With the “Autonomyo” exoskeleton, this dream can now be turned into a reality. An active walker supports weak muscles and allows an intuitive sequence of movements that mimic a natural sequence. Additional power is provided by six micro motors. To facilitate harmonious interaction between the external structure and its user, Fulhab Developed an innovative all-in-one engine with torque sensor.
Medical science distinguishes between more than 800 different neuromuscular disorders. As the name implies, they affect both nerves and muscles. Some of them affect the whole body, while others affect it only in certain areas. Fortunately, however, the majority of these disorders are relatively rare. Many affected patients have severe limitation of movement. This is because although these disorders have many different causes and develop in many different ways, they all have one thing in common: muscle weakness (muscular dystrophy), which in many cases is progressive.
“If weakness occurs in the muscles of the legs, walking becomes increasingly difficult, and eventually it becomes impossible to lean on something,” explains Mohamed Bouri, head of the research group Rehabilitation and Assistive Robotics (REHA Assist) at the Swiss Technical University. Lausanne (EPFL). “The muscles are still working but they cannot muster enough strength for patients to stand still or move their legs independently. As expected, this has a tremendous impact on a patient’s range of motion and quality of life. Effects are similar to those with hemiplegia after a stroke. Our goal was is to overcome these limitations as much as possible using mechanical support – and thus, while still benefiting from the patient’s contribution to their movements.”
Lightweight partial assist
The group leader refers to the traditional exoskeletons already in use that rely on human-inspired technology. These devices enable paraplegics to walk without a crutch, but their weight is more than 40 kg. Weighing only 25 kg, the “Autonomyo” developed by REHA Assist is lighter, works with the weak musculoskeletal system but still partially works.
The device is attached to a corset around the torso and bracelets around the user’s legs. On each side, three motors enable movement by providing the strength that muscles lack. In each case, one motor is responsible for flexing and extending the hip, and another motor does the same for the knee. The third motor supports abduction and adduction of the leg at the hip joint—in other words, the lateral movement of the leg away from the midline of the body. Together, the actuators help the patient maintain their balance and walk straight. In a recent clinical study including people with a walking disability, Autonomyo was shown to work as intended: the exoskeleton provided support while allowing freedom of movement, in accordance with users’ intentions. Joint range of motion and gait cadence were not adversely affected.
Feedback of the magnetic measurement system
It is crucial that the device assists in walking according to the intent of the user. “The initial impulse to change position—that is, to start walking—is expressed as a small change in the position of the lower extremity,” Bury explains. “We discover it by integrating information from an inertial measurement unit, eight load sensors in the sole and motor encoders that act as common positional sensors. All of this data contributes to aiding balance.”
When walking, the interaction between the device and the user is critical. The torque sensor developed by FAULHABER is responsible for sensing this interaction and thus accurately implementing the assist strategy.
“The project to integrate a precise torque sensor into a motor began a few years ago, with the goal of promoting applications such as Cobotics for safe human-robot interactions,” explains Frank Schwenker, FAULHABER’s Advanced Engineering Group Leader. “With Autonomyo, we are able to implement the concept into a challenging assistive technology application for the first time.”
The traditional torque detection technique uses expansion strips on components; These strips are deformed by the force exerted. The weak point in its construction is the adhesive bond with which it attaches. The developers in the Advanced Engineering group have replaced these tapes with a high-precision measuring system.
“This enables us to achieve a deviation of less than 1.5% in the ±30 Nm measurement range,” Schwenker says. “The sensor thus provides a highly accurate value of the response torque in the walking motion.”
This value plays a vital role in controlling the subjective exoskeleton, which is of course also provided with many other values.
“Tweaking the device to fit each individual patient requires very differential calibration of the entire system,” Bury explains. “Using various parameters and feedback from the movement, the software calculates the control signals for the actuators. The type and level of assistance from the actuators is then determined based on this information.”
Engine power and development potential
The six drive units in each machine are provided by FAULHABER. Its main component is a 3274 BP4 brushless motor with a diameter of 32 mm. It provides the maximum power of any engine in the size class available in the market. Its power is transmitted by a 42 GPT planetary gear head with a shaft produced specifically for this application. The IE3 magnetic encoder provides location data to the console. A torque sensor is integrated into the gear heads of the four motors for flexing/extension movements.
Requirements on drive units are typical of the best in the range micro motors. High power with the smallest possible size and weight, as well as accuracy, reliability and long service life are among the most important characteristics of this application.
“It wasn’t hard to find the right supplier,” Albury recalls. “After the specification was made, the selection of possible engines was actually reduced to only a small number of candidates. The interdisciplinary research group of astrophysics faculty at our university is already working with FAULHABER so they have made compelling recommendations, and there has been a really good relationship. In addition FAULHABER was already in a position to develop a torque sensor in a short time frame. This was very important for our project.”
Currently, the component is not a serial product and has so far been produced only for EPFL in small quantities.
However, development engineer Schwenker can imagine many other application areas: “High-precision torque measurement can add significant value in all tactile applications. For example, for all types of robotic assistance in operating rooms where the surgeon directs the instrument and controls the machine for force and precision The sensor can also provide a protective function and is used to limit torque. What’s more, it is ideally suited for QA documentation in all cases where evidence of highly accurate torque values is required.”