AN900

AN900
Controlling 3-Phase AC Induction Motors Using
the PIC18F4431
Author:
Jon Burroughs
Microchip Technology Inc.
The three peripherals and their features are:
1.
INTRODUCTION
Previous Microchip authors (Parekh and Yedamale)
have described the implementation of a 3-phase AC
induction motor (ACIM) control with PICmicro®
devices. The first application note (AN843) detailed
ACIM control with the PIC18F452. More recently, ACIM
control has been implemented with the PIC16F7X7
family of devices (AN889).
2.
This application note describes how the PIC18F4431
may be used to control an ACIM using open and
closed-loop V/f control strategies. The application code
is built incrementally and demonstrates the following
control methods:
1.
2.
3.
Voltage-frequency (V/f) control
Voltage-frequency control with current feedback
Voltage-frequency control with velocity feedback
and PID control
The PIC18F4431 incorporates a set of innovative
peripherals, designed especially for motor control
applications. The utility of these peripherals is demonstrated in both open and closed-loop three-phase
ACIM motor applications.
It is assumed that the reader is already familiar with the
theory and nomenclature of AC induction motors. For an
excellent introduction to the basic concepts of induction
motors control, please refer to Microchip’s application
note AN887, “AC Induction Motor Fundamentals”
(DS00887).
USING THE PIC18F4431 FOR MOTOR
CONTROL
Before getting into actual control applications, we
should understand what distinguishes the PIC18F4431
from other Microchip devices used for motor control.
The core is a set of unique peripherals that simplify
external hardware requirements and also enable
higher levels of motor control capability than the
PIC18F452 or PIC16F7X7.
 2004 Microchip Technology Inc.
3.
Power Control PWM (PCPWM) module:
• Up to 8 output channels
• Complimentary PWM outputs
• Two hardware Fault protection inputs
• PWM resolution up to 14 bits
• Edge-aligned or center-aligned operation
• Flexible dead time
• Simultaneous update of duty cycle and
period
Motion Feedback Module (MFM), comprised of
a Quadrature Encoder Interface (QEI) and an
Input Capture module (IC):
• Standard quadrature encoder inputs (QEA,
QEB and Index) (QEI)
• High and Low Resolution Position
Measurement modes (QEI)
• Velocity Measurement mode using Timer5
(QEI)
• Interrupt with configurable priority on event
detection (QEI)
• Pulse Width Measurement and Period
Measurement modes (IC)
• Edge and state change capture (IC)
High-Speed Analog-to-Digital Converter
(HSADC):
• Two independent sample-and-hold circuits
• Single or multi-channel selection
• Sequential or Simultaneous Conversion
modes
• Four-word FIFO result buffer with flexible
interrupts
As we shall see, each of these features provides a
distinct advantage in implementing more sophisticated
motor control applications.
DS00900A-page 1
AN900
SYSTEM OVERVIEW
In AN843 and AN889, three PWM outputs were used to
drive a 3-phase inverter bridge. In these cases, however,
it was necessary to use external circuitry to generate the
complimentary control signal for the lower leg and insert
the proper dead time between them. For 3-phase
control, what is ultimately needed to drive a 3-phase
bridge is three pairs of complementary PWM outputs,
with dead time between the complimentary channels.
Motor Drive Requirements
Practically speaking, control of a 3-phase AC induction
motor requires pulse-width modulated control of the six
switches of a 3-phase inverter bridge connected to the
motor’s stator windings (Figure 1). The six switches
form 3 pairs of “half-bridges”, which can be used to connect the leg of a winding to the positive or the negative
high-voltage DC bus.
Hardware Overview
The motor control applications described in this
document were developed and tested on a production
model of Microchip’s PICDEM™ MC Development
Board. While the application software has been
designed with this platform’s control and communication requirements in mind, the control methods
discussed are applicable to any 3-phase ACIM control
application based on the PIC18F4431.
As shown in the figure, two switches on the same
“half-bridge” must never be on simultaneously, otherwise the positive and negative buses will be shorted
together. This condition would result in a destructive
event known as “shoot-through”. If one switch is on,
then the other must be off; thus, they are driven as
complementary pairs. It should also be noted that the
switching devices used in the half-bridge (in this case,
IGBTs) often require more time to turn off than to turn
on. For this reason, a minimum dead time must be
inserted between the off and on time of complimentary
channels.
FIGURE 1:
For a complete description of the board and its capabilities as a development tool, please refer to the User’s
Guide for the PICDEM MC Development Board
(DS51453A). To give the reader a more clear idea of
the hardware platform, a brief overview and schematics
of the board are provided in Appendix A: “PICDEM™
MC Board Overview”.
A 3-PHASE INVERTER BRIDGE DRIVEN WITH 6 PWM INPUTS
DC+
PWM1
PWM3
PWM5
Stator Windings
A
PWM0
PWM2
PWM4
B
C
DC-
DS00900A-page 2
 2004 Microchip Technology Inc.
AN900
OVERVIEW OF CONTROL
STRATEGIES
The operation of an ACIM is governed by two
principles:
1.
Open Loop V/f Control
Base speed is directly proportional to the
frequency of the alternating current applied to
the stator and the number of poles of the motor.
Torque is directly proportional to the ratio of
applied voltage and the frequency of the applied
AC current.
AC induction motors are often operated in open loop
with no velocity or position feedback. The V/f ratio is
maintained constant to provide a constant (maximum)
torque over the operating range. This form of control is
relatively inexpensive and easy to implement. Feedback from the rotor is not utilized and the rotor is
assumed to follow the rotating flux generated in the
stator, with a certain amount of slip present depending
upon the load.
2.
To drive an AC induction motor, the 3-phase inverter
bridge is driven by a microcontroller’s PWM outputs, as
shown in Figure 2. By changing the PWM duty cycles
in a regular manner, the PWM outputs are modulated
to synthesize sinusoidal waveforms (three-phase AC)
across the three motor windings (Figure 3).
FIGURE 2:
Therefore, speed can be controlled by varying the input
frequency of the applied alternating current and torque
can be maintained constant by varying the amplitude in
direct proportion to the frequency. These are the two
basic aims of open-loop V/f control.
Speed
Reference
AC is applied to the three stator windings as three
sinusoidal currents, equal in amplitude and frequency,
but offset from each other by 120 degrees. As a result,
the current in the stator windings generates a rotating
magnetic field. This rotating field induces electromotive
force in the rotor, which in turn produces a magnetic
field in the rotor that attempts to align with the rotating
magnetic field in the stator. This causes the rotor to
rotate. See AN887, “AC Induction Motor Fundamentals”
(DS00887), for more details on induction motor
construction and operating characteristics.
FIGURE 3:
OPEN-LOOP V/f CONTROL
BLOCK DIAGRAM
6
V/f Function
PWM
3-Phase
Inverter
Bridge
ACIM
SYNTHESIS OF 3-PHASE SINE WAVE
PWM1 Output
PWM2 Output
PWM3 Output
DC+
Voltage
DC-
Time
 2004 Microchip Technology Inc.
DS00900A-page 3
AN900
V/f Control with Current Feedback
A disadvantage of open-loop V/f control is that the
motor can stall if the speed is ramped up too quickly or
the load otherwise changes rapidly. Without some form
of feedback, it is impossible to detect whether the
motor is turning as expected, or if it is stalled.
A stall causes high currents and the motor loses torque.
By monitoring current, excessive slip can be detected,
and the motor frequency can be adjusted downward
accordingly. A high-current condition may also be
caused by a malfunction of the inverter bridge. If a
high-current condition persists, the drive should be shut
down to prevent motor overheating or other damage.
A conceptual diagram is illustrated in Figure 5. The
speed reference is provided by the user, in this case via
a potentiometer connected to an ADC channel. The V/f
function in firmware calculates the maximum PWM
duty cycle (amplitude) based upon the speed reference. The DC bus (bridge) current is measured using a
FIGURE 4:
shunt resistor, which produces a voltage proportional to
the current through it. This voltage is amplified and
compared with an external comparator to a reference
level that corresponds to the maximum allowable bus
current. The comparator output drives the Fault A input
of the PIC18F4431. If the Fault signal is asserted, the
PWM output is inhibited for the following PWM period.
To detect a persistent overcurrent condition, the number
of times the Fault signal is asserted is monitored in
firmware. For example, if the Fault occurs more than 20
times within the last 256 PWM cycles, the motor is
stopped and an overcurrent condition is indicated by
blinking an LED. (The threshold number of events to
trigger the overcurrent Fault can be changed in the
firmware.)
The shunt voltage can also be monitored by using an
ADC channel to detect increasing current. This way,
corrective action can be taken by decreasing the drive
frequency before the hardware Fault is activated.
CLOSED-LOOP V/f CONTROL WITH CURRENT FEEDBACK
Speed
Reference
PWM
V/f Function
6
Current
Fault
Current Feedback
3-Phase
Inverter
Bridge
ACIM
Current
Limit
V/f Control with Velocity Feedback
In open-loop V/f control, the rotor is assumed to follow
the rotating flux generated in the stator, with a certain
degree of slip present depending upon the load. In
many applications, the load can vary widely and the
resulting motor speed will vary accordingly. To improve
speed control, a form of speed feedback can be added.
A simple implementation of closed-loop speed control is
illustrated in Figure 5. The reference speed is still set by
a potentiometer, as above. However, instead of directly
using the reference speed to determine the drive
frequency, it is compared to the actual motor speed to
generate a speed error signal. Actual motor speed is
established by a speed measurement with either the
FIGURE 5:
The speed error signal is then used as an input to a
Proportional-Integral (PI) controller, which determines
the desired drive frequency to the motor windings. The
standard V/f process determines the amplitude of the
drive waveform. The drive frequency and amplitude are
then used to update the PWM duty cycles of the six
PWM channels that drive the three-phase bridge.
Current feedback may also be used concurrently with
velocity feedback. For clarity, it is not shown in this
example.
CONCEPTUAL BLOCK DIAGRAM OF V/f CONTROL WITH VELOCITY FEEDBACK
Speed
Reference
Speed
Error
PI Controller
_
∫
Speed Feedback
DS00900A-page 4
Quadrature Encoder Interface (QEI) in Velocity mode, or
input capture of a tachometer signal. In this particular
application, the Quadrature Encoder Interface is used.
6
V/f Function
PWM
3-Phase
Inverter
Bridge
ACIM
Velocity
Calculation
 2004 Microchip Technology Inc.
AN900
FIRMWARE OVERVIEW
OVERCURRENT FAULT
In this section, we will see in greater detail how the
three control approaches discussed above are
implemented in firmware. The firmware structure can
be viewed in the flow diagrams shown in Figures 6
and 7. Keep in mind that these descriptions apply to the
specific control applications written for Microchip’s
PICDEM MC demonstration board, particularly with
regards to motor control and Fault display.
A shunt resistor in the negative DC bus gives a voltage
proportional to the current flowing through the three
motor phases. This voltage is amplified and compared
with a reference signal using an external comparator.
On the PICDEM MC board, the reference signal may
be adjusted for a current up to 6.3A. If the DC bus
current signal exceeds the reference level, the Fault A
pin is driven low, indicating an overcurrent Fault.
Channel A is configured in Cycle-by-Cycle Fault mode.
If the Fault occurs more than 20 times in 256 PWM
cycles, then the motor is stopped and an overcurrent
Fault is indicated by blinking LED1.
Initialization
The initialization routine sets the port pins to the
desired states and initializes the peripherals. The
High-Speed ADC, Power Control PWM, hardware
Fault inputs and Quadrature Encoder Interface are
initialized according to the control strategy that is being
implemented (i.e., V/f, V/f with current feedback, V/f
with current and position feedback). Initialization of
these peripherals is discussed in detail below.
Main Loop
The main loop (see Figure 6) continuously checks
Fault status and for key activity. Faults are handled by
the Fault service routine; status is indicated by blinking
LEDs 1 through 3.
Motor control is handled by the key service routine. Two
push button switches toggle the motor between Run and
Stop states and forward and reverse direction. When
switching directions, the motor is first allowed to coast
from its present angular velocity to zero and then accelerated to the reference speed in the opposite direction.
This controlled manner of changing directions prevents
high-current transients that could cause a Fault,
provided that the acceleration rate is set appropriately
for the motor. If PID control is being used, some of the
PID functions may be calculated in this loop.
OVERVOLTAGE FAULT
The DC bus voltage is attenuated using a voltage
divider and compared with a fixed reference signal
using an external comparator. On the PICDEM MC
board, when jumper JP5 is open, the overvoltage is set
to 200V on the DC bus. If jumper JP5 is shorted, then
the overvoltage limit is 400V. The Fault B pin is used to
monitor the overvoltage condition. If the Fault occurs
more than 20 times in 256 PWM cycles, the motor is
stopped and an overvoltage Fault is indicated by
blinking LED2.
OVERTEMPERATURE FAULT
The power module on the PICDEM MC board has a
Negative Temperature Coefficient (NTC) thermal sensor that monitors the junction temperature of the IGBTs.
It gives a 3.3V output for a 110°C junction temperature.
The NTC is connected to AN8 through an analog optocoupler and is continuously measured. If it exceeds
80°C, the motor is stopped and an overtemperature
Fault is indicated by blinking LED3.
ISR Loop
Key Activity Monitoring
The ISR loop handles interrupts from the HSADC, Fault
inputs, QEI and time-base interrupts for three-phase
waveform synthesis and PID control loop.
SW1 and SW2 are monitored and debounced in firmware. SW1 is used to toggle between Run and Stop.
SW2 is used to toggle between forward and reverse.
(Figure 6). This is a subroutine within the main loop.
A/D Channel Conversion
Fault Signals
Three Fault signals are monitored: overcurrent, overvoltage and overtemperature. The overcurrent and
overvoltage Faults use the hardware Fault inputs to
directly inhibit the PCPWM outputs on a cycle-by-cycle
basis.
 2004 Microchip Technology Inc.
AN0, AN1 and AN8 are converted sequentially using
the high-speed ADC. AN0 is used to measure the DC
bus current. AN1 is used as the input for the speed reference. AN8 is used to measure the power module
junction temperature for detecting the overtemperature
Fault condition.
DS00900A-page 5
AN900
FIGURE 6:
MAIN LOOP, FAULT AND CONTROL SERVICE ROUTINES
Direction and Speed Change/Key Service
Initialize
Main Loop
Main Loop
B
Fault activity?
No
No
(Run/Stop Key)
Key activity?
No
Yes
Yes
B
A
Fault Service
FWD/REV key?
No
Is status Run?
Yes
Yes
Accelerate Motor
to Set Speed
Toggle FR_Key
Status
A
No
Overcurrent Fault?
Coast Motor
to 0
Coast Motor
Blink LED1
Yes
Motor speed = 0?
Overtemperature
Fault?
No
Return to
Main Loop
No
Blink LED2
Yes
Yes
Toggle Direction bit
and LED4
No
Overvoltage Fault?
Yes
Blink LED3
Accelerate Motor
to Set Speed
Return to
Main Loop
DS00900A-page 6
 2004 Microchip Technology Inc.
AN900
FIGURE 7:
INTERRUPT SERVICE ROUTINE
ISR High Priority
ISR Low Priority
No
No
ADC ready?
Timer0 overflow?
Yes
Yes
Read Sine Values
from Table*
Calculate New
Duty Cycle based on
Target Speed and
Update Table Offsets*
Calculate New
Target Velocity
Calculate Timer0
Reload Value*
No
Direction change?
Yes
Swap Phase 1 and
Phase 2 Offsets*
Return from
Interrupt
*In actual implementation, a flag is set and operation is performed within the main loop.
 2004 Microchip Technology Inc.
DS00900A-page 7
AN900
INITIALIZING THE POWER CONTROL PWM
MODULE AND HARDWARE FAULT INPUTS
INITIALIZING THE HIGH-SPEED ADC
MODULE
The Power Control PWM module simplifies the task of
driving a 3-phase inverter bridge by providing three pairs
of complementary PWM outputs, with dead time inserted
between complimentary channels. It also provides hardware-based Fault inputs that are capable of shutting
down the PWM outputs completely in a Fault situation.
Three analog values are measured in this application:
To initialize the PCPWM module:
The high-speed ADC incorporates several features,
such as Auto-Conversion mode and a FIFO result
buffer, that reduce the firmware overhead associated
with monitoring multiple analog channels and enhance
ADC throughput.
1.
Configure the PCPWM time base:
a) Select a PWM time base postscale value of
1:1.
b) Select a PWM time base prescale value
input of 1:1 (FOSC/4).
c) Configure the PWM time base for
Free-Running mode (for edge-aligned
operation).
2. Load the PTPERH:PTPERL register pair to
obtain a PWM frequency of 20 kHz. The value to
be used depends on the controller’s clock
frequency; refer to the data sheet to determine
the proper value.
3. Configure the PCPWM output:
a) Enable PWM0 through PWM5 as outputs.
b) Set the PWM I/O pairs (PWM0/1, 2/3 and
4/5) as complementary pairs.
4. Configure the special event trigger:
a) Set the special event trigger postscaler to
1:1.
b) Configure the special event trigger to occur
when the time base is counting upwards.
c) Enable updates from duty cycle and period
buffer registers.
d) Configure for asynchronous overrides from
the OVDCON register.
5. Configure the PCPWM dead time:
a) Select FOSC/2 as the dead-time prescaler.
b) Load DTCON<5:0> with a dead-time value
to achieve a 2 µs dead time. The actual
value depends on the controller’s clock
frequency; refer to the data sheet to
determine the proper value.
6. Disable the output overrides on the PWM pins
by setting bits POVD<5:0>.
7. Clear the special duty cycle register pair
(SEVTCMPH:SEVTCMPL).
8. Clear all of the regular PWM duty cycle register
pairs (PDCxH:PDCxL) to set the duty cycles to 0.
9. Enable the PWM time base.
10. Enable PWM Fault detection:
a) Enable both Fault A and Fault B.
b) Configure both Fault inputs to disable PWM
channels 0 through 5.
c) Configure both Fault inputs to operate in
Cycle-by-Cycle mode.
DS00900A-page 8
• AN0 (DC bus current)
• AN1 (potentiometer input for the speed reference)
• AN8 (IGBT junction temperature in the inverter
module)
To initialize the HSADC module:
1.
2.
3.
4.
Configure ADC operation:
a) Enable Continuous Loop mode.
b) Enable Multi-Channel mode.
c) Configure auto-conversion sequence to
sample sequentially from Group A and
Group B.
d) Assign VREF+ and VREF- to AVDD and
AVSS, respectively.
e) Enable the FIFO buffer.
f) Select the left-justified format for the A/D
result.
g) Set the A/D acquisition time to 12 TAD
(required for sequential conversion).
h) Set the A/D conversion clock to FOSC/32
i) Turn on the ADC.
Configure interrupts and event triggers:
a) Set the A/D interrupt to be generated on
every 2nd and 4th write to the FIFO buffer.
b) Disable external ADC triggers.
Configure input group assignments:
a) Assign AN0 to Group A. This will alternate
with AN8 on every FIFO interrupt.
b) Assign AN8 to Group B.
Configure RA0, RA1 and RE2 as analog inputs:
a) Set the ANSEL0<1:0> and ANSEL<0> bits.
b) Set the TRISA<1:0> and TRISE<2> bits.
 2004 Microchip Technology Inc.
AN900
INITIALIZING THE QUADRATURE ENCODER
INTERFACE
When implementing closed-loop V/f control, a form of
velocity feedback is required. The Quadrature Encoder
Interface can be used in conjunction with Timer5 to
provide very accurate velocity feedback and direction
information. In Velocity Measurement mode, velocity
event pulses are generated on each edge of the QEA
signal. Timer5 counts upward and its value is captured
on each velocity pulse when it is reset to zero. The
captured Timer5 value is stored in the Velocity registers
(VELRH:VELRL), which is used by V/f control with
Velocity Feedback mode.
To enable velocity measurement:
1.
2.
3.
Configure the QEI module for Velocity mode
measurement:
a) Enable Velocity mode (clear the VELM bit).
b) Set the QEI module mode to one of the 2x
capture configurations.
c) Set the pulse reduction ratio to 1:1.
Configure Timer5 as the QEI time base:
a) Disable the Timer5 special event Reset.
b) Enable Continuous Count mode.
c) Set the input clock prescaler to 1:1.
d) Enable Synchronous Timer mode.
e) Enable the timer.
Enable the Timer5 interrupt and set as low priority
V/f CONTROL FIRMWARE
The heart of the ACIM control is accomplished with the
PCPWM peripheral operated in Complimentary mode.
The duty cycle of the three PWM channels are changed
in a regular manner using a Timer0 interrupt to
synthesize the three-phase waveforms that drive the
motor.
A sine table is stored in program memory. It is transferred to data memory during initialization for faster
access. Three registers are used as offsets to the table
through indirect addressing. Each of the offset values
EXAMPLE 1:
In this application, the potentiometer determines the
target motor speed reference signal. Waveform synthesis is identical for open-loop V/f and V/f with velocity
feedback. The difference lies in how the motor drive
frequency is generated from the target frequency.
Depending upon the control strategy, the reference
signal from AN1 is used in one of two ways:
DRIVE FREQUENCY CALCULATION FOR
OPEN-LOOP V/f CONTROL
In this method, the motor drive frequency is directly
calculated from the potentiometer input. Specifically, the
upper byte of the A/D Result register is divided by four to
give the target drive frequency f in Hz. In this application,
the target speed has a lower boundary of 12 Hz and an
upper boundary of 60 Hz. The V/f function determines
the drive amplitude corresponding to that frequency.
Since the synchronous speed (in RPM) for an induction
motor is 120 f/p, where p is the number of stator poles,
the target drive speed (in RPM) can be directly calculated as 30 times the value of ADRESH (120 divided by
times ADRESH divided by 4), divided by p. For the
current application, a motor with two stator poles is
assumed. This reduces to a motor speed equal to 15
times ADRESH.
EQUATION 1:
CALCULATING DRIVE
FREQUENCY AND SPEED
Drive Frequency:
ftarget = (ADRESH/4)
(5 ≤ ftarget ≤ 60)
Calculated Speed:
Ns = (120/f • p) × (ADRESH/4) = (30 • ADRESH)/p
ACCESSING THE SINE TABLE THROUGH INDIRECT ADDRESSING
UPDATE_PWM_DUTYCYCLES
MOVF
TABLE_OFFSET1,W
MOVF
PLUSW0,W
BZ
MULWF
MOVFF
MOVFF
BRA
PWM1_IS_0
MOVLW
MOVWF
points to one of the values in the table, such that there
is always a 120-degree phase shift between the
phases. (Each of the waveforms in Figure 3 is created
by an offset register associated with its PWM.) The
code sample in Example 1 shows how the table is read
using the indirect addressing registers.
PWM1_IS_0
FREQUENCY
PRODH,PDC0H_TEMP
PRODL,PDC0L_TEMP
UPDATE_PWM2
0x02
PDC0L_TEMP
;first update PWM1
;place offset for first sine value inWREG
;use the value in WREG as an offset to FSR0
;FSR0 points to beginning of sine table
;value at sine_table + table_offset1
;is copied to WREG
;check to see if value is 0
;multiply the table value times the frequency
;copy the result to the duty cycle register
;continue on to update PWM2
;If table value is zero,
; make the duty-cycle a small non-zero value
;continue on to update PWM2
 2004 Microchip Technology Inc.
DS00900A-page 9
AN900
DRIVE FREQUENCY CALCULATION FOR V/f
CONTROL WITH VELOCITY FEEDBACK
The microcontroller uses the ADC measurement to calculate the target speed. The speed error is calculated
by subtracting the actual speed (as measured by the
EQUATION 2:
QEI) from the target speed. The velocity error is used
as an input to the PID algorithm, which determines the
motor drive frequency. The V/f function determines the
corresponding drive amplitude.
CALCULATING DRIVE FREQUENCY FROM VELOCITY ERROR
1. Calculate Actual Speed from QEI Velocity Mode:
Factual =
FOSC
QE edges per revolution
×
= factual × 60
4
Value of velocity register pair
where: Factual is the actual rotor speed (RPM) and factual is the actual rotor speed (Hz)
2. Calculate Speed Error:
ferror = (ftarget × (100 – slip)) – factual
where: ftarget is the target drive frequency (Hz), ferror is the frequency error and slip is the expected percent slip
3. Calculate Drive Frequency from PID Algorithm:
ftarget = (Kp × ferror) + (Ki × ∑ferror) + (Kd × (ferror – ferrort – 1))
t
where: Kp is the proportional gain
Ki is the integral gain
Kd is the differential gain
∑ferror is the cumulative sum of frequency errors
ferror – ferror is the difference in error between the
t–1
t
current and immediately previous time period
PWM Waveform Synthesis
3.
The sinusoidal waveform is created by constantly
changing the PWM duty cycle for each output. The
motor drive frequency determines how often the PWM
duty cycle values are updated and thus, the frequency
of the synthesized waveform. The peak-to-peak drive
amplitude corresponds to the maximum PWM duty
cycle, as this generates the maximum voltage output of
each half-bridge of the inverter. The duty cycle
determines the drive amplitude at any given point in the
cycle.
4.
The duty cycle update rate is set by modifying the
Timer0 reload value. This determines the interval until
next Timer0 overflow. The PWM Duty Cycle (PDC)
registers of the three PWM units are modified as follows:
1.
2.
When a Timer0 interrupt occurs, an updated
target drive frequency is determined by either
Equation 1 or Equation 2 (depending on the
control method being used).
The sine value for each phase is read from the
sine table, pointed to by the offset value for that
phase.
EQUATION 3:
5.
6.
7.
The PWM duty cycle for a particular phase is
calculated by multiplying the sine value from the
table by the updated motor drive frequency. The
16-bit product is stored in the PDC register for
that phase. Steps 2 and 3 are repeated for each
phase.
The offset values are updated for the next table
access.
If the direction of rotation is to be reversed, then
the offsets of two phases are swapped. The
offsets of Phase 1 and 2 are swapped for this
purpose.
The Timer0 reload value is calculated based on
the updated motor drive frequency (Equation 3),
where f is the drive frequency. In the current
version of firmware, the number of sine table
entries is set at 19. The reload value determines
the value at which the PWM duty cycle is
updated.
The new PWM duty cycle values take effect at
the beginning of the next PWM period. The duty
cycle determines the drive amplitude at any
given point in the cycle.
CALCULATING TIMER0 RELOAD VALUE
FOSC
4
Timer0 Reload Value = FFFFh – (2 • (sine table entries – 1)) × Timer0 prescaler value × f
DS00900A-page 10
 2004 Microchip Technology Inc.
AN900
PID CONTROL FIRMWARE
The PID functions used in this application note are
described in AN937, “Implementing a PID Controller
Using a PIC18 MCU”.
PID is a well-known, commonly used method of feedback control. As seen in the PID algorithm in
Equation 2, PID generates a control signal by multiplying the error, the integral of the error and the derivative
of the error by individual gains and then summing the
results. The proportional term generates a corrective
signal in proportion to the error. The integral term generates a corrective signal proportional to summation of
the error over time. The derivative term generates a
corrective signal in proportion to the rate of change of
the error. In velocity control applications, the derivative
gain is often set to zero, as PI control is usually
sufficient for achieving well-tuned speed control.
CLOSED-LOOP SLIP CONTROL
In many applications, it is desirable to control slip in
order to optimize for torque, efficiency or power factor
depending upon changing requirements. Figure 8
shows how torque, power factor and efficiency may
vary with the degree of slip for a typical motor. By varying the amount of desired slip, the motor performance
can be optimized for any of these three attributes. For
example, torque may be maximized by allowing a
higher degree of slip; efficiency optimized by allowing a
lesser degree.
To implement V/f control with velocity feedback, the target speed, actual speed and speed error are all calculated as shown in Equation 2. The speed error is
passed to the PID algorithm. Integral error is calculated
in the PID routine by accumulating the speed error over
time. Derivative error is calculated by subtracting the
last error value from the present error value. Since the
routine is called at fixed time intervals, the difference in
the two error values is proportional to the rate of
change of error. In this application, the PWM period
interrupt rate is used to determine the update rate of
the PID calculation.
FIGURE 8:
To control slip, the actual motor speed is compared
against the drive frequency to determine the present
slip frequency. The slip frequency is compared to the
desired slip frequency to produce a slip frequency
error. Drive frequency and amplitude are modified in
order to minimize the slip frequency error.
Figure 9 shows how a slip control could be implemented. Identical hardware is used as in V/f control
with velocity feedback. Only the algorithm is modified.
TORQUE, POWER FACTOR AND EFFICIENCY VERSUS SLIP
Slip for
max T
Slip for Slip for
max n max PF
Torque (T)
Power Factor (PF)
Efficiency (n)
Torque
0
0.2
Rated
Slip
 2004 Microchip Technology Inc.
0.4
0.6
0.8
1
Slip
DS00900A-page 11
AN900
FIGURE 9:
CONCEPTUAL BLOCK DIAGRAM FOR CLOSED-LOOP SPEED AND
SLIP CONTROL
Speed Feedback
Speed
Error
_
V
PI Controller
∫
+
Speed
Reference
Application
Controller
Optimize
Torque or
Efficiency
Slip
Controller
PWM
Slip
Frequency
6
f
+
ACIM
+
Torque Feedback
Motor
Frequency
COMPARING MICROCONTROLLERS
FOR AC INDUCTION MOTOR
CONTROL APPLICATIONS
Velocity
Calculation
The PIC18F4431 also offers the MFM to measure both
rotor speed and direction with little or no additional
support from external components. With previous
control solutions, external circuitry would be needed to
provide pulse conditioning and direction indication from
the encoder; an additional on-chip timer resource
would also be needed to convert this into velocity data.
The MFM can directly interface with a quadrature
encoder or other Hall sensors and calculate direction
and velocity with a minimum of additional hardware or
firmware overhead.
In developing ACIM applications, an important
consideration is often the total part count required to
implement the solution. For 3-phase control applications using the PIC18F452 and PIC16F7X7, this
means a bridge driver that is capable of generating the
complimentary outputs and inserting dead time.
In contrast, the PCPWM module of the PIC18F4431
can be configured to generate complimentary outputs
with configurable dead time, resulting in a simpler
bridge driver circuit. This can translate into a reduced
part count for the application.
TABLE 1:
3-Phase
Inverter
Bridge
The capabilities of the three different microcontrollers
in 3-phase ACIM control are compared in Table 1. The
range of control strategies for the microcontrollers is
presented in Table 2.
COMPARISON OF HARDWARE CAPABILITIES IN 3-PHASE ACIM CONTROL
PWM Outputs
Dead Time
Complimentary
Signal Generation
Velocity Feedback
PIC18F452
3 total: 2 CCP,
1 firmware generated
External hardware
External hardware
None
PIC16F7X7
3 (CCP)
External hardware
External hardware
None
PIC18F4431
8 (PCPWM)
Built into PCPWM
Built into PCPWM
QEI/IC
Microcontroller
TABLE 2:
COMPARISON OF PICmicro CONTROLLERS AND ACIM CONTROL STRATEGIES
Control Method
Microcontroller Family
PIC18FXX31
PIC18FXX39
PIC16F7X7
PIC18FXX2
Yes
No
Yes, with external
dead-time provisions
Yes, with firmware PWM
and external dead-time
provisions
V/f Control with
Current Feedback
Yes, using hardware
Fault input and HSADC
No
Yes, with external
dead-time provisions
Yes, with firmware PWM
and external dead-time
provisions
V/f Control with
Velocity Feedback
Yes, using QEI or
input capture
No
No
No
Yes
Yes
Yes
Yes
V/f Control
Single-Phase
V/f Control
DS00900A-page 12
 2004 Microchip Technology Inc.
AN900
CONCLUSION
REFERENCES
The combination of PIC18 architecture and thoughtfully
designed peripherals make the PIC18F4431 an
excellent choice for 3-phase AC induction motor control
applications.
P. Yedamale, AN843, “Speed Control of 3-Phase
Induction Motor Using PIC18 Microcontrollers”
(DS00843). Microchip Technology Inc., 2002.
The PCPWM module provides sufficient PWM outputs
and modes to directly drive a gate driver/inverter
module, without the need of additional hardware to
create complimentary channels or insert dead time.
The MFM allows users to easily implement speed and
direction monitoring with minimal hardware or firmware
overhead. All of these features, along with the HSADC,
make it possible to design a wide range of powerful
motor-control solutions with a minimum of parts.
 2004 Microchip Technology Inc.
R. Parekh, AN887, “AC Induction Motor Fundamentals”
(DS00887). Microchip Technology Inc., 2003.
R. Parekh, AN889, “V/f Control of 3-Phase Induction
Motors
Using
PIC16F7X7
Microcontrollers”
(DS00889). Microchip Technology, Inc., 2003.
“PICDEM™ MC Development Board for PIC18FXX31
User’s Guide” (DS51453). Microchip Technology, Inc.,
2004.
DS00900A-page 13
AN900
APPENDIX A:
PICDEM™ MC
BOARD OVERVIEW
A PICDEM MC demonstration board was used to
develop, test and debug the ACIM control code
discussed in this application note. The overall block
diagram is shown in Figure A-1.
The board has a diode bridge rectifier that converts a
single-phase AC input to DC, while a power capacitor
bank provides a stable DC bus. A switching power
supply generates several DC supply levels for digital,
analog and power electronics. A 3-phase IGBT-based
inverter bridge with integrated gate drivers is used to
drive the motor from the DC bus.
The on-board user interface has two momentary push
buttons, a potentiometer and four LEDs to indicate
status. In this application, switch SW1 is used to toggle
between motor Run and Stop. Switch SW2 is used to
FIGURE A-1:
toggle between forward and reverse rotation. The
potentiometer is used for setting the desired speed.
The LEDs are used for indications of different states of
control and as Fault indicators. The board can also be
controlled with a host PC over a serial port using
Microchip’s own Motor Control GUI.
The control circuit and power circuits are electrically
isolated from each other by optoisolators. With the
isolation between power and control circuits, programming and debugging tools can be plugged to the
development board with power connected to the board.
For debugging the code in this application note, an
MPLAB® ICD 2 was connected directly to the PICDEM
MC board during development.
For a complete description, please refer to the
“PICDEM MC Development Board for PIC18FXX31
User’s Guide” (DS51453).
PICDEM MC EVALUATION BOARD FUNCTIONAL BLOCK DIAGRAM
ICD
Connector
Potentiometer
RS-232
Interface
User
Push Buttons
Isolated
Control
Section
RS-232
Connector
Hall Sensor
Connector
PIC18FXX31
LEDs
Quad Encoder
Connector
Comparator
PWM
Optoisolators
Voltage
Monitor
Current
Monitor
Phase Current
Monitors
Temperature
Monitor
Back EMF
Conditioner
PWM
Motor
Terminal Block
IRAMS10UP60A
Gate Driver and
3-Phase Inverter
Power Terminal Block
VBUS
DC
AC
DS00900A-page 14
Power
Switcher
Bridge
Rectifier
+5 VDC
+5 VAC
+15 VAC
D GND
A GND
 2004 Microchip Technology Inc.
 2004 Microchip Technology Inc.
SW1 (ON/OFF)
SW2 (FWD/REV)
0.1 µF 33 pF
RB1/PWM1
RB0/PWM0
VDD
VSS
RA5/AN5/LVDIN
RE0/AN6
RE1/AN7
RE2/AN8
RC7/RX/DT
OSC2/CLKO/RA6
RC0/T1OSO/T1CKI
RC4/INT1
RC3/INT0
33 pF
RD2/SDI/SDA
MCP6002-DIP8
0.1 µF
PIC18F4431
RD1/SDO
RD0/T0CKI/GPCKI RD3/SCK/SCL
RC5/INT2
RC2/CCP1
RC1/T1OSI/CCP2 RC6/TX/CK/SS
RD5
RD4/FLTA
OSC1/CLKI/RA7
RD6/PWM6
RB2/PWM2
RA4/CAP3
VSS
RB3/PWM3
RA3/AN3/VREF+
RD7/PWM7
RB4/PWM5
VDD
RB5/PWM4
RA2/AN2/VREF-
RB6/PGC
RA1/AN1
RB7/PGD
RA0/AN0
0.1 µF
MCLR/VPP
0.1 µF
33 pF
220 µF
0.1 µF
V02
CA2
VCC
V01
V02
GND
AN1
CA1
CA2
AN2
GND
V01
CA1
AN2
VCC
AN1
GND
V02
AN2
V01
CA2
VCC
CA1
AN1
0.1 µF
0.1 µF
0.1 µF
FIGURE A-2:
0.1 µF VREF
RESET
AN900
BOARD SCHEMATIC, PART 1 (PIC18F4431 MICROCONTROLLER, PCPWM
ISOLATORS, CURRENT COMPARATOR AND ASSOCIATED PARTS)
DS00900A-page 15
DS00900A-page 16
33 pF
PWM1
PWM0
VDD
VSS
RC4/INT1
RC2/CCP1
RC3
33 pF
RC6
RC5/INT2
RC1/CCP2
RC7
PWM2
RA4/AN4
RC0
PWM3
RA3/VREF+
OSC2/RA6
PWM5
RA2/VREF-
VSS
PWM4
RA1/AN1
VDD
RB6
RA0/AN0
OSC1/RA7
RB7
0.1 µF
OPTIONAL
1 µF
1 µF
V-
C1-
MCP201
LIN
VSS
TXD
VBAT
FAULT/SLPS
VDD
CS/WAKE
RXD
GND
C2-
C2+
A2IN
C1+
A1IN
A1OUT
VCC
A1OUT
T2IN
T1IN
V+
0.1 µF
1 µF
1 µF
10 ohm
FIGURE A-3:
MCLR/RE3
PIC18F2431
1 µF
AN900
BOARD SCHEMATIC, PART 2 (PIC18F2431 MICROCONTROLLER SOCKET,
USART, CLOCK OSCILLATOR NETWORK AND OPTIONAL LIN INTERFACE)
 2004 Microchip Technology Inc.
AN900
BOARD SCHEMATIC, PART 3 (SENSOR AND MICROCONTROLLER HEADER
CONNECTORS, MONITOR LEDS)
VREF
FIGURE A-4:
 2004 Microchip Technology Inc.
DS00900A-page 17
AN900
BOARD SCHEMATIC, PART 4 (SIGNAL CONDITIONER FOR SENSORLESS
BLDC OPERATION)
V02
GND
AN2
CA2
0.1 µF
0.1 µF
0.1 µF
0.1 µF
0.1 µF
MCP6544
MCP6544
V01
CA1
AN2
VCC
V02
GND
CA2
AN1
V01
CA1
0.1 µF
MCP6544
VCC
AN1
MCP6544
0.1 µF
MCP6544
0.1 µF
FIGURE A-5:
DS00900A-page 18
 2004 Microchip Technology Inc.
 2004 Microchip Technology Inc.
NC
Vss
Vcc
ITRIP
L3
L2
L1
H3
H2
H1
DC-
DC-
DC-
NC
V+
NC
VS1
VB1
NC
VS2
VB2
10 µF
10 µF
10 µF
VBUS+
33 pF
COL
EMT
+LED
-LED
4.7 nF
MCP6002-DIP8
0.1 µF
100 pF
MCP6002-DIP8
I1
+VCCT
+LED
-LED
I2
+VCC2
N/C
N/C
MCP6002-DIP8
FIGURE A-6:
VS3
VB3
AN900
BOARD SCHEMATIC, PART 5 (3-PHASE INVERTER POWER MODULE AND SHUNT
CURRENT MEASUREMENT)
DS00900A-page 19
AN900
BOARD SCHEMATIC, PART 6 (MOTOR TERMINAL BLOCK AND OPTIONAL
CURRENT TRANSDUCER CIRCUITRY)
IN6
IN5
IN4
+5V
0V
R
Y
B
G
33 pF
OUT
IN3
IN2
IN1
OPTIONAL
0.1 µF
33 µF
0V
IN6
IN5
IN4
OUT
IN3
IN2
IN1
OPTIONAL
+5V
0V
OUT
OPTIONAL
IN6
IN5
IN4
IN3
IN2
IN1
+5V
FIGURE A-7:
DS00900A-page 20
 2004 Microchip Technology Inc.
 2004 Microchip Technology Inc.
1.3 ohm
0.01 µF 270 VAC
G N L
S
GND
D
VCC
OCP/FB
750 ohm
47 pF
470 µF
470 µF
VBUS+
33 µF
220 pF
4.7 µF
27 ohm
10 ohm
2.2 nF
C
E
NC
A
1
CA
100 µF
100 µF
0.1 µF
10 µH
100 µF
10 µH
47 µF
47 µF
10 µH
47 µF
FIGURE A-8:
56 pF
DC- DC+
AN900
BOARD SCHEMATIC, PART 7 (POWER SUPPLY)
DS00900A-page 21
AN900
APPENDIX B:
SOFTWARE
DISCUSSED IN THIS
APPLICATION NOTE
Due to size considerations, the complete source code
listing for the applications described here is not
included in the text. A complete version of the source
code, with all required support files, is available for
download as a Zip archive from the Microchip web site
at:
www.microchip.com
DS00900A-page 22
 2004 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
use or otherwise. Use of Microchip’s products as critical
components in life support systems is not authorized except
with express written approval by Microchip. No licenses are
conveyed, implicitly or otherwise, under any intellectual
property rights.
Trademarks
The Microchip name and logo, the Microchip logo, Accuron,
dsPIC, KEELOQ, microID, MPLAB, PIC, PICmicro, PICSTART,
PRO MATE, PowerSmart, rfPIC, and SmartShunt are
registered trademarks of Microchip Technology Incorporated
in the U.S.A. and other countries.
AmpLab, FilterLab, MXDEV, MXLAB, PICMASTER, SEEVAL,
SmartSensor and The Embedded Control Solutions Company
are registered trademarks of Microchip Technology
Incorporated in the U.S.A.
Analog-for-the-Digital Age, Application Maestro, dsPICDEM,
dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR,
FanSense, FlexROM, fuzzyLAB, In-Circuit Serial
Programming, ICSP, ICEPIC, Migratable Memory, MPASM,
MPLIB, MPLINK, MPSIM, PICkit, PICDEM, PICDEM.net,
PICLAB, PICtail, PowerCal, PowerInfo, PowerMate,
PowerTool, rfLAB, rfPICDEM, Select Mode, Smart Serial,
SmartTel and Total Endurance are trademarks of Microchip
Technology Incorporated in the U.S.A. and other countries.
SQTP is a service mark of Microchip Technology Incorporated
in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2004, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received ISO/TS-16949:2002 quality system certification for
its worldwide headquarters, design and wafer fabrication facilities in
Chandler and Tempe, Arizona and Mountain View, California in
October 2003. The Company’s quality system processes and
procedures are for its PICmicro® 8-bit MCUs, KEELOQ® code hopping
devices, Serial EEPROMs, microperipherals, nonvolatile memory and
analog products. In addition, Microchip’s quality system for the design
and manufacture of development systems is ISO 9001:2000 certified.
 2004 Microchip Technology Inc.
DS00900A-page 23
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DS00900A-page 24
 2004 Microchip Technology Inc.
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