📚 SQA Engineering Case Study: Practical Application and Analysis | SQA 工程案例分析：实战演练
This article presents a comprehensive engineering case study focused on the design, analysis, and evaluation of an automated greenhouse ventilation system. We explore how systems engineering, electronics, mechanics, and energy principles combine to solve a real-world problem, mirroring the SQA Higher Engineering Science approach. Each section pairs theory with practical application, giving you the skills to tackle similar case studies confidently in your assessments.
本文围绕一个自动化温室通风系统的设计、分析与评估，展开全面的工程案例分析。我们将探索系统工程、电子、机械和能源原理如何协同解决实际问题，这正反映了苏格兰SQA高级工程科学的考察方式。每个部分都将理论与实践相结合，帮助您掌握应对类似案例分析的技能，在评估中自信作答。
1. Introduction to the Case Study | 案例介绍
The case study involves developing a temperature-controlled ventilation system for a small commercial greenhouse. The system must automatically open a roof window when the internal temperature exceeds 25 °C and close it when the temperature drops below 20 °C. It must also operate on a low-voltage DC supply, incorporate manual override, and meet safety standards. The design will be analysed from multiple engineering perspectives: system block diagrams, electronic circuits, structural mechanics, energy consumption, and microcontroller programming.
本案例要求为一个小型商业温室开发一套温控通风系统。系统需在内部温度超过25 °C时自动打开天窗，当温度降至20 °C以下时关闭天窗。系统必须采用低压直流供电，具备手动超控功能，并符合安全标准。我们将从多个工程角度分析该设计：系统框图、电子电路、结构力学、能耗及微控制器编程。
2. Understanding the System Requirements | 理解系统需求
Before any design begins, engineers must clarify functional and non-functional requirements. Functional requirements include sensing temperature, comparing it to setpoints, and actuating a window mechanism. Non-functional requirements include low power consumption, weatherproof enclosure (IP65), and fail-safe operation (window closes on power loss). A clear requirements specification ensures the design stays on track.
在设计启动之前，工程师必须明确功能需求与非功能需求。功能需求包括感测温度、与设定值进行比较以及驱动窗户机械装置。非功能需求包括低功耗、防水外壳（IP65防护等级）以及故障安全操作（断电时窗户关闭）。明确需求规格能确保设计不偏离方向。
- Functional: Temperature sensing, window open/close, manual override button.
功能需求：温度感测、窗户开/关、手动超控按钮。 - Non-functional: Operate from a 12 V battery, use less than 5 W average power, withstand outdoor humidity.
非功能需求：使用12 V电池供电，平均功耗低于5 W，耐受室外潮湿环境。 - Safety: Limit switch to prevent motor stall, emergency stop.
安全：限位开关防止电机堵转，紧急停止。
3. Block Diagram and Sub-system Identification | 框图与子系统识别
A systems approach breaks the product into interconnected sub-systems. The main blocks are: input (temperature sensor), process (microcontroller), output (motor driver and actuator), and power supply. A manual override provides a parallel input path. Feedback is provided by limit switches that signal when the window is fully open or closed.
系统工程方法将产品分解为相互连接的子系统。主要模块包括：输入（温度传感器）、处理（微控制器）、输出（电机驱动与执行器）以及电源。手动超控提供了一条并行输入路径。限位开关反馈窗户全开或全关的信号。
Input → Process → Output, with Feedback loop from limit switches to Processor
输入 → 处理 → 输出，限位开关反馈至处理器形成闭环
A block diagram helps visualise signal flow. The temperature sensor (analogue voltage) is converted to a digital value by an ADC inside the microcontroller. The microcontroller compares the reading with preset thresholds and sends logic signals to an H-bridge motor driver, which controls the direction of a DC motor operating the window mechanism.
框图有助于可视化信号流。温度传感器（模拟电压）经微控制器内部ADC转换为数字值。微控制器将该值与预设阈值进行比较，并向H桥电机驱动器发送逻辑信号，控制驱动窗户机械装置的直流电机方向。
4. Input and Output Devices | 输入与输出设备
Choosing appropriate input and output devices is crucial. For temperature sensing, a thermistor (e.g., NTC 10 kΩ at 25 °C) in a voltage divider configuration offers a cost-effective, analogue signal. It provides a varying voltage correlated with temperature. The output device is a 12 V DC geared motor with a maximum torque requirement of 0.5 Nm to move the window. Limit switches (micro-switches) detect end positions.
选择合适的输入与输出设备至关重要。对于温度感测，采用热敏电阻（例如25 °C时10 kΩ的NTC）构成分压电路，可提供经济有效的模拟信号。其输出电压随温度变化。输出设备为12 V直流减速电机，驱动窗户所需的最大扭矩为0.5 Nm。限位开关（微动开关）检测终点位置。
| Device / 设备 | Type / 类型 | Function / 功能 | Signal / 信号 |
|---|---|---|---|
| NTC Thermistor / NTC热敏电阻 | Input / 输入 | Senses temperature / 感测温度 | Analogue voltage (0–5 V) / 模拟电压 (0–5 V) |
| Manual Override Button / 手动超控按钮 | Input / 输入 | User command to open/close / 用户开/关指令 | Digital (push-to-make) / 数字（常开触点） |
| DC Gear Motor / 直流减速电机 | Output / 输出 | Actuates window mechanism / 驱动窗户机构 | 12 V, pulse-width modulated / 12 V，脉宽调制 |
| Limit Switches / 限位开关 | Feedback / 反馈 | Detect fully open/closed / 检测全开/全关 | Digital (NC contacts) / 数字（常闭触点） |
5. Process and Control | 处理与控制
A microcontroller, such as an ATmega328P (Arduino Nano), forms the processing core. It reads the analogue temperature voltage via its 10-bit ADC, converting it to a numeric temperature using a lookup table or the beta equation for the thermistor. The control algorithm includes hysteresis: when temperature rises above 25 °C, the motor drives the window open until the upper limit switch is triggered. The window stays open until the temperature falls below 20 °C, at which point the motor reverses until the lower limit switch activates.
微控制器（如ATmega328P——Arduino Nano所用芯片）构成处理核心。它通过10位ADC读取模拟温度电压，利用查表法或热敏电阻Beta方程将其转换为温度数值。控制算法包含迟滞：当温度升至25 °C以上时，电机驱动窗户打开，直至触发上限位开关。窗户保持打开，直到温度降至20 °C以下，此时电机反转，直至触发下限位开关。
The hysteresis band (20 °C to 25 °C) prevents rapid on-off cycling around a single setpoint, which would wear the motor and cause instability. This is a classic control engineering technique. The microcontroller code also includes debouncing for the manual override button and limit switches.
迟滞区间（20 °C至25 °C）可防止系统在单一设定值附近频繁启停，避免电机磨损及系统不稳定。这是经典的控制工程技术。微控制器代码还包含了手动超控按钮和限位开关的消抖处理。
6. Energy and Power Calculations | 能量与功率计算
The system must run from a 12 V, 7 Ah lead-acid battery, recharged by a small solar panel. Engineers calculate energy consumption to ensure the battery lasts through cloudy days. The motor draws 300 mA when moving the window, which takes approximately 15 seconds to fully open or close. Assuming 20 open/close cycles per day, the motor energy per day is:
系统需要由12 V、7 Ah的铅酸电池供电，并通过小型太阳能板充电。工程师需计算能耗以确保电池在多云天气下也能持续工作。电机驱动窗户时电流为300 mA，窗户全开或全关约需15秒。假设每天开闭循环20次，每日电机耗能为：
Emotor = V × I × t = 12 V × 0.3 A × (2 × 20 × 15 s) = 12 × 0.3 × 600 = 2160 J
电机能量 = V × I × t = 12 V × 0.3 A × (2 × 20 × 15 s) = 12 × 0.3 × 600 = 2160 J
The microcontroller and sensor circuit consume about 20 mA continuously. Daily standby energy:
E_standby = 12 V × 0.02 A × 86400 s = 20736 J
待机能耗 = 12 V × 0.02 A × 86400 s = 20736 J
Total daily energy ≈ 22.9 kJ. Battery capacity in joules: 12 V × 7 Ah × 3600 s/h = 302400 J. Even without solar input, the battery could power the system for over 13 days (302400 J / 22900 J/day ≈ 13.2 days). This generous margin ensures reliability.
每日总能耗约为22.9 kJ。电池存储能量（以焦耳计）：12 V × 7 Ah × 3600 s/h = 302400 J。即使没有太阳能输入，电池也能为系统供电超过13天（302400 J / 22900 J/天 ≈ 13.2天）。这一充足裕量确保了可靠性。
7. Structural Analysis of the Mechanism | 结构的力学分析
The window consists of a framed polycarbonate panel hinged at the top, pushed open by a linear actuator driven by the geared motor. The actuator must overcome the weight of the panel and wind forces. Let the panel mass be 2.5 kg, length 0.8 m, and centre of gravity at 0.4 m from the hinge. The required torque to hold the window open at an angle θ (measured from vertical) is:
τ = m × g × d × sin θ
窗户由带框聚碳酸酯板构成，顶部铰接，由减速电机驱动的直线推杆推动打开。推杆需克服面板重量及风力。假设面板质量2.5 kg，长0.8 m，重心距铰链0.4 m。保持窗户开启角度θ（从垂直方向起算）所需扭矩为：
τ = m × g × d × sin θ
At θ = 45°, τ = 2.5 kg × 9.81 m/s² × 0.4 m × sin 45° ≈ 2.5 × 9.81 × 0.4 × 0.707 ≈ 6.94 Nm. However, the actuator attaches 0.15 m from the hinge, so the force required is τ / 0.15 m ≈ 46.3 N. The selected lead-screw actuator provides a maximum force of 80 tor, giving a safety factor above 1.5. A static stress analysis of the hinge pin (steel, 5 mm diameter) shows that the shear stress is well below the yield strength.
当θ = 45°时，τ = 2.5 kg × 9.81 m/s² × 0.4 m × sin 45° ≈ 2.5 × 9.81 × 0.4 × 0.707 ≈ 6.94 Nm。然而，推杆连接点距铰链0.15 m，因此所需力为τ / 0.15 m ≈ 46.3 N。选用的丝杠推杆配合减速电机可提供最大80 N的力，安全系数大于1.5。对铰链销（钢质，直径5 mm）进行的静态应力分析表明，剪切应力远低于屈服强度。
| Parameter / 参数 | Value / 数值 |
|---|---|
| Panel weight / 面板重量 | 24.5 N |
| Max torque at hinge / 铰链最大扭矩 | 6.94 Nm |
| Actuator force required / 所需推杆力 | 46.3 N |
| Actuator max force / 推杆最大力 | 80 N |
| Safety factor / 安全系数 | 1.73 |
8. Electronic Circuit Design | 电子电路设计
The temperature sensing circuit is a voltage divider with the NTC thermistor and a fixed 10 kΩ resistor connected to the 5 V regulated supply. The output voltage Vout = 5 V × Rfixed / (R_thermistor + Rfixed). At 25 °C, R_thermistor ≈ 10 kΩ, so Vout = 2.5 logue signal feeds into ADC channel A0 of the microcontroller. A decoupling capacitor (100 nF) smoothes noise.
温度感测电路是由NTC热敏电阻与固定10 kΩ电阻串联构成的分压器，连接到5 V稳压电源。输出电压Vout = 5 V × Rfixed / (R_thermistor + Rfixed)。在25 °C时，R_thermistor ≈ 10 kΩ，因此Vout = 2.5 V。该模拟信号输入微控制器的ADC通道A0。旁路电容（100 nF）用于平滑噪声。
The motor driver uses an L293D H-bridge IC. Two digital pins from the microcontroller control direction: one high, one low for opening; reverse for closing; both low for brake/stop. Enable pin can be used with PWM to soft-start and soft-stop, reducing mechanical stress.
电机驱动器采用L293D H桥集成电路。微控制器的两个数字引脚控制方向：一高一低使窗户打开；电平反转则关闭；两引脚同为低电平时制动/停止。使能引脚可与PWM配合使用，实现软启动和软停止，减轻机械应力。
9. Programmable Logic and Microcontrollers | 可编程逻辑与微控制器
The microcontroller is programmed in C++ (Arduino IDE). A simplified pseudocode illustrates the logic:
Read ADC, convert to temperature T
If T > 25 and window not fully open → motor forward
If T < 20 and window not fully closed → motor reverse
If manual button pressed → toggle motor direction (with limits)
Check limit switches continuously to stop motor
微控制器使用C++（Arduino IDE）编程。简化的伪代码说明了逻辑：
读取ADC，转换为温度T
若 T > 25 且窗户未全开 → 电机正向运行
若 T < 20 且窗户未全关 → 电机反向运行
若手动按钮按下 → 切换电机方向（受限于限位）
持续检查限位开关以停止电机
State diagrams can also be used to model the controller's behaviour, a skill often tested in SQA exams. The system has three primary states: IDLE, OPENING, CLOSING. Transitions are triggered by temperature thresholds, limit switches, or manual inputs.
状态图也可用于对控制器行为建模，这是SQA考试中经常考察的技能。系统具有三个主要状态：IDLE（空闲）、OPENING（开启中）、CLOSING（关闭中）。状态转移由温度阈值、限位开关或手动输入触发。
10. Safety and Risk Assessment | 安全与风险评估
A risk assessment identifies potential hazards: finger trapping in the mechanism, electric shock from 12 V (though low voltage, short circuits can cause fire), and overheating of the motor if stalled. Mitigation measures include using a sealed actuator cover, adding a resettable fuse (PTC) on the motor supply, and implementing software motor current monitoring via a shunt resistor. The emergency stop button cuts power to the motor entirely. The system also fails safe: if power is lost, the window remains closed due to gravity and the lead-screw's self-locking property.
风险评估识别潜在危险：机械装置夹伤手指、12 V电源的电击风险（虽为低电压，但短路可能引起火灾）以及电机堵转时的过热。缓解措施包括：使用密封推杆护罩、在电机供电回路中加入自恢复保险丝（PTC）、通过分流电阻对电机电流进行软件监控。急停按钮完全切断电机电源。系统还具备故障安全特性：若断电，由于重力和丝杠的自锁特性，窗户会保持关闭。
Compliance with relevant standards, such as BS EN 60335 for household appliances or ISO 13849 for safety-related control systems, would be considered in a professional engineering environment. For the SQA case study, awareness of safety-by-design principles is essential.
在专业工程环境中，还需考虑相关标准，如家用电器标准BS EN 60335或安全相关控制系统标准ISO 13849。对于SQA案例分析，了解设计的安全性原理至关重要。
11. Practical Implementation Challenges | 实际实施挑战
Real-world implementation introduces challenges like environmental sealing, corrosion resistance, and variation in thermistor tolerance. Calibration of the temperature sensor against a known standard improves accuracy. The mechanical linkage may develop backlash over time, requiring periodic maintenance. Power cable routing must avoid water ingress. In prototyping, iterative testing and debugging of the microcontroller code with an oscilloscope and multimeter is essential.
实际实施中会引入诸多挑战，如环境密封、耐腐蚀性以及热敏电阻的容差变化。对照已知标准对温度传感器进行校准可提高精度。机械连杆机构随时间推移可能产生间隙，需要定期维护。电源线布线需避免进水。在原型制作中，使用示波器和万用表对微控制器代码进行迭代测试与调试是必不可少的。
Engineers use the design-build-test cycle to refine the system. Data logging of temperature trends could be added to validate the control strategy over weeks. Such practical reflections demonstrate deep understanding in SQA assignments.
工程师通过设计-构建-测试循环对系统进行改进。可以添加温度趋势数据记录功能，以验证控制策略在数周内的有效性。这些实践反思能在SQA作业中展示出深刻的理解。
12. Conclusion and Exam Tips | 总结与考试技巧
This case study illustrates how SQA Engineering integrates systems thinking, electronics, mechanics, and programming. When approaching similar questions, always identify sub-systems, draw a clear block diagram, calculate relevant parameters (energy, force, voltage), and discuss safety and real-world constraints. Use precise terminology: hysteresis, ADC, H-bridge, fail-safe, structural torque, risk assessment. Show all working for calculations and use appropriate units.
本案例展示了SQA工程学如何融合系统思维、电子、力学和编程。在应对类似问题时，请始终识别子系统，绘制清晰的框图，计算相关参数（能量、力、电压），并讨论安全性与现实约束。使用精确术语：迟滞、ADC、H桥、故障安全、结构扭矩、风险评估。计算时展示所有步骤并使用适当单位。
Time management in exams is critical. Spend 5–10 minutes analysing the brief, listing requirements, then draft your answer logically. If a programming question appears, present a simple flowchart or pseudocode. Practice with past paper case studies to build confidence.
考试中的时间管理至关重要。花5–10分钟分析题目、列出需求，然后逻辑清晰地组织答案。若出现编程题，给出简单的流程图或伪代码。通过历年真题中的案例分析练习来积累信心。
Published by TutorHao | Engineering Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图