Sunday, January 4, 2015

Nova Flight - Unstable and WoD

Seems that WobbleOfDeath is back. This flight was at big field and took it up high. Clear day with Winds 5-8mph. Stock GPS is giving unbelievable sats and hdop.

Seems like the Nova has only flown without WoD a couple of times, since it crash-landed in a tree a couple of months ago. It flew so badly today that I only flew one battery through it ... I was afraid it was going to flip over and crash-down hard.

It flies now (only fairly, seems unstable) if I'm in control. It will hover at 4 feet but that and consecutive AutoTrims continue to take a good amount of stick correction. If I try to let it control itself (loiter or RTL) it starts wobbling pretty bad. It's like it doesn't know where level is or is slow to respond. Also, sometimes when it RTLs (wobbling but finally touches the ground) it can't sense it landed and skates across the ground (thank goodness for prop-guards).

I suppose another symptom is that the quad seems over-sensitive to "within limits" vibrations. I have also noticed that "level" slightly drifts away after a time (on the same desk, it's not perfectly level any more a few days later).

In Terminal/ Tests (AC 3.1.x only) I did test GPS, barometer, compass, and ins (accelerometer/gyro). It really hammers the sensors and you can view real-time data. I have a feeling it might also reset them in some way.  I am NOT getting any "Bad Gyro or Accelerometer Health" messages in MP-HUD, so that's good.

Calibrations work fine. When calibrating the Accelerometer/Gyro be sure quad is perpendicular/level and very still (propped-up, not being touched) at each stage. GPS and Compass seem to be working fine.

I just checked the logs from this flight. Auto-Analysis is mostly ok except for some Pitch/Roll/Thrust Fails due to extreme rocking. There was also a Compass Warning about 34% mag-field change. In normal logs, ATT clearly shows WoD happening. If PerformanceMonitor shows high or spiking LongLoops it could be a sign that the processor is being over-burdened. PM shows no NLon over 4 (out of 1000) which is very good. Curr/VCC (5v) is 5.10v~ 5.18v. However, IMU is showing vibrations again. While I previously got X,Y down very low, they are now reaching the Acceptable Limits of  -3~ +3, . Same with Z axis, which is now again over limit of -15~ -5, at -17~ -3.

Edit 2015-01: If you look closely at above graph, you will see that AccX is also beyond acceptable range. X is really -4 ~ +1. I think this is because (at logging time) the FC's gyro/accelerometer chip is not in true middle of quad.

It currently flies so unstable that if I continue to fly it (even with gradual trouble-shooting changes) I'm pretty sure it's going to flip and crash. A couple of times today already, it Rolled and Pitched very close to 90 degrees. I have come to the conclusion that either the Flight Controller is bad, some component is installed fundamentally wrong, or the frame is just to weak to run the motors within acceptable vibration limits. I'm also wondering if that after a bad crash (and the accelerometer and/or gyro gets jolted to a new limit or g-rating) they become more sensitive? Speaking of the body and/or frame ... it's no secret that to keep the aircraft light, the designers selected a body shell that also acts as the frame. It's purposely barely adequate and is in no-way over-built.

I would like to have a stable flying quad so I can move on to autonomous missions, add a gimbal and better AP camera, and other cool things. I have decided to strip the aircraft down ... and rebuild it (in a very simple and basic config) one last time. I need to verify the Flight Controller is good. If it needs a FC or a stronger after-market frame, I'm not sure if I will try to fix it or just strip it for parts and move on to the next model. All that said, it's been a good first GPS-capable quad. I have learned a lot about APM and quads in general this summer and fall.

Edit 2015-03: The rebuild worked great. Here are the results.


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