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PDF AN970 Data sheet ( Hoja de datos )
| Número de pieza | AN970 | |
| Descripción | Using the PIC18F2431 for Sensorless BLDC Motor Control | |
| Fabricantes | Microchip | |
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AN970
Using the PIC18F2431 for Sensorless BLDC Motor Control
Author: Padmaraja Yedamale
Microchip Technology Inc.
INTRODUCTION
Brushless Direct Current (BLDC) motors have many
advantages over other types of motors available in the
industry. Previously, we have discussed the working
principles of BLDC motors and the basics of their
control in Microchip application note AN885, Brushless
DC (BLDC) Motor Fundamentals. We have also
discussed the use of PIC18FXX31 devices to control a
BLDC motor in both open loop and in closed loop using
Hall effect sensors and quadrature encoders
(Microchip application note AN899, Brushless DC
Motor Control Using PIC18FXX31 Microcontrollers).
This application note describes BLDC motor speed
control without the use of sensors.
Those readers interested in additional applications
may refer to Microchip application notes AN857, Brush-
less DC Motor Control Made Easy and AN901, Using
the dsPIC30F for Sensorless BLDC Control, to learn
how other PICmicro® microcontrollers and dsPIC®
digital signal controllers can be used for BLDC motor
control.
ADVANTAGES AND
DISADVANTAGES OF SENSORLESS
CONTROL
Sensorless control of a BLDC motor calls for
commutation based on the Back Electromotive Force
(BEMF) produced in the stator windings. Sensorless
control has two distinct advantages: lower system cost
and increased reliability.
Hall effect sensors are not required for sensorless
control. Some motors may have the Hall sensor
magnets mounted on the rotor, in addition to the main
rotor magnets. These are used to simplify the process
of mounting the Hall sensors onto the stator. The
sensor magnets are a scaled-down replica of the rotor;
whenever the rotor rotates, they produce the same
effect as the main magnets. The Hall sensors are
normally mounted on a PC board and are fixed to the
enclosure cap on the non-driving end of the motor.
With a sensorless method of commutation, the Hall
sensors, the sensor magnets, the sensor wires and the
PC board can be eliminated. This simplifies the motor
construction and reduces cost. Inherently, systems with
fewer components are more reliable. Applications like
the compressor control in a refrigerator or in HVAC
systems, which generates heat and an elevated
temperature, may accelerate Hall sensor failures.
Using sensorless control improves the reliability of total
system.
There are two disadvantages to sensorless control:
• The motor must be moving at a minimum rate to
generate sufficient BEMF to be sensed.
• Abrupt changes to the motor load can cause the
BEMF drive loop to go out of lock.
As the BEMF signal is proportional to the speed of
rotation at standstill and at low speeds, the amplitude
of the BEMF poses a challenge to determine the zero
crossover point. In order to overcome this problem, the
motor is normally run in open loop during starting and
is accelerated to a speed where the BEMF has
sufficient amplitude to detect and then synchronize with
the zero crossing point. Running the motor in open loop
makes it difficult to know the rotor position, when
starting from standstill. So, the rotor can rotate in either
direction when energized.
Applications which require very low motor speed may
be forced to use sensors for control. There are many
applications in appliances, automotive, and industrial
applications where very low speed is not an issue, and
the motor needs variable speeds of about 25% of the
rated speed. Sensorless control is ideal for such appli-
cations.
Another requirement is that the sensorless control
needs extra resources on the control circuit side.
Depending upon the method used, it may require a fast
A/D converter on the microcontroller, or comparator
and filter circuits.
© 2005 Microchip Technology Inc.
DS00970A-page 1
1 page  
AN970
HARDWARE
Figure 5 shows the block diagram for the sensorless
BLDC control system discussed here; a complete sche-
matic of the prototype board is provided in Appendix A:
“Motor Control Circuit Schematics”. The board is
capable of using either the PIC18F2331/2431 Microcon-
troller or a dsPIC30F2010 digital signal controller (only
the PIC18F2431 is discussed here). Toggle switch S2 is
used for changing the rotational direction and switch S1
is used for changing the state between run and stop. A
potentiometer (R14) is used to set the speed reference;
it is connected to A/D converter channel AN1.
Shunt resistor R26 is connected in the return path of the
DC bus. The voltage drop across this resistor varies
linearly with respect to motor current. This voltage is
amplified by a 1:10 ratio using op amp circuit U10A. This
amplified voltage (motor current) is measured using A/D
channel AN0. The motor current is compared with a
current limit set by another potentiometer (R60). If the
motor current exceeds the limit set, the output of U7D
will toggle, indicating an overcurrent fault. This Fault is
connected to Fault A input pin on PIC18F2431, which is
an active-low pin.
A switched-mode block power supply is used to gener-
ate 24V (shown in bold lines in Figure 5) from an AC
input. Using linear voltage regulators VR1 and VR2, +5V
and +15V are generated, respectively. The former pow-
ers the microcontroller, all op amps and related logic
circuits, while the latter is used to power the MOSFET
drivers (U6, U8 and U9).
Note:
For a complete list of motor control
application
notes
refer
to
www.microchip.com/motor.
BEMF Sensing Circuit
In this application note, we are following the second
method of determining the BEMF zero cross detection
shown in Figure 3. As shown in the schematics in the
Appendix, low-pass filter circuits and potential divider
circuits are implemented on all three motor terminals.
The low-pass filter is necessary to filter out the high
frequency noise that is generated by the high frequency
PWM switching. The potential divider reduces the
voltage from high motor voltage to the level MCU can
measure. After this, the BEMF signals are connected
together using R27, R30 and R40 to form a virtual
neutral point. The BEMF signal from each phase is com-
pared with this neutral point using the op amp
comparators. In order to use this configuration, jumpers
J7, J11 and J13 should be bridged across pins 1 and 2,
and jumpers J8, J12 and J14 should be bridged. By
doing this, the comparator outputs are connected to the
input capture pins IC1, IC2 and IC3 on the PIC18FXX31.
The board also provides the option for connecting the
BEMF signals to the A/D channels. The BEMF signal is
filtered using the low-pass filters comprised of R34/C17,
R41/C19 and R49/C19 on motor terminals M3, M2 and
M1, respectively. The potential divider circuits comprised
of R34/R36, R41/R44 and R49/R52 on motor terminals
M3, M2 and M1 reduce the BEMF voltage to TTL level.
To route the BEMF signals to the A/D channels, J7, J11
and J13 should be bridged between pins 2 and 3.
FIGURE 5:
MOTOR CONTROL HARDWARE BLOCK DIAGRAM
Motor
Speed
IC1
IC2
IC3
S2
S3
ICD2
RS232
+5V
Regulator
+5V
+15V
Regulator
+15V
AN1 PWM5
RA2 PWM4
RA3 PWM3
RA4
PWM2
RC5
PWM1
RC4
PIC18F2431 PWM0
PWM5
PWM4
PWM3
IGBT
Driver
PWM2
PWM1
PWM0
+24V
3-phase
Inverter
Bridge
AN0
FLTA
24V
Power Supply
AC input
BLDC
Motor
Back EMF
Signals
Comparator
Amplifier
Imotor
Overcurrent Limit
Current
Signal
Shunt Conditioning
IC1
IC2
IC3
© 2005 Microchip Technology Inc.
DS00970A-page 5
5 Page 
RESOURCE USAGE
The BLDC application consumes CPU resources as
shown in Table 1.
TABLE 1:
RESOURCES USED IN THE
BLDC (USING PIC18F2431)
Resource Type
Used
ROM
RAM
MIPS
800 bytes
65 bytes
4.5 MIPS
CONCLUSION
Sensorless control of BLDC motors has several advan-
tages, such as reduction of total system cost and
increased reliability. However, it requires extra
resources on the control circuit side. The specialized
peripherals on PIC18FXX31 microcontrollers
(PCPWM, Motion Feedback Module and the
High-Speed A/D converter) can simplify the sensorless
control up to a great extent. This application note dem-
onstrates the implementation of sensorless control of a
BLDC motor using the 28-pin version of the
PIC18FXX31 microcontroller family, the PIC18F2431.
AN970
© 2005 Microchip Technology Inc.
DS00970A-page 11
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet AN970.PDF ] | |
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