How do you handle process gain nonlinearity in control loops?

Process gain nonlinearity (gain varies with operating point) causes inconsistent control performance. Solutions: equal percentage valves (compensate for process gain increase with flow), gain scheduling (change controller parameters based on operating point or throughput), adaptive control (automatically adjusts tuning), characterization (linearize valve signal with function block), or model predictive control. Identification: run step tests at multiple operating points, compare responses. For severe nonlinearity, single tuning provides acceptable performance only over limited range.