How do you design inferential control systems using soft sensors?

Inferential control uses calculated (soft sensor) values when direct measurement is unavailable, expensive, or slow (e.g., product quality). Design: identify correlating measurements, develop model (empirical regression, first-principles, neural network), implement online calculation, validate against lab or analyzer data, and design bias update from intermittent measurements. Key considerations: model accuracy vs complexity, update frequency, input validation, and bias correction scheme. Applications: distillation composition from temperature profile, reactor conversion from heat balance, crude assay from density and temperature. Implement confidence indication and fallback strategy.