Ironless winding: Improving the comfort and mobility of prosthetics

Matt Dean, medical sales engineer at DC drive system specialist maxon UK and Ireland, explores the possibilities of ironless winding and how it is driving innovation in the medical industry.

Powering everything from toys to car mirrors and windscreen wipers, iron core DC motors are chosen because they are cost effective to manufacture. However, for more demanding applications that require accurate positioning and control — for example, in prosthetics — they have their limitations.

Prosthetics are essential for improving the quality of life of people living with limb loss, and as motor technology has evolved, so has the functionality of the prosthetic. To maximise comfort, stability and mobility, prosthetists are taking advantage of the performance of the ironless winding.

What’s the purpose of ironless winding?

Manufacturing the ironless winding is more complex and costly, compared to iron core technology, so why bother? Well, for applications where the highest power density is needed, ironless motors are a cut above the rest.

An iron core rotor in a DC motor consists of copper windings wrapped around several layers of iron laminations. The laminations support the winding, making it simple and cost effective to manufacture, and act as a heat sink for the copper coil, protecting the rotor from thermal overload. While these are benefits, the iron core is much heavier, meaning the rotor has a higher inertia and the motor is less responsive. The laminations also introduce cogging, which will stop a prosthetic hand from carrying out precise movement and control.

As the name suggests, motors with ironless winding don’t have an iron core. The rotor mass and inertia are much lower for ironless winding brushed motors, allowing for lower mechanical time constants and more responsive motors. The motor has no cogging, which minimises excess vibrations, allows for precise movement and a longer service life. The reduction in mass is also crucial to applications like prosthetic hands, where space and weight is limited. There may be multiple motors present in a prosthetic hand powering different movements in the fingers and wrist. For comfort, these motors must be small and lightweight, and have the highest power density possible.

In modern myoelectric prosthetic hands, flexes in an amputee’s residual arm are picked up by electrodes in the prosthetic. These electrical signals tell the controller in the prosthetic what the arm muscles are instructing the hand/fingers to do. The controller powers the motors, allowing the user to complete the desired hand/finger movement.

Because of the low inertia offered by the ironless winding technology, these motors have a more dynamic response. The user can complete the desired hand movement with less delay and the functionality of the prosthetic is more efficient, mimicking the human total cognitive response time of 150-200 milliseconds. Precision is also fundamental to mobility, which is why each motor shaft in a prosthetic is fitted with an encoder to provide electronic feedback. This, coupled with the low inertia, allows fine finger control to be achieved.

Why not use ironless winding technology all the time?

Despite its clear advantages in performance, ironless winding technology does have some downsides. They are more complex and costly to manufacture, and because of the absence of the iron laminations, the coil has a shorter thermal time constant. However, challenges like these are easily overcome by working with a reliable custom drive specialist.