CHAPTER 17 ■ DC MOTORS
Little noise ripples do exist even in the power-off state, but contrast them with the no-load spikes. Look
back at Figure 17-21 and notice how small the no-load spikes are on the escap motor—especially relative to
the power-off-state spikes in that same figure.
The capacitors that protect chips during motor start-up can also dampen electrical noise generated
during normal motor operation. Some robot designers go farther by providing separate batteries for the
motors and electronics. The designers add special optical isolators to communicate commands to the
motors without a common electrical connection.
Until recently, none of my robots had ever suffered a glitch due to motor noise. My circuit boards tend
to have a lot of capacitors and I tend to choose nicer quality motors whenever possible. It could also be that
my robots are fairly small, so the amount of wasted power going towards electrical noise is not enough to trip
up modern chips.
I solved the one problematic experience that I encountered with motor noise by switching from an
ultra-low power chip to a normal (not as efficient) chip. Despite my best efforts, I could not reduce the noise
coming from the inexpensive motor, so I simply opted for more tolerant electronics.
Mass Characteristic of DC Motors
Motors weigh a lot. It wouldn’t be unexpected for motors to represent a third of a robot’s mass. Sandwich’s
motors are 37% of the line-follower’s mass.
When building a robot for a contest or some application where the robot’s mass is restricted to a
maximum, avoid selecting motors with a combined weight of more than 40% of the total. Otherwise, there
won’t be enough weight available for the batteries or a structurally decent robot body.
Dimension Characteristic of DC Motors
DC brush motors come in a variety of shapes and sizes. As long as the other characteristics of the motor
aren’t totally out of whack, the physical size of a motor is probably the most important characteristic.
Motor size usually dictates the robot’s size, rather than the other way around. Many builders snap up
decent motors whenever they appear in surplus sales. As such, builders have a private stash of motors that
they select from at the beginning of a project. The motors available then dictate the minimum dimensions of
the robot body. The maximum torque of the selected motors then dictates the maximum weight of the total
robot.
Figure 17-22. Oscilloscope trace of current flow through toy motor