Philip Caspers

Dynamic Substrate for the Physical Encoding of Sensory Information in Bat Biosonar

Horseshoe bats have dynamic biosonar systems with interfaces for ultrasonic emission (reception) that change shape while diffracting the outgoing (incoming) sound waves. An information-theoretic analysis based on numerical and physical prototypes shows that these shape changes add sensory information (mutual information between distant shape conformations <20%), increase the number of resolvable directions of sound incidence, and improve the accuracy of direction finding. These results demonstrate that horseshoe bats have a highly effective substrate for dynamic encoding of sensory information.

A dynamic ultrasonic emitter inspired by horseshoe bat noseleaves.

The emission of biosonar pulses in horseshoe bats (family Rhinolophidae) differs from technical sonar in that it has dynamic features at the interface to the free field. When the horseshoe bats emit their biosonar pulses through the nostrils, the walls of a horn-shaped baffle (anterior leaf) are in motion while diffracting the outgoing ultrasonic wave packets. Here, biomimetic reproductions of the dynamic emission shapes of horseshoe bats have been studied for their ability to impose time-variant signatures onto the outgoing pulses. It was found that an elliptical sound outlet with rotating baffles that were attached along the direction of the major axis can be well suited for this purpose. Most importantly, concave baffle shapes were found to produce strongly time-dependent devices characteristics that could reach a root-mean-square-difference between beampatterns of almost 6 dB within a rotation angle of 10°. In contrast to this, a straight baffle shape needs to be rotated over 60° for a similar result. When continuously rotated in synchrony with the emitted pulses, the concave biomimetic baffles produced time-variant device characteristics that depended jointly on direction, frequency, and time. Since such device properties are so easily produced, it appears well worthwhile to explore their use in engineering.

A design for a biomimetic dynamic sonar head

The biosonar system of horseshoe bats (family Rhinolophidae) has been shown to employ unusual dynamics upon the emission as well as the reception of the ultrasonic pulses. Non-rigid changes to the shapes of the noseleaves (emission baffles) as well as the outer ears (pinnae, reception baffles) have been demonstrated to affect the properties of the emitted and received ultrasonic signals. These effects have been found in the results of numerical simulations as well as experimentation with physical prototypes. In the present work, a next-generation prototype of a biomimetic sonar head inspired by horseshoe bats is being developed. The goals for this system are to create more comprehensive and life-like dynamic baffle shape geometries as well as a better acoustic coupling between the ultrasonic transducers and the time-variant baffle shapes. Particular attention has been paid to geometry of the transition between nostrils and the noseleaf baffle. A single biomimetic system that incorporates these dynamic emission and reception baffles will enable an experimental investigation of how these two dynamic stages could be used in an integrated fashion to enhance sonar performance in real-world sonar sensing scenarios.

A Unified Analysis of Structured Sonar-Terrain Data Using Bayesian Functional Mixed Models

ABSTRACT Sonar emits pulses of sound and uses the reflected echoes to gain information about target objects. It offers a low cost, complementary sensing modality for small robotic platforms. Although existing analytical approaches often assume independence across echoes, real sonar data can have more complicated structures due to device setup or experimental design. In this article, we consider sonar echo data collected from multiple terrain substrates with a dual-channel sonar head. Our goals are to identify the differential sonar responses to terrains and study the effectiveness of this dual-channel design in discriminating targets. We describe a unified analytical framework that achieves these goals rigorously, simultaneously, and automatically. The analysis was done by treating the echo envelope signals as functional responses and the terrain/channel information as covariates in a functional regression setting. We adopt functional mixed models that facilitate the estimation of terrain and channel effects while capturing the complex hierarchical structure in data. This unified analytical framework incorporates both Gaussian models and robust models. We fit the models using a full Bayesian approach, which enables us to perform multiple inferential tasks under the same modeling framework, including selecting models, estimating the effects of interest, identifying significant local regions, discriminating terrain types, and describing the discriminatory power of local regions. Our analysis of the sonar-terrain data identifies time regions that reflect differential sonar responses to terrains. The discriminant analysis suggests that a multi- or dual-channel design achieves target identification performance comparable with or better than a single-channel design. Supplementary materials for this article are available online.

A design for a dynamic biomimetic sonarhead inspired by horseshoe bats

The noseleaf and pinnae of horseshoe bats (Rhinolophus ferrumequinum) have both been shown to actively deform during biosonar operation. Since these baffle structures directly affect the properties of the animals biosonar system, this work mimics horseshoe bat sonar system with the goal of developing a platform to study the dynamic sensing principles horseshoe bats employ. Consequently, two robotic devices were developed to mimic the dynamic emission and reception characteristics of horseshoe bats. The noseleaf and pinnae shapes were modeled as smooth blanks matched to digital representations of a horseshoe bat specimens noseleaf and pinnae. Local shape features mimicking structures on the pinnae and noseleaf were added digitally. Flexible baffles with local shape feature combinations were manufactured and paired with actuation mechanisms to mimic pinnae and noseleaf deformations in vivo. Two noseleaves with and without local shape features were considered. Each noseleaf baffle was mounted to a platform called the dynamic emission head to actuate three surface elements of the baffle. Similarly, 12 pinna realizations composed of combinations of three local shape features were mounted to a platform called the dynamic reception head to deform the left and right pinnae independently. Motion of the noseleaf and pinnae were synchronized to the incoming and outgoing sonar waveform, and the joint time-frequency properties of the noseleaf and pinnae local feature combinations and pairs of pinnae and noseleaf thereof were characterized across spatial direction. Amplitude modulations to the outgoing and incoming sonar pulse information across spatial direction were observed for all pinnae and noseleaf local shape feature combinations. Peak modulation variance generated by motion of the pinnae and combinations of the noseleaf and pinnae approached a white Gaussian noise variance bound. It was found the dynamic emitter generated less modulation than either the combined or reception scenarios.

Horseshoe bat inspired reception dynamics embed dynamic features into speech signals

Horseshoe bats can alter the shape of noseleaves and outer ears (pinnae) at the time of emission and reception of biosonar pulses. The shape changes are a result of specific muscular action and significantly change the emission and reception beams by a spatial redistribution of energy in mainlobe and sidelobes. Here, we explore the potential of embedding dynamic features into the speech signals by means of bat inspired reception dynamics to improve the speech recognition accuracy and speaker localization. For this analysis, a digit dataset with 11 U.S. English speakers was recorded through a robotic setup that mimics pinna dynamics as observed in bats. The utterances were obtained through open-source speech corpora and amplitude modulated (carrier frequency: 55 kHz) to shift to ultrasonic band. These utterances were then re-recorded through the dynamic pinna set up and later demodulated to extract the original signal. It was found that reception dynamics have been able to enrich the speech signals with dy...

Integration of emitter and receiver dynamics in a biomimetic sonar head

Horseshoe bats (family Rhinolophidae) transmit and receive sonar information about the environment using morphologically complex and dynamic noseleaf and pinnae. Both local shape features and dynamic deformations of the noseleaf and pinnae contribute to properties of the bat biosonar system. This work systematically explores the effect of local shape features on the dynamic properties of emission and reception baffles using a biomimetic robotic system inspired by the horseshoe bat. Parametric models of the greater horseshoe bat (Rhinolophus ferrumequinum) noseleaf and pinnae with selectable combinations of local shape features were developed. Baffles with feature realizations were cast out of flexible silicone and mounted to a platform to dynamically actuate the emission and reception baffle surfaces with motions patterns similar to the greater horseshoe bat. Motions of the baffle surfaces were synchronized to the outgoing and incoming sonar waveform, and the time-frequency properties of the emission and ...

Dynamic periphery in a biomimetic sonar system introduces time-variant signatures into targets echoes

Bats have developed unique and refined systems of echolocation throughout the course of their evolutionary history, giving them the ability to navigate and hunt in extremely cluttered environments. While the mechanisms behind many of these abilities remain unknown, it has been observed that the most effective biosonar systems in bats use a variety of dynamic sensing mechanisms. One conspicuous manifestation of this dynamics can be seen in changes to the shapes of the baffle that diffract the emitted biosonar pulses (noseleaves) and the returning echoes (outer ears). Using numerical predictions as well as measurements with biomimetic hardware, our own prior work has established that the dynamics in these baffles can create time-variant emitter and receiver characteristics. However, it has yet to be demonstrated that these time-variant device characteristics have a substantial impact on the received echoes. To address this question, a biomimetic sonar head with dynamic emission and reception baffles was use...

Seeing the world through a dynamic biomimetic sonar

The outer baffle surfaces surrounding the sonar pulse emission and reception apertures of the biosonar system of horseshoe bats (family Rhinolophidae) have been shown to dynamically deform while actively sensing the environment. It is hypothesized that this dynamic sensing strategy enables the animal, in part, to cope with dense unstructured sonar environments. In the present work, a biomimetic dynamic sonar system inspired by the biosonar system of horseshoe bats has been assembled and tested. The sonar head features dynamic deforming baffles for emission (mimicking the bats’ noseleaf) and reception (pinnae). The dynamic baffles were actuated to change their geometries concurrently with the diffraction of the emitted ultrasonic pulses and returning echoes. The time-variant signatures induced by the dynamic baffle motions were systematically characterized in a controlled anechoic setting and the interaction between emission and reception dynamic signatures was investigated. The sonar was further tested in...

Micro-aperture bio-inspired broadband sonar model and system for underwater imaging applications

Conventional angular sonar imaging is based principally on the correlation of signals received across individual elements in an array. Thus, array signal processing simply matches a particular set of time delays (or equivalently, phase shifts) to the corresponding arrival angles. The angular resolution of such conventional systems is fundamentally limited by the aperture-to-wavelength ratio. The biosonar of echolocating bats and dolphins provide inspiration that we can significantly overcome these aperture-to-wavelength limits by two orders of magnitude. The key to success lies in the use of multiple octaves of bandwidth. Previous work in bat echolocation has shown how multiple overlapping echoes in range can be deconvolved through spectral pattern matching of broadband interference notches. Recent modeling and simulation results show how these bio-inspired broadband interferometric techniques can be extended to the angular imaging problem despite having only two elements spaced at 1 to 4 λ. Acoustic tank...