CHAPTER 17 ■ DC MOTORSThe escap motor was received second-hand, so I don’t know the nominal voltage. Assuming 12 V, this
high-quality motor exhibited operation from 12.5% voltage to 150%. I would have tried up to 200%, but my
previously mentioned mounting putty starting flying all over the room at 6000 RPM.
Unfortunately, controlling speed with voltage only works well when you have the ability to swap in
different batteries. Furthermore, the voltage to the escap motor would need to be around 0.39 V to provide
the 137 RPM speed needed by Sandwich. There are better ways (gears and pulse-width modulation) to vary
the speed of the motors than directly controlling the voltage provided to them.
Watching Out for the Relationship Between Voltage and Speed
As the robot wanders around, the batteries will drain. As the batteries drain, their voltage decreases. As
you’ve just learned, as the voltage decreases, so does the motor speed.
For many robots, the loss of speed is inconsequential to viable operation. A line-following robot still
works. In fact, the line-follower will be able to turn tighter corners and complete more difficult courses since
the brains and sensors can more easily outpace the motors.
For other robots, the loss of speed can be disastrous. Robots that follow preset courses using timing no
longer operate because the slower motor speed doesn’t carry them the same distance in a same amount of
time. If necessary, the declining speed can be counteracted, such as by receiving feedback as to the actual
extent the wheel has turned.
Current Characteristic of DC Motors
Recall that the amount of current flowing through a circuit has a direct impact on how long the batteries last.
Look out! Electric motors really suck up the juice.
You may spend hours tweaking resistor values on your LEDs and circuits. You’re proud to save a few mA
here and there. And then, you discover that even a single motor drains more than the entire rest of the robot
combined.
Larger DC motors rate in the amp range, such as half an amp to hundreds of amps. Lunchbox-size robot
motors consume a fraction of that amount, with a usual range from 4 mA to 250 mA for each motor.
The quality of the motor makes a big difference. Case in point: When connected to 3 V, a toy motor
(see Figure 17-20) uses 125 mA but an escap motor uses only 4.5 mA. At 12 V, the escap motor has superior
speed and torque, yet still only uses 7 mA.
Figure 17-19. Graph of escap 26 mm DC motor showing speed increases linearly with voltage