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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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