Supercoiled Polymer Research Articles

Supercoiled Polymer Sensors From Twisting and Coiling Silver-Plated Nylon Threads

Supercoiled Polymer Sensors From Twisting and Coiling Silver-Plated Nylon Threads

A variable stiffness soft robotic manipulator based on antagonistic design of supercoiled polymer artificial muscles and shape memory alloys

This paper introduces a soft robotic manipulator with variable stiffness capability. A novel compact hybrid and antagonistic actuation method is proposed, inspired by the muscle structure of creatures/organs such as octopus and human wrist. The manipulator consists of three pairs of shape memory alloy (SMA) springs–supercoiled polymer (SCP) strings in an antagonistic configuration. Both SMA spring and SCP string are thermally actuated. When the two types of actuators are driven, they are tensioned to increase stiffness, and the variable stiffness feature of the manipulator is further amplified by the antagonistic configuration of the actuators. A kinematic modeling of the manipulator is conducted, extending the classic constant curvature approach to six-actuators case. The effectiveness of this hybrid antagonistic variable stiffness mechanism and in reducing hysteresis are demonstrated experimentally. The performance of the manipulator is further validated by integrating a three-finger gripper for grasping and an endoscope for monitoring at its end, showing the potential of the proposed design for application in minimally invasive surgery (MIS).

Numerical modelling and experimental study of a super-coiled polymer for dual actuator-sensor applications

Numerical modelling and experimental study of a super-coiled polymer for dual actuator-sensor applications

Design, Implementation, and Observer-based Output Control of a Super-coiled Polymer-Driven Two Degree-of-Freedom Robotic Eye.

The prevalence of ineffective corrective surgeries for ocular motor disorders calls for a robotic eye platform in aiding ophthalmologists to better understand the biomechanisms of human eye movement. This letter presents the first hardware design and implementation of a 2-DOF robotic eye driven by super-coiled polymer (SCP) artificial muscles. While our previous work designed and simulated a deep deterministic policy gradient (DDPG) learning-based controller that requires full-state feedback of the SCP-driven robotic eye, measuring the temperature states of the slender SCPs is generally impractical for the ubiquitously aimed robot. To address this predicament, this letter proposes a reduced-order state observer to estimate the temperature of SCPs given the kinematic measurements. Combining the designed observer and the learning-based controller, the closed-loop output feedback control is implemented on the robotic eye prototype to examine its performance on three classical types of eye movements: visual fixation, saccadic pursuit, and smooth pursuit. The experimental results are presented which successfully validate the observer-based output control of the SCP-driven robotic eye.

Comparison of the Hysteresis Response of Super-Coiled Polymer Actuators

Abstract In recent years, super-coiled polymer (SCP) actuators have been investigated as a higher yield, low-cost alternative to conventional actuators. A major challenge has been finding appropriate general and industry uses for these actuators due to their hysteretic response. This paper seeks to compare the response for SCP actuators based on different thread weights and sizes and thus suggest possible directions for their use. The actuators were produced through coiling and heat training controlled via Arduino. The responses were obtained by supplying a controlled voltage to the system. The response was filmed and processed using matlab; the resulting displacement and strain were used to observe the hysteretic characteristics. Based on the characteristics noted, different applications were recommended for these SCP actuators. It is recommended that this research into SCP actuators be furthered by developing hysteresis models for compensation, control, and exploring their use in robotic systems and tactile mechanisms.

Overtwisting and Coiling Highly Enhance Strain Generation of Twisted String Actuators.

Twisted string actuators (TSAs) have exhibited great promise in robotic applications by generating high translational force with low input torque. To further facilitate their robotic applications, it is strongly desirable but challenging to enhance their consistent strain generation while maintaining compliance. Existing studies predominantly considered overtwisting and coiling after the regular twisting stage to be undesirable-nonuniform and unpredictable knots, entanglements, and coils formed to create an unstable and failure-prone structure. Overtwisting would work well for TSAs when uniform coils can be consistently formed. In this study, we realize uniform and consistent coil formation in overtwisted TSAs, which greatly increases their strain. Furthermore, we investigate methods for enabling uniform coil formation upon overtwisting the strings in a TSA and present a procedure to systematically "train" the strings. To the authors' best knowledge, this is the first study to experimentally investigate overtwisting for TSAs with different stiffnesses and realize consistent uniform coil formation. Ultrahigh molecular-weight polyethylene strings form the stiff TSAs, whereas compliant TSAs are realized with stretchable and conductive supercoiled polymer strings. The strain, force, velocity, and torque of each overtwisted TSA were studied. Overtwisting and coiling resulted in ∼70% strain in stiff TSAs and ∼60% strain in compliant TSAs. This is more than twice the strain achieved through regular twisting. Finally, the overtwisted TSA was successfully demonstrated in a robotic bicep.

A Novel Variable Stiffness and Tunable Bending Shape Soft Robotic Finger Based on Thermoresponsive Polymers

In this study, a soft robotic finger with variable stiffness and tunable bending shape capability is proposed. The finger employs shape memory polymers (SMP) as the variable stiffness element and polyimide (PI) electrothermal films as the flexible heaters for segmental heating. SMP and PI electrothermal films constitute the variable stiffness structure. The SMP part exhibits a reversible transition from glass state to rubber state during the heating/cooling process. The whole transition process results in an elastic modulus range of three orders of magnitude. The finger can obtain multiple bending shapes and adapt to the outer contours of diverse objects because of the segmental stiffness modulation capability of the variable stiffness structure. Supercoiled polymer artificial muscles (SCPAMs) are used as actuators for the finger. SMPs and SCPAMs are thermoresponsive and can be activated by thermal heating with electricity, making the whole finger structure compact. Experimental results show that the stiffness of the finger at room temperature (25 °C) is 10.54 times that at 70 °C when the actuators are on. The performance of the finger is further validated via the sequential motion demonstration and grasping tests with variable stiffness and bending shape control capabilities, showing the proposed design’s potential in soft grippers and robotic design.

A Proprioceptive Soft Robot Module Based on Supercoiled Polymer Artificial Muscle Strings.

In this paper, a multi-functional soft robot module that can be used to constitute a variety of soft robots is proposed. The body of the soft robot module made of rubber is in the shape of a long strip, with cylindrical chambers at both the top end and bottom end of the module for the function of actuators and sensors. The soft robot module is driven by supercoiled polymer artificial muscle (SCPAM) strings, which are made from conductive nylon sewing threads. Artificial muscle strings are embedded in the chambers of the module to control its deformation. In addition, SCPAM strings are also used for the robot module’s sensing based on the linear relationship between the string’s length and their resistance. The bending deformation of the robot is measured by the continuous change of the sensor’s resistance during the deformation of the module. Prototypes of an inchworm-like crawling robot and a soft robotic gripper are made, whose crawling ability and grasping ability are tested, respectively. We envision that the proposed proprioceptive soft robot module could potentially be used in other robotic applications, such as continuum robotic arm or underwater robot.

Parameter Estimation and Adaptive Control of Super-coiled Polymer Artificial Muscles

Super-coiled polymer (SCP) artificial muscles demonstrate desirable properties such as high-power density, compliance, and low-cost. However, their performance is dependent on the ambient environment. It is necessary but difficult to re-identify key SCP parameters to maintain desired performance. In this work, an adaptive parameter estimation method is proposed to identify unknown SCP parameters. The SCP model identification demonstrates parameter convergence which indicates that SCPs could be controlled effectively and used in environments with variations. We further develop a control law using direct model reference adaptive control (MRAC) on the output position of the SCP actuator. The MRAC performance is compared to a proportional-integral (PI) and proportional-derivative (PD) with feedforward controllers to demonstrate the ability to reduce tracking errors. The MRAC showed superior performance in comparison to the two controllers when the SCP actuator parameters were unknown.

Toward long and strong electroactive supercoiled polymer artificial muscles: Fabrication with constant-load springs

Toward long and strong electroactive supercoiled polymer artificial muscles: Fabrication with constant-load springs

Design, characterization, modeling, and comparison of helically wrapped super-coiled polymer artificial muscles

Super-coiled polymer (SCP) artificial muscles demonstrate a number of desirable properties, like high power density, inherent compliance, light weight, and lowcost. However, the realization of their full potential in achieving appreciable robot motion is hindered by their limited strain or force generation capabilities. Fabricated with multi-thread nylon filament, non-mandrel-coiled SCP actuators can only generate up to 10–20% strain, and mandrel-coiled SCP actuators are often fragile and fail to produce large force consistently. This paper presents the design, characterization, and modeling of helically-wrapped SCP (HW-SCP) actuators. HW-SCP actuators exhibit the advantages of mandrel-coiled and non-mandrel-coiled SCP actuators simultaneously—they can produce 15–55% strain, and consistently lift more than 90 g of weight. The design parameters of HW-SCP actuators can be conveniently adjusted to tune the actuator's performance. Furthermore, this work presents the first comprehensive comparative study of non-mandrel-coiled, mandrel-coiled, and HW-SCP actuators based on conductive multi-thread nylon filament. The correlations between strain and constant voltage were accurately captured by Preisach hysteresis models. The correlations between force and constant voltage were modeled by three low-order polynomial models. This work contributes to the development of high-performance artificial muscles and may facilitate optimal selection of SCP actuators under scenario-specific future studies.

Hysteresis with lonely stroke in artificial muscles: Characterization, modeling, and inverse compensation

Artificial muscles are inherently compliant actuators that exhibit high power density and impressive stroke and force outputs. They have found numerous applications in advanced mechatronic and robotic systems. Termed as lonely stroke, the first quasi-static input–output cycle of artificial muscles is inconsistent with subsequent cycles that are repeatable and exhibit hysteresis. The lonely stroke not only affects the behavior of artificial muscles, but also presents coupling with the subsequent repeatable hysteresis cycles. While lonely stroke has been reported in a number of artificial muscles, existing studies lack systematic experimental characterization procedure. Furthermore, no models or compensation schemes have been proposed. In this work, we propose the first study to systematically characterize, model, and compensate for the hysteresis with lonely stroke property in artificial muscles. The experimental procedure to characterize the hysteresis with lonely stroke behavior is presented. A modeling approach is developed through the expansion of the input range of the Preisach operator, a widely adopted hysteresis model, to physically infeasible region. The effects of such expansion on the model accuracy and computational cost are studied. An iterative algorithm is proposed to compensate for the hysteresis with lonely stroke by approximately inverting the proposed expanded Preisach operator. The effectiveness of the proposed scheme is validated by comprehensive simulation and experimental results. While this study primarily considers super-coiled polymer (SCP) actuators, the proposed model is also validated for several popular artificial muscles.

A Lightweight Soft Gripper Driven by Self-Sensing Super-Coiled Polymer Actuator

This study proposes a soft gripper driven by a simple, lightweight, low-cost, self-sensing super-coiled polymer (SCP) actuator with a high power-to-weight ratio. SCP generates an untwisting motion when heated, and therefore, is suitable for designing the actuator. The actuator is fabricated with a multifilament sewing thread woven by the self-coiling method and yields a high output force. To enhance the accuracy of the actuator in measuring higher temperatures, we propose an improved temperature self-sensing method that uses a dedicated metal wire wound around the SCP. To provide a theoretical basis for the control of the actuator, we construct through experiments a thermoelectric model and a thermomechanical model for the SCP. The resulting kinematics of the soft gripper are analyzed, and a nested control system is established to realize a precisely control of the soft gripper. Finally, we build a prototype to test the grasping ability of the soft gripper.

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.

Realization of a Simultaneous Position-Stiffness Controllable Antagonistic Joint Driven by Twisted-Coiled Polymer Actuators Using Model Predictive Control

Stiffness adjustability is one of the key characteristics that help humans achieve safe and reliable motions. This paper aims to realize the simultaneous position-stiffness control of an antagonistic joint driven by a type of super-coiled polymer (SCP) artificial muscles, made from a combination of Spandex and nylon fibers. A nonlinear model that can exhibit the variable stiffness characteristics of the actuator is first proposed. The model, whose parameters are identified and verified experimentally, is suitable for estimating the actuator's stiffness as a function of the length and temperature. Based on a linearized model of the antagonistic joint system, a model predictive control (MPC) is applied to control joint angle and stiffness simultaneously. By controlling both actuators' temperatures, simultaneous position-stiffness control is realized. Our contribution also includes enhancement of MPC control by incorporating time delay estimation to estimate model variations and external disturbances. The control performance is verified with both step command and sinusoidal reference with composite frequencies of 0.02Hz to 0.1Hz. Experimental results show that the system can achieve high accuracy of joint position control performance with the maximum position error of 0.42 degrees while varying joint stiffness 46.5%.

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.

Effect of Temperature on Electrical Resistance-Length Characteristic of Electroactive Supercoiled Polymer Artificial Muscle

Effect of Temperature on Electrical Resistance-Length Characteristic of Electroactive Supercoiled Polymer Artificial Muscle

Modeling of a Bending Supercoiled Polymer (SCP) Artificial Muscle

As a recently developed artificial muscle, supercoiled polymer (SCP) actuators exhibit many desirable properties such as inherent compliance, large linear tensile actuation, and high energy and power densities. By embedding linear SCP actuators into flexible beams and activating them, bending SCP actuators can be created with outstanding deflection performances. However, their full potential as bending robotic muscles is challenged by their hysteresis nonlinearity and the complex coupling between the beam and the linear SCP actuator. In this study, we propose a nonlinear model that can accurately capture and estimate the steady-state displacement of the bending SCP actuator. The proposed model is constructed by coupling a Preisach hysteresis model and nonlinear beam equations. Numerical solution is obtained using a bisection technique-based algorithm and Runge-Kutta method. For comparison purposes, a linear model is also considered. Both simulation and experimental investigations are conducted. The effectiveness of the proposed modeling approach is confirmed.

S-Shaped Nonlinearity in Electrical Resistance of Electroactive Supercoiled Polymer Artificial Muscle

S-Shaped Nonlinearity in Electrical Resistance of Electroactive Supercoiled Polymer Artificial Muscle

Comparison of Super Coiled Polymer actuator made using a twisting machine and the one made by conventional procedure

Comparison of Super Coiled Polymer actuator made using a twisting machine and the one made by conventional procedure