General Motion Control

Stepper motor basics: stable shaft positions

Author: Bob Parente | June 15, 2011

Understanding how a stepper motor steps and the mechanism that causes it to stall will give us tremendous insight into the functionality of any standard stepper motor system as well as the new Hybrid Motion Technology™ which takes the stepper control functionality to the next level by eliminating stalls.

The typical two phase stepper moves 1.8 degrees per full step or 200 full steps in one revolution. The two phases (A & B) have two full current conditions. (There’s a one on sequence too, but we won’t get into that cycle) The current in the phase, for now, can be full-on in one direction or the other. Two phases with two current conditions (using high powered math 2 x 2 = 4) gives us a total of four stable shaft position combinations.

1st step position

Now let’s power our stepper up and assume that the current flow through the windings is as shown in Figure 1 for stable position #1.

Figure 1: Phase current at stable position 1 – Step 1

The motor’s shaft snaps to its stable holding position when power is applied and doesn’t move after that initial alignment.If you grab the shaft and try to rotate it, the energized motor will produce torque that tries to keep the shaft in that stable place.

Now change the current direction in phase B as shown in Figure 2 for stable position #2.

2nd step position

The motor’s shaft snaps to its stable holding position when power is applied and doesn’t move after that initial alignment. If you grab the shaft and try to rotate it, the energized motor will produce torque that tries to keep the shaft in that stable place.Now change the current direction in phase B as shown in Figure 2 for stable position #2.

Figure 2: Phase current at stable position 2 – Step 2

And the shaft moves 1.8 degrees for our discussion clockwise (CW).

3rd step position

Change the current direction in phase A as shown in Figure 3 for stable position #3:

Figure 3: Phase current at stable position 3 – Step 3

And the shaft moves another 1.8 degrees CW.

4th step position

Flip the current direction in phase B again as shown in Figure 4 for stable position #4

Figure 4: Phase current at stable position 4 – Step 4

The shaft moves CW 1.8 degrees again.

1st (5th) step position

One more current change to phase A produces the stable condition shown if Figure 5.

Figure 5: Phase current at stable position 5 (1) – Step 5 (1)

The shaft moves 1.8 degrees CW

Wait a minute; I thought we had only four stable current conditions?

Where did the fifth one come from? Well, as you can see in the step number column the 5th step’s current direction is the same as the first step’s. The difference is that fact that the shaft is now 7.2 degrees away from its starting point.

Repeating the four step sequence in this fashion over and over again produces motion in the CW direction. Increasing the step frequency increases the shaft’s rotational speed. Switching the phase current in a 4-3-2-1-4-3-2-1 sequence produces counter clockwise motion. Figure 6 illustrates this sequence in a different manner.

Pretty simple, huh? No wonder that positioning systems that use stepper motors are the most cost effective. Remember that the motor is brushless too. The only rotating parts are the rotor and the bearings. This contributes to making a stepper system very robust.

Another interesting observation is that since step #5 is the same as step #1, we could twist the shaft manually, overcome the holding torque and move the shaft from its zero degree position to its 7.2 degree position and let go of the shaft.

The shaft would now be happy (I’m really not sure how happy it is) to stay in that new stable position. Repeat this forcible movement and we find that we have 50 stable positions in one revolution (using our high powered math again, 360 degrees / 7.2 degrees per stable position = 50 stable positions) for that phase current combination. The motor doesn’t care which of the 50 stable positions it’s in, we may, but it doesn’t.

Stepping sequence visualized

Another view of the stepping sequence and current flow is shown in Figure 6.

Figure 6: Stepper motor step sequence showing stable positions

Summary

Remember that the motor is inductive, if you put a current probe on the winding it wouldn’t look as clean as one shown in Figure 6, This one looks like a pure resistor, but you get the idea, right?

Remember the 50 stable positions and the four step sequence, because we’ll expand on it with the next few postings.

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