Design

Exo or Not?

There are a lot of types of robots out there, and while I’m partial to exoskeletons, which one I would choose to build depends fully on the task at hand.  There are many places where other robots have the advantage over exoskeletons.  In more static environments, arm robots provide speed and accuracy that humans cannot match.  In robotic-assisted surgery, robots enable smaller incisions and more stable control of the end effector.  And in terrains where humans may not be able to go, such as space or the bottom of the ocean, robots become our means to access these places.  To our credit, we engineers have built robots that have extended our reach on Earth and throughout the universe and augmented our human skills to perform incredible feats. However, there are some instances where we engineers have missed the mark..

When carrying supplies into a warzone, we thought it was a waste of energy to have a soldier carry their own gear.  In response, Boston Dynamics developed Big Dog, a robot that could carry a lot of weight to ease a soldier’s burden of carrying.  Big Dog was a brilliant robot with complex sensing and actuation that allowed it to remain stable when kicked and move over rough terrain. However, it turned out that Big Dog was too noisy for the job and soldiers wanted to keep their essential gear on their bodies where they cannot be separated from it.

Big Dog Robot, Boston Dynamics (bostondynamics.com/legacy)

In response, HULC and GaurdianXO (Sarcos) were created to allow soldiers to carry the load right on their bodies, keeping it safe and close.  Utilizing some of the power of the soldier’s body to accomplish the tasks meant less power was needed by the robot, which resulted in a quieter device that fulfilled the user’s needs.

Fulfilling the user’s needs should be our priority. For some assembly environments, we have created robots that can paint, move parts, tighten bolts, and weld seams. These robots fulfill the user’s needs usually by sitting in one place and doing mind-numbing, joint-damaging repeated work.  In other environments, this is not what is needed. When assembling a ship, for example, a base for an arm robot cannot be erected just to finish the welds along the inside corridor or to carry the cabling from one end to the other.  In this instance, exoskeletons fulfill the user’s needs. Exoskeletons fit close to a person and allow for assistance without the large footprint or installation requirements.  The lack of large footprint and installation is also a benefit in the medical rehabilitation setting. Exoskeletons can fulfill the user’s needs in a wide variety of situations.

One of the most important reasons to utilize exoskeletons and other robotic devices is to leverage the user. As with all engineering, and robotic engineering in particular, it is critical that we first examine the requirements of the user before determining what type of robot to build, what that robot will do, and what the robot needs to look like.  The best way to do that is get out from behind our computers, get out of our labs, get out of our offices and visit our users in the environment they need our device. We need to listen carefully to what they are saying and ask them questions and listen to their answers. This is difficult, expensive, and time consuming, but the effort pays off in the product people want to use.

Fitting an Exoskeleton

Exoskeletons derive their name from the exoskeletons we find in nature.  Those seen on insects fit precisely along the body, enhancing the strength of the insect while allowing for motion.  The walking stick shown here gains strength from this supportive structure, but the joints are intricately developed to work with the muscles and tendons to allow for a very large range of motion at the joint.

photos: ©Matthew Strausser IG: @strausserphotography

A rehab exoskeleton must likewise become an extension of the patient, working with both their own muscles and the actuation of the device.  Therefore, the joint alignment and sizing is critical to comfort and functionality.  But our challenge of time remains and we must consider the tradeoffs between accurate sizing and speed.  If we adjust every aspect of the device perfectly, the device will fit snugly, but each patient will either need custom pieces or set up times will become prohibitive.  If we do not have sufficient resolution, however, we risk pressure sores, abrasion, discomfort, or muscle strain.  We as engineers are thus challenged to develop adjustments that are fast, accurate, and secure and use compliance where rigid attachment may not be necessary.

EksoNR utilizes adjustments at the upper and lower leg and hip width as well as velcro soft goods to achieve a safe and comfortable fit while being relatively quick to set up. 

Next, the patient must get into the device.  For some devices, the patient can put on pieces while sitting or may need to put an AFO or similar brace into their shoe.  In others, they transfer into the device, and still for others a bodyweight support system may be used to help get that patient in. Locking in the adjustments may require specialized tools, and we must consider their maintenance as well.  It is advantageous to have adjustments that an untrained user, such as a therapy aid, can make as this decreases the time out of the therapists’ session.  We also consider what adjustments may need to be made during the session and how to accomplish those with minimal disturbance.  We also have to provide the therapist a way to determine if the adjustments are correct whether through visual clues, messages from the device itself, or tests that can be run to check.

Currently, most devices use traditional mechanisms for locking and positioning, such as nuts and bolts, friction clamps or pins.  Improved materials and manufacturing processes will provide us new opportunities for improving and streamlining adjustments. For example, printing carbon fiber or even metal components allows us to gain strength while using intricate patterns for tool-less locking.  There are also soft sensors and soft actuators in development that will improve fit and self-adjust support and bracing.  The challenge here becomes balancing the complexity and cost with the benefits gained in terms of therapy outcomes.  As these technologies advance, the balance may come to tip in the favor of additional sensing and robotic controlled adjustment, but I will leave that for a future design project.

The Legacy

The Legacy

I was introduced to exoskeletons as a graduate student at the University of California, Berkeley where I was fortunate enough to study with Professor Kazerooni whose work focuses on enhancing human capability through exoskeletons.  The lab developed devices like BLEEX, which give able-bodied individuals added strength by wrapping a mechanical structure around the body and adding power.  

The initial devices were large and required offboard power or loud engines which limited their use.  Then the lab developed HULC, the Human Universal Load Carrier, which was a breakthrough exoskeleton developed to help soldiers carry load through unknown terrain. 

 It followed the same model of BLEEX and other exos in mimicking and adding to the body’s structure, but it utilized gravity and joint alignment and actuation in a way that enabled the device to be run off of on-board power.  HULC’s design transfers the load weight around the human body to the ground and added power to the legs only where needed. Professor Kazerooni saw that this technology could be extended to add power where there was a deficit, not just where excess power was needed.  My colleagues and I began work in developing the concept of a medical exoskeleton and were met with many challenges- some expected, some not.  I was excited by this challenge and the potential to help those who had suffered from a stroke or spinal cord injury, and while I knew the challenge would not be an easy one, it took some time to understand the difficulty of transitioning from developing a device for able-bodied load-carriage applications to one that provides mechanisms for gait rehabilitation.  

An exoskeleton for load carriage does not need to provide assistance in all directions.  Gravity helps with knee flexion (like when squatting down), so power can be limited to extension.  In a rehabilitation device, the patient lacks power or control in both flexion and extension so both must be powered.

In a device built for rugged terrain, the soldier must have full flexibility.  Therefore, joints that are not controlled must be free, such as hip abduction to allow for side-stepping.  In therapy, we must control all joints even at the cost of reduced freedom by locking them out completely.  

HULC was built for use by the army, to be used outside and with trained soldiers.  Service could be completed by the users, so regular maintenance was not an issue therefore allowing us to utilize hydraulics to optimize the location of components.  In the medical setting, however, regular maintenance and potential leaks were not acceptable while the distribution of weight was less critical, so a more traditional motor system was chosen.  

A soldier must be able to quickly take off the exoskeleton for full maneuverability in case of emergency or attack, thus the robot has minimal connection points.  In a clinical setting, the patient must be tightly coupled to the device for safety and while donning and doffing must be fast, it does not require the speed a military device does.  However, the user must be able to transfer into it with limited to no motion in the legs.

There are many additional challenges that face rehabilitation exoskeletons.  While some are similar to those faced in the military or industrial applications, many are unique to the users and settings found in rehab.  When I entered that lab more than a decade ago, I did not realize how much this problem would draw me in and become my passion.  The challenge of building a robot for gait rehab is one that challenges the engineering mind while never letting us forget who we create for.