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HomeAircraftHelicopterMain Discussion › 8417 Center Pulse Length?
06-29-2015 05:13 AM  3 years agoPost 1
Ladymagic

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South Korea

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Does any happen to know the pulse length for the JR 8417 servo? I noticed some strange pulsing today and I think it might have something to do with an incorrect pulse setting in my Vx1n governor. There isn't much data that I could find online. Maybe someone here could help.

Right now I have it set to 1520, but it could be 960 or maybe even 760?

thanks in advance.

Mellisa

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06-29-2015 05:37 AM  3 years agoPost 2
TMoore

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Cookeville, TN

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The 8417 has a pulse center of 1520.

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06-29-2015 08:05 AM  3 years agoPost 3
EEngineer

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06-29-2015 04:02 PM  3 years agoPost 4
Ladymagic

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Thanks!

So, my problem is elsewhere in my setup...hmmm

Mellisa

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06-29-2015 05:57 PM  3 years agoPost 5
AirWolfRC

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The spec reference is interesting but it does not give that answer.

So . . . why is center at 1.52ms instead of 1.50ms like the original standard from back in the mid 60's ?

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07-25-2015 04:29 AM  3 years agoPost 6
Ladymagic

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Well guys I ran my 8417 at 1520 and I popped it after just two flights.

Fortunately, the servo didn't fail fully open but it didn't fail near idle either. I was able to land and manually shut the engine down. It's not fun trying to get to a fuel clip with 695s swinging on a fully engaged .91 engine that won't shut down and still has 3/4 tank of fuel and a full header tank left to burn.

Needless to say that the pulse width is NOT 1520 for that servo.

I can't seem to find any useful specs for JR's older servos so I think the best bet for anyone thinking of doing what I did is to run something with accurate and verifiable specs if your setup calls for pulse width specific settings. I know for sure that the 8717 is good for 1520 and 6v+ so I will just pull one off a junk heli have and I'm back up.

Thanks for the input guys

Mellisa

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07-25-2015 04:47 AM  3 years agoPost 7
EEngineer

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TX

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Anytime....

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07-25-2015 07:24 AM  3 years agoPost 8
TMoore

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Cookeville, TN

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The pulse center is 1520us the frame rate is max 250hz.

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07-25-2015 03:22 PM  3 years agoPost 9
Four Stroker

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Atlanta

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Exactly as Terry says. The frame rate pops servos. The correct center pulse just makes them work. It is really obvious when the frame rate is too high. The servos are jittery and jumpy. But maybe you can't see this with a FBL unit ? Other things that pop servos are too high of a voltage and binding.

How much damn difference is there between 1520 and 1500 microseconds. The ANALOG pot in the servo does not know the difference.

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07-25-2015 03:53 PM  3 years agoPost 10
Dr.Ben

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Richmond, VA, USA

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All a slight difference in center pulse will do is shift servo neutral a tiny bit. That servo blew in two flights from some other variable besides center pulse. As noted, drivng frequency is the first place to look.

Ben Minor

Peak Aircraft/Team Minicopter Team Futaba Team Kontronik USA

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07-25-2015 11:27 PM  3 years agoPost 11
AirWolfRC

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The 8417 is rated for 250 hz but the center is pretty much standard.

You can also look here,

https://rc.runryder.com/t69138p1/

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07-30-2015 09:14 PM  3 years agoPost 12
Ladymagic

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South Korea

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FourStroker, you are exactly correct. just before i popped the 8417, it was fine. During all my bench checks and a couple of runs before it was smooth as butter. But a couple of runs later, i noticed my throttle started sparatically surging at idle, right after startup. So i shut it down and came here. I thought it was just the pulse width or a gain issue, but now frame rate sounds more like it.

The servo lead looks like it got hot, melted the insulation, and may have shorted out. There is also something loose inside the case now and the case looks like it has an area that melted too. This was a perfect servo before i installed it.

Mellisa

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07-31-2015 04:34 PM  3 years agoPost 13
raholek

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Zachary, Louisiana

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Root Cause
Ladymagic...have you verified an incorrect frame rate?

I hope you found something. Keeps me up at night when I can't find my mistake.

I'm sure RR will help you.

www.redstickrc.net ama#: 968515

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08-07-2015 02:58 AM  3 years agoPost 14
Ladymagic

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South Korea

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Raholek,

I havent nailed it down, but im pretty sure it was the frame rate. I cant adjust frame rate settings in the Vortex that I know of. The 8717 seems to be doing fine though. It could have also just been a bad servo when i installed it. Cant say for sure though.

Mellisa

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08-09-2015 01:13 PM  3 years agoPost 15
tailwhip

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Southern WI

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I have the same setup in a vibe 50 that I'm chasing too. My throttle surges and I changed the governor gain and it won't go away.I talked to angelos from spartan at ircha yesterday, and he was thinking it is my pickup for the governor. But I'm thinking it may be the pulse rate of my 8417 on the throttle.
Angelos told me on some of the spektrum
Pickups use a little different reference voltage and that is throwing the VX1E off. I purchased a new spartan pickup sensor. I'm going to change out the servo too because of the pulse rate issue.

Dennis M

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08-09-2015 04:10 PM  3 years agoPost 16
AirWolfRC

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Get a standard servo tester (50 Hz pulse rate - 20 ms spacing) and hook up the 8417 on the bench. Run it through the works . . . see if it screws up.

If you're wondering where that "standard" came from . . .

Back in the early 60's, when digital proportional was invented, the idea was that 8 channels would be enough for anybody and a center pulse width of 1.5 ms with a range of ± 0.5 ms would do just fine.

With all eight channels at their max of 2.0 ms taking up 16 ms and a frame sync. of at least 4 ms you get 20 ms (50 Hz).

Example 6ch Encoder - Example 6ch Decoder
-

Needless to say, things have changed a bit with 2.4 GHz but the "standard" that radios talk to the servos with is still very much in use.
(A 1.5 ms ± 0.5 ms pulse about every 20 ms. - 50 Hz)

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08-09-2015 10:56 PM  3 years agoPost 17
Ladymagic

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South Korea

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Question
So, if center pulse isnt as big a deal, why does Spartan recommend not plugging in certain servos until after you set the rate correctly? Or maybe I am confusing frame rates with center pulse widths.

Mellisa

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08-09-2015 11:27 PM  3 years agoPost 18
AirWolfRC

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Frame rate is how many pulses per second.
Center refers to the median length of the pulse, usually 1.5 ms.

Again, the "standard " rate is 50 pulses per second.
The 8417 is spec'd. out for 250 pulses per second.

As far as I'm concerned, the servo could have been designed so the frame rate doesn't really matter. For the 8417, it appears it does.

The frame rate is simply how often the servo position information gets updated.

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08-09-2015 11:33 PM  3 years agoPost 19
Aaron29

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USA

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Some drawings to illustrate. OK so the dashes will be the pulse time, and the dots will be "silence" time

Below would be a low frame rate and low pulse width:

-..........-...........-..........-..........-...........-..........-..........-

Next would be the same low frame rate but a higher pulse width:

---........---........---........---........---........---.........---.......---

OK so far. Now here's a higher frame rate with a low pulse width:

-....-.....-....-.....-....-.....-....-.....-....-....-.....-....-.....-....-.....-

And finally, a high frame rate and a high pulse width:

---..---..---..---..---..---..---..---..---..---..---..---..---..---..---..---

Nothing's to scale. My original picture didn't have the dots, just space, but the dots are necessary because the post generator likes to collapse spaces which kills the illustration. Hope it makes sense. Just imagine the dash is the pulse and the dots are space. Just a model to illustrate. Hope someone gets it.

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08-09-2015 11:48 PM  3 years agoPost 20
Ladymagic

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South Korea

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So its likely that something other than the settings i chose caused that servo to pop. Based on what you said, none of the settings really matter as the 8417 is spec'd above the requirements.

In that case, it could have been high voltage that killed it. I am running 6v regulated and if i am correct, the servo is rated at 6v and that should have been fine. The only other culprit would be a latent defect from mfg or age.

For now i will just watch the 8717 and if it popped then i will know something is else is wrong in my setup.

Mellisa

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