Sensors Transducers and Applications MCQ Quiz - Objective Question with Answer for Sensors Transducers and Applications - Download Free PDF

Explanation:

Thermistors

Definition: A thermistor is a type of resistor whose resistance varies significantly with temperature. Thermistors are widely used as temperature sensors, and they can also be used to limit inrush current in power supply circuits.

Types of Thermistors: Thermistors are broadly categorized into two types:

• Negative Temperature Coefficient (NTC) Thermistors: These thermistors have a resistance that decreases as the temperature increases. NTC thermistors are commonly used in temperature sensing and control applications.
• Positive Temperature Coefficient (PTC) Thermistors: These thermistors have a resistance that increases as the temperature increases. PTC thermistors are often used as over-current protectors and self-regulating heating elements.

Working Principle: The working principle of a thermistor is based on the change in resistance with temperature. When the temperature changes, the resistance of the thermistor changes accordingly. This change in resistance can be measured and used to determine the temperature or control the current in a circuit.

• Impedance is the measure of opposition a circuit presents to a current when a voltage is applied. For loudspeakers, typical impedance values are usually in the range of 4 to 8 ohms, although values can vary depending on the design and application. Here's why the other options are less appropriate:
   
• Kilo Ohms (kΩ): This would be too high for loudspeakers. Impedance in the kilo-ohm range is more common in high-impedance headphones or certain types of audio equipment input and output stages rather than loudspeakers.
   
• Mega Ohms (MΩ): This is far too high and not at all typical for loudspeakers. Impedances in the mega-ohm range are found in situations involving very high resistance, such as the input impedance of certain measuring instruments like voltmeters.
   
• Milli Ohms (mΩ): This would be too low and is not typical for loudspeakers. Milliohm impedances might be relevant in contexts involving very low resistance, such as high-power connections and short circuits, but not for ordinary audio loudspeakers.

The correct answer is:

2) Active transducers

Explanation:

• Thermocouples:

  • A thermocouple generates an electrical voltage in response to a temperature difference between its two junctions, based on the Seebeck effect.
  • It does not require any external power source for operation; the voltage generated is a result of the thermoelectric properties of the materials used.

• Active Transducers:

  • Active transducers convert a form of energy directly into electrical signals without the need for an external power source.
  • Examples: Thermocouples, piezoelectric sensors, photovoltaic cells.

• Passive Transducers:

  • Passive transducers require an external power source to operate and typically modify the applied signal (e.g., resistance, capacitance).
  • Examples: Resistive temperature detectors (RTDs), capacitive sensors.

The correct option is 2) Thermocouples

Explanation:

J, K, R, S, and T refer to standard thermocouples and temperature-measuring devices. Each letter represents a specific combination of metals used in the thermocouple:

• J: Iron-Constantan
• K: Chromel-Alumel
• R: Platinum-Rhodium (13%) and Platinum
• S: Platinum-Rhodium (10%) and Platinum
• T: Copper-Constantan

The correct answer is option no "1".
Explanation:

LVDT (Linear Variable Differential Transformer):

• LVDT (Linear Variable Differential Transformer) is to convert the rectangular movement (Displacement)of an object to the equivalent electrical signal.
• LVDT is used to calculate displacement and works on the transformer principle.
• The LVDT alters a linear dislocation from a mechanical position into a relative electrical signal including phase and amplitude of the information of direction and distance.
• The working principle of the linear variable differential transformer or LVDT working theory is mutual induction; The dislocation is nonelectrical energy that is changed into electrical energy.​​

• The inner coil is the primary, which is excited by an AC source as shown.
• Magnetic flux produced by the primary is coupled to the two secondary coils, inducing an AC voltage in each coil.
   

​Applications:

• This transducer can also work as a secondary transducer.
• LVDT is used to measure the weight, force and also pressure.
• Some of these transducers are used to calculate the pressure and load.
• LVDT’s are mostly used in industries as well as servomechanisms.
• Other applications like power turbines, hydraulics, automation, aircraft, and satellites.

Calculation:

Given stain; \(\frac{\text{ }\!\!\Delta\!\!\text{ }L}{L}=5000\times {{10}^{-6}}=5\times {{10}^{-3}}\)

R = 152 Ω

ΔR = 5 Ω

Gauge factor \({{G}_{f}}~=\frac{\frac{\text{ }\!\!\Delta\!\!\text{ }R}{R}}{\frac{\text{ }\!\!\Delta\!\!\text{ }L}{L}}=\frac{5}{152\times 5\times {{10}^{-3}}}=6.58\)

Transducers: Any device which is able to convert one form of energy into another form is called a transducer.

Types of Transducer based on Quantity to be measured

• Temperature transducers (e.g. a thermocouple)
• Pressure transducers (e.g. a diaphragm)
• Displacement transducers (e.g. LVDT)
• Oscillator transducer
• Flow transducers
• Inductive Transducer

Pickups:

• A pickup is a transducer that captures, or senses mechanical vibrations produced by musical instruments, particularly stringed instruments such as the electric guitar, and converts these to an electrical signal.
• This signal is amplified using an instrument amplifier to produce musical sounds through a loudspeaker in a speaker enclosure.
• The signal from a pickup can also be recorded directly.

Strain gauges: A strain gauge (sometimes referred to as a Strain gage) is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured.

Signal generator: A signal generator is an electronic test instrument that creates or generates either repeating or non-repeating waveforms. These waveforms can be of different shapes and amplitudes. Signal generators of all types are mostly used in designing, manufacturing, servicing and repairing electronic devices.

Amplifier: An amplifier or electronic amplifier is an electronic device that can increase the power of a signal. An amplifier is used to amplify the weak signal without changing any information i.e. strengthen or boost the weak signal without changing any input or information.

The thermocouple is an electrical device containing junctions of two dissimilar metal joints. It is used as temperature sensors.

It works on the principle of the Seebeck effect.

Seebeck Effect:

• When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current that flows in the thermoelectric circuit due to the voltage difference between them. 
• This potential difference is used to measure temperatures.
• Seebeck effect is the converse of the Peltier effect.

The piezoresistive effect:

• It is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied.
• In contrast to the piezoelectric effect, the piezoresistive effect causes a change only in electrical resistance, not in electric potential.

Piezoelectric Effect:

• It is the ability of certain materials to generate an electric charge in response to applied mechanical stress.
• When piezoelectric material is placed under mechanical stress, a shifting of the positive and negative charge centers in the material takes place, which then results in an external electrical field.
• When reversed, an outer electrical field either stretches or compresses the piezoelectric material.

Hall's effect: When a conductor or semiconductor with current flowing in one direction is introduced perpendicular to a magnetic field a voltage could be measured at right angles to the current path.

The effect of getting a measurable voltage as discussed above is called the Hall Effect.

Superconductivity: Superconductivity is a physical property observed in certain materials, wherein electrical resistance vanishes and from which magnetic flux fields are expelled. Any material exhibiting these properties is a superconductor.

Thermocouple:

• A thermocouple is a sensor used to measure temperature.
• Thermocouples consist of two wire legs made from different metals.
• The wire's legs are welded together at one end, creating a junction. This junction is where the temperature is measured.
• When the junction experiences a change in temperature, a voltage is created. The voltage can then be interpreted to calculate the temperature.

• Strain Gauge is a passive transducer that converts a mechanical displacement into change in resistance R, inductance L, or capacitance C
• If the metal conductor is stretched and compressed its resistance changes because of both length and diameter of conductor change
• Also, there is change the value of resistivity of the conductors when it strained

Linear Variable Differential Transformer (LVDT)

• These devices are used to measure displacement.
• It is the most widely used inductive transducer that converts linear motion into an electrical signal.
• The LVDT is placed inside a stainless steel housing because it will provide electrostatic and electromagnetic shielding.
• LVDTs operate on the principle of a transformer.
• The transformer consists of a primary winding and two secondary windings wound on a cylindrical former.
• Both the secondary windings have an equal number of turns, and we place them on either side of the primary winding.
• The primary winding is connected to an AC source which produces a flux in the air gap and voltages are induced in secondary windings.
• A movable soft iron core is placed inside the former and the displacement to be measured is connected to the iron core.
• Both the secondary windings are connected in such a way that resulted in output is the difference between the voltages of the two windings.

Linear Variable Differential Transformer:

• Linear variable differential transformers (LVDT) are used to measure displacement.
• LVDTs operate on the principle of a transformer.
• As shown in the figure, an LVDT consists of a coil assembly and a core.
• The coil assembly is typically mounted to a stationary form, while the core is secured to the object whose position is being measured.
• The coil assembly consists of three coils of wire wound on the hollow form.
• A core of permeable material can slide freely through the center of the form.
• The inner coil is the primary, which is excited by an AC source as shown.
• Magnetic flux produced by the primary is coupled to the two secondary coils, inducing an AC voltage in each coil.

Capacitance transducer:

• Capacitive transducers are used for the measurement of displacement, and pressure.
• The capacitive transducer comprises two parallel metallic plates that are separated by a material such as air, which is called a dielectric material.
• It works on the principle of variable capacitances i.e. the distance between the two plates is the variable.

 

The capacitance of the capacitive transducer can change because of the following reasons:

• Change in the overlapping area of plates
• Change in distance between the plates and
• Change in dielectric constant

Mathematically, this is defined as:

\(C = \frac{{\epsilon A}}{d}\)

ϵ: Dielectric constant

A: Area of the plates

d: Distance between the plates

Advantages of a capacitive transducer:

• They are extremely sensitive.
• They have a high input impedance.
• Loading effects are minimum in capacitive transducers.
• They have a high resolution.
• They can be used in the stray magnetic field.
• They have a good frequency response.
• They require a small power to operate.

Photoelectric transducer:  It is a transducer that changes the energy from light to electrical. It can be designed with the semiconductor material. This transducer utilizes an element that can be used for ejecting the electrons as the light beam soaks ups through it.

Inductive transducer: It works on the principle of inductance change due to any appreciable change in the quantity to be measured.

Piezoelectric transducer: It is a device that uses the piezoelectric effect to measure changes in acceleration, pressure, strain, temperature, or force by converting this energy into an electrical charge

Analog transducers:

These transducers convert the input quantity into an analog output which is a continuous function of time

Ex: Thermistor, strain gauge, LVDT, thermocouple

Digital Transducers:

These transducers convert the input quantity into an electrical output which is in the form of pulses and its, output is represented by 0 and 1

Ex: Encoder, digital resolvers, digital tachometers, hall effect sensors, limit switches

Important Points

A transducer is an electronic device that converts energy from one form to another. Common examples include microphones, loudspeakers, thermometers, i.e.

Transducers can be classified into the following types:

1) Active or Passive Transducers

2) Analog or Digital Transducers

	Active Transducer	Passive Transducer
Definition	The transducer generates the output in the form of voltage or current, without any external energy source.	Transducers whose internal parameters like capacitance, resistance, and inductance change because of the input signal.
Additional Energy Source	Does not require	Requires
Working Principle	Draw energy from the measured source	Take power from the external source that changes the physical properties of the transducer.
Design	Simple	Complicated
Resolution	Low	High
Examples	Tachogenerator,  Thermocouple, Photovoltaic cell, etc.	Thermistor, Differential transformer, etc