This is information on the basics of stepping motors and step motion control. The following will give you a brief but general understanding of the operation of stepping motors.

Stepping motors and Stepping motion - Introduction

A system of stepping motor is an electromechanical rotary actuator that converts electrical impulses in the single-axis rotations. This rotation is directly related to the number of pulses.

The speed is synchronous with the frequency of the pulses.

The result is the absolute speed and position. The stepping motors feature bi-directional control, built-in braking, variable torque, power control, precision accuracy, high resolution, the open-loop control, and direct interface to digital systems.

Compared to other servo systems, stepping motors exhibit an excellent power to weight ratio, minimum rotor inertia, no drift and no cumulative errors. Note: The following descriptions begin from the motor and the progress of electronic control.

Stepping Motors - Overview

A stepping motor converts electrical energy in discrete steps or movements. The motor consists of several pairs wrapped in electrical coils (phases) around the outer stationary (stator).

The inner portion (rotor) consists of iron or magnetic disks mounted on an motor shaft and suspension bearings. The rotor has projecting teeth which are aligned with the magnetic fields of the coils. When the coils are energized in the dc sequence, teeth will follow the order to turn a discreet distance to re-align with the magnetic field.

The number of coil combinations (phases) and the number of teeth to determine the number of steps (resolution) of the motor. For example, a step 200 SPR(Step per rev) of the motor has 50 rotor teeth for 4 combinations of the coil to equal 200 SPR.
No brushes between the rotor and stator, a stepper motor is a multipolar (polyphase) brushless DC motor. These multiple coil pairs can be connected either positive or negative result only four full steps. When the coils are winded correctly, the motor rotates forward. When the sequence is reversed, the motor rotates in reverse.

The amount of torque needed to force the rotor position is the holding or static torque. If the rotor slip (loss of step), which is aligned with the combination of coil available: four steps forward or four steps backward.

The amount of torque required to force the rotor from position is the holding or static torque. If the rotor slips (step loss), it will align with the next available coil combination; either four steps forward or four steps backwards.

Stepping motors can be stalled or held indefinitely without damage

If the frequency is faster than the rotor can be moved, the rotor will slide until the frequency is slowed enough for the rotor to lock back into the frequency. The rotor requires a minimum settling time (ringing) to stop when held. This limits the minimum time for the motor to change direction successfully.

PM stepping motors are deposited faster than hybrid stepping motor. If the frequency of the sequence (pass rate) is close to the natural frequency of the coils, the motor will try to resonate in sub-multiples of this period, resulting in the loss of step and abnormal noises (growling).

The point of low frequency resonance of a typical motor is 100 full steps per second or slower, mid-frequency point is between 900 to 1200 SPR. Resonant behavior (phenomena of electro-mechanical feedback) can be minimized by reducing the current (gain reduction), isolating the mechanical coupling (splitting), which reduces the pitch a half step or micro, not the motor operating continuously in resonance bands (ramp)

Electrical types of stepping motors VR, PM, Hybrid

VR (variable reluctance) stepping motor have soft iron rotors with teeth and are used primarily for special applications. Permanent magnet motors have rotors of magnet with no teeth. Hybrid stepping motors are a hybrid of both VR and PM and have magnetic rotors with teeth. Nowdays, the hybrid stepping motor has more widely use than VR and PM motor.

Mechanical types of stepping motors - Pressed Case (PC) and mechanized case

(MC) Pressed case (tin can) motors are sealed, ball-bearing deposits mated. machine other cases have cast aluminum end bells with audiences of the ball, the bodies are stacks of plates attached with screws. A PC is usually the type of permanent magnet and has a 7.5 or 15 degree angle step.

MC motors are typically 1.8, 0.9 or 0.45 degrees and have a positional accuracy of 3 to 5% also the air space (the space between the motor and the stator) is narrower and therefore produces more torque. PC motors because of their fine cases are more limited in the amount of heat they can handle. The torque of a stepper is a function of magnetic field (force guassian) produced by the DC flowing in the coils. The subsequent heating of the coils of the limits of the motor housing with a power that can dissipate the case before the insulation is damaged (temperature increase).

Motor Case Sizes - Size 8, 11, 14, 16, 17, 23, 24, 34, 42 NEMA standards exist for Front View MC case and the holes in the mounting flange. The most common are;

size 8 (0.8 sq. or 20 mm)

size 11 (1.1 sq. or 28 mm)

size 14 (1.4 sq. or 35 mm)

size 16 (1.6 sq. or 39 mm)

size 17 (1.7 sq. or 42 mm)

size 23 (2.3" sq. or 57mm)

size 23 (2.3" dia. or 57mm)

size 24 (2.4" dia. or 60mm)

size 34 (3.4" sq. or 86 mm)

size 34 (3.4" dia. or 86 mm)

and size 42 (4.2" dia. or 110 mm).

Stepping motor windings -

Phase 2, 4-Phase and 5 Phase

The common stepping motors have two sets of coils. Each can be powered either positive or negative, so the minimum number of connections (cables) is four. Four wire motors are 2-phase (bipolar).

For electrical convenience (L / R driver) each coil is center tapped into two coils (bifilar winding). The result is a cable of six or 4.1 motor phase (unipolar). Unipolar can also have the center taps together creating a 5-wire motor. An 8-wire, the most versatile configuration has 8 wires available. 5-phase stepping motors with 5 coils (10 cables) and require a 5-phase stepping driver.

Electronic of stepping motor driver -

Unipolar and Bipolar
A stepping motor requires an electrical sequencer called a driver, a specialized type of DC power supply. If the driver can reverse the polarity of its outputs, which is bipolar (4 wires). Simple, less expensive (but less effective and efficient) are the drivers that can not reverse the polarity, called unipolar (6 cables). Bipolar can handle 4, 5, 6 or 8-lead motors.

To connect correctly a 5-wire in a bipolar drive, the center tap must be connected to the motor. To convert bipolar 6-wire to bipolar only the center tap and one leg are connected. Half of each coil is not used. 8-wire motors are connected as 6-wire in unipolar and bipolar wires in parallel for bipolar in order to which has more torque and efficiency. Bipolar use 8 transistors two arrays H-bridge, unipolar use 4.

Part two