Rotational twisted string actuator with linearized output: Mathematical model and experimental evaluation

Twisted string actuator is a compliant, light, and mechanically simple module. The actuation principle is based on the contraction of the strings, where a low torque - high speed dc motor is capable of providing high forces. Such actuators are very light, quiet, compliant, mechanically simple and provide a possibility to move motors away from the actu- ating joint. Implementation of these actuators can drastically decrease the weight and size of certain robotic systems such as exoskeletons, manipulators and humanoid robots. However, these actuators have disadvantages such nonlinear transmission ratio, which is found as a ratio between the angle of twisting of the strings and their resulting contraction. In this paper, we propose a linearized output transmission mechanism with rotational motion as an output which incorporates a twisted string actuator. The designed mechanism allows to linearize the relationships between the input and output displacements and forces. The proposed design has been validated theoretically and evaluated through a series of experiments and simulations. A detailed analysis of the performance of the proposed mechanism is presented in this paper.