About Stepper Motors

The most obvious difference between stepper motors and your typical DC motor is that it has more wires. You can find a minimum of 4 to a maximum of 8 wires on stepper motors while there are only two in DC motors. They are very useful in small robotic projects. You can find steppers from old 5.25 floppy disk drives. Larger, more powerful ones can be salvaged from copier machines.

There are two types of stepper motors: unipolar and bipolar. In a unipolar motor, the current flows only in one direction while in a bipolar one, it flows both ways. Chances are the motor you'll find is a unipolar, bipolar types are hard to find because they were used in older disk drives. For now I'll only discuss the unipolar motor. Why? A bipolar also requires a more complicated circuitry and besides it is HARD to find.

Steppers in floppy drives were used to move the drive head. Most floppy drives you'll find are from TEAC and they have five brown wire leads. Others are color coded but color-coded or not, they are still the same. Unipolars also come in 5, 6 and 8 wire versions. A 4-wire stepper is a bipolar and it is not covered here.

We'll just get down on how to move the motor and not bother its internal workings. If you do want to know more, then you can visit SGS-Thompson's website. They have very informative datasheets about stepper motors.

For now it is enough that you do know that unipolars are rated like this: it takes 200 steps to complete one revolution, it moves 1.8 degrees per step, it operates at 12 volts and consumes more or less 160 mA, there is 75 ohms per coil. I found this rating written on a sticker from a unipolar made by Applied Motion Inc. Most manufacturers dont usually include this and they dont even post it on the internet. If you do find something about their ratings, please let me know.

Fig. 1 (Stepper Motor Coils)

Five-wire Setup

First, we must determine which is common between the five wires. This common wire will be connected to +12v while the remaining 4 wires will be connected to a circuit that will sequentially switch the stepper motor on (more on this sequential switch later). The six-wire and eight-wire types are simplified to become a five-wire.

You will need an ohmmeter to measure the resistance between the coils. Look at Figure 2:

Fig. 2 (Measuring Resistance)

If you will place one probe of the ohmmeter at point D of Figure 2 and the other probe at point C, you will measure a resistance of 75 ohms. Holding the probe still at point D, place the other probe at point B, again there is 75 ohms. When you place the probes at points B and C you will measure a resistance of 150 ohms, meaning the resistances of the two coils were added. So it is point D which is common to both points B and C. Point A is the same as point D.

If you place probe one in pin 1 and probe two be placed one at a time to the remaining four wires, it will always give out 75 ohms. The wire at probe one is the common wire. In Figure 1, the common wire is pin 1. Whether you read the table vertical or horizontal, pin 1 gives 75 ohms.

Table 1 (Summary of Values for 5-wire)

Six-wire Setup

If you would look at Figure 1b, the coils were grouped by twos, all you have to do is to connect the two common leads and now you're back to five.

Table 2 (Summary of Values for 6-wire)

Notice that pins 1, 2, 3 are separated from 4, 5, and 6. So if you place the probe at either pins 1, 2 or 3 and the other probe at pins 4, 5 or 6, you get nothing. Now pin 1 and pin 4 both give 75 ohms so they will be joined together to become the common wire.

Eight-wire Setup

This is easy, you simply have to find both ends which will read 75 ohms. There will be no reading on your ohmmeter if you get the wrong one. On Figure 1c, each coil is independent of the others. Take the odd numbered or if you want to, the even numbered wires, connect them to form a common. You can even take any combination as long as only four wires and one end of each coil are shorted.

I'll include some photographs as soon as I get a replacement USB cable for my digicam.

Moving The Motor

What we are going to do here is to check the sequence of the four wires with respect to the common. Sequential meaning the motor steps in one direction only (clockwise or counter clockwise).

You will need a DC supply or even a 9-volt battery will do. First, connect the common wire to the [ + ] of the battery or supply. Next, connect one of the remaining four wires, which we would call 1, to the [ - ] of the battery (Fig. 3A) and observe the sligth movement or step of the motor. The movement is best felt with your hand than using your eyes, note its direction. It will only click once then you have to remove 1 and take another wire 2, connect it to the [ - ] again (Fig. 3B). Check the direction, if the movement is opposite of the first then the two wires are not in sequence. Do this to the remaining wires until you can move the motor in one direction.

The moving part of the motor is called armature. The red arrow is the armature of the motor. It will point to the coil that has current. Note that current moves the motor not voltage.

Also, stepper motors operate at a maximum frequency of 600 Hz. Any rate faster than that will do no good, the motor wont even move. This is an important area where most beginners do not bother to check. There is a minimum time delay that the current must stay at a coil. This time delay is computed as:

f = 1 / t or t = 1 / f

where:

f = frequency (in Hertz)

t = time (in seconds)

therefore:

t = 1 / 600 Hz

t = 0.00166~ Seconds or 1.67 milliseconds

The current must stay on the coil at a minimum of 1.67 milliseconds or else the current will not have enough time to excite the coil so the motor will not move. This will be the fastest rotation of the motor.

On Fig. 3A, the [ + ] of the battery or power supply is connected to common while the [ - ] is connected to wire 1. Therefore a closed-circuit is formed between the supply and the coil. Current passes through the coil and the armature is excited and points to coil 1.

On the following figures (3B, 3C, 3D), as you apply current one at a time on each coil, the armature follows the progression. This is called single coil excitation.

Fig. 3 (Full Stepping)

Fig. 3A (Connect [ - ] of battery to wire 1)

Fig. 3B ([ - ] of battery to wire 2)

Fig. 3C ([ - ] of battery to wire 3)

Fig. 3D ([ - ] of battery to wire 4)

Table 3 (Full Stepping)

On Table 3, a 0 means the wire is connected to [ - ] and the [ ^ ] means hanging or not connected. Note that we moved or stepped the motor six times in one direction. Steps 5 and 6 are identical to steps 1 and 2, you see a pattern here. If you start from wire 1 to wire 4 and then back again to 1, the motor moves is one direction. If you want the motor to move in the other direction, you simply start from wire 4 to wire 1 and back to 4 and so on.

To make the motor continuously rotate, you have to repeat the sequence of steps.

We made the motor step 1.8 degrees, equivalent to a full step, each time we applied 0V to a wire. Full stepping is the easiest way to move the motor.

There is another way of moving steppers which makes their movement more precise. The steps per revolution will be doubled and the step angle will be half of the full step.

Half Stepping

In half-stepping the motor you achieve smoother movements. This is done by applying current to two coils at the same time. The coils must be beside each other. On Fig. 4A, coils 1 & 2 are excited (wires 1 & 2 are shorted and connected to [ - ]). Instead of the armature pointing directly to a coil, the armature points to the space between the two coils. This is called two-coil excitation.

Fig. 4 (Half Stepping)

Fig. 4A (Connect [ - ] to wires 1 & 2)

Fig. 4B (Connect [ - ] to wires 2 & 3)

Fig. 4C (Connect [ - ] to wires 3 & 4)

Fig. 4D (Connect [ - ] to wires 4 & 1)

The wires that are shorted together must be side by side. You cannot skip, there must be no coils in between the two. If you connect wires 1 & 3 or 2 & 4, nothing happens. Connecting wires 1 & 4 as in Fig. 4D is possible because after 4 comes 1.

Table 4 summarizes the sequence for half stepping the motor. We added two coil excitation with single coil excitation to achieve half stepping.

Table 4 (Half Stepping)

Notice that steps 1, 3, 5, 7 are full steps and 2, 4, 6, 8 are added to make the armature stay in between the two coils. Step 9 is the same as step 1.

The degree per step becomes 0.9 degrees. and the steps per revolution becomes 400.

However, you will need twice the current when you use half stepping because unlike full stepping, half stepping uses two coils therefore effectively doubling the current needed. More current means more power.

If you want smaller degrees per step then use a microstepper.

Now you're ready to make the control circuitry for the motor. Read the next article about how to control stepper motors.

revised 07-04-2002

orig 04-25-2001