Tail Servo Problems

I’m testing a 450 heli with a Pixhawk controller. I program it with Mission Planner. I have it mounted on a test frame, All appears to be working OK for about a couple of minutes, when the tail servo goes hard over and locks up, causing the heli to spin. If I then release the servo, all starts up OK again. The servo limits are set so the slider does not lock on full travel. (It over rides these limits when it locks up)
Servo detail GOTECK Digital BB

Been doing some more tests, If I remove the blades and do a run up, the tail works fine, I can move the swash to extremes, and take it up to full power. I’m getting a slight tail drift. Put the blades back on and retry, The tail servo goes to over full travel after a varied time. :cry:

When you say the tail goes full over, does this happen slowly, gradually? Or does it happen suddenly?

The tail is around mid position and holding the head, then it goes suddenly and turns clockwise.
I checked for vibration, the Y ones is slightly over 3% the X and Z are within the recommended limits.

Hi again
On the very odd occasion the swash servo’s have gone over the limits setup in ‘Heli setup’ on Mission Planner, at about the same time as the tail one.

Is this in flight, or on the bench?

Can you get me some logs. Make sure that ATT and MOT(motors) logging is turned on.


Hi I’ve attached photo’s of my test stand.
At 4am this morning, I worked out what might be the problem. I was supplying the power to my servo’s from the ESC, to cut down on hardware on the 450 heli,fine, when there was no load on the ESC, but under load it must have been causing spikes to the 5v supply.
So to-day I rigged up a UBEC to supply the servo power, and Hey Presto, all worked fine.
So, thank you very much for trying to assist me, with this problem, as I was about to put it all in the bin.

I’m in the UK, collecting my pension

Huh, interesting. I’d like to understand what’s going on better.

Did you have the output from the BEC, powering the servos, and the PX4? That’s probably not a good idea.

IMO, the flight controller should NOT be on the same BEC supply as the servos. Period. This has caused problems with APM in the past, as well as other FBL controllers.

I have the 3DR power unit supplying the Pixhawk. I did have the BEC on the ESC powering the servo’s.
I have now fitted a separate BEC to power the servo’s, and the BEC on the ESC is now not in use.
Reading about the power supply, I understand that the Pixhawk uses the 3DR power supply over any other supply to the board.

Ok, yes that should be fine then.

I think the BEC in that ESC is only 3A, and probably couldn’t handle 4 digital servos.