Such structures are also used in a diverse set of industrial and scientific equipment, including various forms of construction cranes, stents and other medical devices, foldable satellites, and certain architectural designs, as in adaptive and morphing building structures ( Del Grosso and Basso, 2010). Engineered expandable structures, a subclass of more general shape-changing technologies, are present in a variety of common consumer products, such as umbrellas, Hoberman spheres, and Origami. Many expandable structures can be found in nature, such as in the leaves of hornbeams, flower petals, and the hind wings of beetles ( Vincent, 2000 Wang et al., 2017).
Many of these systems can be described as expandable structures:Ĭonstructions that can change shape, and size using various linkages and joints ( Pellegrino, 2002). In addition, human-computer interaction (HCI) researchers have investigated shape-changing properties for developing new types of physical user interfaces ( Rasmussen et al., 2012). Several fields have adapted this idea and developed shape-changing structures as solutions to various engineering challenges, leading to innovations in the automobile industry (e.g., roof structures in convertible cars), architecture, and design (e.g., self-inflating life vests). For instance, male magnificent frigatebirds ( Fregata magnificens) inflate their red throats to attract females, while octopuses change their structures to adapt to dynamic changes in their environment or interact with particular objects. Other organisms use size and/or shape changes for different purposes. For example, pufferfish (Tetraodontidae) and the frilled lizard ( Chlamydosaurus kingii) can change size as a self-defense mechanism, with the pufferfish able to expand up to three times their original size to warn predators and the frilled lizard able to expand a large frill around its neck, which is folded most of time, when threatened.
The ability to dynamically change shape and size is a key evolutionary advantage for many biological organisms. To illustrate how these considerations may be operationalized, we also present three case studies from our own research in expandable structure robots, sharing our design process and our findings regarding how such structures enable robots to produce novel behaviors that may capture human attention, convey information, mimic emotion, and provide new types of dynamic affordances. We detail various implementation considerations for researchers seeking to integrate such structures in their own work and describe how expandable structures may lead to novel forms of interaction for a variety of different robots and applications, including structures that enable robots to alter their form to augment or gain entirely new capabilities, such as enhancing manipulation or navigation, structures that improve robot safety, structures that enable new forms of communication, and structures for robot swarms that enable the swarm to change shape both individually and collectively.
In this paper, we survey the emerging design space of expandable structures in robotics, with a focus on how such structures may improve human-robot interactions.
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