Sensing a local pressure in liquid. It has a DC value and then I'll have to measure up to 20-30Hz worth of changes. Very cramped space, hence the
0.010" width.
I can't work on bending (which I am aware is the normal modus operandi of a "singing cap"). I can only work with thickness changes which will eb quite miniscule. Ideally with capacitance change because the signal coming out of it via piezo generation is very tiny, at least here on the bench.
Oh, I've done that a lot :-)
One could buy half a truckload of reels and store them in a gigantic nitrogen cabinet. That would last nearly forever. But the better method is to first establish principle of operation. Then it's time to sit down with a manufacturer. It would not be the first time where the response is "You want to do WHAT?!". After they heard the Dollar numbers involved that usually changed to "Oh, wow, let's see how we can do this".
Would be interesting to see the mechanical configuration that gives you a differential compression force on such a small area with large wavelengths at 20 Hz. without bending.
Can't argue with that. Money talks! Problem is that it only talks to the original people making the agreement. When a cap vendor is making a million caps a minute, your puny order for 10-million caps isn't worth the stamp it costs to reject your request. And even if you get agreement, the guy's successor will back out if it helps his bottom line.
I've had situations where the new guy in my purchasing augmented his bonus by scrapping my "lifetime buy" of specially selected parts. Even if he wanted to ask engineering, there's nobody left who knows why. I learn about it after the production line shuts down and nobody wants to source the "special" part. THAT 2N3904 is exactly the same as THIS 2N3904... NO, IT ISN'T.
I can tell horror stories until you get bored...which is probably about now.
It's easy. Hook up a cap to a low noise connection into a sensitive meter. I used a Fluke 8845A. Which I bought because John Larkin said so :-)
Anyhow, then clap your hands and you'll easily get a bump in the mV range. But capacitive change is a bit more difficult although I have a sensitive setup to measure that. Meantime I have a whole compressor and chamber setup here in the lab.
We have a guy on board who is really, really good with contracts. He could make sure this doesn't happen.
Not at all. I've live through those kind of goof-ups as well. Full line stop, people hollering, crisis meetings, the works.
As I've said, it's about feasibility. Nothing more. If it turns out to work, you'd be amazed what manufacturers will do if your company puts a commensurate pile of money on the table. It's the American way :-)
This is by far not the first time I do this kind of stuff. Sometimes the unorthodox solution is what puts you in front as a company.
I've also had it on a waterfall spectrum display. The bulk of the generated electrical energy was below 100Hz. But that is generation which is not what I am ideally after. I am after capacitance changes.
OK, I'm surprised that a "clap" has much energy at all at 100Hz. Learn something new every day.
I poked a random ceramic disc cap into the scope. It's a Tek 7A22, so I've got sensitivity and filtering out the ying-yang. At the sensitivities required, I couldn't see any "clap" at all. But I could see the effect of my hand waving several feet away. With a random cap and insufficient shielding, my conclusion is...well...inclusive.
Wonder if you could stick the cap in an arbor press and squeeze it until you get the capacitance change you need. That might give you some numbers on what you're up against.
I'm fascinated by this. I took the output of a sound level meter and plugged it into a scope. Tek TDS540 this time. A hand clap is all over in a couple of milliseconds...if you factor out the room echos. The FFT display showed a lot of low frequency energy, but was more affected by the windowing function selected than by the actual clap. I've never determined anything useful at very low frequencies from a scope FFT.
So, I plugged the sound level meter into the PC and CoolEdit. Got pretty much the same result. In the waterfall display, there's a lot of energy (color) showing at low frequencies, but it's unaffected by the clap. The clap shows up above 1kHz. You can't sense something that isn't there.
All I know at this point is that your results differ from mine. And mine line up with my intuition...yeah, yeah, wouldn't be the first time my intuition led me to crash and burn. ;-)
Fascinating...I'm more interested in the pressure measurement than whether caps are microphonic. I'd like to measure differential air pressure with fractional Pascal resolution near zero differential (CHEAPLY). Different animal, but might learn something from what you're doing.
I don't know the 7A22, got the 7A26 here. But you need to be able to measure sub-mV.
You have to use an SMT cap, not something potted in a disk-type structure. IOW the layer stack needs to be directly exposed. I found that leaded parts have very little microphonics.
Yeah, I'll probably do some more experiments but only after I buy some modern very high density caps. Because I need a change in the thousands of pF over a couple of psi.
Oh, I did. The worst was microphonics. Client had tried on their own and even a fancy expensive Stanford Research Analyzer saw ... nothing. So here I was with a laptop and an FFT program. 7-8 Hertz showed up, not too stable. Hmmm, what the heck could that be when every clock in the system is crystal controlled? Told the client it must be the wind pulses from the fan blades causing microphonics in the caps. "Now that's voodoo, you are kidding, right?". So I gently leaned the palm of my hand onto the fan blade center, slowed it down, and sure enough the noise spectrum moved lower in frequency. When I stalled the fan it was gone. Some jaws dropped.
:-)
Try to cup your hands so you get more energy at the low end instead of the high pitched applause type of clap. Or pop an inflated paper bag instead.
That makes two of us. However, as a generator caps can never be very good at low frequencies and there is no DC. Only capacitance change could do that.
Sure. But I can only reveal what's covered in our patent applications at this point. I've actually got a compressor, pressure chamber, pulsating ports, and so on in the lab right now. I wish those compressors wern't so freaking loud.
One of the local used office furniture places has several old printer enclosures in stock for a couple of dozen dollars... a steel box with foam on the inside, for quieting old impact printers. Most of them even come with a free power strip. :) If the compressor is on the order of "blow up beach balls quickly and tires slowly", it would fit in one of these. If it's more like "run 5 half-inch impact wrenches at once", then the usual solution is to banish it to the great outdoors.
Unfortunately, compressors tend to be either very, very expensive,
3-phase and quiet (eg. a Quincy or scroll compressor) or don't give much air (like a Paasche airbrush compressor). I think dentists have some quiet ones too, probably not cheap.
The absolute worst are the oil-less cheap rattle and roar hobby grade compressors.. very antisocial they are.
Best regards, Spehro Pefhany
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That's what we used (and sometimes built) for our old telex machines in the ham radio days. But I just don't have the sapce for it here. So I charge the compressor in the garage, disconnect, and haul it in. Gives me some exercise as well.
But since I am a very social guy I charge it in the garage with the doors down and then carry it inside. The setup is a bit leaky but it holds for about an hour's worth of experimenting.
I know I'm coming late to the party, but let me ask a question. Over some years, the density of ceramic capacitors gets higher and higher. Just a couple of years ago I found a part mis-spec'd on one of my boards that I had to up the voltage without changing the PCB. So we stuck an
0805 on an 0603 footprint. Just recently I found I can now get the proper voltage in an 0603 footprint and get back on the straight and narrow with my assembly house.
So what are they changing that allows this? My understanding is they are making the layers thinner which ups the capacitance in a given volume, not changes to the formulation of the ceramic. But what allows thinner layers? Is it small tweaks to the formulation?
I ask this because if you want to utilize a parameter that is not spec'd by the manufacturer, you need to believe this won't change without notice. If they tweak their formulation at a later date, this may mess up a design depending on the non-spec'd and non-controlled parameter. Or is this a one-off?
I believe it's both the ceramic formulation and also layer thickness, a more controlled production process.
In my case it's feasibility first. Then comes the business part of it. Two avenues for that:
a. (preferred) Line up a deal with a manufacturer where they guarantee a legacy process to be available for x or maybe xx years. Negotiate a high enough per-piece price that makes it worth their while. Not a problem in this case, we'd be prepared to pay several Dollars per capacitor if it performs. Versus the usual $0.005.
b. Buy an obscene amount of caps and store them under nitrogen. CFOs frown upon such ideas but in this case the cost would be relatively miniscule.
One change over the years is a loosening of the voltage coefficient. I dug up some information on some typical X7R parts from about 30 years ago
- the capacitance reductions were much less than what is common with today's small SMT parts. It seems that voltage ratings for these low- voltage parts has never been about "breakdown" - which has typically been MUCH higher than the spec - it's been value tolerance. You can get around this problem by specifying a part with a high-enough voltage, but you may need a physically larger part. There probably have been changes to the ceramic formulation, but they've not been solely responsible for the size reductions.
And as others have said, depending on unspecified characteristics is risky. The importance of risk avoidance is dependent on issues of product volume and production longevity as well as design tractability.
Yep. Many of today's high density X7R caps are almost as bad as Y5V when it comes to loss of capacitances at full rated voltage. Easily 70%, something that used to be unheard of with X7R.
I think there have also been improvements in production precision that allows the manufacturers to specify a higher voltage for a given layer thickness. Maybe more uniform, lower tolerances.
This is one of those projects where risk avoidance would get us nowhere.
Smallness doesn't necessarily help. Physically larger parts thus longer reaches are more likely to experience the stresses to produce strong piezoelectric outputs. Actually i would recommend a modified piezoelectric sounder as the base of the device. Perhaps unpackage some small ones (under 1/2 inch dia case) and see what they can do.
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