The use of sensors in collecting pulse wave signals has become increasingly prevalent in modern healthcare. Pulse wave sensors are typically non-invasive devices that are designed to measure and detect the change in arterial pulse. These sensors are integrated into various
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wearable devices or medical equipments, allowing for continuous monitoring of pulse wave signals. There are different types of sensors used to collect pulse wave signals, including photoplethysmography (PPG) sensors, laser doppler sensors, piezoresistive sensors, piezoelectric sensors, etc.
2.2.1. Photoplethysmography (PPG) sensors
PPG sensors are non-invasive devices that utilize light-emitting diodes (LED) and photodetectors to measure variations in blood volume and flow. They are often placed on the skin, typically at the fingertip or wrist, to capture pulse wave signals.
PPG sensors work based on the principle of optical absorption and reflection. The sensor emits light from an LED onto the skin, and the photodetector measures the intensity of the light that is either absorbed or reflected by the underlying blood vessels. This variation in light intensity is caused by changes in blood volume with each heartbeat, resulting in a pulsatile waveform known as the PPG signal.
Figure 2.5: PPG reflection type working principle Figure 2.4: PPG sensors placements
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- Advantages of PPG sensor:
• Non-invasive: PPG sensors are non-invasive, as they can be placed on the skin surface without the need for penetration or discomfort.
• Continuous Monitoring: PPG sensors allow for continuous monitoring of pulse wave signals, providing real-time information about heart rate and other cardiovascular parameters.
• Cost-Effective: PPG sensors are generally more cost-effective compared to invasive methods or more complex diagnostic equipment.
• Portable and Wearable: PPG sensors can be integrated into wearable devices or mobile applications, enabling convenient and portable monitoring for personal health tracking.
- Disadvantages of PPG sensor:
• Limited Depth of Measurement: PPG sensors primarily capture signals from superficial tissues and are not able to assess deep-seated vascular structures.
• Sensitivity to Motion Artifacts: PPG signals can be affected by motion artifacts, such as movement of the body or the sensor itself, leading to inaccuracies in the measurements.
• Subject to Environmental Factors: External factors such as ambient light, temperature, and skin pigmentation can influence the performance and accuracy of PPG sensors.
2.2.2. Piezoresistive sensors
Piezoresistive strain gauges are widely used as pressure sensors due to their commonality and effectiveness. These sensors operate based on the principle that the electrical resistance of a material changes when it undergoes deformation or strain. The fundamental concept behind a piezoresistive pressure sensor involves utilizing a strain gauge made of a conductive material that exhibits changes in electrical resistance when subjected to stretching. This strain gauge is typically affixed to a diaphragm, which acts as the sensing element and deforms in response to applied pressure. The sensitivity of each material is calculated by a gauge factor, which is defined as the ratio of the relative resistance change and the strain
𝐶𝐹 = (𝛥𝑅
𝑅 ) 𝜖
(2.1) Where strain is defined as the relative change in length:
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𝜖 =𝛥𝐿
𝐿 (2.2)
The strain gauge elements can be made of various materials, including metals such as constantan (a copper-nickel alloy), karma alloy (nickel-chromium), nickel, and semiconductors such as silicon, germanium, or polysilicon. While metal materials offer good mechanical stability, semiconductor strain gauges often have a higher gauge factor than metal foils, providing enhanced sensitivity. However, they can be more sensitive to temperature variations and require careful temperature compensation. In real-life applications, a Wheatstone bridge circuit is commonly employed to measure the change in resistance in the strain gauge sensor. This circuit configuration enables the conversion of small variations in the sensor's resistance into an output voltage.
Figure 2.6: Connected circuit between Piezoresistive sensor and Wheatstone Bridge (Source:
https://www.avnet.com/wps/portal/abacus/solutions/technologies/sensors/pressure-sensors/core- technologies/piezoresistive-strain-gauge/)
The Wheatstone bridge circuit requires an excitation voltage to be applied. In the absence of strain, when all resistors in the bridge are balanced, the output voltage will be zero. However, when pressure is applied, causing resistance changes in the bridge, an output voltage or current will be generated. The calculation for determining this output is provided by the formula below.
𝑉0 = [ 𝑅3
𝑅3+ 𝑅4 − 𝑅2
𝑅2+ 121] 𝑉𝑒𝑥 (2.3)
- Advantages of Piezoresistive sensors:
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• High Sensitivity: Piezoresistive sensors exhibit high sensitivity, allowing them to detect even small changes in pressure or strain.
• Fast Response Time: Piezoresistive sensors have a fast response time, enabling real-time monitoring and measurement of dynamic pressure changes.
• Cost-Effective: Piezoresistive sensors are relatively cost-effective compared to other pressure sensing technologies, making them accessible for a wide range of applications and industries.
• Compatibility with Electronics: Piezoresistive sensors can be easily integrated with electronic circuits and microcontrollers for signal processing and data acquisition.
They can be combined with digital interfaces, allowing for easy integration into larger systems.
- Disadvantages of Piezoresistive sensors:
• Temperature Sensitivity: Piezoresistive sensors can be sensitive to temperature changes, which may affect their accuracy and require temperature compensation techniques to ensure reliable measurements.
• Non-Linearity: The output response of piezoresistive sensors may exhibit non- linear behavior, especially at higher pressure ranges. Calibration may be necessary to obtain accurate and linear measurements.
• Drift: Over time, piezoresistive sensors can experience drift in their output readings, requiring periodic recalibration to maintain accuracy.
• Mechanical Fragility: Piezoresistive sensors can be sensitive to mechanical stress and may be prone to damage if exposed to excessive force or shocks. Proper handling and protection are necessary to ensure their longevity.
2.2.3. Capacitive pressure sensors
Capacitive pressure sensors are devices that utilize changes in capacitance to measure and monitor pressure variations. These sensors consist of two conductive plates or electrodes separated by a dielectric material. The capacitance between the plates is directly influenced by the applied pressure, causing a change in the capacitance value. The capacitance is defined by:
𝐶 = 𝜀𝑟𝜀0𝐴
𝑑 (2.4)
Where:
• 𝜀𝑟 is the dielectric constant of the material between the plates (this is 1 for a vacuum)
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• 𝜀0 is the electric constant (equal to 8.854x1012 F/m),
• A is the area of the plates
• d is the distance between the plates
Capacitive pressure sensors typically consist of a diaphragm or membrane that deforms in response to applied pressure. The diaphragm acts as one of the electrodes, while the other electrode is a fixed plate or backplate. The space between the electrodes is filled with a dielectric material. When pressure is applied to the diaphragm, it deforms, altering the distance between the two electrodes. This change in spacing results in a change in capacitance, as the effective area and separation between the electrodes are modified. The capacitance increases with decreased spacing and vice versa (following to the above equation). The variation in capacitance is measured using appropriate electronics and signal processing techniques. The sensor is connected to an external circuit, which applies an alternating current (AC) or direct current (DC) voltage to the sensor. The resulting capacitance change is converted into an output signal, typically voltage or frequency, that represents the applied pressure.
Figure 2.7: Capacitive pressure sensor structure (Source:
https://www.avnet.com/wps/portal/abacus/solutions/technologies/sensors/pressure-sensors/core- technologies/capacitive/)
- Advantages of capacity pressure sensors:
• High Sensitivity: Capacitive sensors can achieve high sensitivity, allowing them to detect small pressure changes accurately
• Wide Measurement Range: Capacitive pressure sensors can be designed to measure a wide range of pressures, from very low to high values.
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• Good Linearity: These sensors often exhibit good linearity, meaning the output response is proportional to the applied pressure, allowing for precise measurements.
• Low Power Consumption: Capacitive pressure sensors typically consume low power, making them suitable for battery-operated devices and applications where power efficiency is essential.
- Disadvantages of capacity pressure sensors:
• Temperature Sensitivity: The capacitance of the sensor can be influenced by temperature variations, necessitating temperature compensation techniques for accurate measurements.
• Limited Overload Capacity: Exceeding the maximum pressure limit can damage the diaphragm or affect the sensor's performance.
• Susceptibility to Environmental Factors: External factors like humidity, moisture, and contaminants can affect the dielectric properties and introduce measurement errors.
2.2.4. Piezoelectricity sensors
Piezoelectric sensors are electronic devices that can transform mechanical or thermal energy into electrical signals, utilizing the principle of electromechanical coupling. This phenomenon of piezoelectricity occurs in certain materials that generate an electrical voltage when exposed to mechanical stress. Conversely, these materials also produce mechanical stress when subjected to an electrical voltage. The unique capabilities of piezoelectric sensors make them particularly well-suited for applications in Traditional Medicine (TM).
One significant advantage of piezoelectric sensors is their exceptional sensitivity. They can detect even the smallest pressure waves, such as the pulse wave generated by the heartbeat. This high sensitivity allows for precise monitoring and analysis of physiological signals related to the pulse. By accurately capturing and interpreting these subtle pressure variations, piezoelectric sensors contribute valuable insights into a person's health and well-being.
In addition to their sensitivity, piezoelectric sensors boast an impressive response time. The pulse wave is a dynamic signal that changes rapidly, requiring sensors with a fast response time to capture these fluctuations accurately. Piezoelectric sensors excel in this aspect, providing real- time data that is crucial for assessing pulse characteristics in TM. The ability to capture rapid changes in pressure accurately enables practitioners to make informed diagnoses and tailor treatments accordingly.
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Figure 2.8: Piezoelectric pressure sensor construction
Piezoelectric sensor is constructed with a piezo film which is usually made from Zinc oxide or Lead zirconate titanate (PZT). These materials are used because of their larger piezoelectric effect. When pressure or acceleration is exerted on the PZT material, it generates an equivalent amount of electrical charge across its crystal faces. The magnitude of the electrical charge is directly proportional to the applied pressure. Generated electrical charge transmitted through metalized plate can be read by an electrical device. The protective coating is used to cover the metallization and piezoelectric film inside.
- Advantages of Piezoelectric Sensors:
• High Sensitivity: Piezoelectric sensors exhibit high sensitivity, allowing them to detect even small changes in pressure, strain, or acceleration. They can provide precise and accurate measurements.
• Wide Frequency Range: Piezoelectric sensors are capable of measuring dynamic events with high frequency response. They can capture fast-changing signals and vibrations accurately.
• Broad Measurement Range: Piezoelectric sensors can be designed to measure a wide range of pressures, forces, and accelerations, making them versatile for different applications.
• Rugged and Durable: Piezoelectric sensors are known for their robustness and durability. They can withstand harsh environments, high temperatures, and mechanical stress without significant loss in performance.
• Fast Response Time: Piezoelectric sensors have a rapid response time, enabling real-time monitoring and measurement of dynamic events.
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• Wide Application Range: Piezoelectric sensors find applications in various fields, including automotive, aerospace, robotics, medical devices, structural analysis, and industrial monitoring.
- Disadvantages of Piezoelectric Sensors:
• Limited Linearity: The output response of piezoelectric sensors may exhibit non- linear behavior, especially at higher input levels. Calibration or compensation techniques may be necessary to improve linearity.
• Temperature Sensitivity: Piezoelectric sensors can be sensitive to temperature variations, which may affect their accuracy. Temperature compensation techniques may be required for precise measurements.
• Fragile and Delicate: Piezoelectric sensors are relatively fragile and can be prone to damage if subjected to excessive mechanical stress or shock. Careful handling is necessary to maintain their integrity.
• External Power Source: Piezoelectric sensors require an external power source, such as a charge amplifier, to convert their high-impedance output into a usable signal. This adds complexity to the measurement setup.
• Limited Static Measurements: Piezoelectric sensors are primarily designed for dynamic measurements and may not provide accurate static measurements due to drift and hysteresis effects.
• Limited Bandwidth: The frequency response of piezoelectric sensors may be limited at extremely high frequencies, depending on the specific sensor design and materials used.