What is the defining difference between open-loop and closed-loop control?

Closed loop measures the output and uses the error to correct the control action

Open loop uses a sensor but closed loop does not

Open loop compares output with reference, closed loop does not

Why can an open-loop cruise controller not hold speed on a hill?

It never measures the output, so disturbances are not rejected

Which risk is specific to closed-loop systems?

They may oscillate or become unstable if the gain is too high

They drift when parameters change

They cannot reject disturbances

When is an open-loop controller acceptable?

When high accuracy is required under varying load

When the plant parameters change often

When disturbance rejection is essential

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Open loop versus closed loop control

Short answer

Open-loop lacks feedback; accuracy relies on model calibration and disturbance rejection is poor. Closed-loop measures output and corrects errors via feedback, offering high accuracy and disturbance rejection. Closed-loop is superior for most control applications.

Differentiate between open-loop and closed-loop control systems. Which system is better for control system?

An open-loop system applies a control action computed only from the input (reference); it does not measure the output. A closed-loop (feedback) system measures the output, compares it with the reference and uses the error to correct the control action.

Block diagrams

Open loop:
 r --> [Controller] --u--> [Plant] --> y
                              ^
                              d (disturbance)

Closed loop:
 r -->(+)-e->[Controller]-u->[Plant]--+--> y
       ^ -                      ^ d   |
       |                              |
       +------[Sensor / feedback]<----+

Differences

Point	Open loop	Closed loop
Feedback	None	Output is fed back
Accuracy	Depends on a good model and calibration	High; error is corrected
Disturbance	Not rejected	Reduced by feedback
Parameter change	Output drifts	Largely compensated
Stability	Stable if the plant is stable	May oscillate or become unstable if gain is too high
Sensor	Not needed	Required
Cost, complexity	Low, simple	Higher
Examples	Toaster, washing-machine timer	Cruise control, thermostat, servo

Which is better?

Closed loop is better for control. Real plants have disturbances (road slope, wind) and uncertain parameters, so an open-loop controller cannot guarantee the output. Feedback gives small steady-state error and good disturbance rejection, at the price of a sensor and careful design for stability. Open loop is acceptable only when the plant is predictable and disturbances are negligible.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Open loop versus closed loop control

Block diagrams

Differences

Which is better?

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