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Thread: Servo discussion. digital?

  1. #1
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    Servo discussion. digital?

    A few years ago when researching FPV wings there was some old vague talk about avoiding digital servos due to interference issues. That really hasn't been echoed in recent discussions. Was it a issue 8 years ago that's resolved itself through advances in hardware? Most of the new servos being offered are digital...coreless...ect while remaining in the same price range as quality analog servos.

    Anyone had bad experiences with a modern digital servo and FPV. There are still negatives such as higher current draw spikes with them, but most of us run big ECS's on two servo platforms so I would think we should be ok.

    I'm in the market for 4 servos right now so I'm researching all the new ones. Some of them certainly look great on paper, but it would suck to spend $130 bucks just to find out hitec 82mg's would have been much better.

  2. #2
    Pilot Blizzard1287's Avatar
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    In an effort to create a RF "clean" aircraft, Trappy would use analog HS servos because he theorized that the digital switching PWM signals would create RF noise. I believe it wasn't until later that someone with an RF explorer found out that what little RF the digital servos do produce is completely null after about 3" or so. So... for the most part digital vs analog servo thing become a moot point on a large wing where you can separate components. It may become an issue if you are 40+ km out, but I doubt it.

    On hardware side, I know EZUHF had a problem with power spikes on the 5v rail of the Rx causing lockout and RSSI issues... but I am unfamiliar if they fixed that issue or not. In general is is wise to power the Rx and servos off of a dedicated UBEC as it takes the stress (and reduces heat) off the ESC's UBEC. Also you'll still have control of the aircraft if the ESC ever fails.
    Some fly for attitude, others for altitude

  3. #3
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    awesome info thanks! I figured it might be like that these days.

    Lots of nice servos out there....Old known commodities and newcomers with impressive specs. Its tough stuff to research since they usually just work and are forgotten. Its only the failures that make it to posts. I'll probably add some of interest to this post later on with any info I can find.

  4. #4
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    Both 433mhz & 900mhz can cause quite severe servo jitter in digital servos. On fixed wings it's usually easy enough to get enough separation for it not to be an issue, but it caused my a few headaches on my mapping helicopter. (The 915mhz telemetry was the main source )

  5. #5
    Navigator jamesomega's Avatar
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    I'm currently running digitals in my 51" wing about 3-4" away from both my 2.4 Rx and 5.8 VTX and haven't had any video or RSSI issues going from analogs. I could see there being a potential problem if a noisy servo was stacked directly on top of flight controller or anything RF sensitive.

    I'd be interested to see what an RF Explorer picks up comparing magnet/pole servos to coreless ones since some their prices are starting to drop.

  6. #6
    Reckless Video Operator squishy's Avatar
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    Not only rf noise in the air but through the ground line or at least a frequency due to the regulation and draw of a digital over a analog. I remember a long time ago people suggesting Corona 939's and such but they sucked and have horrible resolution and centering. I personally have been flying HS-82 mg's for a long time now and even though there are servos with better numbers, I love how they feel and center and I never had a problem with noise since. ...all I know is those old servos sucked for FPV and there's some which are even better now..the new stuff is tits, servos like the Turnigy TGY series are in need of thorough fpv testing so get some and let us all know what you think...
    Flight Club - "In thrust we trust"

  7. #7
    Navigator parajared's Avatar
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    Digital servos lowered Dbm values on my Dragon link slightly

  8. #8
    Reckless Video Operator squishy's Avatar
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    And that was with ample separation right?
    Flight Club - "In thrust we trust"

  9. #9
    Navigator parajared's Avatar
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    Like in the picture I was getting 100 Dbm with the receiver right next to the digital servos and 105 Dbm separated from the servos by about 100mm or so. I was unable to exceed 105 even with the plane completely off.

  10. #10
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    some good additional info! I dont have a ton of FPV experiences with servos so it makes the process of selection harder. In the past if the control surface moved, then I was more then happy with the servo. Now resolution, centering, torque and weight seem overly important. Servo speed seems something we can be flexible with as long as its reasonable.

    Like squishy is saying, this TGY seems like the king of newish servos. they appear to be made by bluebird so its probably a great servo.

    https://hobbyking.com/en_us/turnigyt...sec-22-5g.html
    https://hobbyking.com/en_us/turnigyt...13sec-16g.html
    the 380max analog servo has been good to me. Lots of crashes and no issues. They have, however, developed slop over two years. They have a lot of hours on them as well so its not a surprize. I wonder if a larger gear set would have worn out slower? The max series are very compact and size isnt much of a concern for me.
    https://hobbyking.com/en_us/bms-385d...sec-16-5g.html

    Then there are some random new comers that look respectable on paper. There isnt much need for a aluminum case on a wing, but they have good numbers just the same.
    https://hobbyking.com/en_us/power-hd...13sec-16g.html
    https://hobbyking.com/en_us/goteck-d...12sec-20g.html

    And the faithful analogs that seem to be the most popular. Probably a very safe bet to be happy with either of these. The only thing stopping the buy button is wondering if there is a better servo for the exact same price. I own HS-225MG's and think they are great, but the weight is really over the top and I dont know what they give in return for the extra weight. Maybe a better gear set...I dont know.
    https://www.amazon.com/Hitec-32085S-.../dp/B0006O3X5O
    https://www.amazon.com/Hitec-32082S-.../dp/B0012YXRJE


    a comparison of a few that are on this website
    https://servodatabase.com/compare?se...,930,1458,2130
    Last edited by Tronski; 18th August 2017 at 04:05 AM.

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