Internal Model Control for Strip Flatness Control in Cold Rolling Mills

Strip flatness is one of the most important quality parameters of rolling products (plate, strip, and foil) used in many industrial branches such as the automotive, construction, household and packaging industries. The considered rolling process is a multivariable system with dominant time delay and subject to partly strong disturbances. This contribution presents, to the first time, the design methodology and on-site experiences of an internal model control (IMC) strategy for flatness control, implemented on an industrial process control system in a cold tandem mill. This includes an orthogonal polynomial decomposition of the flatness error profile for dimension reduction and decoupling of the multivariable system. Feedforward control is also part of the control strategy to compensate for disturbances (entry profile and rolling force variations) in the earlier mill stands. The controller parameters are adapted to the varying strip and plant conditions by gain scheduling.