- freely available
Sensors 2012, 12(11), 14232-14261; doi:10.3390/s121114232
Published: 24 October 2012
Abstract: Animal senses cover a broad range of signal types and signal bandwidths and have inspired various sensors and bioinstrumentation devices for biological and medical applications. Insects, such as desert ants and honeybees, for example, utilize polarized skylight pattern-based information in their navigation activities. They reliably return to their nests and hives from places many kilometers away. The insect navigation system involves the dorsal rim area in their compound eyes and the corresponding polarization sensitive neurons in the brain. The dorsal rim area is equipped with photoreceptors, which have orthogonally arranged small hair-like structures termed microvilli. These are the specialized sensors for the detection of polarized skylight patterns (e-vector orientation). Various research groups have been working on the development of novel navigation systems inspired by polarized skylight-based navigation in animals. Their major contributions are critically reviewed. One focus of current research activities is on imitating the integration path mechanism in desert ants. The potential for simple, high performance miniaturized bioinstrumentation that can assist people in navigation will be explored.
Human and animals navigate for various needs—for finding food, for social reasons, for communication and others. Current navigation devices are mostly dependent on the global navigation satellite system (GNSS), the more fully operational system for global positioning compared to others. The variability of function and integration of the new generation of GNSS has increased the market demand for related products . However its applications may be limited by the low precision of the signal under certain conditions such as in urban areas and in situations of intermittent coverage. Furthermore, the system is always at risk of being shut down during a conflict. A new system should be developed to overcome these limitations.
The development of a GPS-independent navigation system has been inspired by the skylight-based navigation employed by insects. These insects, with their tiny eyes and brains, are capable of navigating distances of hundred of meters (walking insects)  or many kilometers (flying insects) by utilizing the pattern of polarized skylight. Through replicating such insect navigation systems, it may be possible to provide a kind of “navigational sense” to people. The development of a bio-inspired polarized skylight navigation sensor that can expand the human sensory ability towards such a “navigational sense” necessitates the use of highly interdisciplinary bioinstrumentation. In order for this to operate effectively, there is a need to connect external devices to the human body. To connect the system with the human body, miniaturization of the devices needs to be performed. In this article existing bio-inspired polarized skylight-based navigation sensors are reviewed with the intention of examining whether it is possible to upgrade respective devices to miniaturized bioinstrumentation systems that can be utilized for human navigation, potentially in combination with established GPS-based systems, to overcome limitations of both approaches. Both through the development of GPS-independent navigation systems and improvements to current GPS systems bioinspiration of polarized skylight navigation sensors has already yielded tangible results. These devices have been tested for use as a navigational compass that can guide a mobile robot [3,4].
Lambrinos et al. invented a GPS independent polarization compass model that mimics the principle of the desert ant navigation system. Chu and co-workers enhanced this polarization compass principle [4,6–9] and improved the error measurement. Further groups working in this field are Gao and Fan and their co-workers [10–16]. Lu and co-workers implemented polarized skylight detection mechanisms into existing GPS systems [17–19]. Fan et al. implemented a new integrated navigation solution with polarized skylight with geomagnetism and GPS. Advanced miniaturized devices can then be linked to the human body (mainly ex corpore to avoid ethical issues) in order to expand the human sensory perception towards a polarized skylight-based “navigational sense”. This sensor would be very beneficial to humans, especially those who suffer from difficulties with their sight, are wheelchair bound or suffer from Parkinson's disease.
4.1. Algorithm for Measurement and Analysis for Coupled Photodetector-Linear Film Polarizer-Based Polarization Sensor
The output of polarization sensor is described by the following equation :where I is the total intensity given by I = Imax+ Imin.Imax and Imin being the maximum and minimum intensities, respectively. The degree of polarization is denoted by d, Ø is current orientation with respect to the solar meridian, Ømax is the value that maximizes S(Ø), and K is a constant. The delogarithmized polarization sensor outputs of the direction 0°, 60° and 120° respectively are expressed by Equations (2–4):
The algorithm improvement is important for improving the polarization compass. From Equations (2–4), Zhao  eliminated the influence of the polarization degree, d, by presenting a new transform, where after simplification of ti, they got the value of output angle of the polarization sensor Si [Equation (10)] that was independent of d:where Si (I = 1,2,3), Ø1 = 0, Ø2 = π/3, Ø3 = 2π/3. The advantage of the new algorithm is its higher precision with identical input error due to the character of its piecewise function, even though the two algorithms have the same output function style.
The output of the polarization sensor could also be described by using the mathematical description known as Stokes parameters (shown in Equation (11)) . Stokes parameters were first introduced by G.G. Stokes in 1852, who described the polarization state in four quantities. As shown in Equation (11), the four quantities: S0, S1, S2 and S3 could be assumed to be polarization values obtained from Filters 1, 2, 3 and 4 respectively. Here, the first filter is an isotropic filter, passing all states of polarization, Filter 2 is passing the linear state of polarization in the horizontal direction, while Filter 3 passes the linear state of polarization at a 45° direction. Filter 4 filters the circular polarization state only:
The theoretical basis for the system error model of the polarized-skylight angle measurement model (POLAMM) has been developed by Li et al.. The system error model of POLAMM was derived to improve the calculation accuracy of the polarized light navigation sensor. Through this system error model, the system error source parameters could be recognized, and the system error could be compensated to a major extent. From Equation (1), the output of three POL sensors is shown as follows:where Si (I = 1,2,3), Ø1 = 0, Ø2 = π/3, Ø3 = 2π/3.Si(Ø) is denoted as Si. By substituting Equation (15) into Equation (14), Equation (16) is produced: where:
Here:From the differentiation method, the system error model of POLAMM is shown by:
4.2. Algorithm of CMOS-Based Sensor
The measurement in CMOS-based sensors is performed by calculating the Stokes parameters [89,94]. The electromagnetic radiation travel is utilized as input signal. The mathematical representation of an electromagnetic wave propagating in the z direction is given by Equation (22):where E0 is the amplitude, k is the wave constant (k = 2ð/ë), ù is the circular frequency (ù = kc = 2ðc/ë) and is the initial phase. As described in Section 4.1, the polarization state of an electromagnetic wave can be conveniently described using a set of parameters that are known as Stokes parameters (G.G. Stokes, 1852) . These are represented in a column vector as in Equation (11), called as Stokes vector. Different with Lambrinos et al. and Chu and co-workers approach, this CMOS-based polarization imaging sensor is developed in three types of filters. By modifying the Stokes parameters described by G.G. Stokes, S0, S1 and S2 can be represented by Equations (23), (24), and (25) : where I0° is the intensity of skylight after passing through a horizontal linear polarizer, I90° is the intensity after a vertical linear polarizer, and I45° is the intensity after a linear polarizer is placed at 45°. By calculating Equations (22) to (25), the degree of polarization, which is related to maximum and minimum transmitted intensity values, can be obtained.
4.3. Algorithm for Development of Ant Eye Model
Smith [111,112] developed an algorithm for a compass that was applied on robots and drones in light clouds. The working principle of this compass was inspired by the insect compound eye. The algorithm was created by measuring the position of the four points in the sky, where, i.e., the angle χ between the polarized e-vector and the meridian equals ±π/4. The azimuth of these four points is invariant to variable cloud cover, provided that polarized skylight is still detectable below the clouds. The sum of these four azimuth values can be turned into a celestial compass, which is useful for the robot or drone. Compared with the photoreceptor-based design, a compass that uses this design offers a simpler device that offers more accuracy during navigation under cloudy sky.
5. Integration System
Lu et al. introduced the polarized skylight integrated GPS-based navigation system in the three-dimensional world. The polarization measurement unit (POLMU) is a detection unit that consists of the mechanism and components of the polarization direction analyzer that was described in Section 3.1.2. By integrating the POLMU to the integrated GPS/INS navigation system, the attitude error correcting capabilities of the system was improved, producing better precision in the GPS/INS navigation system. Fan et al. implemented a new integrated navigation solution with polarized skylight that assists with geomagnetism and GPS. The output of the analyzed polarization information is used as references in the measurement work of the integration system. By using a Kalman filter, the results of analyzed polarization information are combined with the results of the geomagnetism 3D compass to obtain the smallest error of angle results. By adjusting the results with GPS information, the final output results were obtained. The components of the polarization sensor are polarization direction analyzer, such as that described in Section 3.1.2, with the same mechanism.
The insect navigation system, especially that of the desert ant, Cataglyphis, bees, locusts and crickets have offered useful insights into the development of polarization navigation sensors that have been utilized to assist the navigation of mobile robots (as described in Section 3). By further developing this robot polarization navigation sensor, it is possible that a new kind of human sense, a “navigational sense”, could be fabricated.
Insects, with their tiny eyes and brain, are capable of navigating over hundreds of meters through the utilization of the patterns of polarized skylight . The dynamic properties of skylight polarization provide much useful information to any navigating animal and human utilizing specific devices. GNSS systems do not perform well under some conditions, and polarization based systems do not perform either under other conditions (e.g., indoors, night, artificial light). Both systems can be combined to bring the best of both worlds and cover the deficiencies of each other. The polarized skylight is appropriate to be used as the information in navigation activities because the predictor signal is simple with a static relationship between e-vector orientation and the sun's azimuth .
The qualitatively robust pattern of polarized skylight direction could be obtained under any condition and even in situations when the sun was not visible , such as under canopy and foliage , and during overcasts and heavy haze. This is because only a small section of clear sky is sufficient for the animals to obtain a compass bearing for accurate navigation . The polarization angle pattern of this obscured sky is determined predominantly by scattering on cloud particles themselves . Furthermore, the detection of polarization of downwelling skylight under clouds or canopies is most advantageous in the UV range, where the degree of polarization is lower than the threshold of polarization sensitivity in animals [2,117].
As described in this article, there are three major types of polarization navigation sensor designs that have been utilized in the robotic field: photodiode—linear film polarizer integrated-based design, camera and external polarizer-based design, and DoFP polarimeter-based CMOS sensor design. For the first design, the performance of the devices that were reported in this article is described as shown in Table 4. In the earlier project of this design, the error in the movement of the polarization navigation sensor mounted robot was approximately 13.5 cm, smaller than the error obtained in the latter project, which was 28 cm. However, the output angle in the latter project shows the smaller error than the earlier project, about ±1.3° in difference.
In the second design, the degree of polarization was not compensated. The polarization differential image or polarization summation image is the main task in the third design. This design also studied the image and the variation in the degree of polarization with respect to the orientation. In this design, the degree of polarization behavior with the orientation angle is evaluated. The degree of polarization information from this evaluation could be used to obtain the orientation angle for compass cue application. This third design offers a simpler computation calculation step and provides a system that is easier to integrate with other devices.
In order to develop a human “navigational sense”, polarized skylight sensor research in robotics can be applied to bioinstrumentation research through the use of miniaturization technology. In insects, the most sensitive photoreceptor within polarization detection is UV (bees, flies, ants) and blue photoreceptor (cricket) . UV skylight is in a wavelength range from 330 to 350 nm [41,118], while blue skylight has a wavelength of about 450 nm . As described in Section 3.3.3, for the wire grid micropolarizer to effectively detect the polarization signal, a wire grid pitch should be less than 300 nm. By applying the principle of the central processing stage of insect visual systems (see Section 2.5), the accuracy of the polarization navigation sensor could be increased by raising the number of the polarizing direction axes of the polarizer. Due to the small size and integration process, the multiple polarizing direction axes of the array of wire grid micropolarizers-based navigation sensor could only be developed using nanofabrication technology.
The extremely sensitive e-vector detection system used in crickets can be imitated through the development of a polarization navigation sensor for higher sensitivity. Crickets are active during daytime and at night. The crickets' threshold response possess at lower quantum flux induction than the threshold response by ants and honeybees. In crickets, the threshold response to the radiant quantum flux is about 2.5 × 107 quanta cm−2·s−1 at 433 nm , where the effective quantum flux under the clear, moonless night sky (2 × 108 quanta cm−2·s−1 at 380–500 nm) . During daytime, the threshold for e-vector orientation of honey bees (Apis mellifera) for UV stimulus is about 1010–1011 quanta sm−2·s−1, much higher than that demonstrated by crickets.
7. Conclusions and Outlook·
The implementation of a bio-inspired polarized skylight navigation sensor that can expand the human sensory ability towards a “navigational sense” necessitates the creation of a connection between external devices and the human body. To enable this, miniaturization and integration of existing devices need to be performed, something that is possible using miniaturization technology. Through the application of miniaturization technology, the existing polarized skylight-based navigation sensor that is typically utilized for robot navigation could be enhanced for bioinstrumentation application by integrating them with various techniques, devices and system such as geo-informatics system (GIS) system. Health quality of patient that equipped with the polarization based “navigational sense” could be monitored by the central server at any time continuously or when requested, especially during their outdoor activities . The central server, which is connected to at least three parties (patient, GIS and rescue center), will take first action if the patient's health quality decrease. For example, if the patient have heart problem, the patient's health monitoring device and “navigational sense” will send the health condition data and position data respectively to the central server. As the patient's health quality getting worse, the central server ready to send the ambulance to patient's location.
Weaknesses in the current engineering system when compared with the perfection of natural systems, were highlighted in this review. As such, new theories need to be developed that are capable of improving the approach. MEMS technology can potentially serve to create a system that can more accurately replicate the perfect natural systems with engineering systems or devices that have high functionality and intelligence. A bio-inspired polarized skylight-based MEMS navigation sensor would be very beneficial to humans, especially those who suffer from difficulties with their sight, are wheelchair bound or suffer from Parkinson's disease.
This work was supported by an Arus Perdana Project from the National University of Malaysia. Project number UKM-AP-NBT-16-2010.
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|arthropods||animals lacking a backbone, with jointed limbs and a segmented body with an exoskeleton made of chitin.|
|Cataglyphis||a genus of ants|
|cephalopods||molluscs such as octopus, squid and cuttlefish|
|chromatophore||a cell or cell organelle that contains pigment|
|cornea||transparent part in front of the eye|
|diurnal||active during the day|
|dorsal||the top, back or uppermost surface of an animal oriented with its head forward|
|Gryllus||a genus of crickets|
|Hemiptera||insects such as cicadas, aphids, planthoppers or shield bugs|
|homochromatic||containing or using only one colour|
|iridophore||a pigment cells that reflecting light|
|lamina||a thin layer, plate, or scale of sedimentary rock, organic tissue, or other material|
|Lepidoptera||insects such as butterflies and moths|
|Medulla||the inner region of an organ or tissue|
|microvilli||small ‘hairs’, component of insect photoreceptors|
|monochromatic||containing or using only one colour|
|ocellus||simple eye: an eye having a single lens|
|odometer (odograph)||device that indicates the distance travelled|
|ommatidium||optical unit in compound eyes|
|proprioreceptive||ability to sense stimuli arising within the body regarding position, motion and equilibrium|
|protrusion||something that protrudes|
|rhabdom||light guide in insect compound eyes|
|rhabdomere||reflective inner border of rhabdom|
|tapetum||reflective layer in the eyes of many animals, causing them to shine in the dark|
|tubercle||small rounded projection or protuberance, esp. on a bone or on the surface of an animal or plant|
|ventral||of, on, or relating to the underside of an animal or plant|
|Table 1. Mechanism of polarized skylight detections in various animals. RD–rhabdom, PR–photoreceptor, MV–microvilli.|
|Honey bees(Apis mellifera, Apis cerana) [54,60]||Compound eyes, ommatidia of the dorsal rim area (DRA)||RD: two populations of orthogonally arranged rhabdomeres; not twistedPR: UV receptor absorbing photopigments for polarization detection, maximum sensitivity for skylight polarized parallel to the microvilliMV: aligned in parallel along the length of each photoreceptor cell; microvilli orientation in a fan-like pattern|
|Desert ants(Cataglypis bicolor, C. fortis)[52,54,58,61]||Compound eyes, ommatidia of the DRA||RD: distal tips are dumb-bell shape and fused rhabdomPR: Polarization vision is mediated by UV receptor cells only; mutually perpendicular microvilliMV: aligned in parallel along the longitudinal axes of cells; microvilli orientation in a fan-like pattern|
|Cricket(Gryllus campestris) [18,58]||Compound eyes, ommatidia of the DRA||RD: fused and an elongated triangle rhabdom, contains two orthogonal microvilli orientationsPR: come in two sets that have their microvilli oriented perpendicularly oriented to each otherMV: strictly aligned along the rhadomeres|
|Beetle (Scarabaeus zambesianus)||Compound eyes, ommatidia of the DRA||RD: heart-shaped with orthogonal microvilliPR: seven photoreceptor rhabdomeresMV: the microvilli of photoreceptor 1 are parallel but perpendicular to photoreceptor 2-7|
|Monarch butterfly(Danaus plexippus)[21,55]||Compound eyes, ommatidia of the DRA||RD: wide and short rhabdomsPR: two types of photoreceptor with mutually orthogonal microvilli orientation and well-aligned microvilli in each receptorMV: aligned in different planes to optimize skylight reception at all angles for more global photoreceptor activities|
|Butterflies(Pieris rapae, Papilio crucivora, Colias erate) [46,47]||Compound eyes, ommatidia||RD: fused rhabdomRhabdomere: rhabdomere consists of microvilli containing the rhodopsinPhotoreceptor: nine photoreceptors in three groups according to the position of their rhabdomere and specialized for polarization visionMV: microvilli contain rhodopsin|
|Flies (Calliphora erythrocephala, Musca domestica, Drosophila melanogaster)[14,41,62]||Compound eyes, ommatidia of the DRA||RD: open rhabdomPR: have eight photoreceptor cells, with six of them arranged in a trapezoidal pattern around the tiered rhadom and R7 and R8 specialized for detection of polarized skylight and high polarization sensitivityMV: orthogonally arranged|
|Spider(Drassodes cupreus) ||Secondary eyes, tapetum||Tapetum: acts as a polarizer, canoe-shaped tapeta; microvilli inside tapetumPR: sensitive to the plane of polarization of skylight, orthogonally arranged microvilli|
|Mantis shrimp(Odontodactylus scyllarus)||Compound eyes, ommatidia of the DRA||Ommatidia: form 6 parallel rows, called midbandPR: specialized for UV (linearly polarized), for colour (blue-green) or polarization vision. Cells respond to skylight with an e-vector oriented parallel to the mid-band and with an e-vector oriented perpendicular to mid-band. Orthogonal arrangement of UV-sensitive photoreceptor cells; quarter-wave retarders.MV: parallel microvilli for polarization sensitivity|
|Locust (Schistocerca gregaria)[13,20]||Compound eyes, ommatidia of the DRA||RD: fused rhabdomPR: largely photoreceptors for blue with high polarization sensitivityMV: microvilli of photoreceptor cell 7 are oriented perpendicularly to microvilli of photoreceptors 1, 2, 5, 6 and 8; microvilli photoreceptor 3 and 4 are irregular; microvilli orientation are arranged in a fan-like pattern|
|Cephalopods (squid, cuttlefish and octopus) [64,65]||Complex skin with pigmented chromatophore organs and structural light reflectors (iridophores)||PR: detect linearly polarized skylight by reflectionMV: orthogonal arrangement of microvilliIridophores: contain stacks of protein plates interspersed by cytoplasm spaces, produce colorful linearly polarized reflective patterns|
|Table 2. Development progress of CMOS polarization imager.|
|Project Author, Year||Polarizer on Chip (Pitch)–Metal Type||Pixel Size (μm2)||Pixel Number||Chip Size (μm2)||Extinction Ratio (ER)||SNR (dB)||Micropolarizer Type and Direction||Fabrication Process|
|Tokuda, 2009 ||1,200 μm||20 × 20||30 × 30||1,880 × 1,880||2.03||0.35 mm 2 poly 4 metal standard CMOS|
|Zhao, 2009 ||10 μm -polymer||100||four directions of polarization; 0°, 90°, 45° and −45°||Spin coating and UV photo-lithography|
|Gruev, 2010 ||140 nm-Al||1,000 × 1,000||58||45||micropolarizers with four different orientations offset by 45°|
|Perkin, 2010 ||130 nm-Al||1,000 × 1,000||58||45||four polarizer filter array (0°, 45°, 90°, 135°)|
|Perkin, 2010 ||130 nm-Al||1,000 × 1000||58||45||two polarizer filter array (0°, 45°)|
|Gruev, 2010 ||polymer||18 × 18||100 × 100||13||43.3||Dual tier polymer film with two different orientation offset by 45°||0.5 μm 2 poly 3 metal UMC CIS|
|Sarkar, 2010 [79,80]||480 nm||25 × 25||128 × 128||4,000 × 5,000||22||33||Combination of two types of micro-polarizer (first type: 2 direction of polarization; 0° and 90°; second: 3 direction of polarization; 0°, 45° and 90°)||0.18 μm 1 poly 3 metals UMC CIS|
|Table 3. Nano wire grid polarizers.|
|Year, Ref.||Metal of Wire Grid||Grid Period (Pitch) (nm)||Extinction Ratio (ER)||SNR (dB)||Transmission Efficiencies (%)|
|Table 4. Device performance.|
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