Stepper motor


A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that rotates in a series of small and discrete angular steps. Stepper motors can be set to any given step position without needing a position sensor for feedback. The step position can be rapidly increased or decreased to create continuous rotation, or the motor can be ordered to actively hold its position at one given step. Motors vary in size, speed, step resolution, and torque.
Switched reluctance motors are very large stepping motors with a reduced pole count. They generally employ closed-loop commutators.

Mechanism

s rotate continuously when DC voltage is applied to their terminals. The stepper motor is known for its property of converting a train of input pulses into a precisely defined increment in the shaft’s rotational position. Each pulse rotates the shaft through a fixed angle.
Stepper motors effectively have multiple "toothed" electromagnets arranged as a stator around a central rotor, a gear-shaped piece of iron. The electromagnets are energized by an external driver circuit or a micro controller. To make the motor shaft turn, one electromagnet is first 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. This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one. From there the process is repeated. Each of the partial 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.
The circular arrangement of electromagnets is divided into groups, each group called a phase, and there is an equal number of electromagnets per group. The number of groups is chosen by the designer of the stepper motor. The electromagnets of each group are interleaved with the electromagnets of other groups to form a uniform pattern of arrangement. For example, if the stepper motor has two groups identified as A or B, and ten electromagnets in total, then the grouping pattern would be ABABABABAB.
Electromagnets within the same group are all energized together. Because of this, stepper motors with more phases typically have more wires to control the motor.

Types

There are three main types of stepper motors: permanent magnet, variable reluctance, and hybrid synchronous.
Permanent magnet motors use a permanent magnet in the rotor and operate on the attraction or repulsion between the rotor magnet and the stator electromagnets. Pulses move the rotor clockwise or anticlockwise in discrete steps. If left powered at a final step, a strong detent remains at that shaft location. This detent has a predictable spring rate and specified torque limit; slippage occurs if the limit is exceeded. If current is removed, a lesser detent still remains, holding shaft position against spring or other torque influences. Stepping can then be resumed while reliably being synchronized with control electronics.
Permanent magnet stepper motors have simple DC switching electronics, a power-off detent, and no position readout. These qualities are ideal for applications such as paper printers, 3D printers, and robotics. Such applications track position simply by counting the number of steps that each motor has been instructed to take.
Variable reluctance motors have a soft iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap, so the rotor points are attracted toward the stator's magnetic poles. Variable reluctance motors have detents when powered on, but not when powered off.
Hybrid synchronous motors are a combination of the permanent magnet and variable reluctance types, to maximize power in a small size.

Phases

Two phase

There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.Image:Unipolar-stepper-motor-windings.png|thumb|right|Unipolar stepper motor coils

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 polarity of the common wire, the commutation circuit can be simply a single switching transistor for each half winding. Typically, given a phase, the center tap of each winding is made common: 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.
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 the resistance between 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 motors

Bipolar motors have a pair of single winding connections 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 is common.
A typical driving pattern for a two coil bipolar stepper motor would be: A+ B+ A− B−. I.e. drive coil A with positive current, then remove current from coil A; then drive coil B with positive current, then remove current from coil B; then drive coil A with negative current, then remove current from coil A; then drive coil B with negative current ; the cycle is complete and begins anew.
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 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

Multi-phase stepper motors with many phases tend to have much lower levels of vibration. While they are more expensive, they do have a higher power density and with the appropriate drive electronics are often better suited to the application.

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 a combination of 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.
An additional limitation, often comparable to the effects of inductance, is the back-EMF of the motor. As the motor's rotor turns, a sinusoidal voltage is generated proportional to the speed. This AC voltage is subtracted from the voltage waveform available to induce a change in the current.

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. The resulting current for a voltage pulse is a quickly increasing current as a function of inductance. This reaches the V/R value and holds for the remainder of the pulse. Thus when controlled by a constant voltage 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.
Modern voltage-mode drivers overcome some of these limitations by approximating a sinusoidal voltage waveform to the motor phases. The amplitude of the voltage waveform is set up to increase with step rate. If properly tuned, this compensates the effects of inductance and back-EMF, allowing decent performance relative to current-mode drivers, but at the expense of design effort that are simpler for current-mode drivers.