Stepper - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 254

Introducing Motors
Electric motors generally work by creating electromagnetic i elds from coils,
forcing magnets on an axle to move, therefore driving the axle. By generating
electromagnetic i elds, a motor turns continuously until current is removed.
Servo motors (presented in Chapter 14) function a little differently, but even if
their usage is different, a servo motor is still controlled by an ordinary electric
motor managed by a small microcontroller to ensure the servo motor can move
to a precise position.
Stepper motors are different. They have several coils inside, and the
internal axle is “toothed.” When applying current to one of the coils, the
closest “tooth” is attracted to the coil, and the axle moves by a few degrees.
Current is then removed from the coil and sent through another coil, again
attracting a tooth and moving the axle by a few degrees. By repeating this
operation, a stepper motor can be controlled to turn continuously in either
direction, but this is not normally a stepper motor's main function. Stepper
motors can have precise movement and as such can drive gears with equal
precision.
Imagine a printer. Paper is fed into the printer, and the printer begins to print
one line. A print head moves across the paper and deposits ink in precise loca-
tions according to the image that was sent to it. When the print head arrives
at the far edge of the paper, the paper is fed into the printer, and the printer
heads returns in the opposite direction, continuously printing until the end of
the page. Feeding paper into the printer is extremely precise; too much paper
and white lines appear on the sheet. Too little, and the resulting image will be
squashed. The movement has to be precise and feed exactly the right amount
of paper. Chances are, the motor feeding the paper into the printer is a step-
per motor. Also, because the printer head requires precise positioning, there
is a good chance that the belt used to attach the printer head assembly is also
controlled by a stepper motor.
Stepper motors have several characteristics, but the most important one is
the angle per “step.” This can vary greatly in the different models, but ranges
of between 2-5 degrees are common.
Controlling a Stepper Motor
Stepper motors are different from standard electrical motors, and as such, can
be difi cult to control. They require both software and hardware to be used.
Fortunately, the hardware isn't difi cult to use, and the Arduino software library
is even easier.