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Category: Autonomous Underwater Vehicles

Thrust Module and Environmental Sealing

Preliminary Thrust Module
Preliminary Thrust Module Design

Given the drag results from the CFD analysis, it became possible to begin more rigorous design work on the thrust module, namely to create detailed propeller designs, motor selection and coupling the two. I’ve done some preliminary propeller analysis using JavaProp and have been fiddling with OpenProp to get an idea of the efficiency and rpm of various propeller designs to meet the thrust requirements – Those parameters are sufficient for estimating the sustained torque on the motor. Given sustained torque and rpm, it became easier to size the motor required to drive the AUV at its design points.

One of the tradeoffs made in the AUV design is how to seal the motor from the external environment. Several options were considered, including traditional pump shaft seals, magnetic coupling, and simply running a brushless motor exposed to the elements. All are valid options depending upon the goals of one’s project, but ultimately I settled on using magnetic coupling for a number of key reasons:

Pros:

  • Completely sealed from the external environment, with a lower probability of leakage than a rotary shaft seal. This is a bonus if pressure compensation is needed (e.g. oil filling), as that reduces the probability of environmental contamination
  • Likely no need for pressure compensation at design depth. (Depending upon isolation can design)
  • Acts as a clutch to save the drivetrain from excess torque and stalling
  • No friction losses as with shaft seal
  • Motor is protected from corrosion in ambient environment

Cons:

  • More complicated design
  • Requires precision machining for precise alignment to avoid strong magnetic forces perpendicular to the shaft (the inside magnetic ring essentially sits in an unstable null)
  • Potential eddy-current losses in the surrounding structure and the isolation can if made of metal.
  • Motor needs to be cooled to ambient environment (easy)
AUV Parts _ Magnetic Coupler
Simplified Magnetic Coupler — Isolation can in dark grey, outer ring in dark blue, inner ring in burgundy.

One challenge is designing the isolation can, illustrated in dark grey above. This is the component that isolates the external environment from the internal. The isolation can needs to be as thin as possible, yet as strong as possible to support the pressures acting upon it. Metal seems to be a natural choice, but the rotating magnets will induce eddy currents resulting in reduced efficiency. I’d like to avoid this if possible. I’m thinking of using an engineering plastic such as acetal with an internal pressure design, as analysis indicates that it will stand up to the required pressures without needing pressure compensation.

Past experience with machining thin wall features in acetal indicates that I need to approach this with caution. The extrusion process used to create the stock plastic rods I buy causes a significant amount of stress within the acetal which can cause warping during machining. I will attempt to anneal the acetal prior to machining to see if that will help. Another option I’m considering is casting the isolation can with rigid polyurethane, epoxy or fiberglass composite.

Knowing the torque limits of the drivetrain, and the torque requirements of the propeller, I carried out some analysis on coupler designs using the Finite Element Magnetic Methods (FEMM) software package. The great thing about this software is that I can import DXF files straight from CAD, so I can design, simulate, tweak and update and re-simulate, allowing me to experiment with several design permutations.

2D Simulation of Magnetic Coupler, rotated to point of maximum torque,
2D Simulation of Magnetic Coupler, rotated to point of maximum torque

The rough configuration I’m considering is shown in the simulation results above. The magnets are arranged in an alternating north-south configuration (i.e. each magnet is polarized opposite to its neighbour). This was done so that as the delta angle between the inside and outside increases  with applied torque, the opposite polarized adjacent magnet would provide a repelling force, preventing free rotation. I ran through a series of different angles, and the peak torque occurred when the delta angle between the inner and outer elements was half the angular separation of the magnets. (22.5 degrees for magnets spaced at 45 degrees). The chart below shows the resulting torque vs angle curve for a similar design to that depicted above

 

Another key parameter is that the outside of the outer ring should be magnetic steel. As shown in the simulation, this keeps the magnetic field contained (which should reduce eddy currents in the aluminum tail cone) and actually increases torque in the coupler

Some final tweaks need to be made to the design to match the peak torque of the coupler to that of the motor and gears — As long as the coupler torque is higher than the propulsion service torque and below the max torque of the motor/gears, it should act as failsafe clutch.

With the rough size of the coupler set, now I can put some more effort into designing the rest of the thruster module, to see how I can fit everything I need inside of it — The main motor, the 4x rudder motors and drive electronics.

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Preliminary CFD Results

This post is jumping into the middle of things, as a huge amount of work has gone into getting the AUV design this far, with several revisions of the design and some background work on the sonar to determine how much AUV I’ll actually need to carry it.

A very preliminary design choice was the dimensions of the AUV. The length will be driven by module and payload lengths, leaving the diameter as something needing a tradeoff between how much I could compact the module electronics vs what I could build. I settled on an outside diameter of 5.5 inches, driven largely by the size parts I can realistically make on my small lathe. I would have preferred a larger diameter, but that’s the way the cookie crumbles. Interestingly, with a 5 inch ID (0.25-inch wall thickness), I can just fit 8x 15AH LiFePO4 cells for a 24 volt, 360 Watt-hour battery.

Why imperial units for the diameter when most of the rest of the design is metric? That’s driven by the availability of stock acrylic and aluminum tubing to be used as the main pressure hull body components. (Flooded modules will likely just be fiberglass). Mixing imperial and metric is just a necessary part of life up in Canada. Sometimes with poor results.

Initial Hull Shape
Initial Hull Shape

Knowing the basic dimensions, I’ve been working on detailed 3D CAD models of the entire AUV design, refining the shape and design of the critical components including the Doppler Velocity Log (DVL), drive section, and the universal bulkhead rings (more on those later). The end result is a rough shape of what the AUV will look like in the end. This design will be refined significantly, but is good enough as a starting point, and is shown above. (Note that the antenna on the top wasn’t exported into the flow simulations)

I set up some simulations in OpenFOAM to estimate the drag and flow around the AUV — The goal being to determine enough information to optimize the propulsion design. I’ve been experimenting with both the simpleFoam and pimpleFoam solver. The computational requirements to get a fine enough mesh  to resolve the features properly proved difficult on my home laptop, so I stood up a server on AWS to handle the simulations — After much experimentation with setting up OpenFOAM cases, the AUV hull design, boundary conditions, and meshing, I finally got things set up well enough to run at a high level of detail with a configuration I felt I could trust. 48 hours later, I had a result. I probably could have introduced larger timesteps into the pimpleFoam solver (the maximum Courant number was set to 25), so I’ll experiment with that on future runs.

 

 

 

 

 

The plot above shows the flow over the stern of the AUV. The Reynolds number is fairly high in this design, leading to turbulent flow, and some strange flow along the stern, where you can see it circulating near the surface. The drag pressures appear to settle within 0.4 or so seconds, with a total simulation time of 1 second. The time-steps were dynamic, so prior to settling were very short to avoid the solution diverging. Curiously, the viscous drag remains steady, but the pressure drag is somewhat unsteady.

Plot of the viscous and pressure forces converging over successive iterations.
Plot of the viscous and pressure forces converging over successive iterations.

Previous results had already led me to reduce the tail angle in an attempt to reduce the wake, but ultimately I think this will have to do as narrowing the tail angle too much will result in other design challenges in terms of length of the thrust module.

Streamlines and pressure distribution along hull
Streamlines and pressure distribution along hull

Apart from estimating the drag on the hull, the really cool thing is I can determine the inflow velocity at the propeller disc. Since the hull inevitably has a negative impact on the water speed, designing a propeller for a water flow equal to the vehicle velocity won’t produce an optimal design. The simulation allowed me to extract the velocity profile, which I can feed into propeller design to further optimize the design. If time allows I may optimize the nozzle (it’s currently a vanilla Kort nozzle). However, the purpose of the nozzle in this design isn’t just to attempt to improve the performance, but also to provide a safety guard to protect wildlife and support divers during testing, as well as to reduce the probability of entanglement.

From the results, the hull has a significant impact on the inflow. The Kort nozzle does increase the speed a bit around the propeller tips, but closing into the propeller hub the velocity drops sharply.

Axial velocity through propeller disc
Axial velocity through propeller disc

Interestingly, the horizontal tangential (y-axis) velocity is low but produces an interesting plot showing that the flow is not perfectly axial, but slightly canted inwards. Note that the scale of this image is different than above. This is mostly included because I believed it was a cool picture.

Horizontal tangential flow
Horizontal tangential flow

Of course, I’m not a CFD expert and am learning things as I go, so I need to take these results with a grain of salt. The results seem to be within the order of magnitude of what I can find in literature, so they’re good enough for the next steps in the design process.

Next steps in the CFD work will be verifying the control surface size is sufficient enough to provide good control authority, and to size the motors required to turn them. So far the control surfaces have just been eyeballed, so I’ll have to do some initial calculations to determine what’s required, adjust the design, and simulate.

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Autonomous Underwater Vehicle Intro

Being an avid scuba diver, I’ve wanted to build a sonar to find things to dive on, as well as to document local dive sites, wrecks and reefs. Although side-scan would be the quicker and easier way to achieve this goal, I have set my sights on a Synthetic Aperture Sonar (SAS). The trick? I need to have some sort of stable platform to mount the sonar — And hence my Autonomous Underwater Vehicle (AUV) project. (In case anyone’s curious, I don’t have a boat, and boats aren’t typically stable enough for high-resolution SAS anyways, so it would require some sort of actively stabilized towfish anyways)

I’ve been thinking about, researching and doing some initial design work on this project for years but have been designing in earnest as of late and I finally am in a place where I can start working towards making this project a reality. This blog will jump into the middle of things, as there’s already so much work done that has gotten me to this stage. The main purpose of documenting it here is mainly as a record for myself, but if someone finds it interesting, that’s also great!

Some of the very high-level requirements I’ve set for this project:

Sonar resolution of better than 10cm in azimuth and range. (Goal: Better than 4cm)

The goal is driven by the physics of the design (transmitter dimensions and system bandwidth), but the actual requirement is loosened off due to some of the other challenges with forming SAS images.

A drawback to real aperture sonar such as side scan is that the azimuth resolution drops off with range, so even if this resolution is achievable at close ranges with a side scan, it is not possible across the entire range. With SAS, this resolution can be maintained across the entire image.

Sonar imaging range of at least 50 meters per side (Goal: 100 meters)

Driven by area mapping rate/speed and transmit power tradeoffs, as well as issues raised by the following requirement. Not too much else to say about this yet, other than it’s still in the trade space as I design the sonar. This may become configurable with different beam modes if I end up using a phased array transmitter.

Sonar imaging minimum depth/altitude of TBD meters

This is a challenging requirement to define, as shallow operation of sonar is very tough. Ideally, I’d set the minimums to something on the order of 10/5 meters, respectively, but I haven’t done enough analysis on the sonar yet to know if that’s achievable. This requirement will be refined as time goes on, so for now, the goal is “as shallow as possible”

AUV cruise velocity of up to 2 m/s through the water. Maximum mapping velocity of 1.5 m/s over ground

2 m/s should be enough to get through mild currents (while maintaining 1.5 m/s over ground). The flip side of this is that the propulsion design will need to have enough instantaneous thrust to keep the speed stable in spite of any environmental effects such as surge.

The 1.5 m/s stems from the preliminary sonar design and range requirements. The trade is between the sonar receive array length, range, and velocity. As previously stated, I may opt for configurable modes using a slower speed to achieve greater range.

AUV operational depth of 100 meters sea water.  (Goal: 500 meters)

This is a nice round number, which some of the affordable design choices and material/component selections work well for. It’s also deeper than I’ll probably ever go in my scuba diving career, so even if I found something below that with the sonar, I wouldn’t be able to get to it!

The goal is very loose, would require upgrades (e.g. aluminum body sections as opposed to acrylic, better connectors, pressure compensation of the thruster, etc). The as-built design won’t meet the goal, but will be designed such that if I want to go down the upgrade route in the future, I wouldn’t have to start from scratch.

Likewise, the 100 meters won’t be the calculated buckling limit, but rather will have a very healthy safety margin attached to it — Basically it could go deeper, but gets riskier with any material/manufacturing imperfections causing failure.

2-way communications range of 1km

Both surface (RF) and subsurface (acoustic). Being a comms guy, this will be fun. I’ll probably use a COTS radio for surface communications as only a basic line-of-site datalink is needed, but the acoustic modem should be a fun challenge due to the strong multi-pathing. I’m tentatively aiming for at least 9.6 kbps rate communications, but will scale up as the component selections (transducer bandwidth, mainly) allows.

Modular Design

This is perhaps a poorly written requirement, but the aim is to make the AUV modular so that payloads can be swapped in and out. Potential future ideas are a camera payload, sensor payloads, translational thruster modules, etc. Once the basic requirements are met, I can expand to my heart’s content.

From those requirements, many other lower level requirements will come out, e.g. autonomous operations, mission duration, sonar frequencies, etc, etc. Many will flow out of the high level design process as I research and keep designing.

Ultimately, this is going to be a project that demands a lot of my limited free time, so the project may never get completed as time and resources limit how much I can do. The biggest requirement I’ve levied on myself is that I learn from this project — For my hobbies, the fun is in the journey, not just the destination.

 

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