The Armadillo Aerospace crew has been busy at work on their X-prize prototype, and are having some good success to date. They are now working on getting the single engine rocket to hover in place using control vanes in the exhaust flow. This is really neat, and coming up along the way the DC-X did its flight control, this seems a whole lot simpler.
To make up for the relatively low 10hz update rate of the GPS (I wish I had gone ahead and ordered the 20hz option) and to provide some more graceful failure modes, I combined the inertial position and velocity sensing with the GPS updates, so it gets reset every time a valid GPS packet comes in, but will coast with pure inertial data if the GPS is failing, and provide useful data between GPS updates. This isn’t as good as a truly integrated GPS / IMU system that can use the IMU data to smooth the selection and balancing of different satellite signals before generating a GPS output, but it does several positive things for us. The upwards position / velocity is easy to use the IMU for, because it can auto-orient from the gravity vector while on the ground, but to get north / east data, we now have to orient the vehicle correctly before launching. I have a magnetometer that we could use for an automatic roll orientation, but I haven’t plugged it in for a couple years.I also updated the GPS baud rate, which doesn’t give me any more samples per second, but decreases the latency in getting the updates it does produce.
The auto-hover was somewhat smoother than before, but nor dramatically so. The overshoots are proportional to the control authority times the sum of the sensing latency and the actuating latency with the current control algorithm, so I may just intentionally slow down the valve movement. I may convert it over to a gain based control system like the attitude control, but it is trickier because the vehicle is constantly changing weight and tank pressure as propellant is depleted, so there isn’t a reasonably point on the throttle that a default actuation position could be based on. I don’t want to waste much time on it, because hovering isn’t actually an important part of what the vehicle is supposed to do.
The three videos up are pretty neat to watch.
http://media.armadilloaerospace.com/2004_05_09/autoHover.mpg
http://media.armadilloaerospace.com/2004_05_09/wrongWay.mpg
http://media.armadilloaerospace.com/2004_05_09/autoHold.mpg
Posted by elkaim at May 12, 2004 2:48 PM