AVR172: Sensorless Commutation of Brushless
DC Motor (BLDC) using ATmega32M1 and
ATAVRMC320
Features
8-bit
Microcontrollers
• Robust sensorless commutation control
• Ramp-up sequence
Application Note
References
[1] ATmega32M1 Data sheet
[2] AVR194: Brushless DC Motor Control using ATmega32M1
[3] AVR430: MC300 Hardware User Guide
[4] AVR470: MC310 User Guide
[5] AVR471: MC320 Getting Started Guide
[6] AVR928: Sensorless methods to drive BLDC motors
1 Introduction
This application note describes how to implement a sensorless commutation of
BLDC motors with the ATAVRMC320 development kit.
The ATmega32M1 is equipped with integrated peripherals that reduce the number
of external components required in a BLDC application. The ATmega32M1 is
suitable for sensorless commutation and for commutation with Hall sensors as well,
but this application note focuses on the sensorless commutation.
The AVR928 Application Note describes the theory of the sensorless control
method and must be carefully read first.
Rev. 8306B-AVR-05/10
2 Hardware
The hardware includes the ATAVRMC310 and ATAVRMC300 boards which are the
two parts of the ATAVRMC320 Starter kit.
Please refer to the ATAVRMC300 and ATAVRMC310 user guides :
- AVR430: MC300 Hardware User Guide
- AVR470: MC310 Hardware User Guide
2.1 MC310 jumpers setting
The AVR172 firmware has been developed with the following jumper settings:
Table 2-1.ATAVRMC310 jumpers setting for sensorless control
Designator
Setting
Function
J5
Vm
connect PB4 to Vm’ (motor voltage measurement if necessary)
J6
PFC OC
Connect to overcurrent signal
J7
none
used by CAN applications
J8
ShCo
connect PC5 to ShCo for current measurement
J9
GNDm
connect PC4 to GNDm for current measurement
J12
TxD
connect PD3 to the RS232 driver
MOSI A
Connect PD3 to ISP connector (for ISP use)
RxDUSB
Connect PD3 to RxD1 (for USB interface use)
RxD
connect PD4 to the RS232 driver
SCK
Connect PD3 to ISP connector (for ISP use)
TxDUSB
Connect PD3 to RxD1 (for USB interface use)
J15
none
used by CAN application to add a termination resistor
J21
Cmp-
connect ACMP0- to V+W bemf conditioning
J22
Cmp+
connect ACMP0+ to U bemf conditioning
J23
Cmp-
connect ACMP1- to U+W bemf conditioning
J24
Cmp+
connect ACMP1+ to V bemf conditioning
J25
Cmp-
connect ACMP2- to U+V bemf conditioning
J26
Cmp+
connect ACMP2+ to W bemf conditioning
J28
VCC
supply the on board USB dongle from the board power supply
J13
See also following picture of MC310 Jumpers configurations :
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Figure 1. MC310 Jumpers configuration
2.2 MC300 jumper settings
Table 2-1. ATAVRMC300 jumpers setting for sensorless control
Designator
Setting
Function
J2
none
provide +5V to supply the ATAVRMC310 board
On ATAVRMC300, Vm and Vin connectors can be supplied from the same +12V/7A
power supply. Nevertheless a separate +12V/1A can also be used to supply the Vin
(processor supply voltage).
2.3 Power-supply
This firmware example has been configured according to a power-supply Vm=12V.
This power-supply must be able to provide up to 4A output current.
2.4 Motor
The BLDC motor provided inside MC320 and MC300 Motor Control Kit has the
following characteristics:
Manufacturer : TECMOTION
Number of phases : 3
Number of poles : 8 (4 pairs)
Rated voltage : 24V
Rated speed : 4000 rpm
Rated torque : 62.5 Nm
Torque constant : 35 Nm/A = k_tau
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Line to Line Resistance : 1.8 ohm = R
Back EMF : 3.66 V/Krpm = k_e
Peak current : 5.4A
As Vm=12V, the rated speed will be 2000 rpm.
2.5 ATmega32M1 Configuration
ATmega32M1 must be programmed to run at 16MHz using PLL (set corresponding
Fuse bits).
The CKDIV8 fuse must be disabled.
Extended/High/Low Fuses configurations are : FF/DF/F3
2.6 Technical Advices
2.6.1 Disconnecting the BLDC Motor
The BLDC motor must not be disconnected while it is running or while its coils carry
current. It is allowed to disconnect a BLDC motor if the PWM duty cycle is 0% and the
rotor is at rest so that no current is driven through the coils. Be careful, when stopping
the power supply or PWM, a BLDC motor with a high moment of inertia is able to run
for a relatively long time.
2.6.2 Ground and Power Wirings
One design its own board has to take care of the ground wiring and power wiring. The
power supply of the processor and additional signal conditioning components (e.g.
additional fast comparators, operational amplifiers, …) has to be decoupled from the
motor power supply. The ground connection has to be of low resistance and low
inductance to prevent against voltage drop and noise due to high currents. A ground
plane within a multi layer PCB is recommended for proper operation.
3 Firmware
The example firmware is based on the Sensorless method described in AVR928
Application Note.
It is operating in sensorless mode using the ATmega32M1 internal comparators. Hall
sensor wires of the BLDC motor of the kit can remain unconnected.
The source file directory embeds an html documentation which can be opened
through the readme.html file.
The theory of the different tasks has been detailed in AVR928. The application to
ATmega32M1 is detailed in following sections.
3.1 Main Flow chart
The firmware main flowchart is described below :
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Figure 2. Main flow chart
The tasks are scheduled thanks to the g_tick produced each 1.024ms with Timer0.
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3.2 MS_ALIGN phase
The ALIGN phase forces the motor at a specific position. The time of this phase is
controlled with ALIGN_TIME constant which is the ru_period_counter initial value
(200 for MC310 motor).
3.3 RAMP_UP phase
The ramp-up charateristics (duty-cycles and times) are stored in two tables:
• ramp_up_duty_table[] : which provides the duty_cycle of the step
• ramp_up_time_table[] : which provides the length of the step (ru_step_length)
These two tables are specific to the motor and the application.
The scanning of the step sequences and the monitoring of the step length are
achieved thanks to three independant counters :
- ru_step_length_cntr : which counts the commutation time (up to ru_step_length
variable)
- ru_period_counter : which counts the step length (up to RAMP_UP_PERIOD
constant)
- ramp_up_index : which counts the step numbers (up to
RAMP_UP_INDEX_MAX constant)
The figure below provides a waveform of steps timing :
Figure 3. Steps timing
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3.3.1 Time of steps
The step time is RAMP_UP_PERIOD = 50ms.
3.3.2 Number of steps
The parameter : RAMP_UP_INDEX_MAX = 9, defines 10 steps ramp up.
3.3.3 Parameters tables
In firmware example, the tables have been defined according to the characteristics of
the motor provided in the kit (see parameters in 2.4 Motor section) :
ramp_up_time_table[] = {26,23,20,17,14,11,8,5,3,2,2};
ramp_up_duty_table[] = {122,124,126,129,131,133,135,137,140,143,145};
3.3.4 Sp1/pwm1
The usual parameters described in AVR928 Application Note are:
•
Pwm1 = 50%
•
Sp1 = Sp_max/60
The parameters defined with MC310 Tecmotion motor are:
•
Pwm1 = 48% (= 122/256)
•
Sp1 :
Sp1 is defined thanks to the initialization value of ru_step_length :
ru_step_length = RAMP_UP_STEP_MAX = 40
This variable determines one commutation each 40ms.
So an electrical rotation time is 120ms. As the motor has 4 pairs of poles, the
mechanical rotation time is 480ms. So the rotation speed is 60/0.48 = 125 rpm.
So Sp1 = Sp_max/32.
The second value of ru_step_length is 26 in the time table. It defines the following
commutation time.
3.3.5 Sp2/pwm2
The theorical parameters described in AVR928 Application Note are:
•
Pwm2 = 60%
•
Sp2 = Sp_max/6 = Sp1 / 10
The parameters defined with Tecmotion motor are:
•
Pwm2 = 57% (= 145/256)
•
Sp2 :
Sp2 is defined thanks to the last value of ru_step_length : 2
This variable determines one commutation each 4ms.
So an electrical rotation time is 12ms. As the motor has 4 pairs of poles, the
mechanical rotation time is 48ms. So the rotation speed is 60/0.048 = 1250 rpm.
So Sp2 = Sp_max/3.2.
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This confirms also the usual ratio = 10 between Sp1 and Sp2 which is defined in
AVR498 Application Note.
3.4 LAST_RAMP_UP phase
To avoid a shorten last step, this phase monitors the last ramp-up step to guarantee it
is ended properly before running in closed loop.
3.5 RUNNING Phase
3.5.1 Closed-loop block diagram
The Running phase is a sensorless closed loop which block diagram is following :
Figure 4. Closed-loop block diagram
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3.5.2 Running flowchart
The flowchart is following :
Figure 5. Closed-loop flowchart
•
Motor_state is kept equal to MS_RUNNING
mci_set_ref_speed() function updates the speed setpoint according to the
potentiometer adjustment or the speed command received on serial transmission.
In mc_regulation_loop() function, duty_cycle_reference is the duty_cycle variable
which controls the PWM generator. This variable is the result of following functions :
• In OPEN_LOOP:
mci_set_ref_speed() function
• In SPEED_LOOP:
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mc_control_speed(2*mci_get_ref_speed())
duty-cycle_reference is calculated from
monitored mci_get_measured_speed()
ref_speed
and
from
measured_speed = (KSPEED * 4) / mci_measured_period
with mci_measured_period calculated in the Interrupt vector of
Analog Comparator 1. This interrupt uses Timer 0 to compute the
period.
• In CURRENT_LOOP :
mc_control_current(mc_get_potentiometer_value()
3.5.3 Sensorless Detection and Commutation Management
The analog comparators 0, 1 and 2 are used to detect the zero crossing of the U, V
and W phases.
The timer 1 is used to monitor the time between two consecutive zero crossings. This
time corresponds to one sector of the electrical rotation of the motor. It equals 60° of
the entire electrical period of the motor.
When a zero crossing event occurs, the timer 1 value is stored. Then this value is
divided by 2 (providing the 30° time) and loaded into the Compare A register of timer
1. Then this value is added to the half of itself to provide the 45° time and loaded into
the Compare B register of timer 1.
The timer 1 compare A event occurs 30° after the zero crossing. It activates the next
commutation state and masks the zero crossing to avoid the discharge of the
inductance (demagnetization) pulse generated at the end of a step when the active
switches are released.
Due to the inductance of the motor coils, a voltage equals to -Ldi/dt is generated, the
demagnetization is done through the diodes of the power bridge.
The timer 1 compare B event releases the zero crossing mask : enables the
comparator n interrupt according to the motor_step variable. This Timer1 interrupt
provides the demagnetization mask delay.
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4 RS232 Communication with firmware
4.1 Connecting ATAVRMC310 to use the RS232 interface
Connect PC com port to the ATAVRMC310 RS232 connector through a direct cable.
The serial configuration is:
• 38400 bauds,
• 8 bit data bit,
• 1 stop bit,
• no handshake,
4.2 PC applications
User can communicate with firmware through RS232 with usual PC serial
communication applications (i.e. Hyperterminal) or the Atmel “Motor Control Center”
application which can be downloaded from Atmel web at url : http://www.atmel.com
4.2.1 PC Terminal : RS232 Messages and Commands
At power up the following welcome message is received on terminal :
“ATMEL Motor Control Interface”.
The following commands can be sent to the firmware:
Table 2-1.
List of commands
Command
Action
ru
Run motor
st
Stop Motor
help
Gives help
fw
Set direction to Forward
bw
Set direction to Backward
ss
Set Speed (followed with speed value)
gi
Get ID
g0
Get Status 0
g1
Get Status 1
4.2.2 Motor Control Center
The User Guide is available in Install directory at URL :
C:\Program Files\Atmel\Motor Control Center\help\Overview.htm
The AVR172 Target must be selected first to get the right configuration :
To select a target, execute the File > Select Target command or click the
button in the toolbar. The following dialog pops up:
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Figure 6. Motor Control Center Interface
5 USB communication
Communication can be achieved from PC to USB connector of MC310 board.
The AVR470, MC310 Hardware User Guide details the configuration to be achieved.
Communication port becomes a Virtual Com port. Same tools as described in section
4 (RS232 Communication with firmware), can be used through this Virtual Com port.
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