Stepper motor

Stepper Motors

Stepper motors move a known interval for each pulse of power, making them handy devices for repeatable positioning. Generally they come in two flavours: bipolar and unipolar. The Darwin design uses unipolar motors that are converted to bipolar motors.

Bipolar Motors

These motors are the strongest type of stepper motor. You identify them by counting the leads - there should be four or eight. They are also the type of motors we are using in the RepRap Project's Darwin design. They have two coils inside, and stepping the motor round is achieved by energising the coils and changing the direction of the current within those coils. This requires more complex electronics than a unipolar motor, so we use a special driver chip to take care of all that for us. Some designs (the eight-wire ones) split each coil in the middle so you can wire the motor either as bipolar (short the middles) or unipolar (short the middles and treat the link as the centre tap - see below).

Worldwide Suppliers

ST5709S1208-B stepper motor from Farnell (supplied by Nanotec Gmbh). This is the standard RepRap stepper motor (it is an eight-wire one). It has 400 steps to one revolution (0.9^o per step). It actually has 4 coils (se the unipolar motor design below), but we join up the wires to turn it into a bipolar motor.

Your RepRap machine will need three of these stepper motors. Chop the four white + coloured-stripe wires short - about 50 mm,

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strip them, and twist and solder the red-and-white one to the black-and-white one, and the yellow-and-white one to the green-and-white one. Put heat-shrink sleeve over the connections to insulate them.

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Then solder or crimp a 4-way 2.54mm-pitch (0.1") socket onto the other four plain coloured wires. The pin numbers are:

Pin (Y & Z axes)	Pin (X axis)	Colour
1	4	Red
2	3	Black
3	2	Green
4	1	Yellow

Yes, the X axis motor runs "backwards". What can I say?

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Then you can connect it to your Universal Controller Board.

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Unipolar Motors

Unipolar motors have two coils, each one has a centre tap. They are readily recognisable because they have 5, 6 or even 8 leads. It is possible to drive some unipolar motors as bipolar motors if you ignore the centre tap wires. Their main beauty though is that you can step them without having to reverse the direction of current in any coil, which makes the electronics simpler. Some early RepRap prototypes used this trick. Because the centre tap is used to energise only half of each coil at a time, unipolar motors generally have less torque than bipolar motors.