Sensor Technology Quiz

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Questions and Answers

What type of sensor detects the presence of metallic and non-metallic objects?

Photoelectric Sensor

Magnetic Sensor

Inductive Sensor

Capacitive Sensor (correct)

Which configuration of photoelectric sensors involves an external reflective surface?

Retroreflective

Reflective (correct)

Thru beam

Proximity

What is the primary function of a Hall Effect sensor?

To measure distance using light

To detect the presence of a magnetic field (correct)

To measure temperature changes

To identify electrical current flow

Which of the following sensors is specifically used for detecting metallic objects?

Inductive Sensor Signup and view all the answers

What is the working principle of a linear variable differential transformer (LVDT)?

It functions like a transformer with moving coils. Signup and view all the answers

Which type of proximity sensor can detect liquids?

Capacitive Sensor Signup and view all the answers

What happens to the resistance of a magneto resistor when an external magnetic field is applied?

It increases Signup and view all the answers

Which sensor configuration sends a beam of light directly from the sensor to the target?

Thru beam Signup and view all the answers

Which of the following best describes a sensor?

An element that detects physical parameters and converts them into a signal. Signup and view all the answers

What characterizes active sensors?

They require an external power source known as an excitation signal. Signup and view all the answers

How does a thermocouple operate?

It converts temperature differences into voltage through the Seebeck effect. Signup and view all the answers

What is a mechanical transducer?

A transducer converting energy from one mechanical form to another. Signup and view all the answers

Which sensor type generates an output signal without needing an additional energy source?

Passive sensors like thermocouples and piezoelectric sensors. Signup and view all the answers

What type of sensor is a thermometer in its classical form?

A sensor without an output signal that can be processed further. Signup and view all the answers

Which statement about sensors is incorrect?

All sensors can measure temperature. Signup and view all the answers

Which of the following is an example of an electrical transducer?

Linear variable differential transformer (LVDT) Signup and view all the answers

What is the primary function of a bimetallic strip in thermostats?

To facilitate electrical contacts based on temperature Signup and view all the answers

Which of the following describes the operation of a Light Dependent Resistor (LDR)?

Its resistance decreases as light intensity increases Signup and view all the answers

Which metals are typically used in a bimetallic strip due to their different coefficients of thermal expansion?

Two or more different metals Signup and view all the answers

What is the typical dark current range for silicon photodiodes?

1 uA Signup and view all the answers

What is the general principle behind optical sensors?

Detecting electromagnetic radiation across a broad optical range Signup and view all the answers

Why is a photodiode's casing often transparent or equipped with a clear lens?

To enhance sensitivity to light by focusing it onto the PN junction Signup and view all the answers

Which of the following voltage ranges can a typical calibrated sensor operate within?

4 to 30 volts Signup and view all the answers

What happens to the deflection of a bimetallic strip when temperature changes?

It deflects due to differing thermal expansions of the metal layers Signup and view all the answers

What materials make up CHROMEL in thermocouples?

90% nickel and 10% chromium Signup and view all the answers

How does the resistance of a resistance temperature detector (RTD) change with temperature?

It increases with increasing temperature Signup and view all the answers

Which device is primarily used for circuit protection?

PTC Thermistor Signup and view all the answers

What is the main advantage of using a Wheatstone Bridge Circuit?

It converts resistance changes to voltage changes Signup and view all the answers

What type of sensor is the LM35 temperature sensor?

p-n junction temperature sensor Signup and view all the answers

What characteristic does a Negative Temperature Coefficient (NTC) Thermistor exhibit?

Resistance decreases as temperature increases Signup and view all the answers

What is a common application for thermistors?

Temperature measurement Signup and view all the answers

Which type of thermistor has a resistance that increases with temperature?

PTC Thermistor Signup and view all the answers

What triggers the Hall effect sensor to generate Hall Voltage?

Magnetic flux density around the sensor Signup and view all the answers

Which component is primarily used for measuring strain in a mechanical component?

Bonded metal foil strain gage Signup and view all the answers

What is the function of a mechanical switch?

To provide binary information of touch status Signup and view all the answers

Which of the following is NOT a type of strain gage assembly?

Triaxial Signup and view all the answers

What type of accelerometer uses piezoelectric materials to measure acceleration?

Piezoelectric Accelerometer Signup and view all the answers

What is the primary function of a load cell?

To convert load or force into an electronic signal Signup and view all the answers

What material is typically used for the foil in a bonded metal foil strain gage?

Constantan Signup and view all the answers

Which statement accurately describes the output of a Hall effect sensor?

It generates a voltage when influenced by a magnetic field. Signup and view all the answers

What occurs when light falls upon the junction of a phototransistor?

Hole/electron pairs are formed. Signup and view all the answers

What type of phototransistor is commonly encased in a transparent case?

Photobipolar transistor Signup and view all the answers

What distinguishes an absolute encoder from an incremental encoder?

Each rotational position corresponds to a unique digital word in absolute encoders. Signup and view all the answers

How does a reflective type photoelectric sensor function?

It detects the light beam reflected from the target. Signup and view all the answers

What is measured by an incremental encoder?

Relative displacement of the shaft. Signup and view all the answers

What happens in a thrubeam type photoelectric sensor when a target crosses the optical axis?

The change in light quantity is measured. Signup and view all the answers

Which type of optical encoder is most commonly used?

Digital incremental encoder Signup and view all the answers

What is the function of a photoelectric sensor's light-emitting element?

It emits a light beam. Signup and view all the answers

Study Notes

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.

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Description

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.