Upsets in papermaking ash control: various dead-time compensators

Automation developments in paper manufacturing are driven by both technology and marketplace forces, resulting in the evolution of process control systems. In this paper, the dynamics of ash control loops in a papermachine control system are explored. Initially, a PID algorithm and a traditional Smith compensator are used to control a disturbance upset in the short circulation loop. This short circulation loop is the secondary (slave) loop inside a large dead-time machine loop which is the primary (master) controller in a traditional cascade configuration. Both setpoint and disturbance upsets are then introduced for the large dead-time machine loop, where the integral of time multiplied by the absolute value of error (ITAE) and settling time responses for headbox ash are investigated for PID, traditional Smith predictor, and PI-PD control schemes. As expected, the Smith PI-PD arrangement showed improved response over the traditional Smith compensator for setpoint upsets, but not for disturbances.