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A stepper motor (or step motor) is a brushless, synchronous electric motor that
can divide a full rotation into a large number of steps. The motor's position can
be controlled precisely without any feedback mechanism (see Open-loop
controller), as long as the motor is carefully sized to the application. Stepper
motors are similar to switched reluctance motors (which are very large stepping
motors with a reduced pole count, and generally are closed-loop commutated.)
Fundamentals of operation
Stepper motors operate differently from DC brush 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
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.
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Stepper motor characteristics
1. Stepper motors are constant power devices.
2. As motor speed increases, torque decreases. (most motors exhibit 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 exhibit 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 motors noisier than DC motors.
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. Motors with a greater number of phases also exhibit 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
Steppers are generally commutated open loop, i.e. 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 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.
Types
There are three main types of stepper motors:[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 motors use 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 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 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 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 (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 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[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 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.
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.
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.
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.
Applications
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, stepper motors are used in floppy disk drives, flatbed scanners,
computer printers, plotters, slot machines, and many more devices.
