*3.1.2. Towards a biologically inspired small-scale water jumping robot*

6 Will-be-set-by-IN-TECH

Section 4 of this paper details two different approaches for modeling SMAs; one based on identifying how electrical resistance change as a function of the input current [59], and the other based on identifying how the output torque produced by an antagonistic pair of SMA actuators change as as a function of the applied power [5]. Furthermore, [5] details how to take advantage of phenomenological models for simulating overheating problems when a SMA wires are overloaded. Attempting to perform this analysis on the real SMA actuators might cause several damage to the structure. Phenomenological models are really useful for

The control methods presented in [59] and [5] have been conceived for controlling a pair of antagonist SMA actuators. The antagonistic configuration is useful for having SMA actuators where each direction of motion can be controlled. In [59], the antagonistic pair of actuators must bend the structure of the fish robot, whereas in [5] the antagonistic SMA actuators are connected to a joint for providing the rotational motion. Other approaches in [11], [30], [41], [28] have demonstrated the advantages of using an antagonistic arrangement in terms of controllability. When the active actuator is being heated while the passive (antagonistic) is cooling, hysteresis effects are reduced due to the external stress that the active actuator applies

The use of SMAs as artificial muscles allows for more realistic bio-inspired actuation presented in nature [26]. SMA wires acting as muscle fibers can respond upon electrical signals, taking advantage of the large pull force and its excellent strength-weight tradeoff. Currently, the use of SMAs in biomimetic robotic systems [3],[13],[9] can be found in ground, water and air robots, in many sizes, including those micro-robots or microstructures [25],[78],[53],[35],[63],[82],[37],[37]. In the following sections, we describe the most representative bio-inspired robots and structures that integrate SMAs as muscle-like actuation

Biologically inspired robots that operate in water can be found in two categories. Firstly, robots that use SMAs for actuating appendices (fins), and secondly, robots that use SMAs to actuate the robot's body. In the latter body actuation is used for undulatory motion (fish-like robots). Some animals can move by bending their body in such a way to produce a backward-propagating propulsive wave. The movement obtained by bending a continuous structure is much more natural than others where joints are presented. In Section 4.2 our

In [78] it has been proposed a micro robot fish that uses a flexible biomimetic fin propeller with embedded SMA wires to mimic the musculature and flexible bending of squid fin. The propulsion consists of an active component (the biomimetic fin) and a passive component (the caudal fin). The biomimetic fin-based propulsion mechanism is an actuator that combines the

*3.1.1. A micro-robot fish with embedded SMA wire actuated by flexible biomimetic fin*

determining the upper limits of applied input heating currents.

on the inactive one above the austenite finish temperature.

mechanisms.

**3.1. Water**

bending structure prototype is presented.

SMA wire and an elastic substrate.

**3. Bio-inspired robots with SMA muscle-like actuation**

In [63], the locomotion description of a water-jumping robot that mimic the ability of the water striders and the fishing spider to jump on the water surface. This biomimetic robot achieves a vertical jumping motion by pushing the water surface. The motion is triggered with a latch driven by the SMA actuator.

As a result of the research, quantification of Re = 260 (Reynolds number is the ratio of inertial over viscous forces), Bo = 0.0054 (Bond number is the ratio of the buoyancy to the surface tension) and We = 4.7 (Weber number is the ratio of the inertia to the surface tension) and the *Ba* (Baudoin number is the ratio of the body weight to the surface tension) suggest that the physics of jumping in this robot is similar to those of the fishing spider. The Bond, Weber number and Baudoin numbers are explained by [15], [24]. In terms of actuation, the SMA allows the robot to be extremely light (mass of 0.51*g*), which it is essential to ensure the buoyancy on water. The maximum jumping height is 26*mm*, 26% of the height reached when jumping on ground (53.1*mm*). This prototype is the first concept of jumping robot that integrates SMAs within a structure with an overall mass of 1*g*. The robot requires 2*W* of power consumption in order to generate a force of 1.35*mN*.

### *3.1.3. A micro biomimetic manta ray robot fish actuated by SMA*

In [79] a manta ray robot fish actuated by SMA wires is designed. Figure 2 shows the prototype of the robot. Two pectoral fins arranged in triangular-shaped made of latex with a thickness of 0.2mm form the fin surface.

This micro manta ray was the first prototype that uses SMAs to generate thrust. This robot is capable to swim forward and turn. The sweep back angle of the pectoral fins is 20◦. A maximum swimming speed of 57*mm*/*s* was achieved and the maximum amplitude of the motion was 40*mm*. All the biomimetic fins are open-loop controlled.

#### *3.1.4. Controlling a lamprey-based robot with an electronic nervous system*

In [81] a sea Lamprey has been developed. The robot consists of a cylindrical electronics bay propelled by an undulatory body axis. SMA actuators generate propagating flexion waves in five undulatory segments of a polyurethane strip. The lamprey robot Figure 3(a) consists on a cylindrical hull that houses the electronics and battery pack. In this application, the authors use a neuronal network that allows the robot to be controlled in real time. This neuronal

8 Will-be-set-by-IN-TECH 60 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Figure 2.** Micro biomimetic manta ray robot fish [79].

network also drives the control of the SMAs. The results have shown the system can reject disturbances thanks to the robustness of the nonlinear controller [55]. Each SMA wire drains 1.5*A* of electrical current when activated.

**Figure 3.** (a) Lamprey Robot with sonar array, (b) Lateral view of tail segment showing nitinol actuator, Teflon vertebra and tensioning nuts and (c) Lateral view of pitch mechanism [81].

#### *3.1.5. A biomimetic robotic jellyfish (Robojelly) actuated by shape memory alloy composite actuators*

The newest and more advanced aquatic robot that uses SMA actuators is a jellyfish robot designed by [68]. The hydrogen-fuel-powered robot called "Robojelly" mimics the propulsion, morphology, kinematics and physical appearance of a medusa (jellyfish); the Aurelia aurita species. The bio-inspired actuators are made of silicone, SMA wires and spring steel.

The development of Robojelly has introduced a systematic method for the design and fabrication of SMA-based actuators called BISMAC (bio-inspired shape memory alloy composite). This method allows for bending the structure of the robot by means of SMA contraction [77]. Thanks to the BISMAC SMA arrangement, this robot was capable to mimic the physics and swimming characteristics of jellyfish in terms of A. aurita's bell geometry, passive relaxation mechanism, neutral buoyancy, frequency of motion, and deformation-to-flap motion profiles. The structure can be bended by the SMAs actuators (deformation), and then a flap motion of the bell-segment structures provide the propulsion. The Robojelly was able to produce enough thrust to propel itself and achieve a proficiency of 0.19*s* − 1 which is comparable to the natural medusa at 0.25*s* − 1. The robot consumes an average of 16.74W over its 14th cycle of actuation. This robot confirms the fact that most aquatic biomimetic robots use SMA wires combined with other materials to create SMA-based actuators. This characteristic shows the flexibility of the SMA to work in combination with other materials.

**Figure 4.** A CAD representation of the tail of Suleman's Tuna [65].

### *3.1.6. Design and testing of a biomimetic tuna using shape memory alloy induced propulsion*

In [65], an SMA actuated tail inspired by a Bluefin tuna is proposed. Figure 4 shows the tail cutaway. This fish-like robot has length of 1*m* capable of the biological carangiform swimming mode. The maximum tail beat frequency was 0.5*Hz* due to the limitations of the SMA. Even at this low frequency, the power requirements were significant. The minimum and maximum power consumptions were calculated to be 292.8*W* and 333.6*W* respectively.
