Abstract
Actuation is possibly the most fundamental aspect of any robotic system; a robot without any means to interact with the physical world is simply a computer. As such, new actuation methods are valuable additions to the field of robotics, and optimization of such methods can help advance the field as a whole. Twisted string actuators (TSA) are a relatively novel type of actuator, which convert the rotary motion of an electric motor to a tensile linear motion by twisting two or more strings around one another thereby shortening the length. When the twisting element is composed of high tensile strength fibers (such as high modulus polyethylene fiber), these actuators have the potential to possess many desirable characteristics of actuators, such as high force, high speed, high specific power (power to weight ratio), and high power density (power to volume ratio). Furthermore, because the transmission element is string, these actuators can be designed and built very cost efficiently. One of the major drawbacks of TSAs is their endurance life. While many engineering components operate in the millions of cycles, twisted string actuators operate in the range of tens of thousands of cycles. For some applications, this low endurance life is no issue, because the strings in the actuator are cheap and easily replaced. For many applications, however, this low of endurance life is unacceptable. The purpose of this thesis is to explore the effects of the end termination of the twisted element on the endurance life of the actuator. The basic architecture and concept of twisted string actuators is presented, as well as the mathematical description of their behavior. A hypothesis concerning the endurance life of the twisted element with respect to the end termination of the twisted element is presented and supported. A potential solution to the problem of low endurance life of twisted string actuators is presented and tested alongside two other common architectures. The results from this test are presented and discussed. When different architectures of twisted strings were tested and compared under identical conditions (i.e. style of string, material of string, resting length of twisting element, loading of the actuator, and actuation speed) it was found that the separator plate architecture outperformed both the pulley-to-pulley architecture, and the proposed architecture. The pulley-to-pulley tests resulted in a median endurance life of 36.5 cycles, the separator plate architecture resulted in a median endurance life of 364 cycles, and the proposed architecture resulted in a median endurance life of 115 cycles. During testing, the phenomenon of axial twist of individual strings, in addition to the twisting of the string pair driven by the motor, was observed, and identified as a potential explanation for the limited endurance life of the proposed architecture when compared with that of the separator plate architecture.