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We offer excellent stepper motors, Stepper Motor Driver, DC Servo Motor,
Stepper Motor Power Supply, Stepper Motor Kit, CNC router. These are the
best motors on the market. Stepper Motor is used on CNC router,CNC
Lathes,small to mid-sized CNC mills & milling machines, Laser Engravers and Laser
Cutters, Vinyl Sign Cutters, CNC Plasma Cutters, and CNC Foam Cutters. The
Stepper motor also has been used in precision telescope positioning systems and
robots. Low inductance and high torque make these Motor the best choice.
Stepper Motor Power Supply, Stepper Motor Kit, CNC router. These are the
best motors on the market. Stepper Motor is used on CNC router,CNC
Lathes,small to mid-sized CNC mills & milling machines, Laser Engravers and Laser
Cutters, Vinyl Sign Cutters, CNC Plasma Cutters, and CNC Foam Cutters. The
Stepper motor also has been used in precision telescope positioning systems and
robots. Low inductance and high torque make these Motor the best choice.
Fundamentals of operation
Stepper motor operates differently from DC brush motors, which rotate when voltage is
applied to their terminals. Stepper motor, on the other hand, effectively has multiple
"toothed" electromagnets arranged around a central gear-shaped piece of iron. The
electromagnets are energized by an external control circuit, such as a microcontroller. To
make the motor shaft turn, first one electromagnet is given power, which makes the
gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth
are thus aligned to the first electromagnet, they are slightly offset from the next
electromagnet. So when the next electromagnet is turned on and the first is turned off,
the gear rotates slightly to align with the next one, and from there the process is
repeated. Each of those slight rotations is called a "step", with an integer number of
steps making a full rotation. In that way, the motor can be turned by a precise angle.
Stepper motor characteristics
1. Stepper motor is constant power devices.
2. As stepper motor speed increases, torque decreases. (most motor exhibits maximum
torque when stationary, however the torque of a motor when stationary 'holding torque'
defines the ability of the motor to maintain a desired position while under external load).
3. The torque curve may be extended by using current limiting drivers and increasing the
driving voltage (sometimes referred to as a 'chopper' circuit; there are several off the
shelf driver chips capable of doing this in a simple manner).
4. Steppers exhibits more vibration than other motor types, as the discrete step tends to
snap the rotor from one position to another (called a detent). The vibration makes
stepper motor noisier than DC motor.
5. This vibration can become very bad at some speeds and can cause the motor to lose
torque or lose direction. This is because the rotor is being held in a magnetic field which
behaves like a spring. On each step the rotor overshoots and bounces back and forth,
"ringing" at its resonant frequency. If the stepping frequency matches the resonant
frequency then the ringing increases and the motor comes out of synchronism, resulting
in positional error or a change in direction. At worst there is a total loss of control and
holding torque so the motor is easily overcome by the load and spins almost freely.
6. The effect can be mitigated by accelerating quickly through the problem speeds
range, physically damping (frictional damping) the system, or using a micro-stepping
driver.
7. Motor with a greater number of phases also exhibits smoother operation than those
with fewer phases (this can also be achieved through the use of a micro stepping drive)
Open-loop versus closed-loop commutation
Stepper is generally commutated open loop, i.e. the stepper driver has no feedback on
where the rotor actually is. Stepper motor systems must thus generally be over
engineered, especially if the load inertia is high, or there is widely varying load, so that
there is no possibility that the motor will lose steps. This has often caused the system
designer to consider the trade-offs between a closely sized but expensive
servomechanism system and an oversized but relatively cheap stepper.
A new development in stepper control is to incorporate a rotor position feedback (e.g. an
encoder or resolver), so that the commutation can be made optimal for torque generation
according to actual rotor position. This turns the stepper motor into a high pole count
brushless servo motor, with exceptional low speed torque and position resolution. An
advance on this technique is to normally run the motor in open loop mode, and only
enter closed loop mode if the rotor position error becomes too large — this will allow the
system to avoid hunting or oscillating, a common servo problem.
Stepper Motor Types
There are three main types of stepper motor:[1]
1. Permanent Magnet Stepper (can be subdivided in to 'tin-can' and 'hybrid', tin-can
being a cheaper product, and hybrid with higher quality bearings, smaller step angle,
higher power density)
2. Hybrid Synchronous Stepper
3. Variable Reluctance Stepper
4. Lavet type stepping motor
Permanent magnet motor uses a permanent magnet (PM) in the rotor and operate on the
attraction or repulsion between the rotor PM and the stator electromagnets. Variable
reluctance (VR) motors have a plain iron rotor and operate based on the principle that
minimum reluctance occurs with minimum gap, hence the rotor points are attracted
toward the stator magnet poles. Hybrid stepper motors are named because they use a
combination of PM and VR techniques to achieve maximum power in a small package
size.
Two-phase stepper motor
There are two basic winding arrangements for the electromagnetic coils in a two phase
stepper motor: bipolar and unipolar.
Unipolar motor
A unipolar stepper motor has two windings per phase, one for each direction of magnetic
field. Since in this arrangement a magnetic pole can be reversed without switching the
direction of current, the commutation circuit can be made very simple (e.g. a single
transistor) for each winding. Typically, given a phase, one end of each winding is made
common: giving three leads per phase and six leads for a typical two phase motor. Often,
these two phase commons are internally joined, so the motor has only five leads.
A microcontroller or stepper motor controller can be used to activate the drive transistors
in the right order, and this ease of operation makes unipolar motors popular with
hobbyists; they are probably the cheapest way to get precise angular movements.
Unipolar stepper motor coils
(For the experimenter, one way to distinguish common wire from a coil-end wire is by
measuring the resistance. Resistance between common wire and coil-end wire is always
half of what it is between coil-end and coil-end wires. This is because there is twice the
length of coil between the ends and only half from center (common wire) to the end.) A
quick way to determine if the stepper motor is working is to short circuit every two pairs
and try turning the shaft, whenever a higher than normal resistance is felt, it indicates
that the circuit to the particular winding is closed and that the phase is working.
Bipolar motor
Bipolar motor has a single winding per phase. The current in a winding needs to be
reversed in order to reverse a magnetic pole, so the driving circuit must be more
complicated, typically with an H-bridge arrangement (however there are several off the
shelf driver chips available to make this a simple affair). There are two leads per phase,
none are common.
Static friction effects using an H-bridge have been observed with certain drive topologies
[citation needed].
Because windings are better utilized, they are more powerful than a unipolar motor of the
same weight. This is due to the physical space occupied by the windings. A unipolar
motor has twice the amount of wire in the same space, but only half used at any point in
time, hence is 50% efficient (or approximately 70% of the torque output available).
Though bipolar is more complicated to drive, the abundance of driver chip means this is
much less difficult to achieve.
An 8-lead stepper is wound like a unipolar stepper, but the leads are not joined to
common internally to the motor. This kind of motor can be wired in several configurations:
* Unipolar.
* Bipolar with series windings. This gives higher inductance but lower current per winding.
* Bipolar with parallel windings. This requires higher current but can perform better as the
winding inductance is reduced.
* Bipolar with a single winding per phase. This method will run the motor on only half the
available windings, which will reduce the available low speed torque but require less
current.
Higher-phase count stepper motor
Multi-phase stepper motor with many phases tend to have much lower levels of vibration,
although the cost of manufacture is higher. The motor tends to be called 'hybrid' and
have more expensive machined parts, but also higher quality bearings. Though they are
more expensive, they do have a higher power density and with the appropriate drive
electronics are actually better suited to the application[citation needed], however price is
always an important factor. Computer printers may use hybrid designs.
Stepper motor drive circuits
Stepper motor performance is strongly dependent on the drive circuit. Torque curves
may be extended to greater speeds if the stator poles can be reversed more quickly, the
limiting factor being the winding inductance. To overcome the inductance and switch the
windings quickly, one must increase the drive voltage. This leads further to the necessity
of limiting the current that these high voltages may otherwise induce.
L/R drive circuits
L/R drive circuits are also referred to as constant voltage drives because a constant
positive or negative voltage is applied to each winding to set the step positions. However,
it is winding current, not voltage that applies torque to the stepper motor shaft. The
current I in each winding is related to the applied voltage V by the winding inductance L
and the winding resistance R. The resistance R determines the maximum current
according to Ohm's law I=V/R. The inductance L determines the maximum rate of change
of the current in the winding according to the formula for an Inductor dI/dt = V/L. Thus
when controlled by an L/R drive, the maximum speed of a stepper motor is limited by its
inductance since at some speed, the voltage U will be changing faster than the current I
can keep up. In simple terms the rate of change of current is L X R (e.g. a 10mH
inductance with 2 ohms resistance will take 5 ms to reach approx 2/3 of maximum torque
or around 0.1 sec to reach 99% of max torque). To obtain high torque at high speeds
requires a large drive voltage with a low resistance and low inductance. With an L/R drive
it is possible to control a low voltage resistive motor with a higher voltage drive simply by
adding an external resistor in series with each winding. This will waste power in the
resistors, and generate heat. It is therefore considered a low performing option, albeit
simple and cheap.
Chopper stepper drive circuits
Chopper stepper drive circuits are also referred to as constant current drives because
they generate a somewhat constant current in each winding rather than applying a
constant voltage. On each new step, a very high voltage is applied to the winding initially.
This causes the current in the winding to rise quickly since dI/dt = V/L where V is very
large. The current in each winding is monitored by the controller, usually by measuring
the voltage across a small sense resistor in series with each winding. When the current
exceeds a specified current limit, the voltage is turned off or "chopped", typically using
power transistors. When the winding current drops below the specified limit, the voltage is
turned on again. In this way, the current is held relatively constant for a particular step
position. This requires additional electronics to sense winding currents, and control the
switching, but it allows stepper motors to be driven with higher torque at higher speeds
than L/R drives. Integrated electronics for this purpose are widely available.
Phase current waveforms
A stepper motor is a polyphase AC synchronous motor (see Theory below), and it is
ideally driven by sinusoidal current. A full step waveform is a gross approximation of a
sinusoid, and is the reason why the motor exhibits so much vibration. Various drive
techniques have been developed to better approximate a sinusoidal drive waveform:
these are half stepping and microstepping.
Different drive modes showing coil current on a 4-phase unipolar stepper motor
Wave drive
In this drive method only a single phase is activated at a time. It has the same number of
steps as the full step drive, but the motor will have significantly less than rated torque. It
is rarely used.
Full step drive (two phases on)
This is the usual method for full step driving the motor. Two phases are always on. The
motor will have full rated torque.
Half stepping
When half stepping, the drive alternates between two phases on and a single phase on.
This increases the angular resolution, but the motor also has less torque (approx 70%)
at the half step position (where only a single phase is on). This may be mitigated by
increasing the current in the active winding to compensate. The advantage of half
stepping is that the drive electronics need not change to support it.
Microstepping
What is commonly referred to as microstepping is actually "sine cosine microstepping" in
which the winding current approximates a sinusoidal AC waveform. Sine cosine
microstepping is the most common form, but other waveforms are used [1]. Regardless of
the waveform used, as the microsteps become smaller, motor operation becomes more
smooth, thereby greatly reducing resonance in any parts the motor may be connected to,
as well as the motor itself. Resolution will be limited by the mechanical stiction, backlash,
and other sources of error between the motor and the end device. Gear reducers may
be used to increase resolution of positioning.
Step size repeatability is an important step motor feature and a fundamental reason for
their use in positioning.
Example: many modern hybrid step motors are rated such that the travel of every full
step (example 1.8 Degrees per full step or 200 full steps per revolution) will be within 3%
or 5% of the travel of every other full step; as long as the motor is operated within its
specified operating ranges. Several manufacturers show that their motors can easily
maintain the 3% or 5% equality of step travel size as step size is reduced from full
stepping down to 1/10 stepping. Then, as the microstepping divisor number grows, step
size repeatability degrades. At large step size reductions it is possible to issue many
microstep commands before any motion occurs at all and then the motion can be a
"jump" to a new position.
Stepper Motor theory
A step motor can be viewed as a synchronous AC motor with the number of poles (on
both rotor and stator) increased, taking care that they have no common denominator.
Additionally, soft magnetic material with many teeth on the rotor and stator cheaply
multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design,
having both permanent magnets and soft iron cores.
To achieve full rated torque, the coils in a stepper motor must reach their full rated
current during each step. Winding inductance and reverse EMF generated by a moving
rotor tend to resist changes in drive current, so that as the motor speeds up, less and
less time is spent at full current — thus reducing motor torque. As speeds further
increase, the current will not reach the rated value, and eventually the motor will cease to
produce torque.
Pull-in torque
This is the measure of the torque produced by a stepper motor when it is operated
without an acceleration state. At low speeds the stepper motor can synchronise itself with
an applied step frequency, and this pull-in torque must overcome friction and inertia. It is
important to make sure that the load on the motor is frictional rather than inertial as the
friction reduces any unwanted oscillations.
Pull-out torque
The stepper motor pull-out torque is measured by accelerating the motor to the desired
speed and then increasing the torque loading until the motor stalls or misses steps. This
measurement is taken across a wide range of speeds and the results are used to
generate the stepper motor's dynamic performance curve. As noted below this curve is
affected by drive voltage, drive current and current switching techniques. A designer may
include a safety factor between the rated torque and the estimated full load torque
required for the application.
Detent torque
Synchronous electric motors using permanent magnets have a remnant position holding
torque (called detent torque or cogging, and sometimes included in the specifications)
when not driven electrically. Soft iron reluctance cores do not exhibit this behavior.
Stepper motor ratings and specifications
Stepper motors nameplates typically give only the winding current and occasionally the
voltage and winding resistance. The rated voltage will produce the rated winding current
at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting
and the drive voltages greatly exceed the motor rated voltage.
A stepper's low speed torque will vary directly with current. How quickly the torque falls off
at faster speeds depends on the winding inductance and the drive circuitry it is attached
to, especially the driving voltage.
Stepper should be sized according to published torque curve, which is specified by the
manufacturer at particular drive voltages or using their own drive circuitry.
Applications
Computer-controlled stepper motor is one of the most versatile forms of positioning
systems. They are typically digitally controlled as part of an open loop system, and are
simpler and more rugged than closed loop servo systems.
Industrial applications are in high speed pick and place equipment and multi-axis
machine CNC machines often directly driving lead screws or ballscrews. In the field of
lasers and optics they are frequently used in precision positioning equipment such as
linear actuators, linear stages, rotation stages, goniometers, and mirror mounts. Other
uses are in packaging machinery, and positioning of valve pilot stages for fluid control
systems.
Commercially, stepper motor is used in floppy disk drives, flatbed scanners, computer
printers, plotters, slot machines, and many more devices.
Stepper motor operates differently from DC brush motors, which rotate when voltage is
applied to their terminals. Stepper motor, on the other hand, effectively has multiple
"toothed" electromagnets arranged around a central gear-shaped piece of iron. The
electromagnets are energized by an external control circuit, such as a microcontroller. To
make the motor shaft turn, first one electromagnet is given power, which makes the
gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth
are thus aligned to the first electromagnet, they are slightly offset from the next
electromagnet. So when the next electromagnet is turned on and the first is turned off,
the gear rotates slightly to align with the next one, and from there the process is
repeated. Each of those slight rotations is called a "step", with an integer number of
steps making a full rotation. In that way, the motor can be turned by a precise angle.
Stepper motor characteristics
1. Stepper motor is constant power devices.
2. As stepper motor speed increases, torque decreases. (most motor exhibits maximum
torque when stationary, however the torque of a motor when stationary 'holding torque'
defines the ability of the motor to maintain a desired position while under external load).
3. The torque curve may be extended by using current limiting drivers and increasing the
driving voltage (sometimes referred to as a 'chopper' circuit; there are several off the
shelf driver chips capable of doing this in a simple manner).
4. Steppers exhibits more vibration than other motor types, as the discrete step tends to
snap the rotor from one position to another (called a detent). The vibration makes
stepper motor noisier than DC motor.
5. This vibration can become very bad at some speeds and can cause the motor to lose
torque or lose direction. This is because the rotor is being held in a magnetic field which
behaves like a spring. On each step the rotor overshoots and bounces back and forth,
"ringing" at its resonant frequency. If the stepping frequency matches the resonant
frequency then the ringing increases and the motor comes out of synchronism, resulting
in positional error or a change in direction. At worst there is a total loss of control and
holding torque so the motor is easily overcome by the load and spins almost freely.
6. The effect can be mitigated by accelerating quickly through the problem speeds
range, physically damping (frictional damping) the system, or using a micro-stepping
driver.
7. Motor with a greater number of phases also exhibits smoother operation than those
with fewer phases (this can also be achieved through the use of a micro stepping drive)
Open-loop versus closed-loop commutation
Stepper is generally commutated open loop, i.e. the stepper driver has no feedback on
where the rotor actually is. Stepper motor systems must thus generally be over
engineered, especially if the load inertia is high, or there is widely varying load, so that
there is no possibility that the motor will lose steps. This has often caused the system
designer to consider the trade-offs between a closely sized but expensive
servomechanism system and an oversized but relatively cheap stepper.
A new development in stepper control is to incorporate a rotor position feedback (e.g. an
encoder or resolver), so that the commutation can be made optimal for torque generation
according to actual rotor position. This turns the stepper motor into a high pole count
brushless servo motor, with exceptional low speed torque and position resolution. An
advance on this technique is to normally run the motor in open loop mode, and only
enter closed loop mode if the rotor position error becomes too large — this will allow the
system to avoid hunting or oscillating, a common servo problem.
Stepper Motor Types
There are three main types of stepper motor:[1]
1. Permanent Magnet Stepper (can be subdivided in to 'tin-can' and 'hybrid', tin-can
being a cheaper product, and hybrid with higher quality bearings, smaller step angle,
higher power density)
2. Hybrid Synchronous Stepper
3. Variable Reluctance Stepper
4. Lavet type stepping motor
Permanent magnet motor uses a permanent magnet (PM) in the rotor and operate on the
attraction or repulsion between the rotor PM and the stator electromagnets. Variable
reluctance (VR) motors have a plain iron rotor and operate based on the principle that
minimum reluctance occurs with minimum gap, hence the rotor points are attracted
toward the stator magnet poles. Hybrid stepper motors are named because they use a
combination of PM and VR techniques to achieve maximum power in a small package
size.
Two-phase stepper motor
There are two basic winding arrangements for the electromagnetic coils in a two phase
stepper motor: bipolar and unipolar.
Unipolar motor
A unipolar stepper motor has two windings per phase, one for each direction of magnetic
field. Since in this arrangement a magnetic pole can be reversed without switching the
direction of current, the commutation circuit can be made very simple (e.g. a single
transistor) for each winding. Typically, given a phase, one end of each winding is made
common: giving three leads per phase and six leads for a typical two phase motor. Often,
these two phase commons are internally joined, so the motor has only five leads.
A microcontroller or stepper motor controller can be used to activate the drive transistors
in the right order, and this ease of operation makes unipolar motors popular with
hobbyists; they are probably the cheapest way to get precise angular movements.
Unipolar stepper motor coils
(For the experimenter, one way to distinguish common wire from a coil-end wire is by
measuring the resistance. Resistance between common wire and coil-end wire is always
half of what it is between coil-end and coil-end wires. This is because there is twice the
length of coil between the ends and only half from center (common wire) to the end.) A
quick way to determine if the stepper motor is working is to short circuit every two pairs
and try turning the shaft, whenever a higher than normal resistance is felt, it indicates
that the circuit to the particular winding is closed and that the phase is working.
Bipolar motor
Bipolar motor has a single winding per phase. The current in a winding needs to be
reversed in order to reverse a magnetic pole, so the driving circuit must be more
complicated, typically with an H-bridge arrangement (however there are several off the
shelf driver chips available to make this a simple affair). There are two leads per phase,
none are common.
Static friction effects using an H-bridge have been observed with certain drive topologies
[citation needed].
Because windings are better utilized, they are more powerful than a unipolar motor of the
same weight. This is due to the physical space occupied by the windings. A unipolar
motor has twice the amount of wire in the same space, but only half used at any point in
time, hence is 50% efficient (or approximately 70% of the torque output available).
Though bipolar is more complicated to drive, the abundance of driver chip means this is
much less difficult to achieve.
An 8-lead stepper is wound like a unipolar stepper, but the leads are not joined to
common internally to the motor. This kind of motor can be wired in several configurations:
* Unipolar.
* Bipolar with series windings. This gives higher inductance but lower current per winding.
* Bipolar with parallel windings. This requires higher current but can perform better as the
winding inductance is reduced.
* Bipolar with a single winding per phase. This method will run the motor on only half the
available windings, which will reduce the available low speed torque but require less
current.
Higher-phase count stepper motor
Multi-phase stepper motor with many phases tend to have much lower levels of vibration,
although the cost of manufacture is higher. The motor tends to be called 'hybrid' and
have more expensive machined parts, but also higher quality bearings. Though they are
more expensive, they do have a higher power density and with the appropriate drive
electronics are actually better suited to the application[citation needed], however price is
always an important factor. Computer printers may use hybrid designs.
Stepper motor drive circuits
Stepper motor performance is strongly dependent on the drive circuit. Torque curves
may be extended to greater speeds if the stator poles can be reversed more quickly, the
limiting factor being the winding inductance. To overcome the inductance and switch the
windings quickly, one must increase the drive voltage. This leads further to the necessity
of limiting the current that these high voltages may otherwise induce.
L/R drive circuits
L/R drive circuits are also referred to as constant voltage drives because a constant
positive or negative voltage is applied to each winding to set the step positions. However,
it is winding current, not voltage that applies torque to the stepper motor shaft. The
current I in each winding is related to the applied voltage V by the winding inductance L
and the winding resistance R. The resistance R determines the maximum current
according to Ohm's law I=V/R. The inductance L determines the maximum rate of change
of the current in the winding according to the formula for an Inductor dI/dt = V/L. Thus
when controlled by an L/R drive, the maximum speed of a stepper motor is limited by its
inductance since at some speed, the voltage U will be changing faster than the current I
can keep up. In simple terms the rate of change of current is L X R (e.g. a 10mH
inductance with 2 ohms resistance will take 5 ms to reach approx 2/3 of maximum torque
or around 0.1 sec to reach 99% of max torque). To obtain high torque at high speeds
requires a large drive voltage with a low resistance and low inductance. With an L/R drive
it is possible to control a low voltage resistive motor with a higher voltage drive simply by
adding an external resistor in series with each winding. This will waste power in the
resistors, and generate heat. It is therefore considered a low performing option, albeit
simple and cheap.
Chopper stepper drive circuits
Chopper stepper drive circuits are also referred to as constant current drives because
they generate a somewhat constant current in each winding rather than applying a
constant voltage. On each new step, a very high voltage is applied to the winding initially.
This causes the current in the winding to rise quickly since dI/dt = V/L where V is very
large. The current in each winding is monitored by the controller, usually by measuring
the voltage across a small sense resistor in series with each winding. When the current
exceeds a specified current limit, the voltage is turned off or "chopped", typically using
power transistors. When the winding current drops below the specified limit, the voltage is
turned on again. In this way, the current is held relatively constant for a particular step
position. This requires additional electronics to sense winding currents, and control the
switching, but it allows stepper motors to be driven with higher torque at higher speeds
than L/R drives. Integrated electronics for this purpose are widely available.
Phase current waveforms
A stepper motor is a polyphase AC synchronous motor (see Theory below), and it is
ideally driven by sinusoidal current. A full step waveform is a gross approximation of a
sinusoid, and is the reason why the motor exhibits so much vibration. Various drive
techniques have been developed to better approximate a sinusoidal drive waveform:
these are half stepping and microstepping.
Different drive modes showing coil current on a 4-phase unipolar stepper motor
Wave drive
In this drive method only a single phase is activated at a time. It has the same number of
steps as the full step drive, but the motor will have significantly less than rated torque. It
is rarely used.
Full step drive (two phases on)
This is the usual method for full step driving the motor. Two phases are always on. The
motor will have full rated torque.
Half stepping
When half stepping, the drive alternates between two phases on and a single phase on.
This increases the angular resolution, but the motor also has less torque (approx 70%)
at the half step position (where only a single phase is on). This may be mitigated by
increasing the current in the active winding to compensate. The advantage of half
stepping is that the drive electronics need not change to support it.
Microstepping
What is commonly referred to as microstepping is actually "sine cosine microstepping" in
which the winding current approximates a sinusoidal AC waveform. Sine cosine
microstepping is the most common form, but other waveforms are used [1]. Regardless of
the waveform used, as the microsteps become smaller, motor operation becomes more
smooth, thereby greatly reducing resonance in any parts the motor may be connected to,
as well as the motor itself. Resolution will be limited by the mechanical stiction, backlash,
and other sources of error between the motor and the end device. Gear reducers may
be used to increase resolution of positioning.
Step size repeatability is an important step motor feature and a fundamental reason for
their use in positioning.
Example: many modern hybrid step motors are rated such that the travel of every full
step (example 1.8 Degrees per full step or 200 full steps per revolution) will be within 3%
or 5% of the travel of every other full step; as long as the motor is operated within its
specified operating ranges. Several manufacturers show that their motors can easily
maintain the 3% or 5% equality of step travel size as step size is reduced from full
stepping down to 1/10 stepping. Then, as the microstepping divisor number grows, step
size repeatability degrades. At large step size reductions it is possible to issue many
microstep commands before any motion occurs at all and then the motion can be a
"jump" to a new position.
Stepper Motor theory
A step motor can be viewed as a synchronous AC motor with the number of poles (on
both rotor and stator) increased, taking care that they have no common denominator.
Additionally, soft magnetic material with many teeth on the rotor and stator cheaply
multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design,
having both permanent magnets and soft iron cores.
To achieve full rated torque, the coils in a stepper motor must reach their full rated
current during each step. Winding inductance and reverse EMF generated by a moving
rotor tend to resist changes in drive current, so that as the motor speeds up, less and
less time is spent at full current — thus reducing motor torque. As speeds further
increase, the current will not reach the rated value, and eventually the motor will cease to
produce torque.
Pull-in torque
This is the measure of the torque produced by a stepper motor when it is operated
without an acceleration state. At low speeds the stepper motor can synchronise itself with
an applied step frequency, and this pull-in torque must overcome friction and inertia. It is
important to make sure that the load on the motor is frictional rather than inertial as the
friction reduces any unwanted oscillations.
Pull-out torque
The stepper motor pull-out torque is measured by accelerating the motor to the desired
speed and then increasing the torque loading until the motor stalls or misses steps. This
measurement is taken across a wide range of speeds and the results are used to
generate the stepper motor's dynamic performance curve. As noted below this curve is
affected by drive voltage, drive current and current switching techniques. A designer may
include a safety factor between the rated torque and the estimated full load torque
required for the application.
Detent torque
Synchronous electric motors using permanent magnets have a remnant position holding
torque (called detent torque or cogging, and sometimes included in the specifications)
when not driven electrically. Soft iron reluctance cores do not exhibit this behavior.
Stepper motor ratings and specifications
Stepper motors nameplates typically give only the winding current and occasionally the
voltage and winding resistance. The rated voltage will produce the rated winding current
at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting
and the drive voltages greatly exceed the motor rated voltage.
A stepper's low speed torque will vary directly with current. How quickly the torque falls off
at faster speeds depends on the winding inductance and the drive circuitry it is attached
to, especially the driving voltage.
Stepper should be sized according to published torque curve, which is specified by the
manufacturer at particular drive voltages or using their own drive circuitry.
Applications
Computer-controlled stepper motor is one of the most versatile forms of positioning
systems. They are typically digitally controlled as part of an open loop system, and are
simpler and more rugged than closed loop servo systems.
Industrial applications are in high speed pick and place equipment and multi-axis
machine CNC machines often directly driving lead screws or ballscrews. In the field of
lasers and optics they are frequently used in precision positioning equipment such as
linear actuators, linear stages, rotation stages, goniometers, and mirror mounts. Other
uses are in packaging machinery, and positioning of valve pilot stages for fluid control
systems.
Commercially, stepper motor is used in floppy disk drives, flatbed scanners, computer
printers, plotters, slot machines, and many more devices.
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