← QCE Digital Solutions Unit 3

Control mechanisms: open-loop and closed-loop

Decides what a system does next. Open loop ignores the result (a timer); closed loop uses a feedback signal from the result (a thermostat).

Part of: QCE Digital Solutions Unit 3 — Interactions Between Users, Data and Digital Systems — Module 15

Understanding Control mechanisms: open-loop and closed-loop in QCE Digital Solutions Unit 3

What it does: Reading the sensor gives the controller the feedback signal it needs to check the result. The heater runs for a fixed 600 seconds then stops, whatever the room temperature is. No feedback is used. A feedback signal returns data about the output (such as the current temperature) so the control mechanism can decide what to do next. Closed loop uses feedback from the result. A timer on its own is the classic open-loop approach. A closed-loop thermostat must compare a sensor reading with the target and then choose to turn the heater on or off.

Common mistakes: A thermostat reads the temperature with a sensor and uses it to decide what to do, so it is a closed loop. The comment should say 'Closed loop: thermostat'. An open-loop timer switches the pump on, waits, then switches it off. Line 4 repeats pump_on(), so the pump never stops. It should be pump_off().

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This module's key terms are also in a printable crossword (Modules 15–119) and word search (Modules 15–176).

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Learning

Duo

A heater runs for exactly 30 minutes and then switches off, no matter how warm the room is. Which type of control mechanism is this?

Closed loopOpen loop

Matches

Thermostat (closed loop): heater.on() for 10 minutes, then heater.off()

↓

This is open loop, not closed loop. It runs for a fixed time and never reads the temperature, so there is no feedback signal.

Applying

Meanings

Term

control mechanism

Meaning

Decides what a system does next. Open loop ignores the result (a timer); closed loop uses a feedback signal from the result (a thermostat).

Practice

LOOP

Testing

Gaps

Check Answer

Spots

1# Open loop: thermostat

2while True:

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Quick Questions

1. A heater runs for exactly 30 minutes and then switches off, no matter how warm the room is. Which type of control mechanism is this?

A. Open loop ✓

B. Closed loop

A timer-based heater does not check the result (the room temperature), so it is open loop.

2. A thermostat compares the room temperature with the target and switches the heater on or off. Which type of control mechanism is this?

A. Closed loop ✓

B. Open loop

The thermostat uses the room temperature as a feedback signal to decide what to do next, so it is closed loop.

3. Which statement about a feedback signal is correct?

A. It sends information about the result back to the control mechanism ✓

B. It tells the system to ignore the result and keep going

A feedback signal returns data about the output (such as the current temperature) so the control mechanism can decide what to do next.

4. Which system is open loop?

A. A sprinkler that stops when a soil moisture sensor reads wet enough

B. A sprinkler that waters the garden for 10 minutes every morning ✓

The timed sprinkler never checks the result (soil moisture). The sensor-based sprinkler uses feedback, so it is closed loop.

5. What does a closed-loop control mechanism have that an open-loop one does not?

A. A timer that runs for a set time

B. A feedback signal from the result ✓

Closed loop uses feedback from the result. A timer on its own is the classic open-loop approach.

Match the Pairs

timer.run(10) → An open-loop control: the heater runs for a set time and never checks the result

if temp < target: heater.on() → A closed-loop control: it uses the temperature reading as a feedback signal to decide what to do next

A feedback signal is a measurement of the result that is sent back to the controller → temp = sensor.read()

An open-loop system acts for a set time or amount and does not check the result → heater.on() and wait(600) and heater.off()

Thermostat (closed loop): heater.on() for 10 minutes, then heater.off() → This is open loop, not closed loop. It runs for a fixed time and never reads the temperature, so there is no feedback signal.

Timer (open loop): if sensor.read() > 25: fan.off() → A timer is open loop, but this code uses a sensor reading as feedback, which makes it closed loop.

Key Term

control mechanism

Decides what a system does next. Open loop ignores the result (a timer); closed loop uses a feedback signal from the result (a thermostat).

Fill in the Blanks

1. heater = ON WAIT 10 heater = OFF

Hint: Type the keyword that pauses for a fixed time without checking anything. This is how a timer (open loop) works.

2. reading = readSensor() IF reading < target THEN heater = ON

Hint: Type the variable holding the desired value that the feedback reading is compared with, as a thermostat (closed loop) does.

3. temp = readSensor() IF temp < target THEN heater = ON

Hint: Type the comparison operator that checks whether the feedback reading is below the target.

4. WHILE temp < target DO heater = ON temp = readSensor()

Hint: Type the function name that collects the feedback signal from the sensor each time round the loop.

5. temp = readSensor() error = target - temp IF error > 0 THEN heater = ON

Hint: Type the variable holding the desired value that the feedback signal is subtracted from to find the error.

Spot the Error

Question 1

A # Open loop: thermostat

B while True:

C temp = read_sensor()

D if temp < 20: heater_on()

Line A — A thermostat reads the temperature with a sensor and uses it to decide what to do, so it is a closed loop. The comment should say 'Closed loop: thermostat'.

Question 2

A timer = 600

B pump_on()

C sleep(timer)

D pump_on()

Line D — An open-loop timer switches the pump on, waits, then switches it off. Line 4 repeats pump_on(), so the pump never stops. It should be pump_off().

Question 3

A target = 22

B temp = target

C if temp < target:

D heater_on()

Line B — A closed loop needs a feedback signal from the real world. Setting temp to target means the heater never sees the actual temperature. It should be temp = read_sensor().

Question 4

A temp = read_sensor()

B if temp > 20:

C heater_on()

D else:

E heater_off()

Line B — The thermostat should turn the heater on when the feedback shows the room is too cold. The test should be temp < 20, not temp > 20.

Question 5

A target = 22

B temp = read_sensor()

C error = target - temp

D adjust(target)

Line D — In a closed loop the adjustment uses the feedback (the error between target and measured value). Passing only target ignores the result. It should be adjust(error).

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