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Basic Technical Knowledge / Basic Stepper motor

![]() 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. Advantages
Disadvantages
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. There are three basic stepper motor types. They are:
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. ![]() Often
referred to as a “tin can” or “canstack” 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 - 24 steps/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. ![]() 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). The hybrid 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 exist some special stepper motor designs. One is the disc magnet
motor. Here the rotor is designed as a disc with rare earth magnets, See
figure 4. 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. ![]() 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 rises65 C above the ambient in
still air. Therefore, if the motor can be mounted to a heat-sink 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. A stepper motor can be a good
choice whenever controlled movement is required. 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,
scanners, high-end office equipment, hard disk drives, fax machines and
many more. When a phase winding of a stepper
motor is energized with current a magnetic flux is developed in the
stator. The direction of this flux is determined by the “Right Hand
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 angle to the fingers), then the thumb will
point in the direction of the magnetic field.” Figure 5 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. ![]() The torque produced by a stepper motor depends on several factors.
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. The
torque 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 magnetic flux is defined
by: H = (N · i) / l where: H = Magnetic field intensity N = The number of winding turns i = current 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. Usually
stepper motors have two phases, but three- and five-phase motors also
exist. A bipolar motor with two phases has one winding/phase and a
unipolar motor has one winding, with a center tap per phase. Sometimes
the unipolar stepper motor is referred to as a “four-phase motor”, even
though it only has two phases. Motors that have two separate windings
per phase also exist, these can be driven in either bipolar or unipolar
mode. A pole can be defined as one of the regions in a
magnetized body where the magnetic flux density is concentrated. Both
the rotor and the stator of a step motor have poles. Figure 5
contains a simplified picture of a two-phase stepper motor having 2
poles (or 1 pole pairs) for each phase on the stator, and 2 poles (one
pole pair) on the rotor. In reality several more poles are added to both
the rotor and stator structure in order to increase the number of steps
per revolution of the motor, or in other words to provide a smaller
basic (full step) stepping angle. The permanent magnet stepper motor
contains an equal number of rotor and stator pole pairs. Typically
the PM motor has 12 pole pairs. The stator has 12 pole pairs per phase.
The hybrid type stepper motor has a rotor with teeth. The rotor is split into two parts, separated by a permanent magnet, making half of the teeth south poles and half north poles. The
number of pole pairs is equal to the number of teeth on one of the
rotor halves. The stator of a hybrid motor also has teeth to build up a
higher number of equivalent poles (smaller pole pitch, number of
equivalent poles = 360/teeth pitch) compared to the main poles, on which
the winding coils are wound. Usually 4 main poles are used for 3.6
hybrids and 8 for 1.8- and 0.9-degree types. It is the relationship
between the number of rotor poles and the equivalent stator poles, and
the number the number of phases that determines the full-step angle of a
stepper motor. step angle = 360 / (NPh · Ph) = 360/N NPh = Number of equivalent poles per phase = number of rotor poles Ph = Number of phases N = Total number of poles for all phases together If the rotor and stator tooth pitch is unequal, a more-complicated relation-ship exists. The following are the most common drive modes.
![]() ![]() For the following discussions please refer to the figure 6.
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 wound
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 energizing two phases at any given
time. The stator is energized according to the sequence AB - AB - AB - 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 - AB - A - AB B -
AB - 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. In Microstepping Drive the currents in the windings are continuously
varying to be able to break up one full step into many smaller discrete
steps. ![]() ![]() The
torque vs angle characteristics of a stepper motor are the relationship
between the displacement of the rotor and the torque which applied to
the rotor shaft when the stepper motor is energized at its rated
voltage. An ideal stepper motor has a sinusoidal torque vs displacement
characteristic as shown in figure 7. Positions A and C represent
stable equilibrium points when no external force or load is applied to
the rotor shaft. When you apply an external force Ta to the motor shaft
you in essence create an angular displacement, Qa. This angular
displacement, Qa, is referred to as a lead or lag angle depending on
wether the motor is actively accelerating or decelerating. When the
rotor stops with an applied load it will come to rest at the position
defined by this displacement angle. The motor develops a torque, Ta, in
opposition to the applied external force in order to balance the load.
As the load is increased the displacement angle also increases until it
reaches the maximum holding torque, Th, of the motor. Once Th is
exceeded the motor enters an unstable region. In this region a torque
is the opposite direction is created and the rotor jumps over the
unstable point to the next stable point. The displacement angle is
determined by the following relationship: X = (Z / 2p) · sin(Ta / Th) where: Z = Rotor tooth pitch Ta = Load torque Th = Motors rated holding torque X = Displacement angle. ![]() Therefore
if you have a problem with the step angle error of the loaded motor at
rest you can improve this by changing the “stiffness” of the motor. This
is done by increasing the holding torque of the motor. We can see this
effect shown in the figure 8. Increasing the holding torque for a constant load causes a shift in the lag angle from Q2 to Q1. One
reason why the stepper motor has achieved such popularity as a
positioning device is its accuracy and repeatability. Typically stepper
motors will have a step angle accuracy of 3-5% of one step. This error
is also noncumulative from step to step. ![]() The accuracy of the stepper motor is mainly a function of the mechanical precision of its parts and assembly. Figure 9 shows a typical plot of the positional accuracy of a stepper motor. The maximum positive or negative position error caused when the motor has rotated one step from the previous holding position. Step position error = measured step angle - theoretical angle The
motor is stepped N times from an initial position (N = 360 /step angle)
and the angle from the initial position is measured at each step
position. If the angle from the initial position to the N-step position
is QN and the error is DQN where: DQN = DQN - (step angle) · N. The
positional error is the difference of the maximum and minimum but is
usually expressed with a ± sign. That is: positional error = ±1.2 (DQMax
- DQMin) The values obtained from the measurement of positional errors in both directions. The
performance of a stepper motor system (driver and motor) is also
highly dependent on the mechanical parameters of the load. The load is
defined as what the motor drives. It is typically frictional, inertial
or a combination of the two. Friction is the resistance to motion due to
the unevenness of surfaces which rub together. Friction is constant
with velocity. A minimum torque level is required throughout the step in
over to overcome this friction (at least equal to the friction).
Increasing a frictional load lowers the top speed, lowers the
acceleration and increases the positional error. The converse is true
if the frictional load is lowered.Inertia is the resistance to changes
in speed. A high inertial load requires a high inertial starting torque
and the same would apply for braking. Increasing an inertial load will
increase speed stability, increase the amount of time it takes to reach a
desired speed and decrease the maximum self start pulse rate. The
converse is again true if the inertia is decreased. The rotor
oscillations of a stepper motor will vary with the amount of friction
and inertia load. Because of this relationship unwanted rotor
oscillations can be reduced by mechanical damping means however it is
more often simpler to reduce these unwanted oscillations by electrical
damping methods such as switch from full step drive to half step drive. The
torque vs speed characteristics are the key to selecting the right
motor and drive method for a specific application. These characteristics
are dependent upon (change with) the motor, excitation mode and type of
driver or drive method. A typical “speed – torque curve” is shown in
figure 10. To get a better understanding of this curve it is useful to
define the different aspect of this curve. The maximum torque produced
by the motor at standstill. ![]() The
pull-in curve defines a area refered to as the start stop region. This
is the maximum frequency at which the motor can start/stop
instantaneously, with a load applied, without loss of synchronism. The maximum starting step frequency with no load applied. The
pull-out curve defines an area refered to as the slew region. It
defines the maximum frequency at which the motor can operate without
losing synchronism. Since this region is outside the pull-in area the
motor must ramped (accelerated or decelerated) into this region. The
maximum operating frequency of the motor with no load applied. The
pull-in characteristics vary also depending on the load. The larger the
load inertia the smaller the pull-in area. We can see from the shape of
the curve that the step rate affects the torque output capability of
stepper motor. The decreasing torque output as the speed increases is
caused by the fact that at high speeds the inductance of the motor is
the dominant circuit element. The shape of the speed - torque curve can
change quite dramatically depending on the type of driver used. The
bipolar chopper type drivers which New JRC produces will maximum the
speed torque performance from a given motor. Most motor manufacturers
provide these speed - torque curves for their motors. It is important to
understand what driver type or drive method the motor manufacturer
used in developing their curves as the torque vs. speed characteristics
of an given motor can vary significantly depending on the drive method
used. The single-step response character- istics of a stepper motor is shown in figure 11.
When one step pulse is applied to a stepper motor the rotor behaves in a
manner as defined by the above curve. The step time t is the time it
takes the motor shaft to rotate one step angle once the first step pulse
is applied. This step time is highly dependent on the ratio of torque
to inertia (load) as well as the type of driver used. ![]() Since
the torque is a function of the displacement it follows that the
acceleration will also be. Therefore, when moving in large step
increments a high torque is developed and consequently a high
acceleration. This can cause over shots and ringing as shown. The
settling time T is the time it takes these oscillations or ringing to
cease. In certain applications this phenomena can be undesirable. It is
possible to reduce or eliminate this behaviour by microstepping the
stepper motor. Stepper motors can often exhibit a
phenomena refered to as resonance at certain step rates. This can be
seen as a sudden loss or drop in torque at certain speeds which can
result in missed steps or loss of synchronism. It occurs when the input
step pulse rate coincides with the natural oscillation frequency of the
rotor. Often there is a resonance area around the 100 – 200 pps region
and also one in the high step pulse rate region. The resonance phenomena
of a stepper motor comes from its basic construction and therefore it
is not possible to eliminate it completely. It is also dependent upon
the load conditions. It can be reduced by driving the motor in half or
microstepping modes. |
