Sensor Technology Quiz

Sensors

• A sensor is a component within a mechatronic or measurement system. It detects the magnitude of a physical parameter and transforms it into a processed signal.
• Often, the active part of a sensor is called a transducer.
• Transducers alter energy from one form to another.
• Mechatronic and control systems use sensors to measure physical quantities, such as position, distance, force, strain, temperature, vibration, and acceleration.
• Mechanical transducers convert physical quantities into mechanical forms.

Active vs Passive Sensors

• Active sensors require an external power source (excitation signal). This signal is modified by the sensor to generate an output signal.
  • Example: Photoelectric sensor
• Passive sensors create an electrical current in reaction to an external stimulus. This serves as the output signal without any extra power source.
  • Example: piezoelectric sensor and thermocouple

Sensor Types

• Temperature Sensors
• Optical Sensors
• Electric and Magnetic Sensors
• Mechanical Sensors
• Microelectromechanical Systems (MEMS)

Temperature Sensors

• A common temperature sensor is a thermometer, but is not usually used for further processing because it's output is read directly.
• A thermocouple uses two dissimilar metals in contact to produce a voltage. This voltage is proportional to the junction temperature — called the Seebeck Effect.
• Different thermocouple materials exhibit differing sensitivities and operating temperature ranges.

Thermocouples Details

• Thermoelectric junctions occur in pairs (thermocouples).
• A circuit diagram shows the connection of two junctions from which a voltage measurement can be taken.
• A table details various thermocouple types and their applications. Example includes highest sensitivity of type E which is less than 1000°C.
• Special wires from metals (e.g. Chromel - 90% nickel and 10% chromium, Alumel - 94% nickel, 3% manganese, 2% aluminum, 1% silicon. Different variations of metals are used in the design) are used for measurements with specific temperature ranges and environmental standards (e.g. Highest sensitivity type E < 1000°C, Non-oxidizing environment, <700°C type J)

Resistance Temperature Detector (RTD)

• An RTD is a metallic wire wound around a ceramic or glass core, hermetically sealed.
• The metallic wire's resistance increases as temperature increases.
• RTDs are wired using various bridge configurations: Two wires and Three wires RTD Bridge

Wheatstone Bridge Circuit

• A Wheatstone Bridge uses a bridge network for accurate resistance measurement, often used to detect small changes in resistance.
• It converts resistance changes into voltage changes.

Thermistors

• Thermistors are semiconductor devices available in different sizes and shapes.
• Resistance changes exponentially as temperature changes.
• Operating ranges are much narrower than RTDs.
• NTC (Negative Temperature Coefficient) thermistors decrease in resistance with increasing temperature.
• PTC (Positive Temperature Coefficient) thermistors increase in resistance with increasing temperature.

p-n Junction Temperature Sensors

• p-n junction temperature sensors use the temperature-dependent current (or voltage) across a forward-biased diode.
• The voltage across the p-n junction (also called bandgap temperature sensor) can used to determine temperature.

LM35 Temperature Sensor

• The LM35 is a precision integrated circuit temperature sensor.
• Output voltage is directly proportional to the Celsius temperature.
• Features include linear + 10 mV/°C scale factor, calibrated in degrees centigrade and operates from 4 to 30 volts.

Bimetallic Strip

• A bimetallic strip consists of two or more metal layers with different coefficients of thermal expansion.
• Deforming the strip due to temperature differences is used in household and industrial thermostats to turn thermal systems on or off.

Optical Sensors

• Optical sensors detect electromagnetic radiation ranging from far infrared to ultraviolet.
• Sensing methods can be direct (e.g., photovoltaic sensors) or converting temperature changes into electrical signals.

Light Dependent Resistor (LDR)

• LDR or photoresistors are widely used photo sensors.
• Resistance of LDR varies continuously with light intensity — inversely proportional. High resistance in dark; low resistance when light.
• Can be used in light-sensitive switches.

Photodiode

• Photodiodes are PN-junction diodes, typically with transparent or lens outer casing.
• Their response to light is specific to infrared and red light, not visible light.
• Dark current (0 lux) is around 10 µA (silicon) or 1µA (germanium).
• The leakage current increases with increased light levels.

Phototransistor

• Phototransistors are bipolar transistors encased in transparent cases.
• Light directly impacts the base-collector junction.

Optical Encoder

• An optical encoder converts motion into a sequence of digital pulses.
• Pulses can determine relative or absolute positions.
• Rotary encoders are common, and available as absolute or incremental.
• Absolute encoders have a unique digital word for each position (of the shaft), like a code disk.
• Incremental encoders produce digital pulses with shaft rotation, allowing relative displacement measurement.

Photoelectric Sensors (Types)

• Reflective type detects light reflected from a target.
• Thru-beam type detects the target crossing the light path/optical axis.
• Retroreflective type detects the reflected light that returns to the light source.

Proximity Sensor (Types)

• Inductive sensors detect metallic objects.
• Capacitive sensors detect metallic or non-metallic objects (e.g. liquids, plastics, wood).
• Photoelectric sensors detect objects using light-sensitive elements.
• Magnetic sensors detect the presence of permanent magnets.

Reed Switch

• A Reed switch consists of two ferromagnetic reeds sealed inside an inert atmosphere in a capsule.
• The reeds are attracted to each other when a magnetic field is present, completing the circuit.

Magneto Resistor

• A magneto resistor is a variable resistor that changes resistance based on applied external magnetic force.

Linear Variable Differential Transformer (LVDT)

• Measures linear displacement.
• Uses primary and secondary windings around a movable iron core.
• Responds to AC excitation to induce an AC voltage signal in the secondary coil.
• Secondary voltage output is proportional to the core position.

Hall Effect Sensor

• Hall effect sensors detect external magnetic fields.
• Output is a voltage proportional to the magnetic flux density.
• When the magnetic field density exceeds a threshold, the sensor generates a Hall voltage.

Mechanical Switch (and Configurations)

• A mechanical switch is a simple displacement sensor, determining whether a target is touching or not.
• Typical components include a lever, roller and cam.

Strain Gauge

• Strain gauges measure mechanical deformation (strain).
• They typically use a thin foil of metal (constantan), deposited on a thin plastic (polyimide) grid pattern on one surface.
• Strain gauges with larger metal pads allow the attachment of wires via solder.

Accelerometer (Types)

• Measures acceleration, the rate of change of velocity.
• Piezoelectric accelerometers use piezoelectric materials (e.g., quartz, PZT) to generate a charge in reaction to force (or deformation).
• Magnetic accelerometers use a permanent magnet and sense the field variations of this magnet using a Hall effect device or a magnetoresistive sensor.

Load Cell

• Load cells measure force by measuring the deflection or strain on sensitive components.
• They use strain gauges (often using differential designs to compensate for temperature effects) mounted on the load-bearing structure.
• A Wheatstone bridge circuit is often employed to get a voltage output that is proportional to force.

MEMS

• MEMS are micro-electromechanical systems integrated through standard IC processes.
• They comprise mechanical and electrical components on a small scale — micrometers to millimeters in size.
• Examples of MEMS include microsensors and micro actuators.

Potentiometer

• A potentiometer (pot or rheostat) is a variable resistor measuring rotational or linear movement.
• It uses a wiper, sliding contact along a resistive element. -Change in resistance is proportional to angular displacement.

PIR (Passive Infrared) Sensor

• PIR sensors use a pyroelectric device (a can-like shape with inner rectangle crystal) to detect infrared radiation.
• The heat emitted from objects affects this sensor. Increased heat increases the radiation and produces a signal.
• Motion detection uses the difference in infrared levels between two halves of the sensor to recognize changes/motion.

Ultrasonic Distance Sensor

• Ultrasonic sensors emit high-frequency sound pulses and measure the echo duration.
• Two openings – one to send the pulse (speaker-like) and another to receive (microphone-like) — are used.
• The echo time pulse is calculated to yield distance measurement.

Test your knowledge on various types of sensors, including photoelectric, Hall Effect, and thermocouples. This quiz covers working principles, applications, and characteristics of different sensors used in various fields. Perfect for students and professionals in electronics and engineering.