AirPods are my favorite consumer electronic product in recent years. This little device has had an unexpectedly significant improvement on my daily life, although through some tiny aspects of life. From the product design perspective, I think AirPods best demonstrate that right designs in key aspects of experience and incremental progresses in technology together also make a revolutionary product, especially when we are still on the way to a revolutionary breakthrough in major technologies.

Apple’s introduction of the battery case, as an essential part of an earphone wireless audio system, avoids the bottleneck in the battery technology and provides a convenient container for storing earplugs and space for functional components. Needless to mention the electronic part which allows a much easier connecting and switching experience via bluetooth, only the structural design of AirPods and their case are fascinating enough. So I wonder if I can do some simulation about AirPods to get to know more about the product (yeah, oddly, simulation has become a way for me to bound with things I’m interested).

Drop and shock tests are a common type of analysis on consumer electronics, which help the structural strength and reliability design of products. I decided to simulate the drop test of AirPods. Structure’s response to drop and shock loads is critically dependent on the structure’s stiffness and material properties. As an outside observer, lacking an accurate CAD model with internal structures and knowledge of component materials, a simulation of this kind cannot be physically meaningful. So my first motive is to learn the workflow of drop test simulation of consumer electronics, and along the way to practice skills in 3D modeling, preparing FEA model from CAD model, and modeling the mechanical details such as hinge connection and magnets. Making the model as physically accurate as possible is my second motive.

Planning

Although this type of simulations is not very difficult, it still involves 3D modeling, geometric model trimming and meshing, and etc.. I plan to accomplish the project through several iterations:

• iteration 1: drop simulation of AirPods case shell
  • build a shell model of AirPods case which captures external geometric feature as accurate as possible, the model does not consider internal structure at this stage
  • mesh the geometric model with shell elements to generate the first FEM model, use elastic material properties for the shell
  • model the hinge connection between case cover and body
  • model the magnetic force which holds the cover in close position
  • do the first drop simulation to have visual impression
• iteration 2: drop simulation of AirPods case
  • refer to IFIXIT teardown and add essential structure internal features
  • mesh the model with shell and solid elements
  • try to estimate and model mass / inertial properties as accurately as possible
  • do the second drop simulation
• iteration 3: drop simulation of AirPods in the case
  • model two AirPods
  • model the magnetic force between AirPods and case
  • do the third drop simulation with AirPods in the case
• iteration 4: figure out some basic ways to validate the simulation results

This note will document outcome of each iteration.

Iteration 1

FEM model of AirPods case

3D geometry modeling and meshing were conducted in Hypermesh. The meshed parts were then imported to Abaqus. The drop simulation were computed by Explicit dynamic analysis in Abaqus.

The hinge connection between the case cover and body were modeled by hinge connector in Abaqus. The case cover can open at a maximum angle of around 115° when the rectangular pin touches the case body and stopes the rotation. In Abaqus this was modeled by adding a stop position to the connector section definition.

Apple has been clever in using magnets. In AirPods, magnets are used as the switch between case cover and body and to pull AirPods staying in the chamber. The magnetic force has a similar form of gravitational force, in which the magnitude of the force is inversely proportional to the square of distance between two magnets. Abaqus does not have a built-in model for this kind of behavior. I thought about using a nonlinear spring. But the built-in nonlinear spring requires a positive spring stiffness. The stiffness of the magnetic force is negative. As a first trail, I used contact property that does not allow separation after contact first builds up to approximately model the behavior.

Below is the mesh of the exterior of AirPods case.

Results

The below animation shows the result of the first iteration drop simulation in which the shell case of AirPods drops to a rigid floor with an initial impact velocity of 0.44 m/s. The results shows that the case cover can rotate in the right direction and snap to close and maintain close afterwards. The high intensity of stress after the cover closes in the animation is where the magnets lie in the case cover and body. Therefore, the hinge connection and the magnetic force between cover and body are successfully modeled. The bouncing behavior is unrealistic due to the lack of internal structures and just for having a visual impression.