Design and preliminary evaluation of a flexible exoskeleton to assist with lifting

Abstract

We present a passive (unpowered) exoskeleton that assists the back during lifting. Our exoskeleton uses carbon fiber beams as the sole means to store energy and return it to the wearer. To motivate the design, we present general requirements for the design of a lifting exoskeleton, including calculating the required torque to support the torso for people of different weights and heights. We compare a number of methods of energy storage for exoskeletons in terms of mass, volume, hysteresis, and cycle life. We then discuss the design of our exoskeleton, and show how the torso assembly leads to balanced forces. We characterize the energy storage in the exoskeleton and the torque it provides during testing with human subjects. Ten participants performed freestyle, stoop, and squat lifts. Custom image processing software was used to extract the curvature of the carbon fiber beams in the exoskeleton to determine the stored energy. During freestyle lifting, it stores an average of 59.3 J and provides a peak torque of 71.7 Nm.

Recent Development of Unpowered Exoskeletons for Lower Extremity: A Survey

“…Spinal exoskeleton robot (SPEXOR) is very practical for repetitive lifting that is a common motion in industrial or daily life. Spinal UEs can be divided into two categories according to the energy elements: (1) composite materials and elastic bands, and (2) air or metal springs [81]. Baltrusch et al developed a SPEXOR to decrease metabolic cost [82] and muscle activity for repetitive lifting or bending tasks [83].…”

Modeling the metabolic reductions of a passive back-support exoskeleton

“…Similar to the without-exoskeleton conditions, the vertical displacement of the box COM for with-exoskeleton conditions was obtained to be 0.55 m (Table B in Appendix A) and the body's COM height change was found by the equations in Table D in Appendix A. We use 𝜂 0.95 based on measurements of our exoskeleton reported in (19). We use 𝑋 59.3 J for freestyle lifting, 𝑋 61.8 J for stoop lifting, and 𝑋 51.5 J for squat lifting, all of which were measured in (19) for the same exoskeleton and box height as in the current experiment.…”

“…(2) exoskeleton interventions (i.e., with and without-exoskeleton). The exoskeleton used was the VT-Lowe's exoskeleton, as previously described in (6) and (19). The exoskeleton has a mass of 4.5 kg, which was 5.7% of the mean body mass for the participants in this experiment.…”

“…This assumes that the exoskeleton creates a torque smaller than that necessary to support the wearer's torso during lifting, i.e., the wearer does not have to fight against the exoskeleton during the lowering phase. This assumption is true for all passive back exoskeletons presented in the literature thus far, as they create peak torques of less than half of the torque required to support the torso (19).…”

“…We use 𝜂 0.95 based on measurements of our exoskeleton reported in (19). We use 𝑋 59.3 J for freestyle lifting, 𝑋 61.8 J for stoop lifting, and 𝑋 51.5 J for squat lifting, all of which were measured in (19) for the same exoskeleton and box height as in the current experiment. Also, note that the calculated (best-fit) 𝐾-values for without-exoskeleton freestyle lifting from Part C were used in this section to validate our metabolic cost model for with-exoskeleton conditions.…”