Gyroscopic Electricity-Generating Tennis Ball
A kinetic energy harvester built inside a regulation tennis ball. A two-axis gimbal holds an inertial mass near-stationary while the shell rotates around it; magnets on the gimbal pass fixed coils on the shell wall, generating current by induction.
Run the model, then drag the ball. The gimbal holds its orientation in world space while the shell turns around it — that is the whole mechanism.
- Gyroscopic harvester — key dimensions
- Ø68.6 — Regulation outside diameter
- Ø52.2 — Cavity — the constraint
- 1.0 mm — Airgap · N42 magnets
- ±35° — Two-axis gimbal
Evidence
- Ø68.6Outside dia.regulation
- Ø52.2Cavitythe constraint
- 1.0 mmAirgap
- ±35°Gimbal traveltwo axes
- 4 × N42Arc magnets
- 3 mmCoM offsetbelow pivot
Case study
The problem
- A tennis ball absorbs a great deal of energy per impact and returns none of it.
- A regulation ball is 68.6 mm across. Once felt, compliant layer and structural shell are accounted for, the cavity is 52.2 mm — and everything has to live inside that.
The design
- Two-axis gimbal holding an inertial mass near-stationary while the shell rotates around it.
- Four N42 arc magnets on the gimbal passing four radial coils fixed to the shell wall, with a 1.0 mm airgap held across the magnetic circuit.
- Mass centre offset 3 mm below the pivot so gravity supplies the restoring torque and the assembly has a preferred attitude.
- Lithium cell and charge circuit bonded to the structural shell opposite the socket.
What I did
- Whole thing — concept, packaging study, magnetic circuit, gimbal, prototype.
- Worked the packaging backwards from the cavity rather than forwards from the generator, which is what made it fit at all.
How it was tested
- Impact testing on the prototype.
- Output computed from the modelled magnetic circuit and compared against what the prototype produced.
Result
- Measurable output during impact testing.
- Same operating principle as Uncharted Power’s SOCCKET football, implemented independently at roughly an order of magnitude less internal volume.
The hardware
- On court
Krishang Raghuvanshi, playing with the prototype and giving it its tagline — Ecocity: Play, Produce, Prosper. A harvester built into a ball has to survive being hit properly, which is the one test a bench cannot run. A clip of a tennis player on a clay court at sunset hitting with the prototype ball, then turning to the camera, holding the ball up and saying the product tagline.
I have played tennis since I was seven. You spend that long hitting something and eventually you wonder where all of it goes — a ball takes a great deal of energy per impact and gives none of it back.
The hard part is packaging. A regulation ball is 68.6 mm across; once felt, compliant layer and structural shell are accounted for, the cavity is 52.2 mm. A two-axis gimbal, four coils, four arc magnets, a cell and a charge circuit live inside that, with a 1.0 mm airgap held across the magnetic circuit. Every other decision follows from the cavity.
Same operating principle as Uncharted Power’s SOCCKET football, implemented independently at tennis-ball scale — which is roughly an order of magnitude less room to work in.
It has a name and a line: Ecocity — Play, Produce, Prosper. The prototype produced measurable output during impact testing, and it was put in front of a player who hits harder than any bench rig does.
Decisions
- 01
Why a gimbal rather than a free-floating mass?
A loose mass in a cavity converts impact into noise and a dent. Two constrained axes give you a defined relative velocity between magnet and coil, which is the only thing induction pays you for.
- 02
Why 1.0 mm of airgap?
Field strength falls away fast with distance, so you want the gap as small as it can be. Below a millimetre, the tolerance stack of a moulded shell, a gimbal bearing and a magnet on an arc stops guaranteeing the parts miss each other.
- 03
Why is the mass offset −3 mm?
Gravity has to supply the restoring torque, and a mass whose centre sits on the pivot has no preferred orientation. Three millimetres low turns the assembly into a pendulum that returns to a known attitude.