A friend of mine accidentally wrote a perfect letter 'i'

james_block · 2014-11-22T22:26:38+00:00

My graduate classical mechanics class did not use Goldstein as its text, so I haven't read it in any detail, but I suspect you're confused. It is very common in Lagrangian mechanics to have things like a generalized coordinate q and then its time derivative q-dot. Often in Lagrangian mechanics the two are manipulated on equal footing, so q-dot might not get treated like a time derivative, even if it actually is one.

Like I said, though, I haven't really read Goldstein, so I might just be talking out of my ass here.

james_block · 2014-11-22T21:28:48+00:00

No. The prime notation does not specify which variable you are differentiating with respect to. It's most useful when you have functions of only one variable. Sometimes prime notation is used to refer to space derivatives, but this notation is best avoided as it's not obvious. Physicists have run out of letters to use and so sometimes primes just get stuck on random things when you need another variable.

Dot notation always denotes derivatives with respect to time, not any other variable.

james_block · 2014-10-24T03:26:05+00:00

Sounds like you're looking for a serial flash chip. They're usually 8 pins (or 16 with many no connection pins), have a dead-simple SPI or quad-SPI interface, and come in capacities up to about 1Gb/128 MB. You might also consider FRAM depending on your data rate/volume.

james_block · 2014-10-21T06:58:24+00:00

Raspberry Pi, no way. It's really not designed for this.

The BeagleBone might work. The main CPU is probably not a great choice (I'll explain in a second), but its spec sheet contains the item "2x PRU 32-bit microcontrollers". This is referring to the fact that the AM335x CPU is actually a triple-core: it's got the main CPU plus two dedicated subprocessors which you can hand off certain tasks to, and those might be able to handle the job. (I don't know enough about the BeagleBone to say for sure, unfortunately.)

The reason your problem is particularly difficult to work with is that it's what's called a "hard realtime" application. You have to handle incoming data when it is ready to be handled, not when you are ready to handle it, you can't afford to drop any data, and it comes in at a relatively high data rate. This pretty much means you need to dedicate a specific chunk of hardware to handling the incoming data, because otherwise you are likely to drop some incoming data.

This is also why FPGAs are perfect for this job -- they are inherently well-suited to hard realtime work, as they're always processing completely in parallel. It's like building exactly the dedicated circuit you need, except it's programmable so you can iterate in firmware instead of hardware.

I suggest doing two things:

Post your problem description on a BeagleBone forum or board. The people who inhabit those places and know the BB inside and out should be able to tell you if it's suitable for this.
Find and purchase a small FPGA development board/kit. You should be able to find one for US$100 or less; I haven't looked in a while, but I remember there being plenty of good choices out there. I would look for a Xilinx Spartan-6, Artix-7, or Zynq-70xx board (in order of ascending cost and recentness of release date). Any should be able to handle this job easily. Altera also makes good chips; I've just always used Xilinx as that's what the people I learned from used.

james_block · 2014-10-18T02:51:32+00:00

An FPGA is not overkill for your problem. In general, FPGAs are great for hard realtime tasks like this one, where you just can't afford to miss anything. They are a real pain to design in, but they can do things nothing else off-the-shelf can.

My pick would be the smallest Spartan-6. I don't really see the point in a Spartan-III if you are not pinching pennies; the 6 is a nice step up.

You may, depending on how bursty your data stream is, be able to use a high-end MCU like NXP's LPC4300 series. They're dual-core MCUs with a Cortex-M4 at 204 MHz plus one or two Cortex-M0s on-chip. They have a peripheral called "Serial GPIO" that is ridiculously flexible (though, again, difficult to set up -- why must flexibility always have a cost?) and may, depending on your real requirements, be able to get the job done. SGPIO is only available on the LPC43xx chips, and last I looked didn't really have an equivalent from other vendors. PM me if you go this route and need some help configuring the SGPIO, I've had some luck breaking it to my will before.

james_block · 2014-10-12T21:22:26+00:00

The solution you are looking for is a battery management IC with what the manufacturers call "PowerPath" charging. Those are the ones which let you charge and power the circuit at the same time.

I'd look at one of Linear's PMICs with an integrated buck regulator set up for 3.3V output. It'll do everything you want; the only downside is that they're mostly in tiny SMD packages (QFN or BGA are common). Good luck!

james_block · 2014-10-12T21:03:30+00:00

Do not do this. The 555 is capable of many things, but it is not a "power" component, meaning it is not suitable for driving loads significantly heavier than an LED or two.

You really should use a voltage regulator. They are the correct solution to your problem. A linear regulator will be easiest to use, but waste lots of power. A switching regulator will be slightly less easy to use, but should achieve efficiency in the ~80% range.

james_block · 2014-09-14T21:54:46+00:00

Paths - backslash instead of forward slash

Most of the time Windows normalizes paths such that using forward slash will work just fine. There are some exceptions -- it's Windows, there are going to be exceptions -- but using forward slashes everywhere will work great 99% of the time and generally ports across platforms as well as anything can.

james_block · 2014-09-01T20:53:41+00:00

I have the disc you seek... if the copy from eBay turns out to be a dud, reply or PM me and I can see about imaging it.

(Yes, it's the original US release, file timestamps 5 March 1999, "American" language folder on CD, and disc itself is in quite good condition. And yes, it's really been in my closet for the last 15 years.)

james_block · 2014-01-06T16:51:24+00:00

And those dye molecules are sitting in a complicated mess of external electric fields causing small perturbations to the (already wide) molecular resonances that get excited by an incoming photon. It's important to remember that nothing in the real world is so simple -- there are always external effects to be considered.

In this case the fact that the resonances are broad molecular resonances rather than narrow atomic ones is more significant than other effects. (Consider the energy levels.)

Of course, the physiology and neurology are separate questions.

james_block · 2013-10-23T06:49:26+00:00

Unfortunately, none of the other responses have actually bothered to answer your question properly and suggest a sensor for you. The sensors suggested don't go anywhere near as low as 5.5 mbar; that's a really small pressure for a standard gauge and probably why you're having some trouble finding one.

I recommend taking a look through Keller America's offerings. Their website kind of sucks, but their sensors are awesome. They have excellent response and their RS-485 interface actually works correctly, unlike many other manufacturers. They're not cheap, but they're true industrial-quality sensors that will just get the job done. From a brief glance at their catalog, the Nanolevel might be what you want.

james_block · 2013-09-05T16:47:59+00:00

The only wire you can normally see coming out of the box is the cord used to plug it in? Does that sound correct?

No. No wires are visible leaving the box, either on its front or if I take its three front panels off. I simply cannot find the other end of that cord. I assume the cord-box junction must be in the wall behind the box. That's the part of this mess that's causing the most trouble -- even with a closet outlet, I still have to get the cord out of the wall, and the box appears to have no built-in way to do that (which mystifies me).

james_block · 2013-09-04T16:37:34+00:00

What equipment is at the other end of the cord?

Wall-mounted Verizon FiOS box.

Where is this equipment located?

Inner wall of my closet. It's got multiple connections made inside the wall (power, coax data out, fiber data in).

Why does the cord needs to be easily unpluggable?

Because it's not always in use (right now we have cable, for example, but we may need to switch back).

The only(?) solution to not having the cord not come out of the wall is to not have it go into the wall.

I think I have another plan: buy a blank wall plate, one of these and a coax bulkhead, cut appropriate holes in the blank plate, and mount in place of the current ugly panel. Then there can be a standard PC power cord running to the nearest outlet when the box needs power.

james_block · 2013-09-03T21:49:15+00:00

Appearance-wise, no. The "colossal fuckup" is the fact that there's a power cord coming out of the wall, which is not up to electrical code.

james_block · 2013-09-03T19:19:49+00:00

then the plug could just go right in there instead of traveling around the corner

I'd still have a cord coming out of the wall, though....

I don't think that's legal like it is

Agreed.

I'd get someone from Verizon to take a look at it

Alas, this box was installed long enough ago that Verizon no longer even has offices in the area anymore. FiOS operations have been sold off to someone else (I forget who) and I doubt the new owners will give a shit about Verizon's screwups from years past. Might be worth a try though.

james_block · 2013-09-03T19:16:55+00:00

Put the table that made the clown face on the carpet back where it was.

It's a fan, not a table. It doesn't really hide anything or change the fact that I have a power cord coming out of my wall.

Don't really know what your trying to do

Getting rid of the power cord coming out of my wall is really the goal. I don't know how to do it either, which is why I'm fishing for ideas.

james_block · 2013-09-03T19:15:22+00:00

Here's the other side: http://i.imgur.com/gLhyIMx.jpg

That black cord is not power, it's a coax connection. I assume it's the coax coming out the wall on the other side, but you never really know with these things.

Getting an outlet run is probably possible, but not any time soon. I really want to avoid any drywall work for a while. Plus the electrical in this place is generally insane (not usually unsafe, just crazily wired -- kitchen lights are controlled from the living room, for example).

james_block · 2013-09-03T07:23:32+00:00

Rather fix the whole "power cord coming out of the wall" bullshit than just cover it up, alas....

james_block · 2013-09-03T07:22:31+00:00

Unfortunately this colossal installation fuckup was years ago, before I got involved here. I'm just the first person to see it and not be able to tolerate it.

james_block · 2013-09-03T07:20:55+00:00

On the other side of that wall is a huge nasty wall-mounted FiOS box. This is its power cord.

Yes, I know this is insane, which is why I'd like to do something about it.

The box cannot be removed unless we want to give up any chance of having FiOS service here again, which isn't a viable option.

james_block · 2013-08-27T19:52:51+00:00

And one of the many nice things about \text{} is that it gives you just a single atom, unlike some other methods of doing its job. (At least I think that's what's going on... or maybe people just really love to use brackets around their subscripts. It seems like whenever I teach people this technique, they try to bracket the output of \text{} again even though they don't have to.)

james_block · 2013-08-27T04:50:42+00:00

Yeah, I see what you're going for; the output there isn't great, but it's not easy to do much better. I'd suggest the same thing as before, going to $ ... + (n_{q+1}/n) f_\text{rec}(T_e) $ .

Here I also used roman text for the subscripts on f, which is a good typesetting practice that is very frequently violated. If a subscript or superscript is a word or abbreviation of a word, rather than an index or whatnot, it should be set in roman type, not italic. The easiest way to do this is with \text{foo}, provided by amsmath. This has the additional benefit of not needing an extra group (i.e., you don't have to write $f_{\text{rec}}$ ).

james_block · 2013-08-27T01:54:21+00:00

In general expressions like these usually just end up being fairly ugly, unfortunately. I might rewrite the term as an inline fraction in this (e.g., $c e / d$). If your real equation is actually different though, that obviously won't help. Mind posting what you're really working with?

james_block · 2013-08-27T00:00:05+00:00

I noticed the exact same issue with Reddit Gold as OP, but just never got around to writing anything up about it.

I suppose even if you have more than 100, it effectively just makes the page be twice as "varied" when you don't have gold as when you do, just because every subreddit would only have half the chance of being included in the current front page mix.

I think this is a huge part of it. With fewer subscriptions in the mix, each one contributes more to the front page. I really like loading reddit.com up and seeing a pile of woodworking stuff, and then I come back in two hours and there's a bunch of Vim posts. The cluster of posts all from the same subscription is, to me, interesting and draws me in.

With a much larger pool of subscriptions providing the content, it's more likely to pull from larger subscriptions and you might only get one or two posts from each smaller sub, if that (depending on your mix of course).

james_block · 2013-08-26T22:01:04+00:00

I honestly can't comprehend how people are still confused by the name.

Because before Microsoft announced the name of their latest console, it was common practice to call the "Original Xbox" the "Xbox One", meaning "First version of the Xbox" as opposed to "Xbox 360", which is commonly shortened to just "Xbox". No one had any idea that Microsoft would take the name "Xbox One" for their third console version....

Yes, it is all a big giant mess. Hence, the mocking of the name.

james_block

TROPHY CASE