Direction and fluid flow velocity sensors are essential for different industrial, medical, and environmental uses. Flow sensors quantify the direction and rate of gas and liquid flow in applications such as density measurement, viscosity measurement, flow pattern determination, and wall shear stress determination. In addition to the important requirements of flow parameters for the induction scale such as direction, temperature, speed and rate, the characteristics of the various guidelines to be detected in the liquid or gas also constitute an obstacle to planning accurate, low-power and inexpensive sensors.
MEMS flow sensors
In recent years, microelectromechanical systems (MEMS) technology has offered a wide range of opportunities for the production of flow sensors for different applications. MEMS was first proposed in the 1960s after research into the piezoresistive capabilities of silicon and germanium. In the early years, MEMS flow sensors were manufactured using polymer and silicon components, as well as different sensor components and construction methods. Thermal, drag-based, and torque-based flow sensing are probably the most prevalent sensing technologies.
In this article, a comprehensive overview of the various nano- and micrometer-scale MEMS flow sensors that have been built to date will be provided. According to its sensing theory, these three broad categories are MEMS piezoresistive, thermal, and piezoelectric flow sensors.
Piezoresistive flow sensors
When subjected to external strain or stress, the element that shows a change in resistivity is called a piezoresistive profile. As a result of the applied strain, the internal lattice and atomic orientation of the data are changed, and thus the resistivity is also changed. Piezoresistive materials are widely used by manufacturers to develop MEMS flow sensors. Because of their ability to change the resistance when stress is applied, they have gained traction in flow sensing. When used with MEMS flow sensors, the change in resistance is mapped to a voltage signal that fluctuates with the change in flow rate.
MEMS Warm Current Sensor
Flow sensors that measure flow velocity using the strength of heat transfer are called warm flow sensors. This phenomenon guarantees higher accuracy and sensitivity with less drift in the output signal. In addition, the good thing about such sensors is that they do not need to physically move any miniature parts to work. MEMS warm current sensors typically consist of two main components: the sensing and the heating element. The heat transfer difference between the workflow and the heater is sensed by the sensing element, so that the sensitivity of the system is added as the remaining heat energy is transferred to the operating fluid.
One of the main constraints affecting the accuracy of a specification temperature-based flow sensor is often the temperature of the sensing component. The inability to account for low flow rates is another problem associated with MEMS warm current sensors. The sensing components in conventional thermal film sensors and hot-wire sensors have large specific heat capacities, which makes it difficult to track low convective heat transfer, which then results in low frequencies or poor frequency response.
Based on various thermal management methods and different evaluation methods, three types of MEMS warm current sensors can be identified. H-type sensors are the first type. These are thermal film sensors and hot wire sensors that account for flow by regulating the thermal power at a stable temperature or regulating the temperature at a stable temperature. The difference between them is due to their construction: the line resistor in the thermal film sensor is placed on a diaphragm next to the flux, while the resistor is independent of the bottom layer and is located in the H-type hot wire sensor. The second type of C-type sensor is a calorimetric sensor, which measures the flow rate by measuring the change in heat dispersion on the heater.
MEMS piezoelectric flow sensors
Certain man-made and natural dielectric materials have piezoelectric properties, which allow them to generate an electrical charge when a mechanical load is applied. This is also known as the direct piezoelectric effect. On the other hand, if the data is exposed to an external electric field, it will affect the scale or a slight change in shape. This effect is called the inverse piezoelectric effect. in polymers such as polyvinylidene fluoride (PVDF) and including barium titanate (BaTIO3) or lead zirconate titanate (PZT). MEMS piezoelectric flow sensors are automatic, so no external power supply is required to complete the sensor output signal. In addition, they are mainly composed of polyvinylidene fluoride (PVDF) and lead zirconate titanate (PZT) data.
Scope of use
MEMS flow sensors have a variety of uses, such as sensing ambient flows, industrial gas flow monitoring, flow sensing in biomedical use, and marine hydrodynamic sensing. The compact size, high sensitivity, low price, and mass production capabilities of MEMS flow sensors make them extremely attractive for commercial and industrial use. In addition, some MEMS soft polymer sensors are biocompatible, opening the door to sales for clinical and biomedical use.
MEMS flow sensors are in increasing demand for therapeutic respiratory flow sensing tasks in ventilators, nebulizers, oxygen systems, and sleep apnea diagnostic devices. Wearable breath monitors with integrated MEMS flow sensors are able to track flow and flow rates. They are commonly used to monitor the performance and recovery of athletes. In addition, in flow rate tracking in intravenous infusions, MEMS sensors will play a key role in completing the useful dispensing of drugs for gravity injection, as well as avoiding accidental misuse of drugs that may arise from infusion pump defects.
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