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July 28, 2001 Meeting Notes

July 28, 2001 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Bob Norwood


Today’s Flight Test Video




Gyro Bias Drift


We definitively solved a significant problem today.


On Tuesday, we noticed that the gyro rates drifted noticeably when all four solenoids were open, causing the attitude sense to drift somewhat in proportion to the throttle position even when the vehicle was flat on the ground.


We were able to see in the logs that the bias point voltages moved when the solenoids were drawing their 35 amps of current, which also dropped the main voltage level a couple volts, so I programmed the computer to read both the rate signal and the reference level each frame, and subtract them for “software differential A/D”. This made a minor difference, but the problem wasn’t easy to replicate on the bench.


If I dropped the bench power supply from 12v to 9.5v, the bias moved slightly, but not nearly as much as it appeared to when the solenoids were operating. If I dropped the voltage below about 8.5v, the gyros went into a very fast roll, much more noticeable than the problem we saw. I checked the specs, and the gyros take 9v – 18v, so it isn’t too surprising dropping below that caused problems. Those problems were not addressed by the software differential A/D.


I added tracking of the minimum voltage seen at any time during a frame, so we could see if it was ever dropping down to the 8.5v level that causes major gyro problems.


I started with a freshly charged battery today, and was puzzled to find that the rates still drifted when the solenoids were firing, even though the minimum voltage was still over 10v. It clearly wasn’t just a problem of voltage, although I still worried that with a little bit of battery discharge, we might drop under 9v with all the solenoids on.


We moved a few wires and cables around and retested, but to no effect.


We finally decided to run the solenoids from a separate battery from the rest of the electronics. I had wanted to avoid multiple batteries, but it was worth trying.


Luckily, we had everything we needed on hand to rework the power supply, and the battery that was intended for the master cutoff computer became the main electronics battery, while the larger battery was left to solely power the solenoids. We still had a common ground, but we had two completely separate +12v sources.


We tested it again, and while the voltage minimum logged by the electronics box didn’t change at all during firing, the gyro rates still moved when the solenoids went on.


We took it back apart, and separated the grounds. The solid state relays are optically isolated, so there is now no connection whatsoever between the solenoids and their battery, and the rest of the electronics.


We tested it again, and it now worked perfectly. No change whatsoever in the rates for any combination of solenoid activations.


Lesson learned: keep your electronics COMPLETELY separated from any high-amperage solenoids.


Damn spooky analog crap.



Flight Tests


We taped a new little inner tube on top of the lander tank, because the last one sacrificed itself for the cause on the last flight with the power outage.


I made two changes to the flight control logic since last flight: the gain has been turned down a bit so it shouldn’t kick itself back and forth as hard, and all the sensor values are averaged over an 8 msec time to reduce the noise we currently get from our A/D board.


We did a couple hover tests, and I would say that it is in general a slight improvement, with the oscillations being reduced to about three a second, and of somewhat lower amplitude. The filtering adds some additional latency, and it is clearly a crutch for a not-working-well A/D system.





Compare against the July 7 tests:






We did our first untethered flight (the mpeg at the top), involving flying over a little median in the parking lot.




It worked fine, and was straightforward to control.


I had purchased a force-feedback joystick with the intention of using a stick vibration as the propellant timer warning, because beeps from the computer just weren’t audible when the rocket was flying. I really just wanted something crude like the Nintendo rumble packs, so the Microsoft Sidewinder force feedback joystick is massive overkill in terms of sophistication, but it was all I found. I haven’t written the code to actually use the special features yet, so I was just using it as a normal joystick today. I find that it doesn’t offer as good of self centering as a normal joystick, and it is easy to pull it a little off center without knowing it. We switched back to the other joystick to let Russ fly a hover test.


Russ is going to cut all the engines down so we won’t need the spacers in the chambers, and also fix the threads on the top closure that is visibly leaking in the video.


Our next flight test will involve a new joystick interface for piloted roll, allowing 360 degree or greater rotations, instead of just +/-20 degrees like the other axis have. There will be a couple subtleties to doing this, but it shouldn’t be much trouble.


I am also ready to test a flight control option that recalculates the engine levels every millisecond, instead of every PWM frame, allowing it to change an engine level without waiting for the next frame in some cases. This should have a slight benefit in reducing latency related oscillations.


I may also try the accelerometer-as-inclinometer initial angle setting soon.


Manned Lander


We got the foam pads under the frame today. They are a bit stiffer than we were expecting, but they should work fine.


Russ and Phil finished the rest of the attitude (70 pound thrust) engine machine work today. We will make cat packs, injector plates, and retaining plates, then test fire each of them on Tuesday.


We need to get slightly smaller clamps to hold the engines to the frame.


We made connectors and cables for all the solenoids to the electronics box.


Russ has the metal stock for the main engine.


We are going to wait until we have all the plumbing in place and everything nicely routed and tied down, then Bob is going to take it back and add brackets for the throttle and joystick, and get it powder coated.


We may be hopping the frame on just its attitude engines (without a person) in three weeks or so.


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