Page 112 - Hacking Roomba

Chapter 5 — Driving Roomba 93

Radius

The second parameter is the turn radius value, which is specified in millimeters. This radius
describes a circle upon whose circumference the Roomba will drive. Driving in arcs enables
Roomba to make easy and efficient turns around many things like wall corners, table legs, and
so on.
Positive radius values turn the Roomba toward the left, and negative radius values turn it
toward the right. Postive velocity values turn Roomba in a forward direction, and negative
velocity values have it back up as it turns. Depending on the sign of the velocity and radius
values, there are four different ways the Roomba can move in a curve (see Figure 5-4).

-velocity status +velocity
+radius power clean +radius

-velocity max spot +velocity
-radius detect dirt -radius

FIGURE 5-4: Types of turns depending on the sign of radius and velocity

The turn radius always makes the Roomba travel along a circle. The only exceptions to this are
the three special case radius values of: straight (32768, which is also known in hexadecimal as
0x8000), spin left (1, which is also known as 0x0001), spin right (-1, which is also known as
0xffff). If you’ve done a little homebrew robotics where you directly control each wheel, this
method of dealing with driving two motors in a robot seems a little strange. A common first
task in homebrew robotics is to have direct control of each wheel, allowing you to drive only
one wheel at a time. How is it possible to drive the Roomba in this way using the velocity and
radius values of the DRIVE command?