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90 PCB007 MAGAZINE I JUNE 2024 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 old- est analog computers. Most were mechanical, using gears to create the analog functions, and could weigh several tons 1 . 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. Figure 2: Left, differential equations model the flight of the lunar lander in their textbook form. Right, how they are encoded into the elements provided by "The Analog Thing" (THAT), such as integrators and comparators. The number inside circles refers to the potentiometers used to set the value of param- eters 2 . My Experience Aer 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 ana- log 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% solu- tion 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 omit- ted the "how we solved the design" as we knew that this was a consulting job for the profes- sor, and he was being paid to solve this design, which we had just done for him. Figure 3: The EAI PACE TR-10 analog computer was a tabletop unit from 1965 with 10 operational amplifi- ers for solving differential equations and other ana- log modeling. (Source: EAI Computers)