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Analog Computers

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Re: Analog Computers

#51

Post by Koshkin »

robotresearcher wrote: post #26
Miky wrote: post #14

Earlier quoted context omitted.

Not really. FPGAs are fundamentally digital and pretty much give you a bunch of logic gates to work with ("Field-Programmable Gate Array"). The author's proposed architecture would instead provide an array of components that perform analog operations, such as summing, multiplication, and integration or differentiation, over analog voltages.

FPGAs are fundamentally analog, depending on if 'fundamental' means what was in the designer's head or what you actually fabricated. You are thinking about them and using them as if they were digital. Adrian Thompson at Sussex University used a genetic algorithm to auto-design FPGA circuits in the early 90s. Since no one told the GA that FPGAs were supposed to be logic circuits, it happily used the FPGA as an analog…

I think it is an interesting reminder that the world we live in is wholly analog, and while this includes systems with discrete sets of equilibrium states, using digital devices to perform analog functions looks like hacking rather than as something that one would normally be advised to do. The reason for that is that while such devices may all behave as designed, identically, in the digital domain, their analog characteristics may vary so wildly between versions and even different specimens, as well as with, say, temperature, and do it in a completely unspecified way, that any attempt to do a serious analog design based on them would seem impractical.

Re: Analog Computers

#52

Post by wfunction »

cr0sh wrote: post #16

A good summary of analog computers can be found on the wikipedia article: https://en.wikipedia.org/wiki/Analog_computer Takeaway: Analog computers are limited in precision and by "analog noise"; the precision of the components used determine the precision of the output. Usually no more than 3 or 4 decimal places are possible, at least with the tech that was used in their heydey. I would say that is still close to the…

I seem to recall there were clever but well-known techniques in analog to get higher accuracy than that of your actual components, through negative feedback IIRC. So why is it correct to say that the precision of the components is what limits output precision? Wouldn't the technique potentially make a difference? (and yeah I know accuracy != precision but I'm using them loosely... the distinction doesn't seem relevant here)

Re: Analog Computers

#53

Post by adamnemecek »

moyix wrote: post #50
adamnemecek wrote: post #21

Earlier quoted context omitted.

Read unlimited as arbitrary precision. I'm familiar with the Turing thesis but he's wrong.

Have you read Scott Aaronson's NP-Complete Problems and Physical Reality [1] ? He goes into some detail about why analog computing is not thought to be physically realizable (Section 6). [1] http://www.scottaaronson.com/papers/npcomplete.pdf

I'm passingly familiar. I'm not convinced. I can't really explain to you why. I feel like makes many assumptions about the architecture and workings of such a machine.

I know Scott Aaronson and all, but I won't believe it until someone tries to build one and fails.

Re: Analog Computers

#55

Post by peterburkimsher »

My boss at my previous job at Fisher & Paykel Healthcare in New Zealand is using analogue computing today.

He developed Pertecs, which is a rudimentary analog computer paradigm written in C.

http://tcode.auckland.ac.nz/~mark/Signal%20Processing%3A%20P...

He got me to write some code to compile schematic diagrams into the XML config files. He's also done something similar now to compile from LaTeX, and he ported the controller from a Mac Mini to a Raspberry Pi.

Pertecs is being used to control an artificial lung, which is used for research into obstructive sleep apnoea (snoring).

The problem is, he's retiring, and I'm probably the only other person in the world who knows how to use his program. I would move back there, but the immigration policy got more difficult (minimum salary of $75k), so I'm seriously wondering whether I should stay in Taiwan longer and try to naturalise here.

Re: Analog Computers

#57

Post by sleet »

peterburkimsher wrote: post #55

My boss at my previous job at Fisher & Paykel Healthcare in New Zealand is using analogue computing today. He developed Pertecs, which is a rudimentary analog computer paradigm written in C. http://tcode.auckland.ac.nz/~mark/Signal%20Processing%3A%20P... He got me to write some code to compile schematic diagrams into the XML config files. He's also done something similar now to compile from LaTeX, and he ported the c…

I believe the $75k minimum salary is for non-skilled jobs (the point being, if you're in a non-skilled but 'high' paying role we're still interested in you). The minimum salary for skilled jobs is $50k [1][2]

That and I doubt you would find many people working at F&P Healthcare that earn less than $75k.

[1] https://www.immigration.govt.nz/about-us/media-centre/news-n... [2] https://www.immigration.govt.nz/employ-migrants/hire-a-candi...

Re: Analog Computers

#58

Post by gugagore »

wfunction wrote: post #52
cr0sh wrote: post #16

A good summary of analog computers can be found on the wikipedia article: https://en.wikipedia.org/wiki/Analog_computer Takeaway: Analog computers are limited in precision and by "analog noise"; the precision of the components used determine the precision of the output. Usually no more than 3 or 4 decimal places are possible, at least with the tech that was used in their heydey. I would say that is still close to the…

I seem to recall there were clever but well-known techniques in analog to get higher accuracy than that of your actual components, through negative feedback IIRC. So why is it correct to say that the precision of the components is what limits output precision? Wouldn't the technique potentially make a difference? (and yeah I know accuracy != precision but I'm using them loosely... the distinction doesn't seem relevan…

I haven't thought it through, but feedback lets you do a few (perhaps connected?) things: 1. explore a trade-off between gain and bandwidth, 2. Reject disturbances and nonlinearities.

So you could have a high gain but "low precision" (in the sense of deviating from an ideal, not in the sense of not being noisy) component, and through feedback you can make a low gain, high precision (having desired properties, not low noise) component.

Re: Analog Computers

#59

Post by wfunction »

gugagore wrote: post #58
wfunction wrote: post #52

Earlier quoted context omitted.

I seem to recall there were clever but well-known techniques in analog to get higher accuracy than that of your actual components, through negative feedback IIRC. So why is it correct to say that the precision of the components is what limits output precision? Wouldn't the technique potentially make a difference? (and yeah I know accuracy != precision but I'm using them loosely... the distinction doesn't seem relevan…

I haven't thought it through, but feedback lets you do a few (perhaps connected?) things: 1. explore a trade-off between gain and bandwidth, 2. Reject disturbances and nonlinearities. So you could have a high gain but "low precision" (in the sense of deviating from an ideal, not in the sense of not being noisy) component, and through feedback you can make a low gain, high precision (having desired properties, not low…

You know, I think that might have been what it was -- it was using e.g. 5%-accurate resistors to get a 2%-accurate circuit.

Funny enough, that is the difference between accuracy vs. precision, which I thought was irrelevant here. Thanks for pointing that out. :)

Re: Analog Computers

#60

Post by kurthr »

flavio81 wrote: post #33
robotresearcher wrote: post #25

Earlier quoted context omitted.

> Usually no more than 3 or 4 decimal places are possible By that do you mean accurate to 1 part in 100 (3dp) or 1000 (4dp) or what? Since the scale of a representation is arbitrary, I'm not sure what dp means here.

Good question. I guess precision is finally going to be fractions of the max voltage swing allowed by the computer. For example, if voltage goes from -5 to +5 volts, the voltage swing is 10v, and if noise allows 0.1mV of precision, then the precision is 1/100000 of the full voltage swing. This could be expressed, at the end, simply in decibels, though. Signal-to-noise, as in classic analog systems.

I think it usually refers to accuracy out of a range of 1. typically 3 decimal places means 1000ppm and 4dp means 100ppm.

The typical problems with analog computers are many... precision of components (e.g. gain or attenuation) is limited to ~0.1% for resistors and ~1% for capacitors (inductors aren't typically used). You can try to tune things (ratiometrically) to get higher accuracy, but at the cost of increased noise and temperature sensitivity. The more complex the system, the more things can go wrong... so you end up needing simple systems or simple tools (digital).

The typical problem is that if you build a filter (e.g. a transfer function with a summer or differencer) then you will tend to clip the dynamic range or either with a maximum voltage (integrators) or a minimum noise level (differentiators) pretty quickly. You can play some games with log converters, but accuracy really still matters and drift or gain error with time is rarely an option.

The best way to use analog computers is with negative feedback to null the input. They do that amazingly well... so you can build a temperature controller, missile tracker, or actuator that only minimizes an error so that high gain corrects for any inaccuracy or offset.

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