-- Recalling how a steam engine's piston and escapement are used to drive a wheel, connecting a motor to a wheel's axle and adjusting the stroke of the piston via the escapement should allow you to get the travel required to get the volume of air you need to generate the pulse.
How much volume ?
Would one of those el-cheapo air compressors you see in auto parts places for inflating tyres do ? 12 volt motor, so easy to DC control. At least you have a motor already hooked to a cylinder and piston. Replace the motor with a stepper ?
-- Regards, Adrian Jansen adrianjansen at internode dot on dot net Note reply address is invalid, convert address above to machine form.
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.
-- Regards, Joerg http://www.analogconsultants.com/
I am testing a very tiny pressure sensing device for accuracy and bandwidth, against a known-good large regular silicon pressure sensor.
About 1/50th of a gallon to me moved. Total air is a bit more, but not much.
What I have right now is the test vessel which needs 1/50th of a gallon of air pushed in and out. In order to accomplish that I have about 2" or
1/4" hose connecting from there to an intermediate tank of roughly 1/3rd gallon capacity. The source for that is a long compressor hose connecting to a 3 gallon tank at 80psi max, but with a regulator in between. Works well but is noisy as heck. All the less noisy electrical constant speed air pumps I've tried aren't powerful enough to keep up.Yes, absolutely. But I'd need this sort of "ready to roll", without it turning into a major science project.
-- Regards, Joerg http://www.analogconsultants.com/
That's how my compressor works :-)
But it makes a huge racket and so I'd like a simple, less noisy solution.
-- Regards, Joerg http://www.analogconsultants.com/
1/50th of a gallon each pulse/
Tried that, too weak. Even smaller medical styl;e compressors are too wimpy for this, it needs a half-horse motor or something like that.
If there would be a speaker that can move this much plus stand the pressure that would be great. My big one couldn't quite get the amplitude but made decent pressure pulses. Then the surround rubber tore :-(
-- Regards, Joerg http://www.analogconsultants.com/
I never had much luck with craft projects like that. I'd first have to find the right material and then the surround would become big and probably pop because of the pressure.
That could indeed work. Not sure how many cycles such a balloon would survive though. I'd have about 60 per minute.
It can be open loop, I can pre-distort the arb gen input accordingly. Mine came with a sort of computer-based Etch-a-Sketch.
-- Regards, Joerg http://www.analogconsultants.com/
Get a plastic bag of some sort, or an inner tube covered with a piece of plywoodwood, and jump on it, drop a brick on it, something like that. All you need is a waveform to compare, no?
Or put both sensors in a car, and slam the door.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
You may want to look at Tom Danley's Servodrive speakers for inspiration.
Kevin Gallimore
OK, then you could hit them both with a 'step' and that should be do the time response. (Like popping a pressure vessel with a balloon on the end.)
Well if you can keep the volume down then you might be able to get away with a 'fish air pump' type thing. We sell this floating pool ball apparatus that needs a fair bit of pressure...it took a while to find the right pump. I'll have to look up the name and specs tomorrow.
George H.
If you had a ft cube box, with a 12" subwoofer on 4 sides, or even all
6, wouldn't that give you the required pressure?A few of these:
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
Oh, forget my earlier idea then. Way too wimpy.
George H.
Grin, I might be nice to have something 'semi' repeatable. Mylar or Al foil over the end of pvc tubing might work. If the rupture is faster than time response of the detectors then the exact details (of the rupture) may not matter.
George H.
How about a C02 cartridge, looks to be about 100psi or so. but you can regulate that and pulse the output with a small valve. Probably better off getting the Soda bottle with the C02 Fitting and put a nipple on that. Should get hundreds of shots off a 12gram cartridge. You can get valves that respond in < 20ms at least that?s what they are spec'd at. 1/4" barb
Disclaimer: some experimentation required.
Cheers
Fire a gun. Impulse response.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
I've did a similar thing years ago to calibrate hi pressure piezo pressure transducers.
We used an astable driving a solenoid that vented to ambient when closed. The solenoid was supplied with a pressure regulator. Since you are using low pressures big bore solenoid valves may be necessary - perhaps you could get away with reticulation valves.
Alternatively what about hacking an electric / battery breast pump? Usually available cheaply second hand.
Get one of those linear actuators that moved the head assembly on old
70's era disk drives. I remember playing with one of those. They could break your arm. Maybe attach a plastic bellows from a beach-ball inflator as the pump.Reality check for all the "engineers" in this thread (myself included). ;-)
1/50th gal is 2 * 0.0378 L, and 3 PSI is about 1/5th of 100kPa, or: W = 0.02 x 10^6 N/m^2 * 37.8 x 10^-6 m^3 = 0.756 J peak energy. At 40Hz CW, the peak power would be 2*pi*F*W or ~250W. That much isn't required, but still, a speaker in the 50W range probably wouldn't be unreasonable.Speaker efficiency is normally quoted against propagating atmospheric conditions, so is usually quite terrible. I don't know what the efficiency of a linear motor into a well-coupled acoustic load would be; should be possible to get over 10% at least, and I'd think 90% wouldn't be unreasonable (a motor is a motor?). So that's 60 to 500W.
A little 2" subwoofer would have to handle 10 pounds of backforce at that pressure, while pushing a peak displacement of 1.9 cm (about 3/4"). Imagine a piston cylinder 2" dia. by 1.5" tall, and pump that up and down with 10 pound springs attached. That's a lot of displacement even for the bigger subwoofers, and I don't know that any would support that much pressure (insane professional types aside). So it isn't surprising that it isn't good enough. Also unlikely that other small pumps would deliver enough power (e.g., breast pump, amusing idea though).
To get the force up, a bunch of tiny subwoofers might be paralleled. As another suggested, a bellows would get a user-defined diaphragm smaller than the cone area(s), providing some mechanical advantage. Length and width of the bellows determines pressure/displacement matching, so any combination of voice coils could be used, including a single big one from a 500W+ speaker, say.
If you can find one of those super-low inertia capstan motors from the Old Days of Tape, you might get that kind of bandwidth there as well. Even a regular servo motor might not be too bad. Still need a lever and diaphragm or bellows to couple, of course.
Tim
-- Deep Friar: a very philosophical monk. Website: http://seventransistorlabs.com "Joerg" wrote in message news:b9ug38F583bU1@mid.individual.net... > 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/
For a project I worked on, for pulsed holographic detection of lumps in bre ast tissue, a pulse of air was needed. A linear actuator from a old DEC ha rd drive drove a flat plate into a box. No problem reaching two PSI. The t echnique is similar to tire testing, two flashes of the laser, one before, and one during the air blast. When exposed on the same plate, the result is fringes where there is a stiffness in the tissue. This could detect lumps far smaller then x-ray mammography. Just a vigorous puff of air, and a high peak power laser. The laser beam is diffused, no danger of burns at all.
Same concept is used for tire testing.
If you cant find a DEC hard drive, you use a small chamber between two fast solenoid valves. Fast valves are cheap, if you know where to look. Clippar d makes some, there are other vendors.
Steve
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