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Prosthetic limbs have historically been fixed appendages that offer wearers limited or no movement. Although today’s prosthetics are made from high-tech materials, these passive prostheses, like their wooden predecessors, still do not allow users to walk with a normal gait.
However, new, microprocessor-controlled prosthetic ankles and knees take advantage of miniature DC motor technology to give users the opportunity to walk in a way that feels very similar to natural movement. These active prosthetic devices provide amputees with increased mobility, safety and comfort.
ACTIVE SYSTEMS ENABLE STABLE, EFFICIENT LOCOMOTION
Tendons and muscles are the body’s natural damping system. During each step a person takes, they adapt limb movement for proper motion. This is completed quickly and automatically by the body, based on the person’s weight, balance and the type of movement being performed, such as running or climbing. Passive prosthetic systems are unable to reproduce the same natural feeling, so amputees tire quickly when walking and struggle to keep a normal daily routine.
Active systems offer a better option. Equipped with an intelligent microprocessor, active prostheses offer advanced technology features such as enhanced stability performance and situational awareness. Such smart prosthetic knees or ankles allow walkers to navigate many different and even difficult terrains with stability and efficiency, no matter the speed. Active systems can also incorporate inductive charging technology to eliminate the need for cords, and their durable construction enables reliable operation for as much as 5,000 hours or two years. The result: greater convenience and ease-of-use for the wearer.
TYPICAL ACTIVE PROSTHESIS SYSTEM DESIGNS
Active prostheses are much more complex than passive devices. In addition to the microprocessor, they typically incorporate both mechanical and electrical components — particularly a motor, damping system and battery — coupled with the limb interface to achieve a more natural gait.
The microprocessor receives live data about the movement phase from high-resolution angle and pressure sensors. Using this information, it adapts the hydraulic damping mechanism, which employs a high-performance miniature DC motor to mechanically adjust damping spring forces in real time. Active prosthetic knee and ankle designs require a miniature motion system that can deliver:
| • | Small size and light weight for end user comfort and more balanced movement |
| • | Low inertia for quick response |
| • | A feedback sensor |
| • | Low noise operation |
| • | Resistance to shock and vibrations |
| • | An operating lifetime of approximately 5,000 hours (or two years) |
Despite their complexity, prosthetic legs and feet also have to be economical so as many amputees as possible can use them. To satisfy all these needs, active prosthetic devices typically incorporate a motion system that includes a DC motor, gearbox and encoder.
CORELESS BRUSHED DC MOTORS DELIVER HIGH PERFORMANCE IN A SMALL SIZE
Although an iron core brushed DC motor can be an economical choice for this application, coreless brushed DC technology with precious metal commutation can provide the same performance in a smaller size, lightweight unit.
The benefits of brushed DC motors for active prostheses include:
| • | Smaller size. A small, lightweight motor makes the prosthetic limb more comfortable for the wearer. Coreless brushed DC motors are available in sizes up to 30 percent smaller than their iron core counterparts. |
| • | Smooth rotation. Without the cogging effect typical of iron core motors, coreless brushed DC motors operate smoothly and quietly while offering the user greater control. |
| • | Easy integration. Coreless brushed DC motors can be combined with a gearbox and encoder, creating a smaller assembly that can easily fit into the required space. |
| • | Higher system efficiency. Users of active prosthetics do not want to recharge their devices during the course of their day. Thanks to their low current draw and no iron losses, coreless brushed DC motors last a full day on a battery charge to avoid this nuisance. |
| • | Rugged construction. These motors allow active prosthetic limbs to withstand shock and vibration during everyday use. |
Active prosthetic system designers can leverage these benefits with an assembly consisting of Portescap’s cost-effective 13N coreless brushed DC motor combined with its R13 gearbox and magnetic encoder. The 13N motor can deliver up to 3.3 mNm continuous torque, and the gearbox provides peak torque output up to 0.5 Nm. The coreless motor technology, together with the planetary gearbox, provides exceptional power density for its miniature 13-millimeters diameter size and high efficiency of 75 percent to extend the charge of the battery for longer operation.
It’s also durable. The 13N miniature DC motor features a machined metal tube body, a strong commutation system for an extended operating life and a full metal gearbox that is more durable than plastic options. The integrated magnetic encoder provides greater resistance to harsh conditions than an optical version. In addition, this assembly can operate over a wide temperature range of -15 to +85°C and can endure the shock and vibration that occurs with daily use. The highperformance bearings and smooth operation at low speeds significantly reduce noise, making this assembly an ideal choice for advanced prosthetics.
When selecting a motor, gearbox and encoder combination for your prosthetic limb application, be sure to work closely with your miniature motor manufacturer. Not only can they suggest the best motor to suit your torque requirements, but they can also tailor your motor to your needs with custom lead wires, output shaft diameters and lengths, shaft attachments, modified coils and special housings.
SOLVING MOBILITY CHALLENGES FOR AMPUTEES
Portescap motors and motion products deliver an ideal combination of small size, low noise and weight, high power density, efficiency, reliability and cost that allows prosthetic manufacturers to develop innovative microprocessorenabled knees and ankles. The benefits of a motion device based on coreless brushed DC motor technology, combined with an active prosthetic’s simple and safe induction charging and rugged construction, alleviate the many challenges amputees face with traditional prosthetics and help deliver better quality of life.
Figure 2. Portescap’s 13N coreless brushed DC motor (left) in combination with the R13 planetary gearbox (right)
provides high power density and efficiency to extend battery operation for active prosthetic limbs.
Figure 3. A coreless brushed DC motor, gearbox and magnetic
encoder create a durable and compact assembly that can easily
fit into the prosthetic.