Lexikon's History of Computing


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What is an Analog Computer?

An analog computer is a device that performs computations using continuous physical variables which are analogs of the actual items being computed. Analog computers might, for example, use the continuous rotation of gears or the angular movements of mechanical or electromechanical parts to perform computations.

The following are some examples of Analog Computers.


TDC Mark III (Torpedo Data Computer)

The TDC Mark III was an electromechanical analog computer used to solve targeting problems for torpedos during World War II. The TDC Mark III was used in U.S. Naval submarines, such as the Pampanito, currently in a marine museum in San Francisco. The Mark III aboard the Pampanito is undergoing restoration (1996). (See Restoration Article)

The TDC Mark III consisted of two sections: (1) the position keeper used to track the position of the target, and (2) the angle solver adjusted the gyro of the torpedos to focus on the predicted position of the target based on information from the position keeper.

The TDC is in a class of computers called "computational mechanical analog computers." They were of great use up to and during World War II, at which time, the invention of digital computers made the use of analog computing much less desirable. (PHOTO)



In 1938, inventor George Philbrick developed an electronic analog computer. The computer utilized an oscilloscope as an output device. The large, round oscilloscope display gave it the appearance of a one-eyed beast, and so it was named "Polyphemus" after the Greek mythical cyclops.



The GEDA A-14 was an analog computer developed in 1957 by Goodyear Aircraft Corporation for large and small problem solving.



Analog Computer Kit (GE) (1961)

EAI Analog Computer 1964 (see Photo)

Compumedic Analog Computer 1971 (see Photo)



Westinghouse Electric Company (founded in 1886) produced the ANACOM analog computer in 1948.


Restoration of Pampanito's Rare Torpedo Data Computer

The following article is reprinted with permission of Terry D. Lindell (author), Russell Booth, and the National Maritime Museum Association.

The article describes the fascinating process of restoration of the TDC Mark III analog computer aboard the U.S. submarine Pampanito, located at Pier 45 in San Francisco, California.

You can visit the Pampanito as well as other fascinating ships and maritime exhibits in San Francisco.

For more information, contact the National Maritime Museum Association.

Restoration of Pampanito's Rare Torpedo Data Computer

By Terry D. Lindell (1995)

The Torpedo Data Computer (TDC), Mark III aboard the USS Pampanito has been successfully restored to operating condition. The TDC is the electromechanical analog computer that solved the torpedo targeting problem in the fleet submarines during World War II. The restoration project took over 18 months to complete, and was done with the support of Russell Booth, manager of the USS Pampanito. The TDC Mark III computer is one of the two remaining examples of the TDC Mark III still installed in a museum fleet submarine. We believe that restoring this historically significant device to an operating condition is the best means of preservation.

How it Worked

The TDC was unique in World War II. It was the computational part of the first submerged integrated fire control system that tracked a target and continuously aimed torpedoes by setting their gyro angles. The TDC Mark III gave the U.S. fleet submarine the ability to fire torpedoes without first estimating a future firing position, changing the ship's course, or steering to that position.

During World War II a torpedo's gyro angle was set mechanically while it was in the tube. After being fired, the torpedo traveled on a straight course for a known distance called the "reach." A delay in the release of the torpedo's gyro steering mechanism by a threaded shaft determined the magnitude of the reach. Once engaged, the steering mechanism brought the torpedo to a new course based on the angular offset of the gyroscope.

The Mark III computer consisted of two sections, the position keeper and the angle solver. The position keeper tracked the target and predicted its current position. It automatically received input of the ship's own course from the gyro compass, and its own ship's speed from the pit log. The position keeper had hand cranks on its face that set the target length, estimated speed, and estimated gyro angle on the bow. It also contained a sound bearing converter that calculated the target's location based on sonar measurements.

The position keeper solved the equations of motion integrated over time. The result was a continuous prediction of where the target was at any instant. Successive measurements of the target's position were compared to the position keeper predictions and corrections for error were introduced with the hand cranks. The predicted target position became more accurate as more measurements made the corrections smaller. The angle solver automatically took the target's predicted position from the position keeper, combined it with the tactical properties of the torpedo, and solved for the torpedo gyro angle. The gyro angle automatically went to each of the torpedo rooms and set into the torpedoes continuously. The TDC controlled both torpedo rooms and all 10 torpedo tubes at once.

The U.S. Navy thus had a system that would point the torpedoes at a target as the fire control problem developed. The TDC Mark III was the only torpedo targeting system of the time that both solved for the gyro angle and tracked the target in real time. The comparable systems used by both Germany and Japan could compute and set the gyro angle for a fixed time in the future, but they did not track the target. Thus the idea of the position keeper, and its iterative reduction of target position error was unique to the U.S. Navy and represented a distinct advantage.

TDC Restoration

This restoration effort would have been impossible without the TDC Mark III manual available in the Pampanito's library. In addition, working with other Pampanito volunteers such as fleet submarine veteran Joe Senft, who was familiar with fleet submarine wiring, was invaluable.

The first order of business was to restore AC shore power to the TDC heater circuit. All TDCs have an electric heater to maintain an even temperature of 74 degrees inside the position keeper case. This prevents the buildup of moisture and maintains the mechanical tolerances required for accurate operation.

Hundreds of gears, shafts, bearings, and closely machined surfaces must match each other perfectly for the TDC to work. Every moving shaft and gear turns on finely made miniature ball bearings. The surfaces of the integrator wheels look like mirrors because of their finish.

After manually checking the machine's operation, the next problem was lubricating a machine that had not seen an oil can in 30 years! We were able to obtain a copy of OP 3000-U.U. Navy Lubrication from the library of USS Cobia in Manitowoc, Wisconsin. This document has a table that converts the 1944 Navy lubrication numbers used in the TDC manual into the names of lubricants available today. A large number of Gier tubes feed oil by capillary action into key places inside the very close recesses of the TDC. Lubrication was introduced over a period of several months to ensure that the oil had time to penetrate, by capillary action, the fairly long distances into the machinery.

The single largest challenge to the restoration of the TDC was providing electrical power. The TDC uses two power sources. One source is DC 115 volt at 10 amps required to run the time motor in the position keeper section. The other, used by the angle solver section, is single phase 115 volt AC cycle power.

Restoring power required that someone understand the wiring of Pampanito's IC switchboard. Much of the restoration time was spent wedged behind the IC switchboard tracing wires and checking continuity. Fortunately, Pampanito's cabling systems have well-preserved circuit number tags which sped up the task. Slowly, an IC switchboard wiring diagram was developed.

Power for the TDC time motor on Pampanito could come from three separate sources, and one of those sources was an AC to DC selenium rectifier stack. Although age had caused the selenium crystals to break down, it was possible for Joe Senft to replace them with a solid state device. After considerable testing of the remaining wires, and some repair to the original circuits, we were able to provide both AC and DC power to the TDC for the first time in forty years.

Operating the TDC

As the position keeper computes the current position of its own ship and the target, the results are forwarded to the angel solver as rotating shafts. The angle solver in turn computes the gyro angle and a projected pseudo run for the torpedo to hit the target. The results of the calculated torpedo's run are fed back to the position keeper as a new input.

Once DC power was applied to the time circuit the time motor started to compute the progress of an imaginary target represented by the current settings of the hand cranks. Adding AC power caused the machine to start computing the total solution. Because most of the mechanism was out of alignment, many of the dials started to rapidly turn in every direction at once. In a few seconds, the dials started to slow down, and in a few seconds more they started to seek equilibrium.

Once the machine settled into a steady state, the generating light came on and the machine began to track a solution. This was quite remarkable after so many years of inactivity! In order to test the accuracy of the TDC, we set up the most extreme test problem available in the manual: the target and submarine are approaching each other at high speed. We shut down the machine and set the initial measurements into the hand cranks.

Upon starting up the computer with these extreme initial conditions loaded, the TDC did remarkably well. Most of the variables change at a high rate of speed as the target and submarine passed each other. It was fascinating to watch the machine compute continuous solutions to simultaneous differential equations with rapidly changing variables. The TDC produced a result that was acceptably close to the required answer. It is most amazing when one realized that this machine is mostly wheels, gears, and shafts, and pre-dates the invention of the digital computer.

What is Next?

Computational mechanical analog computers had a very short history. They were only prevalent for the 50 years between the turn of the century and the invention of the digital computer at the end of World War II. These devices played a significant role in many of the historic events of the period. The fact that they were built changed the rules. By understanding these devices, we can start to see how the ability to compute with machines fueled the desire for machines with greater abilities.!


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