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twinturbostang

Any diagram/picture of inside of HS-81 servo?

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I'm planning on modifying my HS-81 pan servo for 180 degree motion (resistor mod). Right now I've got a servo max module on it. It gives about 90* rotation to one side, but only about 45*-50* rotation to the other side before it starts to bind. I'm assuming there is a mechanical stop in there that needs to be removed for full 90* motion in that direction. While I was in there, I figured I'd also mod the servo for 180* to reduce the weight/clutter of the servomax module. But before I do this, does anyone have a diagram or picture of the inside of the HS81 servo? Just in case I end up with any "extra" parts left over. :D

Thanks,

Brian

Oh, and Happy New Year everybody! B)

Edited by twinturbostang

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Let us (me) know how this works out for you. I too have an HS81 that I'm currently using a servomax module with, but would like to convert it internally.

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If you remove the mechanical stop you risk damaging the POT if the wiper goes too far. Be careful.

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I do not removed the stop when I modify the servos, I only add one 1Kohm resitor on each pot leg(not on the middle leg)

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I believe I need a lesson in servo theory. I can't figure out what's going on. I removed the stop, and now there's no binding. BUT, the servo still stops in the same location as it did before. It just stops moving, even though I'm moving the transmitter stick farther. Then when I return the stick back, it starts moving again normally. It's like a dead band or something. I get a full 90* motion to the right (clockwise), but only about 45-50* to the left (counter-clockwise).

What exactly goes on inside a servo? What does this "feedback" pot do? Where is this "wiper"? Hope I didn't damage anything.

Edited by twinturbostang

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See here how to modify a Futaba s-148 servo for 180 degrees operation.

http://mypage.yhti.net/~dmcdnld/s148retract.htm

And more

http://www.kronosrobotics.com/an116/GAN116.shtml

And see the attached picture of the internal functioning of a servo.

Sorry that I need to translate the names on the picture but I couldn't modify the JPG.

in=in

uit=out

pulsverglijker=comparing pulses.

pulsingang=puls input from transmitter.

pulsgenerator=pulsgenerator.

electronische schakelaars= electronic end switches.

pot meter=variable resistor.

as you see dutch is not that difficult :blink: :blink:

The conclusion from the drawing is when you modify the fdbk signals from the pot meter you will fake the comperator. So when you send and angle of 45 deg to the servo the motor will start turning until the fdbk will give the same value as 45 deg. But when you place 2 resistors in series with the pot-meter the fdbk signal will be 45deg when the actual movement is 90 degrees. see the first web page that I posted.

good luck

post-8-1167683480_thumb.jpg

Edited by ahjwinth

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Thanks for the info. Ok, I was a little confused at first because I did not see the pot buried down inside the servo. I first assumed it was part of the motor, but that didn't make much sense. I now see it's a separate piece and it looks like it might be connected to the output shaft. If that's the case, then it makes sense.

So I'm guessing that the possible problem is that the pot has a certain angular motion before it locks. If it goes past that, then it could possibly break. I'm still not sure why the servo just stops without any other kind of indication as to what happened. Any ideas? Maybe that's the end point of the pot??

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Did some more poking around. Found that the output shaft is notched so that the output spline can only go on one way. And it is then directly connected to the pot. The pot appears to have more than 180* of travel. But for some reason it's offset to one side when the servo is centered with all trims set to zero. Not sure why this is. I know Futaba likes to put center position at 1.52 ms instead of 1.50. But that seems like such a small offset. It's hard to believe that's what the cause is. Take a look at the pictures below to see how the output shaft looks when rotated to each extreme (lock to lock on the pot) and centered.

Further testing also showed that when the servo is rotated counter-clockwise until it stops moving, it has not reached the travel limit of the pot. I'm starting to think that the servo clips pulsewidths at some point from center position. So if that's the case, to get 180* out of it, it looks like I'll have to do the resistor mod. Since the pot will go farther than 180*, I should be able to get it to move as far as I originally wanted. Since 1K Ohm seems to be the magic number, I will first attach a secondary pot to each end lead on the servos pot, and start out at 1K. Then I will tweak it slightly until I find the resistance that gives me the desired travel.

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Update...

In the interest of saving time, I just went ahead and stuck two 1K resistors in there. It worked, although I don't get as much travel as I wanted. Somewhere around 130-140 degrees I'm guessing. I can't see the wing tips. I'll have to measure it to see exactly how much travel I'm getting. I really would like to get a true 180 degree motion.

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Back when I developed PanCam, I experimented with many popular standard sized servos. The goal was to see what the typical limits were, since I wanted to expand the throw range. My conclusion was that some could not go beyond 140 degrees without risk to stalling in at least one direction. Stalling (hitting an end stop) is a major motor killer and can damage gears.

I suspect the mini and micro servos are not much different. Max travel range will vary with the resistor hack, so if one particular brand/model does not work out, you should try another. Be vary careful that you do not allow the servo to stall.

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It's not a case of hitting a physical stop though. The mechanical stop has been removed. So the only other stop is the mechanical limits of the pot itself. And I have verified that this is farther than 180 degrees.

I found something interesting though. When probing the pins of the pot, I noticed that on one particular side the resistance increases as I turn the pot... to a point. It gets to somewhere around 2K Ohm if I remember right (I'll have to double check that though), but then the resistance drops as I keep advancing the pot. The resistance peak happens before it reaches the travel limit. When looking at the other side, it does the same thing, only with the rotation in the other direction. That's very strange. I thought that resistance always increased (or decreased, depending on direction of rotation) as you advance the pot, until you reach the limit. I wonder if that creates an electrical "barrier" of sorts. Any ideas on this?

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I thought that resistance always increased (or decreased, depending on direction of rotation) as you advance the pot, until you reach the limit.

The resistive traces on pots are designed for their specified physical travel range. Once outside this area, it is no-man's land. Also, these are very low cost pots. Maybe yours is out of spec, so try another servo.

Go too far and the wiper may be damaged. Keep in mind that the mechanical end stop for the servo gears is not the same place as the end stop for the pot's wiper. They can differ by several degrees due to the servo construction/assy.

Edited by Mr.RC-Cam

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The resistive traces on pots are designed for their specified physical travel range. Once outside this area, it is no-man's land. Also, these are very low cost pots. Maybe yours is out of spec, so try another servo.

Yeah, I'm sure the pots in these things are very low quality. How could they sell whole servos for $15 otherwise!

Go too far and the wiper may be damaged. Keep in mind that the mechanical end stop for the servo gears is not the same place as the end stop for the pot's wiper. They can differ by several degrees due to the servo construction/assy.

Don't worry, I'm on top of that. The gear limit was somewhere less than 180 degrees. Pulling the thing apart, I was able to determine the actual motion of the pot was more than 180 degrees. If I can get the thing to move exactly 180 degrees, then I should be fine. I'd say there's at least 10 degrees more per side of physical rotation of the pot. But I'm not so sure I can get 180 out of it now, due to the electrical properties. I've got another HS81 on the bench, so I'll take a look at it also.

Edited by twinturbostang

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Making some progress. I've got two external pots hooked up to the two outside leads of the servos pot. I can vary both independently to find the proper resistance values that need to be added so that we achieve 180 degrees rotation.

On the bench, I've been able to get 180. What's interesting, is the two values are not the same. Looks like somewhere around 2K for the green wire, and 3K for the red wire. I'm also having to play around with the end stops in the Tx. Decreasing one side, and increasing the other side.

But I stripped the nylon output gear by accident. Must have had one of the pots set too high when I plugged the servo in to the receiver. I'm going to get a metal gear version of the HS81 (and a replacement gearset for this servo). Then I'll be set to rip out the pot too! :D

Stay tuned...

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Replacement gears are all of AU$2.50, nice and cheap :)

I wonder if it would be possible to find a replacement pot?

Yeah, I'm calling ServoCity today to get a set.

Don't know about the replacement pot. I'm sure somebody makes them though. So if you looked hard enough, you might find the supplier.

BTW, does anyone know exactly what the microcontroller does with the resistance values? If I knew the algorithm that it uses to calculate position, I'd probably be able to also calculate what the resistance values should be for 180 degree operation.

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Possibly. Although you will have to find a very specific one. Not only are the resistance values important, but the size of the pot allows it to fit snugly in the servo housing. Any difference in size and it would either not fit, or be loose. Plus the shaft has to be a certain length and thickness, and be keyed for the output spline gear.

So... let me know when you find some like that. :D

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Success! I now have an HS-81 that goes exactly 180 degrees. Give me a couple of days and I'll have a bunch of pictures put together. I'll do a how-to on the steps needed to do the 180 modification.

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Well, I can break it down real quick. It's not that difficult really once you know what needs to be done. Basically just remove the end stop located inside the upper housing. Solder in-line 2K Ohm resistors on both the red and green wires on the pot. And that's it for the hardware changes. However, be VERY careful advancing the servo to max rotation in each direction. You need to precisely adjust the end stop limits in your transmitter so that you don't over extend the pot. It WILL go 180 degrees, but you have to be very careful. I messed up two output gears until I understood what I was doing! lol

Oh, and the metal gear HS-81 is NOT recommended for 180 degree operation. In fact, it will NOT do 360 continuous mode either. And that is because the output gear does not have teeth all the ways around it (the nylon gear does however). If you're only going out to 160 degree or so, then it's ok. But any more than that, and you could end up in a case where the servo binds. I will post pictures explaining this in more detail also.

Edited by twinturbostang

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