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Old Oct 05, 2012, 07:46 AM
Seafax ROV
WalneyCol's Avatar
United Kingdom, Barrow-in-Furness
Joined Feb 2011
371 Posts
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Running 4 servos off 1 receiver channel?

Hi.

I want to run 4 servos off my tricopter rudder channel...

A 13g servo for the yaw mechanism
plus three 9g servos for
(1) a steerable tail wheel
(2) a dolls head and
(3) the dolls arms movement.

I'm guessing that this number of servos might draw an undue amount of current through my reciever's rudder channel. Does anybody have experience of running more than 2 servos off one channel? I'm aware of "servo boosters" but I'm not sure if that's what I'd need.

Col.
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Old Oct 05, 2012, 01:16 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,602 Posts
The simple answer is to try it. If you've got three Y harnesses around then it's easy to do. Try it with one servo and add the others with motion testing as you plug in each servo. If there is going to be a problem it'll show up when you load down the output by too much with one or more of the servos beginning to jitter and not follow the signals accurately.

Actually a good way to do it is to use 4 Y harnesses and plug in 5 servos. The thing being you don't know how close you are with the 4 servos. Adding a 5th and finding all is well is just piece of mind so you know that any minor change in the receiver channel isn't going to suddenly produce troubles.

The weight of the servos doesn't matter on how much "power" they pull. The receiver board will do fine for the power and ground. It's the signal line that matters. If you load it to where the voltage peak of the signal pulse is pulled down too far then the servos will begin to not "see" the pulse correctly and motion can become jittery or slow or even cease altogether. A jumbo servo will typically have the same "load" value on the signal line as a 1.5gm micro. For this signal line size simply isn't a good indicator. But it's not safe to assume that every brand and model of servo has the same load value for the signal line either. Different servo chips or how the inputs are configured can produce different load values. And that's why it's important to test with the brand and model of servos you want to use.

If it runs with 4 but fails with a 5th servo I'd say get a multi servo buffer. If it runs fine with 5 then you'd be fine.
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Old Oct 05, 2012, 02:06 PM
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dedStik's Avatar
United States, VA, Virginia Beach
Joined Feb 2012
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This could also easily be accomplished with some crafty use of pushrods without the cost of added weight from extra servos.
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Old Oct 05, 2012, 02:21 PM
Seafax ROV
WalneyCol's Avatar
United Kingdom, Barrow-in-Furness
Joined Feb 2011
371 Posts
Quote:
Originally Posted by dedStik View Post
This could also easily be accomplished with some crafty use of pushrods without the cost of added weight from extra servos.
Thanks for the suggestion but the canopy that houses the doll is an optional extra (removable) so I'd still need a minumum of two, and probaby three, given that the dolls head and the tiller are in such awkward and only vaguely physically connected places.
Col.
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Old Oct 05, 2012, 02:43 PM
Seafax ROV
WalneyCol's Avatar
United Kingdom, Barrow-in-Furness
Joined Feb 2011
371 Posts
Quote:
Originally Posted by BMatthews View Post
The simple answer is to try it. If you've got three Y harnesses around then it's easy to do. Try it with one servo and add the others with motion testing as you plug in each servo. If there is going to be a problem it'll show up when you load down the output by too much with one or more of the servos beginning to jitter and not follow the signals accurately.

Actually a good way to do it is to use 4 Y harnesses and plug in 5 servos. The thing being you don't know how close you are with the 4 servos. Adding a 5th and finding all is well is just piece of mind so you know that any minor change in the receiver channel isn't going to suddenly produce troubles.

The weight of the servos doesn't matter on how much "power" they pull. The receiver board will do fine for the power and ground. It's the signal line that matters. If you load it to where the voltage peak of the signal pulse is pulled down too far then the servos will begin to not "see" the pulse correctly and motion can become jittery or slow or even cease altogether. A jumbo servo will typically have the same "load" value on the signal line as a 1.5gm micro. For this signal line size simply isn't a good indicator. But it's not safe to assume that every brand and model of servo has the same load value for the signal line either. Different servo chips or how the inputs are configured can produce different load values. And that's why it's important to test with the brand and model of servos you want to use.

If it runs with 4 but fails with a 5th servo I'd say get a multi servo buffer. If it runs fine with 5 then you'd be fine.
Hrm...

I've just tried running three off a single reciever channel (I need it to run 4 eventually) and it didn't work. It worked fine with 2 but I think when I added the third the voltage on the signal wire must have dropped below the acceptable range.

Given that Spektrum advise against using them it looks like I'm going to have to try a servo signal booster after all. Can anyone please give me a link to a good one - preferably one that's already known to work with Spektrum AR7000 receiver.

Cheers, Col.
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Old Oct 05, 2012, 07:31 PM
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epoxyearl's Avatar
United States, MD, Elkton
Joined Oct 2011
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I applaud your question-it's thinking like that,and the testing you did,that supplies answers to a bunch of ideas.

Many years ago,I tried to 'sister' two servos together,that had five-wire connectors.....I cut,tinned soldered and insulated for an hour,but the servos didn't 'center'...Eventually one ended up at full up,the other at full down.
But I tried....woulda' been the first model with two servos on the elevator that I owned !
Cheers
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Old Oct 14, 2012, 10:47 PM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,953 Posts
My FMS 1.4 meter STuka runs 4 9 gm servos off of the flap channel, with no problems. I do run a ubec, though, as there are another 4 servos running the rest of the plane. I have encountered no ill effects.------Metalguy
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Old Oct 15, 2012, 09:02 PM
Registered User
United States, TX, Leander
Joined Sep 2003
2,798 Posts
I use a JR matchbox to run 4 servos off one channel, in my case 4 throttle servos. You have the option of running a separate battery on the matchbox. It also gives you lots of fine tuning ability for each servo.
Edwin
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Old Oct 16, 2012, 01:17 AM
Seafax ROV
WalneyCol's Avatar
United Kingdom, Barrow-in-Furness
Joined Feb 2011
371 Posts
Quote:
Originally Posted by edwin1 View Post
I use a JR matchbox to run 4 servos off one channel, in my case 4 throttle servos. You have the option of running a separate battery on the matchbox. It also gives you lots of fine tuning ability for each servo.
Edwin
Thanks for the suggestions guys. The most cost effective way I've come across so far has been to have two of servos on another reciever channel and slave that channel to the rudder with a very heavy mix.

Col.
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Old Oct 16, 2012, 08:49 AM
Registered User
United States, TX, Leander
Joined Sep 2003
2,798 Posts
There are a couple of other options. A split power bus cable you make yourself. You make a 4 way extension cable with the red wire disconnected at the receiver. You provide a separate battery connection for that one channel. OR, you buy a split power bus from servo city. http://www.servocity.com/html/servo_power_boards.html
This runs the receiver on its own battery, and the servos on a separate battery. I use these on planes with lots of servos or power hungry servos. More batteries to charge, but thats not my concern. I need the right amount of power for the job. My big bipe uses 4 aileron servos and has a 3600ma battery just to power the servos.
Edwin
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Old Oct 16, 2012, 05:40 PM
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United States, TX, Midland
Joined Mar 2005
51 Posts
2 rx's with a Y-harness on each. Could that work for you?
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Old Oct 19, 2012, 11:00 AM
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Toysrme's Avatar
Birmingham, Alabama
Joined Jun 2002
2,970 Posts
yes, youll be fine slaving three or four wimpy servos on a channel.
its common with heavy duty giant scale servos to slave dual elevators dual rudders and dual ailerons. also, with things like dolls, you can use multiple push/pull or pull/pull setups off just ONE setup.
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