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Use of SBUS/PPM converters in a wing.

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David Hall 907/05/2020 10:02:58
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I have just completed my "Inside F5J" glider and wired the six servos in the wing, along with the connection into the fuselage. I had read of others planning to use an SBUS/PPM output from their Rx to drive a converter in the wing (or attached to the wing).

This would allow the wing-fus connection to be a single three pin servo connector.

It's too late for my six servo wing which is now ready, but my future (probably) four servo wings could use this setup. Four/eight channel converters are readily available and quite cheap. I can run the inexpensive FLYSKY iA6B (6CH Rx) from my Taranis Tx via the multimodule, and the later firmware for this Rx supports 8CH SBUS.

I can wire the three channels to the Thr/Ele/Rud as normal using the first three channels, then pick up channels 5 to 8 inside the wing on the converter via SBUS. I'm sure that SBUS/PPM is reliable, has anyone had experience with these cheap converters?

Edited By David Hall 9 on 07/05/2020 10:04:24

Dickw07/05/2020 10:09:13
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I can't coment on the converters as I have not used them, but are you happy running 6, or even 4, servos via a single 3 pin servo plug? The signal would be OK, but I would suggest something more robust for the + and -.

Dick

Edited By Dickw on 07/05/2020 10:10:08

Bob Cotsford07/05/2020 10:16:05
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I use the X series FrSky receivers which give up to 16 channels through SBus. In your example I'd use an X4RSB which gives channels 1-3 through the normal receiver outputs and a 4th connector gives all 16 channels in SBus form. Using FrSky's 4 channel SBus decoder I'd set that to output channels 4 to 7 using the FrSky Servo Sbus Channel Changer or channels 5 to 8 or any 4 random channels - whatever you've set your wing servos as on the transmitter. For a two piece wing I'll use a decoder in each wing. As long as your not flying giant 3D type models the decoders will be fine running a couple of servos and a pair of electric retracts or whatever you want up to a couple of amps draw.

I've not tried the Banggood sourced units but others have, I'm sure they'll be along in due course.

In answer to Dick's point, I've run 4 servos and two electric retracts through an XPS X10+ decoder but on that I used a dedicated power lead. 

Edited By Bob Cotsford on 07/05/2020 10:21:22

Chris Bott - Moderator07/05/2020 10:27:30
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Yep we'll be along very quickly

As far as I remember there's one more caveat with the FrSky 4 channel decoder - It outputs a servo frame rate that matches the incoming S.Bus frame rate. This is 9mS from FrSky receivers, which makes the servo outputs only suitable for digital servos. Analog ones may work, but may also burn out.

The Bangood ones that I've used all appear to have a frame rate of 18mS which is good for all servos.
They've been very good for me.

As Bob says, just don't forget that all servos in the wing will be drawing power through a single servo plug. As long as total current isn't high, they're very useful.

Bob Cotsford07/05/2020 10:31:20
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Good point re the digital servos Chris, and it's worth bearing in mind that some digital servos seem to draw higher peak current pulses than analogues.

stu knowles07/05/2020 11:14:56
615 forum posts
53 photos

Chris, Ref the bit about the FrSky sbus unit, that's the first that I have heard that digi servos are needed. Is there a link to somewhere that I can read up about this?

I have just finished a model which uses a single unit in the wing to drive 2 x flap servos and a L/light switch. They are not digital servos so I'd like to know what the score is now rather than after picking up the pieces.

regards

sk

Chris Bott - Moderator07/05/2020 12:22:11
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Stu, I'm not sure I've seen any official documentation but it keeps coming up in threads here and other forums.

Have a good read through this one, for example.

I've had an aileron servo burn out in flight in a warbird and was using the FrSky decoder. This was after 10s of good flights. It landed OK but was making quite a whining noise and was to hot to touch when the hatch was removed. Of course I can't know if it was the frame rate or just a faulty servo.

PS - There's a post of mine in the thread linked, that mentions how to use a multimeter to check frame rate.

David Hall 907/05/2020 12:24:13
290 forum posts
17 photos

Thanks for the warning about possible problems using analogue servos. I quite like to use analogue at most, but not all control surfaces, so tend to end up with a mix.

Without a scope or such, I guess that a stalled analogue servo making the "normal" (old) buzzing noise around 50hz will be receiving a suitable frame rate, but if it buzzes with a significantly higher note, there's a problem?

 

EDIT:  just noticed a "Hz" position on my multimeter...  The meter shows 50Hz for the output from my iA10B FLYSKY Rx...  handy...   

 

 

Edited By David Hall 9 on 07/05/2020 12:34:52

Ron Gray07/05/2020 12:50:56
2373 forum posts
965 photos

Whilst I do use the FrSky SBus decoders (the Tigercat has one for each wing half as there are 4 servos in each) I have now started to use these connectors where I have 3 or fewer in each wing.

Bob Cotsford07/05/2020 13:12:10
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Those are useful connectors Ron, I first saw them as 'Ashlok' connectors, then they went out of business about the time that I started using SBus. I know FighterAces also sell the components for DIY leads and they will make sets up for you if you ask nicely.

stu knowles07/05/2020 14:07:17
615 forum posts
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Thanks for the info Chris, I will go study that thread.

This potential problem is some thing that has completely passed me by.

stu knowles11/05/2020 13:48:15
615 forum posts
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I have pulled out the wing that has an Sbus converter on the two flaps and L/Light switch. I plugged it into the fus and switched on and Lo! one flap servo, a HiTec 645 dead as a dodo.

Getting them out means cutting into the wing covering but at least I now know that it isn't a servo problem. Without reading this I would have pulled the dead one out and replaced it with another of the same type.

Lesson learned. Thanks for bringing this up

Guy Nicholas31/07/2020 14:06:39
1 forum posts
Posted by Dickw on 07/05/2020 10:09:13:

I can't coment on the converters as I have not used them, but are you happy running 6, or even 4, servos via a single 3 pin servo plug? The signal would be OK, but I would suggest something more robust for the + and -.

Dick

Edited By Dickw on 07/05/2020 10:10:08

For most of my electrics I power everything from the BEC on the speed controller and all of that runs through a single server plug to the receiver. Given that, I feel comfortable running my 4 wing servos through a single 22 gauge wire.

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