Servo - April 2013

uses the bumper sensors as inputs. The output of DoEscape, however, goes to the little circle labeled “S.” The S stands for suppresses. Therefore, DoEscape suppresses the output of DoWander and takes over the motors. When DoEscape is done with its task, it will return control to Do Wander or whatever other behavior has the highest currently active priority.

In general, higher priority behaviors are at the top of the diagram. That doesn’t have to be true, however. All you need to do is look for the behavior that points to the suppressor that has the final say to see which one has the highest priority. When you have all of your behaviors coded, all you need to do in your code’s main loop is call everything, and each behavior will handle itself. Listing 2 shows what our Roomba behavioral program’s main Arduino loop looks like. Simple, isn’t it?

If you look at Listing 2 again, you’ll note that I have more behaviors there than what this article is talking about. The code (which you can find at the article link as RoombaBehave1.zip) has other behaviors stubbed in that can be added later. I am leaving these as an exercise for the reader. (Maybe I’ll add some details in a later column, we’ll see ...)

I have placed these behaviors in ascending order of priority with the lowest priority called first. Each of these behaviors are modified finite state machines (MFSM). There is a standard way to represent FSMs in the programming world.

SELECTED READING

"Cambrian Intelligence, The Early History of the New AI" by Rodney A. Brooks, MIT Press, 1999.

"Mobile Robots, Inspiration to Implementation" by Jones and Flynn, A K Peters LTD, 1993.

"Vehicles, Experiments in Synthetic Psychology" by Valentino Braitenberg, MIT Press 1986.

would block the function call until the delay is finished. Instead — as you can see in Listing 3 — we use the Arduino background milliseconds counter to determine if we are done. If not done, the function exits, leaving the state machine in the same state until the next call of the behavior function.

In the C language, a switch/case construct is the simplest way to implement an FSM. Note the timer function millis(); I use this as my background timer to know when certain states are ready to change.