Biologjia Online

Faced with the problem of controlling a physical system, an engineer will identify a model for the system, and then use this model to design a process for automatically actuating some of the system's input signals in order that the behavior of the system follows a desired profile. The most common control processes use feedback. Consider the example of designing an autopilot function that controls an aircraft to fly at constant altitude (see Fig. 1). A mathematical model of the aircraft dynamics (the plant) is designed, which describes how the aircraft's altitude, pitch, and roll angles (the outputs) change when the elevators, ailerons, and throttle (the inputs) are manipulated. By using sensors to measure the outputs, the engineer uses the model to calculate how these measurements should be employed to automatically adjust the inputs, so that if the aircraft deviates off course, it is guided, quickly and smoothly, back to the desired altitude. This function, which maps the measurements to the input adjustment through the feedback path, is known as the controller. The signal that carries the desired profile is fed into the system in the feedforward path. The entire system composed of the functional parts plant, feedback, feedforward, and controller is referred to as the closed loop system. The hallmark of a good feedback control design is a resulting closed loop system that is stable and robust to modeling errors and parameter variation in the plant and achieves a desired output value quickly without unduly large actuation signals at the plant input. Some insightful recent papers advocate a similar modular decomposition of biological systems according to the well defined functional parts used in engineering (1–5) and, specifically, engineering control theory (6–10). Yi et al. (6) reported that adaptation