Miniature curved artificial compound eyes

Abstract

In most animal species, vision is mediated by compound eyes, which offer lower resolution than vertebrate single-lens eyes, but significantly larger fields of view with negligible distortion and spherical aberration, as well as high temporal resolution in a tiny package. Compound eyes are ideally suited for fast panoramic motion perception. Engineering a miniature artificial compound eye is challenging because it requires accurate alignment of photoreceptive and optical components on a curved surface. Here, we describe a unique design method for biomimetic compound eyes featuring a panoramic, undistorted field of view in a very thin package. The design consists of three planar layers of separately produced arrays, namely, a microlens array, a neuromorphic photodetector array, and a flexible printed circuit board that are stacked, cut, and curved to produce a mechanically flexible imager. Following this method, we have prototyped and characterized an artificial compound eye bearing a hemispherical field of view with embedded and programmable low-power signal processing, high temporal resolution, and local adaptation to illumination. The prototyped artificial compound eye possesses several characteristics similar to the eye of the fruit fly Drosophila and other arthropod species. This design method opens up additional vistas for a broad range of applications in which wide field motion detection is at a premium, such as collision-free navigation of terrestrial and aerospace vehicles, and for the experimental testing of insect vision theories.

Keywords: Micro-opto-electromechanical systems; bioinspired robotics; optic flow sensor; wide-angle vision.

Fig. 1.

Artificial and natural curved compound eyes. (A) Image of the CurvACE prototype. The entire device occupies a volume of 2.2 cm³, weighs 1.75 g, and consumes 0.9 W at maximum power. (B) Illustration of the panoramic FOV of the fabricated prototype. The dots and circles represent the angular orientation and acceptance angle Δρ of every ommatidium, respectively. Compound eye of the extinct trilobite Erbenochile erbeni (22) (C) and of the fruit fly Drosophila melanogaster (D). [(C) Reprinted from ref. with permission from AAAS; (D) Reprinted from ref. with permission from AAAS.]

Fig. 2.

CurvACE design and assembly. (A) Scheme of the three layers that compose the CurvACE artificial ommatidia: optical (microlenses and apertures), photodetector (CMOS chip), and interconnection (PCB). (B) Accurate alignment and assembly process of the artificial ommatidia layers in planar configuration. (C) Dicing of the assembled array in columns down to the flexible interconnection layer, which remains intact. (D) Curving of the ommatidial array along the bendable direction and attachment to a rigid semicylindrical substrate with a radius of curvature of 6.4 mm to build the CurvACE prototype. Two rigid circuit boards containing two microcontrollers, one three-axis accelerometer, and one three-axis rate gyroscope are inserted into the rigid substrate concavity and soldered to the sides of the ommatidia through dedicated pads (Figs. S3D and S4).

Fig. 3.

Characterization of CurvACE angular sensitivity. (A) Measured ASF along the middle (equatorial) row (black curves), with the corresponding interommatidial angles Δφ_{h eq} (red triangles) and mean acceptance angles Δρ_h (blue circles) of the CurvACE ommatidia averaged along every column. Error bars display SDs. (B) Mean horizontal interommatidial and acceptance angles averaged along every row of artificial ommatidia as a function of the elevation angle α. The black curve shows the theoretical Δφ_h values obtained using Eq. S10 with a constant Δφ_{h max} of 4.2°. (C) Schematic representation of the acceptance angle Δρ of an ommatidium and the interommatidial angle Δφ calculated from the peak ASFs of two neighboring ommatidia. (D) Measured ASFs along a single column of artificial ommatidia (black curves), mean vertical interommatidial (red triangles), and acceptance angles (blue circles) averaged along every row of artificial ommatidia. a.u., arbitrary units; deg., degree.

Fig. 4.

CurvACE autoadaptation to ambient light at the single ommatidium level. Steady-state (red dots) and transient (green dots) responses of the adaptive analog VLSI photodetectors [design based on a circuit proposed by Delbrück and Mead (27)]. Each of the four dynamic operating curves (in green) shows the V(log I) response, averaged over 11 ommatidia (photodetectors with optics) of one column, to step increments and decrements of irradiance (Fig. S6) about four steady levels (red circles).

Fig. 5.

Optic flow fields from the CurvACE prototype. Cylindrical equidistant projections of the optic flow field calculated with a modified version of the Lucas–Kanade method (29, 30) from the visual signals obtained by the CurvACE prototype subjected to roll motion (Fig. S8B) at 32° per second and at a distance of about 1 m to a wall displaying random black and white patterns (A) or to linear translation (Fig. S8C) at 3 cm/s toward the patterned wall at a distance of 1 cm (B). The red spot displays the center of rotation (A) or the focus of expansion (B).

Fig. 6.

Characterization of CurvACE motion detection capabilities. (A–C) Angular speed characteristics of CurvACE calculated with a method based on the time-of-travel scheme (32) (Fig. S9) assessed by applying steps of yaw rotational speed Ω_yaw to the sensor at 10° per second, lasting 10 s each, with the prototype placed at the center of a 105-cm diameter arena lined with prints of a natural image. The dashed line displays the theoretical trend.