Mobility without bounds

Design & Automation Update - Cover Story | Design and Automation Update

DC motor technology in active prosthetic devices provides amputees with increased mobility, safety, and comfort.


Prosthetic limbs historically have been fixed appendages offering wearers limited or no movement. Although today’s prosthetics are made from high-tech materials, these passive prostheses, like their wooden predecessors, still don’t allow users to walk with a normal gait. However, new, microprocessor-controlled prosthetic ankles and knees take advantage of miniature DC motor technology to let users walk in a way that feels very similar to natural movement.

Stable, efficient locomotion

Tendons and muscles are the body’s natural damping system. They adapt limb movement for proper motion during each step a person takes. The body quickly and automatically moves, based on the person’s weight, balance, and type of movement, 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 enhanced stability performance and situational awareness. Such smart prosthetic knees or ankles allow walkers to navigate different and difficult terrain with stability and efficiency. Active systems can also incorporate inductive charging technology, eliminating the need for cords and enabling reliable operation up to 5,000 hours or two years.

Complex designs

In addition to the microprocessor, active prostheses 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 from high-resolution angle and pressure sensors. Using this information, it adapts the hydraulic damping mechanism and its high-performance miniature DC motor to mechanically adjust damping spring forces in real time. Active prosthetic knee and ankle designs also require a miniature motion system that delivers:

• Small size, light weight

• Low inertia, quick response

• Feedback sensor

• Low noise operation

• Resistance to shock, vibrations

Despite their complexity, prosthetic legs and feet also must be economical to allow as many amputees as possible to use them. To satisfy all these needs, active prosthetic devices typically incorporate a motion system that includes a DC motor, gearbox, and encoder.

Portescap’s 13N coreless brushed DC motor (above) in combination with R13 planetary gearbox motor provides high power density and efficiency to extend battery operation for active prosthetic limbs.
Portescap’s 13N coreless brushed DC motor.
A coreless brushed DC motor, gearbox and magnetic encoder create a durable and compact assembly that can easily fit into the prosthetic.

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 include:

Smaller size, lighter weight. Coreless brushed DC motors are available in sizes up to 30% 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 with greater control.

Easy integration. Coreless brushed DC motors can be combined with a gearbox and encoder, creating a smaller assembly to easily fit into the required space.

Higher system efficiency. With low current draw and no iron losses, coreless brushed DC motors last a full day on a battery charge.

Rugged construction. Active prosthetic limbs can withstand shock and vibration during everyday use.

Active prosthetic system designers can leverage these benefits with an assembly consisting of Portescap’s 13N coreless brushed DC motor combined with its R13 gearbox and magnetic encoder. The 13N motor delivers up to 3.3mNm continuous torque, and the gearbox provides peak torque output up to 0.5Nm. The coreless motor technology, with the planetary gearbox, provides exceptional power density for its miniature 13mm diameter size and high efficiency of 75% 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’s 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 from -15°C to 85°C and endure the shock and vibration occurring with daily use. The high-performance bearings and smooth operation at low speeds significantly reduce noise.

When selecting a motor, gearbox, and encoder combination for your prosthetic limb application, work closely with a miniature motor manufacturer. Not only can they suggest the best motor to suit your torque requirements, they can also tailor a motor to your needs with custom lead wires, output shaft diameters and lengths, shaft attachments, modified coils, and special housings.