Final Design




The structural design can be broken up into three components: the base, the wheels and gearing, and the shaving cream and pushing mechanism.


The base of our current structure is much more stable than that of the version we built over Wintersession. (Click here to see our Wintersession Design.) It has a solid floor made of beams which are braced together. The front of the structure is wide enough to hold the can of shaving cream and the two gear trains, powering the independent motors. The back of the base is slightly narrower, allowing room for the two crickets, two battery packs, and many different sensors at varied locations.

The robot uses five distance sensors to determine the surroundings of the robot. We initially planned to use seven distance sensors (two facing left, two facing right, two facing back, and one facing forward), but we discovered that having more devices attached to the bus ports increased the likelihood that the sensors would give faulty readings or that the cricket would randomly fail.

In order to reduce these errors, we were able to cut back to five distance sensors, with two facing left, two facing right, and one facing forward. The two facing left are used in keeping the robot parallel to the left wall, and sensing doorways on the left side. The two on the right are used to sense doorways on the right side. The one on the front is used to sense when the robot is approaching a wall. We have not found that we have lost any abilities or functionality by reducing the number of distance sensors we are using. This challenge has allowed us to think more creatively to come up with a more efficient solution than we initially realized possible.


This view of the back of the robot shows both of the crickets, two battery packs, and the two back sensors (facing left and right). The crickets have the capacity to run on a single 9-volt battery, however, we discovered that random errors are more likely to occur when the battery is getting weaker. Additionally, the strength of the current supplied to the cricket varied based on how new the battery was. By using a battery pack, of six 1.5-volt batteries, the current supplied to the robot is more consistent.  

In the final version of the robot, we only used one battery pack, attached to the cricket controlling both driving motors and all five good bathroom sensors.  Additionally, we added one more cricket.  For an explanation of this decision, see the Control Flow page.


Wheels and Gearing

Our initial gearing included a pulley mechanism as the first gear reduction. This had to be replaced because it was causing significant slippage. We went through several iterations of the gear train before constructing the current one. The gear trains are encased in two towers which also provide the base for the pushing mechanism. In order to accommodate the gearing system, other minor modifications had to be made to the structure of the gear towers. We are using a gear reduction of 9:1, which gives enough torque to propel the weight of the structure while still maintaining ample speed to navigate through maze within the allotted six minutes. The gear train is comprised of 8-, 16-, and 24-tooth gears: an 8-to-24 gear pair from the motor, then a 16-to-16 pair for purposes of spacing, and finally another 8-to-24 pair connecting directly to the wheel.

We began by using Lego tires, with a plastic center and a rubber outer ring, as our wheels. Under the weight of the robot, the the flexible rubber part of wheels bulged, causing a good deal of friction. In order to make the wheels more solid, we filled the rubber outer rings with hot glue. Filling this space lessened the bulging and decreased the surface area in contact with the floor. The wheels then ran with less friction, but it was difficult to maintain an even thickness of glue and to mold the it such that the tire treads were straight.

The current robot uses a comercially designed wheel with a thicker tire and a more solid construction. We extended the base to allow for an extra beam on the outside of each wheel to stabilize the wheels to keep them perpendicular to the floor. In addition to the two wheels located near the front of the robot, the base is supported by two casters, one at the back of the base and one at the very front. These allow for easy turning with minimal added friction.

The picture to the right is a view of the bottom of the robot. Since this picture was taken, a slight modification has been made, changing the placement of the two casters.

We were somewhat concerned with the distribution of weight in the original design, in which both casters were at the back of the base. Most of the weight of the vehicle is contained in the shaving cream can, the columns that hold the gear trains and support the mechanism, and the pushing mechanism itself. All of this weight was concentrated at the front of the base.

Thus, if the robot was knocked off balance at all, it may very well tip over and not be able to recover and continue its run. Another factor to consider is that we plan to enter the robot in the contest in the "Non-Dead Reckoning" mode, in which ramps will be placed in the hallways; we had to ensure that the robot will be able to successfully go down the ramp without bottoming out or tipping over. To alleviate this problem, we moved one of the two casters from the back to the front, and centered both of the casters.


Shaving Cream and Pushing Mechanism

We were pleased with the pushing mechanism that we created over Wintersession, and have kept it fully intact. From the columns housing the gear trains, we built up two hollow columns to which the pushing mechanism is secured. The additional height was necessary because the shaving cream nozzle must be higher than the 6" to 8" candle. Moreover, the base of the pushing mechanism is deeper than the gear columns, so we made the hollow columns larger to allow for a more secure attachment of the pushing mechanism. As stability is one of our main goals in this structural design, we felt that the additional size and weight of the two hollow towers were worth the added stability they provided.

The can of shaving cream sits on top of a simple platform built up from the base so that the nozzle is high enough, anchored by Lego bricks at the corners of the platform. Interestingly, the kind of shaving cream makes a important difference. We found that Gillette Foamy, Original, shoots significantly farther than any other brand we tried, allowing the robot to extinguish the candle from a safe distance. Even other kinds of Gillette Foamy (e.g., Sensitive Skin) failed to shoot far enough. We still need to figure out exactly how far away the robot needs to be from the candle in order to extinguish it reliably.


This front view of the robot shows the two hollow columns onto which the pushing mechanism is attached. The shaving cream is raised to the proper height to extinguish the candle, and the space under the can is being used to house the front distance sensor. This seemed like a great place for that sensor for a number of reasons. Since the distance sensors are accurate in ranges greater than about 5cm, we wanted to ensure that a wall was never closer than 5cm to the sensor.



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