The goal of this assignment is to build and program an autonomous robot that can locate a "fire" in a burning "house" and put it out.

Assignment Details

From the Assignment Webpage

The House and Fire:

The "house" will be a maze consisting of rigid walls and obstacles. The layout of the house won't be known beforehand but we can count on the following:

  • Except at wall junctions, there will be no walls less than 24 inches apart.

  • The house will be enclosed (so that the robot will not be able to wander off into the space beyond the outer walls).

  • The walls and obstacles will always run horizontally or vertically (in the coordinate system of the outer walls). (Thus, they will always meet at right angles, or as close as we can devise.)

  • The walls will be one foot high.

Here is an example of a bird's eye view of one possible house (about 8' x 8'):

Some part of the house will be set on fire. Rather, to simulate this, a lit candle will be placed on top of a six-inch platform either against a wall, in a corner, or between two walls in the house. Thus, the height of the flame will be six inches plus whatever the height of the candle is. (Leeway will be made to raise or lower the candle if it's the "wrong" height for a particular mechanism a team has constructed.) The robot will start in one of the four corners of the house and will be aligned initially with the walls. The goal of the robot will be to seek out the candle and extinguish it. This is the highest-level goal -- other goals include maintaining the robot's health and keeping it from getting stuck.


  • The robot should not push or ram any wall/obstacle continuously, i.e., the intent is to keep the house in its initial configuration. If a robot seems to be pushing against a wall, it will be reset.

  • We can devise whatever mechanism we want to extinguish the candle, as long as no water or fluids are involved and no harm comes to the robot.

  • We may not use any glue or epoxy on our robot (tape is OK) -- all equipment needs to be returned in its original state.


The computational brains of the robots are MIT's handyboards, based on the 68HC11 microcontroller. Each team will have one handyboard and a copy of Fred Martin's technical reference for the handyboard. In addition, each team will have at least:

  • Three Lego motors

  • Two light sensors

  • One infrared sensor

  • Three touch sensors

There are spare sensors and possible spare motors. We can use these as we see fit. Finally, there is a pool of lego blocks with which to build the body of the robot.

Interactive C:

The programming environment for the Handyboards is Interactive C. Here is an IC
Online Reference.

The Contest:

On Monday, February 5th each team will have to demonstrate its robots' abilities. There will be at least three trials for each robot on its own.

The first trial will involve a setup like the one below. The robot is indicated by the green rectangle (positioned upper left, facing toward the wall at the bottom of the diagram), and the candle is represented by the yellow circle (in the middle on the right).

The second trial will involve a similar set up, in that the maze and robot starting position will be identical, but the candle will be in an unknown location in the maze.

The third trial will involve an unknown maze (though not substantially different from the one above) and unknown positions for the candle and robot.

Finally, if it seems feasible with the robot designs that the teams have created, there will be a head-to-head run with both robots located in an unknown (but symmetric) environment at equal distance from the candle.

Judging Criteria:

In each case, the robots will be judged according to these criteria:

  • Does the robot drive without getting stuck or endangering itself?

  • Can the robot find the candle and indicate that it knows that fact?

  • Can the robot extinguish the candle?

  • How quickly can it do so?

Our Solution

Our main philosophy when desiging our robot (affectionately known as Mr. Bizzaro) was the KISS mantra (Keep It Simple, Stupid). In terms of the chassis, we aimed for something small, agile, and relatively fast. Our initial idea involved using a light sensor to detect the candle and a snuffer to put the candle out (although our plans eventually changed to using a fan instead of a snuffer). From the beginning we knew that our program would be behavior based. Initially we aimed for two behaviors: wander, which instructs the robot to explore the maze until the candle is detected, and avoid walls, which instructs the robot to reposition itself if it runs into a wall. Our final program ended up using four behaviors, but it operates using the same basic idea as our original software concept.

The Chassis

The chassis started out as a simple three-wheeled, short, squat, frame. The wheels were arranged in a triangle shape, with the two larger back wheels connected to Lego Mindstorm motors and the front wheel left alone as an independent castor. Next a container was built to hold the handyboard vertically on top of the frame. Two bump sensors were added to the "front," and a light sensor tube was added to the top of the container. At this point the robot became a little top heavy and unbalanced, so a fourth castor was added in the back, making the frame of wheels diamond-shaped. The only piece of hardware left to add was the snuffer. We were a little perplexed about how to get the snuffer to extend and retract until in a brilliant stroke of genius Aaron constructed a crank-piston system entirely out of legos. After testing our robot several times, we realized that getting our robot to stop at the correct snuffing distance away from the candle would be extremely difficult. We also realized that a fan would be much more practical as it could be turned on as soon as light was detected and we wouldn't have to worry about stopping at all. After this insight we decided to essentially re-build the entire chassis. Not only did we solve the problem of how to easily extinguish the candle, but we were also able to go back and fix some of the design flaws that were bugging us (high center of gravity, too narrow base, off balance wheels, stiff castors, etc.)

The base frame of the new chassis is very similar to the original frame of the old chassis. There are three wheels in a triangular formation, two large ones in the front and a trackball mounted in a cylinder in the back. The handyboard is mounted in the back, on edge, with all of the ports facing out. The two bump sensors are mounted at the front. The light tube sits on top of a platform that is exactly six inches above the ground (the height of the candle). The fan is attached to the top of the light tube and is angled downwards. This allows the air generated by the fan (which is focused through a rectangular tube) be aimed directly at the flame.

Here are some pictures of Mr. Bizzaro:

This is the "front" of the robot. You can see the two bump sensors in the front, as well as the two larger wheels (they're gray and narrow). The fan and rectangular focusing tube are sitting on top of the light tube. You can click on all of these pictures (as well as the pic of the prototype above) to enlarge them.

This is the back of the robot. The white square at the bottom of the picture is the trackball housing. Above that, you can see the handyboard. The back of the fan is at the very top.

This is the left side of the robot. You can see the gray wheel at the very bottom. The long, black, rectangular thing at the top is the side of the light tube. The thing covered in masking tape is the left edge of the handyboard, and the white square is the trackball housing.

This is the right side of the robot. Again you can see the wheel at the bottom, the light tube and focusing tube at the top, and the edges of the handyboard and trackball housing at the back.

This is a closeup of the back. You can see the trackball housing and handyboard.

This is a closeup of the left wheel.

This is a closeup of the front. You can mainly see the two bump sensors edge on, as well as the two gray wheels.

The Program

Our program uses basic subsumption architecture to regulate the robot's actions. We have four main behaviors:

  1. Avoid Walls

  2. Find Light

  3. Wander Spin

  4. Wander Drive

The behavior with the lowest priority is wander drive. The robot executes the wander drive behavior by driving forward for a set amount of time (right now, that set amount of time is set to five seconds). In wander drive the left motor is set to spin slightly faster than the right. This effectively gives the robot a drift to the right. Eventually will hit the right wall of the maze, back up, turn slightly, and travel forward (again with a drift to the right). This results in the robot effectively following the right wall (perhaps we should have called this behavior follow wall?). The next behavior in the hierarchy is wander spin. After the set amount of time in the wander drive behavior has passed, the robot will stop and turn in a full 360 degree circle. If the light sensor detects light, the robot will enter the next highest behavior, find light. When this behavior is entered the fan is turned on and the robot moves in the direction that the light was detected. If the sensor doesn't detect light in wander spin, the robot will go back into the wander drive behavior. The behavior with the highest priority is avoid walls. If at any time one of the robot's bump sensors is tripped, the robot will immediately stop, back up, turn (the direction of the turn depends on which bump sensor was tripped), and then re-enter whichever behavior it was previously in.

You can find a copy of our code here.

Problems We Encountered

By far the biggest problem we encountered was our battery running out after ~20 minutes of use. This ended up preventing us from fully testing our robot. Other than that, most of the problems we encountered had to do with various aspects of the chassis. A lot of these problems were fixed when we re-designed the chassis.

The first problem we encountered was the coefficient of friction on our castor being too high. Everytime our robot switched from turning in a circle to moving towards the light, it would quickly get off track. This was caused by the castor remaining in a turned position and not straightening out when the robot began to move forwards. We fixed this problem by making the back wheel a trackball. Now, whenever the robot moves out of a turn and towards the light it moves in a straight path.

Another problem we had was with the robot's center of gravity being way too high. This was caused by us adding hardware to the robot in a solely vertical direction. We fixed this by re-building the chassis and making the frame of the robot wider and squatter.

A relatively major problem we had was that the fan was too weak. Even with the fan physically touching the candle, the force of the air was not enough to blow the candle out. We fixed this problem by creating a rectangular focusing tube. Now the fan will extinguish the candle approximately 80% of the time.

We discovered a fairly serious problem this evening (2/5/01) when our robot tried to navigate a corner of the maze. First it would run into one wall of the corner setting off the left bump sensor. The robot would then back up, turn right, and move forward, hitting the other wall and setting off its right bump sensor. Then the robot would back up, this time turn left, move forward, and hit the first wall again, setting off the right bump sensor. The robot ended up effectively trapped in the corner. We solved this problem by making the robot turn more (now ~90 degrees instead of the previous ~45) after it backs up following a collision.


Because the battery kept running out before we could thoroughly test our robot, we are not certain that he can naviagate the maze and accomplish his goal of extinguishing the candle 100% of the time. Most of the problems with the robot's performance have to do with the fan. Preliminary tests indicate that once the light is detected, the robot can put out the candle (with the use of the fan) about 80% of the time. The other 20% of the time the robot ends up running into the candle and knocking it over before the fan can put out the flame. The air resistance flowing around the candle as it falls from the platform is enough to extinguish the flame, and while this technically accomplishes our goal, we do not feel it does so in the safest manner possible.

In our tests the robot detected the light about 50% of the time. This, however, was using an old wander behavior. The battery ran out tonight (2/5/01) before we could fully test our new "follow wall" wander behavior. The only problem with the old behavior was that it was so random that we didn't have the patience to sit around and see if the robot would get lucky enough to wander near the light. We do feel, however, that given an infinite amount of time, the robot could have found the light using the old wander behavior. As soon as we have tested our new follow wall behavior, I will post an updated evaluation here.

Mr. Bizzaro attempted to demonstrate his amazing extinguishing abilities this afternoon (2/6/01), but he seemed to suffer from a bit of stage fright. After testing out our new "follow wall" code we noticed that the drift was not pronounced at all, and that the robot was spinning more than 360 (and thereby getting off track) when searching for light. After making the drift much more pronounced and reducing the time the robot spent spinning, we set him at his task again. We noticed pretty much immediately that there was a significant drawback to the drift. Because the robot was constantly bumping into the wall, it never achieved the full 5 seconds of uninterrupted driving, and therefore didn't go into many spins searching for light. We placed the robot close to the candle to see if it would detect the light and put it out (which it has done accurately many times before). It recognized the light, turned on the fan, and drove towards the candle, but due to the fact the other team had slightly lowered the platform (grr!), the fan was not at the correct angle and wasn't able to extinguish the flame (of course, the fact that the fan motor was really wussy could also have affected its extinguishing abilities...) We also noticed that if the robot didn't immediately extinguish the candle, it would run into it and enter the avoid walls behavior, which made it back up, turn, and lose the candle from its line of sight. It would have been nice if the robot could distinguish between running into the candle and running into a wall, but I'm not sure how we could've implemented that accurately. In retrospect, using IR sensors to follow the wall would've been much more effective. Also it seems that our random wander behavior which relied on the robot getting lucky was more effective than our follow wall behavior.

Mr. Bizzaro in Action

Mr. Bizzaro will probably be demonstrating his abilities and competing against the other team sometime before this thursday (2/8/01). At the demonstration I'll take some digital movies of Mr. Bizzaro in action and post them here.


Mr. Bizzaro was created by Aaron Boyer, Joshdan Griffin, and Seema Patel for a lab project in their CS robotics class at Harvey Mudd College. The class is taught by Professor Dodds.