---
id: wiki-2026-0508-feedback-control-systems
title: Feedback Control Systems
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Closed-Loop Control, Control System, PID, MPC]
duplicate_of: none
source_trust_level: A
confidence_score: 0.92
verification_status: applied
tags: [control-theory, systems, dynamics, automation]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: Python
  framework: python-control, scipy.signal, do-mpc, casadi
---

# Feedback Control Systems

## 매 한 줄
> **"매 sensor → comparator → controller → actuator → plant → sensor 매 closed loop 으로 setpoint 를 maintain"**. James Watt 의 governor (1788) → Bode/Nyquist (1930s) → state-space + Kalman (1960s) → 매 modern: **MPC, ADRC, learning-based control (Koopman/MPC, RL)**. 매 SRE autoscaler, drone, EV motor, vaccine cold-chain, fab DRIE — 매 universal.

## 매 핵심

### 매 canonical block diagram
```
        ┌─────────┐
r ──+──▶│Controller│──u──▶┌─────┐──y──┐
   ─    │  C(s)   │       │P(s)│     │
        └─────────┘       └─────┘     │
              ▲                       │
              └──── −  ◀── sensor ◀───┘
e = r − y_meas      (error)
```

### 매 PID intuition
- **P (proportional)**: 매 instant correction; 매 too high → oscillation.
- **I (integral)**: 매 eliminate steady-state error; 매 too high → wind-up.
- **D (derivative)**: 매 damping; 매 amplify noise.

### 매 modern 분류
- **Classical**: PID, lead-lag, root-locus, Bode design.
- **State-space**: pole-placement, LQR, observer / Kalman filter.
- **Robust**: H∞, μ-synthesis (uncertain plant).
- **Adaptive**: MRAC, gain-scheduling.
- **Predictive**: **MPC** (constraint-aware, multi-step optimize).
- **Learning-based** (2026): Koopman-MPC, Gaussian-Process MPC, **safe RL with control-barrier functions**.

### 매 stability
- 매 LTI 의 Routh-Hurwitz, Nyquist, gain/phase margin (≥ 6 dB / 30°).
- 매 nonlinear 의 Lyapunov.

## 💻 패턴

### PID class
```python
class PID:
    def __init__(self, kp, ki, kd, dt, u_min=None, u_max=None):
        self.kp,self.ki,self.kd,self.dt = kp,ki,kd,dt
        self.u_min,self.u_max = u_min,u_max
        self.i, self.prev = 0.0, 0.0
    def __call__(self, sp, pv):
        e = sp - pv
        self.i += e*self.dt
        d = (e - self.prev)/self.dt
        self.prev = e
        u = self.kp*e + self.ki*self.i + self.kd*d
        if self.u_max is not None and u > self.u_max:
            u = self.u_max; self.i -= e*self.dt   # anti-windup
        if self.u_min is not None and u < self.u_min:
            u = self.u_min; self.i -= e*self.dt
        return u
```

### Ziegler–Nichols tuning
```python
# 1) Set ki=kd=0; raise kp to find Ku where output sustains oscillation at period Tu.
# 2) Classic ZN: kp=0.6Ku, ki=2kp/Tu, kd=kp*Tu/8
def zn_classic(Ku, Tu): return 0.6*Ku, 2*0.6*Ku/Tu, 0.6*Ku*Tu/8
```

### Plant simulation (python-control)
```python
import control as ct, numpy as np, matplotlib.pyplot as plt
P = ct.tf([1], [1, 3, 2])           # 1/(s²+3s+2)
C = ct.tf([2, 1], [1, 0])           # PI: 2 + 1/s
L = ct.series(C, P)
T = ct.feedback(L, 1)
t, y = ct.step_response(T, T=np.linspace(0, 10, 500))
plt.plot(t, y); plt.axhline(1, ls="--")
```

### State-space LQR
```python
import numpy as np, control as ct
A = np.array([[0,1],[-2,-3]]); B = np.array([[0],[1]])
Q = np.diag([10, 1]); R = np.array([[0.1]])
K, _, _ = ct.lqr(A, B, Q, R)
# u = -K x
```

### Kalman filter
```python
import numpy as np
def kf_step(x, P, z, A, H, Q, R):
    # predict
    x = A @ x; P = A @ P @ A.T + Q
    # update
    y = z - H @ x
    S = H @ P @ H.T + R
    K = P @ H.T @ np.linalg.inv(S)
    x = x + K @ y
    P = (np.eye(len(x)) - K @ H) @ P
    return x, P
```

### Model Predictive Control (do-mpc, level tank)
```python
import numpy as np, do_mpc
from casadi import vertcat
m = do_mpc.model.Model("continuous")
h = m.set_variable("_x","h"); u = m.set_variable("_u","u")
m.set_rhs("h", -0.1*h + u); m.setup()

mpc = do_mpc.controller.MPC(m)
mpc.set_param(n_horizon=20, t_step=0.1, store_full_solution=True)
mpc.set_objective(mterm=(h-1)**2, lterm=(h-1)**2 + 0.01*u**2)
mpc.set_rterm(u=0.1)
mpc.bounds["lower","_u","u"] = 0; mpc.bounds["upper","_u","u"] = 2
mpc.bounds["lower","_x","h"] = 0; mpc.bounds["upper","_x","h"] = 1.5
mpc.setup()
mpc.x0 = np.array([0.0]); mpc.set_initial_guess()
u0 = mpc.make_step(np.array([0.0]))
```

### Autoscaler-as-PID (SRE)
```python
class CPUAutoscaler:
    def __init__(self, target=0.6, k=10, max_r=20):
        self.pid = PID(kp=k, ki=k/30, kd=0, dt=15, u_min=-3, u_max=3)
        self.target, self.max_r = target, max_r
    def step(self, cpu, replicas):
        delta = self.pid(self.target, cpu)
        return max(1, min(self.max_r, replicas + round(delta)))
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Single SISO, slow plant | **PID** (90% of industry) |
| MIMO, known model | **LQR / state-space** |
| Constraints + multi-step lookahead | **MPC** |
| Large model uncertainty | **H∞ / robust control** |
| Time-varying gain | **Gain-scheduling / adaptive** |
| Unknown dynamics, lots of data | **Koopman-MPC / GP-MPC / safe RL** |
| Stochastic measurement | **+ Kalman / EKF / UKF** |

**기본값**: 매 SISO + slow plant → **PID with anti-windup + ZN-tuning**, 매 constraint-rich multi-input → **MPC**.

## 🔗 Graph
- 부모: [[Control-Theory]] · [[Cybernetics Foundations|Cybernetics]]
- 변형: [[PID]] · [[MPC]]
- 응용: [[Robotics]]
- Adjacent: [[Feedback-Loops in Systems]] · [[Kalman-Filter-and-State-Tracking|Kalman-Filter]] · [[Reinforcement-Learning]]

## 🤖 LLM 활용
**언제**: 매 PID 의 tuning, 매 plant 의 transfer-function 의 derivation, 매 MPC objective 의 formulation, 매 stability 의 quick check.
**언제 X**: 매 safety-critical real-time control 의 untested LLM-generated code 의 직접 deploy — 매 formal verification + HIL test 필수.

## ❌ 안티패턴
- **No anti-windup**: 매 saturated actuator + integral 의 huge overshoot.
- **D term on noisy measurement**: 매 amplifies high-freq noise — 매 derivative-on-measurement + low-pass filter.
- **Sample rate ≪ bandwidth**: 매 dt 의 ≥ 10× system bandwidth 의 violate → 매 instability.
- **Tuning by gut**: 매 reproducible (ZN, model-based, autotuning) 의 사용.
- **Open-loop "feedback"**: 매 sensor 없이 매 not feedback control.
- **Ignoring delay**: 매 transport delay → 매 Smith predictor / phase margin 의 reserve.

## 🧪 검증 / 중복
- Verified (Bode 1945; Åström & Murray *Feedback Systems* 2nd ed.; Rawlings, Mayne & Diehl *MPC* 2nd ed.; Brunton & Kutz *Data-Driven Science and Engineering* 2022).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 placeholder |
| 2026-05-10 | Manual cleanup — classical→MPC + 7 patterns + decision matrix |