SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Jun 24, 2014, 07:03 PM
Registered User
United Kingdom, Scotland, Edinburgh
Joined Jun 2014
22 Posts
Discussion
custom quadcopter randomly shaking

Hi,

Hope someone can cast a light on my issue I've been having with my custom built quadcopter.

It basically flies as it should but every few seconds it starts to shake quickly like the Kp was too high. I tried to decreasing Kp gain but the shaking does not go away, instead at some low Kp it starts to wobble slowly as the Kp is too low.

Interestingly, the quad only shakes diagonally on "front left - back right" axis. The "front right - back left" axis seems to be rock solid. (The quadcopter is in X configuration). However, the shaking is not continuous. It kicks in randomly and settles down within second or two. It is not related to stick position. The shaking does not really hinder the flight but it will ruin any camera/fpv.

I tried also to put my MPU6050 board on damping foam, but this does not seem to help (http://www.hobbyking.co.uk/hobbyking...dProduct=15214)

What else should I suspect?

My configuration:
frame: HobbyKing Mini Quadcopter Frame V1 - 539mm
motors: turnigy 2209 (980kv)
propellers: 80x39
ecs: afro 20amp with simonk firmware
weight: 650-800g (depending on battery used)
loop frequency: 200hz

This issue is unrelated to flight mode. It is present when flying Rate/Acro or Stab.

My current PID configuration:
Kp: 600
Ki: 0
Kd: 0

However I do not think this is to do with PIDs. It only goes into quick shake every few seconds or so. As mentioned before I went through the entire spectrum of Kp from 300 to 800 and around 800 I see constant high frequency oscillations, and around 400 the quadcopter is not stiff.
I tried to increase Kd but this does not remove the shaking - at high point the motors start to pulsate and at low point I get smoother flight but shaking still is present.

Any advise will be very much appreciated

Thank you,
Gregory
gregd72002 is offline Find More Posts by gregd72002
Last edited by gregd72002; Jun 24, 2014 at 07:17 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Jun 24, 2014, 07:36 PM
Team WarpSquad
United States, PA, Doylestown
Joined Jan 2014
3,501 Posts
Vibration can cause weird issues. Is the motors and props balanced?
waltr is offline Find More Posts by waltr
Reply With Quote
Old Jun 25, 2014, 12:37 AM
Registered User
United States, WA, Twisp
Joined Sep 2013
472 Posts
I had a similar issue at one point with my quad. With 9 or 10" 3 blade props it flies great, put a set of 11" 2 blade props on and it bucks like a wild bronc. Later.

Dave
daved20319 is offline Find More Posts by daved20319
Reply With Quote
Old Jun 25, 2014, 01:40 AM
Walkera Beta tester.
mattjk's Avatar
United States, CA, Azusa
Joined Mar 2014
2,488 Posts
In order for p to function, you need to turn up d. D stops P.

Try .01 and go up from there.

Then you also need to turn up I. I helps with wind and self leveling
mattjk is offline Find More Posts by mattjk
Reply With Quote
Old Jun 25, 2014, 06:15 AM
Registered User
United Kingdom, Scotland, Edinburgh
Joined Jun 2014
22 Posts
Thanks for all the answers.

Here are a few videos of the copter with Kp being 700, 600, 500 respectively.


Kp set to 700 the quad is stable but does shake with high frequency:
Kp700 (0 min 50 sec)


Kp set to 600 the quad is still reasonably stable, shaking is not as strong as with Kp 700:
Kp600 (0 min 57 sec)


At Kp 500 the quad is getting unstable and still shakes but with lower frequency:
Kp500 (0 min 37 sec)



Ki was set to 1500 (around 3000 I'm getting very bad slow oscillations).
Kd was set to 0.006 (around 0.015 the quad is getting extremely sluggish).


From the videos you can see that the Kp 600 seems the best. If you watch it towards the end (40sec+).

It somehow sounds and looks to me like there is something else going on than PID configuration... maybe really propellers are not right size?

The propellers are balanced (checked them before making the video). The motors sound good to me too.
gregd72002 is offline Find More Posts by gregd72002
Last edited by gregd72002; Jun 25, 2014 at 06:49 AM.
Reply With Quote
Old Jun 25, 2014, 09:21 AM
Registered User
United Kingdom, Scotland, Edinburgh
Joined Jun 2014
22 Posts
or can this be due to a bit flimsy frame or vibrations?

How to distinguish this two behaviors?
gregd72002 is offline Find More Posts by gregd72002
Last edited by gregd72002; Jun 25, 2014 at 12:24 PM.
Reply With Quote
Old Jun 26, 2014, 04:38 AM
Registered User
United Kingdom, Scotland, Edinburgh
Joined Jun 2014
22 Posts
Right, I started to log the vibrations. They seem to be within (0.65 / -0.65) G region.

Is this much?
gregd72002 is offline Find More Posts by gregd72002
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion A war between shaking and not shaking..:) FriFlyers Scratchbuilt Multirotors 0 Feb 11, 2014 05:04 PM
Sold 450mm ArduPilot Quadcopter with lots of extras + a FPV custom ground station/LCD! FlySkyVideoInc Aircraft - Electric - Multirotor (FS/W) 1 Sep 24, 2013 09:10 PM
Question Quadcopter shake cessna20122013 Multirotor Talk 4 Jul 26, 2013 08:57 PM
Discussion Quadcopter shaking in flight? iCrap Multirotor Talk 8 Jun 14, 2012 09:32 PM
Discussion Finally a good use for my FPV Quadcopter... Watching my wife "shake it" benderfly FPV Talk 10 Jun 04, 2012 10:32 AM