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Essay: MEMS based flow sensor that simultaneously measures flow rate and direction

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  • Published: 15 September 2015*
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  • Words: 1,335 (approx)
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Abstract: A micromachined cantilever-based flow sensor is designed, depending on the detection of surface strain on the cantilever caused by the mass flow. In the continuous flow mode, the deflection of cantilever is directly proportional to the flow rate. The working mechanism of the strain beam in the flow sensor is analyzed using INTELLISUITE as the finite element tools to investigate the structural deformation and stress distribution. As fluids travels over the surface of cantilever structure, the upstream cantilevers are deflected in the downward direction, while the downstream cantilevers are deflected in the upward direction. According to the simplified stress analysis and simulation results, the flow sensor has a sufficient structural strength and it will achieve ideal static characteristics that meet the requirements of practical applications. The deflection of beams depends on both the flow rate and the orientation of the beam relative to the direction of the gas flow. By varying the design of the cantilever, the measurement range and sensitivity of the sensor can be changed.
Keywords: MEMS, Cantilever, Differential Pressure, Flow Measurement.
Fluid mechanics deals with the study of all fluids under static and dynamic situations. Fluid mechanics is a branch of continuous mechanics which deals with a relationship between forces, motions, and statical conditions in a continuous material. This study area deals with many and diversified problems such as surface tension, fluid statics, flow in enclose bodies, or flow round bodies (solid or otherwise), flow stability etc.The fluid is mainly divided into two categories: liquids and gases.The main difference between the liquids and gases state is that gas will occupy the whole volume while liquids has an almost fix volume Flow measurement is an essential task in many fields, including environmental monitoring, process control, medical instrumentation, air conditioning systems and weather forecasting systems. Flow sensor is a classical device in measurement technology, and it is widely used in industrial process control, life sciences and commercial applications. Micromachined flow sensors are attractive for their small size, low power consumption and high resolution. The most common flow sensors are thermal ones and pressure ones.However thermal flow sensors have a lot of inherent defects, such as high power consumption, measurement error, long response time and zero drift with environment temperature caused by thermal conductivity of the substrate. Comparing with the thermal flow sensors, the sensors based on pressure measurement have a better performance and universality, and a micromachined cantilever based flow sensor is developed utilizing the principle of differential pressure. Broadly speaking these sensors can be categorized as either thermal or non-thermal, depending upon their mode of operation. Thermally actuated flow sensors generally utilize some form of resistor arrangement to evaluate local temperature changes. In such devices, the temperature differential between different resistors within the sensor varies as the fluid flows through the sensor and can therefore be used to estimate the fluid flow rate. In recent years, non thermal gas flow meters have attracted considerable interest due to their lower power consumption and easier integration with other micro scale systems than thermal gas flow sensors. This study develops a MEMS based flow sensor capable of obtaining simultaneous measurements of both the flow rate and the flow direction.
To design the micro-sensor, the static and dynamic characteristics are considered firstly. Meantime, the structure should have enough strength, convenient for external connection and easy to install. As shown in Figure the gas flow direction sensor has the form of four free-standing micro cantilevers arranged in a cross-form configuration. Each cantilever has dimensions of (4,000 ?? 400 ?? 1 ??m) and is patterned with a platinum with a length and width of 4,000 ??m and 50 ??m, respectively. As air flows over the sensor, the cantilever beams in the upstream direction deflect in the downward direction, while those in the downstream direction deflect in the upward direction. The deflection of the beams induces a change in the cross sectional areas of the platinum resistors patterned on their upper surfaces and therefore produces a measurable change in their output signals. Since the amount by which each beam deflects is directly related to its orientation relative to the direction of the gas flow, the gas flow direction can be simply determined through an appropriate manipulation of the output displacements.
It is observed that each of the cantilever beams has a marked deflection in the upward direction as a result of the residual stress induced within the beam during the deposition and etching procedures in the fabrication process. In practice, the curved characteristic of the cantilever beams is highly beneficial since it increases the tendency of the beams to deflect in either the upward or downward direction as air travels over the upper surface of the sensor, and therefore yields an improvement in the measurement sensitivity of the device.
In the present design the sensor consists of central membrane, cantilever and pedestal. Thickness of the central membrane and the cantilever are the same, while the width of the central membrane is two times larger than that of the cantilever, which can increase the sensitivity of the sensor. When there is a kind of fluid flow through the sensor, differential pressure occurs between top and bottom surface of the membrane, the cantilever produces deformation. When there is a kind of fluid flow through the sensor, differential pressure ??P occurs between top and bottom surface of the membrane that can be written as follows where V is the flow velocity, ?? is the fluid density and CD is the drag force coefficient, which is related to the Reynolds
number, and the Reynolds number Re is where ?? is the kinematic viscosity, ACH and S are the area and perimeter of the flow channel, respectively. According to the differential pressure ??P, the force ??F on top surface of the membrane can be calculated as follows where the AM is area of the central membrane. As the cantilever is connected with the central membrane, we can get F ‘ ??f
When the force F caused by the flow acts on the cantilever, it causes a displacement d which is given by following relation as
It demonstrates that the output displacement is proportional to the square volume flow rate of the fluid, so the volume flow rate can be represented by the output displacement value.
The fabrication process typically involves undercutting the cantilever structure to release it, often with an anisotropic wet or dry etching technique.
The MEMS cantilevers are commonly made as unimorphs or bimorphs. In present case the fabrication process commenced by depositing a thin (1.0 ??m) low stress Si3N4 layer on either side of a double side polished silicon wafer utilizing a PECVD technique. Using an electron-beam evaporation process, a chromium layer with a thickness of 0.03 ??m is deposited on the upper Si3N4 surface to serve as an adhesion layer for the subsequent deposition of a Pt layer with a thickness of 0.1??m. Platinum resistors for the flow direction sensor, flow velocity sensor and ambient temperature sensor were then patterned in a one-pass lithographic process.The same deposition and patterning techniques were then re-applied to create thin (0.1 ??m) gold lead electrodes on the end portions of each of the Pt resistors. Finally the cantilever diaphragm structures and back etching nitride mask were patterned and the freestanding structures were then released using a KOH etching agent.
Furthermore, in order to make sure the sensor has sufficient structural strength within its measure range, the simulation is done to study the deformation of the cantilever.The software INTELLISUITE is used to calculate the stress. Fig.—- shows the distribution of the von Mises stress on the cantilever with the differential pressure obtained by simulation. We can get the sensor has sufficient structural strength within its measure range. Fig—shows the stress distribution along the path of y-axis and z-axis.
All the simulation results are generally conformed to the calculated results, which indicate the sensor will achieve ideal static characteristics that meet the requirements of practical applications.

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