How do you identify and address control system performance degradation?

Degradation identification: trend key metrics over time (variance, valve travel, controller mode time, oscillation frequency), detect drift in process model parameters, monitor maintenance records (calibration results, valve overhaul). Common causes: valve wear/stiction (increasing hysteresis), sensor drift, fouling changing process dynamics, changed operating conditions. Address through: regular loop audits (annual assessment), automated monitoring software, proactive maintenance (valve overhaul, sensor calibration), model update for APC systems. Implement benchmark testing after maintenance to verify restoration. Economic justification through variability reduction and constraint optimization.