A New Catapult for Feed the Fish

Back in November I made this design for Feed the Fish.

Feed the Fish Catapult, Version 3

At the time, I was pleased with it.  Initial tests went well, and I had occasions where the catapult delivered five rounds into the aquarium in one shot.  Since then, however, I have become less satisfied with the design.  Here are the problems with it:

  • I misinterpreted the initial results.  The catapult can deliver five shots into the aquarium.  But it would be lucky to get three consecutive deliveries of five shots.  The catapult needs to deliver five shots almost every time for this game to be viable without hundreds of attempts.
  • Adjustments to the catapult are too difficult to make.  It is difficult to add or subtract energy storage (by adjusting the rubber bands) since it is necessary to partially dismantle the device.  Adjusting the trajectory is also tricky since it is necessary to fight the rubber bands whilst doing it.
  • The catapult generally fires to the left.  This might be because the trigger acts on one side of the catapult arm rather than through the middle.  It might also be because the rubber band is tighter on one side than the other.  I would like a design that fires straight!
  • The turntable only moves +/-10 degrees: in November I thought this would be about right.  But the catapult fires too far left for this to work.  Since I want the turntable design for our Nerf gun, I think the turntable is worth revisiting, and I think +/-30 degrees is a better choice.
  • It is difficult to set the robot up with this implement because it has the camera fitted.  It is necessary to juggle the lid, catapult and battery during installation: you need lots of hands.  This is probably not a problem for PiWars@Home, but it feels like the camera cable would be easy to break in the process.
  • There are quite a few aspects of the design which probably contribute to inconsistent results: the arm bearing has too much play, the turntable is not stable enough and the trigger does not act centrally on the catapult arm.

So, here is version 4.

Feed the Fish Catapult, Version 4.

These are the changes:

  • The trigger now acts centrally on the catapult arm.
  • The turntable has been re-worked to move +/-30 degrees.  It is also easier to tighten to remove excess play.
  • The rubber band can now be adjusted with a thumbscrew via a worm gear.
  • The trajectory can be adjusted by the two long bolts mounted on the sides of the catapult.
  • The arm now pivots on ball bearings rather than a bolt so it has less play.
  • The system uses the hull mounted camera.

That’s the theory anyway.  I’m currently printing the new parts to try it.

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