Twisted String Actuators Research Articles

Demonstrator of a Low-Cost Hand Prosthesis

Upper limb absence has an impact on both physical and mental health of a human being. Nowadays, the costs of commercial, externally powered prosthetic hands range between 25.000 € and 70.000 €. A first demonstrator of a lightweight low-cost prosthetic hand is produced with 3D-printing technology (Fused Deposition Modeling). Due to integration of single-axis solid-state joints the five fingers can be printed in one piece. Thermoplastic polyurethane is used for this purpose. The flexion of all fingers is achieved by moving cords which are positioned on the palmar side of each finger. Two twisted string actuators are integrated to allow the movement of the thumb and the remaining four fingers. These actuators consist of two polyethylene strands which are twisted along their main axis by a DC motor, providing a tensile force to bend the fingers. In order to achieve simultaneous but differential actuation of the four fingers (small finger, ring finger, middle finger and index finger), a differential mechanism is used. The thumb is driven by a separate unit. In order to open the hand, the elasticity of the joints’ material is taken advantage of. With the developed mechanics it is possible to perform precision and cylindrical grasps with an opposed thumb. Weights up to 260 g can be held according to the shape and size of an object. To increase the adaption to different sizes and weights of objects, the design should be modified. It can be concluded, that the presented device is a promising basis for a lightweight, low-cost prosthetic hand in the future.

Robust control algorithm using time delay estimation for speed mode of twisted string actuator

Twisted string actuators (TSAs) have been used where conversion of the rotational motion of a motor into a translatory motion by twisting two strings to control the length of the actuator is needed, e.g., in robot applications. Speed mode TSA (SM-TSA) improves the translatory motion achieved in previous TSAs by adding a shaft between two strings. However, the nonlinear response of the translatory displacements remains a problem. Modeling has been one approach, but payload changes or disturbances make it difficult to solve the nonlinear response of SM-TSA through modeling. Here, proportional–integral–derivative (PID) control and time delay control (TDC) in SM-TSA are evaluated as feedback mechanisms during translatory displacements. By following the desired trajectory through PID control and TDC under conditions of payload changes and spring disturbances in SM-TSA and evaluating tracking results, we show that both control methods can provide somewhat precise positional control in SM-TSA even if there are payload changes or spring disturbances. However, TDC shows smaller tracking errors and yields more robust performance against payload changes and disturbances compared to PID control.

Highly Durable Bidirectional Joint With Twisted String Actuators and Variable Radius Pulley

Twisted string actuation is an actuation concept which uses the twisting motion of strings to convert the rotational motion of a motor into a linear motion. By replacing the heavy, high friction, and high-cost gearheads with light, silent, compliant, and low-cost strings, twisted string actuators (TSAs) can provide powerful tendon-based transmission, making it a unique solution for human–robot interactive tasks. Despite such advantages, TSAs exhibit highly nonlinear force/displacement characteristics when compared to conventional geared motors, thus limiting their performance in bidirectional joints. In order to implement TSAs in a bidirectional joint, two motors had to be used to compensate for the nonlinearities, which eventually increased the control complexity, weight, and overall cost of the robotic system. In addition, TSAs show poor durability, which also limits its applicability. To address these issues, a high-performance bidirectional joint has been proposed by implementing optimized TSAs with lubrication, and a variable radius pulley (VRP). By implementation of specially designed TSAs, we achieved 1) extended service time of the strings. Furthermore, by replacing the conventional circular pulley with a VRP, we achieved 2) improved range of motion; 3) increased payload; and 4) better tracking performance, even with a single motor.

Single-Motor-Based Bidirectional Twisted String Actuation With Variable Radius Pulleys

In this letter, we proposed a variable radius pulley (VRP) to linearize input-output behavior of twisted string actuators (TSAs). With the help of a VRP, it is possible to make the inherently nonlinear transmission ratio of twisted string constant. This enables design of bidirectional transmission systems driven by a single motor, much like is done in conventional belt transmissions. Herein, we analyze the limitations on employing constant-radius pulleys in bidirectional twisted string transmission systems and propose a VRP design methodology based on TSA kinematics. We manufactured the pulley and evaluated its performance experimentally in a series of unidirectional and bidirectional actuations. The experiments demonstrated that the resultant transmission ratio from motor to load remained nearly constant as expected from the simulation results. Different statistical measures for error between the experimental and simulated results were also calculated.

Effect of Vibration on Twisted String Actuation Inside Conduit at High Curvature Angles

This paper studies an effect of vibration on twisted string actuation inside conduit at high curvature angles. In our previous work. we have mentioned that twisted string actuators can be used to transmit power even at significant curvature angles of the conduit. However, several undesirable effects, namely pull-back, hysteresis, and chattering, were present during actuation due to friction between strings and the internal sheath of the conduit. This paper reports the results of experimental study on effects of vibration on twisted string actuation inside curved conduits. We have demonstrated that applying vibration generated near natural frequency of the system during the stages of twisting and untwisting cycles helped reduce pull-back and hysteresis and increase string contraction. In case when sheath was deflected by 180° under a constant load of 3 kg, we were able to achieve over 40% decrease in pull-back and 30% decrease in hysteresis, compared with no vibration case.

Accurate modeling and nonlinearity compensation in the speed mode of a twisted string actuator

A twisted string actuator (TSA) is an effective method that can change the rotational motion of a motor into a linear motion, as well as control the speed and stiffness of the actuator. In particular, because the speed mode of the TSA (SM-TSA) can adjust the rotation–linear motion ratio by changing the diameter and length of the twisting shaft, it is a good to increase the usability of the TSA. However, the SM-TSA has a significant limitation in that it demonstrates a nonlinear translatory motion with respect to the constant rotational motion of the motor in terms of its operating principle. To solve this problem, a more accurate modeling method of the SM-TSA should be applied to predict the nonlinearity and compensate for the nonlinearity. Herein, we analyze the tendency of the twisted strings of the SM-TSA and propose a more precise modeling method of the nonlinear SM-TSA. In addition, a nonlinearity compensation algorithm using the proposed modeling of the SM-TSA is developed to linearize the nonlinear translatory motion. Through various experiments of the SM-TSA, we validated that the proposed model exhibits the nonlinearity of the SM-TSA more precisely when compared to the previous model. Additionally, we confirmed that the nonlinearity compensation algorithm using the proposed model can perform more accurate linearization of the translatory motion of the SM-TSA.

Consideration of distributed arrangement of multiple Twisted String Actuators (TSA)

Consideration of distributed arrangement of multiple Twisted String Actuators (TSA)

Graphene-Based Composite Protective Coating for Twisted Strings and Its Experimental Evaluation

Twisted string actuators (TSAs) have found extensive use in various areas of engineering and in consumer products due to their mechanical simplicity and flexibility. Despite huge future potential of TSAs, reliability of the strings operating in stringent conditions has remained a significant concern. The increased string wear due to friction between the fibers and strands is the main reason of comparatively short lifetime of the strings in TSAs. However, very limited research has been done in this area, thus methods to extend the string lifetime remains an open question. Similarly, despite intensive research efforts on graphene over the past decade, its potential as reinforcement in coating material remains comparatively unexplored. In this paper, we report the preparation and implementation of a unique graphene-based composite coating for TSAs. The graphene sample was analyzed by both structural and chemical characterization procedures before the preparation of the coating material. The coated strings were also analyzed thoroughly using both spectroscopic and microscopic techniques. The experimental studies demonstrated that we could increase lifetime of the newly coated strings by approximately 100 $\%$ in comparison with the coated strings procured commercially. The development of this new graphene-based coating material may usher a favorable outlook in the application of TSAs in future.

A 2-Speed Small Transmission Mechanism Based on Twisted String Actuation and a Dog Clutch

In this letter, we propose a 2-speed small transmission mechanism based on twisted string actuation (TSA). The proposed mechanism achieves a compact and simplified two-stage transmission ratio by adjusting the radius of the twisted string. This transmission mechanism does not require the complex mechanical parts and mechanisms of conventional transmission systems. In addition, the mechanism can achieve a braking effect without an additional mechanical component via the nonlinearity of the rotational-linear transmission provided by TSA. The prototype is sufficiently small to be embedded in a robot hand and can be easily installed in a small robotic system. This mechanism can provide an energy-efficient driving strategy and a wider driving range for small robot systems.

Design of a Twisted-String Actuator for Haptic Force Rendering

Abstract In this paper, the design of a Twisted-String Actuation (TSA) module suitable for a wide set of robotic applications is presented. The TSA module is characterized by an integrated force sensor based on optoelectronic components and embedded controller with power electronics. The proposed actuation system is particularly suited for very compact, light-weight, wearable and low cost robotic devices, such as haptic devices, wearable systems and rehabilitation exoskeletons. The whole module design and the basic force sensor working principle are illustrated and discussed. Particular attention is given to the integration of the force sensor into the TSA module to solve the drawbacks of previous solutions. Moreover, the structure of the TSA module electronics and embedded controller is presented. Finally, the actuation module has been experimentally tested, and the ability of the system of controlling the actuation force has been demonstrated.

Experimental Verification of Variable Radius Model and Stiffness Model for Twisted String Actuators (TSAs)

Experimental Verification of Variable Radius Model and Stiffness Model for Twisted String Actuators (TSAs)

Design and Analysis of the Active Dual-Mode Twisting Actuation Mechanism

This paper presents an active dual-mode twisting actuation mechanism, which is an automatic transmission system based on twisted string actuation. This mechanism produces two different transmission ratios by adjusting the radius of a twisted string. Automatic mode switching between the two transmission ratios is implemented using an active control mechanism. Then, the effect of the proposed mechanism is derived via kinetic and kinematic analyses. Mathematical models of the twisted string are derived and a control algorithm for the mode transition is proposed. The performance of the proposed mechanism is experimentally verified using a prototype of the active dual-mode twisting actuator.

Corrigendum to “A Wearable Robotic Device Based on Twisted String Actuation for Rehabilitation and Assistive Applications”

Corrigendum to “A Wearable Robotic Device Based on Twisted String Actuation for Rehabilitation and Assistive Applications”

Designing Anthropomorphic Robot Hand With Active Dual-Mode Twisted String Actuation Mechanism and Tiny Tension Sensors

In this letter, using the active dual-mode twisted string actuation (TSA) mechanism and tiny tension sensors on the tendon strings, an anthropomorphic robot hand is newly designed in a compact manner. Thanks to the active dual-mode TSA mechanism, which is a miniaturized transmission, the proposed robot hand dose has a wide range of operation in terms of the grasping force and speed. It experimentally produces maximally the fingertip force 31.3 N and minimally the closing time of 0.5 s in average. Also, tiny tension sensor with the dimension of 4.7 (width) × 4.0 (height) × 10.75 (length) mm is newly presented and embedded at the fingertips in order to measure the tension on the tendon strings, which would allow the grasping force control. The kinetic and kinematic analyses are performed and the performance is verified by experiments.

A Wearable Robotic Device Based on Twisted String Actuation for Rehabilitation and Assistive Applications

The preliminary experimental study toward the implementation of an arm rehabilitation device based on a twisted string actuation module is presented. The actuation module is characterized by an integrated force sensor based on optoelectronic components. The adopted actuation system can be used for a wide set of robotic applications and is particularly suited for very compact, light-weight, and wearable robotic devices, such as wearable rehabilitation systems and exoskeletons. Thorough presentation and description of the proposed actuation module as well as the basic force sensor working principle are illustrated and discussed. A conceptual design of a wearable arm assistive system based on the proposed actuation module is presented. Moreover, the actuation module has been used in a simple assistive application, in which surface-electromyography signals are used to detect muscle activity of the user wearing the system and to regulate the support action provided to the user to reduce his effort, showing in this way the effectiveness of the approach.

Note: Position/torque control of antagonistic robot joint with high-compliant twisted string actuators (TSAs).

A tendon-driven robot joint that has a low inertia compared with a conventional drive system is proposed. The robot joint displaces the drive system toward the robot base, and it is driven by twisted string actuators (TSAs), which are a substitute for the conventional heavy drive system. The design of the proposed robot joint is based on an antagonistic scheme that is actuated by two motors. The torques from the motors are transmitted to the robot joint through the TSAs. Based on the geometric analysis of TSAs, strategies for position and torque control are proposed for an antagonistic robot joint driven by TSAs. To verify the proposed control strategies, several control experiments are conducted using a developed prototype of a robot joint.

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.

Twisted string-based passively variable transmission: Concept, model, and evaluation

This paper presents a study on a novel passively variable transmission (PVT) which is capable of operating in a wide range of transmission ratios. The developed device is light, bidirectional, and compliant, which is achieved by incorporating a twisted string actuator (TSA) in it. TSA is a cheap and mechanically simple actuator, in which twisted strings contract as a result of twisting and therefore act as a linear transmission. If one introduces a physical offset between the twisted strings, this changes the ratio between the output speed and force provided by the actuator. By introducing a passive element that changes the offset between the strings depending on the magnitude of external load force, we developed a PVT which inherited the advantages of conventional TSAs. This paper describes the basic operation principle and presents mathematical model and experimental evaluation of the proposed variable transmission mechanism. Advantages such as light weight, compliance, and passive bidirectional adjustment of transmission ratio make the proposed device attractive for implementation in various areas of haptics, teleoperation, and wearable and lightweight robotics.

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.

Compensation for Cross-Coupled Dynamics of Dual Twisted-String Actuation Systems

Twisted-string actuation devices have been adopted in various robotic systems due to their advantages of compact size and simple structure. To precisely control the displacement of such devices, a dual-direction actuating mechanism, which provides both extension and contraction of two strings simultaneously, must be implemented. Due to the physical properties of twisted string, the actuator has problems of nonlinear length variation and cross-coupled relationships between two strings. In this study, two controllers (PID-FC and LQR-FC) were synthesized with the consideration of cross-coupling dynamics between the two axes. The experimental results demonstrate the performance of both tracking and synchronization responses of these two types of controllers.