Untitled Quiz

Unit 4 - Transducers and Sensors

• Basic principles and classification of Instruments: Moving coil instruments, moving iron instruments, digital multimeter, digital storage oscilloscope.
• Transducer Classification: Capacitive, inductive, LVDT (Linear Variable Differential Transformer), thermistors, thermocouple, piezoelectric transducers, photoelectric transducers, Hall effect transducers.
• Opto-electronics Devices: Light Dependent Resistor (LDR), photodiodes, phototransistors, photovoltaic cells (solar cells), optocouplers, liquid crystal display (LCD), proximity sensor, IR sensor, bio sensor.
• Sensors for smart building: Introduction to sensors used in building automation, such as for security, energy management, temperature regulation.

Classification of Instruments

• Methods of Classification: By the quantity measured (e.g., voltmeter, ammeter), by the principles used for their working (e.g., moving iron), and by the method of measurement used (e.g., deflecting).
• Deflecting Torque: The torque acting on the instrument's moving system to cause the required deflection, proportional to the magnitude of the measured quantity.
• Opposing Torques:
  • Inertia Torque: Due to the moving system's inertia, the deflecting torque must overcome this.
  • Control Torque: Always present, opposing the deflecting torque, and essential to return the system to zero position after disconnection. It's usually produced by springs or gravity.
  • Damping Torque: Produced during instrument operation, ensuring the moving system reaches its final position in the required time.

Moving Coil Instruments (PMMC)

• Principle: A current-carrying coil placed in a magnetic field experiences a force, causing it to rotate.
• Construction: A permanent magnet, soft iron core, coil of many turns, spindle, helical springs, and pointer.
• Working: The magnetic field from the permanent magnet interacts with the current in the coil, producing a torque proportional to the current or voltage being measured. The pointer indicates the value on a calibrated scale.
• Deflecting Torque, Control Torque & Damping Torque: Deflecting torque is proportional to current, control torque provided by springs, and damping due to eddy currents.

Dynamometer Type Moving Coil Instrument

• Principle: The operating fields are produced by currents, not permanent magnets. Both fixed and moving coils carry current.
• Construction: Fixed coil in two sections, a moving coil placed between them, spindle, pointer, calibrated scale. Spring for control torque. Piston inside air chamber to create damping.
• Working: The interaction between the currents and magnetic fields in the fixed and moving coils creates a deflecting torque proportional to the square of the voltage or current measured.
• Deflecting Torque, Control Torque & Damping Torque: Deflecting torque proportional to current squared, control torque provided by springs, damping by air friction.

Moving Iron Instruments (Attraction & Repulsion Type)

• Principle: A soft iron piece (armature) is placed in the magnetic field produced by the current carrying coil. The movement due to attraction/repulsion forces proportionally to the square of the current or voltage.
• Construction: A working coil, soft iron disc/armature, spindle with pointer on a calibrated scale, control torque by spring/gravity.
• Working: The current in the coil produces a magnetic field, thus the magnetic field magnetizes the soft iron disc towards or away from the centre of the coil, causing it to rotate. The pointer indicates the value on a calibrated scale.
• Deflecting Torque, Control Torque & Damping Torque: Deflecting torque proportional to square of the current, control torque provided by springs/gravity, damping by air friction.

Digital Multimeter (DMM)

• Principle: Measures voltage, current, and resistance using digital display. Combines multiple meters in one device.
• Construction: Constant current source, rotary switch, input probes, buffer amplifier, calibrated attenuator, rectifier circuit, Analog to Digital converter, digital display.
• Working: The input is routed to the appropriate circuit based on settings, which feeds the result to the display.
• Advantages & Disadvantages: Advantages are high accuracy, zero parallax error, digital output. Disadvantages include poor response to fluctuating measurements, more expensive, can be difficult to find specific models.
• Special Functions: Continuity test, diode test, battery test.

Digital Storage Oscilloscope (DSO)

• Principle: Stores a digital copy of the waveform for signal analysis, including signal processing.
• Construction: Amplifier, digitizer, memory, analyzer circuitry, waveform reconstruction, horizontal amplifier, time base circuitry, trigger, and clock.
• Working: The input signal is amplified, digitized, stored in memory, and processed by the analyzer circuit. The processed data is converted back into an analog form. This signal is applied to the CRT, displaying the waveform on screen.
• Advantages & Disadvantages: High accuracy, fast analysis, minimal error. Disadvantages include expensive, performance can be influenced if the sampling rate is not appropriate for the signal being measured.

Basic Requirements of a Transducer

• Linearity: Input-output characteristics should be linear.
• Ruggedness: Should withstand overloads with protective measures.
• Repeatability: Produces identical signals for same input under same conditions.
• High Stability and Reliability: The output should not be affected by temperature, vibrations, etc., with minimum error.
• Good Dynamic Response: Should respond efficiently to input variations.
• Convenient Instrumentation: The signal should be strong enough to be measured.
• Good Mechanical Characteristics: Should maintain their form and not deform under conditions.

Active and Passive Transducers

• Active Transducers: Develop their own output voltage or current from the input signal/energy. e.g., thermocouples, piezoelectric transducers.
• Passive Transducers: Require an external power source to convert input signal into an output. e.g., potentiometer, resistance strain gauges.

Other Sensor Types

• Thermocouples: Measure temperature differences based on the Seebeck effect.
• Piezoelectric Transducers: Convert pressure, force, acceleration, into electrical signals.
• Hall Effect Transducers: Measure magnetic fields.
• Light-Dependent Resistors (LDRs): Resistance changes in response to light.
• Photodiodes: Generate current in response to light incidence, typically in reverse bias.
• Phototransistors: Amplify the photocurrent signal, commonly used in light detection.
• Photovoltaic Cells (Solar Cells): Convert light into direct current energy.
• Optocouplers: Provide electrical isolation between circuits using light.
• Liquid Crystal Displays (LCDs): Modulate light to create images and text.
• Proximity Sensors: Detect the proximity of an object without physical contact.
• Pressure Sensors: Convert pressure to electrical signals.
• IR (Infrared) Sensors: Detect infrared radiation.
• Biosensors: Combine biological elements with transducers to detect analytes.