Not many exoskeletons really evolved their balance. But in this article, you will learn more about two exoskeletons that are. In the Cybathlon, around 12 teams from all over the world participate with their own exoskeleton. Only Wandercraft, originated in Paris, claims to have a self-balancing exoskeleton. Of course, there are also a lot of exoskeleton producers that don’t participate in the Cybathlon. Rex Bionics is one of them. As they say themselves: ‘Rex stands alone’. When standing in this exoskeleton, your hands are free for functional exercise and activities of daily living. Sounds promising, but how do they achieve this? Let’s take a closer look!

 

Let’s start with some more information about ATALANTE, the exoskeleton of Wandercraft. Before we dive into the self-balancing mechanism, we need to get a better feel of this exoskeleton. Roughly, how is this exoskeleton built up? Their website mentions that the fourth generation of the exoskeleton has a mass of 60 kilograms. Besides that, it has electric motors included at the hips, knees, and ankles. It also has a large battery and microcomputer placed at the back. Fixtures consist of straps at the waist, hips, knees, and ankles. The feet are placed on metal walking pads with rubber grips. To stand up, the pilot has to bend at the waist. To walk, you also need to make a gesture (or the therapist can select the right program). Also, a sensor vest is named. Which of these features make the exoskeleton self-balancing though?

 

The used approach descended from advances in bipedal walking robots, which are translated to exoskeletons. These advances include model-based controllers respecting the limits of the system. To realize dynamically stable walking, they make use of a method called ‘Partial Hybrid Zero Dynamics’ (PHZD).  It is an advanced method that uses feedback control to pick a potentially stable gait.

 

Next up is Rex Bionics: a company that makes one of the few exoskeletons that is not only used by persons with a high cervical level Spinal Cord Injury, but is also suitable for those with reduced upper limb strength. And just like ATALANTE, it is self-supporting and offers great stability. However, the exoskeleton is built up differently. Let’s focus on the assembly first. REX consists of ten custom designed linear joints, which provide the power to move REX and its user up to 100 kilograms. Next to that, the REX can be easily adjusted, enabling precise and accurate alignment to the user’s joints. This modularity is important because in this way the device is accessible for a wide range of people. What about the other specifications of this exoskeleton? REX contains a custom fiber pelvis, which provides the strength and stiffness required to support the user whilst minimizing the weight of the system. On top of this fiber pelvis, fixtures can be found. These abdominal straps help support the user and hold them upright. It provides maximum user comfort and is fully adjustable. The cuffs are custom designed and also made from carbon fiber. These hold the legs firmly but without creating pressure points. The removable foam padding allows easy maintenance and compliance with infection control regimens. The legs also need to be held in the exoskeleton through fixtures. Double tethered straps are responsible for this. But what about the self-balancing mechanism? Stability is provided through wide footplates and movement across flat surfaces is more convenient because of this. Just like in the ATALANTE, REX contains sensors with high frequency sampling to continuously detect the location of the moving parts. Per leg, five Degrees of Freedom, two at the hip, one at the knee, and two at the ankle, are featured.

 

 

Rex Bionics set up some clinical trials themselves and created ‘REXEXERCISES’. Together with physical therapists they developed upper and lower limb exercises. Not only static positions, like lunges and squats, are included, but also dynamic activities. These dynamic activities are for example wide squats, leg swings, and sit-to-stand.

Non-technically, the most striking difference between Rex and ATALANTE is the fact that Rex is CE-marked. A special committee, the European Economic Area (EEA), criticizes products to check if it conforms to some rules. When having a CE-mark, conformity with health, safety, and environmental protection is indicated. Next to that, you are allowed to sell the product freely in any part of the EEA.

Unfortunately, because REX is a commercial product, there is not much that can be found about its balancing mechanism. However, we know that the wide footplates provide stability. As we learned earlier, with a big support surface the Center of Mass is more unlikely to go out of this space. Whenever the Center of Mass is above the support surface the object is stable. ATALANTE on the other hand has a rather complex mechanism. The method that is used, is based on the hybrid zero dynamics method which has resulted in dynamic walking on multiple robots. The method of HZD was extended to allow for multi-contact walking for robots with feet in the context of partial hybrid zero dynamics.

 

 

Not many exoskeletons have developed a good balancing mechanism, ATALANTE and REX are just the beginning of a self-balancing future of exoskeletons.

Figure 1: Rex Bionics: “UK Prime Minister Theresa May in New Delhi” by UK in India is licensed under CC BY-NC-ND 2.0

 

Sources:

1. Rex Bionics – Reimagining Rehabilitation. (2020, January 18). Rex Bionics. https://www.rexbionics.com/
2. ATALANTE. (2020, January 24). Wandercraft. https://www.wandercraft.eu/en/exo/
3. Agrawa, A., Harib,  O., Hereid, A., Finet, S., Masselin, M., Praly, L., Grizzle, J. W. (2017). First Steps Towards Translating HZD Control of Bipedal Robots to Decentralized Control of Exoskeletons. IEEE Access. http://ames.caltech.edu/agrawal2017first.pdf