Make Electronics

Experiment 33: Moving in Steps

288 Chapter 5

theory

Inside a stepper motor

If you check the Wikipedia entry for stepper motors, you

may see a very nice 3D rendering showing a toothed rotor

and four coils arrayed around it. Maybe stepper motors

used to be manufactured like this once upon a time, but not

anymore.

Imagine two horizontal rows of coils. In the space between

them is a series of little magnets, like a freight train, that

can move left or right, as shown in Figures 5-111 and

5-112. Each coil has two windings, in opposite directions,

so that current through one winding will create an upward

magnetic force while current through the other will create

a downward force. Each row of windings is connected in

parallel, so that they switch on and off simultaneously.

In Step 1, the negative connection energizes the upper

windings of the upper coils, which creates an upward mag-

netic force. I’ve shown this force using blue-green arrows so

that you won’t mistake it for a flow of electricity. It so hap-

pens that this force attracts the north poles of the magnets

and repels the south poles, so if the magnets begin in the

position shown in Step 1, they will want to move one step

to the right.

This brings them to the position shown in Step 2. Now the

upper windings of the lower coils are energized, and again,

this produces an upward force, which again attracts the

north poles and repels the south poles.

This advances the magnets to their location in Step 3.

Now the lower windings of the upper coils are energized,

producing a downward force. This repels the north poles of

the magnets and attracts their south poles. So the magnets

keep moving.

They reach the position shown in Step 4. The lower wind-

ings of the lower coils are energized, producing a downward

force which continues to attract the south poles while

repelling the north poles. So the magnets move a final step

to the right—which leaves them in the same orientation

shown in Step 1. And the process can repeat all over again.

S N S N S N S N

S N S N S N S N

Step 1

Step 2

Figure 5-111. This sequence shows the first two steps as the rotor

of a stepper motor (shown as a series of north-south magnets)

moves in response to pulses through electromagnets.

S N S N S N S N

S N S N S N S N

Step 3

Step 4

Figure 5-112. After taking another two steps, the motor will be

back where it started at Step 1 in Figure 5-111.