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Happy’s Tech Talk #30: The Analog Computer

Although computer technology has taken enormous leaps, including HPC machines that now exceed the exascale limits, the analog computer is making its comeback. I had the fortune to learn about and program analog computers and programs in early 1966. The various analog computers came into their own during World War II to calculate artillery trajectories and for bombsights (Figure 1a, the Norden Bombsight). Their use continued to grow, and every major aeronautical design and manufacturer, as well as university, had them in the 1950s and 1960s (Figure 1b).

History of Analog Computers
Analog computers have been discovered to be thousands of years old. If you have ever used a slide rule, then you have used one of the oldest analog computers. Most were mechanical, using gears to create the analog functions, and could weigh several tons¹.

How They Work
Analog computers use various devices to integrate or differentiate the applied signal source. For the AC, the operational amplifier is used as it can amplify, differentiate, compare, and integrate the applied analog signal.

My Experience
After learning about analog computers, I used them in 1969 for a senior plant design project. My partner and I were assigned to design a Butadiene Sulfone plant, and I had to design the kinetic reactor and optimize it. To discover the optimum kinetic reaction rate is a trial-and-error of several differential equations and plotting the results of the equations until you have discovered the optimum.

By programming the EAI PACE TR-10 analog computer (Figure 3), varying the reaction rate coefficients using the potentiometers, and watching the reaction rate on the oscilloscope display, I found the optimum and could plot it out. I then backed off 10% on the reaction rate, plotted that curve, and then advanced the coefficients +10% and plotted that curve. In the report, the optimum reaction curve of the reactor was displayed on the page, with two onion-skin charts for the +10% and -10% solution that could be folded over the full page.   

When we got the report back, we had earned the highest grade in the class, with a personal note from the professor asking, “How did you do this? I would have given you an A+ if you had put the procedure in the appendix!” No one else had discovered the optimum, much less showing that it was the optimum. We omitted the “how we solved the design” as we knew that this was a consulting job for the professor, and he was being paid to solve this design, which we had just done for him.

Later, in graduate school, I took an advanced analytical instrumentation course in which we had to build 25 different chemical analytical sensors/analyzers and their electronics from the LEGO-like modular components.  The use of operational amplifiers as differentiators, integrators, and comparators was crucial to the electronics that ran these devices (and the basic components of the analog computer). Suffice it to say, the poor chemists in this course had a very hard time duplicating my setups as they did not have any of the electronics background that I had.

Analog Computers Are Coming Back
An article in the IEEE Spectrum magazine² has highlighted that analog computers are having a comeback, including the new semiconductor analog memory devices. As most of the real world is analog, digital computers first must convert analog signals into digital data (with associated error and distortion) and then back to analog outputs. The new analog IC chips are far faster and more accurate than digital techniques. 

Germany has been a hotbed of continuous analog computer research, and a company there has created the first new analog computer since the 1960s. This unit is called THAT, manufactured by Anabrid Gmbh (Figures 4a, 4b). It is currently selling for about $513.

Summary
It’s remarkable how technology recycles. I never expected to see an analog computer again, yet here they are.  The THAT computer and new AC ASICS (especially the new IBM chips) herald the comeback. It is with delight that I can witness all my engineering associates learning about how to use analog computers to solve differential and integral equations. It gives my control theory heart a new beat.

References

1. Computer History Museum
2. “Try This Brand-new Analog Computer,” by Stephen Cass, IEEE Spectrum, December 2023.

Happy Holden has worked in printed circuit technology since 1970 with Hewlett-Packard, NanYa Westwood, Merix, Foxconn, and Gentex. He is currently a contributing technical editor with I-Connect007, and the author of Automation and Advanced Procedures in PCB Fabrication, and 24 Essential Skills for Engineers. 

This column originally appeared in the June 2024 issue of PCB007 Magazine.