Introduction to Negative Feedback and Control Systems

1 . Introduction Let us consider the common generic problem of causing a network to produce a given desired output signal. A typical block diagram is illustrated in Fig. 1. We are able to select the input vin(s) arbitrarily, and it is desired to produce a certain specified output vout(s). The problem is that there is an unwanted disturbance vd(s) which also affects the output. We have no control over vd(s), and vd(s) varies over some range. Also, the values of the circuit elements are constructed to a certain tolerance (such as ±10%), and so in high-volume manufacturing of the system, the numerical values of the transfer function G1(s) lie in some distribution. Several examples of simple systems of this form will be discussed in class, including an op-amp circuit, a positioning system for computer peripherals or robotics, and a spacecraft solar array power system. You will construct a solar array shunt regulator system in lab, in which the solar array output voltage is kept constant in spite of variations in sunlight intensity or payload current. So we cannot expect to simply apply one input waveform vin(s), and obtain the specified output waveform vout(s) under all conditions. The idea behind the use of negative feedback is to build a circuit that automatically adjusts the input vin(s) as necessary, to obtain the desired vout(s) with high accuracy, regardless of disturbances in vd(t) or variations in component values. This is a useful thing to do whenever there are variations and unknowns that otherwise prevent the system from attaining the desired performance. ++ G2(s)