-
资料翻译
英文资料
Stepper Motor Basics
[n
Zhang
．
DSP-based
microstep controller of stepper
motor
．
Intelligent
Control and Automation,
2004
．
Fifth World Congress on Volume 5, 15-19
June 2004.]
A stepper motor is an
electromechanical device which converts electrical
pulses
into discrete mechanical
movements. The shaft or spindle of a stepper motor
rotates in
discrete
step
increments
when
electrical
command
pulses
are
applied
to
it
in
the
proper
sequence. The motors rotation has several direct
relationships to these applied
input
pulses. The sequence of the applied pulses is
directly related to the direction of
motor shafts rotation. The speed of the
motor shafts rotation is directly related to the
frequency
of
the
input
pulses
and
the
length
of
rotation
is
directly
related
to
the
number of
input pulses applied.
Stepper Motor Advantages and
Disadvantages
Advantages
1. The rotation angle of
the motor is proportional to the input pulse.
2. The motor has full torque at
standstill (if the windings are energized)
3. Precise positioning and
repeatability of movement since good stepper
motors
have an accuracy of 3
–
5% of a step and this
error is non cumulative from one step to
the next.
4. Excellent
response to starting/stopping/reversing.
5. Very reliable since there are no
contact brushes in the motor. Therefore the life
of the motor is simply dependant on the
life of the bearing.
6. The motors
response to digital input pulses provides open-
loop control,
making the motor simpler
and less costly to control.
7. It is
possible to achieve very low speed synchronous
rotation with a load that
is directly
coupled to the shaft.
8. A wide range
of rotational speeds can be realized as the speed
is proportional
to the frequency of the
input pulses.
Disadvantages
1. Resonances can occur if
not properly controlled.
2. Not easy to
operate at extremely high speeds.
Open Loop Operation
One
of
the
most
significant
advantages
of
a
stepper
motor
is
its
ability
to
be
accurately controlled in an open loop
system. Open loop control means no feedback
information
about
position
is
needed.
This
type
of
control
eliminates
the
need
for
expensive
sensing
and
feedback
devices
such
as
optical
encoders.
Your
position
is
known simply by keeping track of the
input step pulses.
Stepper
Motor Types
There are three basic
stepper motor types. They are :
?
Variable
-reluctance
?
Permanent
-magnet
?
Hybrid
Variable-
reluctance (VR)
This type of stepper
motor has been around for a long time. It is
probably the easiest
to
understand
from
a
structural
point
of
view.
Figure
1
shows
a
cross
section
of
a
typical
V.R.
stepper motor. This
type of motor
consists of a soft iron multi-toothed
rotor and a wound stator. When the
stator windings are energized with DC current the
poles
become
magnetized.
Rotation
occurs
when
the
rotor
teeth
are
attracted
to
the
energized stator poles.
Figure 1. Cross-section of a
variablereluctance(VR) motor
.
Permanent Magnet (PM)
Often referred to as a “tin can” or
“canstock” motor the permanent magnet step
motor is a low cost and low resolution
type motor with typical step angles of
7.5°
to
15°
.
(48
–
24steps/revolution)
PM
motors
as
the
name
implies
have
permanent
magnets added to
the motor structure. The rotor no longer has teeth
as with the VR
motor. Instead the rotor
is magnetized with alternating north and south
poles situated
in a straight line
parallel to the rotor shaft. These magnetized
rotor poles provide an
increased
magnetic flux intensity and because of this the PM
motor exhibits improved
torque
characteristics when compared with the VR type.
Hybrid (HB)
The
hybrid
stepper
motor
is
more
expensive
than
the
PM
stepper
motor
but
provides better performance with
respect to step resolution, torque and speed.
Typical
step
angles
for
the
HB
stepper
motor
range
from
3.6°
to
0.9°
(100
–
400
steps
per
revolution). Thehybrid stepper motor
combines the best features of both the PM and
VR type stepper motors. The rotor is
multi-toothed like the VR motor and contains an
axially magnetized concentric magnet
around its shaft. The teeth on the rotor provide
an even better path which helps guide
the magnetic flux to preferred locations in the
airgap. This further increases the
detent, holding and dynamic torque characteristics
of the motor when compared with both
the VR and PM types.
The two most
commonly used types of stepper motors are the
permanent magnet
and the hybrid types.
If a designer is not sure which type will best fit
his applications
requirements he should
first evaluate the PM type as it is normally
several times less
expensive. If not
then the hybrid motor may be the right choice.
There
also
excist
some
special
stepper
motor
designs.
One
is
the
disc
magnet
motor. Here the rotor is designed sa a
disc with rare earth magnets, See fig. 5 . This
motor type has some advantages such as
very low inertia and a optimized magnetic
flow
path
with
no
coupling
between
the
two
stator
windings.
These
qualities
are
essential in some
applications.
Size and
Power
In
addition
to
being
classified
by
their
step
angle
stepper
motors
are
also
classified according to frame sizes
which correspond to the diameter of the body of
the motor. For instance a size 11
stepper motor has a body diameter of approximately
1.1 inches. Likewise a size 23 stepper
motor has a body diameter of 2.3 inches (58
mm), etc. The body length may however,
vary from motor to motor within the same
frame size classification. As a general
rule the available torque output from a motor of
a particular frame size will increase
with increased body length.
Power
levels for IC-driven stepper motors typically
range from below a watt for
very
small
motors
up
to
10
–
20
watts
for
larger
motors.
The
maximum
power
dissipation level or thermal limits of
the motor are seldom clearly stated in the motor
manufacturers
data.
To
determine
this
we
must
apply
the
relationship
P=V×
I
For
example,
a
size
23 step motor may be rated at 6V and 1A per phase.
Therefore, with two phases
energized
the motor has a rated power dissipation
of 12 watts. It is normal practice to rate a
stepper motor at the power dissipation
level where the motor case rises 65°
C
above
the ambient in still air.
Therefore, if the motor can be mounted to a
heatsink it is often
possible
to
increase
the
allowable
power
dissipation
level.
This
is
important
as
the
motor is
designed to be and should be used at its maximum
power dissipation ,to be
efficient from
a size/output power/cost point of view.
When to Use a StepperMotor
A stepper motor can be a
good choice henever controlled movement is
equired.
They
can
be
used
to
advantage
in
applications
where
you
need
to
control
rotation
angle,
speed,
position
and
synchronism.
Because
of
the
inherent
advantages
listed
previously,
stepper
motors
have
found
their
place
in
many
different
applications.
Some of these
include printers, plotters, highend office
equipment, hard disk drives,
medical
equipment, fax machines, automotive and many more.
The Rotating Magnetic Field
When a phase winding of a
stepper motor is energized with current a magnetic
flux
is
developed
in
the
stator.
The
d
When
a
phase
winding
of
a
stepper
motor
is
energized with current a magnetic flux
is developed irection of this flux is determined
by the “Right Ha
nd
Rule”
which
states:
“If
the
coil
is
grasped
in
the
right
hand
with
the
fingers
pointing in the
direction of the current in the winding (the thumb
is extended at a 90°
angleto
the fingers), then the thumb will point in the
direction of the magnetic
field
.”
Figure
2
shows
the
magnetic
flux
path
developed
when
phase
B
is
energized
with
winding current in
the direction shown.
The rotor then aligns itself so that the
flux opposition is minimized. In this
case the motor would rotate clockwise so that its
south pole aligns with the north pole
of the stator B at position 2 and its north pole
aligns with the south pole of stator B
at position 6. To get the motor to rotate we can
now see that we must provide a sequence
of energizing the stator windings in such a
fashion
that
provides
a
rotating
magnetic
flux
field
which
the
rotor
follows
due
to
magnetic
attraction.
Figure 2 Magnetic flux path through
atwo-pole stepper motor with a lag betweenthe
rotor and stator.
Torque Generation
The torque produced by a stepper motor
depends on several factors.
? The step
rate
? The drive current in
the windings
? The drive
design or type
In a stepper
motor a torque is developed when the magnetic
fluxes of the rotor
and stator are
displaced from each other. The stator is made up
of a high permeability
magnetic
material.
The
presence
of
this
high
permeability
material
causes
the
magnetic
flux
to
be
confined
for
the
most
part
to
the
paths
defined
by
the
stator
structure
in
the
same
fashion
that
currents
are
confined
to
the
conductors
of
an
electronic
circuit.
This
serves
to
concentrate
the
flux
at
the
stator
poles.
Thetorque
output
produced
by
the
motor
is
proportional
to
the
intensity
of
the
magnetic
flux
generated when the winding is
energized.
The basic relationship which
defines the intensity of the magneticflux is
defined
by:
H = (N
×
i) ÷
l
where:
N = The
number of winding turns
i = current
H = Magnetic field intensity
l = Magnetic flux path length
This
relationship
shows
that
the
magnetic
flux
intensity
and
consequently
the
torque is proportional to the number of
winding turns and the current and inversely
proportional to the length of the
magnetic flux path. From this basic relationship
one
can see that the same frame size
stepper motor could have very different torque
output
capabilities
simply
by
changing
the
winding
parameters.
More
detailed
information
on how the
winding parameters affect the output capability of
the motor can be found
in the
application note entitled “DriveCircuit
Basics”.
Stepping Modes
The following are the most
common drive modes.
? Wave Drive (1
phase on)
? Full Step Drive
(2 phases on)
? Half Step
Drive (1 & 2 phases on)
?
Micro stepping (Continuously varying motor
currents)
For the following
discussions please refer to the figure 3.
Figure 3
Unipolar and bipolar wound
stepper motors.
In
Wave
Drive
only
one
winding
is
energized
at
any
given
time.
The
stator
is
energized according to the sequence
A
?
B
?
A
?
B
and the rotor steps from position
8
?
2
?
4
?
6
.
For
unipolar
and
bipolar
wo
und
motors
with
the
same
winding
parameters
this
excitation
mode
would
result
in
the
same
mechanical
position.
The
disadvantage
of
this
drive
mode
is
that
in
the
unipolar
wound
motor
you
are
only
using
25% and in the bipolar motor only 50% of the total
motor winding at any given
time. This
means that you are not getting the maximum torque
output from the motor.
In
Full Step Drive you are energizingtwo phases at
any given stator is
energized
according to the sequence AB
?
A
B
?
A
B ?
AB and the
rotor
steps from
position 1
?
3
?
5
?
7
. Full step mode results in the same angular
movement as 1
phase on drive but the
mechanical position is offset by one half of a
full
step. The torque output
of the unipolar wound
motor
is
lower than the
bipolar
motor (for motors with the same winding
parameters) since the unipolar motor uses
only 50% of the available winding while
the bipolar motor uses the entire winding.
Half Step Drive combines
both wave and full step (1&2 phases on) drive
modes.
Every second step only
one phase is energized and during the
other steps one phase on each stator. The
stator is energized according to the
sequence AB
?
B
?
A
B ?
A
?
A
B ?
B
?
A
B
?
A
and the rotor
steps from position 1 ?
2
?
3
?
4
?
5
?
6
?
7
?
8
.
This results in
angular
movements
that
are
half
of
those
in
1-
or
2-phases-on
drive
modes.
Half
stepping can reduce a phenomena
referred to as resonance which can be experienced
in 1- or 2-phases-on drive modes.
The excitation
sequences for the above drive modes are summarized
in Table 1.
Table 1. Excitation sequences for
different drive modes