Robot Builder’s Sourcebook

Actuators-motors 27
mulas intended for these calculations. Most any engineering book has them. The work
goes faster if you have a scientific calculator that can perform trig functions. The
Microsoft Windows Calculator, when switched to Scientific mode, is suitable.
Selecting the Right Stepper Motor
There are many types of stepper motors, but the two encountered most often are the
unipolar and bipolar. On the outside, both motors look the same; inside, the motors differ
by their electrical connections and windings. Both unipolar and bipolar motors are like two
motors sandwiched together and have two sets of windings. In a bipolar motor, the windings
attach to the external power supply or driver circuit at their end points only; in a
unipolar motor there is an extra set of taps at the center of the two windings.
Though variations exist, the typical bipolar stepping motor has four leads; unipolar steppers
have 5, 6, or 8 leads, depending on how the windings are connected internally.
Unipolar and bipolar motors require different actuation techniques. Most self-contained
stepper motor control ICs are designed for unipolar motors, and therefore tend to be less
expensive. Bipolar motors must always be powered in bipolar mode. Unipolar motors can
be powered in unipolar or bipolar mode. When used in bipolar mode, the center taps of
the unipolar motor are left unconnected.
Some additional considerations:
• Bipolar motors provide slightly higher torque, but can't always reach the same top
speeds of unipolar motors.
• Unipolar motors are more common on the surplus and used markets, making them
less expensive.
Motor Sizes
Stepper motors come in lots of sizes, but many are a standardized frame size, with a
standardized mounting plate. The following NEMA (National Electrical Manufacturers
Association) frame sizes are for the height and width of the motor; the length is undetermined
and can vary. The following sizes are for the mounting flange, which provides holes
at each corner for attaching the motor.
Motor Frame Size Mounting Flange Dimension Flange Hole Centers
NEMA 17 40mm, 1.57 inches 1.22 inches
NEMA 23 56mm, 2.22 inches 1.856 inches
NEMA 34 3.25 inches 2.739 inches
Bigger NEMA frame sizes exist, of course, but are used infrequently in small mobile
robots.
The length of the motor varies depending on its internal construction. Stepper motors
are often said to be single-, double-, or triple-stacked, which denotes the number of
“submotors” inside. The more stacks, the longer the motor. Additional stacks are used to
increase the torque of the motor.
Single or Double Shaft
It is not uncommon for stepper motors to have a shaft on both ends of its frame. The
shafts are usually the same diameter and length. One shaft is typically used to drive whatever
load is required by the motor, and the other shaft is used for connection to some