← IGCSE Computer Science Paper 1

Feedback in an automated system

When an actuator's effect changes the next sensor reading, so the system keeps adjusting itself until the stored value is reached, e.g. a heater warms the room until the target.

Part of: IGCSE Computer Science 0478 — 6.1 Automated Systems — Module 8, QCE Digital Solutions Unit 3 — Interactions Between Users, Data and Digital Systems — Module 81

Understanding Feedback in an automated system in IGCSE Computer Science Paper 1

What it does: The actuator's effect (heat) changes the next sensor reading. This is the heart of feedback. The system stops adjusting once the stored value is reached, so the room does not keep getting hotter. When the reading reaches the stored value, the actuator is switched off. Repeating the loop lets the system react whenever the room cools down or warms up.

Common mistakes: The heater should switch on when the temperature is BELOW the stored value. With > the heater would switch on when the room is already warm, so feedback would never bring it up to the target. It should be IF temp < 20 THEN. The program must take a fresh reading from the sensor. Setting temp to a fixed number means the effect of the heater is never seen, so there is no feedback. It should be temp ← READ_SENSOR().

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Learning

Duo

A heater switches on and the room warms up. What happens next in a feedback system?

The sensor ignores the room until the heater is switched off by handThe sensor takes a new reading that shows the higher temperature

Matches

Loop 1: 17 | Loop 2: 17 | Loop 3: 17 (heater ON)

↓

The heater is on but the reading never changes. With feedback, the actuator's effect should change the next sensor reading, so the temperature should rise each loop.

Applying

Meanings

Term

feedback

Meaning

When an actuator's effect changes the next sensor reading, so the system keeps adjusting itself until the stored value is reached, e.g. a heater warms the room until the target.

Practice

WHILE TRUE

Testing

Gaps

WHILE

Check Answer

Spots

2 temp ← READ_SENSOR()

3 IF temp > 20 THEN

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Quick Questions

1. A heater switches on and the room warms up. What happens next in a feedback system?

A. The sensor takes a new reading that shows the higher temperature ✓

B. The sensor ignores the room until the heater is switched off by hand

The actuator's effect changes the next sensor reading. That change is the feedback.

2. Which statement describes feedback correctly?

A. The actuator's effect changes the next sensor reading ✓

B. The sensor changes the stored value after every reading

Feedback links the actuator's output back to the sensor's next input. The stored value stays the same.

3. The stored value is 22°C. The sensor reads 22°C. What does the microprocessor do?

A. Keeps the heater on until the sensor reads 30°C

B. Switches the heater off because the target is reached ✓

When the reading reaches the stored value, the actuator is switched off.

4. Why does the sensor-microprocessor-actuator loop repeat continuously?

A. So the heater always stays on after the first reading

B. So the system can keep checking and adjusting as conditions change ✓

Repeating the loop lets the system react whenever the room cools down or warms up.

5. Stored value: 20°C. Loop 1 reads 15°C and the heater is on. Loop 2 reads 18°C. What should the heater do?

A. Stay on, because 18°C is still below 20°C ✓

B. Switch off, because the temperature has risen

The heater stays on until the reading reaches the stored value. Rising alone is not enough.

Match the Pairs

IF temp < 20 THEN heater ← ON → The microprocessor compares the sensor reading with the stored value and switches the actuator on when the room is too cold.

WHILE true: read sensor again → The loop repeats continuously, so the next reading shows the effect the actuator has had.

The heater is on and warms the room, so the next temperature reading is higher than the last one. → heater ON: 17 -> 19 -> 21

When the reading reaches the target, the microprocessor switches the actuator off. → IF temp >= 21 THEN heater ← OFF

Loop 1: 17 | Loop 2: 17 | Loop 3: 17 (heater ON) → The heater is on but the reading never changes. With feedback, the actuator's effect should change the next sensor reading, so the temperature should rise each loop.

Read sensor once, then heater ON forever → The sensor is read only once and never checked again, so there is no feedback. The system cannot notice the target has been reached or switch the actuator off.

Key Term

feedback

When an actuator's effect changes the next sensor reading, so the system keeps adjusting itself until the stored value is reached, e.g. a heater warms the room until the target.

Fill in the Blanks

1. WHILE TRUE temp ← ReadSensor() IF temp < 20 THEN heater ← ON

Hint: Type the value that keeps the loop repeating continuously so the system keeps checking the sensor.

2. temp ← ReadSensor() IF temp < 20 THEN heater ← ON

Hint: Type the comparison operator that checks whether the room is colder than the stored value.

3. IF temp >= 20 THEN heater ← OFF ENDIF

Hint: Type the state the actuator is set to once the target has been reached.

4. heater ← ON WAIT 1 temp ← ReadSensor()

Hint: Type the function name that takes the next sensor reading after the heater has changed the room.

5. REPEAT temp ← ReadSensor() UNTIL temp >= target

Hint: Type the variable name that holds the stored value the reading is compared with.

Spot the Error

Question 1

A WHILE TRUE

B temp ← READ_SENSOR()

C IF temp > 20 THEN

D heater ← ON

E ELSE

F heater ← OFF

Line C — The heater should switch on when the temperature is BELOW the stored value. With > the heater would switch on when the room is already warm, so feedback would never bring it up to the target. It should be IF temp < 20 THEN.

Question 2

A target ← 22

B temp ← 22

C IF temp < target THEN

D heater ← ON

Line B — The program must take a fresh reading from the sensor. Setting temp to a fixed number means the effect of the heater is never seen, so there is no feedback. It should be temp ← READ_SENSOR().

Question 3

A FOR i ← 1 TO 1

B temp ← READ_SENSOR()

C IF temp >= 22 THEN

D heater ← OFF

Line A — A feedback loop must repeat continuously so each new sensor reading can be checked. A loop that runs only once cannot keep adjusting. The first line should be WHILE TRUE.

Question 4

A WHILE temp > 22

B heater ← ON

C temp ← READ_SENSOR()

D ENDWHILE

E heater ← OFF

Line A — The loop should keep running while the temperature is still below the stored value, so the heater stays on until the target is reached. It should be WHILE temp < 22.

Question 5

A temp ← READ_SENSOR()

B IF temp <= target THEN

C heater ← OFF

D ELSE

E heater ← ON

Line B — The actuator should switch off once the target is reached or passed, so the test needs >=. With <= the heater would switch off when the room is cold. It should be IF temp >= target THEN.

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