ABSTRACT
Multi-input/multi-output systems are usually difficult for human operators to control directly, since changing any one input generally affects many, if not all, outputs of the system. As an example, consider the vertical landing of a vertical take off and landing jet or of a lunar landing rocket. Moving to a desired landing point to the side of the current position requires tilting the thrust vector to the side; but this reduces the vertical thrust component, which was balancing the weight of the craft. The aircraft therefore starts to descend, which is not desired. To move to the side at a constant height thus requires smooth, simultaneous use of both attitude control and throttle. It would be simpler for the pilot if a single control existed to do thismaneuver; hence the interest in controlmethods thatmake the original system behave in a way that is easier to control manually. One example of such technique is when a compensator is sought that makes the compensated system diagonally dominant. If this can be achieved, it is then possible to regard the system as, to first order, a set of independent single-input/single-output systems, which is far easier to control than the original plant. Another approach is that of decoupling, where the system transfer matrix is made to be exactly diagonal. Each output variable is therefore affected by only one input signal, and each input/output pair can then be controlled by an easier-to-design single-input/single-output controller or manually by a human operator.
