An anti-windup-based solution for the low current nonlinearity compensation on the FTU horizontal position controller

In this paper we focus on the nonlinear behavior of the F coil current amplifier at the Frascati Tokamak Upgrade (FTU), which induces closed-loop instability of the horizontal position stabilizing loop whenever the requested current is below a minimum current level. In the paper we first illustrate a modeling phase where nonlinear dynamics are derived and identified to reproduce the open-loop responses measured by the F coil current amplifier. Based on this dynamic model, we then reproduce the closed-loop scenario of the experiment and show that the proposed nonlinear model successfully reproduces the instabilities experienced in the experimental sessions. Next we propose a nonlinear control solution to this instability problem inspired by the so-called anti-windup paradigm. We report on experiments carried out at FTU, testing two versions of the anti-windup solution differing in terms of performance and robustness.