**2. Fabrication**

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

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

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

could fly [12].

236 Recent Advances in Robotic Systems

novel fabrication method to robotics.

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

with insulating fluid.

We utilized a vacuum packaging machine to fabricate a fish-like robot, the entire body of which was composed of a flexible plastic film. We called this fabrication robot packaging [13, 14]. **Figure 2** shows the process used to fabricate robot packaging. The process can be classified into four steps: (a) encapsulation of the internal components, including a microcontroller, a drive circuit, a battery, a servomotor, and an oscillation plate, in a plastic film bag used to package foods; (b) pouring of insulating fluid, specifically industrial oil [13] or cleaning fluid for semiconductors [14], into the plastic bag. This would reduce the quantity of air in the package after the insulating fluid was defoamed and packaged; (c) defoaming the inside of the robot using a vacuum packaging machine; (d) sealing of the plastic film by a sealer within the chamber of the vacuum packaging machine after defoaming. The drive circuit in the body of the robot is not shortened by the insulating fluid surrounding the circuit. Using this method, we were able to easily fabricate the entire body of a fish-like robot at low cost and in a short time because the body of the robot consisted of only a thin plastic film, which was sealed by a vacuum packaging machine to form the entire body of the robot.

**Figure 2.** Fabrication process of the robot packaging method.

Ideally, a plastic film fabricated by a robot packaging method does not break in response to water pressure because the pressure inside the robot is equal to the environmental pressure. The plastic film and the insulating fluid are deformed slightly by water pressure; however, the volume of the insulating fluid does not change markedly due to its high incompressibility. To assess the validity of robot packaging, we can test the pressure resisting feature of a servo motor encapsulated by a transparent plastic film. The pressure test is performed using a transparent acrylic cylindrical pressure tight case, to which a pump funneled water. The pressure tests are performed using images captured by a camera due to the transparencies of the plastic film and the tight case [14]. These images are used to investigate the pressure resistance properties of these robots, based on frequency analyses of movement of the servo motor. **Table 1** shows the motion characteristics of a servo motor (RS304MD; Futaba) with a servo horn at 1 MPa pressurization steps. The angle of the servo horn was determined by the positions of the center of rotation and the LED mounted onto the tip of the servo horn. The amplitudes in **Table 1** were the average amplitudes and the frequencies of the servo horn were computed by frequency analysis utilizing fast Fourier transformation (FFT). As shown by the amplitude in **Table 1**, however, the motor could not move in an environment pressurized at 10 MPa. The frequency calculated by FFT at 10 MPa was not used because the power spectrum was much smaller than the other estimated frequencies at up to 9 MPa in **Table 1**.


**Table 1.** Example of motion characteristics of a pressured servo motor.
