How can I build a better pulsed pressure generator?

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?

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
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One of those old style Wagner paint sprayers with the vibratory motor?

(I think the new ones actually have a turbine :-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Den onsdag den 18. september 2013 22.17.29 UTC+2 skrev Joerg:

solenoid or servo motor and a pneumatic cylinder?

-Lasse

Reply to
Lasse Langwadt Christensen

Those are too wimpy. They can't move this much air around. They can shoot out a quart of paint in 10-15 minutes (unless the motor burns up) but I have to move 1/50th of a gallon of air around in less than a second.

Speakers with a big enough amp can do this but they suffer seriously because they are meant to be operated against a fairly rigid air cushion.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Something like that but I've not seen any solenoid-cylinder combination that is this fast. They all had too much mass and were designed for much higher working pressures (Festo et cetera).

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Speaker driving into a chamber with only outlet your tube? Remember piston area "gain" from Freshman mechanical engineering electives? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Some compact woofers use linear motors.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Are the "cone" and surround durable enough to stand back pressure?

I'm curious, because I've thought of looking at such a scheme for a positioning project.

And: How low a frequency will they work at? I have in mind 1-2 Hz. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

If you didn't care about the "waveform", I'd have just suggested driving a small "lawnmower" engine with an electric motor (plumbing your hose to the openings for the plug(s)). Possibly adding a pressure valve to ensure a greater "leading edge" if need be...

Should be simple to experiment with (assuming you have the time and an old lawnmower/snowblower/chainsaw/etc. handy)

Reply to
Don Y

Try taking apart one of those really small shitty subwoofers... long-throw speakers on the order of a few inches diameter. Turn the enclosure from ported (they usually are) into fully sealed and you get rid of the back wave; arrange suitable means of coupling on the face side.

Assuming AC coupling is fine, of course. If you need DC, you could always bleed in bias from a servo valve and PID.

It would take some work, but you might be able to hack a hard drive head motor onto an especially nice ball valve (normally, they're rather sticky from the o-rings; otherwise, if you can salvage a strong head motor from a

1980s hard drive..). Add encoder for positional feedback, and calibrate output for the nonlinearity of the valve (or feedback loop with a nearby pressure sensor).

Tim

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Deep Friar: a very philosophical monk. 
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Reply to
Tim Williams

That's exactly how I blew up two of them. No smoke, the rubber surround gave up.

In the end the tube doesn't matter much, the total volume of air does. It has to be compressed by xx percent no matter what. With liquids this would be a ton easier to do.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Which ones? Cost? ...

Cone yes, surround no :-(

They work down to DC. You can actually position the membrane and then let it sit at that point of travel. That's also ok for the surround but it sure does not like more or less static pressure.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Or ol'Betsy here :-)

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Well, that's what I wanted to avoid. There has got to be a simpler and more clean solution. One that does not contain oil.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

That's exactly what I did for the first attempt. And promptly killed the surround :-(

I used DC.

I've got a valve thing right now and it works but then I need that loud compressor to generate air pressure. Because the valve works like a waste gate.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Linear electro-hydraulic servo valves have multihorsepower control capability and hundreds of Hz response. One of them should work with high-pressure air.

There is a whole generation of Hispanic hydraulic controls experts created by the low-rider movement.

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John Larkin         Highland Technology, Inc 

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John Larkin

Then there is this:

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But it costs as much as a car so it's out.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

you need a beefy woofer, 3psi is something like 180dB spl

-Lasse

Reply to
Lasse Langwadt Christensen

Might be helpful to know what you're trying to accomplish. A solution for the general case is likely to be big/expensive/complex. How much volume you need to move is critical. The hose can have a major effect. As will the volume at each end.

How about using two independent pistons operating at higher frequency and varying the phase to create the pressure waveform?

Reply to
mike

A bigger speaker, cut off the outer two-thirds of the cone, and make your own surround for the rest?

Or a speaker-coil motor driving a bellows. If accuracy is important you may need to wrap the thing with feedback from a pressure transducer -- bellows tend to change geometry with displacement, so as you work the thing the from differential coil current to differential pressure will change.

You may actually be able to implement the bellows with something as simple as a mylar party balloon and a couple of pieces of plywood (or acrylic sheet -- it looks more professional).

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

Or just trap the acrylic party balloon between the cone of a (repaired) speaker and something flat or suitably curved (probably concave as seen by the speaker). That relieves the speaker cone support from having to stand up to any air pressure, and leaves the -- hopefully strong -- cone working against the party balloon.

If you use one of those servoed things, expect to mess up the loop tuning.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

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