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Position hold, big jet vanes

GPS issues

May 8, 2004 notes


Position hold


We did several more flight tests to track down the GPS loss-of-lock issue.  We replaced the internal 802.11b antenna, which was very close to the GPS antenna, with an external one several feet removed, but that had no impact.  We actually moved the GPS antenna off the vehicle to a point ten feet up the tether to the lift, and it still lost lock almost immediately after liftoff, so we conclude it has nothing to do with the antenna.  We then took the GPS circuit board off of its mounting standoffs and wrapped it in foam and loosely wire tied it down.  We made two perfect flights in a row with this arrangement, so it seems conclusive that some kind of vibration is hurting the RF processing on the board itself.


We then tried an auto-hover with the GPS, and it worked fine.  The vehicle was bouncing up and down a lot in place, because the control authority was a lot higher than the responsiveness of the 10 hz GPS update, but it worked properly.


I had another theory on the GPS problems that sounded plausible: while the entire electronics board is mounted on a foam isolation ring that dampens any vibrations from the frame, it is mounted right above the engine, and the base is directly exposed to the ground reflections from the engine.  Acoustic vibrations might be getting transmitted directly to the board.  We remounted the GPS on the standoffs and glued a foam pad on the bottom of the electronics honeycomb and did more tests.  This seemed to be better, as I was able to get a full flight off, but it still had a cutoff once right as it was leaving the ground, so we went back to the loose mounting of the GPS.  I am fairly appalled that this $8000 GPS system intended for solid fueled missile applications is this sensitive to vibrations.  It sounds like a cracked trace on the board, but I’m not sure if I want to send it back to Thales for another $500 service check that will take two weeks and probably result in them just flashing the bios again and saying it is fine.


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.


http://media.armadilloaerospace.com/2004_05_09/autoHover.mpg (two angles plus on-board camera looking out)


Horizontal position hold, however, would be very useful for us.  Ideally, we want the vehicle to land on the same pad that it launched from.  There was quite a bit of testing and adjusting to get all the axis transformations correct in dealing with inertial space, GPS space, and jet vane angles.  Just like in computer graphics, you almost always wind up getting something backwards the first time, then just throwing in a negation or swapping the order on a cross product.  One point of note is that until we get our differential GPS base station and correction factors working, I am using integrated GPS velocity instead of direct GPS position data for the position hold.  The idea is that a slow drift is much superior to the vehicle instantly thinking itself ten feet off the target spot when the GPS chooses different satellites.


We had a couple exciting runs where the correction factor was backwards, making the vehicle tip into the direction it was moving instead of away from it.  Those runs completely justified the expense of hanging the vehicle off the ground with a lift truck.




Finally, I got everything in the right order, and we made two successful tests with the computer modulating the vanes to hold position.  All I did was say “go up”, and the computer managed the throttle and the joystick, then came down eight seconds later.  The position correction period was about two seconds, and the amplitude was about one foot in Y and only a couple inches in Z.  This being a first random guess at parameters, I’m sure we can cut this down to an extremely smooth position hold with a little tuning.  I am looking forward to doing untethered flights like this at our remote test site and moving on to accelerating boosted hops.


Unfortunately, the engine on the little vehicle was starting to give us problems by the end of the day, with the exhaust staying cloudy on the ground, and spitting some liquid in the air.  It has over twenty runs on it, so it is probably time for us to cut it apart and see how it looks.  We have plenty of ring catalyst, so if the hot pack is just worn out, we can quickly replace it.  If the monoliths in the cold pack (which were scavenged form an even older engine) are worn out, we will have to build a new engine for it.  We are extremely thankful that we only have a single engine to worry about now…





Big jet vanes


We got lots of work done on the big vehicle conversion.


The tank manway came off, and I cut out the old parachute mounting peg and milled the central hole wide enough to take a 2” pipe.  We also added anti-swirl baffles to the inside of the manway.  We still have separate pipes through the manway for fill and drain, but the next one we make will only have a single (probably 4”) pipe coming from the tank, with fill and drain branched off of that below the manway so there are less weakening holes cut in it, and no pockets of trapped propellant in the vertical orientation.


We are going to try mounting the landing gear directly to the manway instead of to our bonded mount points at the tank periphery.  We have some concern about hard side loads on the manway flange, but it makes a lot of fabrication things easier, and it will allow us to just transfer the propulsion and landing gear to any other tank easily.  We needed to add almost two feet of height to the vehicle to clear the big engine and vanes and leave us room for growing to a larger engine.




The big jet vane system is coming together rapidly.  The vanes and actuators are mounted to a stainless plate that is welded directly onto the big nozzle.




We cut off the plumbing and mounts on the new big engine that we had set up for the test stand and made custom flanges for the 2” KZCO valves so we could stack the throttle and master cutoff valves as closely as possible.  The manway feed pipe slides into the top flange with a radial seal, so there is no stress on the plumbing when the engine is mounted.  We still have to build the mounts that will hold the engine to the manway.




We have a pretty good shot at doing a tethered hover of the big vehicle on jet vanes next Saturday.




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