Analog computers are based on principles completely different from digital computers. Problem variables are represented by electrical voltages which can vary continuously within a certain range, usually -10 to +10 volts for a transistor-based machine. Electronic circuit modules allow the variables to be added, integrated (with respect to time) and multiplied by a constant. This makes it is possible to solve a system of ordinary linear differential equations by properly combining a number of adders, integrators, amplifiers and potentiometers using flexible chords and a patch panel (see the examples).
Large machines like the EAI 680 also support non-linear
operations: square-rooting, arbitrary (piecewise linear) function
generation, multiplication of two variables, and comparing the
values of two variables. The last mentioned operation results in a
boolean quantity which, perhaps combined with other boolean results
(using the machine's patchable logic circuitry) can be used to
change the 'program' dynamically. For instance, the simulation of a
bouncing ball requires the solution of one differential equation
when the ball is in free fall, while another equation (describing
the forces occurring during elastic deformation) is applicable when
the ball is in ground contact. The results of the computation can
be shown graphically, in real time, on
an oscilloscope or plotter, or be digitized for being stored or
further processed by a digital computer in a hybrid system. Also
the results can be used directly for the control of some physical
process.
Early special-purpose analog computers were the astrolabe of the Middle Ages, the slide rule, the curvimeter and planimeter, and the harmonic analyzer. In World War II, analog computing mechanisms were of great importance for gunfire control on warships. General-purpose analog computers ("differential analyzers") based on mechanical building blocks (picture below [1]) were first built in the 1930's by Vannevar Bush. The paradigma of the differential analyzer strongly influenced the architecture of the ENIAC, which was indeed designed to replace the differential analyzer for doing ballistic calculations towards the end of World War II.
These mechanical systems were gradually replaced by electronic
machines, first with electron tubes, later with transistors. Analog
computers - sometimes in combination with digital computers - were
heavily used until well in the 70's for applications like
automobile suspension design, chemical process simulation and
control, experimentation with 'world models', aircraft flight simulators, Apollo spacecraft
flight control, and many others. While the analog computer was
ideal for the fast solution of differential equations (because all
operations are effectively done in parallel), very expensive parts
and construction techniques were needed for obtaining a reasonable
accuracy of the solutions. Therefore the analog computer was
quickly superseded by the digital computer in the late 70's.
Numerous programming systems were invented to emulate the analog
computer on a digital machine: well known examples are CSSL
(Continuous Systems Simulation Language) and TUTSIM, developed at
the Technical University Twente (The Netherlands). An early version
of the latter program is running on one of the Museum's PDP-11
computers.
The analog computer paradigm is still alive; many of today's
graphical programming environments for simulation use the idea of
patching together functional units for solving a given problem and
showing the results.
A basic building block of the analog computer is the operational amplifier which is used in almost every functional unit (adders, integrators, etc). It acts as the heart of an electronic servo mechanism which (in an adder) turns the virtual knob of the output voltage source in such a way that the output voltage is accurately equal to the sum of the input voltages. The operation amplifier (featuring a very high gain, high input impedance and low output impedance) is still heavily used today - in a miniaturized version - in a large variety of electronic circuits among which artificial neural networks.
