Any sound field can be described completely by knowing two dimensions: the scalar sound pressure and the vector acoustic particle velocity.
If sound pressure were the acoustic equivalent of voltage, acoustic particle velocity would be the acoustic equivalent of current. Microflown recently introduced sensor technology that turns acoustic particle velocity into a directly measurable quantity.
Based upon MEMS technology, Mircoflown’s sensor uses two extremely sensitive heated platinum wires with minimal heat capacity. If airflow occurs around the wires, heat transfer causes the upstream wire to be cooled down and the air to be heated. As a consequence, the downstream wire is cooled down just a little bit less. The temperature difference that arises in the cross section is a direct measure of the acoustic particle velocity. The sensor is linear and provides output in voltage.
Microflown’s sensor measures the acoustic particle velocity instead of the acoustic pressure, which is measured by conventional sound-pressure microphones. With three perpendicular Microflown sensors placed with a microphone, an acoustic vector sensor (AVS) is constructed.
Sound pressure is a scalar value and therefore sound pressure microphones do not have any directionality. Directional systems that are based on microphones make use of a spatial distribution and the directivity is based on phase differences between the sound pressure at the different locations. There is no directional information found in the amplitude responses. Because the phase shifts are caused by spatial distribution, the method depends on the wavelength and is thus frequency dependent.
AVSs are directional, so making a directional system is relatively straightforward. Because a single AVS measures the sound field at one point, there is limited phase information and the directional information is found in the amplitude responses of the individual particle velocity probes.
Benefits of AVS vs. arrays of microphones are fast setup times, low data-acquisition channel count, and no (lower and higher) frequency limit. The frequency is limited by the sensors (in the order of 0 Hz-120 kHz).
AVS’s are expected to contribute to increased UAV situational awareness (SA) in a number of ways, including use as part of anti-collision systems during flight, ground surveillance for operations, and automatic takeoff and landing (ATOL) systems to start and end the mission.
There are several applications for AVSs in ATOL systems. In one, the acoustic localization of the UAV can be on the ground. Advantages of that scenario are that less equipment can be carried on the UAV and the system can be used for multiple UAVs. Disadvantages include the need for a datalink from the ground to the UAV, and an acoustic system being required at both take off and landing site.
In another scenario, a beacon on the ground can be used to guide a UAV to a landing site. The AVS in the UAV localizes the beacon and uses its location for navigation. To be able to apply the sensor in a UAV it must be used in the presence of flow that is caused by aircraft speed. Apart from that, size and mass are important factors.
If a noise source in the UAV can be applied it is possible to use this signal and localize the reflections of the ground. Such realization can be used if the landing site is unknown. The properties of the soil can be determined (the reflection coefficient) and it can be sensed if the landing site is flat or if obstacles are blocking the landing.
An AVS noise-mapping algorithm (e.g., beam forming) must be used for this to be able to get an “acoustic reflection picture” of the landing area. This is a technique similar to sonar.
AVS resistance to high wind conditions, its small size and low mass, and the ability to find acoustic sources in 3-D space are the ingredients for future R&D toward an acoustic anti-collision system for UAV’s. The difference between ATOL systems is that in anti-collision applications multiple sources and sources of unknown signature have to be located.
Modern aircraft can use several types of collision-avoidance systems to prevent unintentional contact with other aircraft, obstacles, or the ground. Radar or electro-optical systems are the most used techniques, but they are large, heavy, costly, and usually do not have a full spherical coverage.
AVSs are not affected by night or day conditions, rain, fog, or clouds. However, there is a dependency upon meteorological conditions such as wind and temperature. These can be accounted for to a large extent.
An AVS system can help to steer other types of sensors that have limited angle of aperture (narrow field of view) but a higher spatial resolution. Thus, a lightweight sensor configuration with steerable sensors becomes possible.
This article is based on SAE technical paper 2009-01-3249 by Hans-Elias de Bree, Microflown Technologies/HAN University.