Talk
TECHNICAL
ROTASYN
Talk
T
E C H N I C A L
An Absolute Inductive Transducer for
ROTASYN
Brushless Servomotors
The following is based on a technical paper presented by David T. Robinson, VP Engineering,
Admotec Inc., at the Intelligent Motion Systems
’95 conference in Long Beach CA, Sept. 1995.
INTRODUCTION
onal resolvers – is now available. This device utili-
A New Absolute Inductive Transducerzesfor
Brushless
a solid
rotor withoutServomotors
windings and overcomes
Today, frameless brushless (pancake) resolvers
the disadvantages of traditional resolvers. As
are commonly used on AC and brushless DC sersuch, it offers higher speed operation, better relivomotors to provide commutation, position, and
ability.
Introduction
velocity information to the servo controller. These available. This device utilizes a solid rotor without windangular position transducers give a constant sig- ings and overcomes the disadvantages of traditional reToday, frameless brushless (pancake) resolvers are comsuch, it offers
speed operation, better
nal representing the absolute position of the mo- solvers.
AN As
OVERVIEW
OFhigher
ANGULAR
monly used on AC and brushless DC servomotors to protor shaft within one revolution, making them well reliability,
TRANSDUCERS
and lower cost.
vide commutation,
position,
and However,
velocity information
to resolsuited for this
purpose.
traditional
the servo
controller.
These
angular
position
transducers
Overview
of Angular
Transducers
vers are complex and expensive to manufacture, An Flexible
motion
control is unthinkable
without
give a limiting
constant their
signaluse
representing
the absolute
position
in industrial
servomotors.
Rotaprecise information about the position of each
Flexible motion control is unthinkable without precise insyn resolvers
follow
differentmaking
approach
axis. For this purpose, different types of shaft
of the motor
shaft within
one arevolution,
themfor an
formation
the are
position
ofoften
each axis.
absolute
inductive
position
transducer
while
angleabout
sensors
used,
built For
intothis
thepurdriwell suited for this purpose. However, traditional resolvers
pose,
different
types
of
shaft
angle
sensors
are
used,
ofbeing and
mechanically
and
electricallylimiting
compatible
ving motors. On the basis of their physical design,
are complex
expensive to
manufacture,
their
into
the driving
motors. On
thebebasis
of theirinto
with traditional designs. A new type of absolute ten built
these
angular
transducers
can
classified
use in industrial servomotors. A new type of absolute ininductive position transducer – one that is metwo design,
main groups:
physical
these angular transducers can be classiductive position transducer—one that is mechanically and
chanically and electrically compatible with traditi- fied into two main groups:
electrically compatible with traditional resolvers—is now
Rotasyn
Traditional
Resolver
Stator
Stator
Slotted Rotor
with Windings
Rotary Transformer
Solid Rotor
The design advantages of the Rotasyn resolver
admotec
Advanced Motion Technology
Talk
TECHNICAL
ROTASYN
Optical where a phototransistor or other light-sensitive electronic
device countsorlines
a transpaOptical
where a phototransistor
otheron
light-sensitive
rent disk mounted to the rotating shaft. The
electronic device counts lines on a transparent
most common of these devices are incremendisk mounted to the rotating shaft. The most
tal and absolute encoders.
common of these devices are incremental and
absolute encoders.
Inductive
built like small electrical motors, where inducInductive built like small electrical motors, where inductive coupling between a rotating part (the rotive coupling between a rotating part (the rotor) and a stationary part (the stator) generator) and a stationary part (the stator) generates
tes signals indicating shaft position. Resolvers
signals indicating shaft position. Resolvers and
and synchros are the most common devices.
synchros are the most common devices.
Optical transducers, especially incremental encoOptical transducers, especially incremental encoders,
ders, have found wide application because their
have found wide application because their digital outputs
digital outputs can be easily processed by both
can be easily processed by both discrete logic and microdiscrete logic and microprocessors. Nevertheless,
processors.
Nevertheless,
optical
transducers
have a
optical transducers
have
a number
of characterisnumber
of make
characteristics
thatthan
makeoptimal
them lessfor
than
optitics that
them less
many
mal
for
many
applications.
The
built-in
semiconductors
applications. The built-in semiconductors used to
used
to amplify
and format
digital
outputsignals
signals are
amplify
and format
the the
digital
output
are
sensitive
temperature
and
the
LED
light comsoursensitive
to to
temperature
and the
LED
light
sources
ces co
monly employed
are to
susceptible
to aging.
monly
employed
are susceptible
aging. Furthermore,
Furthermore,
applications
that
require
an
absoluapplications that require an absolute output signal
rete
output
signal
require
absolute
encoders,
which
quire absolute encoders, which are much more compliare much more complicated and therefore expencated and therefore expensive.
sive.
From a purely practical standpoint, the precise concenFrombetween
a purely
standpoint,
the precise
tricity
thepractical
encoder disk
and the sensors
required
concentricity
between
the
encoder
disk
and
to maintain accuracy as well as the mere presence ofthe
opsensors
required
to maintain
accuracy
as well
tical
devices
in an industrial
environment
dictate
that aas
the mere presence of optical devices in an industfully enclosed device with bearings and shaft be used in
rial environment dictate that a fully enclosed deall but the crudest applications. Since encoders are typivice with bearings and shaft be used in all but the
cally connected to a shaft having its own bearings, the
crudest applications. Since encoders are typically
user
must payto
forathe
second
set ofitshigh-quality
bearings
connected
shaft
having
own bearings,
the
inuser
the transducer
as
well
as
a
flexible
coupling
to
connect
must pay for the second set of high-quality
the
two shafts.
In many
applications,
especially
brushless
bearings
in the
transducer
as well
as a flexible
servomotor
commutation
or flux
control
of ACapincoupling to
connect the
twovector
shafts.
In many
plications,
especially
brushless
duction
motors,
the additional
lengthservomotor
of the optical commutationshaft,
or flux
vectorand
control
of AC
induction
encoder’s
bearings,
coupling
is too
great and
motors,
the
additional
length
of
the
optical
the optical encoder cannot be used.
encoder’s shaft, bearings, and coupling is too
On
the and
otherthe
hand,
inductive
transducers
such
resolvgreat
optical
encoder
cannot
beasused.
ers are intrinsically absolute and require no semiconducOn on
thethe
other
hand,itself—the
inductiveraw
transducers
such
tors
transducer
output signal
can as
be
resolvers
are
intrinsically
absolute
and
require
no
transmitted over distances of more than 100 meters. In
semiconductors
the primarily
transducer
itself –and
thesteel,
raw
addition,
since they on
consist
of copper
output
signal
can
be
transmitted
over
distances
resolvers are virtually insensitive to temperature over a
of more than 100 meters. In addition, since they
consist primarily of copper and steel, resolvers
are virtually insensitive to temperature over a
wide range. Because no sensitive electronics or
optics are employed, resolvers are often supplied
in anrange.
unhoused
(also
called frameless
wide
Because
no sensitive
electronics or
or pancake)
optics are
configuration and can be mounted directly to the
employed, resolvers are often supplied in an unhoused
shaft whose position is to be measured. Cost and
(also called frameless or pancake) configuration and can
length savings are realized by the user since no
be mounted directly to the shaft whose position is to be
shaft-to-shaft coupling or extra bearings are
measured.
required. Cost and length savings are realized by the
user since no shaft-to-shaft coupling or extra bearings are
required.
While resolvers were originally developed for military and aerospace applications, in recent years
While resolvers were originally developed for military
industrial automation has shown more interest in
and aerospace applications, in recent years industrial
these rugged and precise absolute position transautomation
has shown more
in these
ducers. Nevertheless,
theinterest
expansion
of rugged
the useand
of
precise
absolute
position
transducers.
Nevertheless,
thesiresolvers was often limited by the fact that the
expansion
of the userequired
of resolvers
was often limited
by the
gnal conversion
cumbersome
circuitry
fact
the signal resolver
conversionproduction
required cumbersome
cirandthat
automated
was difficult.
Now,and
however,
inexpensive
and easy-to-implecuitry
automated
resolver production
was difficult.
menthowever,
monolithic
ICs thatand
perform
complete resolNow,
inexpensive
easy-to-implement
ver-to-digital
conversion
are available.
These R/D
monolithic
ICs that
perform complete
resolver-to-digital
converters
give
an
absolute
or
incremental
outconversion are available. These R/D converters give
an
put with a resolution of up to 65,536 counts per
absolute or incremental output with a resolution of up to
revolution. A typical two-chip solution is shown
65,536 counts per revolution. A typical two-chip solution
below.
is shown below.
Resolver
Excitation
Oscillator
(AD2S99)
Digital
Position
Data
R/D
Converter
(AD2S90)
Ref
Sin
Cos
Simple R/D conversion using two monolythic ICs
Simple R/D conversion using two monolythic ICs
The resolver signals are low bandwidth amplitude-moduThe resolver signals are low bandwidth amplitulated sine waves. Since these sine wave signals contain
de-modulated sine waves. Since these sine wave
only a single frequency component rather than the virtusignals contain only a single frequency compoally infinite frequency spectrum of an optical encoder’s
nent rather than the virtually infinite frequency
square
waveof
signals,
they are
inherentlysquare
much more
spectrum
an optical
encoder’s
waveimsimune
to
the
high-frequency
noise
generated
by
PWM to
gnals, they are inherently much more immune
motor
drives and other noise
industrial
machinery.
the high-frequency
generated
by PWM motor drives and other industrial machinery.
Finally, since the resolver itself was handicapped by a
high
levelsince
of manual
labor, some
manufacturers
have
Finally,
the resolver
itself
was handicapped
opted
to transfer
low labor-cost
countries.
by a high
levelproduction
of manualtolabor,
some manufacturers have opted to transfer production to low labor-cost countries.
Talk
TECHNICAL
ROTASYN
While lowering unit costs, this reduces their ability to respond to the market needs and does not
allow them to quickly meet customer demands.
FEEDBACK REQUIREMENTS FOR AC
While lowering unit costs, this reduces their ability to reSERVOMOTORS
spond to the market needs and does not allow them to
quickly meet customer
Commutation
of ACdemands.
and brushless DC servomotors can be done with Hall-effect sensors to
Feedback Requirements for AC
switch current into the proper phase at the proServomotors
per time. However, a tachometer and encoder are
then
required
the velocity
and position
Commutation
of to
ACclose
and brushless
DC servomotors
can
loops
inwith
the Hall-effect
drive. sensors to switch current into the
be done
Using
resolver
theHowever,
number aoftachometer
transduproper aphase
at thereduces
proper time.
cers
on
the
motor
from
three
to
only
one.
and encoder are then required to close the velocityIndeed,
and
from
a single
position
loops intransducer
the drive. it is possible to generate
all three of the required signals: high resolution
Using aposition
resolver reduces
the number
of transducers
on
digital
information
is available
directly
the
motor
from
three
to
only
one.
Indeed,
from
a
single
from the R/D converter; this position data allows
generation
ofpossible
sine wave
signalsallfor
commutation
transducer it is
to generate
three
of the refrom
lookuphigh
table;
and adigital
velocity
signal
is
quiredasignals:
resolution
position
information
available
thefrom
R/D
chipthis
to replace
is availablefrom
directly
theconverter
R/D converter;
position
the
ingeneration
analog drives.
completely
datatach
allows
of sineIn
wave
signals fordigital
comdrives,
the
basic
absolute
shaft
position
informamutation from a lookup table; and a velocity
signal
is
tion
can from
be used
to derive
velocity
and commutaavailable
the R/D
converter
chip to replace
the tach
tion
signals
within
the microprocessor.
In fact,
in analog
drives.
In completely
digital drives, the
basic a
fast DSP can perform the R/D conversion itself by
absolute shaft position information can be used to derive
synchronously sampling the resolver signals.
velocity and commutation signals within the microprocessor. In fact, a fast DSP can perform the R/D conversion
itself by synchronously sampling the resolver signals.
TRADITIONAL BRUSHLESS RESOLVERS
Traditional Brushless Resolvers
Rotary
Transformer
Sin Output
(Secondary)
Cos Output
(Secondary)
Rotary
Transformer
Rotor Input
(Primary)
Secondary
Primary
The traditional brushless resolver consists of a
The traditional
resolver
consistsbelow.
of a wound
rowound
rotor brushless
and stator
as shown
The wintor
and
stator
as
shown
below.
The
windings
on
the
rotor
dings on the rotor generate an AC magnetic field
generate
an AC magnetic
field withThis
a sinusoidal
diswith
a sinusoidal
distribution.
field induces
tribution.
This
field
induces
voltages
in
the
two
stator
voltages in the two stator windings whose ampliwindings
amplitudes
onangle
the rotatudes
arewhose
dependent
onare
thedependent
rotational
of
tional
angleTo
of provide
the rotor. sine
To provide
sine and
cosine sigthe
rotor.
and cosine
signals,
the
Traditional brushless
brushless resolver
Traditional
resolverwith
withwound
woundrotor
rotor
and rotary transformer
and rotary transformer
two secondaries are wound in space quadrature
(90 physical degrees apart) in the stator.
Electrical energy has to be supplied to the rotor
to generate its AC magnetic
T Efield.
C HHowever,
N I C A as
L the
rotor must be able to rotate freely it is not possible to use wires. The use of slip rings is also not
R O T they
A S Yare
N subject to wear,
recommended because
generate signal noise, and compromise the mechanical
of wound
the resolver.
nals,
the tworuggedness
secondaries are
in space quadrature
Traditional brushless resolvers therefore use a ro(90 physical degrees apart) in the stator.
tary coupling transformer to transfer energy from
the stator
to the
rotor.
The primary
of this
rotary
Electrical
energy
has to
be supplied
to the rotor
to genertransformer
is built
the stator.
The secondary
ate
its AC magnetic
field.into
However,
as the rotor
must be
is mounted
on the
and connected
able
to rotate freely
it is rotor
not possible
to use wires.directly
The use
to
the
resolver
primary.
Because
of
the
energy
of slip rings is also not recommended because they are
lost intoenergizing
thissignal
two-stage
transformer
subject
wear, generate
noise, and
compromise(basically
two
transformers
in
series),
many
turns of
the mechanical ruggedness of the resolver.
wire are required to generate usable output sigTraditional
brushlessThe
resolvers
use
rotarymeans
counal amplitudes.
largetherefore
number
ofaturns
thattransformer
a traditional
resolver
is a from
relatively
high-impling
to transfer
energy
the stator
to the
pedance
device,
use at high
excitation
rotor.
The primary
of limiting
this rotaryits
transformer
is built
into
frequencies
or rotational
speeds.
the
stator. The secondary
is mounted
on the rotor and
Becausedirectly
a traditional
resolver
has aBecause
woundofrotor,
connected
to the resolver
primary.
the
its
maximum
speed
is
limited
since
the
windings
energy lost in energizing this two-stage transformer
(basitend
fly out of the
rotormany
due to
centrifugal
cally
twototransformers
in series),
turns
of wire are
force.
Typical
maximum
speeds
are
10,000 RPM
required to generate usable output signal amplitudes.
The
or less.
large number of turns means that a traditional resolver is
a relatively high-impedance device, limiting its use at
high excitation frequencies or rotational speeds.
THE ROTASYN™
Talk
Because a traditional resolver has a wound rotor, its
Unlike the
traditional
brushless
resolver,
maximum
speed
is limited since
the windings
tend the
to flyRotasyn
has
both
primary
and
secondary
windings
out of the rotor due to centrifugal force. Typical maximum
in theare
stator
and
thus
no rotary transformer is
speeds
10,000
RPM
or less.
required – the Rotasyn is intrinsically brushless!
™
The
TheRotasyn
transferred
energy remains magnetic from
the primary coil through the air gap to the sinuUnlike the traditional brushless resolver, the Rotasyn has
soidally shaped poles of the solid rotor.
both primary and secondary windings in the stator and
The Rotasyn is similar to a rotary variable diffethus no rotary transformer is required—the Rotasyn is inrential transformer (RVDT) in which the rotor
trinsically
The transferred
energy remains
acts as brushless!
a magnetic
valve completing
the flux
magnetic
from
the
primary
coil
through
to the –
path. The total flux through the the
gapairisgap
constant
sinusoidally
poles ofthe
theangular
solid rotor.
the rotor shaped
determines
position within
the stator bore where the coupling occurs, and
The Rotasyn is similar to a rotary variable differential
thus the relative amplitudes of the output sigtransformer (RVDT) in which the rotor acts as a magnetic
nals.
valve completing the flux path. The total flux through the
The primary coil is wound circumferentially betgap is constant—the rotor determines the angular posiween the two stators. The two secondary wintion
within
thewound
stator bore
where
the coupling
dings
are
in the
stator
slots inoccurs,
spaceand
quathus
the
relative
amplitudes
of
the
output
signals.
drature (shifted by 90 physical degrees) similar
to a traditional resolver. Hence the induced volThe primary coil is wound circumferentially between the
tage amplitudes correspond to the sine and cotwo stators. The two secondary windings are wound in
sine of the rotor angle as in a traditional resolver.
Talk
T
the stator slots in space quadrature (shifted by 90 physi-
R
cal degrees) similar to a traditional resolver. Hence the
induced voltage amplitudes correspond to the sine and
cosine of the rotor angle as in a traditional resolver.
Primary
Reference Input
(Primary)
Solid Rotor
Cos Output
(Secondary)
Sin Output
(Secondary)
Secondary
Rotasyn resolver
resolver with
Rotasyn
with solid
solidrotor
rotor
This new design (EC patent; US patent pending) gives the
Rotasyn
somegives
uniquethe
advantages
bothunique
traditional
This
design
Rotasynover
some
adbrushless resolvers
and traditional
optical encoders:
vantages
over both
brushless resol-
vers and optical encoders:
More Reliable The rotor coils of traditional resolvers
are subject to very high accelerations
More Reliable
andof
forces
in today’s
high speed,
The rotor coils
traditional
resolvers
arehigh
subhit-rate
applications.
These
forces
can
ject to very high accelerations and forces in
a wire
orhit-rate
wear theapplications.
insulation.
today’s highbreak
speed,
high
Since
its
solid
rotor
has
no
coils,the
the inRoThese forces can break a wire or wear
tasyn
virtually
speed
accelerasulation. Since
itshas
solid
rotornohas
no or
coils,
the
limitations.
And since
the Rotasyn
Rotasyn hastion
virtually
no speed
or acceleration
has only
halfthe
the Rotasyn
windings of
a tradilimitations. And
since
has
only
half the windings
of a traditional
resolver,
tional resolver,
its MTBF (mean
timeits
beMTBF (mean
time
between
failure)
is
double
tween failure) is double that of a tradithat of a traditional
resolver.
tional resolver.
High
Speed
High
SpeedSince the solid rotor has no windings,
thererotor
are no
problems
at very high
Since the solid
has
no windings,
there
speeds.
standard
Rotasyn
are no problems
atWhile
very the
high
speeds.
Whileis
to 30,000
RPM,to
higher
speeds
are
the standardrated
Rotasyn
is rated
30,000
RPM,
higher speeds
areasimply
a matter
of mechanisimply
matter of
mechanical
balance.
cal balance. The
Thetop
top
speed
is primarily
speed
is primarily
limitedlimited
by the
by the excitation
frequency.
excitation
frequency.
E C H N I C A L
Talk
O T A S Y N
TECHNICAL
relatively few turns of wire. Fewer turns
ROTASYN
translates to lower source impedance
which means less susceptibility to noise
pickup andpickup
less sensitivity
to long
runs.
and less sensitivity
to cable
long cable
The Rotasyn
can
be
excited
at
frequencies
runs. The Rotasyn can be excited at fre-up
to 40 kHz quencies
and beyond.
up to 40 kHz and beyond.
Reduced
Ripple Ripple
In traditional resolvers, rotor slots passReduced
ing resolvers,
stator slots induce
pulsations
on thestaIn traditional
rotor slots
passing
output pulsations
signals that manifest
tor slots induce
on the themselves
output sigas velocitythemselves
ripple. The poles
on the Rotanals that manifest
as velocity
ripple. The poles
the
Rotasyn
have no
synon
rotor
have
no slotsrotor
and therefore
slots and therefore
produce
a smooth
produce a smooth
output
with no output
slot
with no slot
ripple
effects.
ripple effects.
Lower
Cost Cost
Since the Rotasyn is mechanically simLower
pler than aistraditional
resolver,
it can be
Since the Rotasyn
mechanically
simpler
manufactured
moreitefficiently
than a traditional
resolver,
can be than
manufacresolvers.
The solid rotor
is
tured moretraditional
efficiently
than traditional
resolvers. The solid
is much
less costly
toamamuch rotor
less costly
to manufacture
than
nufacture than
wound
rotor. Inthe
addition,
wounda rotor.
In addition,
lack of the
lack of slots
on
the
rotor
means
that
stator
slots on the rotor means that thethe
stator
slots can be
wider,
winding
easier witslots
can bemaking
wider, making
winding
hout introducing
slot ripple.
easier without
introducing slot ripple.
Conclusions
CONCLUSIONS
The advantages of this new inductive transducer (Rotasyn)
for absolute positioning make it ideally suited for sinusoiThe advantages of this inductive transducer
dal commutation of AC and brushless DC servomotors as
(Rotasyn) for absolute positioning make it ideally
well as for flux vector control of AC induction motors. This
suited for sinusoidalcommutation of AC and
type of low-cost angular transducer will find broad applibrushless DC servomotors as well as for flux veccation,
not onlyofinAC
industrial
automation
butThis
also as
a of
tor control
induction
motors.
type
component
in
increasingly
sophisticated
automotive
con-not
angular transducer will find broad application,
trol
systems.
only in industrial automation but also as acomponent in increasingly sophisticated automotive
control systems.
admotec
Advanced Motion Technology
LowLow
Impedance
The Rotasyn’s efficient single-stage magImpedance
netic
design brings
the advantage
of
The Rotasyn’s
efficient
single-stage
magnetic
high
electrical
efficiency,
so that
powerdesign brings
the
advantage
of high
electrical
fulthat
output
signals are
produced
usingare
efficiency, so
powerful
output
signals
Admotec Inc.
85 Mechanic Street
Lebanon NH 03766-1500 USA
Tel: +1 603.448.7000
Fax: +1 603.448.7007
produced using relatively few turns of wire.
Fewer turns translates to lower source impedance which means less susceptibility to noise
E-mail: Admotec@Valley.net
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