Capacitive displacement measurement

That's ok, as long as you have a condom on your design tools.

Nice. Nice. No MCU either (I'm still shy of them, which is stupid, because any chepo PIC or so could control the whole thing and take care of the readout as well). All this could be done with a comparator, a counter, a clock generator and some glue logic. The stability of the DAC could of course be an issue, as Phil has pointed out.

Uff -- already too many options to consider.

--Daniel

Can't be. No magnetics allowed.

Yeah, got that. Thanks for all the great suggestions! Now I need to go back to pencil and paper to see how much accuracy I actually *want*.

--Daniel

Gaah. The AFM people down the hall are already unhappy enough with a single fiber.

Stuff that needs to be made and installed and adjusted inside a 2cm³ volume.

When you build stuff that goes inside UHV cryostats you want rugged and simple. A broken wire or somthing getting stuck can cost weeks of time, even if the actual fix is done in five minutes. The capacitive thing consists of a few big, simple, and sturdy parts with three wires. The optical approach would be an option if absolute positioning were top priority.

--Daniel

This system will probably have a couple of lsb's of noise, if not more. That's an advantage here, if you can average, because it will smear out any dac differential linearity errors and give you sub-LSB resolution. So it's nice to use a uP, or interface the data to a pc, and do some averaging.

DAC stability should be excellent, using one of the newish cmos current-steering dacs. Only the output ratio matters.

John

Are these any good to you?

Chris

It is a Ph.D, thesis. The body of thesis is in English, but there is a little introductory test in Dutch.

The idea of the printed circuit versions of the Thompson-Lampard cros-capacitor has been publihsed by the IEEE - I've got a copy of the paper somewhere - but the Ph.D. thesis is cheaper to get at.

While it is reassuring to learn that the universe is made up of all elements that exist, what the hell do inheritance and polymorphism have to do with capacitance?

But what in the world is Fig 3-1 about? And page 16 is especially juicy.

John

How you might break up a system in order to make it easier to design snd more effective in operation - where you chose to go into the digital domain, for instance

Those who can't design do write about the process, and if you do happen to get interested in the subject of system partitioning - which can be interesting - you can add a scholarly gloss to your Ph.D. project by quoting their work.

Please note that Ferry Toth was born in Sydney, though he seems to have got his education in the Netherlands, and thus might have been bitten by whatever got Phil Allison -see page 152 of the pdf document.

A precision ratio transformer would probably better, at least for the coarsest four bits of the digital range. I've long wanted to wind a ratio transformer with round and flat ribbon cable - at least for the voltage-sensing windings. Any widing that carried current would want to be wound with regular enamel-insulated transformer wire, but it is cute to use voltage feedback from a current-free widing to regulate the current through a current winding where you don't have to worry about resistive losses (or capacitative by-passing).

In message , dated Sun, 10 Sep 2006, snipped-for-privacy@ieee.org writes

It appears to be blank. Is there some secret writing there that can only be read by Australians?

Sorry, my mistake. Try page 151.

-- Bill Sloman, Nijmegen

In message , dated Sun, 10 Sep 2006, snipped-for-privacy@ieee.org writes

I don't find that any more informative, except about your powers of concentration, perhaps. (;-)

You have sent me back to the same page.

Your own powers of concentration and memory aren't what they might be. In the original I sent you to page 152, here corrected to page 151, both explicitly and in the re-posted text. Go to page 151 at

You will end up on the page I was referring to, for what that is worth.

In message , dated Mon, 11 Sep 2006, snipped-for-privacy@ieee.org writes

Ah, by correcting it in the re-posted text, you misled me. I expected to see the old number there.

Indeed; I thought there would be something about him being bitten by something. Instead, I find that he's gone from Oz to Ooze, just as you have.

Well, I came from Burnie, Tasmania, via ten years at the University of Melbourne, and twenty-one years of slumming it in the U.K. and I've ended up on one of the non-ooze points in the Netherlands, which - like the Isle of Ely - stuck up out of the swamps long before the Dutch got around to draining the bulk of the Netherlands and East Anglia.

Ferry Toth came from Sydney, possibly evading whatever it is there that has eaten the civilised bits of Phil Allison's brain, and seems to have moved more or less directly to the Netherlands.

I don't think the cases are all that similar. His Ph.D. thesis is certainly a lot more interesting than mine.