Study sheet: Sensors and Measurement Chains

📋 Course Outline

1. Sensors and Transducers
2. Measurement Chain Architecture
3. Passive Sensor Principles
4. Thermoelectric and Piezoelectric Effects
5. Pyroelectric and Photoelectric Effects
6. Hall and Inductive Effects
7. Active Sensor Applications
8. Influence Quantities

📖 1. Sensors and Transducers

🔑 Key Concepts & Definitions

• Sensor : Converts a non-electrical physical phenomenon into an electrical quantity, usually a current or voltage.
• Transducer : Converts one type of energy into another.

★ Must-know

📌 A transducer is considered a sensor when it converts a measurable quantity such as acoustic pressure, optical intensity, or magnetic field into an electrical signal, but it is considered an actuator when it converts an electrical signal into sound or mechanical motion.

Further detail

• Traditional human detection uses the eye, ear, and sense of smell without contact, and taste and touch with contact.

💡 Memory Hook

A sensor converts a physical quantity into an electrical signal, whereas an actuator converts electricity into another form of energy.

📖 2. Measurement Chain Architecture

🔑 Key Concepts & Definitions

• Measurement chain : A set of devices or operations used to detect a physical quantity.

★ Must-know

• 🔄 A typical measurement chain:

  1. acquires the physical quantity with a sensor
  2. conditions the signal
  3. transmits the signal
  4. converts the analog signal into digital form
  5. processes the digital signal
  6. displays or stores the result

• The signal-conditioning block can amplify a weak sensor signal or filter noise in the sensor output.

📌 Because a sensor generally supplies an analog signal, an analog-to-digital converter is required for digital signal processing.

Further detail

• The transmission block sends the information to the processing stage through a wired or wireless medium.

💡 Memory Hook

Sense → condition → transmit → convert → process → display or store

📖 3. Passive Sensor Principles

★ Must-know

📌 A passive sensor detects a phenomenon through variations in the resistance, inductance, or capacitance of an impedance, whereas an active sensor generates electrical energy through a physical effect.

📐 Formula — For a resistive sensor, the resistance satisfies $R=\rho\frac{l}{S}$, where $\rho$ is resistivity, $l$ is length, and $S$ is cross-sectional area.

📐 Formula — For a capacitive sensor, the capacitance satisfies $C=\frac{S\varepsilon}{d}$, where $S$ is plate area, $\varepsilon$ is permittivity, and $d$ is the separation between plates.

Further detail

📐 Formula — For a coil sensor, the inductance satisfies $L=\frac{NS\mu^2}{l}$, where $N$ is the number of turns, $S$ is area, $\mu$ is permeability, and $l$ is wire length.

• Passive sensors use: resistance for light, temperature, pressure, level, or displacement, capacitance for level, displacement, or humidity, inductance for speed, magnetic field, presence, or level

💡 Memory Hook

R-C-L: resistance, capacitance, inductance

📖 4. Thermoelectric and Piezoelectric Effects

🔑 Key Concepts & Definitions

• Piezoelectric effect : The generation of equal and opposite electrical charges on the surfaces of certain materials when mechanical stress deforms them.

★ Must-know

• In the thermoelectric effect, heating one junction of two dissimilar metals connected at their ends produces a continuous current; with the circuit open, a voltage appears that depends on temperature and metal composition.

📐 Formula — For a thermoelectric junction, the generated voltage satisfies $e_{AB}=f(T_1-T_2)$ and depends on the temperature difference and the composition of metals A and B.

📌 In the direct piezoelectric effect, compression produces a voltage and tension produces a voltage of opposite sign, whereas in the inverse effect an applied voltage produces crystal elongation or contraction.

Further detail

• Pierre and Paul-Jacques Curie experimentally demonstrated the piezoelectric effect in 1880.

💡 Memory Hook

Thermoelectricity responds to temperature difference, whereas piezoelectricity responds to mechanical stress.

📖 5. Pyroelectric and Photoelectric Effects

🔑 Key Concepts & Definitions

• Pyroelectric effect : The variation of electrical polarization caused by a temperature change, producing a temporary potential difference that disappears after dielectric relaxation.

★ Must-know

• A pyroelectric motion detector uses at least two heat-sensitive infrared cells to detect a difference in heat between the cells.

📌 The photoemissive effect produces a current when light releases electrons that are collected by an applied electric field, whereas the photovoltaic effect changes the voltage of an illuminated PN junction through the motion of electrons and holes.

Further detail

• The photoelectric effect was discovered in 1887 by the German physicist Heinrich Rudolf Hertz.

• The photovoltaic effect was discovered in 1839 by Antoine Becquerel and his son Alexandre Edmond.

💡 Memory Hook

Temperature change → polarization change → temporary electrical potential in pyroelectric materials.

📖 6. Hall and Inductive Effects

🔑 Key Concepts & Definitions

• Hall effect : The appearance of a voltage perpendicular to the current and magnetic induction when a semiconductor carrying current is placed in a magnetic field.

• Inductive effect : Measures a change in magnetic coupling between primary and secondary windings through the resulting variation in secondary voltage.

★ Must-know

📐 Formula — For a Hall sensor, the Hall voltage satisfies $V_H=R_H\frac{IB}{e}=K_AIB\sin\theta$, where $I$ is current, $B$ is magnetic induction, $e$ is semiconductor thickness, $R_H$ is the Hall constant, and $\theta$ is the angle between current and magnetic field.

📐 Formula — Around a current-carrying conductor, the magnetic field satisfies $B=\frac{\mu I}{2\pi r}$, and a Hall sensor can produce an output $U_S=kB=k'I$.

Further detail

• The Hall effect is used for: contactless position checking, magnetic-field sensing, proximity detection, current measurement without opening the circuit

💡 Memory Hook

Current and magnetic field → Hall voltage; magnetic coupling change → secondary-voltage variation.

📖 7. Active Sensor Applications

★ Must-know

• Active sensors exploit physical effects that convert the energy associated with the measurand into electrical energy.

• Thermoelectricity produces a voltage from temperature, pyroelectricity and photoemission produce a current or charge from optical radiation, photovoltaics produce voltage, and the photoelectromagnetic effect produces voltage from light in a magnetic field.

Further detail

• Piezoelectricity is used for force, pressure, and acceleration measurement by producing electrical charge.

• Induction is used to measure speed by producing voltage, while the Hall effect is used to measure position by producing voltage.

💡 Memory Hook

Temperature, light, force, motion, position, and magnetic field are converted into electrical outputs by different physical effects.

📖 8. Influence Quantities

🔑 Key Concepts & Definitions

• Influence quantity : A parasitic physical quantity, other than the measurand, whose variation changes the sensor's electrical output and cannot be distinguished from the measurand's effect.

★ Must-know

• Temperature can modify the electrical, mechanical, and dimensional characteristics of a sensor.

Further detail

• Pressure, acceleration, and vibrations can deform the sensor's mechanical body or test body.

• Humidity can change the dielectric constant of a material and consequently its resistance, degrading insulation and causing short circuits.

💡 Memory Hook

Temperature, pressure, vibration, or humidity changes → parasitic output variation that can be confused with the measurand.

📊 Synthesis Tables

Passive and Active Sensors

Type	Operating principle	Typical output
Passive sensor	Variation of resistance, capacitance, or inductance	Changed electrical impedance
Active sensor	Conversion of measurand energy through a physical effect	Voltage, current, or charge