Robotic Exoskeleton (EXO)

Lyndon B. Johnson Space Center, Houston, Texas

The Exoskeleton (EXO) is a device that falls into a new classification of robotics called wearable robots. On-orbit applications include countermeasures and dynamometry, allowing for continual assessment of a person’s muscle strength while aboard the International Space Station. Due to its small, compact size and relatively light weight [57 lb (≈26 kg) without batteries], EXO holds great promise as a countermeasure device for missions below low-Earth orbit.

The X1 exoskeleton is compact and lightweight.

The EXO is strapped to the user through a set of cuffs and a backpack, and the current device uses carbon fiber insoles that slip into the shoes of the wearer. The EXO has four active degrees of freedom, at the hips and the knees, and six passive degrees of freedom for internal and external rotation, abduction and adduction, and planar and dorsal flexion.

Each passive degree of freedom is controlled with a belt-driven series elastic rotary actuator with a harmonic drive transmission. A combination harmonic drive and belt drive system was chosen to increase the efficiency of the actuator, as well as to keep the leg hardware thin and close to the body. Applied loads to the joints are sensed using two absolute position encoders as well as a motor encoder on the actuator. Joint data such as position, velocity, acceleration, torque, etc. are all sensed using these encoders as well as the internal spring of the actuator. Motor drivers are located next to each actuator, and are used to control the movement of the joint by reading and analyzing the joint data mentioned above and closing the loop around position or torque at each actuator.

When designing the exoskeleton, safety was of the utmost importance. For this reason, there are several layers of safety built into the system. At the high level is the ability to remove motor power from the device at any time using either of two motion stop buttons. Power to all joints is controlled via a power distribution control board (designed in-house), which ensures there are no faults on the system and all motion stops have been reset before re-enabling motor power. Redundant motor power relays and carefully programmed startup routines reduce the likelihood of a relay failure (first-on, last-off conditions, as well as soft starts to limit high inrush currents), and also protect the user in case such a failure was to occur (if one relay fuses short, the other relay in series will remain open). At any time, soft limits (position, velocity, torque, change in acceleration, etc.) can be placed on the system and will cause the system to “fault” should one of these limits be exceeded. A fault condition results in motor power being removed from the system, in which case the actuators go into a back-drivable state. In this state, the user can very easily overcome the back electromagnetic field of the motors because they are not in a lockedout state. Integrated mechanical hard stops are built into the hips and knees to prevent the device from moving outside of the user’s normal travel.

This work was done by Nicholaus Radford of Johnson Space Center; Rochelle Lynn Rea, Roger Rovekamp, and Christopher Beck of Oceaneering Space Systems; and Peter Neuhaus, Nicholas Payton, Jerryll Noorden, and Travis Craig of The Florida Institute for Human and Machine Cognition. For further information, contact the JSC Technology Transfer Office at (281) 483-3809.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)} to (Oceaneering Space Systems). Inquiries concerning licenses for its commercial development should be addressed to: