Open-loop & closed-loop systems

Feedback is the most fundamental property because it determines whether a system can respond to reality or only execute a plan blindly, which shapes everything about how it behaves in the face of an imperfect, changing world. An open-loop system applies a predetermined action and assumes the result follows; it never looks at the actual output, so it is structurally incapable of knowing whether it achieved its goal. This is fine only in an idealized situation where the action reliably produces the result and nothing disturbs it, but the real world has disturbances (a load on a motor, heat loss from a room, wind on a car), model inaccuracies, and drift, none of which an open-loop system can see or counter. A closed-loop system adds the one thing that changes this completely: it ‍​‌‌​​‌‌​​‌‌‌​​‌​​‌‌​​‌​‌​‌‌​​‌​‌​​‌​‌‌​‌​‌‌‌​​‌‌​‌‌​​​​‌​‌‌​‌‌​‌​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​‌​‌‍measures the output and compares it to the desired setpoint, computing the error, and adjusts its action to drive that error to zero. That single addition, feedback, gives it a capability open-loop fundamentally lacks: the ability to reject disturbances and hold accuracy despite the unexpected. Because it continuously checks the result, it automatically compensates for whatever it never explicitly anticipated, since any deviation shows up in the error and gets corrected. The thermostat holds the room temperature whether or not someone opens a window; cruise control holds speed up hills and down; an open-loop equivalent would just run a fixed action and let reality push the result wherever it likes, with no awareness. So the feedback distinction isn't about complexity or sophistication. It's about whether the system is aware of and responsive to its actual outcome (closed-loop) or blind to it (open-loop). That awareness is what lets automated systems achieve and maintain targets in a messy world, which is why open-vs-closed is the foundational classification underlying every control decision.

Closed-loop isn't automatically better because its power, feedback, comes at a real cost, and that cost is only worth paying when the task actually needs disturbance rejection or accuracy. To close the loop you must measure the output, which means adding a sensor (and its wiring, calibration, and potential failure), comparing to a setpoint, and implementing correcting logic: more hardware, more complexity, more things that can go wrong, and more expense. If the situation doesn't demand it, all of that is wasted: where the relationship between action and result is reliable and disturbances are negligible, an open-loop action will dependably produce the right result without any of the feedback machinery, so the simpler, cheaper, more robust open-loop solution is the ‍​‌‌​​‌‌​​‌‌‌​​‌​​‌‌​​‌​‌​‌‌​​‌​‌​​‌​‌‌​‌​‌‌‌​​‌‌​‌‌​​​​‌​‌‌​‌‌​‌​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​‌​‌‍better engineering choice. A toaster on a timer, a fixed-speed conveyor, a sprinkler that runs for a set time. These don't need to measure anything because the open-loop action is reliable enough for the purpose, and adding sensors and feedback would only add cost and failure modes for no benefit. The decision therefore hinges on two questions: does the task require holding a target despite disturbances, and does it require accuracy that the open-loop action can't guarantee? If yes to either: the room temperature must stay steady though heat is lost, the speed must hold though loads vary, the level must be maintained though inflow changes. You need closed-loop, because only feedback can sense and correct the resulting errors. If no (the action reliably produces an acceptable result and nothing significant disturbs it) open-loop is appropriate and preferable for its simplicity and economy. So you match the approach to the requirement: closed-loop where disturbance rejection or accuracy is essential, open-loop where the action is reliable and feedback would be needless cost. This fit-to-purpose judgement, rather than 'always close the loop', is the mark of good automation design.

The difference between open-loop and closed-loop control is like the difference between driving with your eyes closed for a few seconds and driving with your eyes open. With your eyes closed (open-loop), you just steer based on what you remember: 'the road was straight, so I'll hold the wheel here', and you hope nothing changed. You're applying a fixed action with no way to check the result. That's fine for a moment on a straight, empty road where nothing unexpected happens. It's simple, and you don't need to look. But you're blind: you can't tell if you're drifting. With your eyes open (closed-loop), you constantly see where you actually are versus where you want to be, and you correct the wheel to stay in your lane. You're using feedback, comparing reality to your goal and adjusting. The payoff is that eyes-open lets you handle a curve or a gust of wind that eyes-closed cannot: when the road bends or a crosswind pushes you, you see yourself drifting toward the edge and steer back. Whereas with your eyes closed you'd have no idea anything pushed you and you'd drift right off the road. That's exactly why closed-loop control can hold a target despite disturbances (the curve, the wind) while open-loop just executes its plan and hopes: one watches and corrects, the other acts blindly. (And just as you don't need to stare at the road to butter toast, some simple, reliable actions are fine done 'eyes closed' (open-loop) which is cheaper and simpler when nothing will throw them off.)

Decisions:

Reasoning: - (a) Air temperature: closed-loop: the temperature is constantly disturbed (sunlight, outside conditions, ventilation), so to hold it at a setpoint you need feedback: measure it, compare to target, and adjust heating/cooling/venting to reject those disturbances. Open-loop (a fixed heater schedule) would be blind to the disturbances and let the temperature wander. - (b) Timed irrigation: open-loop: if a fixed daily watering duration reliably delivers an adequate amount and there's no significant disturbance to correct, a ‍​‌‌​​‌‌​​‌‌‌​​‌​​‌‌​​‌​‌​‌‌​​‌​‌​​‌​‌‌​‌​‌‌‌​​‌‌​‌‌​​​​‌​‌‌​‌‌​‌​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​‌​‌‍simple timed action suffices, no sensor needed. Closing the loop here would add cost/complexity for little benefit (the open-loop action is reliable enough). - (c) Soil moisture: closed-loop: moisture is pushed around by evaporation, weather, and plant uptake (disturbances), so to keep it at a target you must measure soil moisture and correct the watering. Feedback rejects those disturbances and holds the setpoint, which open-loop can't.

The open-vs-closed reasoning: use closed-loop wherever you must hold a target despite disturbances (air temperature, soil moisture). The sensor and feedback are worth their cost because the variable is disturbed and accuracy matters; use open-loop for simple, reliable actions (a timed watering) where a predetermined action dependably gives an acceptable result and disturbances are negligible, simpler and cheaper. Match the approach to whether feedback is actually needed. The foundational control decision.