Friday, March 14, 2008

Stepper motors tutorial (figures) 1

The most common problem we face when we use normal DC motors is that we don’t have precise control over how much it rotates. To rotate DC motors through a particular number of degrees what we can do is either calibrate it for a delay based operation i.e. if I switch it on for n seconds it moves 360 degrees; or what we can do is attach n encoder to the shaft which gives us a feedback on how much the motor shaft has rotated so that we can stop it when it rotates through the desired angle. Home made encoders give good results but don’t have such a high resolution and high resolution encoders are costly.

In such cases where we need to control the rotary position of the motor we can use stepper motors. Stepper motors have a tendency to make beginners feel uncomfortable about using them. But lets hope this air of discomfort about using stepper motors disappears once we are done through this article.

Stepper motors are motors available in round, square, rectangular shapes with 4 or more wires coming out of them.As the names suggests a stepper rotates in steps of a particular degree. As compared to DC motors which have continuous movement steppers actually rotate in specified degree of steps. There are steppers available from 0.9 degree step to 6 degrees.

Like all motors stepper motors also have a stator and a rotor. Based on the type of stepper construction the stepper can be of the following types, permanent magnet, variable reluctance and hybrid. As hobbyists and robotics enthusiasts we would limit our discussion to the permanent magnet type because that’s the type that’s easily and cheaply available. Depending upon the arrangement of coils in the stepper motor they can be classified as unipolar or bipolar. Usually the easily or more commonly available stepper motors are the unipolar ones with 5-6 wires. The bipolar has 4 wires.

Well I guess we are now ready to take a look at how Stepper motors actually work.

In the permanent magnet stepper motor the rotor is a permanent magnet and the stator is a set of coils which are energized one after another. In the unipolar motors the direction of current in the coil doesn’t reverse (so UNI) while in the bipolar the current through the coil flows in both the directions (so BI).
In both types of stepper motors there are two coils wound on the stator poles, which gives us 4 wires. The difference is that in unipolar motors there is a center tap from each of the coil winding. These center taps are either brought out individually (which will give us 4+2 = 6 wires) or are shorted together and brought out (which gives us 4+1 = 5 wires)

To rotate a stepper motor we need to energize the coils of the stepper in a specific sequence.
Let’s take a look at unipolar stepper motors first. The first step is to connect the common terminals to the supply voltage. Then we ground the coils in the sequence as shown. This sequence is called as wave drive.

A B A` B`
1 0 0 0
0 0 0 1
0 0 1 0
0 1 0 0

Usually the steppers motors available in the market are ex-stock i.e. pulled out from old printers, floppy drives, etc. and since there is no standard color code for representing the coils of the motors, how do we identify the terminals??
For this we use the most useful and versatile tool in electronics, the multimeter. Keep checking the resistances in between pairs of wires till u determine which is A A`, B B`. (when you try to check the resistance between A and B the meter will go out of range. Resistance between A and A` will be in the range of few ohms to a couple of hundred ohms. ...


Anonymous said...

This is interesting information about stepper motors. Now I know how they work and what they're used for.


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