 Omron Sensors can cover a range from the flutteringof a butterfly's wings to the roar of a typhoon. |
A major advantage of MEMS Flow Sensors is their ability to measure flow speed from 1mm/s to 40 m/s. To put this into perspective, this covers a range from the fluttering ofa butterfly's wings to the roar of a typhoon. At the heart of the MEMS Flow Sensor, thereis a tiny sensor element; the Omron MEMS Flow Sensor chip which is only 1.5 mm square by0.4 mm thick.
Conventional flow sensors use a resistance measurement method based on a naturalcharacteristic that causes the electrical resistance of a material to change due tochanges in temperature. This method has a number of disadvantages though, such as thehigh cost required for the extremely time-consuming adjustment of the resistancebalance.
In contrast, the Omron MEMS Flow Sensor was the industry's first to apply thistechnology by utilizing an element called a thermopile that converts thermal energy intoelectrical energy.
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 Diagram 1. Omron was the first manufacturer toutilize thermopile technology to measure flow rate. |
This revolutionary method delivers a variety of previously nonexistent advantages,including low-cost operation (few adjustments required), low power consumption and highsensitivity.
The chip’s two sets of thermopiles, located on either side of a tiny heaterelement, are used to measure the deviations in heat symmetry caused by gas flowing ineither direction. A thin layer of insulating film protects the sensor chip from exposureto the gas.
When there is no flow present, temperature distribution concentrated around the heateris uniform and the differential voltage over the two thermopiles is 0V (Diagram 1, above leftimage).
 Diagram 2. Omron's crepe structuresilicon etching. |
When even the smallest flow is present, the gas upstream of the heater cools, andwarms the gas downstream of the heater. The difference of temperature appears as adifferential voltage between the two thermopiles, proportional to the mass flowrate.
Omron’s unique etching technologies (Diagram 2) were used tocreate a unique shape that gives their flow sensing chip its superb characteristics. TheCrepe Structure Silicon Etching technology provides a larger sensing area compared toConventional Silicon Etching in the same volume. This cavity design enables efficientheating with low power consumption.
To keep the heater temperature above that of the gas being measured, temperaturecompensating circuitry (which could be described as an “expanded bridgecircuit”) is incorporated in all Omron flow sensors (except the very economicalD6F-V clogged filter/air velocity sensor). This expanded circuit arrangement providesimproved temperature characteristics over an ordinary bridge circuit (Diagram 3). Furthermore,it allows for a factory-adjustable cross point of the temperature characteristicresulting in higher output stability with fluctuating ambient temperatures.
 Diagram 3. Basic diagram ofOmron's temperature compensation circuitry. |
For optimal mass flow readings across the MEMS chip, a uniform, laminar flow throughthe sensor is highly desirable. Omron’s in-line mass flow sensors, such as theD6F-01/02/03/05/10/20/50 series, incorporates a set of screens in the sensor inlets toaccomplish this, resulting in high repeatability.
Pulsing flows can also present a problem for mass flow measurement. The D6F-01A/02Aseries uses an orifice in the outlet side of the sensor to buffer pulsing flows (Diagram 4), often eliminating the need foran external buffer tank.
 Diagram 4. The orifice incorporated in theD6F-01A1-000 and D6F-02A1-000 inline mass flow sensors often smoothpulsating flow allowing for accurate measurement. (Click image for a largerview) |
With the advent of this MEMS technology one must seriously consider utilizing theseflow sensors. They deliver superior repeatable flow rate measurement, lower applied costsince calibration is not necessary, low power consumption and high sensitivity.
Omron has a team of experts ready to help you with any of your design challenges.
For additional information please contact Omron Electronic Components at847-882-2288, by emailing components@omron.com,or by visiting www.components.omron.com