Why closed loop suits modern complex systems

Control system

A control system is a combination of components arranged so that a desired output is obtained by regulating, commanding or directing the system's input, e.g. temperature control of an oven, speed control of a motor.

Why closed loop dominates modern complex systems

Modern systems (aircraft, robots, power grids, process plants) must be accurate and reliable in spite of changing loads, ageing parts and noise. Open-loop systems cannot correct themselves; closed-loop systems can, because they measure the output and act on the error.

1. Accuracy despite parameter changes: the sensitivity of the closed-loop gain to plant variation is

$$S_G^T = \frac{1}{1+GH} \ll 1 \quad (\text{open loop: } S_G^T = 1)$$

2. Disturbance rejection: a disturbance at the plant gives output $\dfrac{G_2}{1+G_1G_2H}D$ instead of $G_2D$, i.e. it is reduced by $(1+G_1G_2H)$.
3. Faster response: a plant $K/(\tau s+1)$ gets time constant $\tau/(1+K)$ in closed loop.
4. Smaller steady-state error and the ability to track a changing reference (e.g. a missile tracking a target).
5. Stabilizing unstable plants: an inverted pendulum, a fighter aircraft or a magnetic levitation system is unstable open loop and can only work with feedback.
6. Automation: it removes the human operator from the loop, which suits large, fast or remote systems.

The disadvantages (higher cost, need for sensors, possible instability if gain is too high) are small compared with these gains, and cheap sensors and microcontrollers have made feedback easy to add. Hence closed-loop control is the normal choice in modern complex systems.