M&C

Monitoring and Control Systems

Understanding sensors, actuators, monitoring, control systems, and feedback loops

Learning Objectives

By the end of this lesson, you will be able to:

  • Explain the difference between monitoring and control systems
  • Describe how sensors work and identify different types of sensors
  • Explain the purpose and function of actuators
  • Understand the role of ADC (Analogue to Digital Converter) and DAC (Digital to Analogue Converter)
  • Explain the importance of feedback in control systems
  • Describe real-world applications of monitoring and control systems
  • Understand how closed-loop feedback systems operate

Key Terms

Sensor

Input device that reads or measures physical properties (temperature, pressure, etc.)

Actuator

Output device that converts electrical signals into physical movement

ADC

Analogue to Digital Converter - converts analogue sensor data to digital format

DAC

Digital to Analogue Converter - converts digital signals to analogue for actuators

Monitoring System

System that watches or monitors external conditions without controlling them

Control System

System that monitors conditions AND controls processes using actuators

Feedback

Process where system output affects subsequent input to maintain desired conditions

Closed-loop System

Control system where feedback directly controls operation

Transducer

Device that converts one form of energy to another (e.g., light bulb, microphone)

Thermocouple

Temperature sensor that outputs electrical voltage changing with temperature

Analogue Data

Continuously changing data without discrete values (from sensors)

Digital Data

Discrete values that computers can process (after ADC conversion)

Sensors and Actuators

Sensors and actuators are fundamental components in monitoring and control systems. Sensors are input devices that read physical properties, while actuators are output devices that create physical movement based on computer signals.

Sensors: The "Eyes and Ears"

Sensors measure physical properties like temperature, pressure, light, and sound. The data they produce is analogue - constantly changing without discrete values.

Real-Life Example: Smartphone Sensors

Your smartphone contains multiple sensors:

  • Accelerometer: Measures movement and orientation
  • Light sensor: Adjusts screen brightness automatically
  • Proximity sensor: Turns screen off during calls
  • Microphone: Detects sound for voice commands

Actuators: The "Hands and Feet"

Actuators accept signals from computers and turn them into physical movement. They are also called output transducers because they convert electrical energy into other forms.

Real-Life Example: Car Systems

Modern cars use many actuators:

  • Electric motors: Power windows, windshield wipers
  • Solenoids: Control fuel injection
  • Hydraulic actuators: Operate brakes
  • Servo motors: Adjust mirrors automatically

ADC and DAC: The Language Translators

ADC: Analogue to Digital Converter

Since computers can only understand digital data, analogue sensor readings must be converted. The ADC performs this conversion:

  • Takes continuous analogue signals from sensors
  • Converts them into discrete digital values
  • Makes data understandable to computer processors
Analogy:

Like translating a continuous melody (analogue) into sheet music with specific notes (digital).

DAC: Digital to Analogue Converter

Actuators often need analogue signals to operate. The DAC converts digital computer signals back to analogue:

  • Takes digital signals from computer
  • Converts them to continuous analogue signals
  • Provides proper signals for actuators to operate
Analogy:

Like converting digital music files (MP3) back into sound waves for speakers to play.

Sensor Types Visualization

Temperature

Thermocouple

Pressure

Pressure sensor

Light

Light sensor

Motion

Infra-red sensor

Sound

Acoustic sensor

Common Sensor Applications: Each sensor type has specific real-world uses. Temperature sensors monitor engine heat, pressure sensors detect intruders, light sensors control street lighting, motion sensors trigger alarms, and sound sensors pick up footsteps.

Types of Sensors and Their Applications

Sensor Type Applications
Magnetic Field Anti-lock braking systems in motor vehicles
pH Monitor/control acidity/alkalinity levels in greenhouse soil; pollution monitoring in rivers
Gas Monitor pollution in rivers/air; measure O₂ and CO₂ in greenhouses; check for CO₂ leaks in power stations
Acoustic/Sound Pick up noise levels in burglar alarms; detect liquid dripping in pipes
Pressure Detect intruders in burglar alarms; monitor/control gas pressure processes
Infra-red/Motion Turn on car windscreen wipers automatically; detect intruders; count people entering buildings
Light Switch street lighting on/off; monitor/control light levels in greenhouses; automatically switch on car headlights
Thermocouple Control central heating; monitor/control chemical processes; control greenhouse temperature
Moisture/Humidity Control/monitor moisture levels in greenhouse soil

Advantages of Using Sensors

  • Readings are generally more accurate than those taken by human operators
  • Can operate continuously without breaks
  • Work in dangerous environments unsafe for humans
  • Provide consistent, error-free measurements
  • Can detect changes too small for humans to notice

Disadvantages of Using Sensors

  • Faulty sensors can give bogus results
  • Can be affected by environmental conditions (dirt, moisture)
  • Example: Dirty sensors on a car's rear bumper may not identify obstacles or give continuous false alarms
  • Require calibration and maintenance
  • Can be expensive to install and replace

Activity 1: Sensor Identification

For each scenario below, identify the most appropriate type of sensor and explain why:

  1. A system that automatically turns on street lights when it gets dark
  2. A greenhouse system that maintains optimal temperature for plants
  3. A burglar alarm that detects footsteps in a building
  4. A car system that prevents wheels from locking during hard braking
  5. A river monitoring system that checks pollution levels
Solution:
  1. Light sensor - Detects light levels to determine when it's dark enough to require lighting
  2. Thermocouple (temperature sensor) - Measures temperature to maintain optimal growing conditions
  3. Acoustic/sound sensor - Picks up noise of footsteps to detect intruders
  4. Magnetic field sensor - Used in anti-lock braking systems to detect wheel rotation speed
  5. pH or gas sensor - Monitors acidity or gas levels to detect pollution in water

Activity 2: ADC and DAC Scenario

A temperature control system in a greenhouse uses a thermocouple (temperature sensor) and a heater (actuator). The system is controlled by a computer.

Task: Explain why both an ADC and DAC are needed in this system. Describe the data flow from sensor to actuator.

Solution:
  1. ADC is needed: The thermocouple produces analogue electrical voltage that changes with temperature. The computer can only process digital data, so the ADC converts this analogue signal into digital format.
  2. DAC is needed: The computer sends digital signals to control the heater. Since the heater requires analogue signals to operate, the DAC converts the digital signals from the computer back to analogue format.
  3. Data flow: Thermocouple (analogue) → ADC → Computer (digital processing) → DAC → Heater (analogue control)

Check Your Understanding: Sensors and Actuators

Answer
  • [1 mark] Sensor is an input device that reads or measures physical properties
  • [1 mark] Actuator is an output device that converts electrical signals into physical movement
  • [Additional] Sensors provide data to the system, actuators perform actions based on system commands
Answer
  • [1 mark] Sensors produce analogue data (constantly changing without discrete values)
  • [1 mark] Computers can only process digital data (discrete values)
  • [Additional] ADC converts analogue sensor readings to digital format that computers can understand
Answer
  • [2 marks] Electric motor - converts electricity into movement (e.g., windshield wipers)
  • [2 marks] Heater - converts electricity into heat (e.g., central heating system)
  • [Additional] Other examples: Lamp (electricity to light), Loudspeaker (electricity to sound), Valve (controls flow)
Answer
  • [1 mark] Magnetic field sensor
  • [2 marks] It detects the rotation speed of wheels to prevent them from locking up during hard braking
  • [Additional] When a wheel rotates too slowly (locking up), the sensor sends data to the microprocessor which reduces braking pressure to that wheel
Answer
Advantage:

Readings taken using sensors are generally more accurate than those taken by human operators. Sensors can operate continuously without fatigue.

Disadvantage:

Faulty sensors can give bogus results. Example: Dirty sensors on a car's rear bumper may not identify obstacles or give continuous false alarms.

Answer
  • [1 mark] A thermocouple is a temperature sensor
  • [2 marks] It outputs an electrical voltage that changes with temperature - higher temperature produces higher voltage
  • [Additional] Used in applications like central heating systems, chemical processes, and greenhouse temperature control

Monitoring vs Control Systems

Monitoring and control systems are closely related but serve different purposes. Monitoring systems only observe and record conditions, while control systems both monitor and actively adjust conditions.

Monitoring Systems

A monitoring system watches or monitors some state external to the computer system. The process is automatic with no human interaction.

  • Creates a record of system conditions over time
  • Detects when a physical property goes outside a desired range
  • Sends warnings or activates alarms
  • Does NOT control any process (no actuators involved)
Real-Life Example: Patient Monitoring

In hospitals, sensors attached to patients measure vital signs (heart rate, temperature, breathing rate). The computer compares these to preset values and sounds an alarm if anything is outside acceptable range. The system only monitors - it doesn't administer medication.

Control Systems

A control system has monitoring capability plus the ability to control a system. It requires actuators and uses feedback.

  • Monitors conditions using sensors
  • Takes action using actuators when needed
  • Uses feedback to adjust actions
  • System output affects next inputs (continuous feedback)
Real-Life Example: Central Heating

A thermostat monitors room temperature. When temperature drops below the set point, it turns the heater on. When temperature reaches the desired level, it turns the heater off. This is a control system with feedback.

How Monitoring and Control Systems Work

Step 1: Sensors take readings

Step 2: ADC converts to digital

Step 3: Microprocessor processes data

Step 4: Warning or control action

Monitoring System Control System
Only observes and records conditions Observes AND adjusts conditions
No actuators involved Uses actuators to control processes
Output does not affect input Output affects input (continuous feedback)
Sends warnings or activates alarms Takes corrective actions automatically
Example: Patient monitoring in hospital Example: Central heating system

Examples of Monitoring vs Control Applications

Monitoring Applications Control Applications
Monitoring a patient in hospital for vital signs Turning street lights on at night and off during daylight
Checking for intruders in a burglar alarm system Controlling temperature in central heating/air conditioning
Checking temperature levels in a car engine Controlling traffic lights at a road junction
Monitoring pollution levels in a river Operating anti-lock brakes on a car when necessary
Controlling the environment in a greenhouse

Advantages of Control Systems

Fast Response

Computers can respond very rapidly to changes in conditions

24/7 Operation

Systems can run 24 hours a day, 365 days a year without breaks

Dangerous Environments

Can operate in places humans would find dangerous or awkward

Consistent Results

Outputs are consistent and error-free compared to human operators

Fast Processing

Computers can process data quickly and machines can operate faster than humans

Automation

Reduces need for human intervention and minimizes human error

Monitoring System Steps

  1. Sensors continuously take readings and send to processor
  2. Analogue sensor readings are converted to digital using ADC
  3. Microprocessor compares sensor reading to stored pre-set value
  4. If data is outside acceptable range, a warning message is sent or alarm activated
  5. Microprocessor has no effect on what is being monitored - it simply "watches"

Control System Steps

  1. Sensors continuously take readings and send to processor
  2. Analogue sensor readings are converted to digital using ADC
  3. Microprocessor compares sensor reading to stored pre-set value
  4. If data is outside acceptable range, microprocessor sends signals to control devices
  5. Output from system affects next inputs from sensors (feedback)
  6. Computer sends digital signal to specific actuator (may need DAC)
  7. Actuator turns device on/off (heater, motor, valve, etc.)

Activity 3: Patient Monitoring System

A patient monitoring system in a hospital works as follows:

  • Sensors are attached to patient to measure temperature, heart rate, breathing rate
  • Sensors constantly send data back to computer system
  • Range of acceptable values for each parameter is preset in the computer
  • Computer microprocessor compares values from sensors with preset values
  • If anything is outside acceptable range, computer sends signal to sound alarm
  • If data is within range, values are shown in graphical form on screen

Task: 1. Is this a monitoring or control system? Explain why.
2. What happens if the heart rate sensor reads 40 bpm (normal range: 60-100 bpm)?
3. Why are sensors attached to computer system rather than monitored by nurses?

Solution:
  1. Monitoring system - It only observes and warns about patient conditions but does not take any action to change those conditions (no actuators involved).
  2. Alarm activation - Since 40 bpm is below the normal range (60-100 bpm), the computer would send a signal to sound an alarm to alert medical staff.
  3. Advantages of computerized monitoring:
    • Continuous monitoring without breaks
    • More accurate than human observation
    • Immediate detection of abnormal values
    • Frees nurses for other tasks
    • Consistent monitoring 24/7

Activity 4: Street Lighting Control

A street lighting system uses a microprocessor to control operation:

  • Lamp is fitted with light sensor which constantly sends data to microprocessor
  • Data value changes according to weather (sunny, cloudy, raining, night)
  • Light sensor sends data to ADC which converts analogue to digital
  • Microprocessor samples data every minute
  • If data from sensor is less than preset value stored in memory, lamp switches on
  • If data from sensor is greater than preset value, lamp switches off

Task: 1. Is this a monitoring or control system?
2. What is the role of the ADC in this system?
3. Why does the system sample data every minute rather than continuously?

Solution:
  1. Control system - It both monitors light levels AND controls the street lamp (turns it on/off) using the microprocessor and switching mechanism (actuator).
  2. ADC role - Converts the analogue signal from the light sensor (continuously changing light levels) into digital format that the microprocessor can process and compare with the preset value.
  3. Sampling rationale:
    • Saves processing power and energy
    • Light levels change relatively slowly (not like heart rate)
    • Prevents rapid on/off cycling during transitional periods (dusk/dawn)
    • Minute-by-minute checking is sufficient for street lighting

Check Your Understanding: Monitoring vs Control

Answer
  • [1 mark] Monitoring system only observes and records conditions without controlling them
  • [1 mark] Control system both monitors conditions AND takes action to control them
  • [1 mark] Control systems use actuators while monitoring systems do not
  • [Additional] Control systems have feedback loops where output affects input
Answer
  • [1 mark] Computers can respond very rapidly to change
  • [1 mark] Systems can run 24 hours a day, 365 days a year
  • [1 mark] Can operate in dangerous environments unsafe for humans
  • [Additional] Outputs are consistent and error-free; computers process data quickly
Answer
  1. [1 mark] System activated by keying in password on keypad
  2. [1 mark] Sensors (infra-red, acoustic, pressure) pick up movement, sounds, or weight
  3. [1 mark] Sensor data converted to digital via ADC, microprocessor compares with preset values
  4. [1 mark] If data outside acceptable range, activates siren or flashing lights
  5. [Additional] Continues until system reset with password
Answer
  • [1 mark] When human judgment is required for decisions (e.g., medical diagnosis)
  • [1 mark] When consequences of automatic control could be dangerous if system fails
  • [Additional] When only observation/data collection is needed without intervention
Answer
  • [1 mark] ADC converts analogue sensor data to digital for computer processing
  • [1 mark] DAC converts digital computer signals to analogue for actuator operation
  • [1 mark] Computers work with digital data, sensors produce analogue, actuators often need analogue
  • [Additional] Acts as a "translator" between analogue and digital domains
Answer
  • [1 mark] Define the acceptable/desired range for the measured property
  • [1 mark] Provide reference values for comparison with sensor readings
  • [Additional] Determine when to trigger alarms (monitoring) or take action (control)

Feedback and Closed-Loop Systems

Feedback is essential in control systems. It enables the system output to affect subsequent system input, allowing conditions to be automatically adjusted to meet given criteria.

Closed-Loop Feedback Systems

What is a Closed-Loop System?

A closed-loop feedback control system is a special type where feedback directly controls operation. A microprocessor functions as the controller.

Desired Value

Controller

Actuator

Process

Actual Output

Feedback Loop

Sensor

The controller compares the value for actual output (as read by sensor) with the desired output. It then transmits a value to the actuator which depends on the difference calculated.

Example: Clothes Dryer Closed-Loop System

Desired Dryness (Input) Controller Actuator Process Actual Dryness (Output)
Set dryness level on control panel Compares actual vs desired dryness Heating elements Heat applied to clothes Measured by moisture sensor

Feedback Loop: Moisture sensor measures actual dryness → sends to controller → compares with desired dryness → adjusts heating elements → continues until desired dryness achieved.

Real Example: Anti-Lock Braking System (ABS)

Anti-lock braking systems on cars use magnetic field sensors to stop wheels locking up if brakes are applied too sharply.

How ABS Works:
  1. When a wheel rotates too slowly (locking up)
  2. Magnetic field sensor sends data to microprocessor
  3. Microprocessor checks rotation speed of other three wheels
  4. If they are different (rotating faster), sends signal to braking system
  5. Braking pressure to affected wheel is reduced
  6. Wheel's rotational speed increases to match other wheels
Key Features:
  • Checks rotational speed several times per second
  • Braking pressure constantly adjusted to prevent locking
  • Creates "judder" feeling on brake pedal as system switches on/off
  • If wheel rotates too quickly, braking pressure increased
  • Equalizes rotational speed of all four wheels
Why This is a Closed-Loop System:

Feedback loop: Wheel speed (output) → measured by sensors → compared to desired speed by microprocessor → braking pressure adjusted → affects wheel speed (back to start). Continuous adjustment maintains optimal braking.

Importance of Feedback

Accuracy

Ensures system operates within given criteria by continuously comparing actual vs desired output

Self-Correction

Automatically adjusts conditions without human intervention when deviations occur

Stability

Maintains consistent performance despite external disturbances or changes

Open-Loop Systems (No Feedback)

  • No measurement of actual output
  • No comparison with desired output
  • No automatic correction
  • Example: Simple timer-based street light (turns on/off at set times regardless of actual light level)
  • Less accurate, cannot adapt to changes

Closed-Loop Systems (With Feedback)

  • Continuously measures actual output
  • Compares with desired output
  • Automatically corrects deviations
  • Example: Light-sensor street light (turns on when dark, off when light)
  • More accurate, adapts to changing conditions

Activity 5: ABS Feedback Analysis

Analyze the anti-lock braking system as a closed-loop feedback system:

  • Magnetic field sensors monitor wheel rotation speed
  • Microprocessor checks if any wheel is rotating significantly slower than others
  • If wheel is locking up (rotating too slowly), microprocessor reduces braking pressure to that wheel
  • System checks rotational speed several times per second
  • Braking pressure constantly adjusted to equalize wheel speeds

Task: 1. Identify the sensor, controller, actuator, and process in this system.
2. Describe the feedback loop in your own words.
3. Why is this checking done several times per second?

Solution:
  1. System components:
    • Sensor: Magnetic field sensors
    • Controller: Microprocessor
    • Actuator: Braking pressure control system
    • Process: Wheel rotation/braking
  2. Feedback loop: Wheel rotation speed (output) measured by sensors → data sent to microprocessor → compared to desired rotation speed (similar to other wheels) → if wheel locking up, signal sent to reduce braking pressure → wheel speed increases → continues monitoring and adjusting.
  3. Rapid checking needed because:
    • Braking happens very quickly (milliseconds)
    • Wheels can lock up almost instantly during hard braking
    • Need continuous adjustment to maintain control
    • Prevents skidding and maintains steering control

Activity 6: Designing a Greenhouse System

Design a closed-loop control system for a greenhouse that maintains:

  • Temperature between 20-25°C
  • Light levels sufficient for plant growth
  • Soil moisture at optimal level

Task: 1. List the sensors needed for each parameter.
2. Describe the actuators that would control each condition.
3. Draw a simple feedback loop diagram showing how the system would work.
4. Explain why this needs to be a closed-loop system rather than just monitoring.

Solution:
  1. Sensors needed:
    • Temperature: Thermocouple
    • Light: Light sensor
    • Soil moisture: Moisture/humidity sensor
  2. Actuators:
    • Temperature control: Heater (for low temp), Ventilation fan/cooling system (for high temp)
    • Light control: Artificial grow lights (for low light), Shading system (for excessive light)
    • Moisture control: Water pump/irrigation system (for dry soil)
  3. Feedback loop diagram:
    Desired conditions → Microprocessor (compares with sensor readings) → Actuators (adjust environment) → Greenhouse environment → Sensors (measure actual conditions) → Back to microprocessor
  4. Why closed-loop: Plants need continuous optimal conditions for growth. Manual adjustment would be inefficient and inconsistent. Closed-loop system automatically maintains ideal environment 24/7, responding immediately to changes (weather, time of day), ensuring maximum plant growth and health.

Check Your Understanding: Feedback Systems

Answer
  • [1 mark] Feedback is when system output affects subsequent system input
  • [1 mark] Allows conditions to be automatically adjusted to meet given criteria
  • [1 mark] Enables self-correction and maintains system stability
  • [Additional] Creates closed-loop systems that continuously monitor and adjust
Answer
  1. [1 mark] Desired value set as input to controller (microprocessor)
  2. [1 mark] Controller sends signal to actuator which affects the process
  3. [1 mark] Process produces actual output measured by sensors
  4. [1 mark] Sensor data fed back to controller, compared with desired value, adjustments made
  5. [Additional] Continuous loop of measurement, comparison, and adjustment
Answer
  • [1 mark] Magnetic field sensors monitor wheel rotation speed (output)
  • [1 mark] Microprocessor compares wheel speeds with each other
  • [1 mark] If wheel rotating too slowly (locking up), reduces braking pressure to that wheel
  • [1 mark] Wheel speed increases, system continuously monitors and adjusts several times per second
  • [Additional] Feedback loop: wheel speed → sensor → microprocessor → braking pressure → wheel speed
Answer
Open-Loop:
  • No feedback
  • No measurement of output
  • No automatic correction
  • Example: Timer-based system
Closed-Loop:
  • Has feedback
  • Measures actual output
  • Automatically corrects deviations
  • Example: Sensor-based system
Answer
  • [1 mark] Braking happens very quickly (milliseconds)
  • [1 mark] Wheels can lock up almost instantly during hard braking
  • [Additional] Need continuous adjustment to maintain control and prevent skidding; rapid feedback prevents accidents
Answer
  • [1 mark] Example: Automatic cruise control in cars
  • [2 marks] Feedback loop: Desired speed set → radar/sensors measure distance to car ahead → microprocessor compares actual vs desired distance → adjusts throttle/brakes → changes car speed → continuous monitoring and adjustment
  • [Additional] Other examples: Automatic voltage regulator, water level controller, temperature-controlled oven

Key Takeaways

  • Sensors are input devices that measure physical properties like temperature, pressure, light, and sound, producing analogue data
  • Actuators are output devices that convert electrical signals into physical movement (motors, heaters, valves)
  • ADC converts analogue to digital so computers can process sensor data; DAC converts digital to analogue for actuator operation
  • Monitoring systems only observe and record conditions without controlling them (no actuators involved)
  • Control systems both monitor and control conditions using actuators based on sensor input
  • Feedback is essential in control systems - output affects input to enable automatic adjustment
  • Closed-loop systems use feedback to continuously compare actual output with desired output and make corrections
  • Different sensors have specific applications: temperature (thermocouple), motion (infra-red), pressure, light, sound, magnetic field, etc.
  • Computerized systems offer advantages: rapid response, 24/7 operation, operation in dangerous environments, consistency, error-free operation
  • Real-world applications include: patient monitoring, burglar alarms, street lighting, anti-lock brakes, greenhouse control, central heating
  • Systems follow step-by-step processes: sensor reading → ADC conversion → microprocessor processing → comparison with preset values → action (warning or control)
  • Preset values define acceptable ranges for measured properties and trigger responses when exceeded

Question Bank

Marking Scheme & Answer
Sensors:
  • [1 mark] Input devices that read/measure physical properties
  • [1 mark] Produce analogue data (constantly changing)
  • [1 mark] Examples: Thermocouple (temperature), Light sensor, Pressure sensor, Infra-red sensor
Actuators:
  • [1 mark] Output devices that convert signals to physical movement
  • [1 mark] Also called output transducers
  • [1 mark] Examples: Electric motor (movement), Heater (heat), Lamp (light), Loudspeaker (sound)
Marking Scheme & Answer
ADC (Analogue to Digital Converter):
  • [1 mark] Sensors produce analogue data (continuous, no discrete values)
  • [1 mark] Computers can only process digital data (discrete values)
  • [Additional] ADC converts physical values into discrete digital values computers can understand
DAC (Digital to Analogue Converter):
  • [1 mark] Computers produce digital signals to control devices
  • [1 mark] Many actuators (motors, valves) need analogue data to operate
  • [Additional] DAC converts digital computer signals to analogue format for actuators
Marking Scheme & Answer
Monitoring Systems:
  • [1 mark] Only observe and record conditions
  • [1 mark] No actuators involved
  • [1 mark] Output does not affect input
  • [Additional] Example: Patient monitoring in hospital
Control Systems:
  • [1 mark] Both monitor AND control conditions
  • [1 mark] Use actuators to take action
  • [1 mark] Output affects input (feedback loop)
  • [Additional] Example: Central heating system
Commonality: Both use sensors to measure conditions, both compare readings to preset values, both can activate alarms or warnings.
Marking Scheme & Answer
  1. [1 mark] System activated by keying in password on keypad
  2. [1 mark] Sensors detect intrusions: infra-red (movement), acoustic (sounds), pressure (weight)
  3. [1 mark] Sensor data passed through ADC to convert analogue to digital
  4. [1 mark] Microprocessor samples data, compares with preset values
  5. [1 mark] If data outside acceptable range, activates siren or flashing lights
  6. [Additional] Continues until system reset with password; this is a monitoring system (no control action)
Marking Scheme & Answer
  • [1 mark] Feedback is when system output affects subsequent system input
  • [1 mark] Allows conditions to be automatically adjusted to meet given criteria
  • [1 mark] Enables self-correction without human intervention
  • [1 mark] Maintains system stability and accuracy despite disturbances
  • [Additional] Creates closed-loop systems that continuously monitor, compare, and adjust
Marking Scheme & Answer
  1. [1 mark] Uses magnetic field sensors to monitor wheel rotation speed
  2. [1 mark] When wheel rotates too slowly (locking up), sensor sends data to microprocessor
  3. [1 mark] Microprocessor checks rotation speed of other three wheels
  4. [1 mark] If different (others rotating faster), sends signal to braking system
  5. [1 mark] Braking pressure to affected wheel reduced, wheel speed increases
  6. [1 mark] System checks several times per second, constantly adjusting to prevent locking
  7. [Additional] Feedback loop: wheel speed → sensor → microprocessor → braking pressure → wheel speed
Marking Scheme & Answer
  • [1 mark] Computers can respond very rapidly to changes (faster than humans)
  • [1 mark] Systems can run 24 hours a day, 365 days a year without breaks
  • [1 mark] Can operate in dangerous environments unsafe for humans
  • [Additional] Other advantages: Consistent, error-free outputs; fast data processing; machines operate faster than humans
Marking Scheme & Answer
  1. [2 marks] (a) pH sensor - Measures acidity/alkalinity levels in soil to ensure optimal plant growth conditions
  2. [2 marks] (b) Infra-red/motion sensor OR Acoustic sensor - Detects movement or sounds of intruders in building
  3. [2 marks] (c) Infra-red/motion sensor OR Rain sensor - Detects rain on windscreen to automatically activate wipers
  4. [Additional] Other possible sensors: Pressure sensor for burglar alarm (weight on floor), Light sensor for greenhouse (light levels)
Marking Scheme & Answer
  1. [1 mark] Sensors continuously take readings and send to processor
  2. [1 mark] Analogue sensor readings converted to digital using ADC
  3. [1 mark] Microprocessor compares sensor readings to stored pre-set values
  4. [1 mark] For monitoring: If data outside range, sends warning or activates alarm
  5. [1 mark] For control: If data outside range, sends signals to actuators (may need DAC), output affects next inputs
  6. [Additional] Continuous loop for control systems with feedback
Marking Scheme & Answer

Example: Central heating system

Components:
  • Sensor: Thermocouple (temperature sensor)
  • Controller: Thermostat/microprocessor
  • Actuator: Heater/boiler
  • Process: Room heating
Feedback Loop:
  1. Desired temperature set on thermostat
  2. Temperature sensor measures actual room temperature
  3. Microprocessor compares actual vs desired temperature
  4. If actual < desired, turns heater on
  5. If actual ≥ desired, turns heater off
  6. Continuous monitoring and adjustment
Other examples: Street lighting (light sensor), Greenhouse control (multiple sensors), ABS (magnetic sensors), Patient monitoring (vital signs).