The Structure of Analog Computers
Analog computers had their heyday in the 1940s through the 1970s. Unlike today’s familiar digital computers, which operate on binary number data, analog computers use continuous electronic signals. An analog computer simulates complex real-world phenomena by processing voltages in a variety of electronic circuits. Like a digital computer, an analog system is programmable, so the same machine performs many different tasks.
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Input
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An analog computer works on data in the form of voltage signals. Some of these vary with time, some do not. You set input parameters for the computer by adjusting dials and flipping switches. Some dials set steady voltages, others change the frequency of oscillating voltages. An analog computer can also process voltages from external sources. By carefully changing the dials and observing the computer’s output, you fine-tune the program.
Patching
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To program an analog computer, you patch different circuit modules together with patch cord cables. Each circuit has a set of inputs and output connections; you patch the outputs of one module to the inputs of another. You can drive the inputs of several modules with the output of one circuit; for example, if you want to adjust two modules simultaneously with the adjustable voltage of a dial, you patch the dial’s output to the inputs of the two modules. Circuit rules usually forbid patching inputs to inputs or outputs to outputs; it may damage the circuits or produce unpredictable results.
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Circuit Modules
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An analog computer has a collection of circuit modules that perform a variety of mathematical functions. Many of these circuits contain operational amplifiers, which add, subtract and multiply voltages. An adder circuit, for example, produces an output voltage which is the sum of the voltages present at its inputs. A differentiator performs a first-derivative calculus function on changing voltages on its input. Signal generator circuits produce sine waves, triangle waves and continuous and single rectangular pulses.
Output
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As with the input, the output of an analog computer takes the form of signals. A scientist or engineer interprets the signal patterns produced by the computer's circuits, measuring the output with meters, oscilloscopes and strip chart recorders. Meters indicate steady or slowly changing values. An oscilloscope displays rapid or complex signals on a screen. A strip chart recorder makes a permanent graphical record of signals on a continuous sheet of graph paper.
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References
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