All exoskeletons that are currently available on the market are assistive exercise tools for people that are normally bound to a wheelchair; they allow them to leave their wheelchair and to walk again, but only for short distances and short times.
An ultimate promise is that such impaired wheelchair-bound persons, for example dealing with a spinal cord injury, one day can fully rely on an exoskeleton for their daily life mobility, and would not need a wheelchair at all anymore, but again could stand up and walk like a healthy subject.
However, at this moment, there are still many challenges to achieve this ultimate goal. Current exoskeletons mainly provide two functions: in the first place, they support the weight of the user, so they do not collapse because of the paralysis or lack of strength of their leg, either through supporting both legs (when standing) or only supporting the stance leg (when walking). In the second place, they support the leg swing, moving the “free” swing leg forward, thus allowing to make a step and to achieve walking. But what these exoskeletons do not do is to take care of placing the feet correctly (at the right position on the floor), or control the full body posture in such a way that the person can stand and walk in a stable, safe way, and does not lose balance and falls. For this reason, patients using current exoskeletons always need crutches or other supports to control their balance, and in fact need supervision of a caretaker of family member, to assure their safety. This obviously limits very much the autonomy and comfort for the user of such systems.
One general goal of exoskeleton research and development is therefore to implement balance control support in exoskeletons. In the period 2013-2017, a European consortium of four European universities, one company and two research and technology organizations, have performed research on this topic in the BALANCE project.
In order to support balance through an exoskeleton, many underlying functionalities had and have to be improved. One core element of balance control is the foot placement: during walking it is essential to place the swing foot in the limited area that assures a safe posture. This is especially important in situations where balance may get lost: when we stumble, or try to recover from a not-so-well-placed prior step – then a fast, adequate corrective step is essential.
To implement foot placement in an exoskeleton, we had to work on several functions:
- develop an exoskeleton that has the degrees of freedom (motorized movement capabilities) to control also the sideways hip rotation (ab-/adduction) and the ankle movements, in order to not only move the leg in the forward direction (as all current exoskeletons can do), but also to be able to position the foot on the floor
- we had to implement a method for detection of the quality of the balance/posture to understand when a person is about to lose balance and fall,
- we had to improve our understanding of how healthy people place their foot, especially in situations where they are losing balance (for example after being pushed), and what goes wrong in this foot-placement in stroke patients; to improve such understanding we have performed many pushing experiments with healthy people and patients, to observe how one does and should react when losing balance.
What also had to be improved was the approach to interaction control: how can an exoskeleton actually support the natural movement and control of a human, and not hinder what the human is doing on own effort: how can we collaborate with an assistive exoskeleton?
Another aspect of the work was to adapt a wearable sensor suit, so that it could measure the body posture even in very difficult (metal and magnetic) environments such as indeed an exoskeleton, and to develop specific devices to study balance control in health subjects and neurological patients. Finally, we also have studied how the results of our work could be used to improve the training of stroke patients that often also suffer in their balance control because of hemiparesis (one-sided paralysis). It appeared that our work suggested promising novel training strategies for this group. More results of the project can be found through the documents shared on the project website and the scientific papers that are listed there, as well as through the project movie.
Although the BALANCE initiative achieved important results, the ultimate goal is still far away. The work will be continued in an effort to coordinate the European research in the field in the COST Action “Wearable Robots for Augmentation, Assistance or Substitution of Human Motor Functions”.