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November 17, 2003
Unipolar Stepper Motor Control
external links:
• Stampworks Experiment #26: Stepper Motor Control (PDF)
• ULN2003 spec sheet (PDF)
• BEAM Reference Library article on Stepper Motors
• Jones on Stepping Motors
internal links:
• Basic Stamp program (revised version of StampWorks program)
• circuit diagram (revised version of StampWorks diagram)
• snapshot of small stepper setup
links to components @ All Electronics:
• ULN2003 Darlington Array
• a small 12V 7.5 deg/step unipolar stepper motor
• a larger 5V 7.5 deg/step unipolar stepper motor
• a larger 5V 1.8 deg/step unipolar stepper motor
NOTE: You will need a separate power supply for the motor.
Sorting out the wires:
Unipolar stepper motors have six leads. Internally, they have two windings, each with three of the six leads. One of these three leads is a center tap on its respective winding; this lead gets wired to V+. The other two leads are connected to each end of the winding, and when either is successively grounded, it creates an opposite magnetic field. By alternately energizing the two windings in each of two directions, the rotor can be stepped in a clockwise or counterclockwise direction.
To sort out the leads on a unipolar stepper motor with 6 wires: Use a multimeter to measure the resistance between pairs of leads. Each pair of leads will give one of three possible results: a fairly low resistance, twice this resistance, or a very high resistance (typically in megaohms).
Try to find a trio of leads with low resistance. Of these three leads, the one that gives identical results with the other two is the common. The other two together should give you twice the resistance of either of them with the common. Label the two groups of three leads each 1 and 2 (don't worry about which is which; see below), and label each of the commons as C.
Now, if you connect the common to V+, by momentarily connecting each of the other two leads to ground, you should be able to make the motor shaft jump slightly clockwise and counterclockwise. Call the clockwise lead A, and the counterclockwise lead B. Find the A and B leads for both groups 1 and 2.
Each wire is now labeled: 1A, 1B, 1C and 2A, 2B, 2C. Connect 1C and 2C to V+, and connect the other wires to your control pins in this order: 1A, 2A, 1B, 2B. If clockwise and counter-clockwise are flipped once you wire everything up, then switch 1 and 2 (ie, 2A, 1A, 2B, 1B).
Posted by Perry at November 17, 2003 09:10 PM
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