Lecture_6_Sensing elements

SENSING ELEMENTS PRINCIPLE OF RESISTIVE SENSORS

  • Resistive Sensor: An electromechanical device converting mechanical changes (like displacement) into electrical signals.

  • Resistance Formula: R = (Resistivity * Length) / Area; R = ρ L / A

  • Factors Affecting Resistance:

    • Composition

    • Temperature

    • Length

    • Cross Sectional Area

  • Composition and temperature changes do not simply affect resistivity.

  • Major Types of Resistive Sensors:

    • Potentiometers

    • Strain Gauges

    • Resistance Temperature Detector (RTD)

    • Thermistors

    • Light Dependent Resistor (LDR)

1. POTENTIOMETER

  • Connection: Object displacement measured via:

    • Rotating shaft (angular displacement)

    • Moving rod (linear displacement)

    • Stretched cable

  • Electric Circuit:

    • Voltage equation: VA = I * RA

      • Where I = VS / (RA + RB)

    • Substituting gives: VA = VS * RA / (RA + RB)

Advantages:

  • Low cost and simple operation

  • Effective for large amplitude measurement

  • High electrical efficiency

Disadvantages:

  • Requires significant force to move sliding contacts (wipers) which can lead to wear, contamination, misalignment

  • Limited lifespan of sliding contacts

Applications:

  • Control systems with feedback loops (machine-tool controls, elevators, etc.)

  • Manufacturing processes (injection molding, robotics, etc.)

2. PROVING RINGS

  • Used For: Measurement of force, weight, or load through deflection measured by electrical sensors.

  • Typically paired with LVDT for deflection measurement and used in load cell applications.

3. STRAIN GAUGE

  • Principle: Resistance changes when a metal conductor is stretched or compressed due to variations in length, diameter, and resistivity (piezoresistive effect).

  • Applications: Load cells, torque meters, pressure gauges, etc.

  • Strain Definition: Fractional change in length due to applied force, expressed as micro strain.

  • Sensitivity: Gauge factor (GF) relates fractional resistance change to strain.

Gauge Factor:

  • For metallic gauges: Typically around 2

Semiconductor Strain Gauge:

  • Advantages:

    • High sensitivity, can measure strain as small as 0.01 microstrain

    • High fatigue life and excellent resolution

Disadvantages:

  • Sensitive to temperature changes and poor linearity

  • More expensive and difficult to attach

  • Typically used in bridge configurations for accurate readings.

4. RESISTANCE THERMOMETER (RTD)

  • Electronic device measuring temperature from resistance variance in metals.

  • Metals Used: Copper, Nickel, Platinum (widely used due to wide temperature range)

  • Resistance Equation: R_T = R_0(1 + α)(T - T_0)

5. THERMISTOR

  • Highly sensitive semiconductor with large resistance changes over small temperature ranges.

  • Types: Positive (PTC) and negative (NTC) temperature coefficient thermistors.

  • Applications emphasize precision and control in various temperature-sensitive environments.

Construction:

  • Composed of metallic oxide mixtures, available in various forms (beads, rods, discs).

6. HOT WIRE ANEMOMETER

  • Measures fluid velocity and direction by heat loss in a wire.

  • Constructed with conducting wire and Wheatstone bridge to assess resistance changes.

7. RESISTANCE HYGROMETER

  • Definition: Measures humidity using materials whose electrical properties change with moisture.

  • Humidity Types: Absolute and relative humidity.

8. PHOTO RESISTIVE SENSOR (LDR)

  • Function: Resistance changes with light intensity, often used in ambient light detection.

  • Common materials include Cadmium Sulphide (Cds).

Characteristics:

  • Dark resistance of about 10MΩ decreases to around 100Ω in light.

9. MULTIPLE CHOICE QUESTIONS

  1. Change in sensor output with input change: Sensitivity

  2. Non-zero sensor output with zero input: Offset or bias

  3. Smallest detectable change: Resolution

  4. Energy conversion device: Transducer

  5. Consistent readings ability: Precision

10. ASSIGNMENTS

  1. Explain construction, principle of operation, circuit, and applications of Strain Gauge with diagrams.

  2. Describe Thermistors, their construction forms, and application with diagrams.

UNIT II - INDUCTIVE & CAPACITIVE TRANSDUCERS

  • Definition: A transducer converts one form of energy into another.

  • Advantages of Electrical Transducers:

    • Amplification, low power, easy transmission, and digital compatibility.

  • Transducer Classification: Active, Passive, Analog, Digital, Primary, Secondary, Inverse, etc.

5. LVDT

  • Function: Measures linear displacement using electromagnetic induction.

  • Construction: Cylindrical transformer with primary and secondary windings.

Output Voltage:

  • Eo = Es1 - Es2 depending on core position.

Applications:

  • Used for precision displacement measurements in various fields.

12. CAPACITIVE TRANSDUCERS

  • Function: Measures displacement based on capacitance changes due to distance or area variations.

  • Advantages: Sensitive, good frequency response, minimal force operation.

  • Applications: Level monitoring, shape error measurement in industrial applications.

13. CAPACITIVE PROXIMITY SENSORS

  • Detects changes in capacitance due to nearby objects.

  • Operation: Utilizes an electrostatic field, can sense both conductive and non-conductive materials.

  • Advantages: Contactless detection and wide material range.

Disadvantages: Low range and higher cost compared to inductive sensors.

KEY QUESTIONS

  1. In a LVDT, secondary voltages vary dependent on core position.

  2. Bill of materials and functionality in standard applications for sensors.