Controlling a unipolar stepping motor 

This simple little project requires only a small adaptor to make it more convenient to connect the stepper motor to the Velleman board. As can be seen from the illustration on the left and the circuit diagram below, the motor is connected via 6 wires seen on the right of the picture. The two common wires of the motor coils are at the top and they connect to the positive supply seen on the bottom and the clamp input of the Velleman board. The four motor wires are then connected to outputs O5 to O8.

Be careful not to do this with a motor that draws too much current - half an amp per coil is the maximum I would risk, even though the Darlington driver fitted to the Velleman board is rated at 1A per line.

There is a suggested PCB layout on the right, though the circuit is so simple a veroboard would do just as well.

The wires of the motor must obviously be connected in the right sequence. A simple way to do this is to test them with a 9V battery or something similar. Connect the two commons together. These are usually fitted in the centre of the coil, but they can be identified with an ohmmeter. The centre tap has the same resistance against both halves of the coil, as shown below.

To determine the correct sequence of wires simply connect the two commons to one side of the battery and any one of the other four wires to the other side. You have a three in four chance that the motor will move by a single step either left or right. If this doesn't happen, try another wire.

Repeat the process with a second wire. If the motor moves again in the same direction, the sequence is correct. If it turns in the other direction, you got it wrong and you have to start again from the beginning.

Generally a few tries will establish the correct sequence. Once established, I tend to fasten the wires in correct order to a small connector, as seen in the photograph above on the right of the picture.

A word about timing. To change the speed of the motor, a time wasting loop is needed. For various reason I picked a simple FOR -NEXT loop that does nothing. Depending on the motor in use, the fastest speed my be too much for your motor or, alternatively, your motor might be able to turn even faster.

The program

My program will turn the motor in both directions at different speed in three basic modes: Single step, half step and two-phase. Single step mode sends the pattern

0001, 0010, 0100, 1000

to the four output lines. This mode uses the least power. To reverse the motor, send the pattern

1000, 0100, 0010, 0001

Two-phase mode uses two coils at the same time. This increases the torque but doubles the current consumption. The pattern is

0011, 0110, 1100, 1001

and so on.

Half-step mode produces a half step by turning on two adjacent coils so that the rotor  moves half way between them. The pattern is

0001, 0011, 0010, 0110, 0100.1100, 1000, 1001

I have stored these values in three arrays (lines 990 to 1190 below). When you run the program, you may find that your motor turns left when the program says it should turn right. This simply means the wires of your motor are in a different sequence from the wires of my motor.

As usual, I supply both a program file and a self executing file should you not have BBC BASIC for Windows.

K8055_onestep_1_0.zip

Below are the same two files adapted for the K8061

K8061_onestep_1_0.zip

Back to the start