A stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps.
See also Electric Motors
A stepper motor is a brushless, synchronous electric motor
that can divide a full rotation into a large number of steps. When
commutated electronically, the motor's position can be controlled
precisely, without any feedback mechanism (see open loop control).
A stepper motor's design is virtually identical to that of a
low-speed synchronous AC motor. In that application, the motor is
driven with two phase AC, one phase usually derived through a phase
shifting capacitor. Another similar motor is the switched reluctance motor, which is a very large stepping motor with a reduced pole count, and generally closed-loop commutated.
Stepper motor characteristics
Stepper motors are constant-power devices (power = velocity x
torque). As motor speed increases, torque decreases. The torque curve
may be extended by using current limiting drivers and increasing the
Steppers exhibit more vibration than other motor types, as the
discrete step tends to snap the rotor from one position to another.
This vibration can become very bad at some speeds and can cause the
motor to lose torque. The effect can be mitigated by accelerating
quickly through the problem speed range, physically dampening the
system, or using a micro-stepping driver. Motors with greater number of
phases also exhibit smoother operation than those with fewer phases.
Fundamentals of operation
Stepper motors operate much differently from normal DC motors, which
rotate when voltage is applied to their terminals. Stepper motors, on
the other hand, effectively have multiple "toothed" electromagnets
arranged around a central metal gear. 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." In that way, the motor can be turned a
precise angle. There are two basic arrangements for the electromagnetic
coils: bipolar and unipolar.
Open-loop versus closed-loop commutation
Steppers are generally commutated open loop, ie. the 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 servo system and an oversized but relatively cheap stepper.
A new development in stepper control is to incorporate a rotor
position feedback (eg. 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.
Two-phase stepper motors
There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.
A unipolar stepper motor has logically two windings per phase, one
for each direction of current. Since in this arrangement a magnetic
pole can be reversed without switching the direction of current, the
commutation circuit can be made very simple (eg. 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.
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.
(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 due to the fact that there is actually
twice the length of coil between the ends and only half from center
(common wire) to the end.)
A six lead unipolar motor may be driven by a bipolar driver. In this
case, one of the windings on each phase is wasted as it never carries
Bipolar motors have logically 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. There are two leads per phase, none are common.
Because windings are better utilised, they are more powerful than a unipolar motor of the same weight.
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:
- 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 motors
- Japan Servo three phase steppers.
- Oriental Motor five phase steppers.
- Sanyo Denki two phase steppers.
- Sanyo Denki three phase steppers.
- Sanyo Denki five phase steppers.
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
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 it's inductance since at some speed, the
voltage V will be changing faster than the current I can keep up.
With an L/R drive it is possible to control a low voltage 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
Chopper drive circuits
Chopper 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.
Full step drive (two phases on)
This is the usual method for full step driving the motor. Both phases are always on. The motor will have full rated torque.
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.
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 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.
What is commonly referred to as microstepping is actual "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.
Regardless of the waveform used, as the microsteps become smaller,
motor operation becomes more smooth. However, the purpose of
microstepping is not usually to achieve smoothness of motion, but to
achieve higher position resolution. A microstep driver may split a full
step into as many as 256 microsteps. A typical motor may have 200 steps
per revolution. Using such a motor with a 256 microstep controller
(also referred to as a "divide by 256" controller) results in an
angular resolution of 360°/200/256 = 0.00703125° or 51200 discrete
positions per revolution. However, it should be noted that such fine
resolution is rarely achievable in practice, regardless of the
controller, due to mechanical stiction and other sources of error between the specified and actual positions.
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
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.
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 "pulls Out of synchronism" with the step frequency.
This measurement is taken across a wide range of speeds and the results
are used to generate the stepper motors dynamic performance curve. As
noted below this curve is affected by drive voltage, drive current and
current switching techniques. It is normally recommended to use a
safety factor of between 50% and 100% when comparing your desired
torque output to the published "pull-Out" torque performance curve of a
Synchronous electric motors using permanent magnets have a remnant position holding torque (called detent torque,
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
Steppers should be sized according to published torque curve, which
is specified by the manufacturer at particular drive voltages and/or
using their own drive circuitry. It is not guaranteed that you will
achieve the same performance given different drive circuitry, so the
pair should be chosen with great care.
Computer-controlled stepper motors are 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, in floppy disk drives, flatbed scanners, printers, plotters and many more devices.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)