MEMS Technology Helps To Measure Flow

Smart technologies are creating compact and lightweight sensing elements. Apart from being optimal, fast, and efficient solutions, these are not limited to only the data input functions as the conventional sensing technologies are. Rather, they integrate the areas of sensing and control while offering high-value information that humans or systems can subsequently process. Several unique and advanced technologies such as MEMS form the concept of sensing and control expertise. For example, flow sensors use the ButterflyMEMS technology to operate.

Flow sensors using the MEMS technology operate with major advantages. For example, they can easily measure flow speed ranging from 1 mm per second to 40 m per second. To understand this better, ButterflyMEMS technology can sense the fluttering of the wings of a butterfly and the roar of a typhoon with equal ease. A tiny MEMS flow sensor does all the work and it is the size of a 1.5 mm square chip, which is only 0.4 mm thick.

Conventionally, flow sensors have been using the method of resistance measurement. The method senses the change in electrical resistance of a filament because of a change in temperature caused by the flow of material across the filament. Balancing the resistance of the filament is a time-consuming method, which forms the major disadvantage of this method and makes it expensive.

In contrast, the MEMS flow sensor utilizes a thermopile, an element that converts thermal energy into electrical energy. This technology offers several advantages not seen earlier. For instance, MEMS technology offers cheaper operation, only a few adjustments, high sensitivity, and low power consumption.

This advanced sensor can even sense the direction of flow. The chip has two sets of thermopiles located on either side of a tiny heater element. The thermopiles measure the deviations in heat symmetry that the gas flow causes. The chip senses the direction of flow based on a positive or a negative deviation. A thin layer of insulating film covers the sensor chip and protects it from being exposed to the gas.

In the absence of flow, temperature distribution remains uniform around the heater and there is no differential voltage between the two thermopiles. With even the smallest flow, the heat symmetry collapses, as the thermopile on the side of the heater facing the flow shows a lower temperature, while the thermopile on the other side is warmer. This temperature difference causes a differential voltage to appear between the two thermopiles. This voltage is proportional to the mass flow rate.

The superb characteristic of the sensing chip comes from an unusual shape created by a unique etching technology. Compared to the conventional silicon etching, this unique etching technology creates a larger sensing area in the same volume. This results in a cavity design enabling heating with greater efficiency while keeping the power consumption low. Additionally, the cross-point of temperature characteristic can be factory adjusted, which results in high output stability even when the ambient temperature fluctuates.

Within the actual sensor, a set of screens in the sensor inlet produces a uniform, laminar flow through the sensor offering optimal mass flow readings. An orifice in the outlet side of the sensor buffers against pulsing flows.