**1. Introduction**

In this chapter, we develop a small-sized lightweight fish-like robot, with its surface com‐ posed of a flexible thin plastic film. In robotics, novel designs have been led to advances. For example, designs of tensegrity structures [1], which are composed of a set of disconnected rigid elements connected by continuous tensional members and have been used to develop lightweight robots such as a crawling robot [2], a robotic arm [3], an underwater vehicle [4], and a fin mechanism [5]. Stream-lined designs of gliding wings have allowed underwater

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robots to energy-efficient wide-area observations [6–8]. Some designs such as a manipulator unit [9] and a mechanical contact mechanism [10], fixed onto a commercial remotely operat‐ ed vehicle (ROV), have improved inspection efficiency of underwater vehicles for undersea landscape inspection. Novel fabrication methods can also lead to advances in robotics. For example, the microfabrication of soft material has been shown to produce a gecko robot that can climb a wall [11]. Moreover, the use of a composite material resulted in a bee robot that could fly [12].

We have applied a vacuum packaging method to fabricating a lightweight fish-like robot [13, 14]. The internal components of this robot consisted of a motor, a drive circuit, a battery, a microcontroller, and an oscillation plate to generate thrust (**Figure 1**). These components were encapsulated by a plastic film bag using a vacuum packaging machine. Vacuum generators have been studied in various industrial settings, including food packaging [15], object gripping by a mechanical hand [16], material formation [17, 18], and casting materials [19]. In robotics, engineering the utilization of a vacuum has included the construction of robots with suction cups for wall climbing [20–23] and a handling tool for nanorobots [24]. This research should not only contribute to the development of a fish-like robot but represents the application of a novel fabrication method to robotics.

**Figure 1.** Concept of a fish robot encapsulated by a plastic film.

Recently, small-sized underwater robots are especially required to improve the spatial resolution of these measurements, resulting in high-quality data. Biomimetic designs to improving small-sized underwater robots have included the development of a mechanical pectoral fin [25], fish-like robots [26–29], and snake robots [30]. These robots can swim through water by creating undulations oscillating their bodies. The entire body of the fish-like robot we proposed also generates thrust by the body flexure. The plastic film encapsulating the internal components is inflected by the oscillation plate fixed on the servo motor. To improve the lubricity between the oscillation plate and the plastic film and to simplify the waterproofing and pressure resistance properties of the fish-like robot, its internal components can be filled with insulating fluid.

Most of the underwater robots are encased in a solid, pressure-resistant structure made of metal, such as a stainless-steel and titanium alloy to improve the waterproofing features. The weight of these robots will therefore tend to be greater due to the density of these metal components. To overcome this drawback, we have designed a fish-like robot, the entire outer layer of which is composed of a plastic film, resulting in a lightweight body with low elasticity. In developing the prototype, we selected a low force/torque actuator by utilizing a thin plastic film. This film was flexible, but had lower elasticity for bending than deformable materials such as silicone.

To achieve autonomous control, underwater robots must detect obstacles under water. In traditional underwater robots, sensors such as a camera [31] and a photodetector [32] to detect obstacles are arranged in pressure tight cases. This study was designed to evaluate the electromagnetic-wave-transmitting properties of the thin plastic film. These properties can enable noncontact sensors to be arranged within the encapsulating plastic film (see **Figure 1**). Similar to the other internal components of our robot, these sensors did not require special waterproofing. Additionally, we were able to easily determine the arrangement of these noncontact sensors because the entire surface of the fish-like robot was composed of an electromagnetic-wave-transmitting film, thus enhancing the design flexibility of its internal components.

Underwater robots also require three-dimensional nonholonomical movement to move over wide areas under water. For underwater robots, several vertical depth control techniques must be implemented, including throwing the ballast [33] and changing the volume [34]. Difficulties may be overcome by attitude changing schemes, including use of a movable weight in the body [35], a movable float on the body [10], the reaction force of internal rotors [36], the gyro effect of a flywheel in the body [37], and thruster forces for a neutral buoyant underwater robot [38]. This study involved changing the position of the floating block in the robot body, allowing the selection of a low torque motor.

This chapter is organized as follows: the next section briefly outlines the fabrication of an underwater robot encapsulated by a plastic film. This film was applied by a vacuum packaging machine used in the food industry. We also utilized insulating fluid to simplify the pressure resistance properties of the robot. Section 3 discusses the methods used to control our fish-like robot with a plastic-filmed body. We first investigated the performance of an infrared sensor, taking into account the influence of water and a plastic film. We showed that the signals from the infrared sensors could direct simple autonomous locomotion of our robot. We also developed an attitude control mechanism, based on the geography of the floating block in the body filled with insulating fluid. We showed that changes in the densities of the floating block and the insulating fluid can change attitude. Section 4 summarizes our conclusions.
