Flying Fish Food

We've filmed our final arena challenge - Feed The Fish. This was always the one that troubled us the most as we really weren't sure our fish food cannon could do the job. 

Nanny McPi's Fish Cannon Ready to Fire

This picture shows the final design viewed from the top. It's constructed from 3mm plywood (as per the rest of the robot) glued and bolted together to form a rubber band powered cannon. The plunger that holds the Nerf Balls fish food is pulled back against the rubber bands and held in place with an electro magnet. When the electro magnet is disabled, the cannon fires. 

Evolution of a Fish Food Flinger

When we entered the competition, we had no idea how we were going to get the fish food to the fish. 

Most of our ideas start from lego, so we tried building a spring loaded catapult, but couldn't get the spring power enough to launch the balls correctly. 

The next idea was to take an old pair of tongs - bolt a plastic cup to it and use that to launch all the balls. Because the tongs were metal, we though holding it back with an electro magnet would be a good way of launching it without any clever linkages. 

So we built a mock up, but it was very difficult to angle it to get the balls to go in the right direction - they would either go straight up in the air or backwards. 

Looking at a simple spring powered nerf gun we started to think about having a barrel to hold the balls and use elastic bands to power a plunger which pushes them up the barrel towards the target.  

So we designed a prototype cut it out of wood with the scroll saw. Below you can see the pieces we cut out - before they were glued together into a tall barrel.

Pieces for the original cannon

Once put together, we tested but it really didn't work. As we fired the cannon, the balls squeezed together against each other and the side of the barrel. One ball at a time was OK, but we didn't want to load each ball separately. 

So the next thought was that if we made it wider, and put the balls side by side, it would behave as if they were separate balls. 

So - using the same design, but making everything wide enough to hold 5 balls in a row, we build another version. Keeping our idea of using an electro magnet for firing, we also built a chassis for it to test it properly with the magnet. This was better, but even with this, we found the balls seemed to be getting stuck and sometimes not coming out. 

The thinking was that the balls were being squashed against the side of the barrel again. We tried building a little paper surround for the plunger to separate the balls from the barrel during the initial acceleration and that seemed to behave better. So back to the scroll saw and we built another cannon - this time with an enclosed plunger, and wider barrel sides to accommodate it. The picture below shows the new plunger. 

A better launcher

The bits of green tape at the bottom are to secure 2 metal disks to the bottom of the plunger which are used to hold it against the electro magnet for firing. 

After putting it all together we tested it again and it was better, but very inconsistent. We'd be lucky to get a single ball in each time. 

So we decided to take a break. At this point the competition delay was announced, so we decided to leave it and concentrate on other things. 

The Final Design 

The design we used for filming is exactly the same version that we stopped development for earlier in the year. 

We painted everything black to match the robot - except the inside of the barrel which we've left plain wood and polished with beeswax to reduce friction. 

We were using cheap imitation Nerf balls and noticed that they were very inconsistent. Some were better than others, but it was difficult to find a good set. So we decided to splash out on a genuine pack of Nerf Rival Balls. This was a good decision. The difference in quality is very noticable, with less of a seam and they were much more consistent. 

The other change we made was to reduce the angle of the cannon and use weaker elastic bands so the balls travelled in a low arc - rather than the original near vertical position where the balls were fired high in the air. This proved much more accurate. 

So here's the final design. 

The Final Fish Cannon Design

The wing nuts at the side allow the angle to be changed and then locked into place. We're using loom bands for the power - by adding or subtracting bands we can adjust the power. 

The picture below shows the electro magnet mounted underneath the cannon barrel. 

The Magnet Mounted At the Bottom Of the Cannon

We're using a L298N motor controller to power the magnet. This was intended to be a temporary solution whilst we created a relay circuit, but the L298N did a good job - so we decided to keep it. 

There is a noticeable delay when disconnecting the power and the magnet releasing the cannon plunger, so we need to account for this in the software. Early attempts had the robot turn to come back to the side of the arena before firing the food. 

Automating the Challenge

We're automating all of the arena challenges, and this is no exception. 

The automation is very similar to the 'Tidy The Toys' challenge. We first put the camera in 'Horizon Mode' to drive out of the home area straight towards the opposite wall until we get to a certain distance. 

Then the robot will turn left and switch the camera into block mode. Because the camera is mounted so low, we can't see much of the fish tank so, because we're not using the opening at the bottom, we've put a sign in front to indicate 'no golfing' and can use the colour of this as a target.

Locating The Fish Tank

We then drive slowly towards the fish tank until we reach the optimum distance, and then release the magnet to fire the fish food. 

Once this is done, the robot will turn left and drive back, stopping close to the arena wall where it can be retrieved and placed back in the start zone. The cannon is reloaded and re-armed, and we press a button on the controller to start the next run.  

The Results 

Without giving too much away we're quite happy with how the challenge went. We didn't get all the balls in, but were more successful that we thought we would be. Here's a short video of the cannon firing.