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Mutual documentation of the undocumented yaw servo (Read 199 times)
Aug 27th, 2017 at 6:28am

Jared Yates   Offline
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Hickory, NC

Posts: 26
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I have not yet been able to get the autopilot working acceptably, and I think it is primarily because of the airplane having adverse yaw. This isn't an airplane that can be flown with aileron only, and when the autopilot tires to do that, it fails just as a live pilot would. I was about ready to give up and remove it all together when Dynon announced the new yaw servo option, so I installed the third servo before the documentation came out, thinking that like many Dynon things, it would be intuitive. And, that the documentation would be out shortly.

Both of those assumptions seem to have proven incorrect, so maybe we can help each other out and combine what we do know.

As of today, I have made a few calls to tech support and gleaned a few details each time, but my main problem is that the yaw servo keeps dropping offline. I'll just be flying along without having the autopilot on at all, and I'll get a "yaw servo offline" warning. This might happen two minutes into the flight, or it might happen 30 minutes into the flight. I've had lots of trouble calibrating the servo also. It will show up in the network fine, but then if the servo goes offline during the calibration, it disappears from the network for some unspecified period of time before I can try again. In the current configuration, it was able to stay online long enough to get through a calibration and test. TS suggested a bad connection, which did sound quite plausible, so I used a single length of Dynon autopilot wire bundle that was left over from installation. I put ends on it and ran it up and over the seats in the cabin to the back of the plane, and reproduced the same problems. I hooked the yaw servo up to the pitch servo wiring and attempted a 2-axis calibration, pretending that the yaw servo was the pitch servo, but then the pitch servo dropped offline.

So here's what I've been able to figure out so far about the yaw servo, please add in if you have been able to figure out any more, or anything to the contrary of the above.

The disconnect button must be wired separately from the pitch and roll, to a momentary pushbutton switch that connects the disconnect line to ground. Is one press supposed to revert it to YD, and a subsequent press supposed to turn it off? Or something like that? Or is it that YD comes alive automatically, and we have to push buttons to disable it? I'm not at all interested in having a YD function when the autopilot is off. I'm really only after two conditions in flight: autopilot on, and autopilot completely off. I have installed the extra push button as directed by the phone techs, and it does not seem that as of now we have any option for disabling YD in the software.

Not really sure what to do with the yaw servo settings. One thing that I have not been able to find in the printed documentation is the general relationship between sensitivity and gain. At Oshkosh one of the Dynon people said that gain was a multiplier for sensitivity. So a sensitivity of 20 on a gain of 2 was like having a sensitivity of 40 on a gain of 1, functionally speaking. That sounded really easy and intuitive, but then there are threads here that contradict that.

When is the "ball" supposed to turn green? Is it when the YD is active, or when it feels like it is doing a good job?

I've never gotten a "yaw slip" which makes me wonder if the software has the capability of annunciating a slipping yaw. One theory I have about the servo going offline is that Dynon did not write a "yaw slip" into the programming, but the servo is slipping, and eventually giving up. What in the code would cause the yaw servo to go offline? Is there a condition for slipping that would?

It seems to me that the next logical solution is to physically remove the yaw servo from the airplane and send it in, but it is installed back in the tail and that's a 1-hour miserable job for removal and for installation. If it would solve the problem I'd be glad to do it, but I'd be pretty disappointed if I went to all that trouble just to realize that it was actually a shortcoming in the software. This servo was one that came directly from Dynon in a swap, but has not been proven to be successful by me, so there is a chance that it is bad and they didn't catch it. This makes me even less enthusiastic about spending all of that time removing, shipping, waiting, and reinstalling. If their test regimen didn't find a problem before, and there is an intermittent internal problem, what's to say it will find it this time? Do they subject servos to a test regimen?

I see that some folks with long wire runs have found that the power wire was too small, which caused servo offline messages. My roll and pitch servos never go offline, so another option is for me to swap the servo from yaw to pitch to see if the length of the wire run (extra 8 feet or so) is causing a voltage drop that is making the servo go offline. But the yaw is a capstan and the pitch is an arm, and combining that with the inaccessibility of the servos, I'd be looking at half a day's work to make the swap. Is there a way to investigate the diagnostic data to rule out this possibility? Is there some sort of log file that would show the incoming voltage to that servo?

So to summarize, I have lots of possible troubleshooting steps, but don't know enough about the system to know if these are the only steps, or which is the best step to take. I welcome input about troubleshooting, and any tidbits that anyone else has been able to catch about the whole yaw situation.
 
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Reply #1 - Aug 28th, 2017 at 6:09am

RayInGA   Offline
Senior Member
I love aviation!
Richmond Hill, GA

Gender: male
Posts: 286
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I don't believe the yaw damper in any plane is intended to deal with adverse yaw, though you might get it to work in some planes, with a lot of tweaking, where it is not so pronounced.

Beyond that, I'd suspect either a connector pin is not well seated or your theory that the wire run is such that it needs a larger gauge wire. There are charts on the internet that can help with this determination. Always err on the side of larger gauge when nearing the limit if the smaller gauge wire for its intended use.

The disconnect button should disable the servo. If it works like the standard servos, holding the button for 3+ seconds will re-enable it.

This is from another AP thread.

Sensitivity = Correction Accuracy
Gain          = Aggressiveness
 

Ray Eaker
RV-7A flying since 27 Jan 2017
Dual Skyview 1000T with all available Dynon VFR goodies
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Reply #2 - Aug 28th, 2017 at 6:00pm

Jared Yates   Offline
Frequent Poster
Hickory, NC

Posts: 26
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Thanks Ray. So I suppose that is a question that I didn't ask... is the yaw servo only intended to dampen yaw oscillations, or is it intended to add a third axis to the autopilot? I was assuming the latter, but perhaps that is not the case? At what point do those two functions cease to overlap? The wire run is fine per the published specs, but perhaps the servo needs something above and beyond the chart specs.

I feel like I'm getting closer to adding a couple thousand dollars to my wallet and removing half a dozen pounds from my airplane.
 
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Reply #3 - Aug 29th, 2017 at 7:26am

RayInGA   Offline
Senior Member
I love aviation!
Richmond Hill, GA

Gender: male
Posts: 286
****
 
It really depends on the airframe, IMO, and how fast the servo must react to adverse yaw. I'd expect it to work great in an RV with little adverse yaw but maybe not in a Luscombe, which requires roll and yaw at the same time to avoid it.

Hopefully the official response will come soon from Dynon but I wouldn't give up on the AP until you've done more tuning. Unfortunately, the documentation around the yaw damper is still in the works and likely competing with other business priorities.
« Last Edit: Aug 29th, 2017 at 12:38pm by RayInGA »  

Ray Eaker
RV-7A flying since 27 Jan 2017
Dual Skyview 1000T with all available Dynon VFR goodies
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Reply #4 - Aug 29th, 2017 at 12:05pm

Jared Yates   Offline
Frequent Poster
Hickory, NC

Posts: 26
**
 
I think you are right, here's hoping we'll hear from them soon!
 
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Reply #5 - Aug 29th, 2017 at 1:54pm

Dynon Support   Offline
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Dynon Employee
Dynon Technical Support

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Dynon's yaw damper is designed to both stabilize yaw, but also to auto-coordinate the aircraft. In our experience with aircraft like an Europa with significant adverse yaw, it makes a tremendous difference in the autopilot's performance. It can also be independently engaged and used even when hand flying pitch and roll.

Technically, there is no tie between the yaw damper and the rest of the AP (other than activation/deactivation behavior). The YD is either on or off, and it has no idea what the AP is doing, but by centering the ball, the AP is much more effective as it flies roll.
« Last Edit: Aug 30th, 2017 at 4:33pm by Dynon Avionics »  

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Reply #6 - Sep 2nd, 2017 at 6:52pm

Jared Yates   Offline
Frequent Poster
Hickory, NC

Posts: 26
**
 
That is good news, thank you. Any input on the sporadic "yaw servo offline" issue?
 
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