Fault detection for iterative learning control of time-varying systems

A method of reducing periodic disturbances in a cast strip metal product in a twin roll casting system having a pair of counter-rotating casting rolls producing the cast strip at a nip between the casting rolls, the nip being adjustable by a casting roll controller, an iterative learning control algorithm generating a-control signals, and a fault detection algorithm generating a fault detect signal;

• the method comprising:
• providing the control signal to the casting roll controller;
• measuring at least one parameter of the cast strip;
• calculating an error signal based on the control signal and the at least one parameter of the cast strip;
• providing the control signal and the error signal to the fault detection algorithm;
• indicating a fault condition via a fault detection signal when the fault detection algorithm determines that an infinity-norm of the control signal exceeds an upper threshold;
• generating the control signals for the casting roll controller by the iterative learning control algorithm based on the error signal and the fault detection signal;
• wherein the casting roll controller adjusts the nip in response to the control signals from the iterative learning control algorithm to reduce the periodic disturbances.

A twin roll casting system, comprising:

• a pair of counter-rotating casting rolls having a nip between the casting rolls and capable of delivering a cast strip downwardly from the nip, the nip being adjustable;
• a casting roll controller configured to adjust the nip between the casting rolls in response to control signals;
• a cast strip sensor capable of measuring at least one parameter of the cast strip and generate strip measurement signals;
• and
• an iterative learning control (ILC) controller coupled to the cast strip sensor to receive the strip measurement signals from the cast strip sensor and coupled to the casting roll controller to provide the control signals to the casting roll controller, the ILC controller including:
• a fault detection algorithm receiving the control signals and the strip measurement signals and generating a fault detection signal to indicate when a fault condition is detected;
• and
• an iterative learning control algorithm to generate the control signals based on the strip measurement signals and the fault detection signal;
• wherein the fault detection algorithm indicates a fault condition when it detects that the ILC controller is operating a state that is not guaranteed as stable or when the ILC controller is not meeting predefined performance specifications.