Tail servo sticking to last position

[GAntonjo]

Today I was out on a field flying my mini for the first time (excluding the hovering in my garden). Had a nice time flying it at first, it felt locked in and I could fly some nice 8 figures. Getting more confident I tested a little more aggressive flying, and at one point I had a “touch and go” after saving it from a crash. It flew nicely after this and not long after I managed to tilt it too far forward at low speed and altitude, causing a nose dive into the grass field. This time the tri ended upside down and I quickly disarmed it before going for it. Nothing broke, the front arms only moved a centimeter back, so I thought it was good to go, so I just put it down after moving the arms to their front positions. First thing I noticed was that the tri would not arm after the crash, so I disconnected the battery and connected it again. This time the tri armed and I tried to take off. Now the problem was that the tail servo seemed to live it’s own life. It felt like it did not listen to the commands from my transmitter, and if it reacted it would be by full yaw in either direction. Looking for obvious physical damage did not reveal any cause for the problem, so I decided to head home. At home I disconnected the props and connected the tri to my PC. I connected the battery and turned on my transmitter. At first it all looked OK, the servo did move by the stick control, but I soon noticed that if I held the Yaw stick long time to one side, the servo would move the corresponding way as expected. However, when I let go of the Yaw stick, the servo would not go back to neutral. Same happened for both directions. Sometimes it stuck just a little to one side of the middle, other times it stuck at maximum angle. Giving just a little yaw to either side will cause the servo to move slowly in the commanded direction. When letting go of the yaw stick, the servo will stay put at the position it had when I let go of the stick. The same behavior is seen both armed and unarmed.

Note: Wiggling the tail left or right causes the servo to try to counter the movement.

Anyone else who have experienced this behavior and have a solution? I guess I will have to attach a BlackBox and record what the FC is doing.

[Rustypaint]

Try moving the servo back and forth with the power off. It should feal smooth and not “chunky”. If it is not smooth, I would suspect broken gears.

[LeoTheHuman]

Everything you described is normal, it should not stop yawing when you release the stick. Basically, it knows it didn’t yaw when commanded (I term saves that difference between current position and commanded one), so it keeps yawing. If after releasing the stick you slowly yaw it where it wants to go it will center again.
I suspect that there was some force applied to the servo during the crash, which might have damaged it either permanently or temporarily. If it works now as you describe, it’s probably fine. But if one of the teeth on the gears sheared off, you might encounter more problems with it. I would try flying it again and try yawing fully to both sides at about 1m above the ground and see what happens.

[GAntonjo]

Thank you both for your comments.

@rustypaint: The servo feels smooth with power off, I also thought of that possibility. Will anyway test the servo with my servo tester tonight when kids are sleeping.

@leothehumangb: I though it could be something like that. However, when I tried to take off after resetting the the FC at the field, the tri was not controllable. It yawed pretty violently to either side straight after takoff. I did not dare flying it more yesterday. Is there a difference in this behavior in Horizon vs Angle mode? I was using Horizon mode when testing.

I will have to test more tonight and possibly test another servo just to be sure.

[lauka]

I had similar issues with my V3 Bluebird servo after a crash. The servo was moving freely and fine on bench, but in flight it was acting very weirdly, much like you explained “it lived it’s own life”. The servo must have taken some damage internally as replacing it fixed the issue for me. I planned to open the servo and check what’s wrong but I forgot about it once I got the new servo :).

[GAntonjo]

The servo did not hit the ground very hard. Due to the moist grass on my flying field I like to have my copter a little higher from the ground. Therefore I have mounted a set of half 11cm PVC rings as landing gear. Yes, I gain some height, but at the same time my mini is a little eager to flip if not landing perfectly straight down :-p I guess I soon will modify the landing gear a little.

As it was soft grass where it flipped with motors off, I cannot understand the “strong” servo should get damaged. I could even say that “no straw of grass was damage at the flip”.

[GAntonjo]

So, the results from my servo tester. The servo moves smoothly each 2 uS from:

• Full left tilt, -50 degrees, 1054 uS
• Middle , 0 degrees, 1554 uS
• Full right tilt, +50 degrees, 2064 uS

I guess I should put in these numbers in the servo calibration. Or, should I use the uS values for +-40 degrees?

• Left tilt, -40 degrees, 1154 uS
• Middle , 0 degrees, 1554 uS
• Right tilt, +40 degrees, 1984 uS

Bottom line: The servo seems to be working as it should, no proof of physical so far 😀

[LeoTheHuman]

I would stay with 40 degrees. I doubt that the tilt mechanism can actually deal with 50 without binding and 2064 seems a little high anyway. Also, you almost never need even 40.

[GAntonjo]

Thank you @leothehumangb. I have another question. Looking at the Servos tab, I can set minimum til to -40, as in the attached picture. However, when looking at the CLI dump, I see:

# servo
servo 0 1000 2000 1500 90 90 100 -1
servo 1 1000 2000 1500 90 90 100 -1
servo 2 1000 2000 1500 90 90 100 -1
servo 3 1000 2000 1500 90 90 100 -1
servo 4 1000 2000 1500 90 90 100 -1
servo 5 1154 1984 1554 216 40 100 -1
                        ^---Hmmmm???
servo 6 1000 2000 1500 90 90 100 -1
servo 7 1000 2000 1500 90 90 100 -1

My maths don’t match with what I see. Where does this 216 come from?

Attachments:

1. 

   screenshot.367.png
   

[LeoTheHuman]

In their terms, up is 90 degrees. So your min and max should be 50 and 130. Luckily, no one uses these parameters yet. (:
216 = -40 + 256, basic byte underflow.

[GAntonjo]

Thanks @leothehumangb. Ahhh… of course 😮
Then there is a bug in the the GUI versus the CLI of CleanFlight Configurator 0.66. Because it is not possible to enter 1230 degrees, only negative values are accepted where I put -40. Or it is a “feature” – so that 0 degrees is up when board is placed upside down, which makes +-40 degrees valid numbers.

[LeoTheHuman]

No idea. Just ignore it for now, it’s not relevant for triflight anyway. We were going to use these endpoint later, but with this in mind, we might postpone that till that’s fixed in GUI.

[lauka]

You could open a ticket in configurator Github so we have better chance of getting it fixed.

Or is it so that you shouldn’t enter negative values there. 50 (-40) and 130 (40) instead. Haven’t checked the new servo config.

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