Stiffness Actuator Research Articles

Encrypted Simultaneous Control of Joint Angle and Stiffness of Antagonistic Pneumatic Artificial Muscle Actuator by Polynomial Approximation

Encrypted Simultaneous Control of Joint Angle and Stiffness of Antagonistic Pneumatic Artificial Muscle Actuator by Polynomial Approximation

Analysis and Verification on Backstepping Control of Antagonistic Variable Stiffness Actuators

Analysis and Verification on Backstepping Control of Antagonistic Variable Stiffness Actuators

A Twisting Mechanism With Parallel Springs for Series Variable Stiffness Actuator

A Twisting Mechanism With Parallel Springs for Series Variable Stiffness Actuator

A Compact Variable Stiffness Actuator for Agile Legged Locomotion

A Compact Variable Stiffness Actuator for Agile Legged Locomotion

Design and Experiment Investigation on Soft Grippers with Modular Variable Stiffness Structure.

Soft grippers have good adaptability and flexibility for grasping irregular or fragile objects, and to further enhance their stiffness, soft grippers with variable stiffness have been developed. However, existing soft grippers with variable stiffness have the disadvantages of complex structure and poor interchangeability. Here, a soft gripper with modular variable stiffness is proposed that has flexible Velcro embedded in the bottom layer of the soft actuator and one side of the variable stiffness cavity respectively, and both the general and variable stiffness grasping modes are achieved by separation or combination. First of all, according to the neo-Hookean model and the assumption of constant curvature, a free bending model of the soft actuator is established and optimal structural parameters of the soft actuator are obtained by the Genetic Algorithm. Then, influence of the driving pressure on the soft actuator stiffness is investigated, and a mathematical model of the variable stiffness is established. Finally, correctness of the statics model and the stiffness model were verified by experiments. Experimental results indicate that the proposed soft gripper with modular variable stiffness structure has excellent adaptability and stability to different objects, outstanding load bearing capacity, and stiffness adjustment capability.

Mechanical Design and Experiments of a New Rotational Variable Stiffness Actuator for Hybrid Passive–Active Stiffness Regulation

To improve collision safety in robot–human collaborative applications, increasing attention has been paid to rotational variable stiffness actuators. A new rotational variable stiffness actuator, which works in two stages, is proposed for hybrid passive–active stiffness regulation. The passive stage is based on the motions of springs driven by the rack-and-pinion systems, elastically converting the shaft’s rotation into the inner shell rotation fixed to the internal gear of the active stage. The active stage is designed to achieve the movement of the pivot point located on the roller actuated by the adjustment motor, providing the output angle of the output shaft. The two pairs of rack-and-pinion systems of the passive stage and the two pairs of planetary gears of the active stage are designed for side-by-side placement, improving the stability and balance of the stiffness regulation process. Two symmetrical cam-slider mechanisms acting as leverage pivots ensure the synchronous movements of the two rollers. The variable stiffness actuator is designed and validated by simulations and experiments. Strength analysis and stiffness analysis are presented. The designed actuator can obtain the range of stiffness adjustment of 35–3286 N·mm/deg. The range of the angle difference between the input and output shafts is ±48°.

A Lightweight Series Elastic Actuator With Variable Stiffness: Design, Modeling, and Evaluation

This article proposes a lightweight variable stiffness actuator (LVSA) driven by a novel mechanism with four sliders on a shared crank (FS2C). The FS2C mechanism allows the LVSA to simultaneously regulate the preload of four springs using only one motor and hence achieves a wider-range continuous stiffness adaption with reduced weight. A cable transmission system is developed to remotely place motors and further reduce the influence of the LVSA on the mass distribution. A dynamics model is established to study the torque-deflection and the stiffness-deflection relations. Based on the model, a torque-stiffness controller is proposed. Experiments are carried out to validate the performance of the dynamics model, the controller, and the LVSA. The results indicate that the LVSA provides a range of stiffness from 0 to 988 Nm/rad with a weight of 0.412 kg, and the controller is accurate in adjusting the output torque and stiffness at relatively high speeds. The proposed actuator provides a solution for actuation systems that have to be lightweight with variable stiffness, such as wearable robotics and assistive exoskeletons.

A Systematic Review of Soft Actuators in Agriculture

Industrial machines and robots use actuators to facilitate repetitive tasks. Conventionally, stiff actuators are used to enable precise position control in a high-pace system. They are usually placed in human-free environments because of safety concerns as they do not comply in a collision. On the other hand, there has been a growing interest in human-robot interactions that require robots to be placed alongside humans. These applications need a soft actuator. An example of a soft actuator is a pneumatic muscle actuator (PMA) which produces a one-way motion and force as a result of the muscle’s contraction under air pressure. In this paper, the aim is to present a systematic review covering the main published solutions of soft actuators including PMA in agricultural applications. This paper provides a useful foundation on the soft actuators, their main applications in agriculture, their challenges and opportunities, as well as supporting new research works in the soft actuators' field.

IrisVSA: Infinite-rotation infinite-stiffness Variable Stiffness Actuator towards physical human–robot-interaction

In Human–Robot-Collaboration applications, variable stiffness actuators provide an attractive actuation solution due to their well-established capabilities in ensuring accuracy and safety. Although, several existing VSAs have achieved an infinite-range of stiffness, those are generally relatively bulky, incapable of attaining an infinite-range of rotation, and their stiffness adjustment rate is rather slow. In this paper, we introduce a new infinite-rotation infinite-stiffness Variable Stiffness Actuator (irisVSA). The novelty of this actuator lies in its design topology, where two long-armed torsional springs are mounted on the output link and connected to the input link via a Force Contact Point (FCP). The variation in stiffness is realized by altering the relative distance from the FCP to the joint rotation center (effective arm). This unique topology offers a wide range of stiffness ranging from transparency (0 stiffness) to physically rigid. Moreover, the rate of stiffness adjustment is considerably fast (0.7 sec) through the utilization of a hypocycloid gear train. The working principle, design, prototyping, and characterization process for the irisVSA are detailed here. A Nonlinear Proportional–Integral–Derivative Controller is proposed to ensure the system’s accuracy. The safety of the actuator was evaluated by subjecting it to both impact force criterion experiments as well as Head Injury Criterion.

Application of Modeling and Control Approaches of Piezoelectric Actuators: A Review

Piezoelectric actuators find extensive application in delivering precision motion in the micrometer to nanometer range. The advantages of a broader range of motion, rapid response, higher stiffness, and large actuation force from piezoelectric actuators make them suitable for precision positioning applications. However, the inherent nonlinearity in the piezoelectric actuators under dynamic working conditions severely affects the accuracy of the generated motion. The nonlinearity in the piezoelectric actuators arises from hysteresis, creep, and vibration, which affect the performance of the piezoelectric actuator. Thus, there is a need for appropriate modeling and control approaches for piezoelectric actuators, which can model the nonlinearity phenomenon and provide adequate compensation to achieve higher motion accuracy. The present review covers different methods adopted for overcoming the nonlinearity issues in piezoelectric actuators. This review highlights the charge-based and voltage-based control methods that drive the piezoelectric actuators. The survey also includes different modeling approaches for the creep and hysteresis phenomenon of the piezoelectric actuators. In addition, the present review also highlights different control strategies and their applications in various types of piezoelectric actuators. An attempt is also made to compare the piezoelectric actuator’s different modeling and control approaches and highlight prospects.

Stiffness-Tunable Soft Bellows Actuators by Cross-Fiber Jamming Effect for Robust Grasping

Soft actuators typically exhibit low stiffness and low load-bearing properties due to the intrinsic limitations of soft materials. Stiffness modulation is an effective means to improve the performance of soft actuators. However, most stiffness-tunable mechanisms are nonstretchable, and have difficulty in independently adjusting the tensile stiffness of soft elongation actuators. Here, a universal cross-fiber jamming mechanism with both elongation and bending stiffness-tunable capability is proposed for soft bellows actuators. It is composed of two semicircular symmetrical fiber bundles in an up-and-down crossed arrangement, and can follow the tensile state of the soft bellows actuator through the passive staggered motion of the layered fiber bundles. Furthermore, a soft gripper with wide grasping range and sufficient grasping stability is also developed by applying soft bellows actuators with cross-fiber jamming mechanisms. Experiments are conducted to show the related performance of the proposed cross-fiber jamming mechanism. The results show that it has fast antiimpact damping response, strong shaping ability and adjustable stiffness. Such efficient performance will promote the ability of grasping robots, and is expected to be used in daily life and industry.

Linear multi-degree of freedom variable stiffness actuator for robotic assembly tasks

Linear multi-degree of freedom variable stiffness actuator for robotic assembly tasks

Tendon-Driven Variable-Stiffness Pneumatic Soft Gripper Robot

In recent times, the soft robotics field has been attracting significant research focus owing to its high level of manipulation capabilities unlike traditional rigid robots, which gives room for increasing use in other areas. However, compared to traditional rigid gripper robots, being capable of controlling/obtaining overall body stiffness when required is yet to be further explored since soft gripper robots have inherently less-rigid properties. Unlike previous designs with very complex variable-stiffness systems, this paper demonstrates a soft gripper design with minimum system complexity while being capable of varying the stiffness of a continuum soft robotic actuator and proves to have potential applications in gripping objects of various shapes, weights, and sizes. The soft gripper actuator comprises two separate mechanisms: the pneumatic mechanism for bending control and the mechanical structure for stiffness variation by pulling tendons using stepper motors which compresses the actuator, thereby changing the overall stiffness. The pneumatic mechanism was first fabricated and then embedded into another silicon layer during which it was also merged with the mechanical structure for stiffness control. By first pneumatically actuating the actuator which causes bending and then pulling the tendons, we found out that the actuator stiffness value can be increased up to 145% its initial value, and the gripper can grasp and lift a weight of up to 2.075 kg.

Pneumatic cells toward absolute Gaussian morphing.

On a flat map of the Earth, continents are inevitably distorted. Reciprocally, curving a plate simultaneously in two directions requires a modification of in-plane distances, as Gauss stated in his seminal theorem. Although emerging architectured materials with programmed in-plane distortions are capable of such shape morphing, an additional control of local bending is required to precisely set the final shape of the resulting three-dimensional surface. Inspired by bulliform cells in leaves of monocotyledon plants, we show how the internal structure of flat panels can be designed to program bending and in-plane distortions simultaneously when pressurized, leading to a targeted shell shape. These surfaces with controlled stiffness and fast actuation are manufactured using consumer-grade materials and open a route to large-scale shape-morphing robotics applications.

Performance Analysis and Optimization of a Modified Stewart Platform for the Qitai Radio Telescope

This paper presents a modified Stewart platform that enables precise adjustment of the sub-reflector for the Qitai Radio Telescope (QTT). QTT demands a parallel platform capable of carrying a heavy load (≥4000 kg), and moving in elevation from 5° to 88° together with the primary reflector while precisely adjusting the sub-reflector’s position in five degrees of freedom. To meet these requirements, a modified Stewart platform with two separated actuators is proposed, and the comprehensive performance is analyzed and optimized for practical application. The performance of the modified platform is compared to that of the traditional Stewart platform, and the results demonstrate that the modified platform has superior load-bearing capacity and stiffness over the entire elevation angle with more uniform actuator load and stiffness distributions. These results indicate that the modified Stewart platform is well-suited for practical application in QTT.

The model and design of a Spring-Embedded Planetary Dual-Motor Actuator to reduce energy losses

The electrification of many mobile applications such as wearable robots has led to the need for carrying batteries to store electrical energy. However, low-efficient actuators require larger batteries that have to be carried along, which limits the range and performance of such devices. In addition, collaborating with robots require safe cobots and actuators. To address this challenge, parallel and series elastic actuators have been adopted in many applications to increase safety and energy efficiency. Meanwhile, some industries, including the automotive industry, have put much research effort into dual motor actuation to decrease the energy consumption of the overall drivetrain.In this paper, we introduce a Spring-Embedded Planetary Dual-Motor Actuator (SEP-DMA) which combines both technologies using a planetary gear unit and a spring. We derive and validate a dynamic model for all possible configurations and discuss their operating modes, including active and passive actuation. To demonstrate the potential of the actuator, we compare it with a traditional stiff actuator consisting of a gearbox and electric motor. The former shows a 40% decrease in the overall energy consumption for a standard task of lifting and oscillating a mass.

High-precision dynamic torque control of high stiffness actuator for humanoids

The high stiffness actuator (HSA), applied to each joint of an electrical driven humanoid robot, can directly affect the motion performance of the torque-controlled humanoid robots. For high control performance of HSA, a high-precision dynamic torque control (HDTC) is proposed. The HDTC consists of two phases: (1) A novel dynamic current control is used to linearize high stiffness actuator torque control system, which can estimate and compensate the nonlinear coupling parts; (2) An enhanced internal model control is designed to ensure high tracking accuracy in the system containing noisy torque signal and even numerical differentiation signals. Benefitting from dynamic current control and the enhanced internal model control, the proposed HDTC is accurate and adaptable. Finally, the superiority of the HDTC is verified with comparative experiments.

Design, Modeling and Experiments of a Variable Stiffness Soft Robotic Glove for Stroke Patients With Clenched Fist Deformity

Soft wearable devices for hand rehabilitation can assist activities of daily living. While existing soft robotic gloves can help with finger flexion, they are less suitable for patients who cannot extend their fingers due to poststroke increased muscular tension. In this letter, we proposed a variable stiffness pneumatic actuator that effectively increases the elasticity of the actuator to facilitate finger extension. The actuator consists of a fiber-reinforced actuator and a multi-stage articulated elastomer. It bends and extends by pneumatic actuation, with multi-stage articulated elastomer providing stiffness adjustment to accommodate varying joint resistance. The variable stiffness mechanism was achieved by the multi layered discrete metal plates which can gracefully fit the unique structure of human fingers. Analysis models were established to quantify the actuator's air pressure, vacuum pressure, and bending angle, also to predict the actuator's stiffness under different vacuum pressures. Experiments and finite element simulation were carried out to evaluate the bending performance of the actuator. Meanwhile, the output force and variable stiffness characteristic of the actuator were tested. The results show that the actuator can achieve 1.54 N fingertip output force and 2.4 times stiffness change, which satisfy the requirement of stroke patient. In addition, a clenched artificial hand with similar damper forces to human hand was applied to further verify the extending effect of the variable stiffness actuator. Finally, a rehabilitation glove integrated with multiple actuators was tested to assess typical rehabilitation gestures and the ability to grasp daily objects.

An Anthropomorphic Robotic Hand With a Soft-Rigid Hybrid Structure and Positive- Negative Pneumatic Actuation

Anthropomorphic robotic hands are seeking to achieve key features such as multi-degree-of-freedom motion ability, bi-directional actuation, high adaptability, and sufficient stiffness. In this research, we propose a 10 active degrees-of-freedom anthropomorphic robotic hand with a soft-rigid hybrid structure and positive-negative pneumatic actuation. The fingers of the hand utilize pneumatic actuators composed of soft bellows to actuate a rigid skeleton for bending motion with unidirectional-stretchable nylon elastic fabric as a strain limiting layer. The positive-negative pneumatic actuation mimics the flexors and extensors of the human hand to implement bi-directional finger actuation. We present a prototype implementation of this robotic hand and provide a preliminary evaluation. The experimental results show that the combined positive-negative pneumatic actuation can extend the workspace of the finger and increase the finger stiffness compared to using only positive pneumatic actuation. The proposed robotic hand has good comprehensive performance (generated a blocking forceof 7.5 N, scored 7 in theKapandji test, achieved32 of the 33 grasp postures from the Feix taxonomy, and could implement 5 human grasping strategies) compared to other robotic hands in literatures.

A configuration-optimisation method for passive-active-combined suspension design

Active control can improve the performance of a passive vehicle suspension, but it comes with a high power consumption and large actuator forces. To balance dynamic performance and power/force needs, both the passive and active parts in the suspension should be designed together and work synergistically. Previous approaches that combine passive and active design have been limited to a narrow range of suspension structures, such as passive-active-parallel layouts. This leaves many other structures unable to be explored (e.g., passive-active-series layouts), and thus the current identified design may be far from optimal. Another limitation is that these previous approaches assume an ideal active part and do not consider the parasitic effects of a physical active actuator and its transmission system, such as backlash, friction and inertia. This simplification would hinder their practical application in real-life situations. To address these two limitations, this work introduces a novel design method for passive-active-combined suspensions. Firstly, it allows for the enumeration of all possible suspension designs consisting of a pre-determined number of stiffness, damping, inertance and active actuator elements. Secondly, the method takes into account the parasitic effects that arise from physical realisation when identifying the optimal suspension design. The effectiveness of this method is demonstrated through a quarter-car case study, where the skyhook controller is adopted as an example control strategy. It is found that compared to the traditional combined suspension, the identified design achieves a significant improvement in the trade-off between ride comfort and required active force. The obtained results are verified experimentally.