Ward says that energy capture begins when the shank of the leg is perpendicular to the foot, or just before. As the cord goes taut it extends the ball screw, which in turn compresses the spring. This action by the ball screw produces energy by spinning the motor/generator at a high rpm and at the same time stores energy in the spring. When the foot pushes off for another step, the cord goes slack, releasing the ball screw and decompressing the spring, which releases its energy into the motor/generator. The sequence repeats in the other leg.
The original version of SPaRK weighed 1.4 kg (3.08 lb), but Ward says the current version is 860 grams (30 oz), and believes it can be reduced to 430 grams.
Ward says that in emergencies more metabolically taxing motions would produce greater amounts of energy. Standing in place and doing knee bends could generate 26 watts, enough to power a radio or, possibly, get a networked soldier back online.
Such an action may seem difficult, but Ward says that for soldiers on a battlefield with no other means of activating critical electronic systems, it would be an important option.
As for the 85 minutes it takes to recharge two AA batteries, Ward says that with a comfortable and lightweight SPaRK system, walking that long wouldn’t be a problem on multi-day missions.
The power generated tends to come in bursts rather than an evenly regulated flow, due to the mechanics of walking. Ward and his team have built a capacitor circuit that filters these energy spikes into a usable form for recharging AA batteries. The next phase will be to improve the efficiency of the charging electronics.
The project is concluding its Phase 2 SBIR (small business innovation research) design stage. SpringActive is seeking additional funding for Phase 3. Depending on the funding award, Ward says, the company could within 12 months build several prototypes for field-testing by the Army.
“Soldier load is a huge issue,” Ward says. “As troops become more networked, power demands go up. There will have to be some technologies to meet this demand without increasing the number of batteries a soldier carries. Harvesting energy at the ankle is a great way to do this.”