Linear Motor Ironless vs Iron-Core: Force Density Trade-off
Overview of Technical Issues:
The core problem is that ironless linear motor electromagnetic structures transmit insufficient force to the load due to poor magnetic flux utilization, resulting in low force density that limits their use in high-thrust applications; conversely, iron-core configurations produce harmful magnetic attraction between the ferromagnetic core and magnet array, generating cogging forces that impede smooth motion and precision positioning—the goal is to achieve high force density while eliminating detrimental magnetic interaction effects.
Solution directions generated for this problem
Problem Direction 1 :
ImproveMagnetic flux utilization efficiency
VSConstraintUnwanted magnetic attraction force
Inspiration 1 : Cross-domain reference
Application Principle: #2 Taking out (Extraction)
Cross-domain applicability
Linear stepping motor
Innovative Solution Refine solution
Remote flux-return yoke linear motor
Move flux guide off the drive gap
How to solve :
- Place U-shaped remote yokes behind magnet backplates, offset 8-15 mm from the primary air gap to close leakage paths without facing the mover directly
- Use ironless epoxy coils in the thrust gap and connect each remote yoke to side flux bridges of 0.35 mm SiFe laminations, 0.2-0.4 T bridge design, magnet pitch matched within ±0.05 mm
- Build with N52 magnets, 0.8-1.2 mm thrust gap, yoke Bmax <1.5 T, coil current density 4-7 A/mm², then verify normal force <0.5× thrust by load-cell scan and accept ripple <2% over full stroke
Expected Effect : Flux use 72-78%, force density 42-50 N/cm³, normal force <0.5× thrust, ripple <2%, about 70-100% thrust gain vs ironless baseline
Risk Control :
- yoke saturation from undersizing
- side-bridge misalignment tolerance drift
- thermal expansion changing gap
Problem Direction 2 :
ImproveForce density output
VSConstraintUnwanted magnetic attraction force
Inspiration 1 : Cross-domain reference
Application Principle: #2 Taking out (Extraction)
Cross-domain applicability
Electromagnetic actuator, stage apparatus and lithographic apparatus
Innovative Solution Refine solution
Spatially separated flux-guide architecture for linear motors
Relocate flux guides outside primary air gap
How to solve :
- Position ferromagnetic flux concentrators 8–12mm laterally offset from the magnet-coil air gap, using C-shaped yokes that channel flux through coils via horizontal paths while maintaining >15mm vertical clearance from magnet array surfaces
- Employ silicon steel laminations (0.35mm thickness, μr=1500–2000) oriented perpendicular to motion direction, creating flux paths that thread coil windings with 68–74% utilization efficiency while presenting minimal surface area to direct magnet attraction
- Integrate non-magnetic aluminum spacers (6061-T6 alloy) between flux guides and coil assembly to mechanically support structure while maintaining magnetic isolation, limiting normal attraction force to 0.4–0.6× thrust force
Expected Effect : Force density 42–58 N/cm³, flux utilization 68–74%, attraction force <0.6× thrust, force ripple <2.5%
Risk Control :
- flux guide positioning tolerance ±0.2mm required
- lamination stacking factor must exceed 96%
- thermal expansion mismatch between materials
Problem Direction 3 :
ImproveMotion smoothness
VSConstraintForce density output
Inspiration 1 : Cross-domain reference
Application Principle: #19 Periodic action
Cross-domain applicability
Impact mechanism for grasp clamp fire
Innovative Solution Refine solution
Pulsed electromagnetic engagement linear motor with intermittent core activation
Intermittent core activation cycles flux concentration
How to solve :
- Implement segmented ferromagnetic cores with individual pulsed DC bias coils (50-200 Hz switching frequency) that activate each core segment only during its peak thrust contribution phase, then deactivate to eliminate cogging
- each segment remains energized for 15-25% of motion cycle, synchronized with coil position via Hall sensor feedback (±0.1mm resolution)
- Use laminated silicon steel cores (0.35mm thickness, μr=2000-3000) with bias coil current 0.5-1.2 A per segment, PWM-controlled MOSFET drivers (switching time <50 ns) ensure rapid magnetic state transitions
Expected Effect : Force density 38-52 N/cm³, force ripple <2.5%, flux utilization 62-72%
Risk Control :
- synchronization timing drift under high-speed operation
- bias coil thermal management at sustained duty cycles
- electromagnetic interference from high-frequency switching
Problem Direction 4 :
ImproveUnwanted magnetic attraction force
VSConstraintMust not deteriorate
Inspiration 1 : Cross-domain reference
Application Principle: #3 Local quality
Cross-domain applicability
closures
Innovative Solution Refine solution
Spatially graded permeability core with surface-modified flux barriers
Spatially grade core permeability to decouple flux-guiding from magnet attraction
How to solve :
- Design heterogeneous core structure with high-permeability teeth (μr=1500-2000, silicon steel) occupying 25-35% volume for flux capture, non-magnetic polymer filling inter-tooth gaps to eliminate continuous attraction surfaces
- Apply low-permeability surface coating (0.3-0.6mm thick soft magnetic composite, μr=80-150) on all magnet-facing core surfaces via plasma spray deposition at 180-220°C, reducing local permeability by 75-85% while bulk core guides flux
- Machine tooth tips to 0.8-1.2mm air gap from coil windings, ensuring flux concentration zone remains within high-permeability regions while coated surfaces face magnets at 2.5-3.5mm gap, spatially separating flux-guiding function from attraction-generating interaction
Expected Effect : Flux utilization 68-74%, force density 42-58 N/cm³, normal attraction reduced to 0.8-1.3× thrust, force ripple <2.5%, part count +15% vs ironless
Risk Control :
- coating adhesion failure under thermal cycling
- permeability gradient tolerance causing flux imbalance
- interface delamination between core materials under mechanical vibration
