Patients that use crutches for ambulation experience forces as high as 50% of body weight and extreme extension angles at the wrist, which increases the risk of joint injury such as carpal tunnel syndrome. We have designed and fabricated a soft pneumatic sleeve to reduce the wrist loading by transferring part of the load to the forearm. The sleeve uses a Fiber Reinforced Elastomeric Enclosure (FREE). FREEs are soft pneumatic actuators that can generate force and moment upon inflation. We have used a contracting FREE, which is wrapped in a helical shape around the forearm as part of the sleeve. Upon actuation, it contracts in length and reduces in diameter, thereby generating a constricting force around the forearm.
In this paper, we describe the modeling of the constricting force generated by the helical FREE. We can model the FREE as a string due to its negligible bending stiffness. The constriction force can be expressed in terms of the axial tensile force generated in the FREE upon actuation and the geometry of the helix. To obtain the axial force, we have used a model previously reported in literature that uses a constrained volume maximization formulation. We validate the string model by comparing with experimental results.