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  Figure 7. Early soft-robotic bat robot with single-point lever actuation for flexible (silicone) noseleaf and pinnae.
Behavioral studies in horseshoe and hipposiderid bats (Yin et al., 2017; Qiu and Müller, 2020) have demonstrated that these animals have far greater variability in their ear motions than what could be accomplished with single-point actuation. Hence, the next generation of soft-robotic bats has been designed to integrate actuators into its noseleaves and pinnae (Figure 8; Eckman et al., 2019). One way to accomplish this is to use small pneumatic actuators in the shape of small
“sausages” that can made from the same flexible materials as the ears (e.g., silicone) and can be filled with pressured air (Sullivan et al., 2019). If one side of the actuator is attached to the surface of the pinna while the other is left unattached, the result will be an asymmetrical stiffness. For example, the side of the actuator that is attached to the pinna is stiffer than the unattached side. With this arrangement, filling the actuator with air will produce a bending motion to deform the attached pinna. In systems designed this way, two actuators have been attached to the noseleaf, one to actuate the tip (“lancet”) and the other to actuate a semicircular baffle around the nostrils (“anterior leaf ”). Flexible biomimetic pinnae have been outfitted with up to four pneumatic actuators to support different deformation patterns (Eckman et al., 2019).
Evolving Beyond Linear Systems
Describing the acoustical effects of static noseleaves and pinna shapes on the spatial sensitivity of the bats’ biosonar systems as outlined in Evolving Beautiful
Acoustics from Ugly Faces can be accomplished by simple linear systems theory, for example, by virtue of a set of transfer functions (i.e., amplitude as a function of
frequency) that also depend on direction. If the noseleaf or pinna deforms, the transfer functions describing the system become time variant. This means that these functions become dependent not only on frequency but also on time, with the latter dependence reflecting the deformation of the physical structure (Meymand et al., 2013). As if this time-variant nature of the bat’s biosonar periphery was not complicated enough already, there are also nonlinear effects to consider. Some bat species have been shown to move their pinnae so fast that the incoming echoes are subject to a Doppler shift on diffraction by the surface of the moving pinna (Yin and Müller, 2019). Bat robots with fast moving pinnae have since been able to create Doppler shifts as well (Yin and Müller, 2019). Whereas research on whether bats actually perceive and use the Doppler shifts created by their own pinnae has yet to be carried out, the bat robots are already ahead of their flesh-and-blood cousins. Thus, using a robotic replica of a fast-moving bat pinna, it has been possible to demonstrate that these motions create Doppler signatures that enter the ear canal and carry useful information on the direction of an incoming sound (Yin and Müller, 2019). Through these findings, observation of the bats’ behavior has advanced technological evolution to a novel, nonlinear principle for telling the direction of a sound source. In return, the evolution of bat robots has generated a new hypothesis for how bats may use the Doppler shifts created by their pinna motions.
 Figure 8. Soft-robotic bat robot with pneumatic actuation consisting of air compressors, control valves, soft actuators (“pneu-nets”), and flexible pinnae.
 36 Acoustics Today • Winter 2020

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