Glossary of Computer Science and Engineering Part 8 - Analog computer | HackTHatCORE

Analog computer

Most natural phenomena are analog rather than digital in nature (see analog and digital ). But just as mathematical laws can describe relationships in nature, these relation- ships in turn can be used to construct a model in which natural forces generate mathematical solutions. This is the key insight that leads to the analog computer. The simplest analog computers use physical components that model geometric ratios. The earliest known analog computing device is the Antikythera Mechanism. Con- structed by an unknown scientist on the island of Rhodes around 87 b . c ., this device used a precisely crafted differen- tial gear mechanism to mechanically calculate the interval between new moons (the synodic month). (Interestingly, the differential gear would not be rediscovered until 1877.) Another analog computer, the slide rule, became the constant companion of scientists, engineers, and students until it was replaced by electronic calculators in the 1970s. Invented in simple form in the 17th century, the slide rule’s movable parts are marked in logarithmic proportions, allowing for quick multiplication, division, the extraction of square roots, and sometimes the calculation of trigono- metric functions.
The next insight involved building analog devices that set up dynamic relationships between mechanical move- ments. In the late 19th century two British scientists, James Thomson and his brother Sir William Thomson (later Lord Kelvin) developed the mechanical integrator, a device that could solve differential equations. An important new principle used in this device is the closed feedback loop, where the output of the integrator is fed back as a new set of inputs. This allowed for the gradual summation or integration of an equation’s variables. In 1931, V annevar B ush completed a more complex machine that he called a “differential analyzer.” Consisting of six mechanical inte- grators using specially shaped wheels, disks, and servo- mechanisms, the differential analyzer could solve equations in up to six independent variables. As the usefulness and applicability of the device became known, it was quickly replicated in various forms in scientific, engineering, and military institutions.
These early forms of analog computer are based on fixed geometrical ratios. However, most phenomena that scien- tists and engineers are concerned with, such as aerodynam- ics, fluid dynamics, or the flow of electrons in a circuit, involve a mathematical relationship between forces where the output changes smoothly as the inputs are changed. The “dynamic” analog computer of the mid-20th century took advantage of such force relationships to construct devices where input forces represent variables in the equation, and nature itself “solves” the equation by producing a resulting output force.
In the 1930s, the growing use of electronic circuits encouraged the use of the flow of electrons rather than mechanical force as a source for analog computation. The key circuit is called an operational amplifier. It generates a highly amplified output signal of opposite polarity to the input, over a wide range of frequencies. By using compo- nents such as potentiometers and feedback capacitors, an analog computer can be programmed to set up a circuit in which the laws of electronics manipulate the input voltages in the same way the equation to be solved manipulates its variables. The results of the calculation are then read as a series of voltage values in the final output. Starting in the 1950s, a number of companies mar- keted large electronic analog computers that contained many separate computing units that could be harnessed together to provide “real time” calculations in which the results could be generated at the same rate as the actual phenomena being simulated. In the early 1960s, NASA set up training simulations for astronauts using analog real- time simulations that were still beyond the capability of digital computers.
Gradually, however, the use of faster processors and larger amounts of memory enabled the digital computer to surpass its analog counterpart even in the scientific pro- gramming and simulations arena. In the 1970s, some hybrid machines combined the easy programmability of a digital “front end” with analog computation, but by the end of that decade the digital computer had rendered analog computers obsolete.

References:

• “Analog Computers.” Computer Museum, University of Amster- dam. Available online. URL: http://www.science.uva.n/ museum/AnalogComputers.html. Accessed April 18, 2007.
• Hoeschele, David F., Jr. Analog-to-Digital and Digital-to-Analog Conversion Techniques. 2nd ed. New York: John Wiley, 1994.
• Vassos, Basil H., and Galen Ewing, eds. Analog and Computer Elec- tronics for Scientists. 4th ed. New York: John Wiley, 1993.