Sense And Avoid Selection

The ability for small unmanned aerial systems (sUAS) to sense and avoid has been a prevalent topic in unmanned aviation and somewhat controversial.  There have been products that have come to market claiming to help to meet this requirement, however the Federal Aviation Administration (FAA) does not recognize these sensors as being able to meet this requirement yet.  This paper will look at ultrasonic sensors as well as advances made in visual sensors.

Ultrasonic sensors utilize high-frequency sound pulses and then by calculating how long it takes for the initial sound to echo to come back, it can then compute a range from the object that reflected the sound (Ultrasonic Distance Sensor, 2016).  As depicted in Figure 1, one part of the sensor will transmit the sound wave, meanwhile the other side “listens,” this is known as the receiver (Ultrasonic Distance Sensor, 2016).  By approximating the speed of sound at 1,100 feet per second this factor becomes a known in the distance equation. The distance equation is as follows: distance (D) equals the time (t) it takes the sound to return and then multiplying it by the known speed of sound (1,100ft/s) and then dividing it by 2 (D = (t x 1100)/2) (Ultrasonic Distance Sensor, 2016). 

Figure 3. Ultrasonic sensors working example.  Courtesy of  Cornell University Electrical and Computer Engineering.

Visual sensors or what we commonly think of as cameras can be used to help system “see” and avoid as well.  Recent software advances have allowed for cameras to be utilized on commercial products, for example Subaru’s “EyeSight” which utilizes stereoscopic cameras to sense range and is available on select Subaru models (SUBARU DEBUTS NEXT GENERATION EyeSight SYSTEM, 2014).  Additionally, companies like Chinese manufacturer DJI, have marketed commercial off the shelf (COTS) solutions with their latest Phantom 4 quadcopter.  The Phantom 4 uses stereoscopic cameras mounted to the front in order to sense and avoid objects in front of it (Sense and Avoid, 2016). 

DJI has also introduced a product know as Guidance as part of their developer series.  This system incorporates both the ultrasonic and visual technologies discussed earlier (Guidance User Manual V1.6, 2015).  It includes five sets of ultrasonic and image sensors and these sensors are all connected by a single core control which can be connected to any DJI control system or other systems via USB or UART (Guidance User Manual V1.6, 2015).  The system requires 11.1-25 volts for power and draws 12 watts of power with all 5 guidance sensors (GUIDANCE SPECS, 2016).  Additionally the system weighs in at 282.4 grams with the guidance core, five sensors, and associated cables (GUIDANCE SPECS, 2016).  Since the systems comes with five sensors, one set could be used fore and aft, one set port and starboard, and on sensor facing down giving five sides of protection.  Lastly the sensor effective range is at maximum 20 meters or just over 65 feet (GUIDANCE SPECS, 2016).  Depending on the processing power of the controller, if an avoidance decision could be made in less than one that could allow for a flight speed of 44 miles per hour, best case scenario.

Currently these systems do not meet the FAA requirements but are being utilized to allow for obstacle avoidance of non-cooperative objects such as birds, debris, or in cases where line of sight position is in question due to viewing angles.  Products such as DJI’s Guidance uses the incorporation of multiple sensors to complete one task.  As time goes, visual recognition software will continue to evolve and progress, and in this authors opinion it will be a combination of sensors such as these that will fit the need of the FAA’s “see” and avoid or for the case of unmanned systems “sense” and avoid requirement. 

References

Guidance User Manual V1.6. (2015 Oct). DJI. Retrieved from http://download.dji-innovations.com/downloads/dev/Guidance/en/Guidance_User_Manual_en_V1.6.pdf

GUIDANCE SPECS. (2016 Jul 12). DJI.com Retrieved from http://www.dji.com/product/guidance/info#specs

Phantom 4 User Manual V1.2. (2016 Mar). DJI. Retrieved from https://dl.djicdn.com/downloads/phantom_4/en/Phantom_4_User_Manual_en_v1.2_160328.pdf

Sense and Avoid. (2016 Jun 29). Dji.com. Retrieved from https://www.dji.com/product/phantom-4

SUBARU DEBUTS NEXT GENERATION EyeSight SYSTEM. (2014 Jan 23). Subaru.com. Retrieved from http://media.subaru.com/newsrelease.do?id=562&mid=123&allImage=1&teaser=subaru-debuts-next-generation-eyesight-system

Ultrasonic Distance Sensor. (2016 Jul 11). Arduino-info.wikispaces.com. Retrieved from http://arduino-info.wikispaces.com/Ultrasonic+Distance+Sensor