Twisted String Actuation for an Active Modular Hand Orthosis

Inhalt:
The treatment of (partially) paralyzed hands continues to reach surgical and therapeutic limits in Germany, marking the supply of patients with mechanical devices becoming increasingly relevant. The aim of the presented work is the realization of a modular orthosis for paralyzed hands. In paresis, an active hand orthosis can be used as a training device to relax spastic paralysis and a support structure to restore flexion and extension. This presented orthosis is intended to functionally support paralyzed fingers regarding the range of motion and achievable force, based on modular mechanical components and drive unit. For the drive unit an actuator principle is required, that permits quiet, lightweight, and inexpensive force generation. The twisted string actuation (TSA) is based on the axial twisting of two polyethylene strings. The twisting reduces the initial length of the string by creating a helix and – given non-elastic behaviour of the material – generates a tensile force against the load. The operating principle requires no additional gears and produces minimal noise. In the first step, the development approach is validated by the movement of two (spastically) paralyzed fingers (index finger, and middle finger) with the orthosis. Two DC-motors of different power are used to move the paralyzed fingers with the required force of 20 N. Two Dyneema(r) strings are used for the twisted string. Various tests are carried out to characterise the parameters (initial length of the twisted string, pre-twisting of the strings, motor behaviour) for operation. The drive unit works according to an antagonistic principle. The motors for extending the fingers are located on the dorsal side of the forearm, the motor for flexion on the palmar side. The motors and the TSA are attached to an orthotic structure that extends from the forearm to the metacarpophalangeal joint. The basic material of this support structure is ORFIT ECO 2.4 mm, a thermoplastic material which can be deformed at 65deg C and adapted to the specific needs of the patient. The TSA is connected to a two-stage mechanical linear guide, which implements the flexion and extension of the fingers. Linear cross roller bearings guide the two carts for movement of the distal and proximal interphalangeal joints in the first step and complete flexion or extension through the metacarpophalangeal joint in the second step. Mechanical pivot joints at the interphalangeal joints and a double joint at the metacarpophalangeal joint allow the flexion and extension of the individual fingers. The joints, the supports for the paralyzed fingers and the two-stage mechanism are printed with polylactide (PLA) using Fused Deposition Modelling (FDM). The constant speed movement of the fingers is triggered by an input from the user. The movement is controlled by limiting the motor current, with feedback from an encoder. If the fingers get in touch with an object, the reaction torque in the motor increases, resulting in the TSA needing to apply more force to bend the fingers. As soon as the motor torque is above a defined maximum, the motor stops and with it a further flexion of the fingers, so that the object is held. A reversal of the direction of rotation causes the fingers to open again. The functionality of the mechanism driven with a TSA and movement over the pivot joints is proven by the first prototype and the movements of the fingers. Currently tests have been performed on a healthy subject and will be extended to subjects with paralysis in the future.