An exoskeleton is an external structure that supports a subject’s movements. Powered exoskeletons are wearable robots or portable devices attached to a subject’s limbs, using principles of electrosimulation to first detect a subject’s movements and then replace or enhance them. These are typically used after an accident, disease or periodic/permanent paralysis, across hospitals and rehabilitation centers that address the needs of elderly and handicapped persons, or even those afflicted with spinal cord injuries or strokes (Fig. 1).

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The motorized joints should be ideally integrated within the exoskeleton structure and provide the forces required in the exoskeleton’s operation. The load profiles can be quite demanding – with extreme peak overloads, like in the example from [1] presented in Fig. 2, thus, the motors should offer the capability to deliver higher peak torques, supporting the demanding movements and loads associated with exoskeleton joints. Ideal integration means the joints should be axially short, i.e. not sticking out too far on the left or right side of the body. The whole joint structure, including the gear, should guarantee the agility and mobility of the exoskeleton, allowing users to move more freely and confidently.

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FiberPrinted™ motors challenging the conventional motor technology

FiberPrinted™ motors demonstrate numerous advantages.

1. They are characterized by intrinsically thin stators, compared to conventional PM machines with iron cores (Fig. 3). This opens opportunity for the integration of gears, brakes, sensors, etc.

2. Benchmarking of the motors on peak torque (Table 1) shows that FiberPrinted motors provide extreme peak torque versus their conventional counterparts. This increased torque output enhances the overall performance and responsiveness of the exoskeleton, enabling smoother and more precise movements. Exoskeleton joints experiencing the short-term overloads like in Fig. 1 can fully exploit the extreme peak torque capability of Alva motors. Reduction in the joint weight coming from the unique torque-to-mass ratio of FiberPrinted motors contributes to the overall agility and mobility of the exoskeleton, allowing users to move more freely and comfortably.

3. Motor torque ripple can be a problem reducing the performance of an exoskeleton. Conventional motors can exhibit quite high torque ripple partially coming from cogging (Fig. 4). Cogging-free operation characteristic of Alva’s motors contributes significantly to the silent and smooth operation of the exoskeleton joints. The absence of cogging ensures minimal vibration and noise generation, providing a more comfortable and immersive experience for the user while reducing wear and tear on the exoskeleton components.

4. Motor constant (See also: The Value of high Km) of FiberPrinted motors is also usually higher than at competitors ensuring high efficiency, low surface temperatures and long battery powered operation.

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Integrated solutions

As mentioned above, FiberPrinted motors have larger in-hole diameters compared to conventional motors. This attribute allows for the seamless integration of larger gearboxes within the joint assembly, thereby enhancing the overall power transmission efficiency and enabling the exoskeleton to exert greater force without sacrificing agility or speed. One of the possible solutions with the thin FiberPrinted motor is presented in Fig. 5 also showing the alternative solution based on the conventional technology. Alva’s solution allows not only larger gear to be fit inside the motor but also better heat removal from the active parts since there are shorter paths for heat flow from the sources to the outer surface of the housing.

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Alva’s motor technology improves exoskeletons

Regardless of the method of integration, the advantages of FiberPrinted™ motors are clear. The combination of higher peak torques, larger in-hole diameters for accommodating larger gearboxes, cogging-free operation, low weight makes Alva’s motors as an optimal choice for exoskeleton applications. This comprehensive set of advantages ensures that exoskeletons equipped with Alva's slotless motors deliver superior performance, comfort, and user experience, making them ideal for a wide range of applications, from medical rehabilitation to industrial assistance and beyond. The role of the technology advantages enabling better exoskeletons are summarized graphically in Fig.6.

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References

1. N. Latif, A. Shaari, Ida S. Md Isa, Tan Chee Jun, Torque analysis of the lower limb exoskeleton robot design, ARPN Journal of Engineering and Applied Sciences, vol. 10, No. 19, October 2015, ISSN 1819-6608.

2. Gan Zhang, Qing Tong, Taixun Zhang, Jinxin Tao, Anjian Qiu, “Design of a high torque density robot joint and analysis of force control method applied for a light exoskeleton”, Electronics 2023, 12(2), 397; (Source).