microstepping a stepper motor

Can the arduino r3 motor shield be used to microstep a stepper motor?

Can anyone provide a reference to source code for my arduino uno + r3 motor shield so I can microstep my 4 wire and 3 wire stepper motors?

Well not at any great speed - steppers really need current-control to get proper microstepping behaviour. PWM will work for static and low speed microstepping, since then current and voltage are pretty much related.

The normal approach is to build a look-up table of sine/cosine terms for your microstepping step size, and then use the magnitude of that to set the PWM rates for each winding. The direction for each winding comes from the sign of the sine/cosine. A step either increases or decrease the angle variable (an index into the table), wrapping around as needed.

That sound like what I'm looking for. I don't want speed. I need as slow and smooth as possible. Does anyone have a sketch (.ino) source code for microstepping? What is the typical maximum number of microsteps between standard steps?

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