Why closed loop preferred despite cost

Closed-loop systems need sensors, comparators and more careful design, so they cost more and can become unstable. Still they are widely preferred because their performance advantages are much larger than these costs.

Justification

1. Accuracy: the output is forced to follow the reference; sensitivity to plant changes is $1/(1+GH)$ instead of 1.
2. Disturbance rejection: load changes and noise are corrected; their effect is reduced by $(1+GH)$.
3. Faster response: time constant $\tau \to \tau/(1+K)$.
4. Can use cheaper, less precise plant components: feedback compensates for their tolerance and ageing, which often saves overall cost.
5. Stabilizes unstable systems and allows full automation with less human labour.
6. Cheap electronics today: sensors and microcontrollers are inexpensive, so the extra cost is small.

Examples

In each case open-loop control would give large errors whenever load or environment changes, so the extra cost of feedback is justified.