784 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 34, NO. 4, JULY/AUGUST 1998
Rotor Position and Velocity Estimation for a
Salient-Pole Permanent Magnet Synchronous
Machine at Standstill and High Speeds
Matthew J. Corley and Robert D. Lorenz, Fellow, IEEE
Abstract— This paper addresses self-sensing (“sensorless”)
control of salient-pole permanent magnet synchronous motors
(PMSM’s). The major contribution of this work is the
introduction of a simple-to-implement estimation technique
that operates over a wide speed range, including zero speed.
The technique achieves simplicity by decoupling the inherent
cross-coupling in PMSM’s. The technique utilizes the dependence
of inductance on rotor position in interior permanent magnet
machines to produce position and velocity estimates both for
field orientation and for all motion control of the drives. The
technique functions in a manner similar to a resolver and
resolver-to-digital converter (RTDC) sensing system, whereby in
the proposed technique the motor acts as the electromagnetic
resolver and the power converter applies carrier-frequency
voltages to the stator which produce high-frequency currents
that vary with position. The sensed currents are then processed
with a heterodyning technique that produces a signal that is
approximately proportional to the difference between the actual
rotor position and an estimated rotor position. This position
error signal and a torque estimate are then used as inputs to
a Luenberger style observer to produce parameter insensitive,
zero lag, position and velocity estimates.
Index Terms—Permanent magnet motor control, self sensing,
sensorless control, sensorless tranducerless field orientation, sen-
sorless vector control, transducerless control.
NOMENCLATURE
Superscripts
ˆ Estimated quantities.
Commanded quantities.
Stator reference frame.
Rotor reference frame.
Estimated rotor reference frame.
Paper IPCSD 98–26, presented at the 1996 Industry Applications Society
Annual Meeting, San Diego, CA, October 6–10, and approved for publication
in the IEEE T
RANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial
Drives Committee of the IEEE Industry Applications Society. This work
was supported by the Wisconsin Electric Machines and Power Electronics
Consortium (WEMPEC), University of Wisconsin, Madison. Manuscript
released for publication March 13, 1998.
M. J. Corley was with Marquip, Inc., Madison, WI 53703 USA. He is now
with the Federal Division, Maxwell Technologies, San Diego, CA 92123-1506
USA.
R. D. Lorenz is with the Department of Electrical Engineering, University
of Wisconsin, Madison, WI 53706-1572 USA.
Publisher Item Identifier S 0093-9994(98)04915-9.
Symbols and Abbreviations
Stator - and -axes voltages.
Stator - and -axes currents.
Stator - and -axes flux linkages.
Permanent magnet flux linkage.
Stator - and -axes inductances.
Stator resistance.
Derivative operator.
Rotor angular velocity and position.
Injected carrier signal frequency.
Average and differential spatial inductances.
Stator - and -axes injected carrier-frequency
voltages.
Stator - and -axes injected carrier-frequency
currents.
Stator - and -axes injected carrier-frequency
flux linkages.
BPF Bandpass filter.
Rotor inertia.
Electromagnetic torque.
I. I
NTRODUCTION
F
OR PROPER motion and field-oriented control of perma-
nent magnet synchronous motors (PMSM’s), knowledge
of position and velocity is required. In general, these feedback
signals come from feedback devices mechanically coupled to
the rotor. This coupling, as well as the electrical connectors
and signal wires from the sensor to the controller, reduce the
mechanical robustness of the overall system. This reduction
in mechanical robustness and the cost of the sensors make
elimination of these devices very desirable.
In recent years, numerous papers have been published that
address the elimination of the mechanical sensors [1]–[17].
These techniques have been primarily based on the fundamen-
tal component voltage model of the machine [1]–[12] or on the
spatial inductance of salient rotor machines [1], [13]–[17]. The
fundamental component model techniques rely on the position
and velocity dependency of the back EMF of the motor.
Because back EMF is zero amplitude at standstill, there is a
lower limit on the rotor velocity for which these techniques can
operate. The methods based on spatial inductance variations
have the desired effect that they can operate at zero speed,
however, at present, many of these techniques have an upper
0093–9994/98$10.00 1998 IEEE
CORLEY AND LORENZ: ROTOR POSITION AND VELOCITY ESTIMATION FOR A SALIENT-POLE PMSM 785
Fig. 1. Model of PM synchronous machine in the rotor reference frame.
limit on the operating speed range due to effects of back EMF
at high speeds.
Jansen and Lorenz [15], [17] proposed an estimation tech-
nique for the induction machine that utilizes an injected
signal at a high frequency to extract position estimates from
a machine that has a high-frequency saliency introduced to
its rotor. This technique was also used on machines with
inherent saliency, such as a buried magnet permanent magnet
synchronous (PMS) machine [16]. Such a method has the
desired zero- and high-speed properties. However, it relies on
a sinusoidal variation in the inductance of the machine. Vari-
ations from a sinusoidal inductance profile result in additional
complexity in estimating rotor position.
The method proposed here is similar to the one proposed
by Jansen and Lorenz in that it utilizes an injected signal
at known frequency to extract information from a spatial
saliency. However, the method described here relies on the
fact that, in a rotor reference frame, the
and axes of the
motor are decoupled from each other. This reduces the number
of additional functions that are needed to produce the position
estimate.
II. PM S
YNCHRONOUS MACHINE MODEL
The – model for a PM synchronous machine in the rotor
reference frame is very helpful in developing the analytical
basis for the proposed method of position estimation. The
stator voltage model in a rotor reference frame is characterized
by (1):
(1)
where
and are the stator - and
-axes voltage, flux linkage, and current in the rotor reference
frame,
is the stator resistance, is the rotor angular
velocity, and
is the derivative operator.
The stator flux linkage viewed in a rotor reference frame
is given by (2):
(2)
where
and are the - and -axes inductance and
is the permanent magnet flux.
Fig. 1 shows this model in block diagram format.
In a rotor reference frame, the
-axis flux is produced by
the
-axis current and the -axis flux is the sum of the flux
produced by the
-axis current and the flux from the permanent
magnets. If the speed voltage is decoupled, then the two axes
are decoupled from each other. Thus, a change in the
-axis
flux should have no effect on the
-axis current. This property
can be utilized to track the position of the rotor.
III. P
RINCIPLES OF A NEW ESTIMATION METHOD
The technique functions in a manner similar to a re-
solver and resolver-to-digital converter (RTDC) sensing sys-
tem, whereby, in the proposed technique, the motor acts as
the electromagnetic resolver and the power converter applies
carrier-frequency voltages to the stator which produce high-
frequency currents that vary with position. The sensed currents
are then processed with a heterodyning technique that produces
a signal that is approximately proportional to the difference
between the actual rotor position and an estimated rotor
position. This carrier-frequency sensing operation is, of course,
occurring simultaneously with the fundamental component
power conversion in the motor/power converter.
A sensing approach based on the property of the
- and -
axes flux being decoupled can be formulated as follows. If a
flux vector at a known carrier frequency is applied only in the
axis of an estimated rotor position, the estimated position
can be made to track the actual position via a controller which
drives the estimated position such that the
-axis current at
the injected carrier frequency is zero in the estimated position
reference frame.
The injected flux vector in the
-axis of the estimated
position,
, reference frame vector can be described by
(3)
where
is the carrier frequency of the injected signal, and
is the magnitude of the injected flux vector.
Transforming the flux vector to the stationary reference
frame yields
(4)
The carrier-frequency stator flux linkage in a stationary
reference frame is given by
(5)
where
and are the carrier-frequency components of
the stator current in a stationary frame and
and are the
average inductance and the amplitude of the spatial modulation
of the inductance, as defined by
(6)
The carrier-frequency components of the stator currents in a
stationary frame can be obtained from (4) and (5), resulting in
(7)
786 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 34, NO. 4, JULY/AUGUST 1998
Fig. 2. Observer block diagram.
where the high-frequency current magnitudes and are
defined by
(8)
The
-axis component of the high-frequency current in the
estimated rotor position reference frame is given by
(9)
which results in
(10)
From (10), it can be seen that a carrier-frequency signal is
produced that is amplitude modulated by the error between the
estimated position and the actual position. This signal takes on
the same form as the error signal generated internally by the
standard RTDC. Thus, position information can be extracted in
the same manner as an RTDC. First, the signal is demodulated
to form a dc signal proportional to the error, and then this error
is fed into an observer that updates the estimated velocity and
position to force the error to zero. A block diagram of the
observer and the heterodyning process is given in Fig. 2.
The observer is based on the model of the mechanical
system of the motor. It uses an estimate of the torque and
inertia of the motor to estimate the velocity and position of
the rotor. With a correct inertia estimate, the estimated velocity
and position will track the actual velocity and position under
acceleration with zero lag. The error signal generated by the
heterodyning process is used to correct the estimates under
load changes and to compensate for inertia estimate errors
during speed transients.
Because we can directly manipulate the voltage applied to
the machine, we calculate the voltage command feedforward
needed to generate the desired decoupled flux vector in (3) and
(4). Since the stator resistive voltage drop is small relative to
the induced voltage at the injected frequency, the stationary
frame voltage that must be applied to the stator can be well
approximated by
(11)
This results in
(12)
Fig. 3. System block diagram.
Since the control is easier to implement in the rotor frame,
it is best to transform these equations to the estimated rotor
frame, designated by the superscript
, as follows. Rewriting
(11), we obtain
(13)
Substituting (3), one can remove the flux variables
(14)
After simplification, the voltage command feedforward terms
become a very simple set
(15)
Fig. 3 depicts the system with the generation of these high-
frequency voltage commands. This figure also includes a block
for a current regulator
. From the diagram, it can be
seen that, if the current regulation is going to be done in
a synchronous frame, only a modest amount of additional
functions are needed to produce the position estimate.
This topology for estimating rotor position has some very
attractive features. First, its steady-state tracking ability is not
dependent on the motor parameters. The term
is dependent
on the inductance of the motor. However, this term is only
a scaling term and should not affect the accuracy of the
position and velocity estimates, since its spatial angle is being
tracked, not the amplitude. Second, the magnitude of
is
also independent of velocity, thus, the observer eigenvalues
will be independent of speed if a linear observer controller is
used. The independence of
from velocity also gives this
estimation technique the inherent ability to work at standstill.
IV. E
XPERIMENTAL RESULTS
A. Experimental Motor
The motor used in the development of the experimental
system is a buried magnet PMS motor. This motor is designed
to be used in servo motor applications and, thus, is deemed
CORLEY AND LORENZ: ROTOR POSITION AND VELOCITY ESTIMATION FOR A SALIENT-POLE PMSM 787
TABLE I
E
LECTRICAL AND MECHANICAL SPECIFICATIONS OF EXPERIMENTAL MOTOR
Stall Torque 367 oz in
Speed 6200 r/min (maximum continuous)
Rated Current 5-A rms
K
t
68.7 oz in/A rms
K
e
29.4 V/kr/min (rms volts of line-to-line sine
wave)
R
2.3
phase to phase
L
25 mH
+
/
0
30% (phase to phase)
J
0.10 kg
1
m
2
2
10
0
3
Fig. 4. Buried magnet PMS motor cross section.
an appropriate motor to study. The electrical and mechanical
data for this motor are given in Table I.
A cross section of the motor used in the experimental tests
is given in Fig. 4. It can be seen from this figure that the motor
has a four-pole rotor with inset permanent magnets. Imbedding
the magnets in the rotor provides a more mechanically robust
rotor capable of higher speed operation compared to a rotor
with magnets mounted on the surface of the rotor. A rotor
constructed in this way also results in a rotor
-axis inductance
that is larger than the
-axis inductance. The flux in the
axis of the motor must cross the magnet airgap as well as the
physical airgap, whereas the
-axis flux can take a path through
the pole face of the rotor, requiring the flux to only cross the
physical airgap. This results in a rotor
-axis inductance that
is greater than the
-axis inductance.
Fig. 5 shows the self inductance of the stator
axis versus
rotor position. From this picture, it can be seen that this motor
contains a great deal of saliency.
B. Implementation of Position Estimation
The actual implementation of the technique involved the
use of two Analog Devices AD2S100 vector rotator chips to
perform the reference frame transformation and an RTDC to
perform the voltage to frequency (V/F) and counter operation.
The demodulation was done by multiplying the high-frequency
signal by a signal at the injected frequency to produce a signal
that consisted of a dc portion proportional to the error, as well
Fig. 5. Self inductance of the stator
d
axis versus rotor position.
Fig. 6. Experimental, dual motor drive test setup.
as a component at twice the injected frequency. The twice-
frequency component is only present when the system is not
tracking correctly.
C. Experimental Test Setup
The experimental setup used for this research is the dual
motor drive system depicted in Fig. 6.
The dual motor drive configuration was set up to allow
continuous and transient loads to be applied at any speed.
Such a configuration is quite helpful in evaluating self-sensing
methods over a full range of operating conditions, including
sustained zero and very-low-speed loaded operation.
The setup consists of an Intel 486-based personal computer
with a Burr Brown interface board consisting of an eight-
channel analog-to-digital converter board, two two-channel
digital-to-analog converter boards, and four 8-b digital I/O
ports. In addition, the computer also contains an encoder
interface board for comparison of actual position with the
estimated position. The position and velocity estimation is
performed in hardware and is interfaced to the computer via
the Burr Brown interfaces. The computer is used to perform the
current regulation and motion control. The setup also consists
of an Electrocraft BRU 200 drive connected to the test motor.
This drive has been modified to allow for direct access to the
voltage command points, thus disabling its hardware current
regulator in favor or our synchronous frame digital regulator.
788 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 34, NO. 4, JULY/AUGUST 1998
Fig. 7. Estimated and measured position at 4.6 r/min.
Fig. 8. Estimated and measured position at 5000 r/min.
The test motor is coupled to a load motor that is controlled with
another Electrocraft BRU 200 drive. This drive has not been
modified and uses an encoder for feedback from the motor.
This encoder feedback from the load motor is also fed to the
control computer for test purposes only. This encoder is a
2000-line encoder, which yields 8000 counts per revolution of
the motor. Both drives operate with switching frequencies of
10 kHz and a 2-kHz carrier frequency used for the estimation.
The magnitude of the carrier-frequency voltage is set at
10% of rated voltage, resulting in a current at the carrier
frequency of about 2% rated current. This results in a 2%
torque oscillation. Because this torque oscillation is at 2 kHz,
the inertia of the motor very effectively filters the effect of this
very small carrier-frequency torque to negligible levels on the
motor velocity.
The observer is tuned for a 100-Hz bandwidth. This and
the observer feedforward input give fast response in estimated
velocity and position under dynamic load changes.
D. Experimental Data
Fig. 7 shows the estimated and actual position measured
with an encoder at low speed. Fig. 8 shows the estimated and
actual position measured with an encoder at high speed. These
two figures show the tracking ability of estimation at low and
high speeds.
This estimation method relies on the property of the
and
axes being decoupled in the rotor reference frame. However,
under load, saturation of the machine can cause the reference
angle to move away from the rotor zero angle. Figs. 9–11
Fig. 9. Position error versus position at no load.
Fig. 10. Position error versus position at a 3-A load.
Fig. 11. Position error versus position at a 6-A load.
demonstrate the effect that this has on the accuracy of the
estimated position. This effect can be seen as a predictable
offset in the position estimate which is easily compensated.
One simple, but effective, method for compensation is with a
position correction term that is a linear function of current.
Figs. 9–11 show that there also exists some load-dependent
error that is harmonic with position. It is believed that this error
is related to unmodeled saliencies. Without including such un-
modeled terms in the shape being tracked, this harmonic error
can cause localized instability and oscillation if the gains on
velocity and position are set too high. The experimental system
was limited to a 10-Hz velocity loop. With the single-saliency
model, the use of the position and velocity information is
limited to commutation or low-performance servo applications.
