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The D-Ankle ankle joint prosthesis from the Polish orthopedic manufacturer Design Pro Technology allows biomechanically correct movements with every step and on any terrain. The artificial foot is actively flexed and extended. An intelligent control finds the appropriate rhythm, driven by a small yet powerful DC servo motor.
Modern prosthetics feature joints, control algorithms, and spring elements made from high-tech materials. With their help, the gait often comes quite close to the natural one. Some are even designed for peak performances: below-knee amputee athletes with carbon prosthetics achieve excellent sprint times. However, such sport prosthetics are designed for fast running, while standing still and performing normal activities can be difficult or even impossible with them. Ankle joint prosthetics for everyday use are therefore constructed entirely differently and more closely resemble natural anatomy: they consist of a lower leg and a foot component connected by a joint. The passive artificial ankle joint keeps the prosthetic in a predictable position but offers only a very limited range of motion during movement.
Support in the swing and stance phases
The key factor is the so-called dorsiflexion, which is the bending of the foot towards the shin during the swing phase. "This increases the distance between the toes and the ground, reducing the risk of tripping," explains Marcin Dziemianowicz. "With a passive prosthetic, the wearer makes a circular movement with the hip or lifts the leg higher to achieve this. These compensatory movements are unnecessary with our prosthetic; walking becomes more natural and less strenuous."
When the prosthetic foot is placed down, the angle changes for the support phase as well; the joint is extended by motor power (plantarflexion). This also contributes to a harmonious gait and saves energy. Only the lateral movements allowed by a natural ankle joint cannot be performed by the artificial one. Instead, they are allowed as passive deformation through the elastic carbon fiber material of the prosthetic foot. This way, the foot has full sole contact even on uneven ground.
Always the optimal foot position
To make this work, the control system integrated into the prosthetic processes signals from multiple sensors to distinguish the different phases of a step cycle. A potentiometer measures the angle between the foot and the lower leg; a bilateral pressure sensor detects the load when the foot strikes as well as the relief in the transfer phase. An accelerometer records the overall movement, including speed, foot inclination, and path slope. "An algorithm combines and evaluates the signals from the most recent steps," Marcin Dziemianowicz explains the functionality. "From this data, the gait rhythm and the optimal position of the foot for each step phase are derived."
For example, when walking uphill, the ankle joint is angled more than on level ground, and the push-off force is increased to make climbing easier. When walking downhill, it is the opposite to achieve the best possible contact between the sole and the ground. Additionally, parameters such as push-off force, pressure sensor sensitivity, or the length of a step cycle phase can be individually adjusted with a smartphone app.
The driving force: DC servo motor with high power density
An integrated electric drive converts the control signals into the corresponding movement. Its core component is a brushless motor from the BP4 series by Faulhaber, which transmits its power to a spindle. The motor and spindle rotate in both directions, achieving the active dorsiflexion and plantarflexion of the foot. The high energy efficiency of the drive allows for an operating time of 12 hours on a single battery charge. The motor also tolerates the considerable heat development that can occur during everyday operation.
"Our requirements were quite ambitious," recalls Marcin Dziemianowicz. "The motor needed to handle a jogging motion, completing the full cycle with dorsiflexion and plantarflexion three times per second. Additionally, quick speed and direction changes should be possible. The application requires very high speed and torque while keeping volume and weight as low as possible." And here, the BP4 motor excelled. The small DC servo motor weighs less than half of what conventional motors with comparable performance do. The reason for the high power density, and also the heart of the motor, is the coil's segment winding.
High copper content increases performance
Individually wound segments are interlocked in an overlapping manner. This allows a particularly large amount of copper to be housed in the coil. The high copper content increases the motor's performance. Additional desired side effects of this coil structure are high winding symmetry with minimal current losses and a correspondingly high efficiency. "We tested various drive solutions from leading motor manufacturers. With Faulhaber, we found not only the most suitable product but also competent technical support," adds Dziemianowicz.
After extensive and successful trials with numerous test subjects, the foot prosthetic was launched on the market at the end of 2023. It can be attached to any modular prosthetic shaft with its standard adapter and is individually fitted by the orthopedic technician. The heel height can be adjusted, allowing the D-Ankle to be worn in a heeled shoe for women. If the battery charge doesn't last after a very long day, the user can still walk as if on a passive prosthetic. "Following the positive experiences with the compact DC motor and the good collaboration with Faulhaber, we have some ideas to use this or similar drive solutions in other prosthetics," concludes Marcin Dziemianowicz.
