MOTORS | STEPPER MOTORS
behaviour. These features, together
with high peak speeds, mean that
any incremental movement is carried
out in the shortest possible time. The
low inertia also means high start/stop
frequencies which save time during
the rst step and solve certain motion
problems without applying a ramp.
Torque is uniform, and many control
and/or positioning schemes rely upon
this linearity for their success – for
instance microstepping and closed
loop operation. Further, there are
no harmonic distortions. Why is this
important? The torque-displacement
curves of stepper motors are supposed
to be sinusoids. Pure sinusoids mean
better accuracy when positioning in
the microstepping mode of operation
where sine and cosine ratios are used.
They also mean no inherent linearity as
positioning current is varied from zero
to maximum, so ensuring a consistent
torque constant.
OVERLAPPING CAPABILITIES
The result of the latest advances in
disc magnet stepper motor technology
is that the application boundaries
between steppers, BLDC motors and
servomotors are blurring more than
ever. With their increased torque, disc
magnet stepper motors can provide
extremely accurate, stiff performance
at low speeds without a gearbox or
other type of mechanical advantage.
The low inertia of disc magnet
stepper motors enables them to
achieve high torque output and higher
speeds, up to 10,000 rpm without the
traditional fall-off in torque.
It is true that the cost of
servomotors has fallen, but even so,
a stepper system will be signi cantly
cheaper, with disc magnet stepper
motors offering near servo
performance beyond the typical
range of applications for steppers. If
an application has predictable loads,
a disc magnet stepper can move them
with high repeatability and reliability.
Ultimately, the choice of the most
appropriate motor technology for a
given application will come down to
a host of different design parameters.
However, with the line between
the different motor technologies
now hazier than ever, the latest disc
magnet stepper motors give design
engineers new options to improve
performance, reduce size and save
costs. !
stepper
motors.
This cost
should be factored
in during speci cation
to determine
whether the long
term performance
improvement will
add enough value
to justify the
premium.
DISC MAGNET
TECHNOLOGY
Evaluating the different stepper motor
technologies, what becomes clear is
that the ideal solution would be a PM
design that could have a high number
of poles and a better magnetic circuit.
Could this be possible?
Back in the 1960s, the development
of quartz-controlled watches required
a new type of stepper motor to drive
the hands. It had to be small and very
ef cient. With rare earth magnets
becoming available, Portescap used
these materials to develop a singlephase
stepper motor with a “thin disc
magnet rotor”. Subsequently used
in many millions of watches, this “2-
phase disc magnet stepper motor”
was presented to the world in 1981 for
more general automation applications.
Today, taking advantage of
developments in rare earth
magnet materials, Portescap
offers disc magnet motors
that provide exceptional
dynamic performance
unparalleled by
other stepper motor
technologies. The rotor
consists of a rare earth
magnet having the shape
of a thin disc which is
axially magnetised.
The magnetisation
method allows for
a high number of
magnetic poles,
giving much
smaller step
angles than
conventional
two-phase PM
stepper motors.
A very low
moment of inertia
results in outstanding
acceleration and dynamic
Stepper motors can be divided
into two basic groups: the rst one
works without a permanent magnet,
the second one uses a permanent
magnet located in the rotor. Variable
reluctance (VR) motors form
the rst group, whereas the
second comprises the
original permanent
magnet (PM) motor, the
hybrid motor and the
disc magnet motor.
VR motors
were favoured
historically
for their capability of having
a high number of steps per
revolution, and therefore a greater
inherent positioning accuracy
capability. However, by introducing
‘microstepping’ (the process of
driving a stepper motor at less
than one full step per movement)
in other designs of stepper motor,
engineers have voided this advantage.
VR motors cannot be driven in
microstepping mode and they do
not have the performance attributes
to compare with other stepper
technologies in other areas.
All of the disadvantages of VR
steppers are addressed in PM motor
designs, at a lower cost of manufacture
and with improved performance.
While it is true that they cannot achieve
as small a step angle, this can be
mitigated in PM stepper motors
with a drive that can operate
in microstepping mode.
Suppose, though,
The low inertia
of disc magnet stepper
motors enables them to
achieve high torque output
and higher speeds, up to
10,000 rpm without the
traditional fall-o
we could combine
the strengths of both
designs and eliminate the
weaknesses? This is the
hybrid stepper motor. Like
PM motors, they contact a
permanent magnet in the
rotor teeth, and like VR motors
they have stator poles. These will
physically modify the airgap as
the rotor moves, and therefore
the inductance of the phase
winding changes with rotor
position.
The result is a motor that
offers small steps (allowing a
simpler drive technology to be
used), increased output torque
and quiet operation. However,
these advantages do come at a
cost, meaning hybrid stepper motors
are more costly than either VR or PM
in torque
26 WWW.EUREKAMAGAZINE.CO.UK | FEBRUARY 2021
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