Measurement Method Tests Fit of Exoskeletons

National Institute of Standards and Technology, Gaithersburg, MD

Exoskeletons, many of which are powered by springs or motors, can cause pain or injury if their joints are not aligned with the user’s. To mitigate these risks, a new measurement method was developed to test whether an exoskeleton and the person wearing it are moving smoothly and in harmony.

Reflective markers are attached to the blue 3D-printed apparatus above and below the user’s knee as well as two metal plates on the exoskeleton leg. Researchers track and compare the movement of the markers to gain insight into how well the exoskeletons fit. In this composite photo, the bottom plate has been added after the original image was taken to show the entire configuration. (Photo: N. Hanacek/NIST)

The method is an optical tracking system (OTS) not unlike the motion capture techniques used by filmmakers to bring computer-generated characters to life. The OTS uses special cameras that emit light and capture what is reflected back by spherical markers arranged on objects of interest. A computer calculates the position of the labeled objects in 3D space. The approach was used to track the movement of an exoskeleton and test pieces, called “artifacts,” fastened to its user.

The ultimate goal is to put these artifacts on a person, put on the exoskeleton, compare the difference in the person wearing the artifacts versus the exoskeleton, and see if they move in concert with one another. If they move differently or it is not fit correctly, adjustments could be made.

Researchers aimed to capture the motion of the knee — one of the body’s relatively simple joints. To assess the measurement uncertainty of the new approach, they constructed two artificial legs as testbeds. One featured an off-the-shelf prosthetic knee, while the other incorporated a 3D-printed knee that more closely mimicked the real thing. Metal plates were also fastened to the legs with bungee cords to represent exoskeletal limbs or test artifacts attached to the body. After fixing markers to the legs and plates, the team used the OTS and a digital protractor to measure knee angles throughout their full range of motion. By comparing the two sets of measurements, they were able to determine that their system was capable of accurately tracking leg position. The tests also established that their system could calculate the separate motions of the legs and exoskeletal plates, allowing the researchers to show how closely aligned the two are while moving.

To adapt the method to be used on an actual person’s leg, the team designed and 3D-printed adjustable artifacts that — like a knee brace — fit to the user’s thigh and shin. Unlike the skin, which shifts due to its own elasticity and contracting muscles underneath, or skintight clothing that may be uncomfortable for some, these artifacts offer a rigid surface to stably and consistently place markers on different people. The team mounted the knee artifacts and a full-body exoskeleton garnished in reflective markers onto a human subject. With the OTS watching, the subject performed several sets of squats. The tests showed that most of the time, the subject’s leg and the exoskeleton moved in harmony. But for brief moments, the subject’s body moved while the exoskeleton didn’t. These pauses could be explained by the way in which this exoskeleton works.

To provide extra strength, it uses springs, which engage and disengage as the person moves. The exoskeleton pauses when the springs shift modes, temporarily resisting the user’s movement. By detecting the nuances of the exoskeleton’s function, the new measurement method demonstrated its attention to detail.

The raw data alone doesn’t always reveal whether a fit is adequate. To improve the accuracy of their method, the team will also use computational algorithms to analyze the positional data. The next steps are to develop artifacts for the arm, the hip, and basically all the joints the exoskeleton is supposed to be in line with and then perform similar tests.

For more information, contact Jonathan Griffin at This email address is being protected from spambots. You need JavaScript enabled to view it.; 301-975-4117.