DC Motor Control Tips n Tricks

DC Motor Control Tips ‘n Tricks
CHAPTER 5
DC Motor Control
Tips ‘n Tricks
Table Of Contents
TIPS ‘N TRICKS INTRODUCTION
TIP #1: Brushed DC Motor Drive Circuits .......
TIP #2: Brushless DC Motor Drive Circuits .....
TIP #3: Stepper Motor Drive Circuits ..............
TIP #4: Drive Software ....................................
TIP #5: Writing a PWM Value to the CCP
Registers with a Mid-Range
PIC® MCU ...........................................
TIP #6: Current Sensing .................................
TIP #7: Position/Speed Sensing .....................
TIPS ‘N TRICKS INTRODUCTION
5-2
5-3
5-4
5-6
5-7
5-8
5-9
Application Note References ............................... 5-11
Motor Control Development Tools ....................... 5-11
Every motor control circuit can be divided
into the drive electronics and the controlling
software. These two pieces can be fairly simple
or extremely complicated depending upon
the motor type, the system requirements and
the hardware/software complexity trade-off.
Generally, higher performance systems require
more complicated hardware. This booklet
describes many basic circuits and software
building blocks commonly used to control
motors. The booklet also provides references
to Microchip application notes that describe
many motor control concepts in more detail. The
application notes can be found on the Microchip
web site at www.microchip.com.
Additional motor control design information can
be found at the Motor Control Design Center
(www.microchip.com/motor).
© 2008 Microchip Technology Inc.
Page 5-1
DC Motor Control Tips ‘n Tricks
Figure 1-3: H-Bridge Drive
TIP #1 Brushed DC Motor Drive
Circuits
V+
V+
All motors require drive circuitry which controls
the current flow through the motor windings.
This includes the direction and magnitude of
the current flow. The simplest type of motor, to
drive, is the Brushed DC motor. Drive circuits for
this type of motor are shown below.
D
A
M
Figure 1-1: High Side Drive
V+
PIC®
Microcontroller
C
B
Digital
Output
A-D are digital outputs from a PIC® MCU.
MOSFET
Driver
M
This drive can control a Brushed DC motor in
one direction. This drive is often used in safety
critical applications because a short circuit at
the motor terminals cannot turn the motor on.
Figure 1-2: Low Side Drive
V+
PIC®
Microcontroller
The H-Bridge derived its name from the
common way the circuit is drawn. This is the
only solid state way to operate a motor in both
directions.
Application notes that drive Brushed DC motors
are listed below and can be found on the
Microchip web site at: www.microchip.com.
• AN847, “RC Model Aircraft Motor Control”
(DS00847)
• AN893, “Low-cost Bidirectional Brushed DC
Motor Control Using the PIC16F684”
(DS00893)
• AN905, “Brushed DC Motor Fundamentals”
(DS00905)
M
Digital
Output
MOSFET
Driver
This is the lowest cost drive technique because
of the MOSFET drive simplicity. Most applications
can simply use an output pin from the PIC®
microcontroller to turn the MOSFET on.
Page 5-2
© 2008 Microchip Technology Inc.
DC Motor Control Tips ‘n Tricks
TIP #2 Brushless DC Motor Drive
Circuits
A Brushless DC motor is a good example of
simplified hardware increasing the control
complexity. The motor cannot commutate
the windings (switch the current flow), so the
control circuit and software must control the
current flow correctly to keep the motor turning
smoothly. The circuit is a simple half-bridge on
each of the three motor windings.
Figure 2-1: 3 Phase Brushless DC Motor
Control
V
V
OA
OC
V
OE
A
B
There are two basic commutation methods
for Brushless DC motors; sensored and
sensorless. Because it is critical to know the
position of the motor so the correct winding can
be energized, some method of detecting the
rotor position is required. A motor with sensors
will directly report the current position to the
controller. Driving a sensored motor requires
a look-up table. The current sensor position
directly correlates to a commutation pattern for
the bridge circuits.
OB
Without sensors, another property of the
motor must be sensed to find the position. A
popular method for sensorless applications
is to measure the back EMF voltage that is
naturally generated by the motor magnets and
windings. The induced voltage in the un-driven
winding can be sensed and used to determine
the current speed of the motor. Then, the next
commutation pattern can be determined by a
time delay from the previous pattern.
OA-OF are digital outputs from a PIC® MCU.
Sensorless motors are lower cost due to
the lack of the sensors, but they are more
complicated to drive. A sensorless motor
performs very well in applications that don’t
require the motor to start and stop. A sensor
motor would be a better choice in applications
that must periodically stop the motor.
© 2008 Microchip Technology Inc.
OD
C
OF
B
A
Motor
C
Figure 2-2: Back EMF Sensing (Sensorless
Motor)
PIC® MCU or dsPIC® DSC
A
ADC
B
Low Pass
Filter
C
Analog
MUX
Page 5-3
DC Motor Control Tips ‘n Tricks
Figure 2-3: Quadrature Decoder (Sensor Motor)
Stepper motors are similar to Brushless
DC motors in that the control system must
commutate the motor through the entire rotation
cycle. Unlike the brushless motor, the position
and speed of a stepping motor is predictable
and does not require the use of sensors.
PIC® MCU or dsPIC® DSC
Digital
Outputs
Digital
Inputs
Drive
Circuit
A
B
Motor
C
Hall
Effect
Motor
Position
Sensor
Sensor
Outputs
Application notes describing Brushless DC Motor
Control are listed below and can be found on the
Microchip web site at: www.microchip.com.
• AN857, “Brushless DC Motor Control Made
Easy” (DS00857)
• AN885, “Brushless DC Motor Fundamentals”
(DS00885)
• AN899, “Brushless DC Motor Control Using
PIC18FXX31” (DS00899)
• AN901, “Using the dsPIC30F for Sensorless
BLDC Control” (DS00901)
• AN957, “Sensored BLDC Motor Control Using
dsPIC30F2010” (DS00957)
• AN992, “Sensorless BLDC Motor Control
Using dsPIC30F2010” (DS00992)
• AN1017, “Sinusoidal Control of PMSM with
dsPIC30F DSC” (DS01017)
• GS005, “Using the dsPIC30F Sensorless
Motor Tuning Interface” (DS93005)
Page 5-4
TIP #3 Stepper Motor Drive Circuits
There are two basic types of stepper motors,
although some motors are built to be used in
either mode. The simplest stepper motor is
the unipolar motor. This motor has four drive
connections and one or two center tap wires
that are tied to ground or Vsupply, depending
on the implementation. Other motor types are
the bipolar stepper and various combinations
of unipolar and bipolar, as shown in Figure 3-1
and Figure 3-2. When each drive connection
is energized, one coil is driven and the motor
rotates one step. The process is repeated
until all the windings have been energized.
To increase the step rate, often the voltage is
increased beyond the motors rated voltage.
If the voltage is increased, some method of
preventing an over current situation is required.
There are many ways to control the winding
current, but the most popular is a chopper
system that turns off current when it reaches
an upper limit and enables the current flow a
short time later. Current sensor systems are
discussed in Tip #6. Some systems are built
with a current chopper, but they do not detect
the current, rather the system is designed to
begin a fixed period chopping cycle after the
motor has stepped to the next position. These
are simpler systems to build, as they only
require a change in the software.
© 2008 Microchip Technology Inc.
DC Motor Control Tips ‘n Tricks
Figure 3-1: 4 and 5 Wire Stepper Motors
Figure 3-4: Bipolar Motor (4 Half-Bridges)
V
Unipolar 5 Wire
V
A
C
B
D
Bipolar 4 Wire
Figure 3-2: 6 and 8 Wire Stepper Motors
Short for
Unipolar
V
Individual coils
wire anyway
appropriate
8 Wire
Unipolar and Bipolar
6 Wire
Figure 3-3: Unipolar Motor (4 Low Side
Switches)
Motor
E
G
F
H
V
V+
Motor
A-H are digital outputs from a PIC® MCU
or dsPIC® DSC.
01
02
03
04
01-04 are outputs from a PIC® MCU or dsPIC® DSC.
© 2008 Microchip Technology Inc.
Page 5-5
DC Motor Control Tips ‘n Tricks
TIP #4 Drive Software
Pulse-Width Modulation (PWM) Algorithms
Pulse-Width Modulation is critical to modern
digital motor controls. By adjusting the pulse
width, the speed of a motor can be efficiently
controlled without larger linear power stages.
Some PIC devices and all dsPIC DSCs have
hardware PWM modules on them. These
modules are built into the Capture/Compare/
PWM (CCP) peripheral. CCP peripherals are
intended for a single PWM output, while the
Enhanced CCP (ECCP) is designed to produce
the complete H-Bridge output for bidirectional
Brushed DC motor control. If cost is a critical
design point, a PIC device with a CCP module
may not be available, so software generated
PWM is a good alternative.
The following algorithms are designed to
efficiently produce an 8-bit PWM output on
the Mid-Range family of PIC microcontrollers.
These algorithms are implemented as macros.
If you want these macros to be a subroutine
in your program, simply remove the macro
statements and replace them with a label and a
return statement.
Example 4-1: 1 Output 8-Bit PWM
pwm_counter equ xxx ;variable
pwm
equ xxx ;variable
set_pwm macro A
;sets the pwm
;setpoint to the
;value A
MOVLW A
MOVWF pwm
endm
update_PWM macro
;performs one update
;of the PWM signal
;place the PWM output
;pin at bit 0 or 7 of
;the port
MOVF pwm_counter,w
SUBWF pwm, w
;if the output
;is on bit 0
RLF
PORTC,f ;replace PORTC with
;the correct port if
;the output is on bit
;7 of the port
;replace the rlf with
;rrf incf
;pwm_counter,f
Page 5-6
Example 4-2: 8 Output 8-Bit PWM
pwm_counter equ xxx
;variable
pwm0
equ xxx
;
pwm1
equ pwm0+1
pwm2
equ pwm1+1
pwm3
equ pwm2+1
pwm4
equ pwm3+1
pwm5
equ pwm4+1
pwm6
equ pwm5+1
pwm7
equ pwm6+1
output
equ pwm7+1
set_pwm macro A,b
;sets pwm b with
;the value A
MOVLW pwm0
ADDLW b
MOVWF fsr
MOVLW a
MOVWF indf
endm
update_PWM macro
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVF
SUBWF
RLF
MOVWF
INCF
endm
;peforms one
;update of all 8
;PWM signals
;all PWM signals
;must be on the
;same port
pwm_counter,w
pwm0,w
output,f
pwm_counter,w
pwm1,w
output,f
pwm_counter,w
pwm2,w
output,f
pwm_counter,w
pwm3,w
output,f
pwm_counter,w
pwm4,w
output,f
pwm_counter,w
pwm5,w
output,f
pwm_counter,w
pwm6,w
output,f
pwm_counter,w
pwm7,w
output,w
PORTC
pwm_counter,f
© 2008 Microchip Technology Inc.
DC Motor Control Tips ‘n Tricks
TIP #5 Writing a PWM Value to the
the CCP Registers With a
Mid-Range PIC® Microcontroller
Example 5-2: Right Justified 16-Bit Macro
pwm_tmp
equ xxx
The two PWM LSb’s are located in the
CCPCON register of the CCP. This can make
changing the PWM period frustrating for a
developer. Example 5-1 through Example 5-3
show three macros written for the mid-range
product family that can be used to set the PWM
period. The first macro takes a 16-bit value and
uses the 10 MSb’s to set the PWM period. The
second macro takes a 16-bit value and uses the
10 LSb’s to set the PWM period. The last macro
takes 8 bits and sets the PWM period. This
assumes that the CCP is configured for no more
than 8 bits.
setPeriod
macro a
Example 5-1: Left Justified 16-Bit Macro
pwm_tmp
equ xxx
setPeriod
macro a
RRF
a,w
MOVWF
RRF
ANDLW
IORLW
MOVWF
MOVF
MOVWF
pwm_tmp
pwm_tmp,w
0x30
0x0F
CCP1CON
a+1,w
CCPR1L
;this variable must be
;allocated someplace
;a is 2 SFR’s in
;Low:High arrangement
;the 10 MSb’s are the
;desired PWM value
;This macro will
;change w
© 2008 Microchip Technology Inc.
SWAPF
a,w
ANDLW
IORLW
MOVWF
RLF
IORLW
MOVWF
RRF
RRF
MOVWF
0x30
0x0F
CCP1CON
a,w
0x0F
pwm_tmp
pwm_tmp,f
pwm_tmp,w
CCPR1L
;this variable must be
;allocated someplace
;a is 2 bytes in
;Low:High arrangement
;the 10 LSb’s are the
;desired PWM value
;This macro will
;change w
Example 5-3: 8-Bit Macro
pwm_tmp
equ xxx
setPeriod
SWAPF
macro a
a,w
ANDLW
IORLW
MOVWF
RRF
MOVWF
RRF
MOVWF
;this variable must be
;allocated someplace
;a is 1 SFR
;This macro will
;change w
0x30
0x0F
CCP1CON
a,w
pwm_tmp
pwm_tmp,w
CCPR1L
Page 5-7
DC Motor Control Tips ‘n Tricks
VSUPPLY
M
CCP
®
PIC MCU
or
dsPIC® DSC
ADC
MOSFET
Driver
MCP601
Op Amp
Current
Sensor
Resistor
-
The torque of an electric motor can be
monitored and controlled by keeping track of
the current flowing through the motor. Torque
is directly proportional to the current. Current
can be sensed by measuring the voltage drop
through a known value resistor or by measuring
the magnetic field strength of a known value
inductor. Current is generally sensed at one of
two places, the supply side of the drive circuit
(high side current sense) or the sink side of the
drive circuit (low side current sense). Low side
sensing is much simpler but the motor will no
longer be grounded, causing a safety issue in
some applications. High side current sensing
generally requires a differential amplifier with a
common mode voltage range within the voltage
of the supply.
Figure 6-2: Resistive Low Side Current
Sensing
+
TIP #6 Current Sensing
Current Sensor
Amplifier
Figure 6-1: Resistive High Side Current
Sensing
Current Sensor
Resistor
VSUPPLY
RS+
V+
ADC
PIC® MCU
or
dsPIC® DSC
RS-
MAX4172
High Side
Current
Sensor
Amplifier
M
1k
Current measurement can also be
accomplished using a Hall effect sensor to
measure the magnetic field surrounding a
current carrying wire. Naturally, this Hall effect
sensor can be located on the high side or the
low side of the load. The actual location of the
sensor does not matter because the sensor
does not rely upon the voltage on the wire. This
is a non-intrusive method that can be used to
measure motor current.
Figure 6-3: Magnetic Current Sensing
CCP
Motor Supply
MOSFET
Driver
VDD
Ferrite
Toroid
ADC
PIC® MCU
or
dsPIC® DSC
Hall
Effect
Sensor
M
CCP
Page 5-8
© 2008 Microchip Technology Inc.
DC Motor Control Tips ‘n Tricks
TIP #7 Position/Speed Sensing
Rotary Encoder Sensing
The motor RPM can be measured by
understanding that a motor is a generator. As
long as the motor is spinning, it will produce
a voltage that is proportional to the motors
RPM. This is called back EMF. If the PWM
signal to the motor is turned off and the voltage
across the windings is measured, the back
EMF voltage can be sensed from there and the
RPM’s can be known.
Rotary encoders are typically used to provide
direct physical feedback of motor position,
and/or speed. A rotary encoder consists of a
rotary element attached to the motor that has
a physical feature, measured by a stationary
component. The measurements can yield motor
speed and sometimes they can provide a motor
position. Rotary encoders are built using many
different technologies. The most common type
is an optical rotary encoder. The optical rotary
encoder is used in the computer mice that have
a ball. It is built with an encoder disc that is
attached to the motor. The encoder disc has
many radial slots cut into the disc at a specific
interval. An LED and a photo detector are used
to count the slots as they go by. By timing the
rate that the slots go by, the speed of rotation
can be determined.
Figure 7-1: Back EMF Motor Speed Sensing
Motor Supply
VDD
Q1
CCP
PIC® MCU
or
dsPIC® DSC
ADC
Back EMF Monitor(1)
M
Note 1: If motor voltage is greater than VDD, an
attenuator will be required. Sample back
EMF while Q1 is off.
Sensing motor position requires a second LED
and photo detector. The second sensor pair
is mounted so the output pulses are 90° out
of phase from the first pair. The two outputs
represent the motion of the encoder disc as a
quadrature modulated pulse train. By adding
a third index signal, that pulses once for each
revolution, the exact position of the rotor can be
known.
An encoder with quadrature outputs can be
used to track relative position from a known
reference point. Another type of encoder uses
a binary encoded disk so that the exact rotor
position is always known. This type of encoder
is called an absolute encoder.
© 2008 Microchip Technology Inc.
Page 5-9
DC Motor Control Tips ‘n Tricks
Figure 7-2: Optical Speed/Direction/Position
Sensing
VDD
A
VDD
M
Drive
B
Older Methods of Motor Sensing
Encoder Wheel
on Motor Shaft
LED
Photo
Transistor
A
PIC® MCU
or
dsPIC® DSC
B
Encoder Wheel
A
B forward
B reverse
Note: Frequency of one signal provides RPM of motor. Pulse count
provides motor position. A-B phase provides motor direction.
Page 5-10
Quadrature sensing can easily be accomplished
in software, but there is generally an upper limit
to the RPM. By using a few gates, the sensing
can be done partially in hardware and partially
in software. The new PIC18FXX31 and dsPIC
16-bit Digital Signal Controller families include
an encoder interface that allows MUCH higher
RPM motors to be measured with an excellent
degree of accuracy.
Resolvers and analog tachometers are two
older technologies for motor position/velocity
sensing. An analog tachometer is simply an
electric generator with a linear output over
a specified range of RPM’s. By knowing the
output characteristics, the RPM can be known
by simply measuring the voltage across the
tachometer terminals.
A resolver is a pair of coils that are excited
by an external AC signal. The two coils are
at 90° to each other so they pick up the AC
signal at different strengths, depending on
their orientation. The result is a sine or cosine
output related to the angle of the resolver in
reference to the AC signal. Inverse cosine/sine
will produce the angle of the sensor. This type
of sensor can be very accurate and is still used
where absolute position must be known.
© 2008 Microchip Technology Inc.
DC Motor Control Tips ‘n Tricks
Application Note References
Motor Control Development Tools
• AN532, “Servo Control of a DC Brush Motor”
(DS00532)
• AN696, “PIC18CXXX/PIC16CXXX DC
Servomotor” (DS00696)
• AN718, “Brush-DC Servomotor
Implementation using PIC17C756A”
(DS00718)
• AN822, “Stepper Motor Microstepping with the
PIC18C452” (DS00822)
• AN843, “Speed Control of 3-Phase Induction
Motor Using PIC18 Microcontrollers”
(DS00843)
• AN847, “RC Model Aircraft Motor Control”
(DS00847)
• AN857, “Brushless DC Motor Control Made
Easy” (DS00857)
• AN885, “Brushless DC (BLDC) Motor
Fundamentals” (DS00885)
• AN899, “Brushless DC Motor Control Using
the PIC18FXX31” (DS00899)
• AN893, “Low-cost Bidirectional Brushed
DC Motor Control Using the PIC16F684”
(DS00893)
• AN894, “Motor Control Sensor Feedback
Circuits” (DS00894)
• AN898, “Determining MOSFET Driver Needs
for Motor Drive Applications” (DS00898)
• AN901, “Using the dsPIC30F for Sensorless
BLDC Control” (DS00901)
• AN905, “Brushed DC Motor Fundamentals”
(DS00905)
• AN906, “Stepper Motor Control Using the
PIC16F684” (DS00906)
• AN907, “Stepper Motor Fundamentals”
(DS00907)
• AN1017, “Sinusoidal Control of PMSM Motors
with dsPIC30F DSC” (DS01017)
• GS001, “Getting Started with BLDC Motors
and dsPIC30F Devices” (DS93001)
Application notes can be found on the Microchip
web site at www.microchip.com.
• PICDEM™ MC Development Board
(DM183011)
Used to evaluate the PIC18FXX31 8-bit
microcontroller family.
• PICDEM™ MCLV Development Board
(DM183021)
• dsPIC30F Motor Control Development System
(DM300020)
Used to evaluate the dsPIC30F 16-bit Digital
Signal Controller family.
• Motor Control (MC) Graphical User Interface
(GUI)
The MC-GUI allows user to configure the
motor and a wide range of system parameters
for a selected motor type.
The MC-GUI is free an can be downloaded at
www.microchip.com
Visit the Motor Control Design Center at:
www.microchip.com/motor for additional
design resources.
© 2008 Microchip Technology Inc.
Page 5-11
DC Motor Control Tips ‘n Tricks
NOTES:
Page 5-12
© 2008 Microchip Technology Inc.
Tips ‘n Tricks
NOTES:
© 2008 Microchip Technology Inc.
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Information subject to change. The Microchip name and logo, the Microchip logo, dsPIC, MPLAB, PIC, PICmicro and PICSTART are registered trademarks of
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