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[Capitaine LaGaffe]

Greetings,

Stupid problem (at least, I feel rather stupid). Radio: Multiplex RoyalPro M-Link, 9ch M-link Rx.  I'm trying to set up a new tug with Graupner Voith-Schneider units. With these, the servo travel must not exceed 4mm either way. No problem, easy to limit the travel using the servo calibration in the Tx. But, and this is a big and annoying but, when I switch on the Rx (after switching on the Tx), all servos travels to one of the ends points (full 45 degrees) before returning to their proper position, and max travel is at least twice what the VSPs allow (and the limits set up in the Tx). Can't find anything in the manual about this merry servo dance on Rx switch on, nothing about what magic settings may prevent this annoying behaviour – and of course, I can't seem to formulate a google query with a relevant hit among the first few hundred.

Is there anyone who has any idea how to get around this? (That is, how to prevent the servos from making a mighty jerk when the Rx is switched on?) It is rather a show stopper.

TIA

[dodgy geezer]

Just design your linkages so that max throw IS 4mm either way, rather than doing it in software...

[catengineman]

I know that trying to get the 4mm throw via the horns with the VS units is almost an impossibility as the control arm is a dual rod at 90* to the control pin in the VS drive and if you position to suite one horn the other is miles out and there is not room to adjust the position of the servos to allow for that modification by using the Graupner parts.
.
If by slim chance the throw is "pulling" then you can make a trombone slide with spring, this will allow the excess travel and the spring will hold in normal position during normal operations.
(Just an idea like)

[Capitaine LaGaffe]

Doing it mechanically rather than by programming was an idea I entertained while designing the boat: I went with what Graupner suggested (doing it in the TX – as is done in Graupner's models); now there is simply no room and bulkheads, motors, decks, and whatnot in the way.

And all servos (though, only analogue servos it seems) turn clockwise as far as they go before returning to wherever they should be, and since I basically designed the smallest possible hull around the VSPs one of the servos actually tries to push its arm through the hull … Somewhat difficult to sail a boat that breaks essential parts when it's switched on.

Yes, replacing all the servos with digital ones could perhaps solve the problem, but four standard size digital servos isn't exactly the cheapest of fixes.

I don't know if this has anything to do with it, but fast response is on (the ESCs in the tug I originally used the Rx in didn't work when it was off), otherwise all is plain vanilla.

[dodgy geezer]

Doing it mechanically rather than by programming was an idea I entertained while designing the boat: I went with what Graupner suggested (doing it in the TX – as is done in Graupner's models); now there is simply no room and bulkheads, motors, decks, and whatnot in the way.

Hmm...

Staying with the mechanical approach, there is a slight possibility in this case that smaller servos would help. Assuming you have 'standard' servos about 2" long, then using small 1" servos would shrink all the throws. It would also shrink the power available, so I don't know how possible this proposal is...

[Andyn]

Assuming you have 'standard' servos about 2" long, then using small 1" servos would shrink all the throws.

Sure about that?

[dodgy geezer]

Sure about that?

Seems likely to me. A standard servo has an arm about 1.2" wide, and I have small servos with arms only about 0.6" wide. A 45deg throw on the bigger one will be about twice as long as on the smaller.

Without seeing pictures we can't be sure, but if standard servos were used initially, and they fitted so tightly that the arms would hit the hull side, then using smaller servos has a good chance of solving that issue as well.

Is there anything wrong with that thinking? 

[Circlip]

Under normal circumstances your thinking is spot on Dodgy, Unfortunately due to constraints in the mechanical "Enviroment" now proffered. it seems that we can't shoehorn a Merlin into a mini.

 Moving the take off point on the servo end closer to to the centre of its rotation or extending that on the VSP would be the obvious mechanical answer but we only have a white stick to poke about with.

 On the other hand, Multiplex, being an e-mail away, may have an electronic genius able to offer a software solution. He can only however offer a "Power bulge" solution to the hull obstruction problem.

  Regards  Ian.

[dodgy geezer]

Ah, well. It was just a thought. Does anyone still make linear servos?

[malcolmfrary]

In fault mode (i.e. no signal from the RX), on switch-on, my servos usually just give a bit of a twitch, then lie there dormant in their new position.  If a servo does a wide ranging hunt, it sounds like it is being instructed to do so, and the only thing telling it what to do is the RX.  Would a switch-on delay circuit operating a relay to keep the servo signal wires separate from the RX output until the RX has settled down be feasible?

[Capitaine LaGaffe]

In fault mode (i.e. no signal from the RX), on switch-on, my servos usually just give a bit of a twitch, then lie there dormant in their new position.  If a servo does a wide ranging hunt, it sounds like it is being instructed to do so, and the only thing telling it what to do is the RX.  Would a switch-on delay circuit operating a relay to keep the servo signal wires separate from the RX output until the RX has settled down be feasible?

The Rx definitely seems to tell the servos to move: if the Tx is off when the Rx is switched on that order seems to be move 45 degrees clockwise regardless, so on-off-on results in 90 degrees clockwise (which is where I chickened out).

I'm not enough of an electronics wizard to design such a circuit (though building one shouldn't be a problem) – a long time since I used to read Practical Electronics and Elektor (nostaliga …).

My current thinking is to get Atlas Digital Servos: (1) digital servos doesn't seem to start with a great spasm, and (2) you can limit travel in the servo, so my hypothesis is that even a neurotic Rx wouldn't be able to do much damage.

[malcolmfrary]

Here's a couple of delay circuits I offered a while back.
For switching logic level signals, a CD4066 could be substituted for the relay with little fiddling about.

[Capitaine LaGaffe]

Thanks!

I received an answer from Multiplex (Germany): "Upgrading the receiver should solve your problem." It didn't: smaller power-on-spasm but large enough to cause damage (I guess I should have bought Mpx or Hitec srvos rather than GWS . Digital servos ordered …