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Bench Talk for Design Engineers

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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics


Ensuring the Electrical Safety of Powered Exoskeletons Alex Misiti

Driven by increased demand in the medical, industrial, and defense sectors, the global market for powered exoskeletons is expected to reach $2.8 billion by 2023—up from $300 million in 2017.

From improving the mobility of patients with spinal cord injuries to helping factory and construction workers lift heavy objects to enhancing soldiers’ capabilities on the battlefield, the benefits of bringing together the world of robotics and kinesiology are becoming hard to ignore. However, as is the case with virtually any wearable technology or electrically powered device, exoskeletons present inherent risks that designers and engineers must address to ensure the health and well-being of their users.

Exoskeleton Design Challenges

Many of the safety risks that are inherent in electronic products become significantly more important when the device in use directly interfaces with the human body for an extended amount of time. In the case of wearables, like earpieces or watches, problems such as overheating or electrostatic discharge are a concern. However, because the user has the option of manually removing the device (typically within a matter of seconds), the likelihood of any serious injury is almost nonexistent.

This is not always the case with exoskeletons, as they are often securely attached to or fully encapsulate a portion of the body, like an arm or leg. In such cases, during a malfunction that leads to overheating or a short circuit, for example, the user may be unable to quickly detach the system, thereby increasing the chance of injury.

Additionally, because many exoskeletons utilize high-torque servo motors to provide sufficient force for movements, a large power source is vital. Lithium-ion (Li-ion) batteries are an option for small exoskeletons that don’t need to produce significant force; however, in many cases (especially in industrial applications) the exoskeleton must be connected to an external outlet and tethered. This connection inevitably exposes the user to high amperage and introduces the risk of electrical shock.

Electrical, Thermal, and Biocompatibility Testing

Electronic devices that interface with skin should not exceed an operating temperature of 37°C, which is the core temperature of the human body. Temperatures over that can cause discomfort to the wearer and in severe cases can cause burning. As a result, many exoskeletons must meet medical electronics standards, which include design requirements to prevent overheating. This is also the case with hazards such as electrostatic discharge and exposure to radiation.  

For many exoskeletons used in medical applications, the US Food and Drug Administration (FDA) Code of Federal Regulations (CFR) 21 addresses the risks and concerns regarding electrical shock, thermal burn, and biocompatibility. Part 890 of CFR 21 defines a powered lower-extremity exoskeleton as “a prescription device that is composed of an external, powered, motorized orthosis that is placed over a person’s paralyzed or weakened limbs for medical purposes.” The directive states the following regarding the testing and safety of these devices:

  • Elements of a device’s materials that may have direct contact with the patient must demonstrate exact biocompatibility.
  • Appropriate analysis/testing must validate the presence of the correct levels of electromagnetic compatibility/interference (EMC/EMI), electrical safety, thermal safety, mechanical safety, battery performance and safety, and wireless performance, if applicable.

Batteries

For exoskeletons that have their own power source, batteries are an area of concern that present one of the biggest safety risks to users. Thermal runaway is a problem that has been well documented relating to devices that use rechargeable Li-ion batteries. It occurs when an increase in temperature causes an exothermic reaction that results in a release of heat. This heat causes an increase in the rate of the exothermic reaction, which in turn releases more heat. Experiencing thermal runaway in the battery of an exoskeleton poses a serious risk.

In recent years, wearable system manufacturers have taken various steps to address safety risks associated with batteries. For instance, some systems utilize high-capacity disposable batteries as opposed to rechargeable Li-ions. Others are exploring advanced power management capabilities that monitor battery health and maximizes its use and life. A more recent development that has made its way into wearable devices with low-power consumption is the use of smart textiles, which harvest sunlight and gentle body movements to generate power internally.

Wire Connectors

Wire connectors are crucially important to the reliability and performance of wearable electronics; however, they have often been overlooked during the development and execution of a design because of their low cost and simplicity. This is particularly the case in exoskeletons, where more expensive integrated circuits and servo motors take precedent.

When examining failures in wearable electronic devices, examiners have discovered that one of the most commonly cited causes is a loss of contact between two conductors. Many times, this is the result of a failed wire connector. Exoskeletons often contain hundreds of connectors for components, including sensors, batteries, and circuit boards, among others. As all these represent a potential point of failure, the selection of proper connectors is critical.

The CP-3.3. Wire-to-Wire Connector System from Molex is an example of a product that is specifically designed for user safety in consumer electronics and industrial and medical wearable applications. The inertial lock on the receptacle housing helps ensure complete, low-insertion-force locking; minimizing the chance of failures and providing a tactile and audible click when mated. Additionally, fully polarized and color-coded plug and receptacle housings allow the use of multiple same-size-circuit connectors in a single application.

Conclusion: Safety First

In recent years, the market for powered exoskeletons in the medical and industrial sectors has grown immensely. While the benefits of these and other wearable electronic devices are becoming hard to ignore, designers and manufacturers must remain watchful of the potential safety risks they pose to users. While many innovative measures are being put in place to address the concerns associated with electronics that interface with the human body, the use of high-quality, high-reliability components, such as connectors and wires, has proved time and time again to be the most effective way to ensure overall success in product performance and reliability.



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