Open and Closed Loop Control
FoundationControl systems come in two kinds: open loop acts on orders only, output never affecting control; closed loop senses its output, compares it with the set value through feedback and corrects itself. Compare a fan with an air conditioner and the difference becomes clear.
- Control systems manage output automatically; block diagrams map their signals
- Open loop acts on orders only — simple, cheap, unable to self-correct
- Closed loop senses, compares and corrects, holding its target through feedback
文體小知識：科技知識說明文以定義、比較和實例講解控制系統概念，令讀者能準確分辨和分析身邊的開環與閉環系統。
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1Household appliances look like strangers to one another, yet most are control systems: they take inputs and manage outputs against instructions or targets. Analyse them with the input–process–output block diagram plus one key question — does the system measure its own output? The answer decides whether it is open or closed loop, and how it behaves.
2Open-loop systems act on orders alone: a fan set to medium stays medium, a toaster heats until the timer pops — blind to the actual result. Simple, cheap and easy to maintain, they suit predictable tasks; their weakness is that any disturbance, from weather to voltage, pushes the output off course with no self-rescue.
3Closed-loop systems add a sensor and a feedback path: the sensor measures the output continuously, the controller compares it with the set value, and any deviation is corrected automatically. The air conditioner is the classic — cooling above the setting, pausing at it, holding the room steady. Fridges, thermostatic irons and dehumidifiers work likewise, and the body's temperature regulation is an exquisite natural loop.
4Both classes have worth: open loop is cheap and easy to repair; closed loop is accurate and disturbance-proof. When designing, first ask whether a target must be held and how often disturbances come — then choose the control, never complexity for its own sake. Analyse every system with sensing, comparing and correcting, and you have stepped through control engineering's door.
1Summary 重點摘要
Control systems come in two kinds: open loop acts on orders only, output never affecting control; closed loop senses its output, compares it with the set value through feedback and corrects itself. Compare a fan with an air conditioner and the difference becomes clear.
2Vocabulary cards Vocabulary
A system that manages its output automatically against instructions or a target.
A system that acts on orders only, its output never affecting control.
A system that measures its output and self-corrects through feedback.
Returning output information to the input to compare with the set value.
A component that measures quantities like temperature, humidity or light.
The target state the user sets, such as a desired temperature.
The part that compares the measured value with the setting to find deviation.
A device that senses temperature and holds the set level.
A system diagram of blocks for processes and arrows for signal flow.
Adjusting the output to bring the system back to its target.
3Step by step Steps
- From system to control systemSecondary 1 introduced systems as input–process–output; a control system is one that manages its output automatically against instructions or a target, with the processing part acting as the commander that decides when the output starts, stops and how strongly. Homes and schools are full of them: rice cookers control heating, lifts control floor stops, traffic lights control traffic flow. Analyse them with the same block diagram: blocks for processes, arrows for signals. For example, pressing a fan's button — the input — starts the motor at the set speed, the signal flowing one way from button to motor.
- Open loop: acting on orders onlyAn open-loop system's defining trait is that the output has no effect on the control: once the instruction is given, the system runs its preset course regardless of the actual result. Simple, cheap and easy to repair, it suits jobs whose outcomes are predictable, but it cannot self-correct when conditions change. For example, a fan set to medium stays at medium however cool the room turns; a toaster heats until the timer pops, blind to whether the bread is golden; a traditional kettle heats at full power and relies only on a preset boil-protection switch, never adjusting to the water's actual temperature.
- Closed loop: correcting itselfA closed-loop system adds a sensor and a feedback path: the sensor keeps measuring the actual output, sends the signal back to the controller, which compares it with the set value and corrects any deviation. Sensing, comparing, correcting — the three elements of the loop. For example, set an air conditioner to twenty-five degrees: the temperature sensor keeps measuring the room; above the setting, the controller runs the compressor; below it, the compressor pauses. Room temperature — the output — returns as part of the input, the control closes into a ring, and the room stays steady.
- The role of feedbackFeedback is the soul of the closed loop: it returns information about the output to the input, so the system acts on reality rather than expectation. The human body runs on feedback: too warm, we sweat to shed heat; too cold, we shiver to make it, holding body temperature in a narrow band. Adjusting a shower by feel is feedback by hand — too cold, turn warm; too hot, ease cool — until satisfied. For example, a dehumidifier senses room humidity, stops at the set level, and stops again when its tank is full: sensing, comparing, correcting, over and over.
- Comparing open and closed loopsBoth types have worth. Open loop is simple, cheap and easy to maintain, suited to tasks with predictable output and little disturbance — fans and door chimes. Closed loop is accurate and copes with disturbance, but complex and costlier, suited to tasks that must hold a target state such as constant temperature or humidity. Design by need, not by novelty. For example, a toilet's exhaust fan needs only on and off — open loop suffices; a refrigerator must hold about four degrees or food spoils — closed loop is required. Made open-loop, a fridge's temperature would swing wildly with weather and door openings.
- Classifying household systemsTo classify a household system, ask one question: does it measure its output and correct automatically? Systems sensing temperature, humidity, light or position are closed loop; systems acting on settings or time are open loop. For example, refrigerators, thermostatic irons, rice-cooker keep-warm functions, auto sensor lights and lifts all sense their actual state — closed loop; fans, door chimes, traditional toasters and timer plugs only follow instructions — open loop. Note that one appliance can mix both: a washing machine's speed sensing is closed loop while its preset wash time is open loop.
- Choosing control for a productBeyond classifying, learn to choose: when adding control to a product, ask whether a target must be held, how often disturbances come, and what the cost allows — then pick open or closed loop. This judgement will power your Secondary 3 automation project and prepares the way for the next lesson on electronic and pneumatic control. For example, a classroom plant-waterer that drips daily works fine and cheap as open loop; to water by soil moisture instead, add a humidity sensor and close the loop — clarify the need first, then choose the technology: that is how engineers think.
4Diagrams Diagrams
5Worked example Example
Example: classify these ten household systems as open or closed loop, and name the feedback source of each closed loop: electric fan, air conditioner, toaster, refrigerator, rice-cooker keep-warm, thermostatic heater, traditional kettle, auto sensor light, door chime, dehumidifier.
- The criterion → does the system measure its output (temperature, humidity, light, position) and correct automatically?
- Climate group → air conditioner, refrigerator, rice-cooker keep-warm, thermostatic heater and dehumidifier all carry sensors: closed loop
- Order-following group → fan, toaster, traditional kettle and door chime never measure their output: open loop
- Sensing group → the auto light triggers on light or body sensing: a simple closed loop
- Answer: Six of the ten are closed loop: air conditioner, refrigerator, rice-cooker keep-warm and thermostatic heater (temperature feedback), dehumidifier (humidity feedback) and the auto sensor light (light or body feedback). Four are open loop: fan, toaster, traditional kettle and door chime — they follow orders without checking results. Hold on to the single criterion — does it measure its output and correct? — and any new system can be classified at a glance.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the greatest difference between open- and closed-loop systems?
Q2In an air conditioner holding room temperature, what are sensing, comparing and correcting?
Q3Which is an open-loop system? A. refrigerator B. electric fan C. dehumidifier
Q4The body's temperature regulation belongs to which class? Give an example.
Q5How would you choose control for a toilet exhaust fan and a refrigerator?
8Recap Recap
- Control systems manage output automatically; block diagrams map their signals
- Open loop acts on orders only — simple, cheap, unable to self-correct
- Closed loop senses, compares and corrects, holding its target through feedback
- A fan is open loop; an air conditioner's thermostatic control is closed loop
- Choose control by need: open loop for predictable results, closed loop to hold a target