All automatic control systems use ve feedback for controlling a physical parameter like position, velocity, torque etc. The parameter which has to be controlled is sensed by a suitable transducers and fed back to the input, for comparison with the reference value (see Figure 20.1. This subtraction of the sampled output signal with that of reference input is called as ve feedback. The difference signal, called the ``error" is then amplified to drive the system (referred to as actuation ) in such a manner that the output approaches the set reference value. In other words the system is designed to minimize the error signal.
All practical loads have inertia and spring constants due to which there is a delay in actuation. Hence, even though a system may be designed for ve feedback, due to inherent time lags, the feedback may turn into ve feedback at certain frequencies. If the loop gain is more than unity at some frequency at which the feedback is ve, the system will oscillate. Hence, in designing control systems great care has to be taken to avoid such situations.