Analog Computers
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By - Richard Wallmann



Analog computers are electro mechanical or electronic computers used to solve engineering problems related to time dependant differential equations especially where non-linear aspects such as discontinuities are involved. Nowhere near as precise as modern digital computers, the solution results are adequate for engineering analysis and occur right away as opposed to all but the latest high speed, large memory digital machines.


Early Development

Starting in the 1930s, MIT and other locations developed electro mechanical machines that were collections of gears, shafts and, to perform integration,  a necessity to solve differential equations, a ball and disk assemble was used . This mechanism would give the integral of the ball position and rotation by the rotation of the disk. This led to the development of several devices used during WW2. Among these are:

        Nordon Bombsight:

This then quite secret device allowed inputs of airplane speed, wind speed, and air temperature combined with optical input by the bpmbardier. The output controlled the plane during the last moments of a bomb run and toggle bomb release. The results were far from perfect but better than other methods then in use.



        B-29 Fire Control:  

Unlike earlier bombers where defensive fire was done by gunners directly operating machine guns, the B-29 used remotely operated guns controlled by separate sighting stations that fed an analog computer with target location, range and relative speed and which calculated the predicted path of the target and fired the guns.

        Anti-Aircraft Gun Control:

Similar to the B-29 fire control. Optical or radar was used as input to an analog computer along with wind and air temp and nature of the shell to predict the appropriate firing time and direction.


Post WW2:

Following WW2, significant development of analog computers took place as the rise of electronics led to the development of very high gain amplifiers allowed for fully electronic analog computers. High gain amps that are linear in the frequency range of signals used in analog computers can sum signals when used with resistive feedback (usually 1 meg-ohm) and as an integrator with capacitive feedback (usually 1 micro-farad). The examples below are typical of analog design and use in the period 1950 thru1979.

        Link Aviation C8

The C8 was a mostly electronic analog flight simulator of a generic, early jet aircraft such as the F80 and used by the USAF in the early 1950s. Some electro-mechanical elements were left motor driven potentiometers primarily were still present but most of the flight characteristics were simulated with vacuum tube amplifiers. This product had no motion capabilities such as modern flight simulators or the legendary ANT-18.


        Boeing Airplane Co:


The Boeing Electronic Analog Computer was developed by Boeing to work on aerodynamic problems. While some thoughts were to make it a commercial product, this effort was dropped when really slick commercial products became available. The computer was housed in standard relay racks with large and awkward connecting cables.

                PACE and EASE

These two analog computers were quite similar and featured exchangeable patch panels. These 24 x 18 aluminum panels allowed the setup of problems so that the computer could be easily switched from one problem to another. Some of the problems that these computers were used for are:

707 Anti-Skid Design

The anti-skid was developed for the 707 braking system and is the forerunner for airplanes and automobiles. The analog computer was used to analyze the forces and sensors needed to relieve the braking pressure when a skid is about to start.

707 Shock Absorber Metering Pin

Airplane shock absorbers are built with a metering pin that regulates the flow of hydraulic fluid depending on the depression and depression velocity of the landing gear. This problem, before computers, was done by controlled dropping an airplane from various heights. This often led to damage to the test airplane.

                        Thermodynamics of a Reentry Ball

The USAF contracted with Boeing to determine what level of thermodynamic problems could be solved with various computing methods. A series of problems of increasing difficulty was produced sand solutions were attempted starting with slide rule and working up to large digital and analog computers. The final one of these was an ablative ball returning from space. The ball was made of several layers with different thermodynamic properties including one liquid layer that would go from liquid to gas when the heat was sufficient. The problem was to graph the heat transfer from outside to center as a function of time. The analog solution was developed considerably faster than the 5digital solution. A copy of the analog solution setup is attached.

                        Wing Loading Analysis Computer

This one of a kind passive element (no    amplifiers) was built to study wing loading for the 707 during the 707 development cycle.


        Tektronix Inc.

In order to improve the design of peaking circuits used in its high performance oscilloscopes, Tektronix built a Heath Kit analog computer and used it to simulate the effects of component variation.


        University of Oregon

The University of Oregon had a requirement for a new science bldg that spanned an extant street and had, because of laboratory requirements, a very irregular pattern of windows. These led to the requirement that the building frame be outside the curtain wall thus forming a pleasing visual aspect. An opinion was formed that an indeterminate truss structure would be best and might save steel quantities. An indeterminate truss is made of elements that are rectangles with additional members between diagonal corners forming an X. If the diagonal members are not tied together the resulting stress loading and bending moments can be solved by analytical methods but, when tied together become indeterminate by analytical methods. A small, passive element, analog computer was built to study the problem with results showing the resulting structure would be very stiff and use much less steel.








Our Syston Donner analog Computer at SMECC


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