Saturday, 28 April 2012

We were planing on testing the thruster again today but since it is raining heavily we decided to postpone to Tuesday

I am still looking for a suitable reamer, I am not sure I will be able to find one so might have to settle for a small (5mm) boring bar. This should be able to cut the divergence with a 8mm throat.

I think I now have a good handle on the dynamics of the vehicle. The problem is that each motor produces torque in pitch and roll, and the way the control system is working now there are 2 PID's taking pitch and roll and outputting one thruster force. It works but not very well. A friend commented that the vehicle was allot like the 3 wheels omni drive robots we made in high school and he is right, what I really needed to do was think in terms of vectors. What I couldn't understand was how I could take two outputs (one from roll and pitch PID's) and transform that into three motor forces. Well it seems obvious now that I just need to have an expression for  roll and pitch, and to just think about the PID's output as a desired moment (although I an not sure what they are). I was sort of doing this before however I got really confused about how the PID constants matlab choose varied depending on if I was commanding "pure torque" to the vehicle or interpreting it as forces which then later got converted back to toques to make the simulation realistic.

Up until now I had been using a simple extruded triangle to represent the vehicle in the simulation. I finally managed to convert our solid edge assembly file of our protype to a VRML file via IGES. It was a huge pain but I finally got it working only to find that VRML viewer built into simulink couldn't render it fast enough (because it is too complicated). It was made in 1997......

So I had to remake the vehicle with simple shapes and while I was making it I thought, why are the engines so far out? Well I couldn't think of a good reason except they night have to be to get the control we need, but from my limited simulator experience I have found that very small thrust variations can induce large rotations quickly. In addition our current design is needlessly complicated. The original plan for fixing the tank to the vehicle was to epoxy brackets to the nitrous tank. I have had some second thoughts about that, mainly because it is a permanent solution. I have come up with a much simpler design:

The basic concept at-least is to bring the engines in as close as possible (model is really only for simulation purposes). We will probably need to have two  triangular plates separated with spacers between which can sit the valves and electronics. The tank can sit upside-down and its neck protrude through the first plate (with syphon removed). We can then have one plate on the top of the vehicle (bottom of tank with threaded rod to clamp the tank to the vehicle.

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