Designing Aerofarms

Project Title: Strap-down Polarization Compass

Design and characterise optical polarization sensors capable of measuring polarized atmospheric light of varying intensity.  The sensors should measure a range of light from moonlit night sky to full sun.  Combine measurements from multiple sensors of different polarization angles to determine heading, given a known position of the sun or moon. The completed compass will provide output data of polarization direction and magnitude.  The compass will be of a compact nature suitable for testing on board an unmanned aerial system (UAS).

 

Project Description

Lord Rayleigh documented the way light is scattered as it passes through the atmosphere [1]. This scattering produces a distinctive polarization pattern, which can be observed as a variation in light intensity by the human eye when looking through a polarizing filter [2]. Similarly, some animals and insects such as bees have evolved to be able to detect the polarized light patterns and have also been shown to use the polarization of light to aid in navigation [2].

Navigation by magnetic compass as is commonly used today, has problems when used close to the magnetic poles, or in environments where there are inconsistent or changing magnetic fields, or strong electro-magnetic interference.  In these situations, navigation by polarization compass can provide an alternative under the right conditions.

Previous published studies have shown how unmanned rovers [3] and aircraft [4] fitted with polarization compasses have been able to obtain accurate heading information for navigation.  These studies have all been conducted using sunlight as the source of the polarized light.  There are also issues with how to maintain the quality of compassing data as the sun moves throughout the day, as the polarization pattern is most usable when the sun is at the horizon, and least usable at its zenith [5].

This project aims to improve on these previous designs in two ways.  Firstly, a sensor suite will be implemented that will be able to produce usable results from full sun, degrading through dull overcast sky and still be effective under moonlit night sky.

Secondly, the project aims to use the completed sensor suite to measure and log the polarization pattern of light over the course of the several days under different conditions to establish how it changes with the position of the light source.  It is expected to be extremely difficult to obtain results when the sun is directly overhead, however the aim is to be able to specify exactly over what conditions the device will be usable.  One option to improve the results would be to mount the compass on a movable gimbal in order to maximise the quality of the measured light.  This project aims not to take this approach, instead relying on mathematical techniques and previously logged data to improve results.

The resultant aim is to produce a compass that is versatile enough to be usable as part of a closed-loop navigation system for an unmanned aircraft.

 

Mission Statement

An integrated polarization compass suitable for mounting on an unmanned aerial system will be tested, calibrated and ready for flight tests by October 2015.

 

Project Scope


The project will involve electronic design of the sensor hardware, including part selection, procurement, circuit prototyping, testing, sensor characterisation and calibration of completed design.

Software will be written to allow acquisition and measurement of sensor data, calculation of polarization direction and magnitude, and output of serialised data to another system.

Testing will be conducted on the prototype design to log polarization readings for various times of day and night in order to establish baseline performance and evaluate effectiveness of design.

The design will not involve any form of gimballing to maintain optimal measurement angle to the sky.

 

Technical Risks

The untreated technical risks have been assessed below, as per the tables in Appendix A.

 

Poor sensor response Sensors unable to perform under full range of lighting conditions Possible Major High
Unsuitable for conditions Prototype unable to be used in required conditions due to size, weather conditions etc. Possible Moderate Medium
Delays in hardware acquisition / construction Delays in acquisition of required components or prototype manufacturing due to regulatory or procurement delays Likely Moderate Medium
Test / calibration facilities unavailable Appropriate facilities for testing and calibration are unavailable due to scheduling conflicts or equipment faults Likely Moderate Medium
Test / calibration facilities unsuitable Appropriate test or calibration facilities or equipment do not exist on campus Possible Major High
Project costs exceed budget Cost of components, manufacturing and testing exceed allocated budget Likely Moderate Medium

 

 

 

References

[1]          L. Rayleigh, “XXXIV. On the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky,” Philosophical Magazine Series 5, vol. 47, pp. 375-384, 1899/04/01 1899.

[2]          D. h. G. Horváth and P. D. D. Varjú, Polarized Light in Animal Vision: Springer Berlin Heidelberg, 2004.

[3]          D. Lambrinos, R. Möller, T. Labhart, R. Pfeifer, and R. Wehner, “A mobile robot employing insect strategies for navigation,” Robotics and Autonomous Systems, vol. 30, pp. 39-64, 1/31/ 2000.

[4]          J. Chahl, M. Burke, K. Rosser, and A. Mizutani, “Development and closed loop flight test of a UAV guided by a polarisation compass,” in AIAC15 : 15th Australian International Aerospace Congress, Melbourne, Australia 25-28 February 2013, 2013.

[5]          T. W. Cronin, E. J. Warrant, and B. Greiner, “Celestial polarization patterns during twilight,” Applied Optics, vol. 45, pp. 5582-5589, 2006/08/01 2006.

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