How to Design Linear Motor for Underwater Propulsion

Overview of Technical Issues:

The core challenge is that the electromagnetic force generation structure must produce sufficient thrust to overcome high hydrodynamic drag forces in the underwater environment, which are significantly greater than in air applications, while the sealing structure provides insufficient protection against seawater ingress that causes harmful corrosion of magnetic components and electrical shorts; the goal is to design a linear motor system that delivers adequate propulsion force, maintains electromagnetic performance despite water exposure, and achieves reliable long-term operation in the corrosive underwater environment.

Solution directions generated for this problem

Problem Direction 1 :

ImproveElectromagnetic thrust force
VS
ConstraintEnergy consumption rate

Inspiration 1 : Cross-domain reference

Application Principle: #35 Parameter changes
Cross-domain applicability Assess applicability
Particle analysis in an acoustic cytometer
Innovative Solution Refine solution

Superconducting coil thermal cycling for zero-resistance thrust generation

Shift coils to superconducting state via cryogenic cooling
How to solve :
  • Replace copper coils with YBCO high-temperature superconductor tape (critical temp 92K) cooled by liquid nitrogen circulation jacket
  • achieve zero electrical resistance below 77K, eliminating I²R losses entirely while carrying 200-400 A/mm² current density for 60% flux density increase
  • Install closed-loop cryocooler (Stirling or pulse-tube type, 80W cooling at 77K) with vacuum-insulated dewar housing the motor stator
  • thermal load budget ≤50W includes conduction via current leads (copper braid, <10W), radiation (multi-layer insulation, <5W), and seawater convection through housing (ceramic thermal break, <35W)
  • Operate coils in persistent current mode: charge superconducting loop to target field via external supply, then disconnect and short-circuit via superconducting switch
  • magnetic field self-sustains indefinitely with zero input power during propulsion, only cryocooler draws 150W vs 800W for resistive coils at equivalent thrust
Expected Effect : Thrust +65%, net energy -72%, 4yr maintenance-free operation
Risk Control :
  • quench risk if temperature exceeds 92K
  • cryocooler reliability in seawater vibration
  • thermal cycling fatigue of solder joints

Problem Direction 2 :

ImproveMagnetic flux density
VS
ConstraintEnergy consumption rate

Inspiration 1 : Cross-domain reference

Application Principle: #28 Mechanics substitution
Cross-domain applicability Assess applicability
Electronic vapour inhaler including a control arrangement that recognizes an inserted cartridge or capsule
Innovative Solution Refine solution

Hybrid permanent-magnet linear motor with field-modulation coils for underwater propulsion

Replace pure electromagnetic excitation with hybrid drive combining permanent magnets and modulation coils
How to solve :
  • Install high-grade NdFeB permanent magnets (N52, Br≥1.45T) in stator slots to provide 70-80% of baseline flux density, eliminating continuous DC current for field maintenance
  • Add compact field-modulation coils (copper wire AWG18, 200-turn per pole) energized only during thrust strokes to dynamically boost flux density by 15-25% in the working gap, achieving target 40-60% total increase
  • Integrate Hall-effect flux sensors (±0.5% accuracy) with PWM controller (switching frequency 20kHz) to activate modulation coils only when thrust demand exceeds 80% peak, reducing coil energization duty cycle to 10-30%
Expected Effect : Flux density +50%, energy consumption -65%, thrust-to-power ratio +180%
Risk Control :
  • permanent magnet demagnetization in seawater temperature fluctuations
  • PWM switching noise inducing electromagnetic interference
  • Hall sensor drift under long-term corrosion exposure

Problem Direction 3 :

ImproveSeal corrosion resistance
VS
ConstraintManufacturing complexity

Inspiration 1 : Cross-domain reference

Application Principle: #1 Segmentation
Cross-domain applicability Assess applicability
Contact plate including at least one bonding connector configured to establish electrical bonds to terminals of at least one group of battery cells in a battery module
Innovative Solution Refine solution

Modular triple-stage fluoroelastomer seal with independent replacement zones

Divide seal into three independent stages for simple assembly and long life
How to solve :
  • Divide seal structure into three independent O-ring stages (primary, secondary, tertiary) using standard FKM-90 fluoroelastomer
  • each stage is a simple compression-molded part requiring no specialized fabrication or multi-layer bonding
  • Design each O-ring with standard AS568 dimensions (e.g., -240 to -260 series, 3.5mm cross-section) installed in separate grooves spaced 8-12mm apart
  • compression ratio 15-20%, ensuring independent sealing paths that block seawater ingress even if one stage degrades
  • Implement tool-free snap-fit housing allowing individual stage replacement in <5 minutes
  • quality control via durometer testing (Shore A 85±5), visual inspection for surface defects (≤0.1mm), and pressure decay testing (≤2% pressure drop over 10 minutes at 0.5 MPa) per stage before assembly
Expected Effect : Corrosion life 3+ years, cost +30% vs complex seals, assembly time -60%
Risk Control :
  • O-ring compression tolerance deviation beyond 15-20% range
  • groove machining precision insufficient causing misalignment
  • fluoroelastomer batch-to-batch hardness variation
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