Page 158 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

138 Making Things Move

FIGURE 6-11 Features of SparkFun’s ROB-09238 stepper motor

• Rated Current Shown as 0.33A, this is the current the winding will draw at
the rated torque. It’s a good idea to size your power supply for a maximum
current higher than this limit, so your motor will hit its own current limit
before hitting the limit of its power supply.

• Holding Torque This is similar to the stall torque in the motors described
previously. The difference is that stepper motors effectively stall every step
because there is a series of wire coils around the motor that are activated in a
sequence. This motor has 2.3 kg-cm (about 2 in-lbs) of holding torque, which
is the torque of the motor when it’s powered and holding its position at one
of the steps. If you try to drive something that needs more torque than the
motor is rated for, it will probably slip and you’ll lose the benefit of precise
positioning. Stepper motors don’t inherently know anything about their
position. They just know to rotate the number of steps you tell them to rotate.
If you exceed the holding torque and the motor slips, all bets are off.