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TRANSCRIPT
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
Recent Researches in Circuits, Systems and Signal Processing
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
Recent Researches in Circuits, Systems and Signal Processing
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
Recent Researches in Circuits, Systems and Signal Processing
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
Recent Researches in Circuits, Systems and Signal Processing
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°
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ISBN: 978-1-61804-017-6 213