A hybrid computer technique for nonlinear function generation

The simulation of a physical system on an analog computer often requires the use of one or more nonlinear function generators. This paper describes a means by which a dig ital computer can be used to simulate the diode function generators of the analog computer in a closed-loop system. The digital computer program utilizes a table-lookup procedure to transform x into f(x). The table is previously prepared by the machine through linear interpolation be tween breakpoints. The breakpoints are the input data for the program. Many simulation problems need a nonlinear function generator as an element in a feedback loop. The digital computer introduces delay into the loop, rendering the simulation inaccurate and perhaps even unstable. A phase- advancing network must be included to compensate for the delay in the hybrid system. A simple and inexpensive technique for accomplishing the phase advance is analyzed and experimentally verified; the derivative of the incom ing waveform is used to predict the wave's value at some later instant of time. Advantages of the digital function generator include rapid setting, perfect reproducibility of functions, no limit on breakpoints, and an effective increase of the analog facility's nonlinear standby capability.