E21: Control Systems & Microcontrollers
Open and closed loop control, microcontroller basics, PIC and Arduino applications
Control Systems & Microcontrollers
Open and closed loop control, microcontroller basics, PIC and Arduino applications
Key Fact: An open-loop control system has no feedback; the output is not checked against the desired value, so errors are not corrected automatically.
Key Fact: A closed-loop (feedback) control system measures the output and compares it with the set point, adjusting the input to reduce any error.
Key Fact: Closed-loop control uses a sensor to measure the output, a comparator to find the error, and a controller to correct it.
Key Fact: A microcontroller is a single-chip computer containing a processor, memory and input/output peripherals for embedded control.
Key Fact: ADC (Analogue-to-Digital Converter) converts a sensor's analogue output into a digital value the microcontroller can process.
Key Fact: DAC (Digital-to-Analogue Converter) converts a digital value from the microcontroller into an analogue voltage to drive actuators.
Key Fact: PWM (Pulse Width Modulation) controls the average power delivered to a load by varying the duty cycle of a digital pulse stream.
Key Fact: A PIC microcontroller (e.g. PIC16F) is a family of low-cost microcontrollers commonly used in education and simple control projects.
Key Fact: Arduino is an open-source microcontroller platform with a simplified programming environment (C/C++), widely used for prototyping.
Key Fact: Microcontroller inputs can be digital (switch, logic signal) or analogue (via ADC); outputs can drive LEDs, relays, motors or displays.
Key Fact: A control program typically reads inputs, processes the data (comparing with thresholds or using algorithms) and updates outputs in a continuous loop.
Key Fact: Hysteresis in a control system (e.g. a thermostat) prevents rapid on/off cycling by having different turn-on and turn-off thresholds.
📋 Key Vocabulary and Concepts
For Control Systems & Microcontrollers, you must know:
- Open-loop control: A control system with no feedback; the controller applies an input without checking the actual output.
- Closed-loop control: A control system that measures the output and feeds it back to compare with the set point, correcting errors automatically.
- Microcontroller: A compact integrated circuit containing a processor, memory and I/O peripherals, designed for embedded control applications.
- ADC: Analogue-to-Digital Converter; a peripheral that converts an analogue voltage into a digital number for processing.
- PWM: Pulse Width Modulation; a technique for controlling average power by varying the proportion of on-time in a digital pulse.
- Feedback: The return of output information to the input of a system to allow comparison with the desired value and error correction.
❓ Practice Questions
Q: Give one advantage and one disadvantage of open-loop control.
Q: Describe the three essential components of a closed-loop control system.
Q: Explain how PWM can be used to control the brightness of an LED.
Q: What is the role of an ADC in a microcontroller-based sensor system?
Q: Why might a thermostat use hysteresis instead of a single temperature threshold?
✅ Answers
- Advantage: simple and low cost. Disadvantage: cannot correct for disturbances or changes in the system since there is no feedback.
- A sensor to measure the output, a comparator to calculate the error between the output and set point, and a controller to adjust the input to reduce the error.
- A PWM signal rapidly switches the LED on and off; increasing the duty cycle (on-time proportion) increases the average current and brightness, while decreasing it dims the LED.
- The ADC converts the analogue voltage from a sensor into a digital number that the microcontroller can read, process and use in its control program.
- Hysteresis creates separate turn-on and turn-off temperatures, preventing the heating from rapidly switching on and off when the temperature is near the set point.
🎯 Exam Tips
- Always draw a block diagram for control system questions, showing sensor, comparator, controller and actuator with feedback path.
- Distinguish clearly between open-loop (no feedback) and closed-loop (feedback with error correction).
- For microcontroller questions, mention the specific peripherals needed: ADC for analogue sensors, PWM for motor/LED control.
- When describing a control program, outline the loop structure: read inputs → process data → update outputs → repeat.
- Explain the purpose of each component in a block diagram rather than just naming it.
📝 Exam Technique
GCSE Electronics Exam Tips — Control Systems & Microcontrollers:
1. For Control Systems & Microcontrollers questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Control Systems & Microcontrollers, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
1. For Control Systems & Microcontrollers questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Control Systems & Microcontrollers, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
⚠️ Common Errors
✗ Describing a closed-loop system as having no feedback. ✓ A closed-loop system uses feedback from the output to correct errors; open-loop systems have no feedback.
✗ Confusing ADC and DAC functions. ✓ An ADC converts analogue signals to digital (for reading sensors); a DAC converts digital to analogue (for driving actuators).
✗ Assuming a microcontroller can read analogue sensors directly without an ADC. ✓ Analogue sensor voltages must be converted to digital by the ADC peripheral before the microcontroller processor can use them.
✗ Stating that PWM changes the amplitude of the output voltage. ✓ PWM changes the duty cycle (on-time proportion), not the amplitude; the average power is controlled by varying how long the signal is high.
✍️ Model Answer
Full-Mark Response
Compare open-loop and closed-loop control systems, and explain how a microcontroller can implement a closed-loop temperature control system.
An open-loop control system applies an input to a process without checking the result; it is simple and cheap but cannot correct for disturbances or component variations. A closed-loop system measures the actual output using a sensor, compares it with the desired set point using a comparator, and adjusts the controller output to reduce the error. This feedback makes closed-loop systems more accurate and robust, though more complex. A microcontroller can implement closed-loop temperature control as follows: a temperature sensor (e.g. thermistor in a potential divider) produces an analogue voltage proportional to temperature. The microcontroller's ADC converts this to a digital value. The control program compares the reading with the set point stored in software. If the temperature is below the set point, the microcontroller turns on a heater (via a transistor or relay); if above, it turns the heater off. Adding hysteresis (separate on and off thresholds) prevents rapid cycling. PWM can provide proportional control rather than simple on/off, adjusting the heater power according to the error magnitude. The program runs in a continuous loop, reading the sensor, calculating the error and updating the output to maintain the desired temperature.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of control systems & microcontrollers, including electronic components, circuit theory and systems concepts relevant to WJEC Eduqas C690QS.
AO2 (Application): Apply knowledge and understanding of control systems & microcontrollers to analyse, design and construct electronic circuits and systems.
AO3 (Evaluation): Evaluate electronic circuits and systems, making reasoned judgements about design choices, performance and practical considerations, constructing supported arguments.
📝 Exam Questions by Topic
🎬 Video Resources
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