General Theory of Stepper Motors (Unipolar)

A stepper motor is very different. If you just feed it power, it will stay where it is. In order to make the motor move, you have to feed it a changing signal.

This is best illustrated by thinking of a magnetic compass with electromagnets around it:
The drawing on the left shows power applied to the N electromagnet, drawing the compass toward it.

On the right, power is instead applied to the E electromagnet, and the needle has rotated toward that side.

Just four electromagnets would give coarse, jumpy motion. Now imagine a similar arrangement with 100 electromagnets around the compass. By energizing each electromagnet in sequence, the needle takes 100 steps around the circle.

But driving 100 individual electromagnets would require complex electronics. So we fake it:
In these drawing, the circled letters represent electromagnets. All the magnets with the same letter are wired together. When you energize that circuit, all of the electromagnets turn on at once.

On the left, there are 8 magnets, but only 4 circuits. Sequencing through the four circuits gives half of a rotation. One more run through the sequence completes the rotation. This setup requires that both ends of the compass needle be north-seeking.

On the right, the same 4 circuits energize 16 magnets. This setup requires 16 steps (4 repetitions of a 4-step cycle) to complete one rotation.

In actual practice, just four control wires can provide just about as many steps as you might want. One of the characteristics of a given stepper motor is the number of steps necessary to make a complete circle, usually expressed as number of degrees per step.

Since computers are great at turning things on and off, let's look at the four electromagnet circuits as a computer might see them.

A	B	C	D	comment
1	0	0	0	take a step clockwise
0	1	0	0	another step clockwise
0	0	1	0	another step clockwise
0	0	0	1	another step clockwise
0	0	0	1	Wait right here!
0	0	1	0	Take a step back.

Here's an assortment of stepper motors.

What Good Is That?

• You can speed up or slow down the motor rotation by adjusting the step rate.
• You can reverse the direction of spin by energizing the coils in reverse sequence.
• Relative positioning is very precise: For a single step, you know exactly how much the shaft will turn.
• This precise relative positioning can be accomplished "open-loop". You don't need feedback to tell you where the position is right now.
• If you stop stepping and still apply power, the motor holds its position well.

There is a down-side if you need absolute positioning. By counting steps, the controlling electronics knows where the motor shaft is, since the counting began. But it doesn't necessarily know where the motor shaft was pointing when the machine was turned on and counting began. In applications where absolute positioning is important, the drive electronics must calibrate the motor position. This may be done by:

• driving the motor all the way in one direction until it encounters a mechanical "stop"
• driving the motor all the way in one direction until it triggers a "limit switch"
• using some form of feedback, such as the current track number on a disk

More Complex Drive

Half Steps

This is a form of "microstepping", and gives improved resolution. A sequence might look like this:

A	B	C	D	comment
1	0	0	0	start here
1	1	0	0	half a step clockwise
0	1	0	0	complete the first full step clockwise
0	1	1	0	another half step
0	0	1	0	complete that step
0	0	1	1	another half step
0	0	0	1	complete that step
1	0	0	1	final half step
1	0	0	0	back to the starting position

Microstepping

In fact, if you want smooth rotation, you can drive the magnets with carefully phased sine waves.

Bipolar Stepper Motors

This is a "bipolar" stepper:
Note that the electromagnets are hooked up in pairs, such that each of the magnets in the pair presents a different pole to the compass needle.

The compass needle aligns itself so that its magnetic field points in the same direction as the field from the coils.

We have turned off the N/S magnets and turned on W/E magnets.

Although magnets on both sides are turned on, the situation is not ambiguous. The needle turns in the only direction that aligns it with the electromagnets.

We have turned off the W/E magnets and turned back on N/S magnets. But this time we apply the power with reverse polarity.

The needle turns in the only direction that aligns it with the electromagnets.

Here's the driving sequence for a bipolar stepper motor:

N/S	W/E	comment
+	0	take a step clockwise
0	+	another step clockwise
-	0	another step clockwise
0	-	cycle complete
+	0	back to the starting position

There are good things and bad things about bipolar stepper motors.

• You can get more torque in a smaller package with bipolar because you are pulling from both ends at once.
• Bipolar steppers require fewer windings, so the motors can be smaller.
• For the same motor power dissipation, bipolar drive provides 40% more torque than unipolar.
• There are fewer electromagnet circuits to deal with.
• The electronics to drive each circuit are more complex, because they must change polarity as well as turning on and off. This is often accomplished using a circuit called an "H bridge".

Here's a tiny bipolar stepper motor. The actual stepper is in the middle of the photo. The part to the left is a lead-screw assembly.

Speed Issues

But there is a limit to the speed.

Think back to the magnetic compass. When you shut off one electromagnet and turn on the next in the sequence, the needle rotates to face the newly energized magnet. But you must wait for the needle to reach the new position before stepping to yet another position. If you step faster than the motor can respond, the driver and motor fall out of synchronization, and you can't be sure exactly where the motor is.

Please note that the maximum stepping rate of a motor depends on the load that it is turning. If your motor is turning a heavy mass, it will take longer to spin into the new position.

Acceleration Issues

The spinning motor has mass in motion, i.e. inertia. When you hook up the motor shaft to turn something, you end up with more mass and more inertia. Going from a full stop to the maximum step rate means all that mass wants to stay at rest. You have to use plenty of torque to get it moving. (Remember Newton's laws of motion.)

But if the motor is already spinning at 90% of its maximum rate, less torque is required to speed it up a little more.

Of course, the same problem exists when you are slowing down and/or reversing.

All this means is that stepper motors can only be accelerated at a given rate. Specifications for this should be available from the manufacturer. Your driving circuitry must take this acceleration into account, in order to keep the motor locked to the driver.

Where can I buy one?

Look in stores that cater to electronic experimenters, such as:

Further Information

NAME	WEB SITE	CONTENT
Douglas W. Jones	http://www.cs.uiowa.edu/~jones/step/index.html	tons of detailed technical information
Ian Harries	http://www.doc.ic.ac.uk/~ih/doc/stepper/	numerous assorted links
Jason Johnson	http://eio.com/jasstep.htm	assorted stepper info
Rustle Laidman	http://209.41.165.153/stepper/Tutorials/UniTutor.htm	assorted stepper info

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