Using a servo in a piston tank?

[gerwalk]

I'm considering a very small and simple piston tank but I have some doubts. Is it possible to use a 180° servo to push and pull a syringe? I mean: the system would consists in only a 180° servo connected to the receiver and a modified syringe. The servo's arm lenght would be enough to load and unload 20ml from a 60 ml syringe.

My doubt is if the servo would be able to keep the piston in possition or the pressure would push it back.
Thanks

[Guest]

Pressure won't be enough to

Pressure won't be enough to push it back, as the servo will push against any pressure,as it would sense a movement in the servo shaft.

I'd choose a high torque servo though, as they're cheap enough these days.

Andy

[gerwalk]

Pressure won't be enough to

Pressure won't be enough to push it back, as the servo will push against any pressure,as it would sense a movement in the servo shaft.

I'd choose a high torque servo though, as they're cheap enough these days.

Andy

Thanks Andy. I will test the idea soon. Im thinking in a simple bench test:

a) 180° Servo connected to the syringe using an adecuate extension arm and everything fixed on a bench.
b)A hose 2m long -or more- connected to the syringe and a funnel attached to the end of the hose.
c) Pour water into the hose: this can be used to simulate different depths conditions by simply rising and lower the hose. Water pressure against the piston (syringe) will be determined by the height of the hose.
d) test the system at different water pressure and see if it works properly.
Of course in this experiment I'm not considering the pressurised air inside the WTC when the piston is full of water.

[anonymous]

Somethink like this.


Gantu

Somethink like this.


Gantu

[raalst]

yep, that's mine. still works

yep, that's mine. still works very nicely after 2 years (=6 events).
I built it into a trumpeter Kilo 1/144.

but please consider that the action is very slow (and strong)
so do not expect 15 sec diving times with a piston tank like that.

I built it with end-switches, because I did not have space to include a lineair
resistor slider.

more info on the construction here : http://home.vianetworks.nl/~raalst/kilo/propkolben.html

re-reading your post : 20 ml = 20 gram of buoyancy, that is awfully
little. mine does about 45 ml, and barely gets the tower into the water.

[gerwalk]

Thanks guys!
I've seen Raalst idea

Thanks guys!
I've seen Raalst idea sometime ago. It is very good indeed but I want something even easier. Maybe I should try that too.
I know that 20ml is very little bouyancy but I'm thinking very small here. Not a 1/144 Kilo maybe something smaller.

[gerwalk]

Raalst,
You said you used limit

Raalst,
You said you used limit switches but the article ends with that as a possibility. How do you finally implemented it?
If you don't use the servo electronics how do you connect it to the receiver?

[raalst]

I do use limit switches,

I do use limit switches, because the potmeter was too inaccurate on the extreme ends, and therefore the piston ran into the wall.

I still use the electronics !
only, the potentiometer is detached from the piston and the switches are added.

I'm not exactly sure how I connected the stuff 2 years ago, but the general idea is this :

you fix the potmeter on neutral so that niothing happens if you do not do
anything.
if you move the stick one way, the servo will start to turn until the potmeter
value is 0 ohm (it will never be, because the piston does not influence it) and the other way until the potmeter has a
very high value (which it will never reach, because it is not influenced by the piston).
now, using the switches instead of the potmeter I can obtain the desired behaviour.

- one (normally open) switch shorts the potmeter -> 0 ohms
- one (normally closed) switch cuts the line as a whole -> a lot of ohms

so, the piston fills while you have the stick above neutral, stopped by an end switch, and it empties while you have the stick below neutral, again limited by a switch.

phew, hope that helps. it's really simpler than explaining it.


for your original approach, maybe using a rack and pinion (e.g. from lego) is feasible ? put a lego gear on the servo disk and operate a "rod with bumps on it" so you get direct lineair motion (rod with bumps is a rack, right ? not sure about the terminology here) . problem there is that the servo is out of the centerline, requiring a larger diameter tube.



btw, you can do some math on what force you need.
you need to overcome the water pressure on the piston disk (the part that moves).
so, 1 bar = 10 meter's depth = 100000 N/m2 =
10 N/cm2
1 meter depth = 1 N/cm2 of pressure.
now suppose the area of your piston is 3 cm2, then you need
3 N of force (= the force to lift 300 grams off the floor), to force the water out when 1 meter deep.
BUT you also need to overcome the friction of the syringe piston itself.

I have bought a very nice lineair servo, but it costs 85 dollars...
And I suppose it is still too clunky for your ambitions.
PM me if you consider buying one. Can't imagine you would ...

[gerwalk]

Thanks raalst! That was very

Thanks raalst! That was very clear! (you are also here to teach man!)
Your idea with the lego stuff is very good.
I was also considering a cd player opening-sliding mechanism (from an old CD player). It has a small motor and all the gear, not sure if the motor is strong enough but the gears seems to be good.

Regards

PS: no, I'm not interested in the linear servo, sorry

[raalst]

there is also a lot

there is also a lot of mechanics in old VHS players.
You can collect those from the scrapheap literally.
might contain useful bits and pieces.

and no problems about the lineair servo

[hakkikt]

I am not sure if

I am not sure if the Lego might not be too flimsy for this purpose.

Norbert Brüggen gives a few data for examples on piston tanks. The smallest looks like this:

tank volume: 20 ml
piston diameter: 30 mm
piston stroke: 28.3 mm
overpressure: 0.3 bar (= 3m depth)
time to move: 1.5 sec
force on piston: 21.2 N (~ 4 pounds)
minimum diameter of rack: 2.4 mm
material of rack: V2A steel
driven by: powerful servo driving rack & pinion, motor power 1.6 W

[gerwalk]

I am not sure if

[quote]I am not sure if the Lego might not be too flimsy for this purpose.

Norbert Brüggen gives a few data for examples on piston tanks. The smallest looks like this]

Thanks for the data!

[raalst]

@gerwalk
check out the "puremtc"

@gerwalk
check out the "puremtc" piston tank (see general forum) !
I have never heard from someone actually using this contraption, however.

[gerwalk]

@gerwalk
check out the "puremtc"

@gerwalk
check out the "puremtc" piston tank (see general forum) !
I have never heard from someone actually using this contraption, however.

Yes, I've seen it. It is very interesting and compact but rather complicated with that thread and spool.

[Guest]

For the tank mentioned in

For the tank mentioned in Norberts book, he specs a sail winch servo. These differ from standard servos in that they have a far wider range of throw.

Andy