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How can I build a better pulsed pressure generator?

J

Joerg

Jan 1, 1970
0
Lasse said:
Den fredag den 20. september 2013 18.55.37 UTC+2 skrev Joerg:

don't use the full rotation, servo the motor like in the woofer thing

It is very tough to get enough gusto out of that, even if I had a big
stepper plus gearbox.

some of those big RC servos for big helicopter and such can do
60deg/0.08sec and the through a gear box, though the probably probably
don't have the power you need

Maybe I could try to obtain one for a drone. But then I'd have some
unmarked cars sitting in front of the house soon :)
 
J

Joerg

Jan 1, 1970
0
A

amdx

Jan 1, 1970
0
I think the pistons in there rely on the lubricating effects of the
hydraulic fluid. They could die quickly in pure air, like a two-stroke
engine driven without oil in the gasoline.

How about a big woofer on the big end of an exponential horn?
About 15 years ago the fellow I worked with built an exponential horn
with cardboard and paper mache, I have no idea of the pressure at the
end of the horn, but it would knock over paper targets at 20 ft.
Mikek
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den fredag den 20. september 2013 19.19.03 UTC+2 skrev Joerg:
It is very tough to get enough gusto out of that, even if I had a big

stepper plus gearbox.

do you need a gear box?

1/50 gallon ~= 75cm^3 -> 40mm piston x ~60mm stroke

3psi ~= 21000 pascal

21000 * 20mm^2*pi ~= 27N

if the crank is only ~60mm you only need few N/m

I think that is doable with a stepper

Maybe I could try to obtain one for a drone. But then I'd have some

unmarked cars sitting in front of the house soon :)



-Lasse
 
J

Joerg

Jan 1, 1970
0
Lasse said:
Den fredag den 20. september 2013 19.19.03 UTC+2 skrev Joerg:

do you need a gear box?

1/50 gallon ~= 75cm^3 -> 40mm piston x ~60mm stroke

3psi ~= 21000 pascal

21000 * 20mm^2*pi ~= 27N

if the crank is only ~60mm you only need few N/m

I think that is doable with a stepper

It is, but only if you have a serious stepper like this one:

http://www.nmbtc.com/pdf/motors/standard_hybrid/34KM-KSTD.pdf

And then one has to hope that the forward-reverse acceleration can do
the 1Hz bas and 25Hz small excursions. When you go this fast stepeprs
can lose oomph.
 
J

Joerg

Jan 1, 1970
0
amdx said:
How about a big woofer on the big end of an exponential horn?
About 15 years ago the fellow I worked with built an exponential horn
with cardboard and paper mache, I have no idea of the pressure at the
end of the horn, but it would knock over paper targets at 20 ft.
Mikek


It's unlikely going to work because in order to create pressure in the
1Hz spectrum it would have to compress the whole big column of air in
the horn. That's a lot, and speakers do not have enough excurion for
that. Also, it would not solve the problem with the failing surrounds.
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den fredag den 20. september 2013 21.17.34 UTC+2 skrev Joerg:
It is, but only if you have a serious stepper like this one:



http://www.nmbtc.com/pdf/motors/standard_hybrid/34KM-KSTD.pdf



And then one has to hope that the forward-reverse acceleration can do

the 1Hz bas and 25Hz small excursions. When you go this fast stepeprs

can lose oomph.

the big one still has 3Nm at 1000hz

and if you used a dual action cylinder you could "preload" the other
side with 3psi so that you only need half the force

though the inertia of the rotor/piston etc. might be the limiting factor
not the force from pressure

-Lasse
 
J

Joerg

Jan 1, 1970
0
Joerg said:
Folks,

I need to generate pressure pulses of 3-4psi above ambient, frequency
around 1Hz, more or less arbitrary waveform with frequency components up
to 40Hz in there (but those are less than 2psi) and delivered via short
hose. Currently just air pressure.

I got this (sort of) running with a compressor but after a while my ears
fall off from all that noise. Can't really place the compressor outside
here. So then I tried various speakers. They don't move enough air
around. When I pushed one of them a bit harder a few minutes ago its
flexible surround rubber burst, that thin stuff doesn't hold much pressure.


In other words I need something similar to an arbitrary function
generator but where the output is air pressure. What I want to avoid is
hacking a motor, removing a valve and using the piston.

Any other ideas? Can one buy something like that at less than 4-digit
prices?

Another idea, maybe someone knows how these are called and maybe even
where to get them:

Some modern cars are advertized as having a system where the electronics
system can apply the brakes if the driver is inattentive or dozing off.
So there probably is some sort of electrically activated piston in the
vehicle. If that is the case, what are these called?
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den lørdag den 21. september 2013 00.05.57 UTC+2 skrev Joerg:
Another idea, maybe someone knows how these are called and maybe even

where to get them:



Some modern cars are advertized as having a system where the electronics

system can apply the brakes if the driver is inattentive or dozing off.

So there probably is some sort of electrically activated piston in the

vehicle. If that is the case, what are these called?

I doubt automatic braking is done with a electrically activated piston,
I would imagine it consist of a pump and some proportional valves

for abs I believe it is called a modulator, I think it is just
valves and a pump

-Lasse
 
J

Joerg

Jan 1, 1970
0
Lasse said:
Den lørdag den 21. september 2013 00.05.57 UTC+2 skrev Joerg:

I doubt automatic braking is done with a electrically activated piston,
I would imagine it consist of a pump and some proportional valves

Possible but a pump would have to come on darn fast or run all the time.
Running all the time makes it prone to wear and failure.

for abs I believe it is called a modulator, I think it is just
valves and a pump

ABS is easier, that just takes pressure away from the individual brake
lines so that the corresponding wheels do not lock up.
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den lørdag den 21. september 2013 01.00.11 UTC+2 skrev Joerg:
Possible but a pump would have to come on darn fast or run all the time.

Running all the time makes it prone to wear and failure.

with an accumulator it only need to run until the pressure is up, it not like you need a continues flow and the volume involved is small
ABS is easier, that just takes pressure away from the individual brake

lines so that the corresponding wheels do not lock up.

I think it has a pump/accumulator so it can get the pressure back up
if you only bleed pressure you'd run out of pedal travel or not brake
enough on a changing surface

-Lasse
 
J

Joerg

Jan 1, 1970
0
Lasse said:
Den lørdag den 21. september 2013 01.00.11 UTC+2 skrev Joerg:

with an accumulator it only need to run until the pressure is up, it
not like you need a continues flow and the volume involved is small

Yes, true, I was still thinking in terms of my old Citroen 2CV which did
not have any brake assist.

Well, I guess that idea won't fly then either :-(

[...]
 
J

Jasen Betts

Jan 1, 1970
0
That's what I am doing right now, except with a hacked pressure switch.
If you valve could do more than one cylce per second, do you remember
what kind and maybe where you bought it?

It doesn't get me away from the compressor noise though.

For quiet you need a different sort of pump.
perhaps scroll or Roots. unlikely to be cheap.

OTOH it may be possible to muffle the intake
with the right sort of accoustic ducting (car exhaust muffler?)
and perhaps soundproof the compressor room.
 
J

Joerg

Jan 1, 1970
0
Jasen said:
For quiet you need a different sort of pump.
perhaps scroll or Roots. unlikely to be cheap.

OTOH it may be possible to muffle the intake
with the right sort of accoustic ducting (car exhaust muffler?)
and perhaps soundproof the compressor room.

Unfortunately I do not have a compressor room. The compressor has to
live in the office for those tests.
 
J

Jasen Betts

Jan 1, 1970
0
Now that could be an idea. I'd have to stagger the controls though, in a
way that each one shoulders about an equal pressure differential on its
surround. Because the weakness of the surround is the only real problem
I have, they work for a short time and the go phhhht on me.

AIUI The "doof-doof car" builders just wire them in series or parallell.

Air looses 1/8 of it's volume if compressed 2PSI you could add a
passive comonent to the driver on the high pressure end to compensate
for the shorter throw and therefore lower innertial load.

But in my opinion 12% is not a large mismatch, simple is probably
better.
 
J

Jasen Betts

Jan 1, 1970
0
Yes, true, I was still thinking in terms of my old Citroen 2CV which did
not have any brake assist.

Later Citroens had full hydraulic brakes.

You could look at electric trailer brakes, that seems to be an
electric-hydraulic system, judging from the appearnce of the hardware
I'd guess ballpark 100W.
 
F

Fred Abse

Jan 1, 1970
0
Later Citroens had full hydraulic brakes.

The 1960s DS model had pumped hydraulic brakes, suspension, steering, and
gearshift.

If the pump stopped pumping, you were left with a mechanical emergency
brake, steering that needed two people, no gearshift, and bottomed
suspension.

Hailed as "The Car of the Future".

De Gaulle was in one that was ambushed, and managed to get away.
 
J

Joerg

Jan 1, 1970
0
Jasen said:
Later Citroens had full hydraulic brakes.

Only the DS series, and maybe XM and GS. For the smaller ones that is
way too expensive.

You could look at electric trailer brakes, that seems to be an
electric-hydraulic system, judging from the appearnce of the hardware
I'd guess ballpark 100W.

I did look. They are unfortunately fully electric, no hydraulic fluids
there.
 
C

Charlie E.

Jan 1, 1970
0
Sounds like a construction project. Wooden box about 18" square, two
hand wound solenoid coils with an iron bar between them, and a sheet
of plywood mounted on the center of the iron bar the same size as the
box.. You power one coil, the sheet of plywood is pulled to one side
and you get a high pressure pulse. Power other coil, and you get a
low pressure pulse. Apply different power levels to each coil, and you
can get interesting waveforms!

Charlie
 
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