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Sky polarization patterns are used by many insects for navigation. Honeybees use celestial polarization to move between the hive and foraging locations (Cronin et al., 2003; Horváth, 2014). Cataglyphis ants (Cronin et al., 2003) and Dung beetles (Dacke, Byrne, Baird, Scholtz, & Warrant, 2011) use the sun and moon, respectively, celestial polarization for similar tasks. At present, Dung beetles maintain travel direction by means of polarization where they are the only animal group known to use the much dimmer polarization pattern shaped around the Moon as a compass cue for orientation (Dacke et al., 2011). See Figure 1.
Figure 1. Polarization in the animal kingdom. From Left to right and top to bottom: Honey Bees, Cataglyphis ant, Dung beetle and Atlantic Salmon (Images adapted from wikipedia.org)
Salmon fishes (Figure 1) may have a similar ability (Hawryshyn, 1992), which allows them to orient in underwater light fields (Novales & Hawryshyn, 1997). Light reflection from water surfaces results in horizontally polarized light which is utilized by water beetles and other insects for orientation. (Schwind, 1991).
Many researchers have made theoretical and experimental investigations on skylight polarization for autonomous orientation and navigation. The sky is polarized due to the scattering of sunlight by particles and air molecules in the atmosphere. The sky polarization patterns present us with the polarization information which can be used as an external compass.
This work surveys the different bio-inspired polarization techniques for robot orientation and navigation. It is a focused discussion of orientation and navigation techniques of (Shabayek, Morel, & Fofi 2013; Shabayek, Morel, & Fofi 2015) where the most recent research in the area of bio-inspired polarization based techniques is covered. The methods are classified according to the type of visual sensors used for orientation and navigation as Photodiodes, CCD/CMOS, and special sensors.
The main contribution of this work is the comprehensive and novel view of polarization based robots orientation and navigation. The techniques are classified according to the used sensor technology. In the light of visual sensors technology, the future research directions in robotics navigation based on polarization will be discussed.
TopPolarization Based Robots Orientation And Navigation
The location of the sun mainly determines the celestial skylight polarization pattern. It is visible and stable even in open sky patches when the sun is occluded by clouds, and it also appears beneath dark objects in air that is illuminated by the sun (Pomozi, Horváth, & Wehner, 2001; Suhai & Horváth, 2004; Horváth, 2014). The benefit of using the skylight polarization pattern, rather than directly using the sun is that only patches of sky are sufficient for orientation task (Cronin, Warrant, & Greiner, 2006; Wu, Gao, Fan, & Xie, 2010).
Firstly, orientation techniques using photodiodes as a primary sensor to read polarization data will be covered. Secondly, as the camera technology became less expensive and more advanced, new techniques using CCD, CMOS, and/or Fisheye lens to obtain polarization information will be covered. Thirdly, water and mud detection for off-road navigation will be covered. Finally, recent techniques for robot navigation using skylight polarization patterns by means of omnidirectional polarimetric sensors will be discussed. All methods require a rotating polarizer as a primary optic combined with the mentioned sensors as will be explained.