Fig. 14. Predicted mast drag as a function of velocity

7. Prototype I: thrust experiments

The proposed design and the preliminary calculations proved to fulfil the function extremely well, with a minimum of resistance to movement. The SMA actuators have also proven themselves capable of generating a wide range of motion in the tail. Moreover, the controller and circuit design proved effective in providing fine motion control of the SMAs. The structural simulations have demonstrated the ability of the tail to move according the prescribed travelling wave motion, and provided the needed input for the design of the control law. The thermodynamic model derived and implemented appears to agree well with experimental results. Combined with a simple thermomechanical model of the SMA wires, controller files were generated for motion of the tail underwater to be used in the testing regime.

Next, the prototype vehicle was manufactured. Figure 15 shows the prototype in motion in the test tank. Swimming motion was achieved and initial thrust measurements were taken. The complicated heat transfer conditions made smooth activation of the SMA wires difficult. The actuator force was also somewhat binary in nature, as the material passed through the transition temperature. This resulted in an uneven "jerky" motion, especially when tested in air. The damping effects of the water and skin lessened these effects, but the motion was not perfectly fluid from port to starboard.

Fig. 15. Prototype Testing in the tank

The speed at which SMA wires can operate as actuators is limited by the rates at which they can be heated and cooled. A further constraint is the software used for the control program,

which has a limited cycle frequency which affects the rate of pulse-wide-modulation (PWM) that can be achieved. Both of these constraints limited the maximum tail beat frequency to 0.5 Hz. Even at this low frequency, the power requirements were measurable. The current sent to each wire was measured using a digital multi-meter and is presented in Table 2. The current required at each section was different because of the different heat transfer conditions along the length of the tail section, but symmetric about the centreline.

Port Wires

Starboard Wires

Vertebra #

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