Sensors, microprocessor and actuator in a control system

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Quick Questions

Shade in the A or B circle for each question.

1. In a greenhouse control system, which device compares the sensor reading with the stored value?

A The actuator

B The microprocessor

2. Which statement about an actuator in a control system is correct?

A It acts when the microprocessor signals it, for example by turning on a heater

B It measures the temperature and sends the readings to the microprocessor

3. A temperature sensor produces an analogue signal. What must happen before the microprocessor can use it?

A A DAC converts it to digital data

B An ADC converts it to digital data

4. In a central heating system, the room is 15°C and the stored value is 20°C. What does the microprocessor do?

A Signals the heater to switch on

B Signals the heater to switch off

5. How often does the sensor, microprocessor and actuator process repeat in a feedback loop?

A Continuously

B Only once, when the system is switched on

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1WHILE TRUE		AThe program runs once and stops. There is no loop, so the system never checks the sensor again and the feedback loop is broken.
2IF temp < 20 THEN heater ← ON		BIF temp >= 25 THEN heater ← OFF
3The sensor reading is analogue, so an ADC converts it to digital before the microprocessor can use it		CKeeps the control loop running continuously so the sensor is read again and again
4The microprocessor compares the reading with a stored value and signals the actuator to turn off when the target is reached		DThe order is wrong. The sensor sends readings to the microprocessor, not straight to the actuator. The microprocessor then signals the actuator.
5Sensor → Actuator → Microprocessor		EThe microprocessor compares the sensor reading with the stored value and switches the actuator on
6temp ← ADC(sensor) IF temp < 20 THEN heater ← ON END		Ftemp ← ADC(sensor)

Definitions

feedback loop

Fill in the Blanks

Complete each line of code by filling in the missing parts.

WHILE ____ DO temp ← READ_SENSOR() ENDWHILE

IF temp _ 20 THEN heater ← ON ENDIF

IF temp > setValue THEN heater ← ___ ENDIF

digital ← ___(analogue) IF digital < setValue THEN heater ← ON

IF temp < ________ THEN heater ← ON ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A WHILE FALSE DO

B t ← READ_SENSOR()

C IF t < 20 THEN heater ← ON

D ENDWHILE

Question 2

A WHILE TRUE DO

B t ← READ_SENSOR()

C IF t > 18 THEN

D heater ← ON

E ENDIF

F ENDWHILE

Question 3

A raw ← READ_SENSOR()

B temp ← DAC(raw)

C IF temp < 18 THEN

D heater ← ON

E ENDIF

Question 4

A setPoint ← 22

B t ← setPoint

C IF t < setPoint THEN

D heater ← ON

E ENDIF

Question 5

A WHILE TRUE DO

B t ← READ_SENSOR()

C IF t > 30 THEN

D sensor ← ON

E ENDIF

F ENDWHILE

Answer Key

Sensors, microprocessor and actuator in a control system

Section A: Quick Questions

B — The microprocessor

A — It acts when the microprocessor signals it, for example by turning on a heater

B — An ADC converts it to digital data

A — Signals the heater to switch on

A — Continuously

Section B: Match the Pairs

WHILE TRUE → Keeps the control loop running continuously so the sensor is read again and again

IF temp < 20 THEN heater ← ON → The microprocessor compares the sensor reading with the stored value and switches the actuator on

The sensor reading is analogue, so an ADC converts it to digital before the microprocessor can use it → temp ← ADC(sensor)

The microprocessor compares the reading with a stored value and signals the actuator to turn off when the target is reached → IF temp >= 25 THEN heater ← OFF

Sensor → Actuator → Microprocessor → The order is wrong. The sensor sends readings to the microprocessor, not straight to the actuator. The microprocessor then signals the actuator.

temp ← ADC(sensor) IF temp < 20 THEN heater ← ON END → The program runs once and stops. There is no loop, so the system never checks the sensor again and the feedback loop is broken.

Section C: Definitions

feedback loop — A sensor sends readings to a microprocessor, which compares them with a stored value and signals an actuator, such as a heater, to act. This repeats continuously.

Section D: Fill in the Blanks

WHILE TRUE DO
  temp ← READ_SENSOR()
ENDWHILE

IF temp < 20 THEN
  heater ← ON
ENDIF

IF temp > setValue THEN
  heater ← OFF
ENDIF

digital ← ADC(analogue)
IF digital < setValue THEN
  heater ← ON

IF temp < setValue THEN
  heater ← ON
ENDIF

Section E: Spot the Error

A — A control system repeats continuously in a feedback loop. WHILE FALSE means the loop never runs, so the sensor is never read. It should be WHILE TRUE DO.

C — The heater should switch on when the temperature is below the stored value. t > 18 would switch it on when the greenhouse is already warm. It should be IF t < 18 THEN.

B — Sensor readings are analogue, so the microprocessor needs an ADC (analogue-to-digital converter). A DAC converts the other way. It should be temp ← ADC(raw).

B — The reading must come from the sensor. Copying the stored value means the comparison never reflects the real temperature. It should be t ← READ_SENSOR().

D — The microprocessor signals an actuator, such as a fan, to act. A sensor only inputs data and cannot cool the greenhouse. It should be fan ← ON.