This study proposes a technique to estimate the output state of twisted string actuators (TSAs) based on payload's acceleration measurements. We outline differential kinematics relationships of the actuator, re-formulate these into a nonlinear parameter identification problem and then apply linearization techniques to efficiently solve it as a quadratic program. Using accurate estimates of string parameters obtained with the proposed method, we can predict TSA position with sub-millimeter accuracy via conventional kinematic relationships. In addition, the proposed method supports accurate estimation under varying operating conditions, unpredictable perturbations, and poorly-excited trajectories. This technique can be employed to improve the accuracy of trajectory tracking when the use of direct position measurements is challenging, with the list of potential applications including flexible and soft robots, long-span cable robots, multi-DOF joints and others.

Twisted string actuators are an emerging type of transmission systems that may benefit various applications of robotics and mechatronics. However, control of TSAs in applications that require high bandwidth has attracted comparatively little interest from research community, mainly due to complexity of twisted string behavior. This paper proposes a new adaptive control methodology that allows to sufficiently increase bandwidth of TSA-based systems. We reformulate mathematical model of the TSA into a suitable form for online parameter estimation, outline adaptive estimation methods and propose a method to design variable controller gain that rectifies nonlinearities in the system. we present experimental comparison of proposed adaptive control strategies with two conventional tsa control techniques. experimental results demonstrated that the proposed adaptive control architecture with feedforward speed term was nearly insensitive to increase in input signal frequency while reducing position tracking error by 80%. proposed algorithm can be applied in any tsa control system that has input and output signal measurements.

Robotic systems for object handling and manipulation are hugely important for modern engineering and industry, with their efficiency, agility and robustness often depending on gripper design and performance. In this work, we investigate a gripper design that, when driven by a twisted string actuator, exhibits nearly-constant transmission ratio throughout its motion range. This allows for design of a highly-compact, modular and efficient robotic gripper driven by a low-power motor. We investigate kinematics of the device, experimentally verify developed models with a practical gripper testbed, and analyze transmission ratio and efficiency of the designed device. The resulting system has a nearly-constant transmission ratio of 550, with the constancy coefficient of 0.985.