Our Project

Our final project is a takeoff of our favorite amusement park game - the Whack-A-Mole. Our six moles come out of their 'holes' in a golf course and passer-bys can 'whack' them "Caddy Shack" style. A counter keeps track of how many moles have been 'whacked' in a sixty-second interval.

 

After brainstorming many ideas, we decided to build a Whack-A-Mole because it sounded fun to construct and it interacts with the audience.

 

Our first difficulty in making the Whack-A-Mole arose from the vertical configuration of most Whack-A-Moles. We couldn't figure out how to get our moles to rise and sink quickly and reliably. We tried several mechanical devices, but in the end none of them were robust enough to carry the moles while being able to move fast enough to make our robot into a playable game. When we hit upon the idea of moving the moles horizontally we realized that not only had we solved our mechanical problems, but we now only had to build three movement devices, not six or nine. (As we would have had to do to raise and lower six or nine moles.)

 

Our next problem was to figure out how the robot would know that a mole had come all the way out of its hole and that the motor driving it should reverse direction. Our first thought was, of course, to program the motors to turn for a set amount of time in each direction. We soon learned that this method only works the first few times the program runs. As friction and imperfections in the track take their toll on the momentum of the mole carrier, it moves less and less each time the motor switches direction. This led ultimately to its grinding to a halt somewhere inside the golf-course box and the cessation of moles emerging from their 'holes.' We solved this problem by gluing touch-sensors to the ends of each track and programming the Handy Board so that when the mole-carrier hit the sensor at the end of the track the motor switched directions. This brings one mole back inside and pushes another mole out.

 

We were having problems with the amount of friction generated by the track - which Robbie and Gardner solved by having the mole carrier run over suspended axles instead of directly on the Lego track. The nearly frictionless track caused problems of its own the first time we started the motors and tested the mole carriers - they had so much momentum that they broke out of the ends of the tracks - sending Legos flying - and shot off the table. We solved this by building the track stronger and using a lot of hot glue to hold the sensor and its reinforcing Legos in place.

 

We also hit obstacles when we tried to build a counter to record how many times the moles had actually been hit. Our original idea was to put touch sensors on the tops of the heads of the moles, but we were concerned that these would only go off if they were hit directly from above. To increase the reliability of the sensors going off when moles were hit, we decided to give the moles heads separated from their bodies and put our touch sensors between the moles' heads and bodies. This worked in that anytime the head went down, the sensor would register a hit. To make this device work when the game was played over and over, the heads had to be a little springy. Our first few attempts at creating springy-headed moles resulted in heads that were so loose that they went flying in all directions when the mole-carriers started to move back and forth. Later versions of springy-headed moles were sturdier but were not springy enough to allow the sensors to depress repeatedly properly. One design which worked well in both respects used a few Legos in the neck to carry the force of the whack down to the sensor - but not only did this give our moles long scrawny necks - making them look like so many eaglets - but it made the moles too tall to fit under the golf course box. After a decapitation resulted from our first trial with the tall moles we realized our solution to the sensor problem would have to be a short one.

 

Our final solution to the counter sensor problem was to take the heads off of the moles altogether. We reglued their eyes onto what had once been their stomachs and glued more fur over the sensors - now placed on the tops of the moles 'heads' again. We increased the reliability of the sensors tripping by making a very large mallet with which to hit the moles and we found that the sensors went off every time a mole was struck. Perhaps the fur suspended over the sensor acted as a kind of force-distributor and depressed the sensor even if the mole was struck off to one side of its head. This device worked very well during the exhibition and held up to the stress of repeated blows.

 

We had a lot of fun building, and playing with, this robot. The problem-solving process involved was educational and we improved our engineering and computer programming skills during construction. There is a special way of thinking that goes with building something that works, and this class was a fun exercise in that left-brain kind of executive-tasking type thinking.