Page 120 - Hacking Roomba

Chapter 5 — Driving Roomba 101

of time, and then stops. What is this speed value though? That’s easy. Define a few more
methods.
public void setSpeed( int s ) { speed = Math.abs(s); }
public int getSpeed() { return speed; }

Now each Roomba has an internal speed that the distance-based At commands will work with.
You don’t often change the speed of the Roomba, so specifying every function is wasteful.
In use, the preceding new methods would be used like this:
roombacomm.setSpeed(500); // 500 mm/sec, fastest possible
roombacomm.goForward(1000); // forward 1 meter = ~39 inches
roombacomm.goBackward(254); // backward 254 mm = ~10 inches
Rotating Specific Angles

So now you can move Roomba forward and backward specific distances. What about turns?
In order to determine how much the Roomba has rotated through a spinLeftAt() and
spinRightAt() command, you need to remember that when spinning in place, the velocity
parameter to the DRIVE command becomes the velocity of the wheels along the circumference
of a circle. That velocity is in mm/s and you need degrees/s. To convert it, you need to figure
out how many millimeters per degree in the circle the Roomba is moving around:
mm/degree = (258 mm × pi) / 360 = 2.2515 mm/degree
Then to get angular speed from Roomba speed in mm/s, you use this formula:
angular_speed in degrees/s = speed in mm/s / (mm/degree)

To figure out how long to wait to spin a specific number of degrees, you flip the equation
around to solve for seconds for a given Roomba speed and angle you want:
wait time = (mm/degree) × (angle/speed)
For example, to do a 180 degree spin at a speed of 300 mm/s, you spin for (2.2515) ×
(180/300) = 1.35 seconds.
In Java, these equations are implemented for spinRight() and spinLeft() as in Listing 5-3.

Listing 5-3: RoombaComm.spinRight() and spinLeft()

int wheelbase = 258;
float millimetersPerDegree = (float)(wheelbase * Math.PI /
360.0);

public void spinRight( int angle ) {
if( angle < 0 ) return;
float pausetime = Math.abs(millimetersPerDegree *
angle/speed);
spinRightAt( Math.abs(speed) );
Continued