A4979 datasheet(37/44 Pages) ALLEGRO

37 / 44 page

Microstepping Programmable Stepper Motor Driver

With Stall Detect and Short Circuit Protection

A4979

A-3

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Figure A2. Half step operation

Starting at the top, panel (a) in figure A1, the current is flowing

down through the phase A winding from top to bottom and there

is no current in phase B. The result is an N magnetic pole on the

A electromagnets and an S pole on the A-bar electromagnets. The

rotor position is such that that the poles of the permanent magnets

align with the poles of the electromagnets, N to S.

In the next panel, panel (b), the current is flowing down through

the phase B winding from top to bottom and there is no current

in phase A. The result is an N pole on the B electromagnets and

an S pole on the B-bar electromagnets. These magnetic poles will

attract and repel the permanent magnets on the rotor producing a

force that moves the rotor from left to right in the diagram until

the poles of the permanent magnets again align with the poles of

the electromagnets.

In panel (c), the current is flowing up through the phase A wind-

ing from bottom to top and there is no current in phase B. This

reverses the pole orientation from the top panel, such that there

is an S pole on the A electromagnets and an N pole on the A-bar

electromagnets. As before, these magnetic poles will attract and

repel the permanent magnets on the rotor producing a force that

moves the rotor from left to right in the diagram, until poles of

the permanent magnets again align with the poles of the electro-

magnets.

The bottom panel, panel (d), shows the final combination with

current flowing up through the phase B winding from bottom to

top and there is no current in phase A. This produces an N pole

on the B electromagnets and a S pole on the B-bar electromag-

nets. As before, these magnetic poles will attract and repel the

permanent magnets on the rotor producing a force that moves the

rotor from left to right until poles of the permanent magnets again

align with the poles of the electromagnets.

Each of the four steps in figure A1 represents a single full

mechanical step of the stepper motor. The four steps together

represent a single electrical cycle.

The step resolution depends entirely on the mechanical construc-

tion of the motor and typically there will be 200 or more full

steps per mechanical revolution of the motor. A 200-step motor

will provide a resolution of 360 / 200 = 1.8° of rotation per step.

Stepping in the opposite direction to that described above is sim-

ply a case of changing the step sequence or inverting one of the

phase current directions.

Microstepping

In many applications it is necessary to improve the resolution of

the stepper motor, for more precise positioning control, or simply

to increase the number of steps per revolution to reduce the

torque ripple and therefore the vibration and noise of the motor.

Fortunately this can be achieved by driving both phases at the

same time in order to move the rotor to a position between two

electromagnets. This is known generically as microstepping.

Figure A2 shows the basic principle of microstepping. Panels (a)

and (c) of figure A2 correspond to panels (a) and (b) of figure

A1. Panel (b) shows each phase energized such that there are now

Stator

Rotor

Stator

Rotor

Stator

Rotor

(a) Same as

figure A1(a)

(b) Half-step

position

figure A1(b)

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