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Sail-winch drum servo modification

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Old 11-27-2002, 06:51 PM
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RobStagis
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Default Sail-winch drum servo modification

Here's the question, with some options in the solution. This is a large sailboat (the Robbe Atlantis) and its genoa (very large foresail) is controlled by a dedicated drum-winch servo that has to drag a traveller the full length of a 48" (or so) track.

Not having the $150 that Futaba wants for its S5801, I bought a HiTec HS725. This winch has more power than the Futaba, but its shortcomings include a lack of adjustability and high current draw at idle. But for a $100+ savings, I'm willing to mess around to get what I need.

So: the drum has a 7 3/8" circumference. To get 48" of sheet travel I'm going to need the drum to make at least 6.5 turns. The winch's standard rotation is 3.5-4.5 and it depends on the transmitter. I can find any number of sites that tell me how to make a standard servo into a continuously rotating servo; how to make a 60 degree servo into a 90 degree one, or reduce the number of turns on a drumwinch servo.

But I can't find a single one that will tell me how to increase the number of turns. The options are that I modify the receiver with adjustable pot (or pots) in the channel circuit, modify the servo with either resistors or pots in the feedback circuit, or throw my hands in the air and whine to the wife,"Gimme $150!" Not being fond of black eyes, I'll kindly ask you all if you can suggest something..

Thanks in advance - Rob
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Old 11-27-2002, 08:15 PM
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Default Sail-winch drum servo modification

...modify the servo with either resistors or pots in the feedback circuit...
If you are good with a soldering iron then this is the cheapest solution. Just pad the outside pot leads with a series resistor (2 required). Try 1K's to start.

Haven't tried this on a winch, but works fine on standard servos.
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Old 11-27-2002, 08:59 PM
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Default Sail-winch drum servo modification

Is there some math behind this? I'm not good with electronics, but putting the resistors in there will increase the turns, right? Any idea how much?

Honestly, I have yet to disassemble the thing - I'm assuming (hoping) that they're using a multi-turn pot - then, as I see it, I could increase the turns with your resistors (thank you, by the way) then tailor the turns with trim pots.....does that sound feasible?

Heh (additional thought) or just limit the stick-travel with some blocks on the transmitter?
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Old 11-27-2002, 11:39 PM
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Default Coming Soon!

Our new servo power gearboxes on our site will soon be offered with a 10 turn pot. They should be completed by late next week and sell for slightly more than the standard. Just as the standard, it will allow you to use a standard servo and increase the power 4 times. Thanks!

www.ServoCity.com
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Old 11-28-2002, 02:52 AM
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Default Sail-winch drum servo modification

putting the resistors in there will increase the turns, right? Any idea how much?
This is a emperically driven task and must be customized for each servo model. Start with 1K's. Just add the resistors then see where it takes you. This resistor padding trick only works on proportional servos.
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Old 12-28-2002, 08:12 PM
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Default Hacking a servo

Hi Rob,

Fancy meeting you here. I think in our other board we discussed hacking a Hitec 715 arm servo from 80 to 160 degrees with adding resistors. I added 2.2k 1/4 watt on either side of the pot on my 715 to get that 160 degree rotation. Should be about the same on the 725-doubling the travel (of which I'll have to try). As there aren't any mechanical limits, should be pretty straight forward. Get back to me on our sailing site and let me know. I will pull apart the 725 just to confirm the mechanical side not having a limit. As to servo city's idea. The gearbox setup they have now doesn't appear to be able to fit this size servo, and if they have one now that will fit, it still does give you great power, but looks to me like it will slow the winch down by a factor of four. Not desirable in sailing!!!

Chris
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Old 12-29-2002, 08:57 AM
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Default Sail-winch drum servo modification

refer to the section =
"Servo & TX alterations, calculators, clonepacs, make an ESC or winch, FAQ." under radio systems at
http://homepages.ihug.co.nz/~atong/
I use a Hitec Saiwinch servo in my Marblehead yatch which inly travels two turns - very easily adjusted via the trimpot fitted tio the TX whilst the yacht is sailing.
regards
Alan T.
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