Building an F450 quadcopter – Removing the Wobble

With plenty of flight modes now available it was time to address the wobble I noticed on the first flight. 

Some Googling pointed to the most likely candidate for wobble and the copter sounding “angry” was that the P-gain was too high.  I don’t know what that means but I figured that this would be easy to take care of.  Take off in Altitude Hold, run Auto Tune and, voilà!  Problem solved.  

Unfortunately it turned out to be more difficult than that.

With my new-found knowledge of how to set up the controller I was able to configure one of the options to be Altitude Hold, and set up a switch to start Auto Tune and I set off to get it done.

It was a somewhat windy day so I chose a local school (it was Saturday) that had a nice square protected on three side by buildings thinking this would give me some protection.

I took off in stabilize, stopped at about 15′ and switched to altitude hold.  Immediately the quad shot up about another 20′.  It was now high enough that it was no longer protected by the low buildings and it started to drift fast!  I immediately pressed down on the throttle and it came down, but much slower than I hoped.  I switched back to stabilize as it got near the ground and it dropped very fast, bounced once and landed up side down.  Bummer!

A quick check and things seemed OK, or so I thought.

I sent her back up again and exactly the same thing happened.  This time I had left myself more room and tried to leave it in alt. hold, but now it was really acting oddly.  Pressing the lever to make it go forwards caused it to climb more each time and, with the wind, it was now high up and drifting over the top of the school buildings.  I was convinced it was going to end up on the roof and, in a last desperate act, pushed forwards all the way.  Finally it responded and shot away from the school.  Bringing it down again was, once again, very slow and with the lever all the way down I switched back to stabilize.  Unfortunately the throttle all the way down in stabilize basically means turn the props off and the quad drop out from the sky like a brick.

Time for some self-analysis and to find the culprit of the problems with altitude hold.

vibrationMore Googling revealed that altitude hold issues are often caused by vibration.  Fortunately the APM captures logs every time you fly and I was able to download those into Mission Planner and (after some experimentation) view the vibration logs. This proved to me that this wasn’t the problem. 

flight_mapIt also showed me that this little APM flight controller is super cool. If you have time it is well worth looking into the logs and what they can tell you.  You can even upload these into Goole Earth and get a 3D picture of your flights complete with color coding to show you which flight mode you were in.  Nice!!

Since Auto Tune was off the cards until I could figure out what was wrong with the altitude hold it was time to start asking for help.  Someone on the Quadcopters group provided me with the settings they had that worked and, in comparing those, I noticed that a TRIM_THROTTLE value of 33% seemed much too low compared with his settings.  This is APM’s estimate of the throttle required to maintain a level hover calculated automatically from the pilot’s throttle input while in stabilize mode.  Mine was way too low and actually outside of the 40-60% that is the mid point for altitude hold.

On a hunch that I hadn’t flown in stabilize mode enough for it to get a good reading I went into the back yard and ran through two fully charged batteries doing nothing but trying to keep it in place in stabilize.  Then it was off to a large open area and, voilà!  Alt hold was now working without any crazy behavior!  I was losing altitude on long runs, but for the most part it was well-behaved.  Checking later I found the  TRIM_THROTTLE value of was now a much more sensible 43%.

While working on identifying the Alt hold issue I had also found out that I could assign a knob on the controller to Ch 6, and set that up such that I could adjust the P-Gains while in flight.  I did that, used the knob to back off the Rate Roll and Rate Pitch P gains until it calmed down.  When I got home I found out I had dropped from .15 to .13 so I locked those values in and saved the configuration file to my hard drive as a baseline.

Oh, and for the record, let me just say that testing altitude hold while tying the quad down to a table gives the barometer false readings and doesn’t work, so don’t try this 🙂

But what about Auto Tune?  Now that I had a copter that could stay in the air I went out the next day with a fully charged battery and ran it.  It is very odd watching your quad do it’s dance with no input from you, but it finished without incident.  The results?  Terrible!  It was back to being funky again and very difficult to control.  The good news – remember above when I said I saved my working results as a baseline?  Well, I just restored those.  My old developer habits still keep me honest.

I still have a problem with the craft losing altitude when flying forwards.  I think I might have improved that by stuffing the APM with cotton wool, since I had lost the little piece of foam.  Sadly that was three weeks ago and I have not had a chance to fly since then when it wasn’t raining, snowing or blowing 35mph winds.  I’ll keep you posted on how that goes.

Advertisements

Leave a comment

Filed under Building Multirotors, Education

Your thoughts?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s