.

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

(a) Motor assambly with rotary encoder

(b) Rotary encoder magnet configuration

BLDC

motor

Rotary

encoder

Magnet

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


ISBN: 978-1-61804-017-6 211

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:

0 er (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:

polepairs

advanceer

0 (5)

- 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:

polepairs

advance 00' (6)

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

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

BLDC

motor

controller

ASIC

3 phase

inverter U

W

V

Laptop

USB

System

microcontroller

S12XF384

H1H2

H3

PWM DIR EN

BLDC motor

Rotary encoderSPI

Fig. 12. Experimental setup block diagram

Table 1. Parameters of the employed BLDC motor

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)

Torque and

speed sensorBrake

BLDC

Motor

Fig. 13. Motor test bench setup


ISBN: 978-1-61804-017-6 212

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.

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.

Acknowledgement

This work was partially supported by the strategic grant

POSDRU/88/1.5/S/50783, Project ID50783 (2009), co-

financed 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), co-

financed 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.

References:

[1] P. P. Acarnley, J. F. Watson, "Review of Position

Sensorless Operation of Brushless Permanent-

Magnet 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

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

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1000 2000 3000 4000 5000 6000 7000

Torq

ue

[Nm

]

Speed [rpm]

+40°

+20°

0°

-20°

-40°


ISBN: 978-1-61804-017-6 213

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