© 2007 Microchip Technology Inc. DS01078A-page 1
AN1078
INTRODUCTION
Designers can expect environmental demands to con-
tinue to drive the need for advanced motor control tech-
niques that produce energy efficient air conditioners,
washing machines and other home appliances. Until
now, sophisticated motor control solutions have only
been available from proprietary sources. However, the
implementation of advanced, cost-effective motor con-
trol algorithms is now a reality thanks to the new
generation of Digital Signal Controllers (DSCs).
An air conditioner, for example, requires fast response
for speed changes in the motor. Advanced motor con-
trol algorithms are needed to produce quieter units that
are more energy efficient. Field Oriented Control (FOC)
has emerged as the leading method to achieve these
environmental demands.
This application note discusses implementation of a
sensorless FOC algorithm for Permanent Magnet Syn-
chronous Motors (PMSMs) using the Microchip dsPIC
®
DSC family.
Why Use the FOC Algorithm?
The traditional control method for BLDC motors drives
the stator in a six-step process, which generates oscil-
lations on the produced torque. In six-step control, a
pair of windings is energized until the rotor reaches the
next position, then the motor is commutated to the next
step. Hall sensors determine the rotor position to elec-
tronically commutate the motor. Advanced sensorless
algorithms use the back-EMF generated in the stator
winding to determine the rotor position.
The dynamic response of six-step control (also called
trapezoidal control) is not suitable for washing
machines because the load is changing dynamically
within a wash cycle, and varies with different loads and
selected wash cycle. Further, in a front load washing
machine, the gravitational power works against the
motor load when the load is on the top side of the drum.
Only advanced algorithms such as FOC can handle
these dynamic load changes.
This application note focuses on the PMSM-based sen-
sorless FOC control of appliances because this control
technique offers the greatest cost benefit in appliance
motor control. The sensorless FOC technique also
overcomes restrictions placed on some applications
that cannot deploy position or speed sensors because
the motor is flooded, or because of wire harness place-
ment constraints. With a constant rotor magnetic field
produced by a permanent magnet on the rotor, the
PMSM is very efficient when used in an appliance. In
addition, its stator magnetic field is generated by sinu-
soidal distribution of windings. When compared to
induction motors, PMSM motors are powerful for their
size. They are also electrically less noisy than DC
motors, since brushes are not used.
Why Use Digital Signal Controllers for
Motor Control?
dsPIC DSCs are suitable for appliances like washing
machines and air conditioner compressors because
they incorporate peripherals that are ideally suited for
motor control, such as:
• Pulse-Width Modulation (PWM)
• Analog-to-Digital Converters (ADCs)
• Quadrature Encoder Interface (QEI)
When performing controller routines and implementing
digital filters, dsPIC DSCs enable designers to optimize
code because MAC instructions and fractional opera-
tions can be executed in a single cycle. Also, for oper-
ations that require saturation capabilities, the dsPIC
DSCs help avoid overflows by offering hardware
saturation protection.
DSCs need fast and flexible analog-to-digital (A/D)
conversion for current sensing—a crucial function in
motor control. The dsPIC DSCs feature ADCs that can
convert input samples at 1 Msps rates, and handle up
to four inputs simultaneously. Multiple trigger options
on the ADCs enable use of inexpensive current sense
resistors to measure winding currents. For example,
the ability to trigger A/D conversions with the PWM
module allows inexpensive current sensing circuitry to
sense inputs at specific times (switching transistors
allow current to flow through sense resistors).
Author: Jorge Zambada
Microchip Technology Inc.
Sensorless Field Oriented Control of PMSM Motors
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