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A stepper motor is a motor that, as the name suggests, moves in steps. Stepping motors are known in German as Schrittmotoren, in French as moteurs pas à pas, and in Spanish as motor paso paso.

Introduction

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Cross Section of a Stepper Motor

A stepper motor is a motor that, as the name suggests, moves in steps. Stepping motors can be viewed as electric motors without commutators. Typically, all windings in the motor are part of the stator, and the rotor is either a permanent magnet or, in the case of variable reluctance motors, a toothed block of some magnetically soft material. All of the commutation must be handled externally by the motor controller, and typically, the motors and controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the other. Most steppers, as they are also known, can be stepped at audio frequencies, allowing them to spin quite quickly, and with an appropriate controller, they may be started and stopped "on a dime" at controlled orientations.

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Stator

Rotor

In other words, a stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied .

Fundamental of operations of a stepper motor

The stepper motor is known by its important property to convert a train of input pulses (typically square wave 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. 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 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. A more common method of stepping is a two-phase approach, which energized both phases, but only switches one polarity at a time. This forces the rotor to align itself between the "average" north and the "average" south magnetic poles. While a two-phase stator draws twice as much power, it delivers 41% more torque

Types of Stepper motors

There are three main types of stepper motors

• Permanent magnet stepper (can be subdivided into 'tin-can' and 'hybrid', tin-can being a cheaper product, and hybrid with higher quality bearings, smaller step angle, higher power density)
• Hybrid synchronous stepper
• Variable reluctance stepper

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.

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Microco stepping

Torque-Speed of stepper motor

$\mathbf {\omega } ={\frac {\psi (s/s)}{6}},(RPM)$ With $\mathbf {\omega ,\psi ,and(s/s)}$ motor speed (RPM) ,step angle in degrees, and number of steps per second respectively

The torque produced by a stepper motor depends on several factors:

• The step rate
• The drive current in the windings
• The drive design or type

In a stepper motor a torque is developed when the magnetic fluxes of the rotor and stator are displaced from each other. The stator is made up of a high permeability magnetic material. The presence of this high permeability material causes the magnetic flux to be confined for the most part to the paths defined by the stator structure in the same fashion that currents are confined to the conductors of an electronic circuit. This serves to concentrate the flux at the stator poles. The torque output produced by the motor is proportional to the intensity of the magnetic flux generated when the winding is energized.

The general formula of torque acting on an object is shown as follow: $\mathbf {T} ={FR},$ Where F is the force in linear direction action on the object and R is radius of the object being rotated.

The electromagnetic torque is the maximum toque, Tg, that can be produced par a stepper or DC motor:

$\mathbf {T_{g}} ={\frac {PZ\phi I_{a}}{2\pi A}}$ Where, P is the number of poles, φ is flux per pole, Z is number of conductors, Ia is the armature current, and A is number of parallel paths.

This torque equation of dc motor can be further simplified as: $\mathbf {T_{g}} ={K_{a}\phi I_{a}}$, where $\mathbf {K_{a}} ={\frac {PZ}{2\pi A}}$, Which is constant for a particular machine and therefore the torque of dc motor varies with only flux φ and armature current Ia.

The torque we so obtain, is known as the electromagnetic torque of dc motor, and subtracting the mechanical and rotational losses from it we get the mechanical torque. Therefore, $\mathbf {T_{m}} ={T_{g}-mechanical,losses}$

The stepper motor and all other dc motor as we all know is a rotational machine, and torque is a very important parameter in this concern, and it’s of utmost importance to understand the torque equation of dc motor for establishing its running characteristics.

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Circuit diagram of a DC motor

Torque-Speed Curve

It is very important to know how to read a torque/speed curve because it describes what a stepper motor can and cannot do. It is also important to keep in mind that a torque/speed curve is for a given motor and a given driver. Torque is dependent on the driver type and voltage. The same motor can have a very different torque/speed curve when used with a different driver. The torque/speed curves in this catalog are given for reference only. The same motor with a similar drive, similar voltage and similar current should give similar performance. Torque/speed charts can also be used to roughly estimate the torque produced using different drivers at varying voltages and currents

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Torque-speed curve

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Torque-speed relationship curve

Torque/speed curves have torque on the Y-axis, measured in N-m (in this catalog), and speed on the X-axis, measured in PPS (pulses per seconds) or Hz.

Holding Torque

Amount of torque that the motor produces when it has rated current flowing through the windings but the motor is at rest.

Detent Torque

Amount of torque that the motor produces when it is not energized. No current is flowing through the windings.

Pull-in Torque Curve

Shows the maximum value of torque at given speeds that the motor can start, stop or reverse in synchronism with the input pulses. The motor cannot start at a speed that is beyond this curve. It also cannot instantly reverse or stop with any accuracy at a point beyond this curve.

Stop / Start Region

Area on and underneath the pull-in curve. For any load value in this region, the motor can start, stop, or reverse “instantly” (no ramping required) at the corresponding speed value.

Pull-out Torque Curve

Shows the maximum value of torque at given speeds that the motor can generate while running in synchronism. If the motor is run outside of this curve, it will stall.

Slew Range

The area between the pull-in and the pull-out curves, where to maintain synchronism, the motor speed must be ramped (adjusted gradually)

Applications

Computer controlled stepper motors are a type of motion-control 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 actuator, stages, |rotation stage, , 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, image scanners, printers, plotter, slot machine, image scanner, disc drives, intelligent lighting, camera lenses, machines and, more recently, in 3D printing.

Advantages

• Low cost for control achieved
• Ruggedness
• Simplicity of construction
• Can operate in an open loop control system
• Low maintenance
• Less likely to stall or slip
• Will work in any environment

Desadvantages

• Require a dedicated control circuit
• Use more current than D.C. motors
• High torque output achieved at low speeds

External links

• http://www.engineering.com/ProductShowcase/StepperMotors.aspx