A Model-Based Approach to the Protection of the Steering Mechanism of High-Power Antennas Placed in a Nuclear Fusion Tokamak

Abstract

In most applications where motors control several degrees of freedom of a mechanical system, there is the necessity to implement protection that is able to stop the motors when the mechanical system reaches the extreme positions of its workspace. When the mechanical system is located in a hostile environment and/or when the limit positions define a complex surface, the protection strategy cannot be always implemented by simply placing sensors and limit switches in the proximity of the limit positions. We present in this paper a solution for the protection problem, which employs a model-based algorithm capable of estimating in real time the system state variables. The idea of the model-based protection scheme is to implement a model of the mechanical system forced by the emergency braking actions in order to predict, using the current estimated state of the system as initial condition, the trajectory of relevant system outputs, together with their uncertainties, during the braking transient. If the probability that the predicted output trajectories intercept the physical boundaries of the workspace exceeds a predefined threshold, the protection trips. With this approach, we aim to reach a greater exploitation of the system workspace than with a fixed-threshold algorithm, since in this case, the threshold is adapted to the current system state. In this paper, the aforementioned algorithm is developed for a particular application, namely, the protection of a set of steering microwave antennas placed in a hostile environment (hard vacuum condition, strong neutron and gamma radiation, and strong magnetic fields) inside a nuclear-fusion reactor. However, it is the opinion of the authors that the same procedure could also find good application in other systems that need protection with similar characteristics. Some simulations and preliminary test results will be also presented in order to better clarify how the protection works, and finally, some implementation issues will be discussed.