A stepper motor is a brushless DC electric motor that divides a full rotation into a
number of equal steps. The motor's position can then be commanded to move and hold at
one of these steps without any feedback sensor, as long as the motor is carefully sized to
the application. Switched reluctance motors are very large
stepping motors with a reduced pole count, and generally are closed-loop commutated.
Fundamentals of operation
DC brushed motors rotate continuously when voltage is applied to their terminals. The
stepper motor is known by its important property to convert a train of input pulses into a
precisely defined increment in the shaft position. Each pulse moves the shaft through a fixed
angle. Stepper motors effectively have 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 magnetically attracts the gear's teeth. When the gear's teeth are 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 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. Types
There are four main types of stepper motors: Permanent magnet stepper
Hybrid synchronous stepper Variable reluctance stepper
Lavet type stepping motor Permanent magnet motors use a permanent magnet
in the rotor and operate on the attraction or repulsion between the rotor PM and the
stator electromagnets. Variable reluctance 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 motors
There are two basic winding arrangements for the electromagnetic coils in a two phase stepper
motor: bipolar and unipolar. Unipolar motors
A unipolar stepper motor has one winding with center tap per phase. Each section of windings
is switched on 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 for each winding. Typically, given a phase, the center tap 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 micro controller 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, the windings can be identified by touching the terminal wires
together in PM motors. If the terminals of a coil are connected, the shaft becomes harder
to turn. one way to distinguish the center tap 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 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 motors have 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. Static friction effects using an H-bridge
have been observed with certain drive topologies. Dithering the stepper signal at a higher frequency
than the motor can respond to will reduce this "static friction" effect.
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. Though a bipolar stepper motor is more complicated
to drive, the abundance of driver chips 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 motors
Multi-phase stepper motors with many phases tend to have much lower levels of vibration,
although the cost of manufacture is higher. These motors tend 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. Computer printers may use hybrid designs.
Stepper motor driver circuits
Stepper motor performance is strongly dependent on the driver 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 driver circuits L/R driver 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 / R. 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 drive circuits Chopper drive circuits are 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, 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.
Wave drive or Full step 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. The animated figure shown
above is a wave drive motor. In the animation, rotor has 25 teeth and it takes 4 steps to
rotate by one teeth position. So there will be 25*4 = 100 steps per full rotation and
each step will be 360/100 = 3.6 degrees. Full step drive
This is the usual method for full step driving the motor. Two phases are always on so the
motor will provide its maximum rated torque. As soon as one phase is turned off, another
one is turned on. Wave drive and single phase full step are both one and the same, with
same number of steps but difference in torque. Half stepping
When half stepping, the drive alternates between two phases on and a single phase on. This
increases the angular resolution. The motor also has less torque at the full step position.
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. In animated
figure shown above, if we change it to half stepping, then it will take 8 steps to rotate
by 1 teeth position. So there will be 25*8 = 200 steps per full rotation and each step
will be 360/200 = 1.8 degrees. Its angle per step is half of the full step.
Microstepping What is commonly referred to as microstepping
is often "sine cosine microstepping" in which the winding current approximates a sinusoidal
AC waveform. Sine cosine microstepping is the most common form, but other waveforms
can be used. 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
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.
Theory A step motor can be viewed as a synchronous
AC motor with the number of poles 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. 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 synchronize 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.
The pull-in curve defines an area called the start/stop region. Into this region, the motor
can be started/stopped instantaneously with a load applied and without loss of synchronism.
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 resonant position holding torque
when not driven electrically. Soft iron reluctance cores do not exhibit this behavior.
Ringing and resonance When the motor moves a single step it overshoots
the final resting point and oscillates round this point as it comes to rest. This undesirable
ringing is experienced as motor vibration and is more pronounced in unloaded motors.
An unloaded or under loaded motor may, and often will, stall if the vibration experienced
is enough to cause loss of synchronisation. Stepper motors have a natural frequency of
operation. When the excitation frequency matches this resonance the ringing is more pronounced,
steps may be missed, and stalling is more likely. Motor resonance frequency can be calculated
from the formula:
Mh Holding torque cN·m p Number of pole pairs
Jr Rotor inertia kg·cm² 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. Steppers should be sized according to published
torque curve, which is specified by the manufacturer at particular drive voltages or using their
own drive circuitry. Step motors adapted to harsh environments
are often referred to as IP65 rated. Applications
Computer controlled stepper motors are a type of motion-control positioning system. They
are typically digitally controlled as part of an open loop system for use in holding
or positioning applications. 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 motors are used in floppy
disk drives, flatbed scanners, computer printers, plotters, slot machines, image scanners, compact
disc drives, intelligent lighting, and camera lenses.
Stepper motor system A stepper motor system consists of three basic
elements, often combined with some type of user interface:
Indexers - The indexer is a microprocessor capable of generating step pulses and direction
signals for the driver. In addition, the indexer is typically required to perform many other
sophisticated command functions. Drivers - The driver converts the indexer
command signals into the power necessary to energize the motor windings. There are numerous
types of drivers, with different voltage and current ratings and construction technology.
Not all drivers are suitable to run all motors, so when designing a motion control system
the driver selection process is critical. Stepper motors - The stepper motor is an electromagnetic
device that converts digital pulses into mechanical shaft rotation. Advantages of step motors
are low cost, high reliability, high torque at low speeds and a simple, rugged construction
that operates in almost any environment. The main disadvantages in using a stepper motor
is the resonance effect often exhibited at low speeds and decreasing torque with increasing
speed. See also
References
External links Zaber Microstepping Tutorial. Retrieved on
2007-11-15. Stepper System Overview. Retrieved on 2012-3-01.
Animation of a stepping motor from Nanotec. Control of Stepping Motors - A Tutorial – Douglas
W. Jones, The University of Iowa Stepping 101