[sbaiddn]

Get a ruler. Draw a line 10 cm long, AB. Now grab end B and draw another another line, BC, with a certain angle $/alpha$ wrt to the first. Now measure the distance AC.

Congratulations! You have just built an analog computer to use the Law of Cosines to solve for line segment AC. Non-dimensionalize your result and its a general LofC solver. A problem that (I suspect) would take the majority of modern day Eng undergrads a week to program without the use of the math lib[1], can be solved by any keen middle schooler.

Now build a robot that measures AC for you and you have an API for your analog computer.

Typically an analog computer is thought of as a set of opamps and diodes, whose currents and voltages solve a set of non-linear ODEs; but thats a very narrow view. An analog computer is, ultimately, any physics experiment whose model is known

Wind tunnel? Navier Stokes analog computer

Cold atoms traveling through a double slit in a magnetic field? Analog Quantum computer

RCL circuit? Analog computer solving the response of a car's suspension.

[1] code reuse and libraries are a big reason why digital computers are more popular to solve models nowadays. Cost, bandwidth, are another. Ostensibly so is reproducibility. But if CS scientists cannot get reproducible builds, what hope does a humble physicist hacking on C or Matlab have?

[agumonkey]

40s navy videos showed many electromechanical computers using simple geometric blocs encoding sin x as a groove.

Coupled together the embodied a mid sized equation (of the other way around), live and reactive as we'd say today.

Kinda blew my mind.

[arbuge]

The issues arise when converting the analog outputs of those physical experiments back to the digital domain for processing. High speed and resolution analog to digital converters are not cheap, and often require different process node technologies for implementation. The cost savings from the elimination of computing the models digitally have to be weighed against the costs arising from the converters' introduction.

[sbaiddn]

I agree, hence my robots API joke.

[mellavora]

> Typically an analog computer is thought of as a set of opamps and diodes, whose currents and voltages solve a set of non-linear ODEs;

Oh oh oh! and also resistor/capacitor circuits! Great for integration or differentiation!

Integration and differentiation have interesting audio effects; maybe our ears might be one of the better analog computers.

[fjkdlsjflkds]

> Integration and differentiation have interesting audio effects

An integrator is just a low-pass filter and a differentiator is just a high-pass filter, so you'll most likely get "boring" audio effects ;)

[sbaiddn]

Our senses are incredibly good.

[noduerme]

>> Now build a robot that measures AC for you and you have an API for your analog computer.

This line made me laugh out loud, but what an amazing explanation and set of examples. Thank you.

The original ECU in my 1980 Datsun ... is digital, right? I just never thought of the insane analog vacuum pot system that controls the cold start valve and the air conditioning selector and the cruise control to also be a "computer" but I guess it does compute things, in its humble way ;)

[sbaiddn]

Glad to get a chuckle. I was more more proud of my reproducible builds jab.

Does a 1980's Datsun have an ECU? That's not obvious to me. Even a 6502 would be an expensive add on at the time given car margins. Maybe a one bit controller?

But there are analog solutions to all these control situations. And you don't need vacuum tubes!

Measuring and adjusting the fuel-air mix is the carburetor's job. The carb doesnt just add fuel. It adds the right amount of fuel. Its needle calibrates it for different conditions (namely pressure).

The spark advance "computer" was super cool. Originally spark advance was a lever controlled by the user. Faster rpm -> adjust for more advance. Eventually the lever was attached to a governor (a set of balls attached to a spinning linkage and a stiff spring. It measures rpm) and the spark advance disappeared under the hood.

The automatic version of your car would have had a (very complicated) hydraulic circuit in the tranny pushing pistons that measure throttle, rpm, velocity and choose the gear accordingly.

The manual version had a vast organic neural network doing the same job but using just sound pitch of the engine. The manual transmission was cheaper because it didn't include the neural network shifter except on the very highest end cars - think RRs - and then only as a service (early SAS model).

German WW2 aircraft were fuel injected so they probably used a governor attached to the throttle wire and pressure gauge to guesstimate the mass flow and therefore the fuel to inject. There's a front page HN submission - the fuel control system of those engines or the original Mercedes Gull-wing!

All of the hydro-mechanical stuff described were slowly transistorized (but kept analog) in the 70s. In the late 80s to 90s everything became digital except the transmissions that took longer.

[rasz]

>Does a 1980's Datsun have an ECU?

of course https://www.classiczcars.com/forums/topic/63812-1980-280zx-e...

picture https://www.electro-tech-online.com/attachments/4-ecu-full-s... a ton of Hitachi chips, big one on top seems to be some kind of ADC.

https://en.wikipedia.org/wiki/JECS build on BOSH license using Hitachi HD46802P (Motorola 6802 clone). Here reverse engineered https://github.com/eccs-reengineering/280ZX-Turbo-ECCS

Bosh shipped analog controlled d-jetronic in 1967 based on US patents/designs from fifties https://jetronic.org/index.php/en/d-jetronic/51-history.

Afaik first mass produced digital ECU was shipped by GM in 1978 (Motorola 6802 based). Ford also shipped some Toshiba ECUs in 1979. Bosh first fully digital ECU (Intel 8051 based) was https://en.wikipedia.org/wiki/Motronic in 1979 like Datsun/Nissan. Ford went full ECU in 1983 (https://en.wikipedia.org/wiki/Intel_8061)