Swash jitters when throttle=0: RESOLVED!
« on: May 10, 2018, 05:48:59 am »
   Hi, all... back with another newbie problem.

This is a bench test only, no components installed in the heli.  Hitec Aurora 9 Tx, Hitec Optima PWM Rx, standard CC3D board running 16.09, Hyperion digital servos.

I'm using both the Cronus and JWgera heli setup guides here for reference.  Now at the point where all the basics are set up, swashplate is properly aligned mechanically, Max and Min values reduced to avoid servo problems.  All servos move nicely in the proper direction in response to Tx stick movements, and also in response to the channel sliders on the Output screen.

Problem:  When I move the throttle stick to full low throttle, the three collective servos start jittering back and forth.  The Yaw servo is not affected. 

Everything I've read says I should set the Throttle and Collective 'neutral' values to just a point or two above their respective 'min' values to correct this phenomenon, but that makes no difference... jitter still occurs at zero throttle (and ONLY at zero throttle).  Moving the throttle stick just the slightest bit away from zero, and all three collective servos stop jittering and behave perfectly.  The same thing occurs regardless of flight mode... Attitude, Rate, or Manual.

All servo output channels are set to 330 Hz and PWM.  Throttle and Flight Mode channels are set to 50 Hz and PWM.
All PID-related values are still at their defaults.

What have I overlooked?

I've attached the .uav file in case anyone asks.
« Last Edit: May 10, 2018, 09:48:58 pm by Bobrcnut »


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Re: Swash jitters when throttle=0
« Reply #1 on: May 10, 2018, 06:14:18 am »
Try setting neutral for throttle and collective to exact the same as their min (1117).
What happens then?

Re: Swash jitters when throttle=0
« Reply #2 on: May 10, 2018, 06:41:11 am »
My fixed wing servos jitter when in stabilized modes too.

(First disable main motor and tail motor somehow or at least remove blades in both places) Maybe go to Output page and do Test Outputs.  If the jitter goes away and it allows you to move the servos with the page sliders, then the jitter is coming from stabilization, a small amount is normal.

If the jitter stays, then it is a digital servo issue, maybe it does not like that update rate.

Also, if it's the first time using digital servos, be aware that some do make more jitter noise at idle than others.

Do we know they are servos designed to run at faster than the standard 50Hz rate?

Re: Swash jitters when throttle=0
« Reply #3 on: May 10, 2018, 08:30:28 am »
Karla:  Setting the 'neutral' values the same as the 'min' values does not alleviate the problem.

TheOtherCliff:  As soon as I check the 'test outputs' box, the jitter disappears, and all servos follow the sliders perfectly throughout their full range of movement.  Turning off 'test outputs' causes the jittering to resume if the Tx throttle is at zero.
The Input screen shows no evidence of any 'jitter' in the incoming Rx signals.

These servos are documented (and proven to my satisfaction) to be able to handle up to 400 Hz, and I have an identical set installed in a heli that uses a Revo Mini.  No evidence of any jitter on that setup.  Also, the tail rotor servo does not exhibit any jitter at all under any conditions... It's only the three collective servos.

I'm familiar with the characteristics of digital servos, with their slight hum and occasional 'vibration' around neutral if their deadband is set too tight.  These same 4 servos behave properly in all other applications.  What I'm seeing in this case is a more significant jitter back and forth around neutral fairly fast, and the jittering is enough to cause them to move around on the work surface.  The jitter does not decrease or 'fade away' as the throttle stick is moved from full off... the jittering just completely disappears if the stick is moved by even the slightest amount.  The servos are not mounted, just sitting stationary, and holding them in a fixed position has no effect on the problem.

Since the same thing occurs with FlightMode = Manual, doesn't that rule out stabilization as a possible cause?

Next step, I'm going to import a .uav file from a different (but almost identical) setup and see if that changes the symptoms at all.

Also, this is just a test bench setup.. no linkages attached to the servos, and no motor connected to the ESC.

Thank you both for your help.    :)

Re: Swash jitters when throttle=0
« Reply #4 on: May 10, 2018, 06:14:28 pm »
Since the same thing occurs with FlightMode = Manual, doesn't that rule out stabilization as a possible cause?

That would have been my next question, and says to me that it is not stabilization.

You say that it only happens at zero throttle?  At above zero (Manual) it no longer jitters?

Re: Swash jitters when throttle=0... RESOLVED!
« Reply #5 on: May 10, 2018, 09:47:47 pm »
TheOtherCliff:  that is correct... if the throttle were set anywhere other than full off, everything worked perfectly.

HOWEVER:  this issue is now resolved.  Chalk it up to inexperience on my part.  After thinking about it last night, I decided it HAD to be some weird relationship between zero throttle/collective (as sent by the Tx) and zero throttle/collective (as set with the Output sliders).  SO.. I re-did the Input calibration, being VERY careful to wait for GCS to establish its Min/Max values for each channel...  and now, NO MORE JITTER.

As I said:  inexperience on my part.