2.2 Sensing and transducer technologies

Sensing and transducer technologies are crucial for wireless sensor networks. They convert physical phenomena into electrical signals, enabling data collection and monitoring. This topic covers various sensor types, from temperature and humidity to motion and orientation.

MEMS technology has revolutionized sensor design, making them smaller, cheaper, and more efficient. We'll explore how different sensing principles like capacitive and resistive sensing work, and their applications in environmental and motion sensing.

Environmental Sensors

Temperature Measurement

Top images from around the web for Temperature Measurement

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

1 of 3

Top images from around the web for Temperature Measurement

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

• 

  JSSS - Temperature estimation of induction machines based on wireless sensor networks View original

  Is this image relevant?

1 of 3

• Temperature sensors measure the amount of heat energy in a source, allowing the temperature to be calculated
• Thermistors are temperature-sensitive resistors that change resistance with temperature (negative temperature coefficient thermistors decrease resistance as temperature increases)
• Thermocouples consist of two dissimilar metal wires joined at one end (the measuring junction) that generate a voltage proportional to the temperature difference between the measuring junction and the reference junction
• Resistance temperature detectors (RTDs) are made from pure metals (platinum) with resistance that increases linearly with temperature, offering high accuracy and stability

Humidity and Pressure Sensing

• Humidity sensors measure the amount of water vapor in the air, which is expressed as relative humidity (the ratio of the current absolute humidity to the highest possible absolute humidity)
• Capacitive humidity sensors use a hygroscopic dielectric material between two electrodes, and as the humidity changes, the dielectric constant of the hygroscopic material changes, altering the capacitance
• Pressure sensors measure the force per unit area applied on a surface and can be used to determine altitude, detect leaks, or monitor weather conditions
• Piezoresistive pressure sensors use a diaphragm that flexes under applied pressure, causing a change in the resistance of the piezoresistive material bonded to the diaphragm

Light Sensing

• Photodiodes are semiconductor devices that convert light into an electrical current, with the current proportional to the intensity of the incident light
• When photons strike the photodiode's depletion region, electron-hole pairs are generated, causing a photocurrent to flow in the presence of a reverse bias voltage
• Photodiodes can detect visible, ultraviolet, or infrared light depending on the bandgap of the semiconductor material used (silicon, germanium, or indium gallium arsenide)
• Applications of photodiodes include ambient light sensing, optical communications, and spectroscopy

Motion and Orientation Sensors

Inertial Sensors

• Accelerometers measure proper acceleration (acceleration relative to freefall) and can detect the orientation, vibration, and shock experienced by an object
• MEMS accelerometers use a proof mass suspended by springs, and as acceleration is applied, the proof mass displaces, causing a change in capacitance proportional to the acceleration
• Gyroscopes measure angular velocity and can determine an object's orientation and rotation
• MEMS gyroscopes use a vibrating mechanical element (proof mass) to sense rotation based on the Coriolis effect, which causes a secondary vibration perpendicular to the original vibration when the gyroscope is rotated

Vibration and Impact Sensing

• Piezoelectric sensors generate an electrical charge in response to applied mechanical stress, making them useful for detecting vibrations, impacts, and changes in pressure
• The piezoelectric effect occurs in certain materials (quartz, lead zirconate titanate) due to the displacement of ions in the crystal structure when stressed, creating a dipole moment and generating a voltage
• Piezoelectric accelerometers use a seismic mass coupled to a piezoelectric element, and when the mass experiences acceleration, it exerts a force on the piezoelectric element, generating a voltage proportional to the acceleration
• Applications of piezoelectric sensors include structural health monitoring, impact detection, and vibration control

Sensing Technologies

Microelectromechanical Systems (MEMS)

• MEMS technology involves the miniaturization of mechanical and electro-mechanical elements using microfabrication techniques derived from the semiconductor industry
• MEMS devices combine electrical and mechanical components on a single chip, enabling the integration of sensing, actuation, and signal processing
• Common MEMS fabrication processes include surface micromachining (depositing and etching thin films on a substrate) and bulk micromachining (selectively etching the substrate itself)
• MEMS sensors offer advantages such as small size, low power consumption, low cost, and high sensitivity, making them well-suited for wireless sensor network applications

Capacitive and Resistive Sensing Principles

• Capacitive sensing relies on measuring changes in capacitance caused by the variation of the dielectric constant or the distance between the plates of a capacitor
• In MEMS capacitive sensors, one plate is fixed (the substrate) while the other plate (the proof mass) moves in response to the measured stimulus, changing the capacitance
• Capacitive sensing is used in applications such as humidity sensing, pressure sensing, and touch sensing due to its high sensitivity and low power consumption
• Resistive sensing involves measuring changes in resistance caused by the deformation or displacement of a resistive element
• Piezoresistive sensors (strain gauges) exhibit a change in resistance when subjected to mechanical stress, allowing them to measure force, pressure, and acceleration
• Resistive temperature detectors (RTDs) and thermistors are examples of resistive sensors used for temperature measurement, exploiting the temperature-dependent resistance of the sensing material