Defense

Yesterday, we installed the rocket launcher. It is a large piece of pvc pipe that is on a tilting mechanism that we built with some Vex parts we had laying around. It will fire model rockets. There is a small neodymium magnet in the little cargo hold of the rocket and another magnet on the outside of the tube. In this way, we can install the rocket motor, attach the ignitor and wires, and then use the magnet on the outside of the tube to hold the rocket in the tube without falling out.

We are also going to be getting a new motor controller. We are going to get the Sabertooth 2x25. Right now, we are using a little BaneBots controller that has a maximum current draw of 9 amps. Since toque is proportional to current draw, our rover has had a hard time going fast or up any incline at all because we have limited its draw at 9 amps. The new controller is really cool. It has a continuous draw of 25 amps and a maximum draw of 50 amps for a few seconds. By measuring the static resistance of the motors we are using and using ohm's law, we got a theoretical maximum draw of about 64 amps. This new controller also has several features like the ability to recharge the battery when the motors slow down or go in reverse. It is 2 channel, so we only need one which is good becuase it is not cheap. Its $125, but we think the performance we will get will be worth it. We will use the old controllers to fire the rocket (supply 12v to the ignitor) and control lights. We plan on having headlights and some underglow provided by some old cold cathodes we got out of some scanners.

I also learned yesterday the difference between PWM and PPM. PWM is pulse width modulation and PPM is pulse position modulation. PWM works by detecting the length of each pulse. The time between the pulses is always the same. PPM works by detecting the ratio of the pulse length to the off time length. Most hobby servos and motor controllers are expecting PPM. When writing Arduino code, analogWrite() sends PWM and the servo class sends PPM. I didn't know it, but we have been using PPM the whole time. The good news is that the new controller will accept both PWM and PPM. We are also going to start using a new servo library called Megaservo. It lets you control a servo from every digital pin, so up to 12 in our case. We will only use about 4 though.

Sorry for the long distance between posts, but in the next week or 2, we will have more time to work on it and add stuff. I'll put pictures up soon.