Preliminary Study of Twisted String Actuation Through a Conduit Toward Soft and Wearable Actuation

Twisted string actuators (TSA) exhibit desirable traits for robotic applications like enabling high-force actuation. It is important to understand TSAs' key performance metrics to allow for ubiquitous usage. However, performance analysis of TSAs is challenging due to the strong coupling between the TSA model parameters. Comparing TSA's performance to motor-based actuators is essential in understanding the trade-offs of TSAs compared to motor-based tendon-driven actuators. This letter presented a model-based performance analysis of TSAs with a focus on four metrics: contraction range, linear velocity, effective torque input, and force output. Furthermore, the performance of TSAs was compared to another motor-based actuation technology namely spooled motor-tendon actuator (SMTA). A comparison framework was proposed by synthesizing models of TSAs and SMTAs. Simulation studies were conducted using the same type of motors to construct TSAs and SMTAs. The results showed that TSAs performed better in force outputs and effective torque inputs, whereas the SMTAs outperformed TSAs in terms of linear velocity and contraction range. In addition, a robotic assistive glove leveraging the advantageous performance of TSAs was realized, demonstrating the importance of the results. The findings suggest that TSAs are more suited for compact, high-force applications while SMTAs are more appropriate for high-speed robotic applications.

Actuators with adjustable transmission ratios are required in a variety of robotics and automation applications, spanning from human assistive devices and mobile robots to general manipulators. Most existing self-adjustable actuators are bulky and mechanically complex, which often makes their implementation challenging. In this work, we propose a novel passively-adjustable transmission mechanism based on a twisted string actuator. Twisted string actuator is a light, cheap, and mechanically simple actuator, in which twisted strings contract as a result of twisting and therefore act as a translational gear. If one introduces a physical offset between the twisted strings, this changes a ratio between the speed and output force provided by actuator. This work introduces a kinematical model of such actuator for the configuration when a physical offset between the strings is present. In order to experimentally verify the proposed mathematical model, we designed and manufactured a twisted string actuator with variable offsets between the strings. Additionally, in this paper we also propose an idea for passively adjustable gear, controlled by twisted string actuator according to the proposed kinematical model. The main advantages of the proposed passively adjustable gear are its light weight, compliancy and mechanical simplicity, which make it attractive for implementaion in various areas of haptics, teleoperation, wearable and lightweight robotics.

Twisted string actuators (TSA) are efficient, compliant cable actuators with high power density that have found their way into many engineering and robotics applications in recent years. They are generally used in the scenarios involving comparatively slow positioning, since quick motions in TSAs often result in large overshoot values, undesired oscillations and loss of cable tension. This work discusses time optimal trajectory generation for point-to-point transitions in TSA. We propose a method to generate smooth trajectories by directly solving an optimal control problem while respecting constraints on motor torque and speed and preventing tension loss on cables. We have conducted an experimental evaluation demonstrating that, in comparison to classical, constrained 'bangbang' transitions, the proposed trajectories result in much faster settling time, nearly-zero overshoot and require almost 60% less motor power for tracking. The proposed method can help TSAs find their way into highly-dynamical applications that have been previously deemed to be too demanding, which include parallel manipulators and antagonistically-controlled systems.

Twisted string actuators (TSAs) convert rotational motion from twisting into linear motion. They are known for high energy efficiency, and large linear strain and stress outputs. Although they have been successfully applied as the moving mechanism for different robot applications, their potential in soft robotics is mainly challenged by two aspects: First, the conventional strings of TSAs are stiff and strong but not compliant. Second, precise control of TSAs predominantly relies on external position or force sensors. Because of these, TSA-driven robots are often rigid and bulky. In this study, we propose the design, modeling, and robotic application of TSAs that are compliant, can produce large strain, and are capable of self-sensing during twisting-induced actuation. The design is realized by replacing conventional stiff strings with compliant, thermally activated, and conductive supercoiled polymer strings. Experiments show that the developed TSAs have normalized stiffness of <50 N, strain >30%, and position self-sensing capability during twisting. The quasi-static actuation and self-sensing properties are accurately captured by the Preisach hysteresis operators. In particular, both the twisting-induced actuation and thermally induced actuation are considered. Finally, the proposed TSAs are successfully demonstrated in a low-cost three-dimensionally printed compliant robotic gripper.

Training techniques and enhanced kinematic modeling for extended range twisted string actuators

A rotary motor combined with fibrous string demonstrates excellent performance because it is powerful, lightweight, and prone to large strokes; however, the stiffness range and force-generating capability of twisted string transmission systems are limited. Here, we present a variable stiffness artificial muscle generated by impregnating shear stiffening gels (STGs) into a twisted string actuator (TSA). A high twisting speed produces a large impact force and causes shear stiffening of the STG, thereby improving the elasticity, stiffness, force capacity, and response time of the TSA. We show that at a twisting speed of 4186 rpm, the elasticity of an STG-TSA reached 30.92 N/mm, whereas at a low twisting speed of 200 rpm, it was only 10.51 N/mm. In addition, the STG-TSA exhibited a more prominent shear stiffening effect under a high stiffness load. Our work provides a promising approach for artificial muscles to coactivate with human muscles to effectively compensate for motion.

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. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs and exoskeletons, since it allows the implementation of powerful tendon-based driving systems, based on small-size DC motors characterized by high speed, low torque and very limited inertia. One of the major limitations of this actuation system is by now related to the fact that the string should not be in contact with any obstacle, because this contact will alter the twisting angle propagation along the string and, eventually, completely stop the string twisting. This design constraint imposes a straight path between the motor and the linear load attached to the other string end. 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.

Exoskeletons that make running easier could increase users’ physical activity levels and provide related health benefits. In this paper, we present the design of a portable, powered ankle exoskeleton that assists running and uses lightweight and compact twisted string actuators. It has limited durability at this stage of development, but preliminary results of its power to mass density and potential for reducing the metabolic cost of running are promising. The exoskeleton can provide high peak power of 700 W per leg, 7 times more than prior twisted-string devices, and high peak torques of 43 Nm. Kinetostatic and dynamic models were used to select mass-optimal components, producing a device that weighs 1.8 kg per leg and 2.0 kg in a backpack. We performed preliminary tests on a single participant to evaluate the exoskeleton performance during both treadmill running and outdoor running. The exoskeleton reduced metabolic energy use by 10.8% during treadmill running tests and reduced cost of transport by 7.7% during outdoor running tests compared to running without the device. Unfortunately, the twisted string wore out quickly, lasting an average of 4 min 50 s before breaking. This exoskeleton shows promise for making running easier if string life challenges can be addressed.

For general grasping, a strong lightweight and compact robot hand needs to be designed as a robot end effector. This paper describes the design of a 3D printed anthropomorphic robot hand that actuates flexible fingers using Twisted String Actuators (TSAs). A total of 6 of these actuators were fitted within a limited confined space, the palm of the hand. This gives the hand 6 Degrees of Freedom (DOFs) 2 in the thumb and 1 in each of the other fingers. A simple modular design was used which allows for rapid prototyping of different finger designs with respect to different requirements at low cost. In this paper, only power grasping is considered for simplicity. The hand is capable of performing both spherical and cylindrical grasps whereby the flexible nature of the fingers allows for forming around the geometry of a target object. The maximum holding load of the hand was found to be 10 kg in performance tests.

Although several studies have been conducted on the modeling and control of twisted string actuators (TSAs), most have been limited to position control; the studies on tension control are limited. This study proposes an indirect tension control method for TSAs. Through comprehensive loading/unloading experiments involving frequency richness and a wide range of tension inputs, the results showed that the conventional static model is no longer valid with a fixed stiffness and constant original length of the string. Therefore, the proposed method allows the stiffness and original length variation in the static tension model, which is then considered in the control by introducing an estimation method. Second, the proposed method indirectly controls string tension by controlling the rotation angle of the motor. The inverse calculation of the desired rotation angle of the motor to achieve the desired tension is equivalent to adaptively changing feedback gains with respect to the nonlinear varying gear ratio of the TSAs. The proposed estimation method simulated the actual string tension from the low- to high-frequency and from the low- to high-tension regions within a tolerable error range. Therefore, the proposed tension controller demonstrated superior regulating and tracking performance compared with the best-tuned direct proportional–integral–derivative feedback controller in a wide range of desired tensions and bandwidths.

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

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.

Similar to biological muscles in nature, artificial muscles have unique advantages for driving bionic robots. However, there is still a large gap between the performance of existing artificial muscles and biological muscles. Twisted polymer actuators (TPAs) convert rotary motion from torsional to linear motion. TPAs are known for their high energy efficiency and large linear strain and stress outputs. A simple, lightweight, low-cost, self-sensing robot powered using a TPA and cooled using a thermoelectric cooler (TEC) was proposed in this study. Because TPA burns easily at high temperatures, traditional soft robots driven by TPAs have low movement frequencies. In this study, a temperature sensor and TEC were combined to develop a closed-loop temperature control system to ensure that the internal temperature of the robot was 5 °C to cool the TPAs quickly. The robot could move at a frequency of 1 Hz. Moreover, a self-sensing soft robot was proposed based on the TPA contraction length and resistance. When the motion frequency was 0.01 Hz, the TPA had good self-sensing ability and the root-mean-square error of the angle of the soft robot was less than 3.89% of the measurement amplitude. This study not only proposed a new cooling method for improving the motion frequency of soft robots but also verified the autokinetic performance of the TPAs.

This paper presents a novel lightweight and simple TSA-based (twisted string actuation) wearable haptic glove (ExoTen-Glove). This system is using two independent twisted string actuators with integrated force sensors and small-size DC motors. The proposed system can provide users force feedback during the execution of grasping virtual objects. The design of the TSA-based ExoTen-Glove, description of the TSA system, the controller and the preliminary experimental evaluation of the proposed system has been presented in this paper. This device has been evaluated by an experiment in virtual reality environment using HTC VIVE headset with 2 degrees of freedom grasping tasks, where the participants were squeezing a real spring with their thumb and index finger and compare it with a virtual spring stiffness. The results prove the applicability of the ExoTen-Glove for rehabilitation and haptic purposes.

Twisted string actuators (TSAs) have emerged as a promising class of soft actuators with diverse applications in fields such as robotics, biomedical engineering, and wearable technology. This review article provides an overview of recent advancements in TSA technology, discusses the challenges associated with their implementation, and explores potential future directions in research and development. Key topics covered include the working principles and mathematical models of TSAs, design solutions, materials selection, fabrication techniques, control, applications, limitations, and avenues for further innovation. By examining the current state of TSA research, this article aims to stimulate discussion and inspire new avenues of exploration in this exciting field.