Page 157 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 6 Options for Creating and Controlling Motion 137

Stepper Motors
The stepper motor combines the precise positioning of standard hobby servos and the
continuous rotation of DC toy and gearhead motors. The central shaft of a stepper
has a series of magnets on it in the shape of a gear, and there are several wire coils
surrounding this gear magnet on the inside of the motor housing. It is a bit like an
inside-out version of the previously described DC motors, which have the coils on the
shaft and the magnets on the housing.

Steppers work by moving in a bunch of little increments, or steps. If you step them
fast enough, it looks like continuous motion. Each time one of the coils is energized, it
pulls one of the teeth on the shaft toward it to complete one step. For example, a
200-step motor moves in a full 360° circle at 1.8° per step.

These motors have four to eight wires you need to use to control the pulses to make
the shaft step continuously, so they’re more complicated to control than the previously
described motors. They are squatter looking than the rest of the DC motor family, and
have less torque than you might expect for their size and weight. However, they’re
also the fastest way to integrate both speed and position control into a project.
Printers and scanners use stepper motors to control the speed and location of the
print head with the ink and rotate the paper through them. So if you see a discarded
printer on the curb on garbage day, you just found yourself at least two free stepper
motors.
A good example of a simple stepper motor is SparkFun’s ROB-09238. Figure 6-11
shows the feature list from the website (www.sparkfun.com/commerce/
product_info.php?products_id=9238).

Let’s step through the list to see what we have here.

• Step Angle This is in degrees of 1.8. If you divide 360 by 1.8, you get 200
steps for one revolution. We’ll talk about how to create these steps in the
“Motor Control” section later in this chapter.
• 2 Phase This stepper is bipolar (4 phase is unipolar). We’ll also look at this
characteristic in the “Motor Control” section.
• Rated Voltage This is 12V, which is just the voltage for which the stepper
was designed. Give it more, and you’re likely to burn out the motor. Give it
less, and it might not turn at all.