Design and Applied Technology Advanced: Core Concepts in Depth
AdvancedThis lesson models products as systems of input, processing and output, contrasts open-loop with closed-loop control, and explains how sensors and feedback drive automated products such as automatic doors and thermostatic appliances.
- The system model has three stages: input receives information, processing decides, output acts.
- Sensors are the soul of input, converting light, temperature, distance and sound into electrical signals.
- Open-loop control follows preset programs — simple and cheap; closed-loop control corrects itself through feedback — stable and precise.
文體小知識：科技知識說明文——剖析「輸入—處理—輸出」的系統模型，比較開環與閉環控制，並說明傳感器與回饋在自動化產品中的角色。
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1Advanced design thinking treats a product as a system. The system model has three links: input receives signals or commands from the surroundings, processing makes decisions according to rules, and output produces the actual action. In an electric fan, the buttons are the input, the speed-control circuit is the processing, and the motor driving the blades is the output; the three links interlock, and none can be missing.
2The heart of the input stage is the sensor, a component that converts physical quantities such as light, temperature, sound or distance into electrical signals. Light-dependent resistors sense brightness, thermistors sense temperature, ultrasonic sensors measure distance and microphones pick up sound. Processing is usually handled by a control circuit or microcontroller, which compares the signal with a set value and decides what to do; outputs include motors, light-emitting diodes, buzzers and heating elements.
3Control comes in two broad kinds, open loop and closed loop. Open-loop control follows a preset program and ignores the actual result; it is simple and cheap, like the speed dial of an ordinary fan, where selecting a setting gives a fixed fan speed. Closed-loop control adds feedback: a sensor keeps measuring the actual output, and the controller corrects the action so the output stays close to the target.
4A thermostatic iron is a classic closed loop: the set temperature is the target, a thermal element measures the actual temperature, the heater switches on when the temperature falls below the target and off when it rises above, so the temperature stays within a narrow band. Automatic doors work the same way: infrared or ultrasonic sensors detect an approaching person, the controller commands the motor to open, and the door closes after the person passes. Feedback makes products responsive.
5To analyse a system question, draw a block diagram: boxes for input, processing and output, arrows for the flow of signals, and a marked feedback path. The diagram simplifies complex products and frames your answer: identify the sensor and the output device first, then judge whether feedback exists, and you can state whether the product is open-loop or closed-loop and why it was designed that way.
1Summary 重點摘要
This lesson models products as systems of input, processing and output, contrasts open-loop with closed-loop control, and explains how sensors and feedback drive automated products such as automatic doors and thermostatic appliances.
2Vocabulary cards Vocabulary
A whole made of interconnected input, processing and output stages.
The stage where a system receives signals or commands from outside.
The stage that compares signals with set values and makes decisions.
The stage that produces the system's action or effect.
A component that converts physical quantities such as light, temperature or distance into electrical signals.
A mechanism that returns the actual result to the input so the controller can correct its actions.
Control that follows a preset program without checking the actual result.
Control that keeps correcting its output using sensor feedback.
A diagram using boxes and arrows to show system stages and signal flow.
3Step by step Steps
- Step 1: Seeing products as systemsAdvanced analysis splits a product into three stages: input receives information, processing decides according to rules, and output acts. This model fits products from electric fans to automatic doors and is the shared language for analysing and designing smart products. For example, in a sound-activated night light, the microphone picking up sound is the input, the circuit judging whether the sound is loud enough is the processing, and the light-emitting diode lighting up is the output — a clear account in system language.
- Step 2: Knowing common sensorsSensors are the system's senses, translating the physical world into electrical signals. Light-dependent resistors respond to brightness, thermistors to temperature, ultrasonic sensors measure distance by echo, microphones turn sound into signals, and humidity sensors track moisture. Choosing a sensor means considering its sensing range, sensitivity and mounting position. For example, designing automatic night lighting calls for a light-dependent resistor to detect darkness, while a collision-avoiding buggy needs an ultrasonic sensor to gauge the distance to obstacles.
- Step 3: Processing and output stagesThe processing stage is the system's brain, usually a control circuit or microcontroller; its job is to compare the input signal with a set value and decide what to do next. The output stage is the system's hands and feet: motors create rotation, light-emitting diodes give light, buzzers sound alarms and heating elements give warmth. For example, in a rice cooker the microcontroller judges from temperature and timing whether the rice is done, then commands the heating plate to warm or stop — processing and output interlock.
- Step 4: The character of open-loop controlOpen-loop control follows a preset program and, once it has acted, pays no further attention to the result. It is simple, cheap and direct, suiting situations where the outcome is easy to predict; its weakness is that it cannot correct deviations by itself, so changed conditions may throw the output off target. For example, a fan's speed dial is open-loop: the middle setting always gives the same fan speed, but a crowded room blocks the airflow and the fan never notices the difference in cooling.
- Step 5: Closed-loop control and feedbackClosed-loop control returns the actual result of the output, via a sensor, to the input side; the controller compares the actual value with the target and keeps correcting its actions according to the gap — this mechanism is feedback. Feedback lets a system correct itself, giving stable, disturbance-resistant output at higher cost and complexity. Take a thermostatic iron as an example: the thermal element measures the temperature continuously, heating when the temperature falls below the target and cutting off when it is reached, so the temperature stays within a narrow band.
- Step 6: Choosing between open and closed loopsEach control type has its stage: when the outcome is predictable and demands are modest, open loop saves money and effort; when the environment changes and precision matters, closed loop trades cost for stability. The key test is whether the product needs to know the result of its own output. For example, traffic lights that change on a fixed timetable manage perfectly well with open loop, while an air conditioner must keep a room at a set temperature, measuring and adjusting its compressor and fan, so it must be closed-loop.
- Step 7: Block-diagram analysis for examsA system block diagram uses boxes for input, processing and output and arrows for signal flow, with the feedback path looping from output back to input. To answer a system question, first identify the sensor and the output device, then trace the signal flow and judge whether a feedback path exists; from this you can name the control type and justify it. For example, in an automatic-door diagram the infrared sensor points to the control circuit and the circuit to the motor; with no path from the door's position back to the circuit, the description is open-loop and misses the real closed-loop design.
5Worked example Example
Worked example: A car park barrier rises automatically when a vehicle approaches and lowers after the vehicle has passed. Identify the input, processing and output stages, and state whether the control is open-loop or closed-loop.
- Find the input: induction loops or infrared sensors detect a vehicle arriving or leaving and convert this into signals.
- Find the processing: the control circuit compares the signals with set conditions and decides to raise, hold or lower the barrier.
- Find the output: a motor raises and lowers the boom, with indicator lamps showing the state.
- Judge the control type: the sensor keeps reporting the vehicle's position, and the system corrects its actions from actual conditions — a feedback path exists.
- Answer: The input is the vehicle-detecting sensor, the processing is the control circuit deciding to raise or lower, and the output is the motor driving the boom. Because the sensor continuously feeds the actual situation back to the controller, the system is closed-loop. Draw the block diagram with the feedback path and explain how feedback keeps the barrier up until the vehicle has fully passed, preventing it from lowering early.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the three basic stages of the system model?
Q2What does a sensor do? Give one example.
Q3What is the fundamental difference between open-loop and closed-loop control?
Q4How does a thermostatic iron keep its temperature steady?
Q5How is the feedback path shown in a block diagram?
8Recap Recap
- The system model has three stages: input receives information, processing decides, output acts.
- Sensors are the soul of input, converting light, temperature, distance and sound into electrical signals.
- Open-loop control follows preset programs — simple and cheap; closed-loop control corrects itself through feedback — stable and precise.
- Thermostatic irons and automatic doors are classic closed loops: sensors measure reality and controllers keep correcting.
- For system questions, draw the block diagram first: identify the sensor and output, trace the signals, and judge whether feedback exists.