Direction and fluid velocity sensors are critical for different industrial, medical, and environmental applications. Flow sensors quantify the direction and rate of gas and liquid activity in applications such as density measurement, viscosity measurement, flow pattern confirmation and wall shear stress measurement. In addition to the important requirements of the flow parameters for the sensing range such as direction, temperature, speed and rate, the characteristics of the various target liquids or gases to be detected also pose an obstacle to planning accurate, low-power and inexpensive sensors.
MEMS flow sensors
In recent years, microelectromechanical systems (MEMS) skills have provided a wide range of opportunities for the production of flow sensors for different purposes. MEMS was first proposed in the 1960s after research into the piezoresistive talents of silicon and germanium. In the early years, MEMS flow sensors were made using polymer and silicon components, as well as different sensor components and construction methods. Thermal, drag-based, and torque-based flow sensing are perhaps the most prevalent sensing skills.
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 their sensing theory, these three 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 positions of the data are changed, and thus its resistivity is also changed. Piezoresistive data is widely used by manufacturers to develop MEMS flow sensors. Because they can 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 by using heat transfer intensity are called warm flow sensors. This ensures higher accuracy and sensitivity with less drift in the output signal. In addition, the advantage of such sensors is that they do not need to physically move any miniature parts to operate. A MEMS warm current sensor typically consists of two primary components: the sensing and the heating element. The heat transfer difference between the working stream and the heater is sensed by the sensing element, so the sensitivity of the system increases as excess heat energy is transferred to the working fluid.
One of the first constraints affecting the accuracy of standard temperature-based flow sensors is often the correct retention of 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 a large specific heat capacity, which makes it difficult to keep an eye on low convective heat transfer, resulting 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 adjusting the thermal power at a stable temperature or by adjusting the temperature at a stable temperature. The difference between them is due to their construction: the line resistors in the thermal film sensor are placed on a diaphragm next to the flux, while the resistors are independent of the bottom layer and are 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 be charged when mechanical loads are 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 size or 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 application
MEMS flow sensors have a variety of uses, such as sensing ambient flows, industrial gas flow monitoring, flow sensing in biomedical applications, and marine hydrodynamic sensing. The compact size, high sensitivity, low price, and mass production capabilities of MEMS flow sensors make them attractive for commercial and industrial applications. In addition, some MEMS soft polymer sensors are biocompatible, opening the door for sale for clinical and biomedical applications.
MEMS flow sensors are in increasing demand for therapeutic respiratory flow sensing tasks in ventilators, nebulizers, oxygen systems, and sleep apnea diagnostic instruments. Wearable breath monitors with integrated MEMS flow sensors can monitor flow and flow rates. They are commonly used to monitor an athlete's performance and recovery. In addition, MEMS sensors will play a key role in completing the efficient dispensing of drugs for gravity injections and in preventing accidental misuse of medications that may be caused by infusion pump shortcomings in flow rate tracking in intravenous infusions.
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