Noninteger-order Control to Reduce Effects of Saturating Actuators in Faulty Marine Engines

Operation and maintenance of marine engines by intelligent and predictive techniques may avoid or reduce the impact of faults of these systems to the environment and humans, hence reduce polluting emissions and risks for safety of navigation. However, in many cases, reactive strategies are used to repair or replace components and systems as faults arise, thus leading to safety risks, emergency situations for critical assets, high costs, unpredictable long downtime, etc. In some other cases, prevention maintenance is considered to schedule regular inspections, but it requires investment, dedicated staff for complex tasks, pause of operation, and may yield unnecessary maintenance with waste of time and resources. To face such issues, control systems may help in achieving fault tolerance. However, many fault-tolerant controllers fail to consider saturation in actuators or faults that may cause control signals that saturate the actuators. These events may lead to further faults, performance reduction, oscillations, and instability. Then, they increase risks and polluting emissions by marine engines. To address this problem, this paper proposes a controller preventing saturation and associated limit cycles in a control loop including a saturating actuator and other linear elements. The two-stage controller employs noninteger orders to extend the properties of the classical phase-lead network and improve robustness. Control design is in the frequency domain and takes advantage of the describing function method to deal with the saturation nonlinearity. Simulation confirms effectiveness and robustness of the noninteger-order controller in preventing limit cycles.