Wireless Charging Pad Mechanical Design and Ground Clearance Study

The problem

Wireless charging pads sit on or in the ground and must survive cars driving over them, water, dust and heat, while keeping the coil gap to the vehicle pad within the range the electronics need. Ferrite is brittle, so the housing must spread tyre loads. Ground clearance varies between vehicles and with load, which affects efficiency and alignment. These mechanical questions are a strong M.Tech topic.

Objectives

• Define pad dimensions, coil and ferrite layout and target air-gap range for an example system.
• Design a low-profile drive-over enclosure with load-spreading structure and drainage.
• Analyse tyre crush loads on the cover and ferrite support.
• Analyse heat from coil and ferrite losses with a thermal study.
• Study how ground clearance and vehicle loading change the coil gap and alignment tolerance.

What you need

Key concepts

Air gap

The vertical distance between ground and vehicle coils. Coupling falls as the gap grows, so the system is designed for a range. Standards such as SAE J2954 define classes, which should be checked in the current edition.

Tyre contact patch

The area where the tyre meets the ground. Wheel load divided by patch area gives an approximate contact pressure on the pad.

Brittle failure

Ferrite cracks without yielding, so stress must be kept well below its strength, and the cover must spread load so ferrite is not bent.

Non-metallic cover

The cover above the coil must be non-conductive and non-magnetic, such as a glass-fibre reinforced plastic, so it does not heat or block the field.

Step-by-step guide

1. 1

   Define the system

   Set up an example pad of about 600 × 500 mm and under 70 mm tall, containing a coil, ferrite tiles, an aluminium shield plate and an electronics compartment. Take an example air-gap range and coil size, noting that the electromagnetic design comes from the electronics team. Define loads: a car wheel load from an example 2,000 kg car with a safety factor, and environmental needs such as water, dust and road salt.

   Checkpoint: A requirement table with pad size, layers, air-gap range, wheel load and environment.

2. 2

   Model the layer stack

   Model the pad as layers: a base plate with mounting points, the shield plate, ferrite tiles in a grid with small gaps, the coil former with a spiral groove, and a reinforced plastic cover. Add a perimeter seal and a sloped edge ramp so tyres roll on smoothly. Use global variables for layer thicknesses so the stack can be varied in a design table.

   Checkpoint: A parametric layer stack that rebuilds with changes in each thickness, with total height within the limit.

3. 3

   Design load paths

   Add support ribs or pillars between the cover and base that pass through gaps between ferrite tiles, so tyre loads go to the ground rather than through the ferrite. Use a compliant pad layer between cover and ferrite to avoid point contact. Make sure no metal parts sit inside the active field area above the shield. Check the rib positions against the coil layout so the ribs never cross the coil windings or sit over the strongest part of the field.

   Checkpoint: A load path sketch showing tyre load going through ribs to the base, with ferrite not in the direct load path.

4. 4

   Analyse tyre crush

   Run a static study with the tyre load applied as pressure on a contact patch, sized from wheel load and tyre pressure, at the worst location such as mid-span between ribs. Use contact between cover, ribs and ferrite. Refine the mesh near the patch and ferrite tile corners, checking convergence. Look at cover deflection, rib stress and ferrite stress. Run a second case with the tyre at the pad edge on the ramp, since edge loading often produces the highest cover bending.

   Checkpoint: Ferrite stress well below its strength with a large margin, cover deflection within a stated limit and converged peak stress.

5. 5

   Iterate the structure

   Use configurations to compare rib spacing, cover thickness and the compliant layer. Record mass, height, peak stresses and deflection for each. If ferrite stress is too high, reduce rib spacing or add a stiffer cover. Choose a design that meets all limits with the lowest height, since height reduces the available air gap. Also record the estimated part cost drivers, such as cover thickness and rib count, because they influence the final choice as well.

   Checkpoint: A comparison table across variants and a chosen design meeting all limits at minimum height.

6. 6

   Run a thermal study

   Apply example coil and ferrite losses as heat power on those bodies, set convection on the top and sides to ambient air, and conduction to the ground through the base. Model a hot case with solar load and a high ambient. Include contact resistance between layers. Check coil and ferrite temperatures against material limits and find the main thermal bottleneck.

   Checkpoint: Coil and ferrite temperatures below limits in the hot case, with the dominant thermal resistance identified.

7. 7

   Design sealing and drainage

   Design a perimeter seal between cover and base and a sealed cable exit. Make the top surface slightly domed or ribbed so water runs off and does not pond. For an in-ground version, add a drained pocket. Check every opening in a sealing map, and state the target IP rating as a test requirement. Model the seal compression in a section view and confirm it stays within the supplier range at both extremes of the stack tolerance.

   Checkpoint: A sealing map with every opening addressed and a drainage path shown.

8. 8

   Study ground clearance and gap

   In Excel, compute the coil gap for several example vehicles: ground clearance unladen, with full load and suspension compression, and with tyre deflection. Include pad height and vehicle pad position. Plot the gap range against the electronics team's allowed range. Add a lateral misalignment estimate from parking accuracy and see whether a guidance aid is needed. Include the effect of tyre pressure changes and a passenger load in the rear, since both shift the vehicle pad height.

   Checkpoint: A chart showing which example vehicles and load cases fall within the allowed gap, and a recommended pad height.

9. 9

   Check installation

   Design surface-mount anchors and an in-ground frame option. Check that the pad can be removed for service, that cable routes to the wall unit are protected, and that edges do not present a trip hazard. Run a quick static check of anchors under braking loads from a wheel on the pad. Model a simple tyre envelope rolling across the pad edge to confirm the ramp angle does not catch a low-profile tyre or a bicycle wheel.

   Checkpoint: Anchor check passes and the pad can be removed with stated tools.

10. 10

    Document and report

    Produce drawings of each layer and the assembly, with materials and seal details. Write a report with requirements, structural and thermal results, mesh convergence, ground clearance study, installation and limitations. State that the electromagnetic performance and real crush behaviour need testing, for example a compression test on a ferrite and cover sample. Include a summary table linking each requirement to the analysis or check that shows it is met, and to any test still needed.

    Checkpoint: A documented design and report with clear validation requirements.

Testing & validation

• Crush study converges and ferrite stress has a large margin to strength.
• Contact pressure from wheel load and patch area is checked by hand.
• Thermal hot case keeps materials below limits.
• Ground clearance study covers laden and unladen cases.
• Physical compression testing is planned or carried out.

Expected results

A low-profile wireless charging pad mechanical design with a load path that protects ferrite, converged crush and thermal results, a sealing and drainage scheme and a ground clearance and air-gap study for example vehicles.

Take it further

• Run a fatigue study for repeated drive-over cycles.
• Model the vehicle-side pad mounting under the floor.
• Test a printed and composite cover sample under compression.

Viva and review questions

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