Parallel Control Structure: From Conventional To Intelligent

Conventional control systems have been known to experience difficulties in achieving the multi-objective faced by controlled systems. Advanced control methods beyond the one-degree-of-freedom (1-DOF) controller have been considered to solve the multi-criteria requirements in the control system design. Two-degree-of-freedom (2-DOF) controllers are renowned for offering flexibility in handling the stability, tracking response, and disturbance or noise rejection requirements of closed-loop systems. Such powerful capabilities are achieved with feedback and/or feed-forward configurations. In addition, the configuration can be carried out with cascade and/or parallel structures. The cascade control structure in the 2-DOF controllers has received a lot of attention over the decades. On the other hand, parallel control structures are less discussed in the literature. In this paper, the parallel control structures are reviewed and discussed along with the development of their potential to deal with today's increasingly complex control problems. The discussion covers conventional to intelligent approaches related to the development of intelligent systems that have been applied everywhere recently.