MEMS Technology At The Heart Of Omron Flow Sensors

A major advantage of MEMS Flow Sensors is their ability to measure flow speed from 1 mm/s to 40 m/s.  To put this into perspective, this covers a range from the fluttering of a butterfly's wings to the roar of a typhoon. At the heart of the MEMS Flow Sensor, there is a tiny sensor element; the Omron MEMS Flow Sensor chip which is only 1.5 mm square by 0.4 mm thick.

Conventional flow sensors use a resistance measurement method based on a natural characteristic that causes the electrical resistance of a material to change due to changes in temperature. This method has a number of disadvantages, though, such as the high cost required for the extremely time-consuming adjustment of the resistance balance.

In contrast the Omron MEMS Flow Sensor, which by the way was the industry's first to apply this technology, utilizes an element called a thermopile that converts thermal energy into electrical energy. This revolutionary method delivered a variety of previously nonexistent advantages, including low-cost operation because there are few adjustments required, low power consumption, and high sensitivity. 
The chip’s two sets of thermopiles, located on either side of a tiny heater element, are used to measure the deviations in heat symmetry caused by gas flowing in either direction. A thin layer of insulating film protects the sensor chip from exposure to the gas. 

When there is no flow present, temperature distribution concentrated around the heater is uniform and the differential voltage over the two thermopiles is 0V (Diagram1). When even the smallest flow is present, temperature on the side of the heater facing the flow cools, and warms up on the other side of the heater - heat symmetry collapses.  The difference of temperature appears as a differential voltage between the two thermopiles, proportional to the mass flow rate.

Diagram 1. Omron was the first manufacturer to utilize thermopile technology to measure flow rate.  

Omron’s unique etching technologies (Diagram 2) were used to create a unique shape that gives their flow sensing chip it’s superb characteristics by  providing a larger sensing area compared to Conventional Silicon Etching in the same volume.  This cavity design enables efficient heating with low power consumption.

To keep the heater temperature above that of the gas being measured, temperature compensating circuitry, which could be described as an “expanded bridge circuit” is incorporated in all Omron flow sensors (except the very economical D6F-V clogged filter/air velocity sensor).  This expanded circuit arrangement provides improved temperature characteristics over an ordinary bridge circuit (Diagram 3). 

Furthermore, it allows for a factory-adjustable cross point of the temperature characteristic resulting in higher output stability with fluctuating ambient temperatures. 

For optimal mass flow readings across the MEMS chip, a uniform, laminar flow through the sensor is highly desirable.  Omron’s in-line mass flow sensors, such as the D6F-01/02/03/05/10/20/50 series, incorporate a set of screens in the sensor inlets to accomplish this, resulting in high repeatability.  Pulsing flows can also present a problem for mass flow measurement.  The D6F-01A/02A series uses an orifice in the outlet side of the sensor to buffer such flows (Diagram 4), often eliminating the need for an external buffer tank.

With the advent of this MEMS technology one must seriously consider taking utilizing these flow sensors. They deliver superior repeatable flow rate measurement, lower applied cost since calibration by the customer is not necessary (they are individually calibrated at the factory), low power consumption and high sensitivity. Omron has a team of experts ready to help you with any of your design challenges. View ALL of Omron's flow sensors by visiting the product page.

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