Page 35 - Winter 2020

Figure 6. Different forms of mobility in bat robots: A: bat robot mounted on a six degree of freedom humanoid robot arm that is capable of translating and rotating the entire bat robot; B: bat robot that can track targets by comparing inputs received from a pair of microphones and can rotate the ears as well as translate the entire head. The target (ball bearing) can be seen in reflections by the emitter (center) and the two receivers (on the side).
of the ear canal, its geometry is of critical importance in determining some of the ear’s acoustic properties. In general, sound diffraction by a surface is determined by the geometry of the surface, its size relative to the sound wavelength involved, and the material. Hence changing the shape will change the properties of the diffraction process. The diffraction process at the pinna determines how sensitive the bat is for sound from any given direction at any given frequency.
The task of reproducing the deformations seen in bat noseleaves or pinnae poses a worthy challenge for the emerging field of soft robotics, where researchers try to build robots using materials that mimic the compliant nature of biological tissues. It is pretty straightforward to create a flexible noseleaf or pinna shape modeled on the geometry of bats from a silicone material, for example.
The tricky parts are getting the shape to deform like a bat ear and controlling the motion. Packing the equivalent of more than 20 muscles onto a small pinna shape is not an easy task given the state of the art in soft robotics. In particular, fitting 20 tiny motors on a structure the size of a bat noseleaf (typically 1 to 2 cm tall) or pinna (typically 1 to 5 cm tall) remains a daunting task. It is possible to provide a bit relief by increasing the size of the noseleaves and pinnae somewhat over what is found in bats as long
as the used frequencies are scaled down simultaneously so that the ratio of structure size and wavelength stays the same. However, this approach has its limits because the environment/sonar targets would also have to be scaled to replicate the context of bat biosonar and hence retain the opportunity to discover or leverage functional principles from the bats.
Early attempts to reproduce the deformations in bat noseleaves and pinnae with a soft robot (Figure 7; Pannala et al., 2013; Caspers and Müller, 2018) have avoided the problem of integrating actuators altogether. Instead, an external motor was connected to the noseleaf or pinna with a lever to insert a point force into the structure. In this approach, the deformation pattern is determined by the geometry and material of the structure as well as by the insertion point for the force. Once the system design covering these parameters is in place, the noseleaf or pinna can only carry out a single deformation pattern that can only be varied in terms of speed and amplitude. Nevertheless, the research carried out with these (severely limited) systems has been able to demonstrate that changes in the geometry of noseleaves and pinnae that occur during the sound diffraction process can encode additional sensory information that is useful to standard sonar tasks such as estimating the direction of a target (Müller et al., 2017).
Winter 2020 • Acoustics Today 35