At one time a scientist or engineer trying to solve a tough problem with electronic computation had the choice of an analog computer, a digital computer, or both together in a hybrid configuration. For really complex math involving differential equations and other messy calculus, the analog computer was king. It could solve differential equations almost instantaneously. And since analogs could do parallel computing naturally, multiple parts of a problem could be solved concurrently—really speeding up the solution. This made the simulation of big, ugly physical systems fast and practical.
Digital computers were really slow back then and while they could be programmed to do calculus and other higher math algorithms, they were essentially impractical for simulation. It was rarely done. But as digital computers got faster, it soon became apparent they were going to give analog computers some competition. Today, almost any digital computer, big mainframe to smallest PC, does calculus and other higher math in a flash and with unparalleled precision. Pretty soon, analogs could no longer compete and just disappeared. Just like so many other electronic technologies, these impressive machines ran their life cycle course.
Nevertheless, it was and still is a cool technology. Analog computers represented constants and variables with proportional analog voltage levels. These were then processed by various electronic circuits that performed the mathematical operations in analog form. The key processing circuit was the operational amplifier. The op amp could be easily reconfigured to perform a wide range of math operations such as addition/subtraction, multiplication/division by a constant, integration and differentiation, and many others. The op amp could even be configured to do logarithmic and trigonometric operations with special non-linear feedback circuits. For operations like multiplication, special four-quadrant modules were available.
To program an analog computer, you simply wrote out the equations you wanted to solve, converted them to a block diagram, then wired the various computing elements together with pluggable cords on a large patch panel. Most problems were simulations of circuits, mechanical assemblies, and even big complex systems involving chemical processes and space travel. With analog components, the computation precision was not as great as that you could get with a digital computer, but it was good enough for most jobs. Answers with an error of no more than a few percent were possible. Lots of really tough problems got solved this way. These computers could give you calculus answers that you could not come up with yourself for lack of a suitable paper solution. When your table of integrals failed to reveal a form to fit your problem and you had an analog computer at your disposal, you were good to go. The math software available today readily takes care of nasty problems like that.
Back in the heydays of analog computers there were quite a few manufacturers. I used to work for the largest analog supplier, Electronic Associates in West Long Branch, NJ. I worked on their big analogs, first the PACE series then the 8800, as well as their 32-bit digital computer called the 8400. A set of AD/DA interfaces was used to connect the two together in a hybrid system. One of the systems I worked on was the one that simulated the first Apollo mission to the moon at NASA Houston. The big digital system simulated the long flight from earth to moon while a couple of analog computers simulated the fast dynamics of the Apollo spacecraft. The ADCs and DACs in the interface let the analog and digital machines exchange data. You can imagine the equations that went into programming this monster.
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