Fast Aquatic Escape With a Jet Thruster
Summary (3 min read)
I. INTRODUCTION
- For an aerial robot, movement in water creates additional structural and propulsive design constraints that can be difficult to overcome [1] .
- The authors are developing an AquaMAV capable of diving directly into the water and retaking flight using a high powered burst of thrust (Fig. 1 ).
- The vehicle could then perform a short take-off (Fig. 1 ), and return to its launch site to submit collected samples and data.
- This approach relies on accurate sensing and control to maintain position while a sample probe is lowered.
- The speed and range of robotic aircraft may not always be required by a mission, but aerial-aquatic locomotion has broader advantages in robot mobility.
A. Principles for Aquatic Escape
- Underwater locomotion is one of the most energetically efficient forms of locomotion in the animal kingdom [11] , principally because a neutrally buoyant animal is not required to resist its own gravity to swim.
- Another drawback is that such a vehicle would require a calm surface to take-off, while the principle advantage would be that it allows multiple swimming and flying phases in a single mission.
- This allows a vehicle to escape the water and accelerate when airborne, where drag is dramatically reduced compared to in water [11] .
- Reconfigurable wings have also been shown to have advantages in jumpgliding [8] , and are features of almost all aerial aquatic animals [1] .
- In the following sections, the authors introduce the physical principles behind water jet propulsion, and detail the key design features of the jet-propelled jumping robot.
II. WATER JET PROPULSION
- 2, 3 and 4 to denote variables relating to the main gas tank, the gas within the water tank, the air-water interface, and nozzle outlet respectively (Fig. 2A ).the authors.
- Decreasing nozzle diameter increases total efficiency, but reduces thrust production, and a minimum of diameter 2mm was set.
- These equations remain valid until all water is expelled, after which the release of remaining gas produces a small amount of thrust.
- When subsonic, the outlet will be at atmospheric pressure, but if the pressure ratio is greater than a critical value (equation 11, different to the valve-specific κ choke value), the flow is choked, M =1, and the nozzle outlet pressure will be greater than atmospheric.
- In both cases, the mass flow out can be computed using a standard Mach number relation (equation 12).
A. Design Domain
- For a given reservoir pressure and valve flow coefficient, the work extracted from the gas can be maximised by varying the water tank size and nozzle diameter.
- Enlarging the water tank increases launch mass, and an optimum tank volume exists.
- This decreases thrust (equation 1) and a very small nozzle will be insufficient to propel the vehicle.
- The design domain was computed by numerical integration (Fig. 3 ), with the specific impulse calculated based on the mass of the thruster alone, excluding the electronics and airframe.
- This gave an optimum tank length of 0.45m, which was fabricated.
III. PLANAR TRAJECTORY MODEL
- The robot is fitted with fins and a collapsible wing for flight.
- To investigate the robustness of the transition to flight from water, the authors implemented a planar trajectory model and simple estimation of the hydrodynamic forces during water exit (section III-C).
- Subscript s refers to skin friction forces.
- The authors also define position vectors, x, within a robot-fixed reference frame rotated by an angle θ about Ẑ from the inertial frame, with its origin at the robot nose and unit vectors x,ŷ,ẑ (Fig. 4A ).
B
- The effect of increasing the depth of the robot beneath the water (50 o start angle), also known as A.
- In order to estimate the viscous force on the wings and fins, the authors use a turbulent flow flat plate skin friction coefficient, using a Reynolds number (Re) based on retracted wing cord to estimate friction on the lifting surfaces (equation 25).
- Neglecting Reynolds number changes, the only fluid specific variable in equations 15-26 is the fluid density.
- The robot floats on the water surface prior to launch, so buoyancy must also be included.
- The resulting equations of motion are integrated numerically using a Runge-Kutta solver in Matlab.
D. Take-off Robustness
- The simple drag model was found to give a good prediction of the acceleration profile of the AquaMAV during aquatic take-offs (section VI).
- The model was therefore used to evaluate the robustness of take-off to external perturbations which can occur in an outdoor environment.
- Increasing depth has a strong effect on the final velocity, as drag greatly limits underwater speed (Fig. 5A ).
- Simulating launches at several different angles and depths, the simulation indicates that the jet will be able to achieve its minimum flight velocity (8.5m/s) regardless of angle as long as it is not submerged more than 0.8BL beneath the surface (Fig. 5B ).
- Launching the robot nearer to vertical results in lower speed (but higher altitude).
IV. PROTOTYPE
- The fabricated thruster has an air and water tank, with sealed screw connections to a centrepiece containing a poppet valve (Fig. 6 ).
- To contain and release the high pressure gas, an NiTi Shape Memory Alloy (SMA) actuator has been developed.
- The gas pressure vessel is constructed from 7075 aluminium according to European standards [20] , with an extra safety factor of 2 applied to the wall thickness to increase safety.
- The water tank is pressurised to less than 10bar (Fig. 2C ) and sustains pressure only briefly, so is instead made from a woven CRFP tube, bonded to an aluminium screw connection (Fig. 6B ) and plastic nozzle.
- The system has a deliberate modular construction, with the centrepiece and valve actuation system entirely self-contained, so that both tanks can be changed according to final mission requirements.
A. Valve Actuation
- The valve is opened by raising the valve stem 1.6mm.
- This produced a force of 40N, which is near-constant over the stroke range.
- To provide an electrical conduit into the pressurised container, the vessel wall is used as a negative earth and an insulated bolt was fastened through the tank end (Fig. 6 ).
- This is passed through the wire from a 7.4V, 200mAh battery, sufficient for over 150 actuations, and controlled using an Arduino microcontroller.
- While the water tank will fill gradually with the tail pointing upward in the water, it will not fill when nose up.
B. Flight Components
- The AquaMAV is fitted with deployable wings for flight.
- Of the six segments, only the leading edge is actuated, while the root segment is fixed to the fuselage.
- The wing hinges are actuated by 4.5 gram servos.
- Control electronics are contained in a separate fuselage section attached to the gas tank which allows the gas tank to be removed easily for charging.
V. STATIC PERFORMANCE
- Static thrust was measured by mounting the robot vertically to a load cell, with force data recorded at 2500Hz (Fig. 7 ).
- The sensor was zeroed with the jet water tank full, so weight reduction as water is expelled was also measured by the force sensor.
- The model output has been modified to show this effect in the predicted force profile shown in Fig. 7 .
- The measured thrust profiles show the expected features of a rapid rise in pressure before the water accelerates, followed Towards the end of the water expulsion, the velocity of the air-water interface increases rapidly as it passes through the nozzle contraction.
- No significant variation in thrust when actuating underwater was expected, which was confirmed by the sixth static thrust test, conducted underwater.
VI. AQUATIC TAKEOFF PERFORMANCE
- After thrust was recorded, the AquaMAV was launched from a water tank into flight.
- To allow longer trajectories, the AquaMAV was also launched from a nearby lake and filmed, although tracking was not possible.
- The use of separately actuated wings often led to asymmetry between the two wings, rolling the robot (Fig. 9B ).
- This model indicates that the robot is able to take-off in the presence of perturbations from surface waves to both its launch angle and depth.
- This includes batteries, electronics, a waterproof fuselage and large lifting surfaces.
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Citations
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Cites background from "Fast Aquatic Escape With a Jet Thru..."
...These included friction of the fluid against the internal walls of the chamber, the thrust produced by the jetting air once the water was evacuated, the irregularity of the water-air interface during jetting, and the effect of water-air spray caused by fluid instabilities at the end of the water jetting phase (35)....
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...S9 the estimated jumping height of the robot at 90o compared with impulsive jumping animals in (15), squid flight in (22), and water-jumping robots (15, 16, 27, 35, 37)....
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36 citations
Cites background from "Fast Aquatic Escape With a Jet Thru..."
...Available: http://blog. modernmechanix.com/denmarks-amazing-submarine-plane/#mmGal [14] R. Siddall and M. Kovač, “Fast aquatic escape with a jet thruster,” IEEE/ASME Trans....
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...Siddall and Kovač [14], [15] built an aquatic micro air vehicle capable of launching from the water like a flying squid by expelling a pressurized jet of water....
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...[21] Y. H. Tan, R. Siddall, and M. Kovac, “Efficient aerial–aquatic locomotion with a single propulsion system,” IEEE Robot....
[...]
...Siddall and Kovač [14], [15] built an aquatic micro air vehicle capable of launching from the water like a flying squid by expelling a pressurized jet of water....
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...[15] R. Siddall and M. Kovač, “Launching the AquaMAV: Bioinspired design for aerial-aquatic robotic platforms,” Bioinspiration Biomimetics, vol. 9, no. 3, 2014, Art. no. 031001....
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35 citations
References
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"Fast Aquatic Escape With a Jet Thru..." refers methods in this paper
...To compute body drag, this coefficient is modified based on the ratio of the body’s maximum width and length [BW/BL, (26)] [18]....
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187 citations
"Fast Aquatic Escape With a Jet Thru..." refers background in this paper
...Many amphibious terrestrial robots have been implemented [3], [4], but these robots are not able to cross large, sheer obstacles, and often can only exit the water on gentle inclines....
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167 citations