Sensors and Transducers: 2 Marks Important Questions with Answers

Basic Electronics and Electrical Engineering

CHAPTER 8: SENSORS AND TRANSDUCERS

Part A ‒ Two Marks Questions and Answers

1. What is the purpose of a sensor?

Sensor is used for an element which produces a signal relating to the quantity being measured. i.e., an electrical resistance temperature element, the quantity being measured is temperature and the sensor transforms an input of temperature into a change in resistance.

2. Define transducer?

The term transducer is often used in place of the term sensor. Transducer is defined as elements that when subject to some physical change experience a related change. Thus, sensors are transducers.

3. What are the terms that define the performance of the transducers?

1. Range and span

2. Error

3. Accuracy

4. Sensitivity

5. Hysteresis error

6. Non‒linearity error

7. Repeatability/ reproducibility

8. Stability

9. Dead band / time

10. Resolution

11. Output impedance.

4. Define static characteristics?

The static characteristics are the values given when the steady‒state conditions occur. i.e., the values given when the transducer has settled down after having received some input.

5. Define dynamic characteristics?

The dynamic characteristics refer to the behavior between the time that the input value changes and the times that the value given by the transducer settles down to the steady‒ state value. Dynamic characteristics are stated in terms of the response of the transducer to inputs in particular forms.

6. What are the terms that you can find from the dynamic characteristics?

1. Response time.

2. Time constant.

3. Rise time

4. Settling time.

7. What is a displacement sensor?

Displacement sensors are concerned with the measurement of the amount by which some object has been moved.

8. What is a position sensor?

Position sensors are concerned with the determination of the position of some object with reference to some reference point.

9. What is a proximity sensor?

Proximity sensors are a form of position sensor and are used to determine when an object has moved to within some particular critical distance of the sensor.

10. What are the two basic types of the displacement and the position transducers?

The two basic types are,

1. Contact sensors

2. Non contact sensors.

11. Write about the strain gauged element?

The electrical resistance strain gauge is a metal wire, metal foil strip, or a strip of semiconductor material, which is wafer like and can be stuck onto surfaces like a postage stamp.

12. Define gauge factor.

The gauge factor is defined as the ratio of per unit change in resistance to per unit change in length.

Gauge factor Gf = ∆R / R

= ∆L / L

13. Write about the gauge factors for different types of strain gauges?

The gauge factor of metal wire or foil strain gauges with the metals generally used is about 2.0. Silicon p- and n‒ type semiconductor strain gauges have gauge factors of about + 100 or more for p‒type silicon and ‒ 100 or more for n ‒ type silicon.

14. What is the capacitance of a parallel plate capacitor?

The capacitance of a parallel plate capacitor is given by,

 C = εA / d = ε_rε₀A / d

Where

 ε = Permitivity of the medium/m

 ε_r = Relative permitivity

ε₀ = Permitivity of the free space = 8.85 × 10^‒12 F/m

A = Area of plates

D = Distance between two plates

15. Define LVDT?

The Linear Variable Differential Transformer consists of three coils symmetrically spaced along an insulated tube. The central coil is the primary coil and the other two are identical secondary coils, which are connected in series in such a way that their outputs oppose each other.

16. Determine the working Principle of LVDT?

When there is an alternating voltage input to the primary coil, alternating e.m.f.s are induced in the secondary coils. With the magnetic core central, the amount of magnetic material in each of the secondary coil is the same.

17. What are the uses of LVDT?

The uses are as follows.

(a) Widely used as primary transducers for monitoring displacements.

(b) Also used as secondary transducers in the measurement of force, weight and pressure.

18. Write about inductive proximity switch?

Inductive proximity switch consists of a coil wound round a core. When the end of the coil is close to a metal object is inductance changes. This change can be monitored by its effect on a resonant circuit and the change used to trigger a switch. It can only be used for the detection of metal objects and is best with ferrous metals.

19. What is an encoder?

An encoder is a device that provides a digital output as a result of a linear or angular displacement.

20. What are the two types of position encoders?

The two types of position encoders are,

 (a) Incremental encoders (b) Absolute encoders.

21. What is an incremental encoder?

An incremental encoder is used for the measurement of angular displacement. A beam of light passes through slots in a disc and is detected by a suitable light sensor. When the disc is rotated, the sensor produces a pulsed output with the number of pulses being proportional to the angle through which the disc is rotated. Hence, the angular position of the disc is determined.

22. Write about absolute encoder?

An absolute encoder is used for the measurement of angular displacement. This gives an output in the form of a binary number of several digits, each such number representing a particular angular position. The rotating disc has three concentric circles of slots and three sensors to detect the light pulses.

23. What are the uses of photosensitive devices?

Photosensitive devices can be used to detect the presence of an opaque object by it breaking a beam of light or infrared radiation, falling on such a device or by detecting the light reflected back by the object.

24. Write about Hall effect sensors?

When a beam of charged particles passes through a magnetic field, forces act on the particles and the beam is deflected from its straight line path. A current flowing in a conductor is like a beam of moving charges and thus can be deflected by a magnetic field.

25. What is Hall co‒efficient?

The transverse potential difference is given by, V = KHBI

Where,

KH = Hall co‒efficient

B = Magnetic flux density at right angles to the plate,

I = Current

26. What are digital signals?

Digital signals are discrete time signals in that they are not continuous functions of time but exist at only discrete times.

27. Why the signal conditioning is required?

Signal conditioning of analogue signals requires components such as amplifiers filter circuits and digital.

28. Define Actuator.

An actuator is an output device which performs useful works

29. Classify actuator based on motion.

1. Linear motion

• Single acting cylinder

• Double acting cylinder

2 Rotary Motion

• Air motors

• Rotary actuators