Compact Bidirectional Acoustic Airflow Meter for Aviation Applications (CBAAM) Navy SBIR FY2010.1 Sol No.: Navy SBIR FY2010.1 Topic No.: N101-016 Topic Title: Compact Bidirectional Acoustic Airflow Meter for Aviation Applications (CBAAM) Proposal No.: N101-016-1258 Firm: Comtech Communication 100 Hogan Point Rd Hilton, New York 14468-8197 Contact: Jeffrey Gutterman Phone: (585) 392-8299 Abstract: The Comtech Bidirectional Acoustic Air Meter uses tuned piezoelectric transducers to send pulse trains of approximately 250 cycles of ultrasonic (40-100 kHz, typ.) acoustic energy diagonally across the air stream. Every ~10 ms, the transducers change roles and send the same acoustic energy in the opposite direction, defined as "upstream" and "downstream". We precisely measure the phase shift between the sender and receiver in both directions. If there is no air flow, the upstream and downstream phase shifts are identical. If there is flow, the upstream phase shift will be slightly longer than the downstream and vice versa. Air velocity is determined using the difference between the two phase shifts. Air temperature is determined from the average of the two phase shifts. Air pressure information is measured independently with a pressure sensor as the speed of sound is virtually independent of ambient pressure. For a known cross sectional area, mass air flow is calculated every ~10 ms by combining the velocity, temperature and pressure information. High resolution is obtained by averaging the phase readings (~100) during each 10 ms reading cycle. Since acoustic energy is sent in both directions, reverse flows and temperatures can be measured with equal resolution. Benefits: The use of continuous pulse trains affords several significant advantages over time-of-flight, hot wire, and venture type meters: 1) The meter is truly bidirectional with equal accuracy in the positive or negative directions. 2) It is nonintrusive and requires no object or element placed directly in the air stream. 3) The sensor does not rely on thermal mass or changes in current flow 4) The resolution is 1 to 2 orders of magnitude higher than conventional meters since ~100 phase readings are taken and averaged in every 10 ms cycle. 5) Extremely low flows in the negative direction, e.g. "stall", can be determined with resolution at about 1/200 of the maximum flow velocity. 6) The signals are sent diagonally across the air stream, increasing the sampling cross section and reducing the effect of any localized turbulence. 7) The signal is insensitive to noise since the send and receive signals are precisely windowed off the same timing circuitry and the output reading is averaged from over 100 individual readings. 8) There is no tradeoff between minimum resolution and maximum flow. The dynamic range can be orders of magnitude. 9) Sensor output is inherently linear with air velocity rates up to Mach 0.1. 10) Sensor reading can be compared with the previous reading to rapidly compute a rate of change of mass air flow and temperature. Return