ST AN1946 Application note
AN1946
APPLICATION NOTE
SENSORLESS BLDC MOTOR CONTROL
AND BEMF SAMPLING METHODS WITH ST7MC
1 INTRODUCTION
Permanent Magnet Brushless DC Motors are replacing brush motors in numerous applications as they offer significant energy efficiency improvements, lower acoustic noise and better
reliability to name a few advantages. To be driven and controlled properly, 3-phase Perma
nent Magnet Brushless motors require a 3 half bridge "inverter" topology to deliver a 6-step or
sine wave signal. They also require the electronic commutation of motor phases to respect the
synchronization between statoric flux and the permanent magnet of the rotor.
Generally, a BLDC motor drive uses one or more sensors giving positional information to keep
synchronization. Such implementation results in a higher drive cost due to sensor wiring and
implementation in the motor. Moreover, sensors cannot be used in applications where the
rotor is in closed housing and the number of electrical entries must be kept to a minimum such
as in a compressor, or in applications where the motor is immersed in a liquid such as some
pumps.
-
Therefore, for cost and technical reasons, the BLDC sensorless drive is an essential capability
of a brushless motor controller. The ST7MC allows various implementations of sensorless
BLDC control with the lowest possible system cost while maintaining the highest performance.
This paper describes in detail these topologies, their advantages and drawbacks, as well as
their practical implementation. Most of the examples in this paper make use of the ST7MCKIT/BLDC Starter kit which allows easy implementation of most topologies described.
AN1946 Rev. 2 1/35
1
Table of Contents
1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 SENSORLESS DRIVE PRINCIPLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 SAMPLING METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
3 SAMPLING AT END OF PWM OFF STATE (ST PATENTED "3 RESISTOR" METHOD) 7
3.1 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
3.2 ST7MC-KIT/BLDC PRACTICAL IMPLEMENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
3.3 PRACTICAL RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
4 SAMPLING DURING PWM ON STATE (INDUSTRY STANDARD “CLASSICAL” BEMF
METHOD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.2 PRACTICAL IMPLEMENTATION OF SAMPLING DURING PWM ON . . . . . . . . . . . . . . . . . .13
4.2.1 Components dimensioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
4.2.2 Size of Reference Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
4.2.3 Size of BEMF sensing network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
4.3 PRACTICAL IMPLEMENTATION OF THE "CLASSICAL" METHOD, USING THE ST7MC-KIT/
BLDC STARTER KIT 17
4.4 PRACTICAL RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
5 SAMPLING DURING PWM ON STATE USING DIGITAL FILTER . . . . . . . . . . . . . . . . . 20
5.1 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
5.2 PRACTICAL IMPLEMENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
5.2.1 Sizing of network on motor phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
5.2.2 Sizing of Network of Voltage Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
5.3 RECORDS OF DIGITAL SAMPLING DURING PWM ON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6 MIXED SAMPLING AT END OF PWM OFF AND DURING PWM ON . . . . . . . . . . . . . . 25
7 BEMF SAMPLING AT HIGH FREQUENCY METHOD (USED IN PULSE AMPLITUDE
MODULATION METHOD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8 CHOICE OF SAMPLING METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9 CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
10 APPENDIX - IMPROVED BEMF DETECTION FOR LOW SPEED AND LOW VOLTAGE
APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
10.1 THEORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.2 PROBLEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
10.2.1Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
10.3 RESULT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
10.4 CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
11 REVISION HISTORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
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2
35
AN1946
2 SENSORLESS DRIVE PRINCIPLE
The sensorless drive is based on the detection of the Back Electro Magnetic Force (BEMF) induced by the movement of a permanent magnet rotor in front of stator winding.
This method also requires the use of a trapezoidal signal in order to have a zero crossing of
the BEMF.
Figure 2 below shows the three BEMF voltages referenced to the neutral point for a motor run-
ning at constant speed without excitation (the motor is not supplied, and the rotor is manually
rotated).
Figure 1. Model of BLDC Motor with the Wire in Star Connection.
A
E
a
E
c
E
b
B
Neutral
C
Figure 2. Phase voltage versus Neutral For each stator winding
Ea
Eb
Ec
For a given fixed motor design (number of stator winding turns, mechanical rotor characteristics and rotor magnet characteristics) the BEMF Amplitude is proportional to the rotor speed.
The sensorless method uses the zero crossing of BEMF to synchronize phase commutations.
To detect BEMF the specific 120° six-step drive is used. "120° six-step drive" forces zero cur
rent twice in each phase during a six step period. This allows BEMF zero crossing to be detected and read.
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AN1946
The step sequence and corresponding motor phase current are shown in Figure 3 and Figure
4 below.
Figure 3. Six-Step current circulation
+HV DC
T5
W
T6
-
Figure 4. Six-Step sequence
T3
V
T4
Current in winding A,B,C
A
T1
U
T2
1
6
4
5
A
B
C
2
3
t
B
C
1234
T1 T1 T3 T3 T5 T5
T4 T6 T6 T2 T2 T4
More specifically, for each step, one phase of the motor is not energized, which allows detection of the BEMF zero crossing in this phase.
For each phase two zero crossings must be detected during a period:
One "rising crossing" when BEMF passes from negative to positive,
One "falling crossing" when BEMF passes from positive to negative.
In the non-energized winding (phase C here), the current is zero and the voltage measured is
the BEMF of the motor (
4/35
Figure 5).
56
t
t
Figure 5. BEMF measurement synoptic.
A
e
a
e
c
AN1946
BUS
e
b
N
C
B
Voltmeter
POWER GND
Based on the above BEMF detection principle, several methods are available to precisely determine the BEMF zero crossing point. All these methods have advantages and drawbacks
which will be discussed in the next section. These methods also take into account the fact that
in most cases the neutral point of the motor is not accessible; either because the motor is
delta-wound, or because no wire is extracted. In any case the methods presented in the next
section allow the designer to select the most suitable approach to meet his application require
ments.
Note 1: Because BEMF is proportional to the rotor speed, this implies that the rotor should
turn at a minimum speed to generate sufficient BEMF. This minimum speed varies from one
motor to another. For very low speeds it may be required to amplify BEMF in order to control
the motor. This is presented in the appendix of this application note.
-
Note 2: As mentioned above, the sensorless BEMF methods described can only be implemented using a trapezoidal signal drive. A sine wave signal drive doesn't provide zero
crossing signals and cannot be implemented with the topologies shown above. It is to be
noted however that motors originally designed to be driven with a sine wave signal (these mo
tors are wound in such way that the stator flux has a continuous variation contrary to BLDC
wound motors which have non-continuous stator flux commutation) can be controlled with a
trapezoidal signal drive and ST7MC, without any impact on performance.
5/35
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AN1946
2.1 SAMPLING METHOD
The ST7MC microcontroller allows the implementation of four methods to sample and detect
BEMF zero crossing to run a BLDC motor in sensorless mode.
Sampling during PWM OFF State at PWM frequency (SPLG=0 & DS[3:0]=0)
“ON”
PWM signal
“OFF”
Sampling order
Sampling during PWM ON state only at PWM frequency (SPLG=0 & DS[3:0]=xxx)
Sampling order
“ON”
PWM signal
d
“OFF”
Programmable Delay
d
d
Sampling during PWM ON state only at High frequency (SPLG=1 & DS[3:0]=xxx)
sampling
“OFF”
f
SCF
d
d
PWM signal
“ON”
d
Programmable Delay
Sampling either during OFF or ON state at High frequency (SPLG=1 & DS[3:0]=0)
f
sampling
SCF
PWM signal
“ON”
“OFF”
These allow for various controls and hardware topologies to be implemented.
Each method is presented below with benefits, drawbacks and rules to select the right method
for a given motor/application.
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AN1946
3 SAMPLING AT END OF PWM OFF STATE (ST PATENTED "3 RESISTOR" METHOD)
3.1 DESCRIPTION
BEMF sampling is fully digital based on the PWM duty cycle. Figure 6 below gives the physical
configuration of a motor when PWM is in an OFF state allowing BEMF to be measured during
the non-energized phase.
Figure 6. BEMF sampling at end of PWM OFF
300V
T1 PWM "ON"
A
V/2
C
B
T4 always ON
GND
Current in A & B Phases
when T1 “ON”
bemf
PWM signal
D2
B
T4 always ON
GND
Current in A & B Phases
when T1 “OFF”
“ON”
“OFF”
300V
T1 PWM "OFF"
A
GND
C
bemf
voltage clamping
+ 5V
ST7MC
Sampling order
During the Off state of PWM, the current circulation in active winding of the motor passes
through D2, the adjacent diode of switch T1.
Due to the fact that the potential of the neutral point is grounded, the voltage comparator obtains complete information about the BEMF voltage of the non-energized phase on its input
via C. Application Note AN1130 gives more information about this sampling method.
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AN1946
Advantages:
To sample the BEMF of the BLDC motor, the ST patented method needs only three resistors
(to limit input current in I/O) as external components. This simplifies the practical implementa
tion which consists of connecting the three resistors from the three motor phases to the MCIA,
MCIB, and MCIC Microcontroller inputs which:
– allows the whole sensing signal for the BEMF to be obtained,
– gives high sensitivity, used to:
– Get a large speed range on the drive motor,
– Run the motor at very low speed,
– Start the motor with maximum torque
– avoids the need for an analog filter, which suppresses filtering delays
– provides high signal to noise ratio
Drawback:
A minimum PWM OFF time is needed and the maximum available duty cycle should be limited.
-
For some applications we need to go up to maximum (100%) duty cycle, this cannot be
reached with the ST detection method and we have to switch to the classical detection method
which is described later.
3.2 ST7MC-KIT/BLDC PRACTICAL IMPLEMENTATION
By default the ST7MC-Kit/BLDC starter kit board is configured to use the ST patented "3 resistors" method.
Figure 7. Sampling at end of PWM off implementation
V
DC
W
U
V
Motor
6_MCOx
ST7FMC
MCIA
MCIB
Three
Phase
Power
Converter
8/35
MCIC
AN1946
The implemented resistors are used to limit the input current in the pad diode which clamps
the voltage on MCIA, MCIB and MCIC microcontroller inputs.
The current in the clamping diode should not exceed 5mA and should typically be around
2mA.
For a high voltage (400V) DC bus, 200k resistors are to be used. For safety and power dissipation, two 100k resistors in serial are implemented on each phase.
Resistors implemented on the starter kit board are two 82k which is suitable both for the 24V
motor included and also for high voltage motors.
3.3 PRACTICAL RESULTS
The sampling below (Figure 8) shows the voltage signal on one phase of the motor drive in 6step mode.
During both steps where the phase is not energized, corresponding to the floating state of the
winding, we can follow the progression of BEMF:
For the "rising transition" between T1 & T2 where BEMF passes from a negative to
positive value;
For the "falling transition" between T4 & T5 where BEMF passes from a positive to
negative value.
We can note the PWM superposition with the BEMF signal for both "ON" and "OFF" states of
the PWM.
For compare values set to zero, we get the zero crossing information only when the PWM is
in an "OFF" state.
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AN1946
Figure 8. Sampling at end of PWM "OFF" state
Back-EMF
Back-EMF
Z ero C ro ssing
Z ero C ro ssing
Floating phase
Floating phase
Conducting phase
Conducting phase
T3
T1
T1
T2
T2
T3
Back-EMF
Back-EMF
Floating phase
Floating phase
T4
T4
T5
T5
The zero crossing detection can also be done when PWM is in an "ON" state if the compare
value is set to the half value of the DC bus. This sampling method is discussed in further detail
later.
The fact that a 100% PWM duty cycle cannot be achieved with this method may be a drawback when the motor needs to be used to the limit of its capabilities.
The main advantage of the "3 resistors method" is that, being fully digital it allows maximum
efficiency over a very wide speed range, typically 1 to 100.
Finally, the key advantage of this method is its cost which is the lowest available on the market
now.
10/35
AN1946
4 SAMPLING DURING PWM ON STATE (INDUSTRY STANDARD “CLASSICAL” BEMF METHOD)
4.1 DESCRIPTION
In this configuration, due to sampling during PWM "ON" state, the neutral point is not
grounded like in the previous method.
Figure 9. Sampling during PWM “ON” State Once
Sampling order
“ON”
PWM signal
d
“OFF”
Programmable Delay
d
d
We have to get an access to this neutral point to be able to measure the BEMF voltage.
In many applications like compressor drives, it is not possible to implement a wire giving access to a neutral point. To get this voltage information we use various methods to rebuild a virtual neutral.
We can already note that both sampling methods during PWM ON (sample at PWM frequency
or sample at high frequency) can be implemented using:
– Discrete frozen analog filter build with resistor and capacitor, or
– Embedded digital filter of ST7MC.
The zero crossing of the BEMF measurement is done using an analog filter.
With this classical analog method the BEMF is detected through two networks.
One network is used to sense the BEMF on the floating phase and allow passing from a high
voltage level to a low voltage level compatible with the comparator input. The switching of the
PWM signal gets on the voltage adaptation request to be filter.
A second network allows the voltage reference of floating phase to be obtained necessary to
do the BEMF detection.
This second network could be realized following three schemas:
a) The rebuild of the virtual neutral motor using three resistors and again a voltage divider and
filter.
b) A voltage divider of High DC bus Voltage to get a reference voltage compatible with the
input voltage of the comparator (2.5V for example) and which follows the DC bus fluctuation.
c) In the event of constant DC bus without fluctuation, this second network can be removed
and be replaced by the internal reference voltage of the microcontroller comparator.
11/35
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