Design and Computational Modeling of a Modular, Compliant Robotic Assembly for Human Lumbar Unit and Spinal Cord Assistance

Abstract

Wearable soft robotic systems are enabling safer human-robot interaction and are proving to be instrumental for biomedical rehabilitation. In this manuscript, we propose a novel, modular, wearable robotic device for human (lumbar) spine assistance that is developed using vacuum driven, soft pneumatic actuators (V-SPA). The actuators can handle large, repetitive loads efficiently under compression. Computational models to capture the complex non-linear mechanical behavior of individual actuator modules and the integrated assistive device are developed using the finite element method (FEM). The models presented can predict system behavior at large values of mechanical deformations and allow for rapid design iterations. It is shown that a single actuator module can be used to obtain a variety of different motion and force profiles and yield multiple degrees of freedom (DOF) depending on the module loading conditions, resulting in high system versatility and adaptability, and efficient replication of the targeted motion range for the human spinal cord. The efficacy of the finite element model is first validated for a single module using experimental results that include free displacement and blocked-forces. These results are then extended to encompass an extensive investigation of bio-mechanical performance requirements from the module assembly for the human spine-assistive device proposed.

Figure 1

(a) Human spinal column representation comprising of the lumbar unit consisting of the vertebrae, intervertebral discs, spinal cord and other components. (b) Representation of the spinal column unit with a vacuum driven soft pneumatic actuator (V-SPA) module. (c) Numerical simulation results for a single V-SPA module. (d) Simulation results for a pressurized V-SPA module assembly with fixed ends. (e) V-SPA assembly undergoing free bending displacement. (f) Schematic representation of replication and assistance of spinal column motion with the help of the robotic assembly.

Figure 2

(a–c) Fabrication of the V-SPA module using a cutout polyurethane foam chamber which is painted with molten Elastosil rubber that acts as a restoring “skin” for the foam. Three such chambers are aligned as shown in (c) to form the 3-DOF module. (d) FE model for a single module containing the three chambers covered with top and bottom end-plates. (e) Fully assembled V-SPA module with top and bottom fiberglass end-plates. (f) A module undergoing free displacement testing, using an inertial measurement unit mounted on the top plate to measure displacements. (g) A module undergoing blocked force testing, using a load cell attached to the plate on the right side.

Figure 3

Stress-strain data for polyurethane foam samples undergoing mechanical testing under different modes of deformation. (a) Stress-strain data from uniaxial compression tests at different strain rates. The sample undergoes compression up to 80% of its original volume. (b) Cyclic test results in uniaxial compression. (c) Test results from simple shear testing. (d) Uniaxial tension test results. (e,f) Stress relaxation test results showing the decay of strain and stress vs. time, respectively.

Figure 4

(a–d) Different eigenmodes of buckling for a single foam chamber subjected to compression loading. (e) Post-buckling collapse of a foam chamber using explicit analysis. (f) Free displacement simulation of the entire V-SPA module under bending conditions, with two foam chambers pressurized. (g) Image of the module exhibiting linear displacement profile with all chambers subjected to vacuum pressure. (h) Image of module exhibiting bending motion profile with two chambers subjected to vacuum pressure. (i–k) Comparison of simulation and experimental results for bending, linear displacement and blocked force tests, respectively. (l) Mesh convergence test results with varying total number of nodes in the system comprising the V-SPA module.

Figure 5

(a) Simulation result for a three column V-SPA module assembly in free displacement condition. The bottom plate is kept fixed. (b) Simulation result for a two column wearable V-SPA assembly made of the same structural units (comprising the foam core and Elastosil skin) as the three column assembly, in blocked ends condition, and under a distributed pressure loading scheme. The stresses generated in the assembly are closely representative of the stresses generated in the spinal column under high compression loading. (c) Displacement profile for the device under the same conditions as in (b).