Can a system be "partially" closed-loop?

Not in the strict control-theory sense for a single control variable — a given loop either measures its controlled variable and feeds it back, or it doesn't. But a larger system can mix strategies: a batch process might run open-loop timed steps for some stages (like a fixed-duration rinse) while running fully closed-loop PID control on temperature or pressure within that same batch.

Is open-loop control always worse than closed-loop?

No. Open-loop control is simpler, cheaper, and perfectly adequate when the process is well characterized and disturbances are minor or irrelevant — a fixed-duration conveyor indexing step, for example. Closed-loop control adds cost (a sensor, wiring, calibration, tuning) and is only worth that cost when disturbance rejection or accuracy actually matters to the outcome.

Does adding a PID algorithm automatically make a loop closed-loop?

No. A PID block is just a computation. It only forms a closed loop if its process variable (PV) input actually comes from a live sensor measuring the real process, and its output actually drives an actuator affecting that same process. A misconfigured PID block reading a fixed or stale value, or one whose output doesn't reach the real actuator, is not functioning as a closed loop even though the algorithm is technically "PID."

What happens if the feedback signal (sensor) fails in a closed-loop system?

The loop effectively degrades toward open-loop or worse — the controller keeps acting on a stale, frozen, or fault-value reading instead of reality, and can drive the actuator to a dangerous extreme trying to correct an error that no longer reflects the actual process. This is why SCADA/DCS systems implement sensor fault detection (rate-of-change alarms, out-of-range checks, redundant transmitters) and fail-safe fallback modes for exactly this failure case.

Is a summing junction a physical component?

Not usually — in modern SCADA/PLC/DCS systems the "summing junction" is just a subtraction operation inside the controller's software (error = setpoint − PV), computed every scan cycle. In older pneumatic or analog electronic controllers it could be an actual physical comparator circuit, but the concept — computing error from setpoint and measured value — is identical either way.

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Concept Explainer · SCADA

Open-Loop vs Closed-Loop Control

Why feedback — not the presence of a controller — is the thing that separates a system that corrects itself from one that just runs a script.

Ask an engineer what makes a control system "closed-loop" and a common wrong answer is: it has a controller, and the controller has a setpoint. Plenty of open-loop systems have both. A sprinkler timer has a setpoint — 20 minutes — and a controller that runs on that schedule. What it doesn't have is any idea whether the soil actually got wet enough. That single missing piece, a measurement of the real output fed back for comparison, is the entire difference between open-loop and closed-loop control.