Definition of an Analog(ue) Computer

From: Philip.Belben_at_powertech.co.uk <(Philip.Belben_at_powertech.co.uk)>
Date: Tue May 5 12:33:59 1998

I have just returned after a long weekend to find my mail had got set to
Postpone again. This has probably all been said but I thought I'd put
my bit in anyway.

Tony:

< A question occurred to me today : Can you have an embedded analogue
< computer, and if so, how many op-amps are needed to have one?

Allison

> Yes, and that's very common. None, a low pass filter(RC) performs a
> function and can be considered analogue.

A filter made of passive components or otherwise I wouldn't consider a
computer, although I agree it is definitely analogue (Americans may omit
the ue where appropriate).

I would say that an analogue computer:

(a) combines two or more signals
(b) does so in a more complex way than by simple addition or
subtraction. (But this could be A+dB/dt, for example)

An analogue computer need not be electrical at all - quite complex
analogue functions can be implemented in cams, for example. A good
example of a simple embedded analogue computer is the ignition
distributor on a petrol engine. This:

     Takes two inputs - camshaft angle and manifold vacuum;
     
     Differentiates camshaft angle to get engine speed (centrifugal
     weights on springs);
     
     Applies some non linear function to engine speed (cams attached to
     the centrifugal weights);
     
     Adds together camshaft angle, function(engine speed) and constant *
     vacuum level;
     
     Compares the result with a reference angle to generate pulses of a
     given width for the ignition.

I claim that is a simple (but actually quite sophisticated) analogue
computer.

< I was looking at the service manual for my Micropolis 1203 hard disk, and
< I read the circuit description of the servo electronics. It's a fairly
< complicated array of op-amps, which combine integral and differential
< forms of the position error, positioner current, etc. I would claim that
< is an embedded analogue computer.

> Valid claim, also a good example of a fairly complex function.

Agreed 100% (As Tony would say). An excellent example of an embedded
analogue computer.

Another example is the convergence circuit in a colo(u)r television.
This takes the two timebases as inputs, multiplies them and their
squares/ first derivatives etc. by user settable constants, and feeds
this back onto the deflection systems of the tube.

< On the other hand, I think it would be stretching the definition to call
< a simple op-amp wired as a voltage follower an analogue computer.

> Correct. However often the buffer is between some function or follows one
> so it's part of the analog system.

Agreed it could be part of an analogue computer. But I think Tony's
point was that it does not by itself make one. Otherwise practically
any analogue circuit becomes a computer (One of Vonada's axioms, I
think: All circuits are amplifiers)

> Other analog systems common to computers:
>
> cassette IO (low pass filter on output) and complex filter/differentiator
> edge/peak detectors for input. Some of the acients used PLLs for clock
> recovery (KANSAS City is one).
>
> Analog to digital conversion (quantification).
> Digtial to analog conversion (filtering)
>
> Disk/tape systems have several layers of analog function for data and
> control.

Definitely analogue systems. But some are merely filters, not
computers. (I think a PLL almost qualifies as a computer, though...)

But in general, I agree with you both - analogue computers are often
small, simple and embedded, and they're a heck of a lot more common than
most people think.

Philip.
Received on Tue May 05 1998 - 12:33:59 BST