CHAPTER 19 ■ WHEELS
It seems that 19 cm is the maximum diameter you could use and still have part of the robot’s body touch
the surface. Although this is technically correct, it would still present problems. Not only would the robot
pop wheelies when starting up, but also the sensors would need to be angled strangely to face the floor.
Determining the Maximum Diameter Based on Speed
Since larger wheels result in greater speed, the maximum diameter of the wheels is probably limited most by
the reaction time of the motors, sensors, and brains. After all, the robot will drive off of the line if it moves too
quickly for the electronics and motors.
The primary factors that determine speed have already been identified: raw motor speed, motor
voltage, motor gear ratio, robot mass (load), and wheel diameter. (More complicated electronics can
manipulate the pulses of power to the motors to reduce speed in spite of the factors stated here.)
Sandwich’s gearhead motors were selected on the basis of size, cost, availability, and final shaft speed.
Although a slightly slower-speed motor would have been desirable, we’re stuck with the limited selection of
motors that met the other criteria.
The most common occasions when a robot’s mass is altered with respect to speed is when a robot’s
mass is lightened to make it faster. Adding deadweight to slow down a robot is an energy-inefficient method.
That leaves motor voltage and wheel diameter as the best remaining factors that we can vary to control
Sandwich’s speed. All of this is a long-winded way of saying that the maximum appropriate diameter of the
wheels is going to be determined by the size at which the robot’s speed begins to cause it to stray off of the
course. Some experimentation is necessary to pick this diameter.
Figure 19-11. A ruler measuring the distance from the front of the robot to the center of the motor shaft