I recently upgraded to 3.1 on a trex 600 running digital servos and I am getting a random twitch. I thought at first it was the ppm encoder so I re-flashed it like I have done before on other APM flight controllers. After doing so I didn't see any changes and it seemed to twitch just randomly like before. 

On another note I did see more twitching if I ran higher voltage to the servos. 

Of course I haven't flow it in this configuration.  

Has anyone seen this problem?

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Hi Sean,

I have the same problem with  my TR 700 E DFC  High voltage servos. But did not have it in v 3.01 at all. The Heli was flying pretty well in 3.01 with most of your param. from July 2013 I believe.

My 700 had the exact setup in 3.1 as Manfred D. did setup his TR 600. ESC got signal direct from RX not from APM. That setup has crashed Manfred's 600 in v3.1 at the first test flight, to a total write off. His TR 450 was fine in v 3.1 before!

I have changed immediately  so APM Ch8 output to ESC. Everything is working but I do have the same twitching in the 3 servos but not at the yaw servo. It is in ARMED or DISARMED and when Motor is running or not. But when I change to ALT HOLD and LOITER it is totally gone. Armed or not Armed.  So this problem appears  in Stabilize and DRIFT only,

Have not tried to fly with those strange appearances.

I can not help you further because I am waiting for Manfred D. to get his opinion first. He has far more experience than me.


Hi Sean,

my problem is solved. The frame rate in the DX8 Transmitter was on 22ms and I changed it back to 11ms for digital servos and  all is good, no twitching anymore. All flight modes working on the bench. Finally I dare to do a first flight test soon.


Are you guys using APM?

I still have an APM on my 450 with 3.1, runing HV servos on 2S direct, and I haven't had any twitching issue.  I have had a twitching problem with my PX4, that I have sort of solved...

Hi Robert

Yes, I am using APM 2.5 in my two TREX Helis. My twitching was a problem in my Spektrum DX8 TX I had set it to the wrong Frame rate for servos. It is definitely gone now, no more twitching.

As you Know Manfred Dickgiesser lost his Trex 600 E because of full negative collective pitch switching into  Alt Hold during testing first time v3.1.

We both have changed since to your recommended way to let the APM control the ESC through Ch8 output channel. We both had the ESC controlled direct from the RX.

Today I have done some testing on the bench with my TR 450 without the blades and discovered that with the APM armed and the motor running for the first time after rebooting, switching from Stabilize into Alt Hold  - the collective pitch jumps full negative. You switch back into stabilize and back into Alt Hold it will not do it again.

You must switch from stabilize into alt hold in the first 15 seconds and it will happen each time after a reboot of the APM.

I e-mailed Manfred who tried this with his  TR 450 - he got the same result.

I am sure it is not likely that somebody takes off that quick and switches into Alt Hold but if somebody is in a hurry and does that another traditional Heli is kaput. This happens wile the APM is controlling the ESC through channel 8.


And if you in the air and by mistake switch the motor off (CH8 low) then you get full negative collective pitch, your heli is dead. To cut the motor for a fraction of a sec would be a big problem, but to get full negative collective is.
And if you in the air and by mistake switch the motor off (CH8 low) then you get full negative collective pitch, your heli is dead. To cut the motor for a fraction of a sec would not be a big problem, but to get full negative collective is.

Hi Rob, I got some random twichting with my PX4 as well, how did you solve it?

Fred, yes, I heard about Manfred's crash, and it is unfortunate.  I can't test for all conditions, and I've been using Ch8 ESC control exclusively for over a year now, and recommending the same.  The problem would also not have occurred if the heli had been armed first.  That's actually the key point to the whole thing, you MUST arm first.  The Ch8 ESC control just ensures that you CANNOT take off without arming first.

As for your other thing here about switching to Alt Hold, this may only be quirk about bench testing.  The system is getting quite sophisticated, and so bench testing isn't always accurate.  But I will try to check it.  Could be that since you weren't actually in the air, it was storing the wrong Throttle Trim value.  

I have occasionally noticed that when switching into Alt Hold the first time, it will jerk down initially, but it doesn't go far, as the APM feels the downward acceleration (which it won't on your bench).  It typically only falls about 1-2 feet.  But I haven't seen that with 3.1 yet, so maybe it's fixed.  I'll try to have a look.  I actually did a bunch of take-offs and landings, right from Alt Hold mode, and it is working fine.  

This is a big, complicated issue.

Initially, I had the servo twitching problem basically all the time on the bench.  I suspected it was a dirty power problem, and on a pure hunch, removed the TI 08080 regulator from the board and replaced it with a Micrel 29300 linear regulator.  This solved the problem for the most part, except for whenever the motor was running.  I then tracked that down to static discharge from the belt drive on this Protos heli.  I did a really good ground job on the heli, involving wire from the tail shaft bearing race to the boom, and then the boom to the motor mount.  The static stopped, and the glitching stopped.

I worked on this almost full time for 2 months.  I suspect that something on the PX4 makes it more sensitive to any electrical noise, but have not really figured out why.  While it is true that I had static discharge on the Protos, this is a new kit which as all the latest changes to reduce static, and typically this isn't a problem with other FBL anymore.  I should not have had to have done the grounding mods.  I worked with Lorenz and Tridge to see if we can find something in the code, and cannot.  Well, they had a lot of ideas, and I tested a lot of code, but nothing seemed to help.

Now that I have it working, I've kind of taken a bit of a break from testing it as I just wanted to fly the darn thing.

Can you get a log for me?  Turn on Attitude_Fast logging, arm it (so that it logs, it doesn't log when disarmed), and do whatever it is on the bench that causes the glitches.  There are a number of other people that have the glitching. Even on multirotors (my quad had it!) and maybe even on Pixhawk.  I'm concerned it's going to be a widespread problem as more people start using these.

Manfred, are you saying that if you switch the motor off, the collective jumps to negative?  I've never seen this.  I've never switched the motor off in the air, but I've done it on the bench.  Which mode?  Stab, Alt Hold?

Rob I think it’s OK, I got a bit twitchy about the collective jumping down and I checked it again.
It’s the RC TX7 taking down the throttle on my 450 when switching the throttle hold switch, which also controls the CH8. I might have to change to another switch on the TX7 which lives the throttle alone when switching CH8.

So much work you have done. I think the cause of the problem is easier to get exposed if more people are involving. PX4 is a very good platform, I can see very good performance with my quad and PX4 combi. Now APM is on the quad and PX4 handles my heli.

The log have three steps: only throttle stick input with motor off and heli sitting on the desk; move the heli around with motor on with low RPM and no stick input (esc signal directly from Rx, still haven't fixed this issue yet); and finally move the heli around with motor off and no stick input.

The glitches of my servos behave like the signal is kind of stop-go-stop-go, the movement of the servos are not smooth , a bit like jumping forward to follow my stick input, and occasionally a bouce up and down. But the servos do move smooth without that stop-go issue if I move the heli around with no stick input, but ocassional bounces do exist.

PX4 with IO only connected with lipo / servos and usb, no telemetry or GPS.

Perhaps a photo of my wiring can help as well. My heli is a Trex 450 pro with aluminium parts and belt drive, and some turnigy servos. Static discharge may exist but I'm not very sure about that.


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