Actuator and Contact Force Modeling of an Active Soft Brace for Scoliosis

Abstract

Scoliosis is an abnormality of the spinal curvature that severely affects the musculoskeletal, respiratory, and nervous systems. Conventionally, it is treated using rigid spinal braces. These braces are static, rigid, and passive in nature, and they (largely) limit the mobility of the spine, resulting in other spinal complexities. Moreover, these braces do not have precise control over how much force is being applied by them. Over-exertion of force may deteriorate the spinal condition. This article presents a novel active soft brace that allows mobility to the spine while applying controlled corrective forces that are regulated by varying the tensions in elastic bands using low-power light weight twisted string actuators (TSAs). This article focuses on the actuator and contact force modeling of the active soft brace (ASB). The actuator modeling is required to translate the twisting of string in terms of contraction of the string's length, whereas the contact force modeling helps in estimating the net resultant force exerted by the band on the body using single point pressure/force sensors. The actuators (TSAs) are modeled as helix geometry and validated using a laser position sensor. The results showed that the model effectively tracked the position (contraction in length) with root mean square error (RMSE) of 1.7386 mm. The contact force is modeled using the belt and pulley contact model and validated by building a custom testbed. The actuator module is able to regulate the pressure in the range 0-6 Kpa, which is comparable to 0-8 Kpa pressure regulated in rigid braces. This makes it possible to verify and demonstrate the working principle of the proposed active soft brace.

Keywords: exoskeletons; modeling; scoliosis; soft brace; twisted string actuators (TSA); wearable robotics.

Figure 1

Active soft brace design and prototype. (a): conceptual design with scoliotic spine description; (b): prototype with actuator module, bands and stretch sensor.

Figure 2

Test setup for actuator modeling (TSA actuation module with elastic band and laser displacement sensor).

Figure 3

Testbed for contact force modeling. (a): Isometric view showing the actuator module, battery, bands and data acquisition board; (b): Top view with three single point pressure/force sensors.

Figure 4

Helix’s geometry model for TSA.

Figure 5

Actuator model validation setup schematic diagram.

Figure 6

Actuator model validation results showing the contraction in string’s length.

Figure 7

Lifecycle test of twisted strings with different twisting regions.

Figure 8

(a) Representation of contact reaction forces from single point force sensors and net resultant force along with (b) sensor hysteresis.

Figure 9

Belt pulley contact force model to model contact force where red represents the band.

Figure 10

Contact force model validation results.

Figure 11

Corresponding pressure range exerted by the elastic bands.