Modern Control System Theory and Design, 2nd Edition

5.4.  STATIC ACCURACY

Static accuracy ranks as the next most important characteristics for a feedback control system. The designer always strives to design the system to minimize error for a certain anticipated class of inputs. This section considers techniques that are available for determining the system accuracy.

Theoretically, it is desirable for a control system to have the capability of responding to changes in position, velocity, acceleration, and changes in higher-order derivatives with zero error. Such a specification is very impractical and unrealistic. Fortunately, the requirements of practical systems are much less stringent. For example, let us consider the automatic positioning system of the missile launcher illustrated in Figure 5.1b. Its functioning is similar to the missile launcher positioning system in Figure 1.9. Realistically, it would be desirable for this system to respond well to inputs of position and velocity, but not necessarily to those of acceleration. In addition, it probably would be desirable for this system to respond with zero error for positional-type inputs. However, a finite tracking error could probably be tolerated for inputs of velocity. In contrast to this system, where the stakes are quite high, let us consider a simpler positioning system, which perhaps is only required to reproduce the angular position of a dial at some remote location. Such a control system would probably be only required to reproduce any positional inputs, but not any ...