Side-by-side comparison
| Parameter | Open Loop | Closed Loop Control |
|---|---|---|
| Feedback | Absent | Present |
| Accuracy | Low; affected by disturbances | High; corrects error continuously |
| Stability | Always stable if designed correctly | Can become unstable if gain is too high |
| Complexity | Simple; fewer components | Complex; needs sensors and error amplifier |
| Cost | Lower | Higher due to feedback hardware |
| Response to disturbance | No correction | Error detected and corrected |
| Typical example | Toaster, stepper motor drive | Thermostat, DC motor speed controller |
| Sensitivity to parameter change | High — output drifts | Low — feedback compensates |
| Design effort | Minimal | Requires stability analysis (Bode, root locus) |
Key differences
Open loop systems apply a fixed control action — a timer-based sprinkler does not care if the soil is already wet. Closed loop systems measure output via sensors (e.g., a tachometer on a DC motor) and feed the error back through an op-amp like the LM741 to drive the actuator. Gain tuning matters critically: too much loop gain in a closed loop system causes oscillation, a problem that simply does not exist in open loop. Closed loop reduces sensitivity to plant parameter changes by a factor of (1 + GH).
When to use Open Loop
Use open loop when the process is well-characterised and disturbances are negligible — a conveyor belt timer or a microwave oven magnetron drive needs no feedback because load variations are predictable.
When to use Closed Loop Control
Use closed loop whenever load disturbances, supply variations, or precision matter — a CNC spindle motor uses a closed loop drive with encoder feedback to hold speed within ±1 RPM despite cutting-force changes.
Recommendation
For any exam or placement scenario involving precision, disturbance rejection, or varying loads, choose closed loop. Open loop is only acceptable when cost dominates and accuracy requirements are loose.
Exam tip: Examiners frequently ask you to state the effect of feedback on sensitivity and bandwidth — memorise that feedback reduces sensitivity by (1+GH) and increases bandwidth by the same factor.
Interview tip: Interviewers at core companies expect you to explain why adding feedback can destabilise a system and to name at least one stability criterion (Nyquist, Bode gain/phase margin) used to prevent it.