WANTED : 2 1/4" x 15" Piston Tank

[raalst]

the rod is threaded M3.

the rod is threaded M3. the resolution of the photo makes it hard to see.
The hole of the servo horn lockscrew was drilled and tapped M3

this means each full turn of the servo moves the rod about half a millimeter.
it takes about 5 minutes to travel the length of the syringe.

[mylo]

Ronald, That's what I'm talking

Ronald, That's what I'm talking about.....right there. Good idea.

Chris, .... you can't help it, you're a tech nut. It's an illness, not your fault. It's ok though, because we need tech nuts to come up with new gadgetty stuff. I'm more of a..... model nut which too is a disease, a very expensive and time consuming disease, and given that, I have built a 9' long U-Boat. Now it's your turn, build something..........neat, like........ a 1:25 scale electric torpedo with a compressed air "warhead" that blows off on impact, spraying air/water in the air......that would be....neat. I know just the boat to put it in too.

It's all good, ....keeps us out of trouble.

Mylo

[cstranc]

Ronald,
Thanks for the info

Ronald,
Thanks for the info that makes perfect sense.

Chris
That torpedo would be neat. But I have to get the Sierra in the water in time for Carmel this year....

So just to keep you thinking... If you used one of those dirt cheap 12v fuel pumps. Take the fuel line and it ends where the air should be, and you make a little hole in the hose below water level too. So when you turn the pump on you get a mix of water and air going through the pump and into the ballast tank. The water goes out the bottom of the tank as the air fills it.

Might be tricky setting it up so you get a good water / air mixture. It would remove the "ooh I don't want to get water in my blower" which sounds kinda painfull.

Chris

[hakkikt]

Ronald,

did you choose a specific

Ronald,

did you choose a specific type of servo for your surgery? I have never taken one apart - is it normal that you can lead that threaded rod right through the innards without hitting anything important? Or am I being fooled by perspective here, and the rod does not not through but alongside the servo?

-Harald

[raalst]

the rod goes through !

the rod goes through !

it depends on the servo make and model, but I believe most will do nicely.
I have butchered one with metal gears (because I would rather use
metal than plastic for this) but that one had teeth that only populated
half the gear wheel. therefore it could not fully rotate by design.

the variable resistor often sits underneath the main axle, and the motor
sits on the opposite side. since the resistor and electronics are removed
this leaves the space underneath the axle free for the rod to feed through.

I also used a bit of brass tubing to guide the rod through the servo box.
I'll make a few more photo's soon, as I will redesign the Kilo for a third time shortly..

in the meantime have a look at version zero and one of the servo piston tank

http://home.vianetworks.nl/~raalst/kilo/propkolben.html

[JWLaRue]

Ronald,

A very nice solution!

Question: how

Ronald,

A very nice solution!

Question: how does the the sensing arm work? I think I see a servo connected to a short piece of rod, which then uses some rubber tubing to connect to the threaded rod of the piston.

-many thanks,

Jeff

[raalst]

this solution with the arm

this solution with the arm was aimed at having a fully proportional
piston tank, but it does not work in practice.

how it works :
the position of the variable resistor indicates to the servo electronics the position of the servo horn. the servo compares this information to the position of the stick on your transmitter, then operates the motor
to make the position of the variable resistor (and hence the servo arm) equal to your stick position.
This is how it works in a non-hacked servo.

Now I fool the servo by letting the variable resistor sense some other movement, in my case the extention of the piston rod. the bit of rubber is
a makeshift joint, and the arm consists of two telescoping brass rods. This is connected to the variable resistor so that the lineair motion of the rod translates to a rotation of the resistor. the resistor was pressed into the
white plastic.
this means that the variable resistor and hence your plunger follows the position of your stick.

This works in theory, but is impractical because :
- the plunger moves far too slow to keep up with the stick/human
- the sensing is less accurate at the extreme ends, due to the angles
involved. a linear variable resistor would work OK but I could not find a
small one which had enough travel to match the length of the syringe.

The piston simply kept running and the syringe was broken off its base.
because this modification makes for an extremely able gearbox...
(the same pushrod idea could be used to move your batteries or another
sizable weight and trim the boat that way btw)

therefore I reverted to end switches. Now the servo does not follow the stick position, but simply keeps moving forward or backward as long as you keep the stick out of neutral. the limit switches prevent damage at the extreme ends.

(I promised Tim a SCR article a long time ago... serves me right !)

[JWLaRue]

Ronald,

Many thanks for the explanation.

Ronald,

Many thanks for the explanation. I can see how the sensing arm would then not work well enough.

What about connecting the threaded rod to a variable resistor pot? Let the rod cause the pot adjustment turn. Seems like a suitable reduction gearing could be arranged to match the throw length of the threaded rod to that of the rotational distance of the pot.....?

-Jeff

[raalst]

yes, that might work, actually

yes, that might work, actually I have seen it in a german boat.
a small gear is turned by the threads of the rod, turning a multi-turn pot.
But in my Kilo this setup did not fit.

[cstranc]

Ronald,
Funny you should mention

Ronald,
Funny you should mention using the same setup for moving a ballast weight. That is what I am doing. But I just use a standard 1/8" rod to go out through the WTC and then some Lego gears and chain to connect to the threaded rod the weight is on.

It moves slowly, but does a great job balancing. I found I had to be carefull and not use too small a Lego gear because any binding and the gear disintegrates. Thankfully Lego is cheap compared to everything else.

Chris

[raalst]

@Jeff,

I've been looking around

@Jeff,

I've been looking around because I had seen a relevant photo.

http://www.postimage.org/image.php?v=aVEmmGS

This concept would work well I guess. attach the pot to the center of the wheel and attach the wires to the pushrod.
The pushrod might have to be longer than
otherwise necessary though.

[JWLaRue]

Ahh...a LineDrive! Is someone still

Ahh...a LineDrive! Is someone still making those?

-Jeff

[raalst]

seems not, I got this

seems not, I got this picture from an old thread where Brian Stark
kindly offered it to anybody interested.

but the principle might be easy to duplicatie : piano wire, a
grooved wheel and two set-screws.
(any old transistor radio with a linear dial would have
something like that built in)