**4.1. iTuna: a bending structure swimming robotic fish**

14 Will-be-set-by-IN-TECH

This pectoral fin was the first and today continues being the only designed with only SMA wires. In fact, most research on fish-like robots is focused on studying propulsion (how to

Novel design principles and technologies for a new generation of high dexterity soft-bodied robots inspired by the morphology and behavior of the octopus are being developed in the

The imitation of the internal muscular structure of octopuses' tentacles is being studies and imitated. Longitudinal cables and transverse SMA imitate the arrangement of muscle fibers, controlling contractions as soft actuators within the robot arm [42]. Moreover, this manipulator is surrounded by a sensitive skin, with contact sensors embedded into silicone rubber, equipped with passive suckers that allow the grasping of objects. SMA actuators are

*3.4.2. Development of a dexterous tentacle-like manipulator using SMA-actuated hydrostats*

used to change the section of the tentacle in several locations, inducing its bending.

**Figure 11.** The SMA-based tentacle (See http://www.octopusproject.eu/).

The development and testing of a biomimetic active hydrofoil using Shape Memory Alloy (SMA) actuators is presented in [16]. This work describes the development and testing of a

In this section, we report our most recent results on two SMA-actuated bio-inspired robots. The first, called iTuna, ia an underwater robot that according to our classification falls into the "full-actuated-boy" category. The second is an aerial robot, which implements the concept of

*3.4.3. Development of a Shape-Memory-Alloy actuated biomimetic hydrofoil*

six-segment demonstration foil and the control schemes used.

**4. Review on recent advances: iTuna and BaTboT**

morphing wings by means of SMA-based muscles.

<sup>1</sup> http://www.octopusproject.eu/

generate thrust), while maneuvers is largely unexplored.

framework of the OCTOPUS-IP project1.

The iTuna [59] is a swimming fish-like robot that apart from the external appearance, imitates some key features of the internal morphology of fishes.

This mechatronic concept takes inspiration from the arrangement of the red or slow-twitch muscles (see inset in Figure 12). In live fishes, such muscles are used for bending a flexible but nearly incompressible axis. Such axis is either composed of a (visco) elastic beam (notochord) or a series of vertebrae connected through intervertebral discs. The main structure of the iTuna robot fish is inspired by the former solution, and is composed by a continuous flexible backbone. The backbone is composed of polycarbonate of 1*mm* thickness actuated by SMA muscles acting as red muscles.

**Figure 12.** Main structure of the iTuna robot fish. a=8.5 cm. Under nominal operatrion, *b* ∼= 96% a = 8.16 cm, h=1.02 cm, b=28◦ [59].

Six SMA-based actuators whose length is 1/3 of the body length are positioned in pairs, parallel to the body in such a way to produce an antagonistic movement on three body segments of 8.5*cm* length. This antagonistic configuration of SMA wires has some advantages in terms of increasing the range of controllable actuation, since both directions of motion (contraction and elongation) can be actively controlled. Figure 12 shows the location of the SMA wires within the skeleton structure of the prototype.

A V-shaped configuration of the wires, where each artificial muscle is composed of a single V-shaped SMA wire, twisted around the tension screw, allows to double the pull force without a significant increase of power consumption. NiTi SMA wires with a diameter size of 150*μm* have been adopted. These have a pull force of 230*grams* − *f orce* at consumption of 250*mA* at room temperature, and a nominal contraction time of 1 second.

Under nominal operation such SMAs can bend the body segments up to 28 degrees (angle *β* of Figure 12), even if SMA wires only contract approximately 4% of their length. By increasing the input electrical current and including a suitable control that handles an overloaded SMA operation, contraction time of 0.5*s* was achieved, and strain could be increased up to 6%, corresponding to a bending of approximately 36◦ (Fig. 13).

#### 16 Will-be-set-by-IN-TECH 68 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

### *4.1.1. SMA control in the iTuna*

After identification, a low-level PID controller has been designed to address two main limitations of SMAs: slack in the fibers, and limited actuation speed. Slack issues appear when SMA wires develop a two-way memory effect during operation [11]. Limitation in actuation speed occurs due to the large switching time between cooling and heating phases. To address such problems, a pre-heating mechanism has been developed that works in conjunction to the antagonistic arrangement. The pre-heating avoids the temperature on both wires drops below the 10% of the maximum applied electrical current, preventing the inactive alloy from complete cooling. On the other hand, the antagonistic arrangement provides an external stress to the cooling wire (provided both by the elastic backbone and by the active antagonistic wire). Working with an already-warm wire allows for a faster stretch and slack issues are avoided. Note that the PID controller is based on the experimental observation that the hysteresis on the electrical resistance curve was smaller than the hysteresis on the temperature curve. Resistance measurements are used as a feedback signal for closed-loop control (see [59] and [60] form more details).

The control developed allows overloading the SMA with up to 350*mA* peak current (note that power signals are sinusoidal, hence overloading only lasts a brief period of time). Overloading has allowed for achieving a 1*Hz* oscillation time (i.e. 0.5 seconds contraction and cooling times) and a bending angle of 36 degrees of each body segment.

**Figure 13.** Bending under SMA overloading [59].

#### *4.1.2. Control architecture*

A key feature of SMAs is the possibility to develop closed loop control systems without the need of external sensor hardware. The feed back signal is provided by the detection of inner electrical resistance, that allows an indirect measurement of the temperature.

The main components are described in the following. A micro controller implements the PID algorithm. The PID controller receives the input reference position (set point) and the feedback of SMA's voltage and current that allows calculating the heating current to drive the SMA actuator. The digital output of the PID controller is converted to a reference current in two steps. First, it is converted into an analog signal using a 2-wire serial 8-Bit DAC (Digital to Analog Converter) with Rail-to-Rail outputs. Then, a Voltage Controlled Current Source (VCCS) transforms the DC voltage in a constant current that feeds the SMA. This stage has a power consumption of less than 10*mA*.

The DAC output ranges from 0 to 5 Volts with a resolution of 0.02*V*. The measured voltage (VSMA) and current (ISMA) on the SMA fiber are fed-back to the micro controller in order to close the control loop. The hardware used (16F690-PIC) had a 12-bits A/D converter with a resolution of 0.537*mV* (considering the maximum voltage measured at the SMA *VSMA* = 0.55*V*.

On the other hand, taking into consideration the maximum current through the wire (500*mA*), SMA resistance variations about 1.074mΩ can be measured. Therefore, since the maximum variation in the SMA length is 0.34*cm*, and the maximum variation of the resistance is 1.6Ω, the theoretical position error of the system based on the SMA length is 0.067%. i.e.,0.12*mm*.
