How unit 2 is examined
This unit covers sensors (definition, sensor node, features, classes, types and errors) and actuators (definition and types); all three past 7-mark questions came from the sensor node, analog/digital sensors and actuator types.
Define Sensor, Basic components and challenges of a sensor node
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>
Definition. <mark>A sensor is a device that detects a physical quantity such as temperature, light or pressure and converts it into an electrical signal that can be measured and processed.</mark> A sensor node is a small device that senses, processes and wirelessly transmits data.
Diagram.
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Key points.
- The sensing unit contains the sensor, which measures the physical quantity, and an ADC, which converts the analog reading into digital form.
- The microcontroller (processing unit) runs the program, filters the data and controls every other block.
- The transceiver sends and receives data over a wireless link such as ZigBee, Bluetooth or Wi-Fi.
- The memory stores the program and temporary readings, and the battery (power unit) supplies energy to all blocks.
- Challenges: limited energy, since batteries are small and hard to replace, so low-power design and sleep modes are needed.
- Challenges: limited memory and processing power, weak security because of light encryption, difficult deployment in harsh places, and costly maintenance.
Answer frame. Open with the definition of a sensor node; draw the block diagram; list the five components with one line each; then the five challenges; close with the point that energy is the main constraint.
Asked: [7 marks] (May 2023) What are the basic components and challenges of a sensor node?
Sensor features, Sensor resolution
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>
Definition. Sensor features are its specifications: range, sensitivity, accuracy, precision, response time and resolution. <mark>Resolution is the smallest change in the measured quantity that the sensor can detect.</mark>
Key points.
- Range is the span of values the sensor can measure, and sensitivity is the change in output per unit change in input.
- Accuracy is closeness to the true value, while precision is the repeatability of readings.
- Resolution of an n-bit digital sensor is $\text{Resolution} = \dfrac{\text{Range}}{2^n}$; for 0 to 100 °C with 10 bits it is $100/1024 \approx 0.098$ °C.
Sensor classes: Analog, Digital, Scalar, Vector Sensors
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>
Definition. <mark>An analog sensor gives a continuous output that varies with the input, while a digital sensor gives a discrete (binary) output.</mark> A scalar sensor measures only magnitude, and a vector sensor measures magnitude and direction.
| Basis | Analog sensor | Digital sensor |
|---|---|---|
| Output | Continuous voltage or current | Discrete, 0/1 or digital word |
| Needs ADC | Yes, before a microcontroller | No |
| Noise | More affected | Less affected |
| Example | LM35 temperature, pressure, LDR | PIR motion, DHT11, IR obstacle |
| IoT use | Room temperature, tank pressure, light level | Intruder alarm, smart-home occupancy |
Key points.
- Analog examples: temperature sensors (LM35) for smart thermostats, pressure sensors for industrial pipelines and weather stations.
- Digital examples: a PIR sensor detects human motion for security lighting, and a digital humidity sensor sends readings over a serial line.
- Scalar examples are temperature and humidity, and vector examples are accelerometer, gyroscope and magnetometer, used in wearables and navigation.
Answer frame. Open with the analog versus digital definition; give the comparison table; then the types with IoT applications; close with the note that analog sensors need an ADC.
Asked: [7 marks] (May 2023) Explain the types of Analog and Digital Sensors with Applications in IoT.
Sensor Types, bias, drift, Hysteresis error, quantization error
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>
Definition. Sensors are grouped by the quantity measured: temperature, pressure, light, proximity, motion, gas and humidity. <mark>Sensor errors are deviations of the output from the true value, such as bias, drift, hysteresis and quantization error.</mark>
Key points.
- Bias is a constant offset added to every reading, and it is removed by calibration.
- Drift is a slow change in output over time with the input constant, caused by ageing or temperature.
- Hysteresis error is the difference in output for the same input when it is approached while rising and while falling.
- Quantization error is the rounding error of the ADC, at most half the step size: $e_{max} = \dfrac{\text{Range}}{2 \cdot 2^n}$.
Actuator
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>
Definition. <mark>An actuator is a device that converts an electrical control signal into physical action such as motion, heat or flow, so it does the reverse of a sensor.</mark>
Key points.
- A sensor senses the environment and an actuator acts on it, together closing the control loop in an IoT system.
- The actuator receives a command from a microcontroller, usually through a driver circuit or relay.
- It needs an energy source such as electricity, fluid pressure or compressed air.
- Examples are a motor, relay, valve, heater and buzzer.
Actuator types: Hydraulic, Pneumatic, electrical, thermal/magnetic, mechanical actuators, soft actuators
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>
Definition. <mark>An actuator type is named by the energy it uses to produce motion: hydraulic, pneumatic, electrical, thermal, magnetic, mechanical or soft.</mark>
Key points.
- Hydraulic actuators use pressurised liquid in a cylinder, give very high force, and are used in heavy machinery and industrial presses.
- Pneumatic actuators use compressed air, are fast and clean, and are used in factory automation and pick-and-place valves.
- Electrical actuators such as motors, solenoids and relays convert electricity to motion, and are the most common in IoT, for example a relay switching a smart-home light or a servo turning a camera.
- Thermal and magnetic actuators change shape or move when heated or placed in a magnetic field, for example a bimetallic strip or a shape-memory alloy in a thermostat valve.
- Mechanical actuators use gears, cams, screws or levers to convert one motion into another, such as a rack and pinion.
- Soft actuators are made of flexible materials that bend or expand under air or voltage, and are used in soft grippers and wearable robotics.
Answer frame. Open with the definition and the reverse-of-sensor role; list the types with energy source, function and one IoT example each; close with the point that the choice depends on force, speed and cost.
Asked: [7 marks] (May 2023) What is Actuator? Also explain the different types of Actuators with their functions in IoT.
Last-minute revision
- A sensor converts a physical quantity to an electrical signal, and an actuator converts an electrical signal to physical action.
- Sensor node blocks: sensing unit with ADC, microcontroller, transceiver, memory and battery.
- Sensor node challenges: energy, memory, security, deployment and maintenance.
- Analog output is continuous and needs an ADC, while digital output is discrete.
- PIR is a digital motion sensor, and LM35 is an analog temperature sensor.
- Scalar sensors give magnitude, and vector sensors give magnitude and direction.
- Resolution $= \text{Range}/2^n$, for example 100/1024 = 0.098 °C.
- Bias is an offset, drift is change over time, and hysteresis depends on the direction of approach.
- Actuator types: hydraulic (liquid), pneumatic (air), electrical, thermal, magnetic, mechanical and soft.
Memory hooks
- Sensor node = "SMART B": Sensor, MCU, ADC, Radio, Transceiver, Battery.
- Sensor senses, actuator acts: input side versus output side.
- Hydraulic = heavy liquid force, pneumatic = light fast air.
- Bias is fixed, drift is slow, hysteresis is the direction.
Coverage checklist
- Define Sensor, Basic components and challenges of a sensor node: components and challenges of a sensor node.
- Sensor features, Sensor resolution: no past question, resolution formula given.
- Sensor classes: Analog, Digital, Scalar, Vector Sensors: analog and digital sensor types with IoT applications.
- Sensor Types, bias, drift, Hysteresis error, quantization error: no past question, errors defined.
- Actuator: no past question, definition and reverse role covered.
- Actuator types: Hydraulic, Pneumatic, electrical, thermal/magnetic, mechanical actuators, soft actuators: actuator definition and types with IoT functions.