Twisted String Actuators Research Articles

Development of an EMG-based Elbow- Exoskeleton with Twisted String Actuation

Abstract This paper presents an exoskeleton for upper extremity rehabilitation of stroke patients. In order to make rehabilitation widely available, the goal is an inexpensive, portable, and easy-to-use therapy for paralysis of the arm. The exoskeleton is actuated by twisted string actuators (TSA). To reduce the length of the TSA a motor with hollow shaft was chosen, as this allows the string to run through the motor. The control of the exoskeleton is designed for predefined elbow flexion exercises. The start of the movement is triggered by a threshold value in the sEMG of the m. biceps brachii. To evaluate the controller, the exoskeleton performed a flexion movement (5° to 115°) in 2 s and then an extension back to 5° in the same time after a 1 s wait. To simulate different force requirements, additional weights up to 1500 g were attached. During both, the flexion and extension movement, that angle tracking error was sufficiently small and the steady state error was zero. The angular velocity of the elbow joint showed pronounced oscillations with increasing weights. The results show that TSAs can be used to actuate elbow exoskeletons. Thus lightweight, ergonomic, simple and small rehabilitation devices can now be built on this basis. The elasticity of the system must be considered in future work. sEMG must be further implemented to control more sophisticated functions of the rehabilitation device.

Actuator and Contact Force Modeling of an Active Soft Brace for Scoliosis.

Scoliosis is an abnormality of the spinal curvature that severely affects the musculoskeletal, respiratory, and nervous systems. Conventionally, it is treated using rigid spinal braces. These braces are static, rigid, and passive in nature, and they (largely) limit the mobility of the spine, resulting in other spinal complexities. Moreover, these braces do not have precise control over how much force is being applied by them. Over-exertion of force may deteriorate the spinal condition. This article presents a novel active soft brace that allows mobility to the spine while applying controlled corrective forces that are regulated by varying the tensions in elastic bands using low-power light weight twisted string actuators (TSAs). This article focuses on the actuator and contact force modeling of the active soft brace (ASB). The actuator modeling is required to translate the twisting of string in terms of contraction of the string’s length, whereas the contact force modeling helps in estimating the net resultant force exerted by the band on the body using single point pressure/force sensors. The actuators (TSAs) are modeled as helix geometry and validated using a laser position sensor. The results showed that the model effectively tracked the position (contraction in length) with root mean square error (RMSE) of 1.7386 mm. The contact force is modeled using the belt and pulley contact model and validated by building a custom testbed. The actuator module is able to regulate the pressure in the range 0–6 Kpa, which is comparable to 0–8 Kpa pressure regulated in rigid braces. This makes it possible to verify and demonstrate the working principle of the proposed active soft brace.

Corrections to “Highly Durable Bidirectional Joint With Twisted String Actuators and Variable Radius Pulley” [Feb 20 360-370

In Table I, the dXmax was writtenas \begin{equation*} d{X_{\text{max}}} = \ {C_{dx}}\ \cdot L. \end{equation*}

Active-Type Continuously Variable Transmission System Based on a Twisted String Actuator

This letter presents an active-type twisted string actuator (TSA)-based continuously variable transmission (CVT). The proposed mechanism improves the advantage of the intrinsic CVT nature of TSA by compensating its nonlinear transmission ratio (TR) with the active and continuous conversion of the TR during contraction at a constant load as well as variable load. To change the TR, the proposed CVT adjusts the ratio of rotation between individual twists and overlap twists, which are two different types of string-twisting patterns. The performance of the TSA-based active CVT was experimentally assessed with the fabricated prototype and compared with a mathematical model. This adjustment in the rotation ratio between individual and overlap twists is realized using a planetary gear. By adjusting the rotating speed ratio between the sun and ring gear, the ratio of rotation between individual and overlap twists can be adjusted. Unlike the conventional TSA-based CVT, the proposed CVT can actively and continuously change the TR independent of the external load, even during operation. Consequently, it allows an increase in the operating range of the TSA while maintaining a high contraction speed.

The MSC Prosthetic Hand: Rapid, Powerful, and Intuitive

Amputees suffer from the weight, insufficient power, and uncomfortable control methods of their prostheses. Recent studies have introduced many ideas for both hardware and software to tackle these problems. In this letter, the authors introduce a preliminary platform of a robotic prosthetic hand system called the MSC hand that integrates effective mechanical mechanisms and intuitive control methods. The hand adopts mode-switchable twisted string actuators to provide a wide range of grasping speed (closing speed of 0.8 s) and grasping force (pinch force of 45 N) with a light weight of 390 g. All the fingers and the thumb flex and extend actively, and the thumb is also able to abduct and adduct passively. The active fingers are controlled by surface electromyographic signals, and a learning-based neurophysiologic model is used to estimate human intention for each finger. The model provides independent intentions for each finger, so the simultaneous and proportional control of multiple fingers is possible in real time. The performance of the MSC hand was verified through standardized experiments such as online simulation and the box and block test. In addition, a demonstration of gripping various objects was performed. The results showed rapid and precise gripping and intuitive control over the tasks.

Active Soft Brace for Scoliotic Spine: A Finite Element Study to Evaluate in-Brace Correction

Scoliosis is a spinal disorder that is conventionally treated using rigid or soft braces. Computational methods such as finite element-based models are used to investigate the mechanics of the spine and the effect of braces. Most spinal braces are either passive, static, or rigid and do not allow mobility to the spine, resulting in muscle atrophy, skin deterioration and other spine complexities. Lack of control over the amount of force being exerted by braces on the human spine could have adverse effects. Therefore, developing an active soft brace which allows mobility to the spine while applying controlled corrective forces could be a promising solution. This study presents finite element analysis (FEA) of an active soft brace that applies corrective forces using elastic bands. The pressure exerted by the brace on the spine can be controlled by varying the tensions in the elastic bands. The elastic band tensions are controlled using low-power, lightweight, and twisted string actuators (TSAs). This study aims to demonstrate the immediate corrections induced by the soft active brace using a scoliotic spine finite element (FE) model. A FE model of the patient’s trunk was created and validated with in vitro study. The brace model was installed on the simulated trunk to evaluate in-brace correction in both sagittal and coronal planes. The brace was evaluated under various load cases by simulating the actuator action.

Twisted string actuation with position feedback for robotic endoscopy

Abstract A twisted string actuator (TSA) is a small, strong, lightweight, and low-cost gear, transforming rotation into a linear pulling movement. The TSA consists of two or more strings that are twisted along their common longitudinal axis. The helix formed in this process becomes shorter the further the bundle is twisted. A possible application is a tendon-based endoscopic robot. To control the movement of the endoscope, a precise contraction of the tendon is necessary. Since the strings of the TSA show an elastic behaviour, position feedback is needed to determine the exact movement of the TSA. In this paper, a TSA with a closed-loop position control by a low-cost displacement sensor is presented.

A Twisted String, Flexure Hinges Approach for Design of a Wearable Haptic Thimble

Wearable haptic devices in the shape of actuated thimbles are used to render the sense of touch in teleoperation and virtual reality scenarios. The design of similar devices has to comply with concurring requirements and constraints: lightweight and compactness, intensity and bandwidth of the rendered signals. Micro-sized motors require a mechanical reduction to increase the output force, at the cost of noise and vibrations introduced by conventional gear reducers. Here we propose a different actuation method, based on a miniaturized twisted string actuator and a flexure hinge transmission mechanism. The latter is required to transmit and transform the pulling force of the twist actuator to a pushing force of the plate in contact with the fingerpad. It achieves a lightweight and noiseless actuation in a compact mechanism. In this work, we present design guidelines of the proposed approach, optimization, and FEM analysis of the flexure hinge mechanism, implementation of the prototype, and experimental characterization of the twist actuator measuring frequency response and output force capabilities.

A compact, compliant, and biomimetic robotic assistive glove driven by twisted string actuators

A compact, compliant, and biomimetic robotic assistive glove driven by twisted string actuators

Design and Feasibility Analysis of a Foldable Robot Arm for Drones Using a Twisted String Actuator: FRAD-TSA

This paper presents a feasible analysis of a foldable robot arm for drones (FRAD) for payload lifting without drone landing. The arm design is aimed to be light, foldable and energy-efficient. FRAD is designed with a scissor mechanism in a parallel arrangement actuated by a twisted string actuator (TSA) with a guide. The robot arm can fold, unfold, and easily deploy on drones by having benefit from inherent TSA advantages. We propose a linkage design approach to provide a solution to mechanical singularity and to improve a space-saving capability. The nonlinear pulling force of twisted strings' transmission ratio (decreasing with twisting) closely matched the typical force profile requirement for folding the arm. This fact was used in the foldable robot arm for its optimal actuation design. The required string torque and twist angle were calculated to generate a pulling force to fold the arm with a 0.3kg payload from the unfolded to the fully folded situation. Preliminary experimental trials demonstrated that the proposed FRAD could generate the required lifting force and fold the arm without a high torque motor.

A Compact High-Torque Twisted String Actuator Using Parallel Bundle-Driven Actuation With an Asymmetric Behavior Compensator

The twisted string actuator (TSA) has been extensively studied as an alternative to metallic gear reducers due to its low-cost, lightweight, compliance, and highly adjustable actuation characteristics. Owing to these advantages, TSAs are utilized to realize lightweight human–robot interactive robots requiring safe interaction, such as wearables and prosthetics, at a relatively low cost. However, because its force transmission ratio is almost proportional to the length of the string, the string of the conventional TSA must be elongated to achieve high force output. Thus, it is difficult to obtain high force capacity while maintaining a small size. To overcome this limitation, in this article, we propose a novel TSA based on parallel bundle-driven actuation to significantly reduce the length of the TSA while achieving high force output. Furthermore, in order to boost the proposed TSA performance, we also present a passive mechanism of an asymmetric behavior compensator. The experimental results show that the force transmission ratio of the proposed TSA is increased by 21%, compared with that of the conventional TSA. Consequently, the proposed TSA has a 44% higher torque capacity with a 37% shorter string than those of theconventional TSA.

Experimental Characterization and Modeling of the Self-Sensing Property in Compliant Twisted String Actuators

Twisted string actuators (TSAs) have exhibited great promise in robotic applications by generating high translational force with low input torque. Despite great success, it remains a challenge to reliably estimate the strain of TSAs using compact solutions while maintaining actuator compliance. The inclusion of position sensors not only increases system complexity but also decreases system compliance, a property often crucial in soft robots. We recently constructed a compliant TSA with self-sensing capability by adopting conductive and stretchable super-coiled polymer (SCP) strings; however, only quasi-static measurements of the strain-resistance correlation were obtained. This study proposes a strategy to experimentally characterize and model the transient self-sensing property in compliant TSAs. The correlation between resistance and strain is characterized under different motor twisting sequences and step durations, and exhibited transient decay, hysteresis, and creep. A self-sensing model that consists of a log-based nonlinear term, a rate-dependent Prandtl-Ishlinskii hysteresis term, and a creep term is proposed for the compliant TSAs. Experimental results confirm the high effectiveness of the proposed self-sensing approach, with the average model validation error less than 0.036 cm.

ねじれ紐式アクチュエータと分散協調駆動機構

ねじれ紐式アクチュエータと分散協調駆動機構

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.

Compliant, Large-Strain, and Self-Sensing Twisted String Actuators.

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.

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.

Experimental investigation of a twisted string actuation for usage in active catheter.

The improvement of the steerability and controllability of catheters is highly required because catheter-based interventional diagnosis and therapy have become increasingly popular. However, active catheters are often complex and expensive. This study attempts to control catheter tip by a twisted string actuator. Experiments are carried out to analyze the full actuation and bending characteristics of the active catheter. It is indicated that the expected bending motion can be achieved by twisting the strings. Hysteresis behavior in backward and forward motion is also discussed. High repetitive positional accuracy and reverse position accuracy can be obtained when the number of rotation is low. When the number of turns is 25, the maximum tip position errors are 4, 1.8 and 1 mm in X, Y and Z directions, respectively. It is expected that the presented method will prove useful for the application of twisted string active catheter.

Elastomeric Continuously Variable Transmission Combined With Twisted String Actuator

Electric motors, with a fixed reduction ratio, have a large unused operating region when mimicking muscle movements owing to the difference between the force-velocity curves of electric motors and muscles. This unused region can be reduced by changing the reduction ratio according to the external force. However, the conventional continuously variable transmissions (CVTs) are large and heavy. Elastomeric CVT (ElaCVT), a new concept relating to CVT, is designed in this study. The primary purpose of ElaCVT is to expand the operating region of a twisted string actuator (TSA) and duplicate the force-velocity curve of the muscles by passively changing the reduction ratio according to the external load applied to the end of the TSA. A combination of ElaCVT and TSA (ElaCVT-TSA) is proposed as a linear actuator that mimics the characteristics of muscles. The deformation of elastomer changes the reduction ratio without the need for complicated mechanisms. This enables the CVT to be small and lightweight so that it can be applied to various robotic systems. The performance of the ElaCVT-TSA was evaluated by means of the experiments, and the results show that the reduction ratio was passively and continuously adjusted as the external load changed. The ElaCVT has a cylindrical shape with a length of 27 mm, a diameter of 24 mm, and weighs 12 g. The reduction ratio in the maximum velocity mode is approximately 2.31 times the reduction ratio in the maximum torque mode.

A sEMG-Driven Soft ExoSuit Based on Twisted String Actuators for Elbow Assistive Applications

The scope of this letter is to show the applicability of the Twisted String Actuators (TSAs) for lightweight, wearable and assistive robotic applications. To this aim, we have developed a novel surface electromyography (sEMG)-driven soft ExoSuit using the TSAs to perform both single and dual-arm elbow assistive applications. The proposed ExoSuit, with an overall weight of 1650 g, uses a pair of TSAs mounted in the back of the user, connected via tendons to the user's forearms to actuate each arm independently for supporting external loads. We confirm this new light-weight and customizable wearable solution via multiple user studies based on the biceps and tricep' sEMG measurements. We demonstrate that user's muscles can automatically activate and regulate the TSAs and compensate for the user's effort: by using our controller based on a Double Threshold Strategy (DTS) with a standard PID regulator, we report that the system was able to limit the biceps' sEMG activity under an arbitrary target threshold, compensating a muscular activity equal to 220% (related to a single arm 3 kg load) and 110% (related to a dual arm 4 kg load) of the threshold value itself. Moreover, the triceps' sEMG signal detects the external load and, depending on the threshold, returns the system to the initial state where it requires no assistance from the ExoSuit. The experimental results show the proposed ExoSuit's capabilities in both single and dual-arm load compensation tasks. Therefore, the applicability of the TSAs is experimentally demonstrated for a real-case assistive device, fostering future studies and developments of this kind of actuation strategy for wearable robotic systems.

Accurate Dynamic Modeling of Twisted String Actuators Accounting for String Compliance and Friction

This letter presents derivation and experimental evaluation of extended dynamical model of twisted string actuator (TSA). The proposed model supports more precise estimation of required motor torque for both low-speed and high-speed (up to 2 Hz) motion by considering effects of elastic deformation of strings and frictional forces. Analysis of the experimental data demonstrated that the relative error between theoretical and practical curves never exceeded 7% for the whole range of tested motion frequencies, the accuracy that is impossible to achieve with state-of-art mathematical models. The proposed dynamical model can be used for accurate model-based control of the TSA-based systems and for efficient TSA motor selection.