Microstepping Data

Microstepping of Stepper Motors
Let's now look at what current ratios are needed to produce a particular
step angle. The Microstep angle can be graphically represented with
a Phasor Diagram. (See diagram below) The X and Y axis indicate
the current level in two respective coils A, B. A vector (ray from origin
to coordinate X,Y) shows the resultant angle and Torque (magnitude
of the vector) when some current is applied to both coils. Keep in mind
that this diagram shows the 'sub-angle' between natural whole steps
(poles) of the motors. On a typical 200 step per revolution motor this
is 1.8 degrees. The graph below is a representation of how that angle
can be further sub-divided.
http://www.stepperworld.com/Tutorials/pgMicrostepping.htm

Microstepping Tutorial
If the controller is designed with the capability to control the magnitude
of the current in each winding, then microstepping can be implemented.
The phase diagrams below all show different implementations of "divide
by 4" microstepping. Note that it is the phasor angle (not it's length) that
determines the microstep position.

http://www.zaber.com/wiki/Microstepping_Tutorial

Stepping Motor Physics
Microstepping allows even smaller steps by using different currents
through the two motor windings

For a two-winding variable reluctance or permanent magnet motor,
assuming nonsaturating magnetic circuits, and assuming perfectly
sinusoidal torque versus position curves for each motor winding
http://www.suc-tech.com/technology/stepcontrol2.htm