In the previous section mention was made of how a stepper motor can be used in an open loop mode. What this implies is that you tell the motor to do something, like rotate a set distance, and it does it with out any need for feedback. It is the subject of this section to explore just how this is done.
The first thing that you notice about a stepper motor is that it has more than two wires leading into it. In fact various versions have 4, 5, 6 and sometimes more. Also when you manually rotate the shaft you will experience a cogging sensation as if the bearings in the motor are faulty.
In the simplest sense the interior of a stepper motor looks like Fig 22
The rotor in this case is a bar magnet that pivots about its center. You see two loops of wire, each loop forming its own electromagnet and each end having a different polarity.
If we apply a voltage such that pole piece A is South and B is North (it must be because of the way they are wound) the rotor magnet will line up as shown. (remember our earlier theory about unlike poles attracting each other. This theory is very important in understanding stepper motor operation).
You see not only why the rotor lined up in this position but that it will stay in this position as long as there is voltage applied to the coil. This holding position will stay as long as there is not unreasonable force applied against it and the voltage is sufficient to provide a large enough current through the coil and consequent magnetic attraction.
Now to get the rotor to turn. If we remove the voltage from the second loop and apply it to the fi st loop, pole pieces A and B will have no magnetic attraction and pole pieces C and D will have. (Fig 23).
We will assume that pole piece C will go South and D will go North. You can see how the magnet will take up a new position and be rotated 90 degrees clock wise.
This is a very crude movement but at least it has moved and will now hold this position. To obtain further clockwise movement we remove the voltage from the first coil and reapply it to the second coil but this time in the reverse direction such that pole piece A is North and D is South (Fig 24).
The magnet rotates a further 90 degrees. To get it to move again we remove the voltage from the second coil and reapply it to the first, again in a reverse direction to when it was originally applied (Fig 25)
This time pole piece C will be North and D will be South. Again another 90 degree movement. To arrive back at where we started we remove the voltage from the first coil and reapply it the second coil in a direction such that pole piece A is South and D is North (Fig 22)
To obtain another rotation we repeat the sequence. This has given us a clockwise rotation. How do we get it to rotate in the opposite direction? Simple. We remove the voltage from the second loop and reapply it to the first but in this case in the opposite direction such that pole piece C is North and D is South. Fig 26 shows the anti-clockwise sequence.
You can see that we have set up a rotating sequence in the electromagnet that the rotating rotor magnet follows around. We have to be careful that the load attached to the shaft is not so high as to not allow the rotor to move. This could be because the load is too high or that the voltage applied to the coils is not strong enough. If this happens there will be a tendency for the shaft to just move back and forwards and the controller that is providing the driving sequence will lose positional information i.e. will think that the rotor is in a position other than where it actually is.
The rate at which the driving sequence is applied to the motor is also critical, particularly when accelerating and decelerating. There is a certain amount of rotor and load inertia that needs to be overcome and it is important that the rotor not be accelerated too quickly. Decelerating is a reversal of this.
This is known as a full step and there are four steps per revolution. If we were to go to a half step sequence we could obtain twice as many steps per revolution of 45 degree steps.
How this done is covered in Stepper motor Basics part 2 (Half Step)
