Page 209 - Hacking Roomba

190 Part II — Fun Things to Do

Digital Sensors

All of the distance sensors Roomba employs are digital sensors. Whether it is the distance to
the floor, to the wall, or from the wheels to the ground, all these distances are distilled down
to a single Boolean value. Instead of “How far away is the wall?” the question is just “Is there
a wall nearby?” The single-mindedness of these types of sensors makes them reliable but also
harder to use for other purposes. However, by combining the readings from multiple sensors,
or reading a single sensors multiple times, it may be possible to gather additional data. For
example, by reading a button bit over time, you can determine how long the button was held
down. A quick tap would mean one thing, but a longer hold would mean something else.

Analog Sensors
The only external sensor with a graduated value is the dirt sensor. This sensor doesn’t seem as
accessible to hacking because it appears to be tuned to the normal vacuuming environment
(brushes moving, air moving past, and so on). It’s difficult to get readings from the dirt sensor
when Roomba is running its vacuum and brushes. The next most interesting analog sensor is
the current drain value. By watching this value and the motor over-current sensor values, it may
be possible to detect how hard Roomba is working as it moves its wheels. This could prove use-
ful if the wheels are purposefully strained in a known way.
The distance and angle sensor values are a derived analog value from the digital sensors in the
wheels. They offer high resolution but because they are “cooked” in a way that the other sensor
values aren’t, the distance values can sometimes be hard to use.

Using Roomba as a Mouse

The original computer mouse created in 1970 was a wooden box with two wheels mounted on
its underside at right angles. When dragged along a desktop, one or both of the wheels would
rotate in correspondence with the motion. The well-known ball mouse came soon after. The
wheels were moved inside and a small rubber ball carried the mouse motion to the wheels.
Roomba has two wheels with sensors almost exactly like the sensors in a ball mouse. These
sensors work and the data is available through the DISTANCE and ANGLE ROI commands even
when Roomba isn’t being driven. This means that the computeRoombaLocation() method
used in several of your previous Roomba programs can be used verbatim. The difference in use
now is that instead of using the rx,ry position pair from that function to represent the on-screen
position of a controlled Roomba, you can use it as a virtual mouse pointer (in lieu of the mouseX,
mouseY position pair) to represent how you are moving the robot.
Recall from Chapter 5 that Roomba only moves in straight or circular paths. This applies to
it being either driven by its motors or positioned by you moving it. Figure 10-1 demonstrates
some of the preferred ways Roomba moves. As discussed in Chapter 9, however, you can
approximate almost any curve with many circle segments.