Spring Actuator Research Articles

Variable Stiffness Spring Actuators for Low-Energy-Cost Human Augmentation

Theoretical studies suggest and experimental evidence confirms that maintaining and changing human joint stiffness by coactivated antagonistic muscles are metabolically expensive, even if muscles do not perform net mechanical work. Based on this observation, we posit that effective human augmentation can be achieved by actuators operated in parallel to human joints, even if these actuators only supplement joint stiffness without doing net mechanical work. In this article, we present a prototype variable-length leaf-spring actuator capable of large-range stiffness modulation. The key feature of the actuator is that it provides intrinsically low-energy-cost stiffness modulation even for large output deflection, by keeping the force on the driving motor low. Variable stiffness actuators use two motors to provide both stiffness and equilibrium position modulation as they are designed to do net mechanical work. The proposed actuator conceptually differs from variable stiffness actuators because first, it uses a single motor to only provide stiffness modulation, second, it does not provide equilibrium position modulation, and third, unless externally loaded, it cannot do net mechanical work. Using this actuator, we demonstrate stiffness augmentation during human–machine collaboration in challenging postural stabilization and weight-bearing tasks. Our results indicate that the proposed actuator can be used to complement a biological system by restoring or extending its functionality with low energy cost, and that variable stiffness spring actuators could effectively augment humans by doing no or a limited amount of mechanical work.

Passive hovering of a flexible -flyer in a vertically oscillating airflow

We numerically investigate the passive flight of a flexible$\unicode[STIX]{x039B}$-flyer in a vertically oscillating airflow with zero mean stream. The flexibility of the flyer is introduced by a torsion spring installed at the hinged joint. We study the effects of spring stiffness, density, resting angle and actuation efforts on the hovering performance. The results suggest that the occurrence of resonance in flexible flyers can result in significantly different performances in flexible and rigid flyers. It is found that flexibility can have two opposing effects, reducing or increasing the actuation efforts for hovering, depending on the range of driving frequency. This result is explained by the modulation of relative motion between the flyer and the imposed background flow due to the involvement of passive angular oscillation. The angular oscillation patterns, the wake symmetry properties and the postural stability behaviours under different driving conditions are also explored. Based on the findings of the present study, the ideal parameter values for stable hovering are suggested. The results of this study offer novel insight into the mechanism by which the flexibility of the flyer affects the passive hovering performance.

Beyond power amplification: latch-mediated spring actuation is an emerging framework for the study of diverse elastic systems.

Rapid biological movements, such as the extraordinary strikes of mantis shrimp and accelerations of jumping insects, have captivated generations of scientists and engineers. These organisms store energy in elastic structures (e.g. springs) and then rapidly release it using latches, such that movement is driven by the rapid conversion of stored elastic to kinetic energy using springs, with the dynamics of this conversion mediated by latches. Initially drawn to these systems by an interest in the muscle power limits of small jumping insects, biologists established the idea of power amplification, which refers both to a measurement technique and to a conceptual framework defined by the mechanical power output of a system exceeding muscle limits. However, the field of fast elastically driven movements has expanded to encompass diverse biological and synthetic systems that do not have muscles - such as the surface tension catapults of fungal spores and launches of plant seeds. Furthermore, while latches have been recognized as an essential part of many elastic systems, their role in mediating the storage and release of elastic energy from the spring is only now being elucidated. Here, we critically examine the metrics and concepts of power amplification and encourage a framework centered on latch-mediated spring actuation (LaMSA). We emphasize approaches and metrics of LaMSA systems that will forge a pathway toward a principled, interdisciplinary field.

A computational and experimental study of shape memory alloy spring actuator

A computational and experimental study of shape memory alloy spring actuator

Finite element simulation of thermomechanical training on functional stability of shape memory alloy wave spring actuator

Pre-service thermomechanical training is of great significance to achieve functional stability for shape memory alloy device. This article presents a finite element simulation of the training behavior of a shape memory alloy wave spring actuator using a thermomechanically coupled and finite-strain shape memory alloy model (Wang et al., 2017a). The model is implemented into ABAQUS/Explicit by means of a user-defined material subroutine VUMAT. The introduction of a finite-Hencky-strain return-mapping integration scheme substantially improves the numerical efficiency and stability. Model predictions are validated against the experimental data. The good agreement between both demonstrates the capabilities of the model of well describing the training behavior of shape memory alloy when subjected to large cyclic thermomechanical loading. Simulation results illustrate several primary thermomechanical characteristics during training process, such as the expansion of the phase transformation zone, the accumulation of the residual deformation, and the concentration of the internal stress. The present finite element approach provides a powerful tool in design and optimization of shape memory alloy wave spring actuator, especially to improve the geometric precision and to enhance the two-way shape memory effect.

A Versatile Soft Crawling Robot with Rapid Locomotion.

This article presents a versatile soft crawling robot capable of rapid and effective locomotion. The robot mainly consists of two vacuum-actuated spring actuators and two electrostatic actuators. By programming the actuation sequences of different actuators, the robot is able to achieve two basic modes of locomotion: linear motion and turning. Subsequently, we have developed analytical models to interpret the static actuation performance of the robot body, including linear and bending motions. Moreover, an empirical dynamic model is also developed to optimize the locomotion speed in terms of frequency and duty cycle of the actuation signal. Furthermore, with the help of the strong electroadhesion force and fast response of the deformable body, the soft robot achieves a turning speed of 15.09°/s, which is one of the fastest among existing soft crawling robots to the best of our knowledge. In addition to the rapid and effective locomotion, the soft crawling robot can also achieve multiple impressive functions, including obstacle navigation in confined spaces, climbing a vertical wall with a speed of 6.67 mm/s (0.049 body length/s), carrying a payload of 69 times its self-weight on a horizontal surface, crossing over a 2 cm (0.15 body length) gap, and kicking a ball.

Dynamic Compression Garments for Sensory Processing Disorder Treatment Using Integrated Active Materials

Many medical conditions, including sensory processing disorder (SPD), employ compression therapy as a form of treatment. SPD patients often wear weighted or elastic vests to produce compression on the body, which have been shown to have a calming effect on the wearer. Recent advances in compression garment technology incorporate active materials to produce dynamic, low bulk compression garments that can be remotely controlled. In this study, an active compression vest using shape memory alloy (SMA) spring actuators was developed to produce up to 52.5 mmHg compression on a child's torso for SPD applications. The vest prototype incorporated 16 SMA spring actuators (1.25 mm diameter, spring index = 3) that constrict when heated, producing large forces and displacements that can be controlled via an applied current. When power was applied (up to 43.8 W), the prototype vest generated increasing magnitudes of pressure (up to 37.6 mmHg, spatially averaged across the front of the torso) on a representative child-sized form. The average pressure generated was measured up to 71.6% of the modeled pressure, and spatial pressure nonuniformities were observed that can be traced to specific garment architectural features. Although there is no consistent standard in magnitude or distribution of applied force in compression therapy garments, it is clear from comparative benchmarks that the compression produced by this garment exceeds the demands of the target application. This study demonstrates the viability of SMA-based compression garments as an enabling technology for enhancing SPD (and other compression-based) treatment.

Design and Fabrication of a Minimally Invasive Surgical Device with Customized Shape Memory Alloy Spring Actuator

A shape memory alloy (SMA)-based spring actuated gripper (SAG) was designed and developed for facilitating minimally invasive surgeries (MIS). Specifically the research consists of design and development of a SMA spring which acts as the actuator for the gripper. A novel mechanism was also developed to transfer the actuator force to the gripper jaws. Further, the actuator force was characterized by a testing apparatus which uses a full-bridge load cell as the force sensing element. The activation temperature of the SMA actuator was obtained by performing differential scanning calorimetry (DSC) tests on the samples.

Design of Shape Memory Alloy Coil Spring Actuator for Improving Performance in Cyclic Actuation

Performance of the shape memory alloy (SMA) coil spring actuator in cyclic actuation as an artificial muscle is strongly related to the mechanical design of the coil geometry. This paper proposes a practical design method for improving the frequency and efficiency of the SMA coil spring actuator; by designing the SMA coil spring to have large index (coil diameter/wire diameter) and pitch angle (LIP), cooling characteristics can be improved (increasing the actuation frequency) and large deformation can be obtained. The LIP design process is based on the two-state static model that describes the displacement-force relationship of the SMA coil spring in two states—a fully austenite phase and a fully martensite phase. The design process gives accurate design parameters of the SMA coil spring actuator that satisfy the required stroke and force. The model of the fully martensite phase of the SMA coil that includes the stress-induced detwinning enables the use of maximum shear strain of the SMA. The design method reduces the mass of an SMA without changing the stroke and increase the power density and efficiency. The cyclic actuation experiments demonstrate that the LIP design doubles the maximum frequency of SMA coil actuator with one-sixth the mass of the non-LIP design.

Polymer Optical Fiber for Angle and Torque Measurements of a Series Elastic Actuator's Spring

Series elastic actuators (SEAs) can provide low output impedance, bandwidth close to the human movement, and direct measurement of torque through the spring deflection. These advantages enable the application of SEAs in human active orthosis and exoskeletons. However, conventional technologies to measure the spring deflection are bulky, inhibit natural pattern of movement or are sensitive to misalignments. This paper presents the application of polymer optical fiber (POF) as a sensor to measure the spring deflection to overcome some of the issues of conventional technologies, since it is compact, lightweight, and have electromagnetic immunity. Furthermore, the spring is employed to validate a torque sensor based on POF stress-optic effects. Results show high linearity of both sensors and mean squared errors below the encoder resolution employed as a reference on dynamic measurements.

Modeling and Simulation of a Shape Memory Alloy Spring Actuated Flexible Parallel Manipulator

Parallel platforms are known for its positioning ability. Adding flexibility to a robotic system gives an advantage to better interact with the environment. In this paper a 2 DOF flexible parallel platform has been developed which consist of a triangular top and base plate connected by a universal joint at its centroid. The Shape memory alloy (SMA) springs are used as an actuator because of its large strain and are connected between the top and base plate on all the three vertices of the platform. The top plate provides desired orientation with respect to base plate on SMA actuation. The work intends to understand the system behavior by developing a mathematical model of the system. Two separate models, one to understand the kinematics and dynamics of the platform and other to understand the dynamics of the SMA spring actuator has been developed. The models were then integrated to understand the complete system behavior. The paper focuses on the integration of various system level equations to develop a general model of the SMA actuated platform and the algorithm incorporated for the same has been discussed in the work. The mathematical models were developed in MATLAB/Simulink. The kinematic model of the platform was verified on the experimental set up. The dynamic model of SMA actuator described by Liang was used and the results obtained were found to characterize the SMA. The integrated model simulation results were also verified on the experimental set up.

Active Stiffness Tuning of a Spring-Based Continuum Robot for MRI-Guided Neurosurgery

Deep intracranial tumor removal can be achieved if the neurosurgical robot has sufficient flexibility and stability. Towards achieving this goal, we have developed a spring-based continuum robot, namely a Minimally Invasive Neurosurgical Intracranial Robot (MINIR-II) with novel tendon routing and tunable stiffness for use in a magnetic resonance imaging (MRI) environment. The robot consists of a pair of springs in parallel, i.e., an inner inter-connected spring that promotes flexibility with decoupled segment motion and an outer spring that maintains its smooth curved shape during its interaction with the tissue. We propose a shape memory alloy (SMA) spring backbone that provides local stiffness control and a tendon routing configuration that enables independent segment locking. In this work, we also present a detailed local stiffness analysis of the SMA backbone and model the relationship between the resistive force at the robot tip and the tension in the tendon. We also demonstrate through experiments, the validity of our local stiffness model of the SMA backbone and the correlation between the tendon tension and the resistive force. We also performed MRI compatibility studies of the 3-segment MINIR-II robot by attaching it to a robotic platform that consists of SMA spring actuators with integrated water cooling modules.

Biomimetic design of an ultra-compact and light-weight soft muscle glove

Wearable robotic hand devices can support people doing hand-intensive tasks by reducing the physical stress and strain on the human hand. For the safety and comfort of the user, such a device should be compatible and inconspicuous. Based on these two requirements, this paper presents a biomimetic design of a wearable robotic hand device called soft muscle glove, aiming to restore the salient features and functionalities of the human hand. Inspired by the hand musculature, the soft structure of the glove contains strings, bands and shape memory alloy (SMA) spring actuators to replicate the functionalities of tendons, pulleys and muscles in the human hand. The low-mass and small-size SMA spring actuator allows an ultra-compact and light-weight design of the glove with high dexterity. The glove weighs in total 85.03 g inclusive of the actuators and microcontroller. The performance of the muscle glove was experimentally investigated through hand function tests. The experimental results suggest that the glove can achieve functional range of motion of the human hand and can perform a wide range of grasp types defined in grasp taxonomy. Moreover, the grasping performance of the muscle glove with coupled and uncoupled flexion of the finger joints was compared. The uncoupled control shows a better matching between the grasp posture and the objects form, contributing to more efficient force transmission. This confirms the benefits of the proposed highly biomimetic design.

A novel model-based approach for resistance estimation using rise time and sensorless position control of sub-millimetre shape memory alloy helical spring actuator

Shape memory alloy shows considerable strain during heating and cooling. This effect is due to its phase transformation with temperature. Due to this property, shape memory alloys can be deployed for physical actuation in place of conventional actuators in bio-medical and bio-mimicking robots. Sub-millimetre diameter shape memory alloy wires wound as helical springs are also used for this purpose. Due to their small size, it is difficult to use sensors for temperature or displacement measurements of shape memory alloy springs. This article attempts to demonstrate that the rise time of the current through a sub-millimetre diameter shape memory alloy helical spring is directly proportional to its displacement. To characterize the rise time–displacement hysteresis, a constant current drive with overcurrent protection is developed. The data generated are utilized to implement an open-loop sensorless control. A method to estimate the resistance from the rise time is proposed with which the temperature of the shape memory alloy during actuation can be obtained. The design avoids using an analogue-to-digital converter for the direct measurement of voltage and current for measuring the resistance variation in the shape memory alloy under actuation. This helps in the development of a new sensorless control using only the digital Input/Output pins of a microcontroller/microprocessor.

A hybrid dynamic model of shape memory alloy spring actuators

This paper presents the development of a hybrid model that describes the temperature, elongation and inner force relationships in a spring Shape Memory Alloy (SMA) actuator. The temperature-inner force relationship obeys a hybrid structure in which a sigmoid function correlates the variation of temperature for the SMA and the force executed by the spring actuator. The hybrid nature of the model describes the regular hysteresis behavior of the SMA. The switching law of the hybrid model depends on the time derivative of the temperature. A multivariable model depending on temperature, and inner and external forces was developed in order to characterize the Shape Memory Effect (SME). A set of experiments was carried out to obtain the parameters used to characterize the model. The application of the Levenberg-Marquardt method resulted in the parametric estimation procedure. An averaged correlation factor of 0.95 between the model response and experimental results justifies the proposed modeling approach.

Toward the Development of a Flexible Mesoscale MRI-compatible Neurosurgical Continuum Robot.

Brain tumor, be it primary or metastatic, is usually life threatening for a person of any age. Primary surgical resection which is one of the most effective ways of treating brain tumors can have tremendously increased success rate if the appropriate imaging modality is used for complete tumor resection. Magnetic resonance imaging (MRI) is the imaging modality of choice for brain tumor imaging because of its excellent soft-tissue contrast. MRI combined with continuum soft robotics has immense potential to be the next major technological breakthrough in the field of brain cancer diagnosis and therapy. In this work, we present the design, kinematic, and force analysis of a flexible spring-based minimally invasive neurosurgical intracranial robot (MINIR-II). It is comprised of an inter-connected inner spring and an outer spring and is connected to actively cooled shape memory alloy spring actuators via tendon driven mechanism. Our robot has three serially connected 2-DoF segments which can be independently controlled due to the central tendon routing configuration. The kinematic and force analysis of the robot and the independent segment control were verified by experiments. Robot motion under forced cooling of SMA springs was evaluated as well as the MRI compatibility of the robot and its motion capability in brainlike gelatin environment.

A thermomechanically coupled finite deformation constitutive model for shape memory alloys based on Hencky strain

This paper presents a new thermomechanically coupled constitutive model for polycrystalline shape memory alloys (SMAs) undergoing finite deformation. Three important characteristics of SMA behavior are considered in the development of the model, namely the effect of coexistence between austenite and two martensite variants, the variation of hysteresis size with temperature, and the smooth material response at initiation and completion of phase transformation. The formulation of the model is based on a multi-tier decomposition of the deformation kinematics comprising, a multiplicative decomposition of the deformation gradient into thermal, elastic and transformation parts, an additive decomposition of the Hencky strain into spherical and deviatoric parts, and an additive decomposition of the transformation stretching tensor into phase transformation and martensite reorientation parts. A thermodynamically consistent framework is developed, and a Helmholtz free energy function consisting of elastic, thermal, interaction and constraint components is introduced. Constitutive and heat equations are then derived from this energy in compliance with thermodynamic principles. Time-discrete formulations of the constitutive equations and a Hencky-strain return-mapping integration algorithm are presented. The algorithm is then implemented in Abaqus/Explicit by means of a user-defined material subroutine (VUMAT). Numerical results are validated against experimental data obtained under various thermomechanical loading conditions. The robustness and efficiency of the proposed framework are illustrated by simulating a SMA helical spring actuator.

Finite element simulation of ferromagnetic shape memory alloys using a revised constitutive model

In this article, a previously developed constitutive model for ferromagnetic shape memory alloys is phenomenologically enhanced using experimental observations. A modified phase diagram along with a method for calibration of the required model parameters is further presented. The model is implemented into a user material subroutine to equip commercial finite element software ABAQUS with the capability of simulating magneto-mechanical behaviors of ferromagnetic shape memory alloys. A combined convergence scheme is employed to solve the implicit equations. The proposed model together with the presented numerical solution is shown to be able to study shape memory effect and pseudoelasticity at different constant magnetic fields. The simulated magnetic loading/unloading cycles at different constant stresses are found to be well-fitted to the experimental findings. As a practical application of the ferromagnetic shape memory alloy coupled magneto-mechanical response, a spring actuator (a bias spring serially connected to one ferromagnetic shape memory alloy element) is investigated, and the numerical predictions are shown to be in a good agreement with available experimental results. As a novel case, geometrically graded NiMnGa elements are also introduced and are simulated with the use of this approach.

An experimental comparison of the pedestrian safety performances of a spring actuator and a pyrotechnic actuator for deploying an active hood lift system

This research experimentally investigates the pedestrian safety performance of an active hood lift system of a passenger vehicle by adopting two different actuators: a spring actuator and a pyrotechnic actuator (gunpowder). After briefly introducing the working principle of the active hood lift system with the two different actuators, experiments to measure the deployment time of the system are carried out to evaluate the pedestrian safety. Subsequently, headform impact tests on the hood are performed to generate the impact force, and hence the mitigation of pedestrian injuries is investigated for the two different actuators. By comparing the measured performances obtained from both actuators, it is shown that the pyrotechnic actuator can provide a faster deployment system time. It is also identified that the spring actuator can provide a better safety performance for protecting adult pedestrians, whereas the safety performance of the pyrotechnic actuator is relatively low. Consequently, the pyrotechnic actuator is redesigned and manufactured to improve its safety performance and tested again. Then, it is shown that the modified pyrotechnic actuator can provide a better protection effect for an adult pedestrian than the spring actuator can.

Evaluation of Human Exposure to Vibration Subjected to Active Suspension Actuators

This paper details the assessment of human response to vibration through modelling of seated human body using seven degrees of freedom lumped mass model. Continued human exposure to chronic vibrations may subsequently leads to person’s discomfort. To avoid this discomfort, an active suspension with combination of electro-hydraulic, pneumatic or air spring actuator is introduced between sprung mass and the unsprung mass which is controlled by a PID controller. For the simulation, ISO D-class road is given as input for the designed Matlab Simulink model and the results were compared. The simulation result shows that air spring actuators based active suspension can effectively attenuate the vertical vibration acceleration and increase the riding comfort.