A compact, compliant, and biomimetic robotic assistive glove driven by twisted string actuators

Abstract

It is desirable but difficult to develop wearable robotic devices that have a compact form factor, sufficient range of motion to perform activities of daily living and produce enough force to allow for the manipulation of the environment and objects. Conventional actuation approaches produce high force and power output but require external pressure sources or are heavy and bulky. Twisted string actuators produce high force amplification from low torque motors and require no complicated appendages, making them attractive for wearable robotic devices. However, studies that quantify the performance of twisted string actuators to conventional spooled-motor configuration are lacking. Further, it is difficult to implement bi-directional drive using twisted string actuators without making the system more complicated. In this paper, an in-depth comparison of twisted string actuators to spooled-motor configuration is presented. In addition, a robotic assistive device that leverages the advantages of twisted string actuators is presented. Supercoiled polymer strings are used in the device for both monitoring the position change through resistance changes and simultaneously provide passive force for the glove to return to neutral position. Fingertip force of more than 7 N is achieved with the glove that weighs 186 grams including the electronics. Further, picking everyday objects is demonstrated.

Anthropomorphic Twisted String-Actuated Soft Robotic Gripper with Tendon-Based Stiffening

“…For example, SMTAs have been utilized to drive high performance soft grippers and to realize tunable stiffness [5], [29]- [32]. However, due to their low force outputs, SMTAs requires high-torque motors or high-reduction gearboxes [20], [25]. High-torque motors may be bulky and heavy, whereas high-reduction gearboxes would significantly decrease the actuation speed.…”

“…Due to these reasons, TSAs are more efficient than the motor and spool configuration. A brief study on the comparison of torque outputs of TSAs and SMTAs was conducted in [20], which concluded that TSAs can produce more than 5 times the force output of similarly driven SMTAs. This allows for less-powerful and lighter motors to be used in TSAs to generate similar forces.…”

“…This is likely because TSAs require motors, which are difficult to be incorporated into soft structures. Meanwhile, the successful applications of TSAs in exoskeletons and assistive devices [20], [26], [27] demonstrate their strong promise in areas where safe interaction with humans is necessary. Although the strings used in traditional TSAs are not largely stretchable, their flexibility could be very useful in soft robots.…”

Twisted String Actuation Through a Conduit: Undesirable Behaviors and Pre-twist Effects

“…Others have developed surface electromyography (sEMG)-driven soft exosuit using TSAs to perform single-and dual-arm elbow assistive applications. The exosuit uses a pair of TSAs mounted on the back of the user, connected via tendons to the user's forearm to actuate each arm independently to support external loads (Hosseini et al, 2017(Hosseini et al, , 2020Tsabedze et al, 2021). For lower extremity applications, a soft knee exosuit was developed to provide stair-climbing assistance (Zhao et al, 2019) and a back support exosuit to reduce the lower back's physical load.…”

Anthropomorphic Twisted String-Actuated Soft Robotic Gripper With Tendon-Based Stiffening

“…For these reasons, TSAs are more efficient than the motor and spool configuration. A brief study comparing the torque outputs of TSAs and SMTAs was conducted in [20], which demonstrated that TSAs produce significantly higher force with less input torque than SMTAs with the same motors. This allows for less-powerful and lighter motors to be used in TSAs to generate similar forces.…”