2. TRANSDUCERS notes in english

 

2. TRANSDUCERS

A transducer is a device that converts one form of energy into another. In instrumentation, transducers are essential because they convert physical quantities (such as temperature, pressure, flow, etc.) into measurable electrical signals, making them easier to process, analyze, and control. The various types of transducers are used in different applications depending on the type of energy conversion needed.

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2.1 Distinguish Between:

2.1.1 Primary and Secondary Transducers:

• Primary Transducer:
  • Definition: A primary transducer directly converts a physical quantity (such as temperature, pressure, or displacement) into an electrical signal without any intermediate step.
  • Example: A thermocouple (converts temperature into voltage) or a strain gauge (converts strain into a change in resistance).
  • Characteristics: It operates on the principle of directly sensing the physical phenomenon.
• Secondary Transducer:
  • Definition: A secondary transducer receives the output from a primary transducer and then processes or converts it into a usable form, such as further amplification or display.
  • Example: A voltmeter used to measure the output from a thermocouple, or a digital display that shows the output of a strain gauge.
  • Characteristics: It does not sense the physical quantity but instead works on the electrical signal received from the primary transducer.

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2.1.2 Electrical and Mechanical Transducers:

• Electrical Transducers:

  • Definition: Electrical transducers convert physical quantities into electrical signals such as voltage, current, or resistance.
  • Example: A thermocouple (temperature to voltage), LVDT (displacement to voltage), or strain gauge (strain to resistance).
  • Characteristics: They provide output in an electrical form that is easy to process and control.

• Mechanical Transducers:

  • Definition: Mechanical transducers directly convert physical quantities into mechanical displacement or force.
  • Example: Bourdon tube (pressure to mechanical displacement), diaphragm (pressure to mechanical displacement).
  • Characteristics: They are often used in situations where an electrical signal is not practical or desirable.

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2.1.3 Analog and Digital Transducers:

• Analog Transducers:

  • Definition: Analog transducers produce continuous output signals that are proportional to the measured physical quantity. These signals can vary in a continuous manner.
  • Example: Thermocouple (produces a continuous voltage signal), potentiometer (continuous position signal).
  • Characteristics: The output is continuous, and the measurement is real-time and continuous.

• Digital Transducers:

  • Definition: Digital transducers produce discrete output signals that represent the measured quantity in a digital form (usually binary code).
  • Example: Digital thermometers, tachometers (measuring speed), and pressure sensors with digital output.
  • Characteristics: The output is in a digital form, and it is often more suitable for processing by digital computers or microcontrollers.

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2.1.4 Active and Passive Transducers:

• Active Transducers:

  • Definition: Active transducers require an external power source to operate and generate their output signal. They can produce their own electrical signal in response to a physical input.
  • Example: Thermocouple, piezoelectric transducer.
  • Characteristics: These transducers do not require an external signal conditioning system as they generate their own output signal.

• Passive Transducers:

  • Definition: Passive transducers do not generate an electrical output but rather modify an external electrical signal in response to a physical quantity.
  • Example: Strain gauges (change in resistance), LVDT (change in inductance).
  • Characteristics: They require an external source of power or signal conditioning for their operation.

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2.1.5 Mechanical Devices:

• Definition: Mechanical devices are instruments that use physical movement or displacement to measure a quantity. These devices are usually part of the system that converts mechanical energy into electrical signals or other forms of energy.
• Example: Mechanical pressure gauges, Bourdon tubes, and mechanical manometers.
• Characteristics: Mechanical devices are typically not as sensitive or accurate as electrical devices but are often used for direct measurement in rugged or non-electrical environments.

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2.2 Advantages of Electric Transducers:

Electric transducers have several advantages over mechanical devices:

1. High Sensitivity: They can detect very small changes in the measured quantity.
2. Easy to Process: Their electrical output is easy to amplify, process, and store.
3. Accuracy: They offer better precision and reproducibility in measurements.
4. Remote Sensing: Electric transducers can be used to remotely monitor and control systems via electrical signals, often allowing long-distance transmission of data.
5. Flexibility: Their output can be easily interfaced with other devices like computers, controllers, and displays.
6. Compactness: They are generally more compact and can fit into smaller spaces.
7. Cost-effective: Electric transducers often cost less to maintain and operate than mechanical transducers.

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2.3 Factors Affecting the Choice of Transducers:

The selection of a transducer depends on several factors:

1. Range and Sensitivity: The transducer should be able to measure the required range of the physical quantity with adequate sensitivity.
2. Accuracy and Precision: The accuracy of the transducer should be suitable for the application.
3. Output Type: The output type (analog or digital) should be compatible with the control or measurement system.
4. Environmental Conditions: The transducer should be suitable for the operating environment (e.g., temperature, humidity, vibration).
5. Power Requirements: The power requirements of the transducer should be considered, especially for portable or remote systems.
6. Cost and Maintenance: Transducers with low cost and easy maintenance are often preferred in industrial applications.
7. Size and Weight: In certain applications (e.g., aerospace), the size and weight of the transducer are critical factors.

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2.4 Construction and Principle of Resistive Transducers

Resistive transducers work on the principle that resistance changes with the change in the physical quantity being measured, such as temperature, displacement, or strain.

2.4.1 Potentiometer – Variac

• Construction: A potentiometer consists of a resistive element with a sliding contact (wiper) that moves along the resistance element. As the wiper moves, the resistance changes, generating an output voltage.
• Working Principle: The output voltage is proportional to the position of the wiper, which changes in response to the physical quantity.
• Application: Used for measuring displacement, position, or angular rotation.

2.4.2 Strain Gauges

• Definition: A strain gauge is a sensor used to measure the strain (deformation) on an object.
• Formula for Gauge Factor: $$G = \frac{\Delta R / R}{\Delta L / L}$$ where:
  • $G$ is the gauge factor
  • $\Delta R$ is the change in resistance
  • $R$ is the original resistance
  • $\Delta L$ is the change in length
  • $L$ is the original length

2.4.2.1 Types of Strain Gauges:

1. Unbonded Strain Gauges:

   • Construction: These gauges consist of a wire or foil placed on a non-deformable base. They are not attached directly to the object being measured.
   • Application: Used for measuring larger strains or when less precision is acceptable.

2. Bonded Strain Gauges:

   • Construction: These gauges have a metal foil pattern bonded directly to the object being measured. The resistance changes as the object deforms.
   • Application: Common in structural and mechanical testing due to their precision.

3. Semiconductor Strain Gauges:

   • Construction: Uses semiconductor materials that exhibit significant changes in resistance when strained.
   • Application: Used for high-precision measurements but can be more sensitive to temperature variations.

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2.5 Construction and Principle of Following Transducers

2.5.1 L.V.D.T (Linear Variable Differential Transformer)

• Construction: An LVDT consists of a primary coil and two secondary coils symmetrically placed around the primary. A movable core is placed inside the coil assembly.
• Working Principle: As the core moves in response to displacement, the induced voltage in the secondary coils changes, creating a differential voltage that is proportional to the displacement.
• Application: Used for accurate linear displacement measurements.

2.5.2 R.V.D.T (Rotary Variable Differential Transformer)

• Construction: Similar to the LVDT but used for angular displacement. It has a rotating core instead of a linear one.
• Working Principle: As the core rotates, the voltage in the secondary coils changes, which is proportional to the angular displacement.
• Application: Commonly used for measuring rotational positions or angles.

2.5.3 Photoconductive Cells

• Construction: Photoconductive cells are made from materials like cadmium sulfide (CdS) that change their resistance when exposed to light.
• Working Principle: When light falls on the material, its resistance decreases, which can be measured as a change in electrical current.
• Application: Used in light-sensitive applications like automatic lighting systems.

2.5.4 Photovoltaic Cells

• Construction: A photovoltaic cell is a semiconductor device that converts light energy directly into electrical energy.
• Working Principle: When exposed to light, electrons in the semiconductor material are excited, generating a current.
• Application: Commonly used in solar energy systems, light meters, and other solar-powered devices.