We have already alluded to the general nature of feedback control, and the need to provide sensors that ascertain the state of the vehicle. When human pilots are in control, their eyes and kinesthetic senses, aided by the standard flight information displayed by their instruments, provide this information. (In addition, of course, their brains supply the logical and computational operations needed, and their neuro-muscular systems all or part of the actuation.) In the absence of human control, when the vehicle is under the command of an autopilot, the sensors must, of course, be physical devices. As already mentioned, some of the state information needed is measured by the standard flight instruments—air speed, altitude, rate of climb, heading, etc. This information may or may not be of a quality and in a form suitable for incorporation into an automatic control system. In any event it is not generally enough. When both guidance and attitude-stabilization needs are considered, the state information needed may include:
Position and velocity vectors relative to a suitable reference frame.
Vehicle attitude (в, ф, ф).
Rotation rates (p, q, r).
Aerodynamic angles (a, /3).
Acceleration components of a reference point in the vehicle.
The above is not an exhaustive list. A wide variety of devices are in use to measure these variables, from Pitot-static tubes to sophisticated inertial-guidance platforms. Gyroscopes, accelerometers, magnetic and gyro compasses, angle of attack and sideslip vanes, and other devices all find applications as sensors. The most common form of sensor output is an electrical signal, but fluidic devices have also been used. Although in the following examples we tend to assume that the desired variable can be measured independently, linearly, and without time lag, this is of course an idealization that is only approached but never reached in practice. Every sensing device, together with its associated transducer and amplifier, is itself a dynamic system with characteristic frequency response, noise, nonlinearity, and cross-coupling. These attributes cannot finally be ignored in the design of real systems, although one can usefully do so in preliminary work. As an example of cross-coupling effects, consider the sideslip sensor assumed to be available in the gust alleviation system of Sec. 8.9. Assume, as might well be the case, that it consists of a sideslip vane mounted on a boom projecting forward from the nose. Such a device would in general respond not only to (3 but also to atmospheric turbulence (side gusts), to roll and yaw rates, and to lateral acceleration av at the vane hinge. Thus the output signal would in fact be a complicated mathematical function of several state variables, representing several
feedback loops. The objective in sensor design is, of course, to minimize all the unwanted extraneous effects, and to provide sufficiently high frequency response and low noise in the sensing system.
This brief discussion serves only to draw attention to the important design and analytical problems related to sensors, and to point out that their real characteristics, as opposed to their idealizations, need finally to be taken into account in design.