Simulation, once the exclusive domain of analog computation, substitutes a model—usually differential equations—for a real dynamic device whose motion is too large or remote for convenient study. Originally, the operational amplifier was the only device capable of “real-time” calculation because its integrators could be adjusted to an R-C time constant of 1 second. Despite its noise, saturation, and inability to adequately multiply or generate functions, there was no other means in the 1940s and 1950s fast enough. Eventually, however, digital technology overcame its slowness, permitting discrete integrators to run in real time, and so digital simulation has largely replaced analog methods. An example (Figure 8-1) is an aircraft’s flight-control system, where the human pilot deflects the surfaces causing motion simulated by a computer. Visual and motion cues are presented to the pilot closing the loop.
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