In order to improve control, the Plié microprocessor-controlled knee determines the degree of dampening based on the angle of the leg to the ground.
A lifeguard sees a child fall into a swimming pool. First, there is the visual cue that is delivered to the brain. The brain processes the cue, and, in turn, sends a message to the legs, which respond by springing into action. Something similar occurs when a person with a prosthetic knee takes a step. Biosensors represent the eyes. They send a cue to a microprocessor (the brain), which in turn sends a message to a motion controller (the knee joint).
“The problem is speed,” says Charles Bisbee III, vice president of smart technology at Freedom Innovations (Irvine, CA; www.freedom-innovations.com). “For the patient to feel secure and in control, the duration from when he decides to move to when his leg actually moves has to be very short.”
The Plié microprocessor-controlled knee was developed by Freedom Innovations to improve response time. Processor speeds have been steadily climbing over the years, but fast processors have diminishing returns for prosthetic knees. “It doesn’t matter how fast your processor is if you don’t have actuators and valves that can keep up,” Bisbee says.
In designing the Plié to maximize motional control response time, the company turned to Turner Technology Group (Victorville, CA; www.fundamentalindustries.com). The company owned proprietary hydraulic dampening technology originally developed for shock absorbers on land vehicles. “The technology allows for automatic manipulation of resistance based not on the speed, but on the angle of the shaft,” says Roy Turner, president.
In vehicles, the technology functions to resolve two opposing goals: good handling and braking for driver safety, and isolation of the passengers from as many bumps and vibrations as possible. The technology works similarly in the Plié knee. As with automobiles, safety is paramount, and a critical function of a prosthetic knee is to prevent stumbles and falls. To achieve this, most prosthetic knees generate an unnatural amount of resistance with the ground, leading to a somewhat stilted gait. The Plié, however, determines its resistance based on the angle of the leg (the shaft) to the ground, providing levels of balance and resistance closer to that of a human knee.
“It monitors and softens transitions between steps predictably and reliably,” Bisbee says. “This improves control and allows decisions about speed to be made midstep.”
Needing a valve suitable to work in tandem with the dampening technology, Freedom Innovations tapped Turbo Jet Products (Rosemead, CA; www.turbojetproducts.com), a firm used to supplying high-speed valves for similar shock applications from its work in the aviation industry. The company prototyped and manufactured a solenoid valve specifically for the project. “[The valve] had to be both leak-free and provide the cycle life necessary for this type of device,” Bisbee says.