Twisted String Actuation System Research Articles

Hybrid Control Strategy for Force and Precise End Effector Positioning of a Twisted String Actuator

In this article, we propose a new control approach for the twisted string actuation system; a hybrid control scheme consisting of two nested loops, considering the motor angle and the axial force at the clamping point of the actuator as the system outputs. We show that measuring the axial force at the base of the actuator, we can effectively estimate and compensate the coupling torque between the motor and the external load. Then, compensation of the coupling torque allows formulating a linear control law to control the angular position of the actuator and, at the same time, implement an external control loop to regulate the force applied by the actuator. Simulated and experimental results validate the proposed original control scheme. Furthermore, we highlight the contributions of this article in the broader context of control of twisted string actuators.

Modeling of Linear Displacement and Design of Adaptive Robust Controller

Twisted-string actuation method has attracted much more attention due to its compact size and considerable output force. Many studies have been proposed to estimate its linear displacement of actuation based on the angular measurement of the rotary device. However, the estimated results are not always accurate, which requires an additional process to make the estimation accuracy. In this paper, an ARC control scheme with force feedback has been developed for a high-accuracy twisted-string actuation system.

A compact two-phase twisted string actuation system: Modeling and validation

In this paper, we propose a compact twisted string actuation system that achieves a high contraction percentage (81%) on two phases: multi string twist and overtwist. This type of system can be used in many robotic applications, such as robotic hands and exoskeletons. The overtwist phase enables the development of more compact actuators based on the twisted string systems. Furthermore, by analyzing the previously developed mathematical models, we found out that a constant radius model should be applied for the overtwisting phase. Moreover, we propose an improvement of an existing model for prediction of the radius of the multi string system after they twist around each other. This model helps to better estimate the bundle diameter which results in a more precise mathematical model for multi string systems. The model was validated by performing experiments with 2, 4, 6 and 8 string systems. Finally, we performed extensive life cycle tests with different loads and contractions to find out the expected life of the system.

Experimental Evaluation of Guided Twisted Actuation

Abstract: In this paper, an ongoing work for verifying the behavior of a twisted string actuator in contact with a sliding surface or guided through a sheath is presented. After the presentation of the basic properties of the twisted string actuation system, the model of the twisted string in contact with a sliding surface is discussed. The behavior of the system has been then experimentally verified and discussed. A preliminary evaluation of control strategies for compensating the side effects generated by the contact of the twisted string with the sliding surface is also presented.

The UC Softhand: Light Weight Adaptive Bionic Hand with a Compact Twisted String Actuation System

In this paper, we present the design and development of the UC-Softhand. The UC Softhand is a low cost, Bionic and adaptive hand that takes advantage of compliant joints. By optimization of the actuation strategy as well as the actuation mechanism, we could develop an anthropomorphic hand that embeds three actuators, transmission mechanisms, controllers and drivers in the palm of the hand, and weighs only 280 g, making it one of the lightest bionic hands that has been created so far. The key aspect of the UC Softhand is utilization of a novel compact twisted string actuation mechanism, that allows a considerable weight and cost reduction compared to its predecessor.

Modeling and Control of the Twisted String Actuation System

The innovative actuation concept presented in this paper allows the implementation of powerful, simple, compact, and light-weight tendon-based driving systems, using as actuators small-size dc motors characterized by high speed and low torque. Due to its properties, this actuation system is very well suited for implementation in highly integrated robotic devices. The basic working principle of this novel actuation system is introduced, and the constitutive equations of the system are given, together with their experimental validation. Driven by the necessity of controlling the actuation force in the robotic hand, the problem of tracking a desired force profile is tackled. With the aim of guaranteeing a high level of robustness against disturbances, a control algorithm based on a second-order sliding manifold has first been evaluated by means of simulations and then validated by experiments. The results obtained with this simple and compact actuation system demonstrate its suitability for use in robotic devices such as robotic hands.