Closed-Loop Sensitivity Identification for Cross-Directional Systems

TL;DR

This paper presents a novel method for estimating closed-loop sensitivity in cross-directional systems, enabling better performance evaluation and fault detection in large-scale feedback control systems like those at synchrotron facilities.

Contribution

It introduces a reference signal-based approach to decouple and identify the system's sensitivity, accommodating strong directionality and noise, applicable to large-scale systems.

Findings

Enables indirect closed-loop sensitivity estimation

Facilitates fault detection in complex systems

Applicable to various industrial cross-directional processes

Abstract

At Diamond Light Source, the UK's national synchrotron facility, electron beam disturbances are attenuated by the fast orbit feedback (FOFB), which controls a cross-directional (CD) system with hundreds of inputs and outputs. Due to the inability to measure the disturbances in real-time, the closed-loop sensitivity of the FOFB can only be evaluated indirectly, making it difficult to compare FOFB algorithms and detect faults. Existing methods rely on comparing open-loop with closed-loop measurements, but they are prone to instabilities and actuator saturation because of the system's strong directionality. Here, we introduce a reference signal to estimate the complementary sensitivity in closed loop. By decoupling the system into sets of single-input, single-output (SISO) systems, the reference signal is designed mode-by-mode, accommodating the system's strong directionality. Additionally, a lower bound on the reference amplitude is derived to limit the estimation error in the presence of disturbances and measurement noise. This method enables the use of SISO system identification techniques, making it suitable for large-scale systems. It not only facilitates performance estimation of ill-conditioned CD systems in closed-loop but also provides a signal for fault detection. The potential applications of this approach extend to other CD systems, such as papermaking, steel rolling, or battery manufacturing processes.