Sensors: Static Characteristics

• Sensors measure physical quantities and convert them into electrical signals.
• Examples include temperature, displacement, fluid, light, motion, and velocity sensors.

Sensor Characteristics

• Sensor characteristics can be divided into static and dynamic types.
• Static characteristics relate to the steady-state relationship between sensor input and output.
• Dynamic characteristics relate to the sensor's behavior when the measured quantity changes rapidly.

Static Characteristics

• Range: Defines the limits within which the input can vary, for example, a thermocouple with a range of 25-225°C.
• Span: The difference between the maximum and minimum input values, such as a thermocouple with a span of 200°C (225-25).
• Error: The difference between the measured and true values; for instance, a sensor reads 29.8 mm when the actual displacement is 30 mm, resulting in a 0.2 mm error.
• Sensitivity: The ratio of change in sensor output per unit change in input; a temperature sensor may have a sensitivity of 10 mV/°C, with a 1°C rise resulting in 10 mV.
• Non-Linearity: Indicates the maximum deviation of the actual measured curve from the ideal curve.
• Hysteresis: Error defined as the maximum difference in output for a measurement value, within the sensor's specified range, when approaching a point with increasing, then decreasing input.
• Stability: The ability of a sensor to provide the same output for a constant input over time.
• Dead band: The range of input values for which there is no output.
• Repeatability: How well a sensor provides the same output for repeated applications of the same input value under the same conditions.
• Accuracy: The closeness of the measurement to the actual value.
• Precision: The ability of a sensor to reproduce a certain set of readings within given accuracy, which relies on repeatability.
• Output Impedance: Impedance measured at the output of the sensor, important as the electrical output interfaces with an electronic circuit.

Dynamic Characteristics

• Response time: The time it takes for a sensor to provide an output corresponding to a specified percentage (90-95%) of its steady value after a constant input.
• Time constant: Measures the inertia of the sensor, indicating how fast it will react to input changes, equal to 63.2% of the response time.
• Rise time: Measures the time it takes for the output to rise from 10% to 90% of its steady value.
• Settling time: The time it takes for the output to settle within a small percentage (2%) of its steady-state value.

Classification of Sensors

• First Classification: Active sensors (self-generating) do not require external power (e.g., thermocouples), while passive sensors (external supply) do (e.g., photodiodes).
• Second Classification: Analog sensors produce a continuous output signal (e.g., LDR, strain gauge), whereas digital sensors work with discrete or digital data (e.g., IR, PIR).
• Third Classification: primary, containing mechanical and electrical components, convert physical quantities into mechanical signals, and secondary, deployed in cascade with primary, convert mechanical signals into comprehensible electrical signals (e.g., Bourdon tube and LVDT).

Sensor Selection Parameters

• Operating principle, availability (source location, delivery schedule, payment and supply continuation), cost (sensor and delivery).
• Performance figures: range, ease of use, power requirements, accuracy, and hysteresis effect.

Displacement Sensors

• Used to measure movement; the electrical signal is in the form of Resistance/Capacitance/Inductance.
• Classified into resistive, capacitive, and inductive types based on their electrical output.

Resistive Displacement Sensors

• Potentiometer Construction: includes a resistance element with a sliding contact moved over its length; the element is wire wound or conductive plastic.
• Wire wound tracks have a resolution of ~0.5 mm, conductive plastic ~0.1 µm.
• Potentiometer Operation: Relates the change in linear or rotary position to a change in resistance, converted into a proportional voltage change, which is represented as: Vout=kVs*(linear displacement).
• Strain gauge: passive transducer converting mechanical displacement into resistance change, used for force, torque, pressure, or acceleration measurement.
• Strain Gauge Working Principle: the metallic wire changes dimension and thus changes resistance when strain is applied.
  • Working principles that strain gauges are based on: Mechanical, Electrical, and Piezoelectric effects.
  • Mounting principles include Bonded and Unbonded strain gauges
  • Most commonly used is the electrical strain gauge.
  • Its construction includes a grid-shaped sensing element of thin metallic resistive foil (3 - 6µm thick), a base of thin plastic film (15 - 16µm thick), and laminated with a thin film.
• Strain Gauge Operation: bonded tightly to a measuring object, the sensing element stretches or contracts, resulting in a change in length.
• The relationship that defines the change in length and change in resistance is as follows: R=ρl/ A.
• The correlation between applied strain and resistance is: ΔR/R = Gε.
  • R is the original resistance (Ω), and ΔR is the change in resistance (Ω)
  • ε is the strain
  • G is the proportional constant (gage factor).
  • The gage factor is 2 for metal wire/ foil strain gauges, -100 for N-type semiconductors, and +100 for P-type semiconductors.
• Applications of strain gauges Displacement, force, residual stress, vibration, torque and bending measurements.
• Displacement Measurement with Strain Gauge: attached to a flexible element (cantilevers, rings, or U-shapes).
• When force is applied to the cantilever beam and it is displaced, the strain gauges are strained; one gauge is in tension, the other in compression that cause measurable resistance changes
• This setup measures linear displacements of 1-30mm with ±1% non-linearity error.

Inductive Displacement Sensor

• Example: Linear Variable Differential Transducer/Transformer (LVDT).
• LVDT Principle: relies on mutual induction to convert displacement into electrical energy.
  • Construction : cylindrical former with a primary winding in the center flanked by two secondary windings, wired opposite to each other. Secondary coils are noted S1 and S2.
  • An iron core moves within the former.
  • Excitation voltage: 5-12V AC, with 50-400 Hz frequency.
• LVDT Operation:
  • Case 1: core is in null position, equal voltage is induced in secondary windings, so net output=0 (V0=V1-V2=0).
  • Case 2: core moves to the left, voltage rises in secondary coil 1, so net output is positive (V0=V1-V2= +ve).
  • Case 3: core moves to the right, voltage rises in secondary coil 2, so net output becomes negative (V0=V1-V2= -ve).
• LVDT applications: measures displacements from a fraction of a millimeter to centimeters and, as a secondary transducer, can be used for force, weight, and pressure measurements.

Force Sensor (Load Cell)

• Strain Gauge Load Cell Principle: A steel cylinder changes dimension when force is applied; bonded strain gauges are stretched or compressed, causing resistance changes.
  • These resistance changes give a measure of the applied force.
• Construction: a steel cylinder with four identical strain gauges; two (R1, R4) gauge the applied load, while the other two (R2, R3) measure circumferential changes at right angles.
• Load Cell Operation: Connect the four gauges to form a Wheatstone Bridge to convert resistance changes into voltage.
  • When force is absent from cylinder, all four gauges have equal resistance, then the bridge is balanced and output voltage is zero.
  • When force is applied, vertical gauges (R1, R4) compress (decreasing resistance), while horizontal gauges (R2, R3) undergo tension (increasing resistance).
  • This unbalances the bridge, generating non-zero output voltage proportional to the applied force.
• Applications: vehicle weighing, force dynamometers, and wire tension measurement.

Temperature Sensors

• Detect temperature changes and convert them into electrical signals and include bi-metallic strips, thermocouples, RTDs, thermistors, thermodiodes, and thermotransistors.

Bi-Metallic Strips

• Created by bonding two metals with different thermal expansion coefficients (e.g., brass and steel).
• This bimetallic strip principle is used in thermal switches.
• Advantages of bi-metallic strips: no power source needed, easy to use, and cheap.
• Disadvantages: not accurate; limited to 500°C. Not good for low temperatures or precision use.

Resistance Temperature Detectors (RTDs)

• Principle: the electric resistance of metal changes with temperature which typically follows a linear relationship, therefore the correlation is R=Ro(1+αΤ).
• Applications : air conditioning, refrigeration, food processing, stoves, plastics processing, electronics, and exhaust gas temperature sensing.

Thermistors

• Resistors that measure changes in temperature by relying on the change in resistance.
• ΔR(resistance change) is proportional to kΔT(temp change).
• Types:
  • NTC (negative temperature coefficient) thermistors made of metal oxides (chromium, cobalt, iron) in disk or rod shapes.
  • PTC (positive temperature coefficient) thermistors made of barium, lead, and strontium titanite, are used for motor and transformer winding protection.

Thermocouple

• When a junction of different metals is heated, it produces an electric potential (Seebeck effect).
• Uses Chromel (90% nickel, 10% chromium) and Alumel (95% nickel, 2% manganese, 2% aluminum, 1% silicon).
• Applications : Monitoring temperature and chemistry in steel making, testing temperatures in chemical and petroleum plants, device and oven safety testing, gas turbine temperature measurement, and metal production/smelting.

Thermodiodes and Thermotransistors

• Semiconductor pn junction diodes are widely used as temperature sensors due to how temperature changes the mobility of doped semiconductors.
• This alters the rate at which electrons/holes diffuse across the junction and leads to a potential difference, V, that's temperature-dependent.
  • Integrated circuits like LM3911 supply temperature sensors with signal conditioning, producing 10mV/°C.
• The voltage across base-emitter junction of a thermotransistor depends on temperature.
• Circuits such as LM35 give compact sensor with an output voltage linearly proportional (± 10mV/°C) to the Centigrade temperature.
• Features of LM35: calibrated directly in Celsius, ±0.5°C accuracy (at 25°C).
• LM35 Rated for -55°C to 150°C, suitable for remote or low-cost applications. Operates over 4-30 V, with Non Linearity Only ±¼°C.
• Applications include air conditioners, incubators, microwave ovens, and poultry farming.

Light Sensors

• Convert light energy into electrical energy and light dependent resisters (LDR), photodiodes and phototransistors are examples.

Light Dependent Resistor (LDR)

• Resistance depends on the amount of light, also known as photoresistors.
• Construction: semiconductor materials such as cadmium, lead sulfide, and indium antimonide.
  • Cadmium sulfide is used because of its spectral response and controllability with a torch
• Working principle of photo-conductivity, when light photons is exposed to LDR, electrons gain energy proportional to conduction band which increases conductivity.
  • LDR devices are known as dark resistance (order of several MΩ) when there is is no light.
• Applications include: Street lights, bar code scanners, packages, light intensity meters, and burglar alarms.

Photodiode

• Converts light energy into electrical energy and referred as photo-detector/sensor.
• Photodiodes are semiconductor diodes exposed to detect UV or X-rays/packaged with optical fiber.
• Photodiodes are classified into PN, PIN, and Avalanche types.
  • PN Junction Photodiode are designed to work in reverse bias. When light enters the photodiode, the carriers also generate a photoelectric effect with generated carriers, forming current with photons.
• Applications consist of CD players, smoke detectors, space and optical coms.

Phototransistor

• Tri-terminal or bi-terminal semiconductor devices with light-sensitive base regions which are optimized for photo apps.
• Devices are made with diffusion to give larger region compared to transistors.
• The transistor can be either homojunction (silicon, germanium) or heterojunction (non-identical materials, III-V i.e. GaAs).
• The circuit symbol for npn phototransistor has arrows pointing at the base, indicating sensitivity to light.
• Applications include use in detection on paper, punch cards, CD players, counting change, and security features.

Proximity Sensors

• Detect nearby objects without contact, emitting electromagnetic fields (infrared) and detecting changes.
• Target called proximity sensor, proximity sensors include inductive, capacitive, optical, and ultrasonic types.

Inductive Proximity Sensor

• Contactless electric proximity sensor detecting metallic objects by using high frequency oscillator & coil that that a field.
• Oscillation amplitude is the operating area that corresponds to the presence of metal. Threshold circuit recognizes shifts that control the sensors output.
  • Operating of sensor distance depends on coil and size of target. It has sensing ranges under cm and has no directionality involved.
• Common detection apps metal detectors, cars, and automation systems. There is no physical contact that requires access to specific aspects.

Capacitive Proximity Sensor

• They are used to non-contact detecting metallic & non-metallic things (liq, wooden, plastics etc). They depend on varying capacitative between item and when the set point is the sensitive side of an object.
• Inside electronic circuit starts to pulsate when target object gets close, and threshold amp is drove for the set pt.
• Applications: touch sensors digital audio devices, track pads, and cellphones. Capacitative has had robustness and has lowered cost to change electrical switches.

Optical Proximity Sensor

• Typically more than inductive counterparts in terms of costs and used a great deal in three classifications as a component of automated systems.
• Types: through beam, retroreflective, and diffuse reflective.
• In all types of sensor if output is ON only object is present otherwise output is OFF.

Ultrasonic Proximity Sensor

• Optical sensors used to the same level but high freq. The sound is a wave that typical is beyond human capable of hearing and commonly frequents at a 40 Khz.
• Common frequencies comprise of.
  • less than 20hz
  • between 20hz & 20khz -.Beyond 20khz.
• Operation is to wait for the echoes and emission. This sensor picks the target after reflection returns and by measuring delays the gap between a person and device is accounted for.
• Applications include the detection variety, controls of liquid height in containers, removal, and detection for vehicle.