Feedback Control
Source: Nicolas Minorsky, 1922; Norbert Wiener, Cybernetics, 1948; Hendrik Bode, 1945
Finding
A feedback control system measures the difference between a desired state and an actual state, and applies corrective action proportional to the error. The PID controller uses three terms: P (proportional to current error), I (proportional to accumulated error), D (proportional to rate of change). Failure modes are structural: overshoot (too much correction), undershoot (too little), instability (corrections amplify error). Wiener’s insight: this structure — measurement, comparison, correction — is universal: thermostats, cruise control, biological homeostasis, economic policy.
Pattern Mapping
Proportion — The PID controller is proportion incarnate. The P term: correct in proportion to error. The I term: correct for accumulated drift. The D term: correct in proportion to rate of change. Every term is a proportionality relationship. Overshoot and undershoot are proportion failures.
Alignment — The entire purpose is alignment: making actual state match desired state. The gap between desired and actual IS the error signal.
Honesty — The sensor must honestly report current state. A temperature sensor reading 20C when the room is 30C causes the controller to heat an already warm room. Sensor honesty is prerequisite to control system function.
Connections
- Safety Factors in Structural Engineering — both calibrate response proportionally
- Le Chatelier’s Principle — chemical equilibrium feedback is the same structure (→ Meta-Pattern 09)
- Autoimmune Disease — immune regulation is a feedback system; autoimmunity is when it malfunctions
- Addiction — addiction is a hijacked feedback loop: reward system oscillating between excess and deficit
- The Arms Race — arms races are positive feedback loops without proportional control
Status
Control theory is established engineering (Astrom and Murray, Feedback Systems, 2008). Minorsky (1922). Wiener’s Cybernetics (1948) is foundational. The characterization as proportion engineering is this project’s interpretation, though P-proportionality is definitional.
The mapping to the five properties is this project’s structural interpretation.